1. Introduction

SUMMARY: Coral bleaching has become a serious threat to coral reefs worldwide, and mass coral bleaching is caused by elevated sea surface temperature (SST). NOAA's satellites measure SST in near-real-time, and Coral Reef Watch (CRW) uses this information to pinpoint areas around the world where corals are at risk for bleaching. Our suite of operational data products are produced at 0.5 degree (approximately 50-km) spatial resolution twice-weekly. Data and images are available for free on our website.

Coral reefs are presently Earth's largest biological structures and have taken thousands of years to form. However, in recent decades, coral reef ecosystems have been declining at alarming rates worldwide. Coral bleaching has been one of the most significant contributors to this increased deterioration (Wilkinson, 2008).

Coral bleaching occurs when the symbiotic relationship between algae (zooxanthellae) and their host coral breaks down under certain environmental stresses. As a result, the host expels its zooxanthellae, exposing its white calcium carbonate skeleton and the affected coral colony becomes stark white or pale in color. This is known as "coral bleaching." Coral bleaching can be triggered and sustained under various environmental stresses. Anomalously warm water temperatures have been observed as one of the major causes of mass coral bleaching worldwide. Ambient water temperatures as little as 1 to 2 °C above a coral's tolerance level, indicated by summer monthly mean temperatures, can cause coral bleaching (Berkelmans and Willis, 1999; Reaser et al., 2000). Corals that are partially to totally bleached for long periods often die. Severe bleaching events have dramatic long-term ecological and social impacts, including loss of reef-building corals, changes in benthic habitat and, in some cases, changes in fish populations on the reef. Even under favorable conditions, it can take decades for severely bleached reefs to fully recover (Wilkinson, 2008).

The need for improved understanding, monitoring, and prediction of coral bleaching has become imperative. Satellite remote sensing, which provides synoptic views of the global oceans in near-real-time and monitors remote reef areas previously known only to wildlife, has become an essential tool for coral reef managers and scientists. As early as 1997, NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) began producing web-accessible, satellite-derived, global near-real-time nighttime sea surface temperature (SST) products to monitor thermal conditions conducive to coral bleaching and to assess the intensity of bleaching stress around the globe. This activity evolved into a crucial part of NOAA's Coral Reef Watch (CRW) program in 2000 (Strong et al., 2004; Liu et al., 2005). CRW's earliest "experimental" products were the outgrowth of earlier work by Goreau and Hayes (1994) and by Montgomery and Strong (1995). Between September 2002 and February 2003, after successfully providing early warnings of coral bleaching to the global coral reef community for several years, most "experimental" products were transitioned to "operational" status. These "operational" products are now supported and delivered by NESDIS on a 24-hour/7-day basis, permitting almost constant global monitoring of environmental conditions that can cause coral bleaching. This satellite thermal stress monitoring technique has been successful in now-casting coral bleaching episodes around the globe (e.g., Goreau et al., 2000; Wellington et al., 2001; Strong et al., 2002; Liu et al., 2003; Coral Reef Watch, 2003; Liu et al., 2005; Skirving et al., 2006); and Eakin et al., 2010).

CRW's current satellite coral bleaching monitoring and assessment product suite includes: SST, SST Anomalies, coral bleaching HotSpots, coral bleaching Degree Heating Weeks (DHW), Bleaching Alert Areas, Virtual Stations, SST/DHW time series, and Satellite Bleaching Alerts (SBA). These products are produced by CRW in near-real-time using NOAA/NESDIS operational composite nighttime POES (Polar Operational Environmental Satellites) AVHRR (Advanced Very High Resolution Radiometers) SSTs at 0.5-degree (approximately 50-km) spatial resolution.

The products are updated twice-weekly every Monday morning (using observations from the previous Thursday through Sunday) and Thursday morning (observations from the previous Monday through Wednesday), U.S. Eastern Time. The data and images are date-stamped with the end-date of the half-week period. These products are described in detail in the following sections. Data and images are available for free on the CRW website at http://coralreefwatch.noaa.gov.

