Influence Of Sea Surface Temperature Research Articles

Pattern and trend analysis of temperature in a set of seasonal ensemble simulations

[1] Eighteen years of seasonal 10-member ensemble simulations have been performed with observed sea surface temperature (SST) to assess the ability of dynamical models to predict seasonal-interannual climate variations during boreal summer. In addition, test cases have been designed to assess the role in climate trends, predictability of land initial conditions, systematic errors of precipitation and radiation fluxes at the land surface, and increasing CO2. The model reproduces a global warming trend in surface temperature similar to that observed. This appears to be attributable to the influence of SST. However, surface flux replacement and atmospheric initialization can reduce the SST-driven trend even while improving the spatial pattern of surface air temperature. The results also show that land surface interaction has an effect both on surface temperature and the higher levels of the troposphere. However, realistically increasing CO2 concentrations have little impact in these seasonal simulations.

Influence of the North Atlantic SST tripole on northwest African rainfall

The sea‐surface temperature (SST) tripole, with warm anomalies off the east coast of the United States and cold anomalies north of 40°N and south of 25°N, is the leading mode of interannual variability in wintertime North Atlantic SST. Its influence on northwest African rainfall is investigated by using a large‐ensemble of GCM simulations. Firstly the modeled basin‐scale rainfall impact is displayed, and the results suggest: in early‐mid winter (November–January), a positive SST tripole causes a reduced rainfall extending from the tropical North Atlantic northeastward to Mediterranean while a negative SST causes a south‐north increased rainfall across the central Atlantic from the subtropics to the midlatitude. In late winter (February–April) a positive SST tripole causes a reduced rainfall in the central Atlantic from the subtropics to the midlatitude while a negative SST tripole induces a zonal increased rainfall from the subtropics to Mediterranean. The asymmetry and seasonal dependence of the SST influence on the basin‐scale rainfall is consistent with the nonlinear response of the large‐scale atmospheric circulation. Under the large‐scale impact background, northwest Africa regional rainfall response is also nonlinear and seasonally dependent. In early‐mid winter a positive SST tripole causes reduced rainfall, while a negative SST has little effect. In late winter a negative SST tripole induces increased rainfall, while a positive tripole has little effect. A similarly large‐scale asymmetric association between SST and rainfall‐circulation exists in observations in late winter, while the observed seasonal dependence of this association is relatively weak. Also, a similar SST tripole association with the regional rainfall over the northwest coast of Africa exists in observations.

Climatic Influence of Sea Surface Temperature: Evidence of Substantial Precipitation Correlation and Predictability

Abstract Using a combination of statistical methods and monthly SST anomalies (SSTAs) from one or two ocean regions, relatively strong SSTA–precipitation relationships are found during much of the year in the United States: hindcast-bias-corrected correlation coefficients 0.2–0.4 and 0.3–0.6, on monthly and seasonal timescales, respectively. Improved rigor is central to these results: the most crucial procedure was a transform giving regression residuals meeting statistical validity requirements. Tests on 1994–99 out-of-sample data gave better results than expected: semiquantitative, mapped predictions, and quantitative, Heidke skills, are shown. Correlations are large enough to suggest that substantial skill can be obtained for one to several months' precipitation and climate forecasts using ocean circulation models, or statistical methods alone. Although this study was limited to the United States for simplicity, the methodology is intended as generally applicable. Previous work suggests that similar or...

A study on the intra‐annual variation and the spatial distribution of precipitation amount and duration over Greece on a 10 day basis

