Bibliography - G A Vecchi

1. DiNezion, P, A C Clement, G A Vecchi, Brian J Soden, B P Kirtman, and S Lee, in press: Climate response of the equatorial Pacific to global warming. Journal of Climate. 4/09.
2. Landsea, C, G A Vecchi, L Bengtsson, and Thomas R Knutson, in press: Increases in Atlantic tropical cyclone counts since the late 1800s are likely due to advances in observations and analyses. Journal of Climate. 2/09.
   [ Abstract ]

   Records of Atlantic basin tropical cyclones since the late-19th Century indicate a very large upward trend in storm frequency. This increase in documented cyclones has been previously interpreted as connected to alterations in climate conditions, in particular to those resulting from anthropogenic climate change. However, improvements in observing and recording practices provide an alternative interpretation for these changes, with recent studies suggesting that the number of “missed” tropical cyclones may have sufficient to explain a large part of the recorded increase in storm counts. Of note is that much of the recorded increase in cyclones has been in those that were short-lived and weak. This study, through analysis of numerical model output and observational data, explores the influence of tropical cyclone duration on modeled and observed changes in cyclone frequency. In a global climate modeling framework there is an extremely large sensitivity of tropical cyclone counts to the duration threshold required for a cyclone to be “counted”—much larger than the model’s response of Atlantic tropical cyclone frequency to 21st century greenhouse warming. In the observational record, the increase in cyclone frequency since the late 19th Century is due to an increase in very short-lived tropical cyclones, whose occurrence increased from about once per year in the late-19th/early-20th Century to about five per year since about 2000. Improvements in the quantity and quality of observations along with enhanced analytical techniques allow National Hurricane Center forecasters to better detect initial tropical cyclone formation (and thus incorporate very short-lived systems into the tropical cyclone database) with much more confidence than earlier eras. Finally, a sampling study based upon the distribution of ship observations provides quantitative estimates of the frequency of “missed” tropical cyclones once the very short-lived systems are removed. Upon adding these estimated numbers of missed tropical cyclones to the time series of moderate to long-lived systems, no significant trend in Atlantic tropical cyclones remains, though substantial multi-decadal variability is still present. Such lack of significant increasing trends in the Atlantic tropical cyclone frequency record is consistent with recent simulations of anthropogenic greenhouse warming influence on 21st century Atlantic tropical storm frequency.

3. Lloyd, I D., and G A Vecchi, in press: Submonthly Indian Ocean cooling events and their interaction with large-scale conditions. Journal of Climate. 2/09.
   [ Abstract ]

   The Indian Ocean exhibits strong variability on a number of timescales, including prominent intraseasonal variations in both the atmosphere and ocean. Of particular interest is the south tropical Indian Ocean thermocline ridge, a region located between 12◦S - 5◦S, which exhibits prominent variability in Sea Surface Temperature (SST) due to dominant winds that raise the thermocline and shoal the mixed layer. In this paper, submonthly (less than 30 day) cooling events in the thermocline ridge region are diagnosed with observations and models, and are related to large-scale conditions in the Indo-Pacific region. Observations from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) satellite were used to identify 16 cooling events in the period 1998-2007, which on average cannot be fully accounted for by air-sea enthalpy fluxes. Analysis of observations and a hierarchy of models, including two coupled global climate models (GFDL CM2.1 and GFDL CM2.4), indicates that ocean dynamical changes are important to the cooling events. Further, it is found that cooling events are preconditioned by large-scale, low-frequency changes in the coupled ocean-atmosphere system. When the thermocline is unusually shallow in the thermocline ridge region, cooling events are more likely to occur and are stronger; these large-scale conditions are more(less) likely during La Nina (El Nino/Indian Ocean Dipole) events. Strong thermocline ridge cooling events are associated with changes in atmospheric convection, which resemble the Madden-Julian oscillation, in both observations and the models.