The CRW team at NOAA/NESIDS that develops and generates these coral reef bleaching monitoring products comprises scientists from the Center for Satellite Applications and Research (STAR, formerly the Office of Research and Applications (ORA)) and from the Office of Satellite and Product Operations (OSPO, formerly the Office of Satellite Data Processing and Distribution (OSDPD)).

2. Sea Surface Temperature (SST)

SUMMARY: The CRW near-real-time SST is produced from nighttime-only data, to eliminate the effect of solar glare and reduce variability caused by heating during the day. SST data come from NOAA's polar-orbiting satellites, which measure infrared radiation from the ocean surface across the entire globe every day. This SST product is a twice-weekly composite at 0.5-degree (50-km) resolution. Animations of the most recent SST images are also available online.

NOAA has been measuring sea surface temperatures from satellites since 1972. Monitoring of SST from earth-orbiting infrared radiometers has had a wide impact on oceanographic science. Currently, one of the principal sources of infrared data for SST measurement is the Advanced Very High Resolution Radiometer (AVHRR) carried on NOAA's POES satellites, beginning in 1978. AVHRR is a broad-band, four or five channel (depending on the model) scanner, sensing in the visible, near-infrared, and thermal infrared portions of the electromagnetic spectrum. The POES satellite system offers the advantage of daily global coverage, by making near-polar orbits roughly 14 times daily. In situ SSTs from buoys (drifting and moored) are used operationally to maintain accuracy of satellite SST by removing biases and compiling statistics with time (McClain et al., 1985; Strong, 1991; Montgomery and Strong, 1995; Strong et al., 2000).

The CRW operational near-real-time nighttime SST product includes the most recent satellite global nighttime composite AVHRR-SSTs at 0.5-degree (50-km) resolution produced twice-weekly (see Introduction section for details on the update schedule). Nighttime-only satellite SST observations are used to eliminate daily warming caused by solar heating at the sea surface (primarily at the "skin" interface, 10-20 µm) during the day and to avoid contamination from solar glare. Compared with daytime SST and day-night blended SST, nighttime SST provides more conservative and stable estimate of thermal stress conducive to coral bleaching. Nighttime SSTs also compare favorably with in situ SSTs at one meter depth (Montgomery and Strong, 1995). The 50-km resolution data are calculated by averaging multiple temperature observations (weighted by distance from pixel center, conditionally out to a maximum of 150-km), which are based on 4-km AVHRR Global Area Coverage (GAC) SST acquired daily. Data and images of both near-real-time and archived SSTs are available on the CRW website. Animations of SST images for the past six months are also available.

The color range of temperatures displayed on the SST charts is -2.0 to 34.0 °C. Each color gradation on the color bar is 1.0 °C. Any satellite pixels that have SST values greater than 34.0 °C are displayed in the same color as SST equal to 34.0 °C. An ice mask, courtesy of the NOAA National Center for Environmental Prediction (NCEP), has been incorporated since April 28, 1998.

Charts of the retrospective 1984-1998 monthly mean SSTs are available online at 36-km resolution.

3. Climatology

SUMMARY: Coral bleaching is caused by unusually warm sea surface temperatures. Therefore, the first step in looking for areas at risk for bleaching is to define the "usual" temperatures in the world's oceans. This is accomplished by calculating a long-term mean SST, or climatology. CRW monthly mean SST climatologies are calculated from 7 years of satellite data. The Maximum of the Monthly Mean SST climatology is then defined as the warmest monthly mean value for each pixel around the world, indicating the upper limit of "usual" temperature.. These climatologies are available as images and HDF data files on the CRW website.

Beginning in mid-1996, more accurate monthly mean SST climatologies derived solely from satellite nighttime SST observations became available at a higher spatial resolution, 36-km, than any previous global SST climatologies (at 60- to 100-km) (Strong et al., 1997). This made it possible to generate more accurate, higher-resolution climatologies, which led to improved near-real-time SST Anomaly and coral bleaching HotSpot products from the CRW nighttime SST field.

