 / Climate Monitoring / 2005 / ENSO / Help
Use these links to access detailed analyses of Global and U.S. data. 
|
|
During December 2005, ocean surface and sub-surface conditions associated with the El Niño/Southern Oscillation (ENSO) remained cooler than normal in the equatorial Pacific basin. In the Niño 1+2 region, adjacent to the west coast of South America, the observed Sea-Surface Temperature (SST) anomalies in December varied between -0.5°C and -1°C (-0.9°F and -1.8°F). The cooling trend was also observed in the central and western equatorial Pacific regions due to increased upwelling along the equator. For the month of December, the SSTs decreased in the Niño 3.4 region to reach an anomaly of -0.79°C (-1.42°F), while in the Niño 4 Index region the monthly SSTs also decreased to an anomaly of -0.02°C (-0.036°F). At the end of 2005, these cold SST anomalies in the Niño 3.4 region remained above the threshold to characterize them as weak La Niña conditions in the equatorial Pacific Ocean.
From the atmospheric side of ENSO, the standardized Southern Oscillation Index (SOI) remained negative, but near-neutral, during December, with a monthly averaged index value of -0.2. This was the second consecutive month with a slightly negative SOI value. In contrast to the SOI, the Outgoing Longwave Radiation (OLR) Index remained positive and increased during the past month, with a monthly index value of +1.5. Therefore, despite moderate cooling in the tropical Pacific Ocean, the atmospheric indices were mixed in December, with a near-neutral SOI and the development of a La Niña-type signal in the OLR index. KEYWORDS: ENSO; Niño Regions; Southern Oscillation Index (SOI); Kelvin Waves; Thermocline; Sea-Surface Temperatures (SSTs); Madden-Julian Oscillation (MJO); Walker Circulation |
December
Larger image of December SSTs 
Larger image of December Mixed-Layer Temperatures 
Larger image of December Sea-Surface Temperatures 
Larger image of October-December Sea-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
Since April 2005, the SSTs across the equatorial Pacific Ocean have cooled in the different Niño regions. The coldest anomalies have developed along the South American coast, as reflected in the SST observations from the Niño 1+2 region. Temperatures in the upper ocean and mixed-layer have also decreased, as a layer of anomalously cold water (~1 to ~2°C below normal) has persisted in the central and eastern equatorial Pacific. The monthly mean SSTs continued to cool over the past month, with a December SST anomaly of -0.79°C (-1.42°F) in the Niño 3.4 Index region in the central equatorial Pacific Ocean, and -0.02°C (-0.036°F) in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for December 2005. At the end of December 2005, the 3-month running mean of the Niño 3.4 Index continued to decrease below the long term mean, but remained above NOAA's -0.5°C threshold for classifying the conditions in the equatorial Pacific Ocean as a La Niña. (NOTE: A running 3-month mean SST anomaly below -0.5°C in the Niño 3.4 region is one indicator that a La Niña is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions). |
Larger image of December Zonal Winds 
Pacific Sea-Level Anomalies on 19 December 2005 
Global Sea-Level Anomalies on 19 December 2005 |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds remained above-normal during December across the near-equatorial Pacific, which enhanced equatorial upwelling in the mixed-layer. Stronger-than-average easterly trade winds were reflected in the monthly zonal wind anomalies along the equator, with the largest departures from average of -1 to -2 m/s in the western Pacific region. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. A recent overpass of the Jason-1 satellite on December 19th showed a large region north of the equator in the western North Pacific where positive sea-level anomalies were present. Despite the development of this anomaly in the western North Pacific, no large-scale sea level anomalies had been observed along the equatorial zone at the end of 2005. |
 Larger image of December OLR Anomalies  Larger image of October-December OLR Anomalies  Larger image of December OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during December. In the western Pacific, positive OLR anomalies associated with suppressed convection were observed near the dateline, while negative OLR anomalies associated with enhanced convection were observed over Indonesia. During the October-December period a large east-west oriented region of positive OLR anomalies was observed immediately north of the equator along the Inter-Tropical Convergence Zone (ITCZ), and therefore tropical convection was anomalously below average in the central and eastern equatorial Pacific during the past 3 months. The monthly averaged OLR Index remained postive in December, with a mean value of +1.5 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, although the index has been positive for the past 4 months and has shown an increasing trend. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of December SOI |
Southern Oscillation Index (SOI):
The standardized SOI remained slightly negative in December, with a monthly averaged index value of -0.2. At the end of 2005, the SOI had been negative during 6 months, although it switched signs several times with no definitive longer-term trend. However, since March of 2005 the SOI has remained either neutral or near-neutral, which reflects the lack of any large-scale fluctuations in mean sea-level pressure (MSLP) across the equatorial Pacific basin. |
November
Larger image of November SSTs 
Larger image of November Mixed-Layer Temperatures 
Larger image of November Sea-Surface Temperatures 
Larger image of September-November Sea-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
Since April 2005, the SSTs across the equatorial Pacific Ocean have cooled in the different Niño regions. The coldest anomalies have developed along the South American coast, as reflected in the SST observations from the Niño 1+2 region. Temperatures in the upper ocean and mixed-layer have also decreased, as a layer of anomalously cold water (~1 to ~2°C below normal) has persisted between 50-200 m depth in the central and eastern equatorial Pacific. For the monthly means, the November SST anomaly was -0.49°C (-0.88°F) in the Niño 3.4 Index region in the central equatorial Pacific Ocean, and +0.12°C (+0.22°F) in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for November 2005. The monthly SST anomalies in the Niño 3.4 region have exceeded +0.5°C only during April of this year, after which the 2004-2005 ENSO warm event dissipated. At the end of November 2005, the 3-month running mean of the Niño 3.4 Index continued to decrease below the long term mean. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions). |