4. McPhaden, M J., and G A Vecchi, et al., February 2009: Ocean-atmosphere interactions during cyclone Nargis. EOS, 90(7), 53-60.
   [ PDF ]
5. Zhao, Ming, Isaac Held, Shian-Jiann Lin, and G A Vecchi, in press: Simulations of global hurricane climatology, interannual variability, and response to global warming using a 50km resolution GCM. Journal of Climate. 1/09.
6. Chang, Y-S, Anthony Rosati, and G A Vecchi, in press: Basin patterns of global sea level changes for 2004-2007. Geophysical Research Letters. 12/08.
   [ Abstract ]

   Based on independent observations, we estimate the sea level budget and linear trends for individual ocean basins and the world ocean during 2004-2007. Even though it is confirmed that the seasonal variation of global sea level is balanced by the different sea level components (total sea level change from satellite altimetry equals to the sum of the steric height contribution obtained by Argo profiles and any variability in ocean mass observed from GRACE), the basin sea level budgets show very different characteristics. Sea level budgets over the South Pacific and Antarctic Ocean maintain a good balance both on seasonal to interannual time scales. Only the satellite altimeter data exhibits an abnormal 4-year trend over the Southern Oceans, especially for the Indian Ocean. These basins significantly impact the magnitude of the disagreement for the global sea level budget pointed out by Willis et al. [2008]. Large differences among the nine different gravity fields in the Atlantic and Indian Oceans could be one of the major causes of the imbalance in the global sea level budget.

7. Knutson, Thomas R., Joseph J Sirutis, Stephen T Garner, G A Vecchi, and Isaac Held, 2008: Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience, 1(6), 359-364.
   [ Abstract PDF ]

   Increasing sea surface temperatures in the tropical Atlantic Ocean and measures of Atlantic hurricane activity have been reported to be strongly correlated since at least 1950 (refs 1, 2, 3, 4, 5), raising concerns that future greenhouse-gas-induced warming6 could lead to pronounced increases in hurricane activity. Models that explicitly simulate hurricanes are needed to study the influence of warming ocean temperatures on Atlantic hurricane activity, complementing empirical approaches. Our regional climate model of the Atlantic basin reproduces the observed rise in hurricane counts between 1980 and 2006, along with much of the interannual variability, when forced with observed sea surface temperatures and atmospheric conditions7. Here we assess, in our model system7, the changes in large-scale climate that are projected to occur by the end of the twenty-first century by an ensemble of global climate models8, and find that Atlantic hurricane and tropical storm frequencies are reduced. At the same time, near-storm rainfall rates increase substantially. Our results do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations.

8. Song, Qian, G A Vecchi, and Anthony Rosati, 2008: Predictability of the Indian Ocean sea surface temperature anomalies in the GFDL coupled model. Geophysical Research Letters, 35, L02701, doi:10.1029/2007GL031966.
   [ Abstract ]

   We explore the predictability of the sea surface temperature anomalies associated with the Indian Ocean Dipole/Zonal Mode (IODZM) at a three-season lead, within the Geophysical Fluid Dynamics Laboratory (GFDL) coupled general circulation model (CGCM). In both control simulations and retrospective forecasts of the 1990's in the CGCM, we find that the occurrence of some IODZM events is preconditioned by oceanic conditions and potentially predictable three seasons in advance, while other IODZM events appear to be triggered by weather noise and have low predictability. The results highlight the necessity for future studies to distinguish periods when the IODZM is more or less predictable and search for its precursory pattern in the ocean.

9. Vecchi, G A., and Thomas R Knutson, January 2008: On estimates of historical North Atlantic tropical cyclone activity. Journal of Climate, 21(14), 3580-3600.
   [ Abstract PDF ]