The original 36-km satellite-only reprocessed SST data used for creating the climatologies were generated from the Multi-Channel SSTs (MCSSTs) by the Rosenstiel School of Marine and Atmospheric Science (RSMAS) at the University of Miami (Gleeson and Strong, 1995). In situ SSTs from drifting and moored buoys were used to remove any biases, and statistics were compiled with time to derive the reprocessed SSTs. The monthly mean SST climatologies were then derived by averaging these satellite SSTs during the time period of 1985-1993. Observations from the years 1991 and 1992 were omitted due to the aerosol contamination from the eruption of Mt. Pinatubo. These climatologies were developed at NOAA/NESDIS/STAR (then ORA) before being delivered to NESDIS/OSPO (then OSDPD) for implementation. The 36-km climatologies were finally interpolated into 0.5-degree (50-km) resolution to match the resolution of the operational SST analysis field.

Daily SST climatologies were derived from these 12 operational monthly mean climatologies to produce our operational SST anomaly products. First, the 12 monthly mean SST climatologies were set as the daily climatologies for the 15th days of the corresponding months. Daily climatology of any other day was derived by linearly interpolating between the two temporally closest values in the 12 monthly climatologies described above. For example, the daily climatology for June 30th was calculated by linear interpolation between June 15th and July 15th. If these two closest monthly climatologies are named A and B, with A the earlier one and B the subsequent one, the formula for deriving a daily SST climatology is

      Daily_SST_climatology = day_fraction*(B-A) + A,

where the day_fraction is the ratio of the number of days of the targeted day away from A to the number of days between A and B. For example, say we want to calculate the daily value for May 25th, the May climatology is 26 °C, and the June climatology is 30 °C. May 25th is ten days from May 15th, and there are 31 days between May 15th and June 15th.

      day_fraction = 10/31 = 0.32258
      Daily_SST_climatology = 0.323 * (30 °C - 26 °C) + 26 °C = 27.3 °C

for producing CRW's operational satellite thermal stress monitoring products (HotSpots and Degree Heating Weeks) a specialized Maximum of the Monthly Mean SST climatology was derived from the 12 monthly mean climatologies by taking the highest monthly mean climatology value for each pixel. The MMM SST climatology is static in time but varies in space (Strong et al., 1997).

The operational climatologies at 0.5-degree resolution are available as images and HDF data files on the CRW website.

4. Sea Surface Temperature (SST) Anomaly

SUMMARY: CRW's SST Anomaly is produced by subtracting the long-term mean SST (for that location in that time of year) from the current value. A positive anomaly means that the current sea surface temperature is warmer than average, and a negative anomaly means it is cooler than average. The spatial resolution is 0.5-degree (50-km), and the data and images are updated twice-weekly. Animations of the most recent SST Anomaly images are also available online.

CRW's near-real-time global SST Anomaly product makes it possible to quickly pinpoint regions of elevated SSTs throughout the world oceans. It is especially valuable for the tropical regions where most of the world's coral reef ecosystems thrive. It is also very useful in assessing ENSO (El Niño-Southern Oscillation) development, monitoring hurricane "wake" cooling, observing major shifts in coastal upwellings, etc.

A twice-weekly SST anomaly at a 0.5-degree (50-km) grid is calculated by subtracting the daily climatological SST of the last day of the twice-weekly period at that grid from the corresponding twice-weekly SST (described in Sea Surface Temperature Section). The formula for obtaining the anomaly is

      SST_anomaly = SST - Daily_SST_climatology

The color range of temperature anomalies displayed on the SST Anomaly charts is -5.0 to +5.0 °C (or Kelvin). Areas with SST anomaly values less than -5.0 °C are displayed as -5.0 °C, and areas with values greater than +5.0 °C are displayed as +5.0 °C. Note that these anomalies are somewhat less reliable at high latitudes where more persistent clouds limit the amount of satellite data available for deriving accurate SST analysis fields and climatologies.

Data and images of both near-real-time and archived SST anomalies are available from the CRW website, along with the operational 0.5-degree monthly mean SST climatologies. Animations of SST Anomaly images for the past six months are also available.