Larger image of November Zonal Winds 
Pacific Sea-Level Anomalies on 29 October 2005 
Global Sea-Level Anomalies on 29 October 2005 |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds remained above-normal during November across the near-equatorial Pacific, which enhanced equatorial upwelling in the mixed-layer. Monthly zonal wind anomalies exceeding 1 m/s were observed along the equator in the eastern Pacific region. In the western equatorial Pacific basin, anomalous westerly winds that were present at the beginning of the month relaxed and stronger than average easterly trade winds developed west of the dateline by mid-November (see the November zonal wind loop). Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans are shown to the left. A recent overpass of the Jason-1 satellite on October 29th showed no large-scale sea-level anomalies were present in the equatorial Pacific. However, increased sea levels associated with lower atmospheric pressure and tropical convection were observed just south of the equator along the South Pacific Convergence Zone (SPCZ). |
 Larger image of November OLR Anomalies  Larger image of September-November OLR Anomalies  Larger image of November OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during November. No large-scale OLR anomalies (positive or negative) were observed along the equator or within the equatorial region in the eastern tropical Pacific over the past month. In the western Pacific positive OLR anomalies, associated with suppressed convection, were observed near the dateline, while enhanced convection and negative OLR anomalies were observed over Indonesia. During the September-November period a large east-west oriented region of positive OLR anomalies was observed immediately north of the equator along the Inter-Tropical Convergence Zone (ITCZ) , and therefore tropical convection was anomalously below average in the central and eastern equatorial Pacific during the past 3 months. The monthly averaged OLR Index remained postive in November, with a mean value of +1.1 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, although the index has been positive for the past 4 months. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of November SOI |
Southern Oscillation Index (SOI):
The standardized SOI decreased in November, with a monthly averaged index value of -0.3. So far in 2005, the SOI has been negative during 6 months, although it switched signs several times with no definitive longer-term trend. However, since March of this year the SOI has remained either neutral or near-neutral, which reflects the lack of any large-scale fluctuations in mean sea-level pressure (MSLP) across the tropical Pacific basin. |
October
Larger image of October SSTs 
Larger image of October Mixed-Layer Temperatures 
Larger image of October Sea-Surface Temperatures 
Larger image of August-October Sea-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
Since April 2005, the SSTs across the equatorial Pacific Ocean have cooled to near-normal in the different Niño regions. The coldest anomalies have developed along the South American coast, as reflected in the SST observations from the Niño 1+2 region. Temperatures in the upper ocean and mixed-layer have also decreased, as a layer of anomalously cold water (~1°C or ~2°F below normal) has persisted at 50-200 m depth in the equatorial Pacific. For the monthly means, the October SST anomaly was -0.06°C (-0.108°F) below average in the Niño 3.4 Index region in the central equatorial Pacific Ocean, and +0.24°C (+0.43°F) above the mean in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for October 2005. The monthly SST anomalies in the Niño 3.4 region have exceeded +0.5°C only during April of this year, after which the 2004-2005 ENSO warm event dissipated. At the end of October 2005, the 3-month running mean of the Niño 3.4 Index continued to decrease below the long term mean. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of October Zonal Winds 
Pacific Sea-Level Anomalies on 2 October 2005 
Global Sea-Level Anomalies on 2 October 2005 |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were above-normal during October across the tropical Pacific basin, which enhanced equatorial upwelling in the mixed-layer. Monthly zonal wind anomalies exceeding 1 m/s were observed along the equator near the South American coast and between 170° E and 150° W. In the western equatorial Pacific basin, anomalous westerly winds that were present at the beginning of the month relaxed and easterly trade winds developed west of the dateline by mid-October (see the October zonal wind loop). Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans are shown to the left. The overpass of the Jason-1 satellite on October 2nd showed that increased sea level associated with tropical convection developed near the equator in the western Pacific at the beginning of the month. |
 Larger image of October OLR Anomalies  Larger image of August-October OLR Anomalies  Larger image of October OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during October. No large-scale OLR anomalies (positive or negative) were observed along the equator or within the equatorial region in the tropical Pacific. The 3-month averaged OLR anomalies during the August-October period show that positive OLR anomalies were observed immediately north of the equator along the Inter-Tropical Convergence Zone (ITCZ) and therefore tropical convection was below average in the central and eastern equatorial Pacific. The monthly averaged OLR Index remained postive in October, with a mean value of +0.3 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, with no persistent trend in the index observed so far in 2005. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of October SOI |
Southern Oscillation Index (SOI):
The standardized SOI increased in October, to a monthly averaged index value of +1.1. This was the largest positive SOI value since May 2004. So far in 2005, the SOI has been negative during 5 months. However, since March of this year the SOI has been either neutral or near-neutral, which reflects the lack of any large-scale fluctuations in mean sea-level pressure (MSLP) or the Walker Circulation across the tropical Pacific basin. Therefore, atmospheric indicators during the past few months have continued to reflect the near-neutral status of ENSO in the equatorial Pacific. |
September