   In this study, an estimate of the expected number of Atlantic tropical cyclones (TCs) that were missed by the observing system in the presatellite era (between 1878 and 1965) is developed. The significance of trends in both number and duration since 1878 is assessed and these results are related to estimated changes in sea surface temperature (SST) over the “main development region” (“MDR”). The sensitivity of the estimate of missed TCs to underlying assumptions is examined. According to the base case adjustment used in this study, the annual number of TCs has exhibited multidecadal variability that has strongly covaried with multidecadal variations in MDR SST, as has been noted previously. However, the linear trend in TC counts (1878–2006) is notably smaller than the linear trend in MDR SST, when both time series are normalized to have the same variance in their 5-yr running mean series. Using the base case adjustment for missed TCs leads to an 1878–2006 trend in the number of TCs that is weakly positive, though not statistically significant, with p ~ 0.2. The estimated trend for 1900–2006 is highly significant (+~ 4.2 storms century⁻¹) according to the results of this study. The 1900–2006 trend is strongly influenced by a minimum in 1910–30, perhaps artificially enhancing significance, whereas the 1878–2006 trend depends critically on high values in the late 1800s, where uncertainties are larger than during the 1900s. The trend in average TC duration (1878–2006) is negative and highly significant. Thus, the evidence for a significant increase in Atlantic storm activity over the most recent 125 yr is mixed, even though MDR SST has warmed significantly. The decreasing duration result is unexpected and merits additional exploration; duration statistics are more uncertain than those of storm counts. As TC formation, development, and track depend on a number of environmental factors, of which regional SST is only one, much work remains to be done to clarify the relationship between anthropogenic climate warming, the large-scale tropical environment, and Atlantic TC activity.

10. Vecchi, G A., A C Clement, and Brian J Soden, February 2008: Examining the tropical Pacific's response to global warming. EOS, 89(9), 81, 83.
    [ PDF ]
11. Vecchi, G A., K L Swanson, and Brian J Soden, November 2008: Whither hurricane activity. Science, 322(5902), doi:10.1126/science.1164396.
    [ Abstract ]

    Alternative interpretations of the relationship between sea surface temperature and hurricane activity imply vastly different future Atlantic hurricane activity.

12. Lu, Jian, G A Vecchi, and T Reichler, 2007: Expansion of the Hadley cell under global warming. Geophysical Research Letters, 34, L06805, doi:10.1029/2006GL028443.
    [ Abstract ]

    A consistent weakening and poleward expansion of the Hadley circulation is diagnosed in the climate change simulations of the IPCC AR4 project. Associated with this widening is a poleward expansion of the subtropical dry zone. Simple scaling analysis supports the notion that the poleward extent of the Hadley cell is set by the location where the thermally driven jet first becomes baroclinically unstable. The expansion of the Hadley cell is caused by an increase in the subtropical static stability, which pushes poleward the baroclinic instability zone and hence the outer boundary of the Hadley cell.

13. Seager, R, Mingfang Ting, Isaac Held, Y Kushnir, Jian Lu, G A Vecchi, H-P Huang, N Harnik, Ants Leetma, Ngar-Cheung Lau, C Li, Jennifer Velez, and N Naik, 2007: Model projections of an imminent transition to a more arid climate in southwestern North America. Science, 316(5828), doi:10.1126/science.1139601.
    [ Abstract ]

    How anthropogenic climate change will affect hydroclimate in the arid regions of southwestern North America has implications for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the 1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.

14. Song, Qian, G A Vecchi, and Anthony Rosati, June 2007: The role of Indonesian throughflow in the Indo-Pacific climate variability in the GFDL coupled climate model. Journal of Climate, 20(11), doi:10.1175/JCLI4133.1.
    [ Abstract ]

    The impacts of the Indonesian Throughflow (ITF) on the tropical Indo–Pacific climate, particularly on the character of interannual variability, are explored using a coupled general circulation model (CGCM). A pair of CGCM experiments—a control experiment with an open ITF and a perturbation experiment in which the ITF is artificially closed—is integrated for 200 model years, with the 1990 values of trace gases. The closure of the ITF results in changes to the mean oceanic and atmospheric conditions throughout the tropical Indo–Pacific domain as follows: surface temperatures in the eastern tropical Pacific (Indian) Ocean warm (cool), the near-equatorial Pacific (Indian) thermocline flattens (shoals), Indo–Pacific warm-pool precipitation shifts eastward, and there are relaxed trade winds over the tropical Pacific and anomalous surface easterlies over the equatorial Indian Ocean. The character of the oceanic changes is similar to that described by ocean-only model experiments, though the amplitude of many features in the tropical Indo–Pacific is amplified in the CGCM experiments. In addition to the mean-state changes, the character of tropical Indo–Pacific interannual variability is substantially modified. Interannual variability in the equatorial Pacific and the eastern tropical Indian Ocean is substantially intensified by the closure of the ITF. In addition to becoming more energetic, El Niño–Southern Oscillation (ENSO) exhibits a shorter time scale of variability and becomes more skewed toward its warm phase (stronger and more frequent warm events). The structure of warm ENSO events changes; the anomalies of sea surface temperature (SST), precipitation, and surface westerly winds are shifted to the east and the meridional extent of surface westerly anomalies is larger. In the eastern tropical Indian Ocean, the interannual SST variability off the coast of Java–Sumatra is noticeably amplified by the occurrence of much stronger cooling events. Closing the ITF shoals the eastern tropical Indian Ocean thermocline, which results in stronger cooling events through enhanced atmosphere–thermocline coupled feedbacks. Changes to the interannual variability caused by the ITF closure rectify into mean-state changes in tropical Indo–Pacific conditions. The modified Indo–Pacific interannual variability projects onto the mean-state differences between the ITF open and closed scenarios, rectifying into mean-state differences. These results suggest that CGCMs need to reasonably simulate the ITF in order to successfully represent not just the mean climate, but its variations as well.