Charts of the retrospective 1984-1998 monthly mean SST anomalies are available online at 36-km resolution.

5. Coral Bleaching HotSpot

SUMMARY: Corals are vulnerable to bleaching when the SST exceeds the temperatures normally experienced in the hottest month. This is shown in the Coral Bleaching HotSpot product, which highlights regions where the SST is currently warmer than the highest climatological monthly mean SST for that location. The HotSpot value of 1.0 °C is a threshold for thermal stress leading to coral bleaching. To highlight this threshold, HotSpot values below 1.0 °C are shown in purple, and HotSpots of 1.0 °C or greater range from yellow to red. Global images and data sets are at 0.5-degree (50-km) resolution and are updated twice-weekly. Animations of the most recent HotSpot images are also available online.

CRW's Coral Bleaching HotSpot product measures the occurrence and magnitude of thermal stress potentially conducive to coral bleaching. It is an anomaly product, but not a typical climatological SST anomaly which is based on the average of all SSTs. The HotSpot anomaly is based on the climatological mean SST of the hottest month (often referred to as the Maximum of the Monthly Mean (MMM) SST climatology) (Liu et al., 2003; Liu et al., 2005; Skirving, 2006). This MMM SST climatology is simply the highest of the monthly mean SST climatologies described in the Climatology section. The Coral Bleaching HotSpots became available in 1997 (Strong et al., 1997), using a technique based on the earlier work by Goreau and Hayes (1994). Glynn and D'Croz (1990) showed that temperatures exceeding 1 °C above the usual summertime maximum are sufficient to cause stress to corals. Based on this study, MMM SST climatology was derived as a threshold for monitoring coral bleaching.

The HotSpot value shows the difference between the measured global 0.5-degree (50-km) near-real-time nighttime satellite SST analysis field and the MMM SST climatology:

      HotSpot = SST - MMM_SST_climatology

Only positive values are derived, since the HotSpot is designed to show the occurrence and distribution of thermal stress conducive to coral bleaching. The range of HotSpots displayed on the charts is 0.0 to +5.0 °C. The HotSpot chart highlights (in yellow to red color) anomalies that are at least 1.0 °C greater than the MMM SST climatology as studies have shown that bleaching stress occurs when the water temperatures exceed 1.0 °C above the maximum mean summertime temperature (Glynn and D'Croz, 1990). HotSpot values between 0 and 1.0 °C are displayed in light purple to light blue. Areas with HotSpot values greater than +5.0 °C are displayed in the same color as +5.0 °C.

Animations of HotSpot images for the past six months are also available on the CRW website.

Charts of 1998 50-km HotSpots and their animations are available at a separate web page.

6. Coral Bleaching Degree Heating Weeks (DHW)

SUMMARY: Mass coral bleaching has been shown to be caused by prolonged periods of thermal stress. The DHW product accumulates any HotSpots greater than 1 °C over a 12- week window, thus showing how stressful conditions have been for corals in the last three months. It is a cumulative measurement of the intensity and duration of thermal stress, and is expressed in the unit °C-weeks. DHWs over 4 °C-weeks have been shown to cause significant coral bleaching; values over 8 °C-weeks have caused widespread bleaching and some mortality. The global data are at 0.5-degree (50-km) resolution and are updated twice-weekly. Animations of the most recent DHW images are also available online.

CRW's near-real-time satellite Coral Bleaching DHWs measure the accumulation of thermal stress that coral reefs have experienced over the past 12 weeks (3 months) up to and including the most current product update. While the Coral Bleaching HotSpot provides an instantaneous measure of the thermal stress, there is evidence that corals are sensitive to an accumulation of thermal stress over time (Glynn and D'Croz, 1990)). In order to monitor this cumulative effect, a thermal stress index called Coral Bleaching Degree Heating Week (DHW), was developed by CRW in 2000 (Liu et al., 2003; Liu et al., 2005). Glynn and D'Croz (1990) showed that temperatures exceeding 1 °C above the usual summertime maximum are sufficient to cause stress to corals. This is commonly known as the bleaching threshold temperature. Based on our definition of Coral Bleaching HotSpot (see Coral Bleaching HotSpot section for details), only HotSpot values that are =>1 °C are accumulated. For example, two DHWs is equivalent to one week of HotSpot values at 2 °C, or two weeks of HotSpot values at 1 °C, etc.