Larger image of September SSTs 
Larger image of September Mixed-Layer Temperatures 
Larger image of September Sea-Surface Temperatures 
Larger image of July-September Sea-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
Over the past month, SSTs across the equatorial Pacific Ocean have continued to cool or remain near-normal. The coldest anomalies have persisted along the South American coast, as reflected in the SST observations from the Niño 1+2 region during September. Temperatures in the upper ocean and mixed-layer have also decreased, as a layer of anomalously cold water (~1-2°C or ~2-3.5 °F below normal) persisted at 50-200 m depth in the equatorial Pacific. For the month, the September SST anomaly was -0.18°C (-0.32°F) below average in the Niño 3.4 Index region in the central equatorial Pacific Ocean, and +0.09°C (+0.16°F) above the mean in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for September 2005. In early 2005, ocean surface and subsurface temperatures began to cool in the equatorial Pacific basin. As a result, the monthly SST anomalies in the Niño 3.4 region exceeded +0.5°C only during April of this year, after which the 2004-2005 ENSO warm event dissipated. At the end of September 2005, the 3-month running mean of the Niño 3.4 Index continued to decline sharply and was was approaching zero. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of September Zonal Winds 
17 September 2005 Pacific Sea-Level Anomalies Loop of Global Sea-Level Altimetry |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were above-normal during September across the eastern and central tropical Pacific basin, which helped to enhance equatorial upwelling in the mixed-layer. In the western equatorial Pacific basin, westerly wind anomalies developed during the latter half of the month and these moved east past the dateline by mid-September (see the September zonal wind loop). Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The overpass of the Jason-1 satellite on September 17th showed that increased sea level associated with tropical convection developed near the equator in the western Pacific near the end of the month. This was the first significant sea level rise since late April, but it remains uncertain whether this event generated an oceanic Kelvin wave and whether it will persist and move further eastward. |
 Larger image of September OLR Anomalies  Larger image of July-September OLR Anomalies  Larger image of September OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during September. No large-scale OLR anomalies were observed along the equatorial region in the tropical Pacific, except for a region of negative OLR anomalies associated with enhanced convection and tropical cyclone activity west of the dateline. The 3-month averaged OLR anomalies during the July-September period show that positive OLR anomalies were present just north of the equator in the central and eastern Pacific basin, and therefore tropical convection was suppressed along the Inter-Tropical Convergence Zone (ITCZ). The monthly averaged OLR Index remained postive in September, with a mean value of +0.1 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, with no persistent trend in the index observed so far in 2005. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of September SOI |
Southern Oscillation Index (SOI):
The standardized SOI remained near-neutral in September, despite a slight increase in the monthly averaged index to a value of +0.4. The SOI has been negative during 5 months so far in 2005, with an extraordinarily low value of -4.1 occurring back in February 2005 (which was the lowest SOI value since the peak of the 1982-1983 El Niño event). However, since March the SOI has been either neutral or near-neutral, which reflects the lack of any large-scale fluctuations in mean sea-level pressure (MSLP) across the tropical Pacific basin. |
August
Larger image of August SSTs 
Larger image of August Sub-Surface Temperatures 
Larger image of August Sea-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
Over the past month, SSTs decreased across the western and central equatorial Pacific Ocean. Below normal SSTs have persisted along the South American coast, as reflected in the observations from the Niño 1+2 region during August. Mixed-layer temperatures also decreased, as a layer of anomalously cold water (1-2 °C below normal) developed at 50-200 m depth along the equatorial cold-tongue in the eastern Pacific region between 160°W and 110°W . For the month, the SST anomalies remained near average in the Niño index regions, with an August value of +0.05°C (+0.09°F) above average in the Niño 3.4 region in the central equatorial Pacific Ocean, and -0.03°C (-0.05°F) below the mean in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for August 2005. Over the past year, above average SST anomalies had persisted in the western and central equatorial Pacific basin, with the largest SST anomalies of the 2004-2005 El Niño event observed during November 2004. However, SSTs began to cool in early 2005 and the monthly anomalies in the Niño 3.4 region exceeded +0.5°C only during April of this year, after which the warm event dissipated. At the end of August 2005, the 3-month running mean of the Niño 3.4 Index continued to decline and remained below +0.5°C. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of August Zonal Winds 
21 August 2005 Pacific Sea-Level Anomalies Loop of Global Sea-Level Altimetry |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were stronger-than-normal across the eastern and central equatorial Pacific basin during August, enhancing equatorial upwelling in the mixed-layer. In the far western equatorial Pacific basin west of the dateline, westerly wind anomalies developed during the latter half of the month (see the August zonal wind loop). At monthly and seasonal time-scales, this pattern of stronger than normal trade winds in the eastern Pacific and weaker than normal trade winds in the western Pacific has persisted for several months. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The overpass of the Jason-1 satellite on August 21st showed that no large-scale fluctuations in sea-level were present across the equatorial Pacific basin near the end of this past month. Since late April, there have been no significant sea-level anomalies associated with oceanic Kelvin waves in the equatorial Pacific basin. |