15. Song, Qian, G A Vecchi, and Anthony Rosati, July 2007: Indian Ocean Variability in the GFDL Coupled Climate Model. Journal of Climate, 20(13), doi:10.1175/JCLI4159.1.
    [ Abstract ]

    The interannual variability of the Indian Ocean, with particular focus on the Indian Ocean dipole/zonal mode (IODZM), is investigated in a 250-yr simulation of the GFDL coupled global general circulation model (CGCM). The CGCM successfully reproduces many fundamental characteristics of the climate system of the Indian Ocean. The character of the IODZM is explored, as are relationships between positive IODZM and El Niño events, through a composite analysis. The IODZM events in the CGCM grow through feedbacks between heat-content anomalies and SST-related atmospheric anomalies, particularly in the eastern tropical Indian Ocean. The composite IODZM events that co-occur with El Niño have stronger anomalies and a sharper east–west SSTA contrast than those that occur without El Niño. IODZM events, whether or not they occur with El Niño, are preceded by distinctive Indo-Pacific warm pool anomaly patterns in boreal spring: in the central Indian Ocean easterly surface winds, and in the western equatorial Pacific an eastward shift of deep convection, westerly surface winds, and warm sea surface temperature. However, delayed onsets of the anomaly patterns (e.g., boreal summer) are often not followed by IODZM events. The same anomaly patterns often precede El Niño, suggesting that the warm pool conditions favorable for both IODZM and El Niño are similar. Given that IODZM events can occur without El Niño, it is proposed that the observed IODZM–El Niño relation arises because the IODZM and El Niño are both large-scale phenomena in which variations of the Indo-Pacific warm pool deep convection plays a central role. Yet each phenomenon has its own dynamics and life cycle, allowing each to develop without the other. The CGCM integration also shows substantial decadal modulation of the occurrence of IODZM events, which is found to be not in phase with that of El Niño events. There is a weak, though significant, negative correlation between the two. Moreover, the statistical relationship between the IODZM and El Niño displays strong decadal variability.

16. Vecchi, G A., and Brian J Soden, 2007: Increased tropical Atlantic wind shear in model projections of global warming. Geophysical Research Letters, 34, L08702, doi:10.1029/2006GL028905.
    [ Abstract ]

    To help understand possible impacts of anthropogenic greenhouse warming on hurricane activity, we assess model-projected changes in large-scale environmental factors tied to variations in hurricane statistics. This study focuses on vertical wind shear (Vs) over the tropical Atlantic during hurricane season, the increase of which has been historically associated with diminished hurricane activity and intensity. A suite of state-of-the-art global climate model experiments is used to project changes in Vs over the 21st century. Substantial increases in tropical Atlantic and East Pacific shear are robust features of these experiments, and are shown to be connected to the model-projected decrease in the Pacific Walker circulation. The relative changes in shear are found to be comparable to those of other large-scale environmental parameters associated with Atlantic hurricane activity. The influence of these Vs changes should be incorporated into projections of long-term hurricane activity.