Note that since the DHW is a 12-week accumulated HotSpot, it is possible for a location to have a non-zero DHW value when the HotSpot value is less than 1 °C and even 0 °C. This condition simply means that there has been thermal stress at that location within the last three months, but the local conditions are not currently stressful for corals. Exposure to the previous thermal stress may still have adverse impact on the corals, although recovery may be underway.

A half-week approach is used because the CRW satellite near-real-time coral bleaching monitoring products are updated twice-weekly. With this approach, the DHWs are accumulated based on twice-weekly HotSpots using the following formula,

      DHWs = 0.5 * Summation of previous 24 twice-weekly HotSpots,

where HotSpots have to be at least 1.0 °C to be accumulated. For example, if we have consecutive twice-weekly HotSpot values of 1.0, 2.0, 0.8 and 1.2 °C, the DHW value will be 2.1 °C-weeks because 0.8 °C is less than one and therefore does not contribute to the accumulated value.

The range of DHW displayed on the charts is 0.0 to 16.0 °C-weeks. In the chart, any area with DHW values greater than 16 °C-weeks is displayed in the same color as 16 °C-weeks.

Field observations (many of which are subjective measurements presented as informal reports) with corresponding satellite data are only available for a limited number of years; these observations indicate that there is a correlation with bleached corals when DHW values of 4 °C-weeks have been reached. By the time DHW values reach 8 °C-weeks, widespread bleaching is likely and some mortality can be expected. Since its inauguration in 2000, the DHW product has successfully generated satellite bleaching warnings and alerts (e.g., Goreau et al., 2000, Wellington et al., 2001; Strong et al., 2002; Liu et al., 2005; Coral Reef Watch, 2003; Liu et al., 2003; Skirving et al., 2006; and Eakin et al., 2010).

The timing of the peak bleaching season varies among ocean basins and hemispheres but it is generally during the local summertime. Thus, the peak season is July-September for the northern Atlantic and Pacific Oceans, and January-March for the southern Atlantic and Pacific. The peak is April-June for the northern Indian Ocean and January-April for the southern Indian Ocean.

Animations of the DHW images for the past six months are also available on the CRW website.

Charts of retrospective 1998-1999 50-km 3-monthly DHWs are also available online.

7. Bleaching Alert Areas

SUMMARY: These global maps summarize the current DHW and HotSpot values. At a glance, this product outlines the location, coverage, and potential risk level of the current bleaching thermal stress. Alert levels use the same definition as our Satellite Bleaching Alert email system, but in the Bleaching Alert Area product every pixel has an alert level defined and color-coded. The global data are at 0.5-degree (50-km) resolution and are updated twice-weekly. Animations of the most recent Bleaching Alert Areas images are also available online.

The CRW Coral Bleaching Alert Area product outlines the areas where bleaching thermal stress currently reaches various bleaching stress levels, based on satellite sea surface temperature monitoring. The stress levels defined in the table below are based on current values of the coral bleaching HotSpot and Degree Heating Weeks (DHW) products.

Note that if a location has a status level of "No Stress" or "Bleaching Watch," it is still possible for the corresponding DHW value to be greater than 0 °C-week. This condition simply means that there has been thermal stress at that location sometime over the last 3 months, but the local conditions are not currently stressful for corals. Previous thermal stress exposure may still have and adverse impact on the corals, although recovery may be underway.

The timing of the peak bleaching season varies among ocean basins and hemispheres but it is generally during the local summertime. Thus, the peak season is July-September for the northern Atlantic and Pacific Oceans, and January-March for the southern Atlantic and Pacific. The peak is April-June for the northern Indian Ocean and January-April for the southern Indian Ocean.