 Larger image of August OLR  Larger image of August OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during August. No large-scale OLR anomalies were observed along the equatorial region in the tropical Pacific. The 3-month averaged OLR anomalies during the June-August period show that positive OLR anomalies were present in the central and eastern Pacific basin, and therefore tropical convection was suppressed along the Inter-Tropical Convergence Zone (ITCZ). The monthly averaged OLR Index shifted signs in August, with a mean value of +0.6 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, with no persistent trend in the index observed so far in 2005. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of August SOI |
Southern Oscillation Index (SOI): The standardized SOI remained near-neutral in August, despite a slight decrease in the monthly averaged index to a value of -0.8. The SOI has been negative during 5 months so far in 2005, with an extraordinarily low value of -4.1 occurring back in February 2005 (which was the lowest SOI value since the peak of the 1982-1983 El Niño event). However, since March the SOI has been either neutral or near-neutral, which reflects the lack of any large-scale anomalies in mean sea-level pressure (MSLP) across the tropical Pacific basin. |
July
Larger image of July SSTs 
Larger image of July Sub-Surface Temperatures 
Larger image of July Sea-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs decreased across the western and central equatorial Pacific Ocean this past month. Anomalously cold ocean surface temperatures persisted along the South American coast in July, as reflected in the observations from the Niño 1+2 region during this past month. Surface and subsurface temperatures in the mixed-layer decreased during July, as a layer of anomalously cold water developed near 140°W and along the equatorial cold-tongue in the eastern Pacific region. For the month, the SST anomalies decreased in the Niño index regions: the July value was -0.1°C (-0.18°F) below average in the Niño 3.4 region in the central equatorial Pacific Ocean, and +0.1°C (+0.18°F) above the mean in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for July 2005. Over the past year, above average SST anomalies had persisted in the western and central equatorial Pacific basin, with the largest SST anomalies observed during November 2004. However, SSTs cooled at the beginning of 2005 with monthly anomalies exceeding +0.5°C only during April of this year. At the end of July, the 3-month running mean of the Niño 3.4 Index continued to decrease and remained below +0.5°C. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of July Zonal Winds 
Larger image of 25 July 2005 Pacific Sea-Level Loop of Global Sea-Level Altimetry |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were stronger-than-normal across the eastern and central equatorial Pacific basin during July, which helped enhance equatorial upwelling in the mixed-layer. In the far western equatorial Pacific basin west of the dateline, westerly wind anomalies developed during the latter half of the month (see the July zonal wind loop). Several high frequency fluctuations in the trade winds also occurred during July, as the strength of the easterly flow varied considerably along the equatorial zone. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The most recent overpass of the Jason-1 satellite on July 25th showed that no large-scale fluctuations in sea-level were present across the equatorial Pacific basin near the end of this past month. |
 Larger image of July OLR  Larger image of July OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during July. Negative OLR anomalies, which are associated with enhanced tropical convection, were observed along the equator west of the dateline in the western Pacific. The 3-month averaged OLR anomalies during the May-July period show that tropical convection was suppressed in the central and eastern Pacific basin along the Inter-Tropical Convergence Zone (ITCZ). The July OLR Index was near-neutral, with a monthly mean value of -0.3 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, with no persistent trend in the index observed so far in 2005. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of July SOI |
Southern Oscillation Index (SOI): The standardized SOI was neutral in July, with a monthly averaged index value of 0.0. Similar to the OLR Index, the SOI displayed unusual behavior during the relatively weak 2004-2005 El Niño event. For example, during the latter-half of 2004 the monthly SOI fluctuated several times between a near-zero value (indicating near-neutral conditions) and a more negative index (indicating El Niño conditions), although 2004 ended with seven consecutive negative monthly values. During the past few months the SOI has been near-neutral, which reflects the neutral state of the atmosphere across the tropical Pacific basin. |
June
Larger image of May SSTs 
Larger image of June Sub-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs decreased across the eastern equatorial Pacific Ocean and along the South American coast in June, as reflected in the Niño 1+2 region, although the monthly averaged SST anomalies increased slightly in the central and western parts of the Pacific basin. Above average temperature anomalies, which have persisted over the past six months in the mixed-layer decreased during June, as below average water temperatures expanded along the equatorial cold-tongue in the eastern Pacific region. For the month, the SST anomaly in both the Niño 3.4 and Niño 4 Index regions in the central and western equatorial Pacific Ocean decreased to +0.44°C (+0.79°F) above the mean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for June 2005. Above average SST anomalies persisted throughout the latter-half of 2004 and through the first half of 2005 in the western and central equatorial Pacific basin, which can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). At the end of June, the 3-month running mean of the Niño 3.4 Index remained below +0.5°C. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of June Zonal Winds 
Larger image of 25 June 2005 Pacific Sea-Level Loop of Global Sea-Level Altimetry |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were near-normal across the eastern equatorial Pacific basin during June. In the western equatorial Pacific basin near the date line, westerly wind anomalies developed during the latter half of the month (see the June zonal wind loop). Several high frequency fluctuations in the trade winds also occurred in the central tropical Pacific during June, as the strength of the easterly flow varied considerably along the equatorial zone. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The June 25th overpass of the Jason-1 satellite shows that there were no large-scale sea-level anomalies present across the equatorial Pacific basin near the end of this past month. |