17. Vecchi, G A., and Matthew J Harrison, July 2007: An observing system simulation experiment for the Indian Ocean. Journal of Climate, 20(13), doi:10.1175/JCLI4147.1.
    [ Abstract ]

    An integrated in situ Indian Ocean observing system (IndOOS) is simulated using a high-resolution ocean general circulation model (OGCM) with daily mean forcing, including an estimate of subdaily oceanic variability derived from observations. The inclusion of subdaily noise is fundamental to the results; in the mixed layer it is parameterized as Gaussian noise with an rms of 0.1°C; below the mixed layer a Gaussian interface displacement with an rms of 7 m is used. The focus of this assessment is on the ability of an IndOOS—comprising a 3° × 3° Argo profiling float array, a series of frequently repeated XBT lines, and an array of moored buoys—to observe the interannual and subseasonal variability of subsurface Indian Ocean temperature. The simulated IndOOS captures much of the OGCM interannual subsurface temperature variability. A fully deployed Argo array with 10-day sampling interval is able to capture a significant part of the Indian Ocean interannual temperature variability; a 5-day sampling interval degrades its ability to capture variability. The proposed moored buoy array and frequently repeated XBT lines provide complementary information in key regions, particularly the Java/Sumatra and Somali upwelling and equatorial regions. Since the subdaily noise is of the same order as the subseasonal signal and since much of the variability is submonthly, a 5-day sampling interval does not drastically enhance the ability of Argo to capture the OGCM subseasonal variability. However, as sampling intervals are decreased, there is enhanced divergence of the Argo floats, diminished ability to quality control data, and a decreased lifetime of the floats; these factors argue against attempting to resolve subseasonal variability with Argo by shortening the sampling interval. A moored array is essential to capturing the subseasonal and near-equatorial variability in the model, and the proposed moored buoy locations span the region of strong subseasonal variability. On the whole, the proposed IndOOS significantly enhances the ability to capture both interannual and subseasonal variability in the Indian Ocean.

18. Vecchi, G A., and Brian J Soden, 2007: Global Warming and the Weakening of the Tropical Circulation. Journal of Climate, 20(17), doi:10.1175/JCLI4258.1.
    [ Abstract ]

    This study examines the response of the tropical atmospheric and oceanic circulation to increasing greenhouse gases using a coordinated set of twenty-first-century climate model experiments performed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The strength of the atmospheric overturning circulation decreases as the climate warms in all IPCC AR4 models, in a manner consistent with the thermodynamic scaling arguments of Held and Soden. The weakening occurs preferentially in the zonally asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of the tropical circulation and is shown to induce substantial changes to the thermal structure and circulation of the tropical oceans. Evidence suggests that the overall circulation weakens by decreasing the frequency of strong updrafts and increasing the frequency of weak updrafts, although the robustness of this behavior across all models cannot be confirmed because of the lack of data. As the climate warms, changes in both the atmospheric and ocean circulation over the tropical Pacific Ocean resemble “El Niño–like” conditions; however, the mechanisms are shown to be distinct from those of El Niño and are reproduced in both mixed layer and full ocean dynamics coupled climate models. The character of the Indian Ocean response to global warming resembles that of Indian Ocean dipole mode events. The consensus of model results presented here is also consistent with recently detected changes in sea level pressure since the mid–nineteenth century.

19. Vecchi, G A., and Brian J Soden, 2007: Effect of remote sea surface temperature change on tropical cyclone potential intensity. Nature, 450(7172), doi:doi:10.1038/nature06423.
    [ Abstract ]

    The response of tropical cyclone activity to global warming is widely debated. It is often assumed that warmer sea surface temperatures provide a more favourable environment for the development and intensification of tropical cyclones, but cyclone genesis and intensity are also affected by the vertical thermodynamic properties of the atmosphere. Here we use climate models and observational reconstructions to explore the relationship between changes in sea surface temperature and tropical cyclone 'potential intensity'—a measure that provides an upper bound on cyclone intensity and can also reflect the likelihood of cyclone development. We find that changes in local sea surface temperature are inadequate for characterizing even the sign of changes in potential intensity, but that long-term changes in potential intensity are closely related to the regional structure of warming; regions that warm more than the tropical average are characterized by increased potential intensity, and vice versa. We use this relationship to reconstruct changes in potential intensity over the twentieth century from observational reconstructions of sea surface temperature. We find that, even though tropical Atlantic sea surface temperatures are currently at a historical high, Atlantic potential intensity probably peaked in the 1930s and 1950s, and recent values are near the historical average. Our results indicate that—per unit local sea surface temperature change—the response of tropical cyclone activity to natural climate variations, which tend to involve localized changes in sea surface temperature, may be larger than the response to the more uniform patterns of greenhouse-gas-induced warming.