AbstractThe intra‐annual variation of precipitation amount and duration and their spatial distribution during the year are studied on a 10 day basis for the Greek region, using S‐mode and T‐mode factor analysis. (i) For the intra‐annual variation of precipitation amount, two modes were revealed: the first shows one broad maximum during the conventional winter in stations affected by the sea; the second presents two maxima, the first during late autumn–early winter and the second during late spring, corresponding to the northern mainland stations. (ii) For the spatial distribution of precipitation, three main patterns were revealed: the first one is the ‘winter’ pattern, with the maximum over the west windward area; the second is the ‘summer’ pattern, with a maximum over the north inland region; and the third is the ‘autumn’ pattern, with the maximum over northwestern Greece. (iii) For precipitation duration, two types of intra‐annual variation were revealed. The first one is similar to the first of the analysis for precipitation amount; the second presents two maxima, the first during the beginning of December and the second during the middle of February, corresponding to the areas of northwestern and northeastern Greece. (iv) For the spatial distribution of precipitation duration, three main patterns were revealed: the first is the ‘summer’ pattern, which is similar to the second of the analysis for precipitation amount; the second is the ‘winter’ pattern, with the spatial maximum located over the eastern mainland and western Crete; finally, the third one is the ‘autumn’ pattern, with the maximum in northwestern Greece. During the third 10 day period of October and the second 10 day period of February, precipitation seems to present singularities, possibly due to fluctuations in atmospheric circulation. The above intra‐annual variations and spatial distribution patterns are connected to the seasonal variations of the depression trajectories, the atmospheric instability, the influence of sea‐surface temperature as a cyclogenesis factor, and the windward or leeward character of the various areas (orographic effect). Copyright © 2003 Royal Meteorological Society.

Observational evidence of the influence of Pacific SSTs on winter precipitation and spring stream discharge in Idaho

Forty years of winter precipitation (23 stations) and spring stream flow discharge records (five stations) from across Idaho are analyzed to reveal regional patterns of association with sea surface temperatures (SSTs) in the Pacific Ocean. Results indicate that winter precipitation in the northern Idaho mountains, between 45° and 48°N, is negatively correlated with fall SSTs in the eastern tropical Pacific Ocean (El Nino and La Nina). Winter precipitation north of 45°N, is negatively correlated with winter SSTs in the northern Pacific (Pacific Decadal Oscillation, PDO). Spring stream discharge in Idaho is also negatively correlated with SSTs in the eastern tropical and northern regions of the Pacific Ocean. The association is asymmetric with stronger responses during negative SSTs for both regions in the Pacific Ocean. Wet and dry conditions are most likely associated with the combination of La Nina–negative PDO and El Nino–positive PDO, respectively. The greatest anomalies occur during the optimal combination of La Nina with negative PDO conditions. The revealed connections are valuable for climatic predictions based on the previous season's SST conditions in the eastern tropical Pacific and slowly evolving SSTs in the northern Pacific Ocean.

Empirical modelling of interannual trends in abundance of squid ( Loligoforbesi) in Scottish waters

This study compares alternative empirical models for forecasting interannual variation in squid ( Loligo spp.) abundance around Scotland (UK). Annual abundance indices (1970–1996) were derived from fishery data, for stocks in coastal waters (northern North Sea and West Coast) and the Rockall area. Abundance in coastal waters was correlated with several annual environmental indices, including the winter North Atlantic Oscillation index and the average sea surface temperature (SST) and sea surface salinity (SSS) in the northern North Sea. Regression models, relating fishery abundance to annual environmental indices, and time-series models, predicting abundance from the previous year’s abundance, could both provide reasonable fits to observed abundance data. Model fits obtained for coastal waters were generally better than those for the Rockall area. Regression models produced consistently poor forecasts whereas time-series models produced satisfactory forecasts for the years 1997–1999. Forecast accuracy for the time-series models could be improved by taking into account environmental variables. Thus, adding effects of SST in May in the northern North Sea in the current and previous years improved a time-series model of squid abundance in coastal waters. Improved forecasts were also achieved by fitting time-series models to monthly abundance data. The apparent influence of May SST in the northern North Sea on the coastal fishery could indicate that SST affects recruitment strength. Results for the offshore Rockall fishery suggest that large-scale and longer-term oceanographic processes may be involved.

On the influence of sea-surface temperature on mesoscale flows: an example from the city of Athens

The differences resulting in mesoscale flow estimations when using constant or varying (in time and space) sea-surface temperature information have been investigated, with the aid of appropriate air-flow simulations. Results suggest interesting interactions of land and sea surfaces with the atmosphere, which are expected to influence atmospheric flow modelling and air quality management in coastal areas.