Animations of the Bleaching Alert Areas images for the past six months are also available on the CRW website.

8. Coral Bleaching Virtual Stations

SUMMARY: Coral Bleaching Virtual Stations focus the CRW global satellite data products on selected coral reef sites around the world. Users can think of these as virtual SST buoys; no instrumentation is needed in the water because all of the data are derived from satellite measurements. The Virtual Stations webpages provides near-real-time information for each site: current thermal stress status, current DHW value, historical maximum DHW value for the site, current SST, and the the Maximum of the Monthly Mean climatologies for that site. The data are updated twice-weekly. Currently, CRW has 24 operational Virtual Stations and more than 200 experimental Virtual Stations.

8.1 Operational Virtual Station Webpage

CRW's operational Coral Bleaching Virtual Stations webpage provides near-real-time satellite monitoring information on thermal stress conducive to coral bleaching for 24 selected reef sites around the globe (Liu et al., 2001). The information is extracted from near-real-time satellite global SST measurements and derived indices of coral bleaching related thermal stress (see the sections on SST, SST Anomaly, Coral Bleaching HotSpot, and DHW earlier on this methodology page for detail) from 0.5-degree (50-km) water pixels surrounding or close to the reef sites. Information listed for each reef site includes the reef site name, current thermal stress status, current DHW value in °C-weeks ("Current DHW" in the table on the webpage), historical maximum DHW value and its year of occurrence ("Hist Max DHW"), current SST value in degrees Celsius ("Current SST"), and the Maximum of the Monthly Mean SST climatologies value for that site ("Max Month SST"). A map showing a particular reef site (Virtual Station) and its satellite pixel is accessible by clicking on the reef name. The map page also provides links to other coral bleaching monitoring products, including current satellite ocean surface winds, SST time series, and the Satellite Bleaching Alerts online subscription. Regional DHW and SST charts for each site are accessible by clicking on "Current DHW" and "Current SST," respectively.

The five status levels of thermal stress shown on the Virtual Stations page are the same as those for the Bleaching Alert Area product, defined as:

These levels are defined in terms of the HotSpot and DHW values. When low thermal stress is present at a reef site (0 °C < HotSpot < 1 °C) a Bleaching Watch is posted for the site. A triangular warning icon is also added to that reef site, and the status is displayed in red text. A Bleaching Warning is posted when the HotSpot => 1 °C. At this point, DHWs begin accumulating and a larger triangular icon is displayed. A DHW accumulation of 4 °C-weeks triggers a Bleaching Alert Level 1, and the status is displayed in bold red text. At Bleaching Alert Level 1, bleaching is expected at the site within a few weeks of the alert. An accumulation of 8 °C-weeks triggers a Bleaching Alert Level 2, at which point widespread bleaching and some coral mortality are likely.

Please note that since the DHW is a 12-week accumulation of HotSpot, it is possible for a location to have a non-zero DHW value when the HotSpot value is less than 1 °C or even 0 °C. Hence, at a status level of "No Stress" or "Bleaching Watch," it is possible for the corresponding DHW value to be greater than 0 °C-week. This condition simply means that there has been thermal stress at that location sometime within the last 3 months, but the local conditions are not currently stressful for corals. Previous thermal stress exposure may still have adverse impact on the corals, although recovery may be underway.

Click here to see the list of the 24 operational satellite Virtual Stations. SST and DHW time series graphs and data are available online for these Virtual Stations. We also provide automatic e-mail alerts for these sites and user can subscribe to the e-mail alerts for free at our subscription page.

8.2 Experimental Virtual Station Webpage

In addition to the 24 operational Virtual Stations described in the previous section, CRW now has more than 200 experimental Virtual Stations accessible from its Google Maps webpage. CRW is still evaluating the selections and locations of these experimental sites. These Virtual Stations will be added to our operational system soon.