 Larger image of June OLR  Larger image of April-June OLR  Larger image of June OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during June. Only a few small areas with positive OLR anomalies were measured in the western tropical Pacific during the past month. In addition, negative OLR anomalies were observed just north of the equator in the central Pacific. A significantly different pattern is evident in the most recent 3-month averaged OLR anomalies, with large areas of negative OLR anomalies west of the dateline and along the South Pacific Convergence Zone (SPCZ). Enhanced tropical convection was also observed during the April-June period in the central Pacific basin immediately north of the equator along the Inter-Tropical Convergence Zone (ITCZ). The monthly-averaged OLR Index was near-neutral in June, with a value of 0.3 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past 9 months, reflecting the lack of persistence of El Niño conditions in the atmosphere since the latest warm event developed in mid-2004. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of June SOI |
Southern Oscillation Index (SOI): The standardized SOI was near-average in June, with a monthly averaged index value of 0.1. Similar to the OLR Index, the SOI has shown unusual behavior during the relatively weak 2004-2005 El Niño event. For example, during the latter-half of 2004 the monthly SOI fluctuated several times between a near-zero value (indicating near-neutral conditions) and a more negative index (indicating El Niño conditions), although 2004 ended with seven consecutive negative monthly values. The most recent trend of the SOI over the past few months has been toward a near-zero value. This reflects the transition to an ENSO neutral state in the atmosphere, as a near-average mean sea-level pressure (MSLP) pattern was observed in June across the tropical Pacific basin. Therefore, despite the persistence of anomalously warm SST anomalies in the Niño 3.4 and 4 Index regions during June, the majority of oceanic and atmospheric indices suggests that the current state of ENSO remains in a neutral phase. |
May
Larger image of May SSTs 
Larger image of May Sub-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs decreased across the eastern equatorial Pacific Ocean and along the South American coast in May, as reflected in the Niño 1+2 region, although the monthly averaged SST anomalies increased slightly in the central and western parts of the Pacific basin. Above average temperature anomalies that have persisted over the past six months in the mixed-layer decreased during May, as below average water temperatures expanded along the equatorial cold-tongue in the eastern Pacific region. For the month, the SST anomaly in the Niño 3.4 Index region in the central equatorial Pacific Ocean increased to +0.53°C (+0.94°F) above the mean, while in the western Pacific region the SST anomaly also increased in May, with a monthly averaged Niño 4 Index value of +0.55°C (+0.99°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for May 2005. Above average SST anomalies persisted throughout the latter-half of 2004 and into early 2005 in the western and central equatorial Pacific basin, which can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Despite the slight increase in the monthly averaged SSTs in the central equatorial Pacific this past month, the 3-month running mean of the Niño 3.4 Index remained below +0.5°C in May. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of May Zonal Winds 
Larger image of 29 May 2005 Pacific Sea-Level Loop of Global Sea-Level Altimetry |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were stronger-than-normal across the eastern equatorial Pacific basin during May. In the western parts of the basin near the date line, westerly wind anomalies developed during the latter half of the month (see the May zonal wind loop). Several high frequency fluctuations in the trade winds also occurred in the central tropical Pacific during May, as the strength of the easterly flow varied considerably along the equatorial zone. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The May 29th overpass of the Jason-1 satellite shows that there were no large-scale sea-level anomalies present across the equatorial Pacific basin at the end of the month. |
 Larger image of May OLR  Larger image of March-May OLR  Larger image of May OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during May. Negative OLR anomalies were measured west of the date line in the western tropical Pacific and along the South Pacific Convergence Zone (SPCZ) during the past month. In addition, negative OLR anomalies were observed just north of the equator in the central Pacific, while in contrast positive OLR anomalies occurred along the equator in the central and eastern portions of the Pacific basin. A similar pattern is evident in the most recent 3-month averaged OLR anomalies, with large areas of negative OLR anomalies west of the dateline and along the SPCZ. Enhanced tropical convection also occurred in the eastern Pacific immediately north of the equator along the Inter-Tropical Convergence Zone (ITCZ). The monthly-averaged OLR Index was near-neutral in May, with a value of -0.1 averaged across an area centered over the date line in the western Pacific (between 160° E and 160° W). The recent shifts in sign of the OLR Index reflected the lack of persistence of El Niño conditions in the atmosphere since it developed in mid-2004. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of May SOI |
Southern Oscillation Index (SOI): The standardized SOI decreased in May to a monthly averaged value of -1.2 (note that negative SOI values are consistent with El Niño conditions). Despite the negative SOI value the past three months, none of these monthly values approached the unusually low index in February 2005, which was the lowest monthly value since the peak of the 1982-1983 El Niño event. The SOI has shown some unusual behavior during the relatively weak 2004-2005 El Niño event. For example, during the latter-half of 2004 the monthly SOI fluctuated several times between a near-zero value (indicating near-neutral conditions) and a more negative index (indicating warm event conditions), although 2004 ended with seven consecutive negative monthly values. Therefore, despite the recently observed negative SOI values and the persistence of anomalously warm SST anomalies in the Niño 3.4 and 4 regions during May, the majority of oceanic and atmospheric indices suggest that the current ENSO warm event had transitioned from weak El Niño conditions to neutral conditions during the past month. |