Recent Sea Level and Sea Surface Temperature Changes Along the Maldives Coast

Maldives, a South Asian small island nation in the northern part of the Indian Ocean is extremely vulnerable to the impacts of Sea Level Rise (SLR) due to its low altitude from the mean sea level. This artricle attempts to estimate the recent rates of SLR in Maldives during different seasons of the year with the help of existing tidal data recorded in the Maldives coast. Corresponding Sea Surface Temperature (SST) trends, utilizing reliable satellite climatology, have also been obtained. The relationships between the SST and mean sea level have been comprehensively investigated. Results show that recent sea level trends in the Maldives coast are very high. At Male, the capital of the Republic of Maldives, the rising rates of Mean Tidal Level (MTL) are: 8.5, 7.6, and 5.8 mm/year during the postmonsoon (October-December), Premonsoon (March-May) and southwest monsoon (June-September) seasons respectively. At Gan, a station very close to the equator, the increasing rate of MTL is maximum during the period from June to September (which is 6.2 mm/year). These rising trends in MTL along the Maldives coast are certainly alarming for this small developing island nation, which is hardly one meter above the mean sea level. Thus there is a need for careful monitoring of future sea level changes in the Maldives coast. The trends presented are based on the available time-series of MTL for the Maldives coast, which are rather short. These trends need not necessarily reflect the long-term scenario. SST in the Maldives coast has also registered significant increasing trend during the period from June to September. There are large seasonal variations in the SST trends at Gan but SST and MTL trends at Male are consistently increasing during all the seasons and the rising rates are very high. The interannual mode of variation is prominent both in SST as well as MTL. Annual profile of MTL along the Maldives coast is bimodal, having two maxima during April and July. The April Mode is by far the dominant one. The SST appears to be the main factor governing the sea level variations along the Maldives coast. The influence of SST and sea level is more near the equatorial region (i.e., at Gan). There is lag of about two months for the maximum influence of SST on the sea level. The correlation coefficient between the smoothed SST and mean tidal level at Gan with lag of two months is as high as ~ +0.8, which is highly significant. The corresponding correlation coefficients at Male with the lags of one and two months are +0.5 and +0.3, respectively. Thus, the important finding of the present work for the Maldives coast is the dominance of SST factor in sea level variation, especially near the region close to the equator.

How Do Tropical Sea Surface Temperatures Influence the Seasonal Distribution of Precipitation in the Equatorial Amazon?

Although the correlation between precipitation over tropical South America and sea surface temperatures (SSTs) over the Pacific and Atlantic has been documented since the early twentieth century, the impact of each ocean on the timing and intensity of the wet season over tropical South America and the underlying mechanisms have remained unclear. Numerical experiments have been conducted using the National Center for Atmospheric Research Community Climate Model Version 3 to explore these impacts. The results suggest the following. 1) Seasonality of SSTs in the tropical Pacific and Atlantic has an important influence on precipitation in the eastern Amazon during the equinox seasons. The eastern side of the Amazon is influenced both by the direct thermal circulation of the Atlantic intertropical convergence zone (ITCZ) and by Rossby waves. These processes are enhanced by the seasonal cycles of SSTs in the tropical Atlantic and Pacific. SSTs affect Amazon precipitation much less during the solstice seasons and in the western Amazon. 2) The seasonality of SSTs in the Atlantic more strongly affects Amazon rainfall than does that of the Pacific. Without the former, austral spring in the eastern equatorial Amazon would be a wet season, rather than the observed dry season. As a consequence of the lag at that time of the southward seasonal migration of the Atlantic SSTs behind that of the insolation, the Atlantic ITCZ centers itself near 10°N, instead of at the equator, imposing subsidence and low-level anticyclonic flow over the eastern equatorial Amazon, thus drying the air above the planetary boundary layer and reducing the low-level moisture convergence. Consequently, convection in the eastern Amazon is suppressed despite strong surface heating. 3) Seasonality of the SSTs in the tropical Pacific also tends to reduce precipitation in the eastern Amazon during both spring and fall. In spring, subsidence is enhanced not only through a zonal direct circulation, but also through Rossby waves propagating from the extratropical South Pacific to subtropical South America. This teleconnection strengthens the South Atlantic convergence zone (SACZ) and the Nordeste low, in both cases reducing precipitation in the eastern Amazon. A direct thermal response to the Pacific SSTs enhances lower-level divergence and reduces precipitation from the northern tropical Atlantic to the northeastern Amazon.