Access to the latest updates for these Virtual Stations is through the Google Maps interface, where stations are color-coded to show the current alert status. Users can click on a station to see the current conditions and link to time series graphs and data. There is also an alternate text-based webpage to access the same information for users who prefer a simpler interface. CRW also provides automatic e-mail alerts for these sites. However, the online subscription system is only for the operational Virtual Stations; free subscriptions for the experimental Virtual Stations is available simply by sending an email to coralreefwatch@noaa.gov.

9. SST and DHW Time Series Graphs and Data for Virtual Stations

SUMMARY: Time series graphs show the satellite SST, DHW, and thermal stress conditions since 2000 at CRW's Virtual Stations. The SST climatology for each month, the Maximum of the Monthly Mean SST climatologies (i.e., MMM SST Climatology), and the bleaching threshold temperature are plotted in the central portion of the graphs. In the bottom section of each graph, there is a separate plot of DHW and bleaching alerts for that reef location. There are two types of time series graphs available: single-year graphs and overlapping multi-year graphs. Graphs for the 24 operational Virtual Stations and graphs for the experimental Virtual Stations are accessible through different webpages. The time series data for CRW's SST, SST Anomaly, HotSpot, and DHW products in ASCII text are also available on the corresponding webpages. These graphs and data are updated twice-weekly.

Time series graphs show the 2000-present CRW satellite SST, DHW, and thermal stress condition at the 24 operational Virtual Stations and more than 200 experimental Virtual Stations described in the Coral Bleaching Virtual Stations Webpage Section. The values are extracted from the same dataset used for making the Virtual Stations webpage. Click here for a comprehensive description of the graphs.

In the single-year graphs (click here to open an example in a new window), the monthly mean SST climatologies (light-blue crosses) are plotted on the charts to show the "normal" SST condition at the site and the time of climatologically warmest months. The MMM SST Climatology value (horizontal dashed light-blue line) is the warmest of the twelve monthly mean SST climatologies. The Coral Bleaching Threshold SST (horizontal solid light-blue line) is defined as the MMM SST + 1 °C. Both the MMM SST and Coral Bleaching Threshold SST are location-specific. DHWs, in units of °C-weeks on the right-hand axis, are the accumulation of thermal stress whenever the SST equals or exceeds the coral bleaching threshold SST at the pixel during the 12 weeks up to the given time of the data.

In each time series graph, the corresponding thermal stress condition (see the table in the Coral Bleaching Virtual Stations Webpage section) related to coral bleaching is color-coded and plotted in a bar at the bottom of the time series graphs. The thermal condition is categorized in the five bleaching alert levels. The area below the DHW time series is also filled with colors corresponding to the color- coded bleaching alert levels whenever bleaching related thermal stress is present. At Bleaching Alert Level 1, some bleaching is expected at the site within a few weeks of the alert. An accumulation of DHW of 8 °C-weeks triggers a Bleaching Alert Level 2, at which point widespread bleaching and some coral mortality are likely.

The multi-year graphs, with time axis covering only 12 months, overlap time series from different years for a virtual station (click here to open an example in a new window). In the multi-year graphs, the time series of the bleaching alerts is plotted only for the current year. They provide a convenient way to compare the time series between years.

Time series data for CRW's SST, SST anomaly, HotSpot, and DHW products are also available in ASCII text, along with the data description, for the 24 operational Virtual Stations and the experimental Virtual Stations.

The time series graphs and data integrate and deliver the most comprehensive site-specific information available from the CRW coral bleaching monitoring product suite. Graphs and data for the experimental Virtual Stations will be added to CRW's operational system soon.

10. Satellite Bleaching Alert (SBA)

SUMMARY: SBAs are automatic e-mails that alert subscribers when coral reefs are at risk for bleaching. The alerts are currently available for 24 operational Virtual Station reef sites and more than 200 experimental Virtual Station reef sites around the world, and are based on CRW's measurements of stressfully high sea surface temperatures from satellites. The alerts are free and updated twice-weekly. Anyone can sign up on the CRW website for the operational stations. To subscribe to the experimental staions, simply send a request e-mail to coralreefwatch@noaa.gov.