April
Larger image of April SSTs 
Larger image of April Sub-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs increased across the eastern equatorial Pacific Ocean and along the South American coast in April (note the observed increase in SSTs in the Niño 1+2 region), although the monthly averaged SST anomaly increased only slightly in the central parts of the Pacific basin. In the mixed-layer, above average temperature anomalies moved eastward during the month into the eastern equatorial Pacific, with the largest anomalies reaching over 3°C at 50 m depth between 120°W and the South American coast. The depth of the 20°C isotherm increased in April as well, with the largest depth anomalies in the eastern equatorial Pacific. For the month, the SST anomaly in the Niño 3.4 Index region in the central equatorial Pacific Ocean increased slightly to +0.36°C (+0.65°F) above the mean. In the western Pacific region, the SST anomaly decreased in April, with a monthly averaged Niño 4 Index value of +0.51°C (+0.92°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for April 2005. Above average SST anomalies persisted throughout the latter-half of 2004 and into early 2005 in the western and central equatorial Pacific basin, which can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Despite the slight increase in the monthly averaged SSTs in the central equatorial Pacific this past month, the 3-month running mean of the Niño 3.4 Index remained below +0.5°C in April. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of April Zonal Winds 
Larger image of 20 April 2005 Pacific Sea-Level Loop of Global Sea-Level Altimetry |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The trade winds were near- to above-normal across much of the eastern and central equatorial Pacific basin during April. In the western parts of the basin near the date line, westerly wind anomalies at the beginning of April dissipated as easterly trade winds increased during the latter half of the month (see the April zonal wind loop). Several high frequency fluctuations in the trade winds also occurred in the central tropical Pacific during April, as the strength of the easterly flow varied considerably along the equatorial zone. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The April 20th overpass of the Jason-1 satellite shows that another Kelvin wave had developed and was propagating across the equatorial Pacific basin and into the central equatorial Pacific near the end of the month. |
 Larger image of April OLR  Larger image of Feb-Apr OLR  Larger image of April OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during April. Negative OLR anomalies were measured west of the date line in the western tropical Pacific and along the South Pacific Convergence Zone (SPCZ) during the past month. In addition, negative OLR anomalies were observed just north of the equator in the central and eastern Pacific, while in contrast positive OLR anomalies were observed immediately south of the equator in the central and eastern portions of the Pacific basin. Note that the enhanced tropical convection that occurred in February associated with the formation of numerous tropical cyclones is still reflected in the most recent 3-month averaged OLR anomalies, with large areas of negative OLR anomalies near the dateline and westward along the equator, as well as along the SPCZ. The monthly-averaged OLR Index shifted sign to a negative value in April, with an index of -0.4 averaged across an area centered over the date line in the western Pacific (between 160° E and 160° W). The recent shifts in sign of the OLR Index reflected the lack of persistence of El Niño conditions in the atmosphere since it developed in mid-2004. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of April SOI |
Southern Oscillation Index (SOI): The standardized SOI decreased in April, with a monthly averaged value of -1.0 (note that negative SOI values are consistent with El Niño conditions). This transition to a lower SOI value in the atmosphere followed the near-neutral SOI in March and the unusually low index in February. The SOI has shown some unusual behavior during the relatively weak 2004-2005 El Niño event. For example, during the latter-half of 2004 the monthly SOI fluctuated several times between a near-neutral value and a more negative index associated with warm event conditions (although 2004 ended with seven consecutive negative monthly values). In summary, the SST anomalies and atmospheric indices suggest that the current ENSO warm event remained weak, despite the recently observed warming of the SST anomalies in the Niño 1+2 region during April. |
March
Larger image of March SSTs 
Larger image of March Sub-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs decreased along the South American coast and across the eastern equatorial Pacific Ocean in March, although the monthly anomalies increased slightly in the central parts of the Pacific basin. In the mixed-layer, above average temperatures persisted in the central equatorial Pacific, with the largest anomalies reaching over 4°C at 100-200 m depth between 170°W and 130°W. The depth of the 20°C isotherm increased in March, with the largest depth anomalies in the central equatorial Pacific. For the month, the SST anomaly in the Niño 3.4 Index region in the central equatorial Pacific Ocean increased slightly to +0.32°C (+0.58°F) above the mean. In the western Pacific region, the SST anomaly decreased slightly in March, with a monthly averaged Niño 4 Index value of +0.67°C (+1.21°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for March 2005. Above average SST anomalies persisted throughout the latter-half of 2004 and into early 2005 in the western and central equatorial Pacific basin, which can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Due to the recent decreases in the monthly averaged SSTs, the 3-month running mean of the Niño 3.4 Index declined below +0.5°C in March. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of March Zonal Winds 
Larger image of 21 March 2005 Pacific Sea-Level Loop of Global Sea-Level Altimetry |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The trade winds were above normal across much of the eastern equatorial Pacific basin. A significant transition occurred in the western half of the tropical Pacific during during March (see the March zonal wind loop), as westerly wind anomalies during the first half of the month gave way to stronger than normal easterly trade winds. This led to the observed decreases in the monthly averaged SST anomalies in the Niño 1+2 region. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The March 21st overpass of the Jason-1 satellite shows that the Kelvin wave that propagated across the equatorial Pacific in late February and early March dissipated as it moved into the eastern parts of the equatorial Pacific near the end of the month. |