20. Gnanadesikan, Anand, Keith W Dixon, Stephen Griffies, Ventakramani Balaji, M Barreiro, J A Beesley, W F Cooke, Thomas L Delworth, R Gerdes, Matthew J Harrison, Isaac Held, William J Hurlin, H C Lee, Z Liang, G Nong, Ronald C Pacanowski, Anthony Rosati, J L Russell, Bonita L Samuels, Qian Song, Michael J Spelman, Ronald J Stouffer, C Sweeney, G A Vecchi, Michael Winton, Andrew T Wittenberg, Fanrong Zeng, Rong Zhang, and John Dunne, 2006: GFDL's CM2 Global Coupled Climate Models. Part II: The baseline ocean simulation. Journal of Climate, 19(5), doi:10.1175/JCLI3630.1.
    [ Abstract ]

    The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.

21. Vecchi, G A., 2006: The Termination of the 1997–98 El Niño. Part II: Mechanisms of Atmospheric Change. Journal of Climate, 19(12), doi:10.1175/JCLI3780.1.
    [ Abstract ]

    The mechanisms that drove zonal wind stress (τx) changes in the near-equatorial Pacific at the end of the extreme 1997–98 El Niño event are explored using a global atmospheric general circulation model. The analysis focuses on three features of the τx evolution between October 1997 and May 1998 that were fundamental in driving the oceanic changes at the end of this El Niño event: (i) the southward shift of near-date-line surface zonal wind stress (τx) anomalies beginning November 1997, (ii) the disappearance of the easterly τx from the eastern equatorial Pacific (EEqP) in February 1998, and (iii) the reappearance of easterly τx in the EEqP in May 1998. It is shown that these wind changes represent the deterministic response of the atmosphere to the observed sea surface temperature (SST) field, resulting from changes in the meridional structure of atmospheric convective anomalies in response to the seasonally phase-locked meridional movement of the warmest SST. The southward shift of the near-date-line τx anomalies at the end of this El Niño event was controlled by the seasonal movement of the warmest SST south of the equator, which—both directly and through its influence on the atmospheric response to changes in SST anomaly—brought the convective anomalies from being centered about the equator to being centered south of the equator. The disappearance (reappearance) of easterly EEqP τx has only been evident in extreme El Niño events and has been associated with the development (northward retreat) of an equatorial intertropical convergence zone (ITCZ). The disappearance/return of EEqP easterly τx arises in the AGCM as the deterministic response to changes in the SST field, tied principally to the changes in climatological SST (given time-invariant extreme El Niño SSTA) and not to changes in the underlying SSTA field. The disappearance (return) of EEqP easterly τx in late boreal winter (late boreal spring) is a characteristic atmospheric response to idealized extreme El Niño SST anomalies; this suggests that the distinctive termination of the 1997–98 El Niño event is that to be expected for extreme El Niño events.

22. Vecchi, G A., and D E Harrison, 2006: The termination of the 1997–98 El Niño. Part I: Mechanisms of oceanic change. Journal of Climate, 19(12), doi:10.1175/JCLI3776.1.
    [ Abstract ]

    The 1997–98 El Niño was both unusually strong and terminated unusually. Warm eastern equatorial Pacific (EEqP) sea surface temperature anomalies (SSTAs) exceeded 4°C at the event peak and lasted well into boreal spring of 1998, even though subsurface temperatures began cooling in December 1997. The oceanic processes that controlled this unusual termination are explored here and can be characterized by three features: (i) eastward propagating equatorial Pacific thermocline (Ztc) shoaling beginning in the central Pacific in November 1997; (ii) persistent warm EEqP SSTA between December 1997 and May 1998, despite strong EEqP Ztc shoaling (and subsurface cooling); and (iii) an abrupt cooling of EEqP SSTA in early May 1998 that exceeded 4°C within two weeks. It is shown here that these changes can be understood in terms of the oceanic response to changes to the meridional structure of the near-equatorial zonal wind field. Equatorial near-date-line westerly wind anomalies greatly decreased in late 1997, associated with a southward shift of convective and wind anomalies. In the EEqP, equatorial easterlies disappeared (reappeared) in late January (early May) 1998, driving the springtime extension (abrupt termination) of this El Niño event. The authors suggest that the wind changes arise from fundamentally meridional processes and are tied to the annual cycle of insolation.