CRW's Satellite Bleaching Alert (SBA) system is an automated e-mail alert system designed to monitor the status of thermal stress conducive to coral bleaching via the use of the CRW global satellite near-real-time suite of monitoring products. The SBA was developed as a tool for coral reef managers, scientists, and other interested individuals. The SBA became operational in July 2005 for the 24 operational Virtual Stations. Currently, the alert messages are also available for more than 200 other coral reefs (experimental Virtual Stations) around the world, as described in Coral Bleaching Virtual Stations Webpage section. A sample operational SBA message can be viewed here, and a sample experimental message can be viewed here.

An automated e-mail is sent to a subscriber for one or more reef site only when the status level of thermal stress status level changes, regardless of the current status level. The thermal stress status, described in the Coral Bleaching Virtual Stations Webpages section, is evaluated twice-weekly. All of the information available on the Coral Bleaching Virtual Stations webpage is included in the SBA emails; internet links to time series graphs and global/regional images are provided, along with the previous bleaching alert levels experienced at the site.

The SBA is a convenient data delivery system that allows critical information to reach a user's desktop as soon as the information becomes available, without the user having to manually check the CRW website for updated information. To receive automated e-mail alerts for any of the 24 operational Virtual Stations, go to the subscription page; from this page, you can also view and alter your selection(s) or unsubscribe from the e-mail list. To receive automated e-mail alerts for experimental Virtual Stations, send a request e-mail to coralreefwatch@noaa.gov.

Color-coded alert levels from the SBA system are also plotted on both the operational and experimental SST/DHW time series graphs. Bleaching alert histories for the 24 operational Virtual Stations are available dating back to 2003.

The experimental SBA product will be added to CRW's operational SBA system soon.

11. Satellite Coral Bleaching Monitoring Source Data

The source data for most of the CRW products are available on the CRW website for free download and use. Currently, data are available in the Hierarchical Data Format (HDF) via FTP, HTTP, and OPeNDAP, and THREDDS servers. Preview images (graphic displays) of the data are also provided. Please check the HDF data webpage for data availability and options for data download.

CRW also provides free NOAA software tools (i.e., the CoastWatch Software Library and Utilities) that can be used for visualizing data, viewing data information and values, calculating certain statistics, creating graphical output, and more. Please see CRW's HDF data page and the software page for more information. The software is easy to install and use and is customized for CRW's HDF data files. This software is not required to visualize and manipulate the data; many commonly used computer languages and software packages can read and process HDF files.

Time series data for CRW's SST, SST anomaly, HotSpot, and DHW products in ASCII text, extracted from the global source data described here, are also available at the Coral Bleaching Virtual Stations webpages for operational and experimental Virtual Stations.

12. CRW Products in Google Earth Format

Google Earth can be used to view almost all CRW near-real-time satellite global coral bleaching monitoring products. Some archived products and products developed for special projects are also available in the Google Earth format. For most CRW products, users can display them in Google Earth on their computers by a simple mouse click on a desired product link on the Google Earth product web page. No software installation is needed (except for the Google Earth software itself). For CRW's online-version Google Earth products, internet access is required to display the images. For offline-version products, users can download complete packages containing program files and images, and the products can be displayed without a live internet connection. For detailed instructions, please see CRW's Google Earth webpage.

13. Animation Products

Animations of CRW's SST, SST Anomaly, HotSpot, and DHW images are available to view online in Javascript animation player and in animated gif format. Users can save the animated gif files directly from the animation webpages. Currently, animations are available for a maximum time period of six months, ending at the most recent twice-weekly update.

14. Tutorial for Coral Reef Watch Products

A tutorial for Coral Reef Watch products has been developed to provide background information on satellite remote sensing, coral bleaching, and CRW's monitoring products. This tutorial is written in layman's language and aimed for both the coral reef management community and the general public.

15. References

Berkelmans, R. and B.L. Willis, 1999. Seasonal and local spatial patterns in the upper thermal limits of corals on the inshore Central Great Barrier Reef, Coral Reefs, 18, 219-228.