 Larger image of March OLR  Larger image of Jan-Mar OLR  Larger image of March OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during March. In contrast to the anomalous convection and tropical cyclone activity observed in February, there were no large regions of enhanced convection and associated negative OLR anomalies present in the equatorial Pacific in March. Positive OLR anomalies were observed in the central and eastern portions of the Pacific basin and along the South American coast during the month. However, the convection that occurred in February is reflected in the most recent 3-month averaged OLR anomalies, with large areas of negative OLR anomalies near the dateline and westward along the Equator, and also along the South Pacific Convergence Zone (SPCZ). The monthly-averaged OLR Index shifted sign to a positive value in March, with an index of +0.4 averaged across an area centered over the Dateline in the western Pacific (between 160° E and 160° W). The recent shifts in sign of the OLR Index reflected the lack of persistence of El Niño conditions in the atmosphere since it developed in mid-2004. Therefore, the change in sign in March has been part of a longer period of inconsistent OLR indices since early 2004. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of March SOI |
Southern Oscillation Index (SOI): The standardized SOI was near-neutral in March, with a monthly averaged value of -0.2 (note that negative SOI values are consistent with El Niño conditions). This transition to near-neutral conditions in the atmosphere followed a strongly negative SOI in February. The SOI has shown some unusual behavior during the relatively weak 2004-2005 El Niño event. For example, during the latter-half of 2004 the monthly SOI fluctuated several times between a near-neutral value and a more negative index associated with warm event conditions (although 2004 ended with seven consecutive negative monthly values). In summary, the SST anomalies and other indices suggest that the current ENSO warm event remained weak and confined to the western and central parts of the equatorial Pacific at the end of March. |
February
SOI REACHES LOWEST VALUE SINCE FEBRUARY 1983
Larger image of February SSTs 
Larger image of February Sub-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs decreased across the entire equatorial Pacific Ocean in February, although the monthly values remained above average in the western and central parts of the basin. In the mixed-layer, above average temperatures persisted in the central equatorial Pacific, with the largest anomalies reaching over 4°C at 100-200 m depth between 170°E and 150°W. The depth of the 20°C isotherm decreased in February, with the largest anomalies developing along the equatorial cold-tongue in the eastern Pacific. For the month, the SST anomaly in the Niño 3.4 Index region in the central equatorial Pacific Ocean decreased to +0.13°C (+0.23°F) above the mean. The February monthly mean was significantly cooler than in January, when the SST anomaly was +0.68°C (+1.22°F). In the western Pacific region, the SST anomaly decreased as well in February, with a monthly averaged value of the Niño 4 Index of +0.72°C (+1.66°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for February 2005. Above average SST anomalies persisted throughout the latter-half of 2004 and into early 2005 in the western and central equatorial Pacific basin, and this can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Despite the significant decreases in the monthly averaged SSTs, the 3-month running mean of the Niño 3.4 Index remained slightly above +0.5°C in February. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of February Zonal Winds 
Larger image of 22 February 2005 Pacific Sea Level |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The trade winds were above normal across much of the eastern and central equatorial Pacific basin during February. These easterly wind anomalies increased equatorial upwelling in the mixed-layer, which led to the observed decreases in the monthly averaged SST anomalies in the Niño 1+2 and 3.4 regions. In contrast to the strong trade winds in the eastern Pacific, anomalous westerly winds developed in the western equatorial Pacific during February. Periods of unusual westerly winds along the equator are typically termed a westerly-wind-burst (WWB), and they usually form in the eastern Indian Ocean and over Indonesia in association with Madden-Julian Oscillation (MJO) activity. Anomalously strong westerly winds were observed near the Dateline during the month, and these westerly winds (i.e., positive zonal winds) were evident in both the February monthly means and anomalies measured by NOAA's Tropical Atmosphere-Ocean (TAO) buoy array. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The westerly wind activity that began in January and persisted in February along the Equator in the western Pacific generated an oceanic Kelvin wave in the mixed-layer, which subsequently propagated eastward. The most recent Pacific overpass of the Jason-1 satellite shows that a significant area of positive sea-level anomalies associated with this Kelvin wave were present in the western equatorial Pacific basin near the end of the month (see the 22 February 2005 overpass). |
 Larger image of February OLR  Larger image of Dec-Feb OLR  Larger image of Feb OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during February. In general, a large region of enhanced convection and negative OLR anomalies were present along the equator near the dateline in the western equatorial Pacific region. The most recent 3-month averaged OLR anomalies were negative near the Dateline, and positive in the eastern portions of the basin and the South American coast. The large region with enhanced convection along the South Pacific Convergence Zone (SPCZ) was associated with five tropical cyclones during February (2 tropical storms and 3 major cyclones: see the South Pacific / Australia region Tropical Cyclone page and the tropical cyclone summaries on the February Global Hazards pages for further details). The monthly-averaged OLR Index shifted signs to a negative value in February, with an index of -2.3 averaged across an area centered over the Dateline in the western Pacific between 160° E and 160° W. The recent shifts in sign of the OLR Index reflected the lack of persistence of El Niño conditions in the atmosphere since it developed in mid-2004. Despite the monthly fluctuations, the February negative OLR index was the lowest since a value of -2.6 in December 1994. Therefore, this unusually low OLR index during February, and the associated convection along the Equator, have been the most significant response of the atmosphere to this point during the 2004-2005 El Niño event. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of February SOI |