23. Vecchi, G A., Brian J Soden, Andrew T Wittenberg, Isaac Held, Ants Leetma, and Matthew J Harrison, 2006: Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature, 441(7089), 73-76.
    [ Abstract PDF ]

    Since the mid-nineteenth century the Earth's surface has warmed1, 2, 3, and models indicate that human activities have caused part of the warming by altering the radiative balance of the atmosphere1, 3. Simple theories suggest that global warming will reduce the strength of the mean tropical atmospheric circulation4, 5. An important aspect of this tropical circulation is a large-scale zonal (east–west) overturning of air across the equatorial Pacific Ocean—driven by convection to the west and subsidence to the east—known as the Walker circulation6. Here we explore changes in tropical Pacific circulation since the mid-nineteenth century using observations and a suite of global climate model experiments. Observed Indo-Pacific sea level pressure reveals a weakening of the Walker circulation. The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing. The climate model indicates that the weakened surface winds have altered the thermal structure and circulation of the tropical Pacific Ocean. These results support model projections of further weakening of tropical atmospheric circulation during the twenty-first century4, 5, 7.

24. Vecchi, G A., Andrew T Wittenberg, and Anthony Rosati, 2006: Reassessing the role of stochastic forcing in the 1997–1998 El Niño. Geophysical Research Letters, 33, L01706, doi:10.1029/2005GL024738.
    [ Abstract ]

    We explore the extent to which stochastic atmospheric variability was fundamental to development of extreme sea surface temperature anomalies (SSTAs) during the 1997–8 El Niño. The observed western equatorial Pacific westerly zonal stress anomalies (τ a x ), which appeared between Nov. 1996 and May 1997 as a series of episodic bursts, were largely reproducible by an atmospheric general circulation model (AGCM) ensemble forced with observed SST. Retrospective forecasts using a hybrid coupled model (HCM) indicate that coupling only the part of τ a x linearly related to large-scale tropical Pacific SSTA is insufficient to capture the observed 1997 warming; but, accounting in the HCM for all the τ a x that was connected to SST, recovers most of the strong SSTA warming. The AGCM-estimated range of stochastic τ a x forcing induces substantial dispersion in the forecasts, but does not obscure the large-scale warming in most HCM ensemble members.

25. Bhat, G S., G A Vecchi, and S Gadgil, 2004: Sea Surface Temperature of the Bay of Bengal Derived from the TRMM Microwave Imager. Journal of Atmospheric and Oceanic Technology, 21(8), 1283–1290.
    [ Abstract ]

    The Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) with the capability of measuring sea surface temperature (SST) in the presence of clouds, has been providing an unprecedented view of tropical basin-scale SST variability. In this paper, an assessment of the accuracy of the SST derived from TMI over the Bay of Bengal using in situ data collected from moored buoys and research ships, is presented. The authors find that TMI captures the evolution of the SST of the bay on seasonal time scales with reasonable accuracy. The mean difference between the SST from TMI and buoys is less than 0.1°C, and the rms difference is about 0.6°C. The time scales of the intraseasonal variation of the TMI SST are realistic. However, the amplitude of the SST variation on the intraseasonal scale is overestimated by a factor of about 1.3 when compared to buoy data. It is observed that the SST derived from TMI tends to be lower during periods with deep convection or winds stronger than 10 m s−1, or both. There is better agreement during weak conditions of convection/wind. This leads to a cold bias during convectively active periods when running average SST time series are constructed from SSTs retrieved from the TMI.