Southern Oscillation Index (SOI): The standardized SOI switched signs from a slightly positive value in January to a strongly negative index in February, with an averaged SOI value of -4.1 for the month (note that negative SOI values are consistent with El Niño conditions). The SOI has shown some unusual behavior during the 2004-2005 El Niño. For example, during the latter-half of 2004 the weekly SOI fluctuated several times between a near-neutral value and a more negative index associated with warm event conditions (although 2004 ended with seven consecutive negative monthly values). However, the dramatic decrease in the SOI in February 2005 was almost unprecendented, and the lowest monthly value since February 1983 when the SOI was -4.6 (which was the peak of the 1982-1983 El Niño). Despite the anomalously low monthly SOI in February, the SST anomalies and other indices suggested that the current warm event remained weak and confined to the western and central parts of the equatorial Pacific at the end of the month. |
January
SST ANOMALIES DECREASE IN THE EQUATORIAL PACIFIC
Larger image of January SSTs 
Larger image of January Sub-Surface Temperatures |
Sea-Surface Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs decreased across the entire equatorial Pacific Ocean in January, although they remained above average for the month. In the mixed-layer, above average temperatures persisted, with the largest anomalies reaching 2°C at approximately 100 m depth between 170°E and 140°W. The depth of the 20°C isotherm decreased in January, with negative anomalies developing in the western Pacific. In the central equatorial Pacific, the SST anomaly in the Niño 3.4 Index region decreased to +0.68°C (+1.22°F) above the mean. The western Pacific SST anomaly decreased as well, with a monthly averaged Niño 4 Index of +0.92°C (+1.66°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for January 2005. Above average SST anomalies have persisted throughout 2004 and into January 2005 in the western and central equatorial Pacific basin, and this can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Despite the decreases in the monthly averaged SSTs, the 3-month running mean of the Niño 3.4 Index remained above +0.5°C in January. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.) |
Larger image of January Zonal Winds 
Larger image of 23 January 2005 Pacific Sea Level |
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were above normal across much of the eastern and central equatorial Pacific basin during January, with westerly wind anomalies developing in the western Pacific during the month. The observed easterly trade-wind anomalies increased equatorial upwelling in the mixed-layer, which led to the observed decreases in the monthly averaged SST anomalies in the Niño regions. Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The anomalous westerly wind activity in the western Pacific did not generate another oceanic Kelvin wave in the mixed-layer by the end of January. The most recent Pacific overpass of the Jason-1 satellite shows no significant, large-scale positive or negative sea level anomalies in the equatorial Pacific at the end of the month (see the 23 January 2005 overpass). |
 Larger image of January OLR Larger image of Nov-Jan OLR Larger image of January OLR Index |
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during January. In general, the majority of the central equatorial Pacific region had positive OLR anomalies with respect to the 1979-1995 base period during January. The 3-month averaged OLR anomalies were positive or near-normal along the Equator, from the dateline to the South American coast. The one region that had enhanced convection was the western Pacific region, where large-scale tropical convection and several tropical cyclones have occurred. The monthly-averaged OLR Index value remained positive in January, with a value of +0.2 W m-2 averaged across an area centered over the dateline in the western Pacific between 160° E and 160° W. The recent shift in sign of the OLR Index from a negative value in November to positive one in December reflected the lack of persistence in the index in 2004, during which it shifted sign numerous times. The January positive index value reflected the change that occurred in late 2004. Therefore, the convective response of the atmosphere to the above normal SST anomalies in the western and central equatorial Pacific has been limited, since the OLR Index has shown no consistent trend during the 2004-2005 El Niño. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices. |
 Larger image of January SOI |
Southern Oscillation Index (SOI): The standardized SOI switched signs from negative to positive in January, with an average index value of +0.3 for the month. This switch in sign followed seven consecutive months during which the SOI had remained negative (note that negative SOI values are consistent with El Niño conditions). The trend in SOI during the latter half of 2004 fluctuated several times between a near-neutral value and a more negative index associated with warm event conditions. The observed fluctuations in the SOI during 2004, combined with the recent change in sign to a positive value in January, suggests that the atmosphere never fully responded to the above average SSTs during this weak El Niño event. Given that the El Niño conditions in the equatorial Pacific weakened further in January, it appears that the current warm event peaked during November and December 2004. Therefore, the 2004-2005 El Niño has begun to diminish, and ENSO conditions were approaching near-neutral at the end of January given all of the observed changes in atmospheric and oceanic indices. |
ENSO Links
Questions?
| For all climate questions other than questions concerning this report, please contact the National Climatic Data Center's Climate Services Division: |
|
|
For further information on the historical climate perspective presented in this report, contact: |
-or-
|
NCDC / Climate Monitoring / 2005 / ENSO / Help