26. Vecchi, G A., and N A Bond, 2004: The Madden-Julian Oscillation (MJO) and northern high latitude wintertime surface air temperatures. Geophysical Research Letters, 31, L04104, doi:10.1029/2003GL018645.
    [ Abstract ]

    The Madden-Julian Oscillation (MJO) is the primary mode of large-scale intraseasonal variability in the tropics. Recent work has connected the MJO to atmospheric variability in mid-latitudes. We focus on relationships between the MJO and wintertime surface air temperatures in the Northern Hemisphere high latitudes. The MJO is diagnosed using principal EOF of 850 hPa zonal winds from the NCEP/NCAR Reanalysis for 1979–2002. Station data are used for surface air temperature in Alaska, Canada, the former U.S.S.R., Greenland, and Iceland. The phase of the MJO has a substantial systematic and spatially coherent effect on intraseasonal variability in wintertime surface air temperature through the global Arctic. Composites of geopotential height and specific humidity suggest that radiative and advective effects are important in the observed connections. These statistical connections may be useful for wintertime temperature forecasts. The mechanisms connecting intraseasonal tropical variability with polar and sub-polar variability bear examination.

27. Vecchi, G A., and D E Harrison, 2004: Interannual Indian rainfall variability and Indian Ocean sea surface temperature anomalies In Earth Climate: The Ocean-Atmosphere Interaction, Geophysical Monograph 147, Washington, DC, American Geophysical Union, 247-260.
    [ Abstract ]

    It is shown that interannual variations in Indian continental rainfall during the southwest monsoon can be usefully represented by two regional rainfall indices. India rainfall is concentrated in two regions, each with strong mean and variance in precipitation: the Western Ghats (WG) and the Ganges-Mahanadi Basin (GB) region. Interannual variability of rainfall averaged over each of the two regions (WG and GB) is uncorrelated; however, the rainfall over these two regions together explains 90% of the interannual variance of All-India rainfall (AIR). The lack of correlation between WG and GB rainfall suggests that different mechanisms may account for their variability. During the period 1982-2001, rainfall variability over each of these two regions exhibits distinct relationships to Indian Ocean SST: warm SSTA over the western Arabian Sea at the monsoon onset is associated with increased WG rainfall (r = 0.77), while cool SSTA off of Java and Sumatra is associated with increased GB rainfall (r = -0.55). The connection between SSTA and AIR is considerably weaker, and represents the superposition of that associated with each region. We find the relationship with WG rainfall is robust, while that with GB results from a single exceptional year. Each region also exhibits distinct relationships to El Nińo SSTA indices.

28. Vecchi, G A., S Xie, and A S Fischer, 2004: Ocean-atmosphere covariability in the Western Arabian Sea. Journal of Climate, 17(6), 1213-1224.
    [ Abstract ]

    The western Arabian Sea exhibits strong spatial variability in sea surface temperature (SST) during the southwest monsoon, with changes in SST that can exceed 5°C over 200 km. Exploration of satellite-based and in situ data shows a strong connection between mesoscale SST features and changes in the atmospheric boundary layer. The fundamental relationship is that of weak (strong) wind velocities overlying cold (warm) SST features. There are also coherent changes in other near-surface meteorological parameters, such as the air–sea temperature difference and relative humidity—indicating changes in the stability of the planetary boundary layer over the mesoscale SST features. These relationships are similar to those recently reported over the equatorial Pacific tropical instability wave region. This observed covariability of atmospheric boundary layer structure and SST results in variations of the surface heat and moisture fluxes; latent heat flux is modified by changes in relative humidity (principally through the temperature dependence of saturation specific humidity), wind speed, and boundary layer stability over the cold filaments. The nonlinear dependence of latent heat flux on the three parameters leads to a net enhancement of latent heat flux from the mesoscale features, as compared to that computed using spatially averaged parameters. Additionally, the spatial structure of the heat-flux variability will tend to dampen the mesoscale SST features. The mesoscale wind variability results in strong wind stress curl patterns on the same spatial scales as the oceanic features. The resulting Ekman pumping variations may play an important role in the evolution of the ocean eddy fields in this region. Further examination of the processes controlling the observed covariability, and the oceanic and atmospheric response to the coupling should therefore be undertaken. doi: 10.1175/1520-0442(2004)017<1213:OCITWA>2.0.CO;2

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