A central tropical Pacific coral demonstrates Pacific, Indian, and Atlantic decadal climate connections

While instrumental and proxy‐based climate records describe significant decadal‐scale climate variability throughout the tropical Pacific, Indian, and Atlantic Oceans, the processes responsible for these variations and their interactions are not readily apparent from the observations. A new 112‐yr coral‐based sea surface temperature (SST) reconstruction from Palmyra Island in the central tropical Pacific (CTP) exhibits strong decadal variability with an amplitude of roughly 0.3°C. A 12–13yr‐period signal in this coral record is highly coherent with long equatorial Atlantic and Indian Ocean climate records, implying a unified phenomenon. The Atlantic pattern suggests that it may fall under direct influence of anomalous SST in the CTP, as it does over inter annual timescales, while the Indian Ocean pattern exhibits maximum response during the switch between warm/cold states in the tropical Pacific. The results demonstrate that the CTP has played a significant role in determining the expression of global decadal climate variability over the twentieth century.

Changes in the Spread of the Variability of the Seasonal Mean Atmospheric States Associated with ENSO

Abstract For a fixed sea surface temperature (SST) forcing, the variability of the observed seasonal mean atmospheric states in the extratropical latitudes can be characterized in terms of probability distribution functions (PDFs). Predictability of the seasonal mean anomalies related to interannual variations in the SSTs, therefore, entails understanding the influence of SST forcing on various moments of the probability distribution that characterize the variability of the seasonal means. Such an understanding for changes in the first moment of the PDF for the seasonal means with SSTs is well documented. In this paper the analysis is extended to include also the impact of SST forcing on the second moment of the PDFs. The analysis is primarily based on ensemble atmospheric general circulation model (AGCM) simulations forced with observed SSTs for the period 1950–94. To establish the robustness of the results and to ensure that they are not unduly affected by biases in a particular AGCM, the analysis is ba...

Trends, spectral characteristics, and rainfall relationships of low‐latitude sea surface temperatures at different longitudes

The sea surface temperature (SST) data for low latitudes in the Pacific, Atlantic, and Indian Oceans for 1950–1996 (47 years) showed different seasonal variation patterns at different longitudes. When the seasonal patterns were subtracted from the monthly values, the deseasoned residuals showed considerable anomalies (interannual variability). In the Pacific the main features were the El Niño events. In the Atlantic, North and South Atlantic SST showed dissimilar anomalies, and these did not have any fixed lag or lead relationships with the Pacific events. The same was true for the low‐latitude Indian Ocean SST. The correlation of Pacific SST with Atlantic or Indian Oceans' SST was less than ∼0.65, yielding a common variance (square of the correlation) of less than ∼40%. Thus, whereas SST anomalies might have some common origin, the manifestation of SST anomalies at different longitudes was erratic, with no preference for any longitude to start with, nor any definite sequence of occurrence in the Pacific relative to the Atlantic or Indian Oceans. A spectral analysis showed that all regions had quasi‐biennial, quasi‐triennial, and higher periodicities, but the exact values of these periodicities differed significantly at different longitudes. All parameters had long‐term trends. These were mostly nonuniform, almost negligible in the first half (1950–1973) and mostly upward in the second half (1973–1996), indicating warming in recent decades, which is also reflected in decreases in snow cover area in the Northern Hemisphere. Rainfalls in various regions are considerably influenced by local SST regimes. For northeast Brazil, Atlantic SST influence is overpowering and often operates independently of the Pacific SST (El Niños). Hence the emphasis given in mass media (press, radio, and television) to the role of El Niño events only in influencing the rainfalls may turn out to be misleading, as seems to have happened for the 1997 El Niño. This El Niño started in early 1997 but did not have any significant influence on rainfall in northeast Brazil and India. It continued strongly in 1998, causing a severe drought in northeast Brazil but no drought in India.

Assessment of the impacts of the 1997–98 El Niño on the Asian‐Australia Monsoon

Singular Value Decomposition (SVD) analyses of rainfall and sea surface temperature (SST) are carried out globally over the entire tropics and regionally over the Asia‐Australian (AA)‐monsoon domain. Contributions to monsoon rainfall predictability by basin‐scale SST forcing and regional SST coupling are evaluated from cumulative anomaly correlation with dominant regional SVD modes. The observed 1997–98 AA‐monsoon anomalies are found to be very complex with approximately 34% of the anomalies of the Asian (boreal) summer monsoon and 74% of the Australia (austral) monsoon attributable to basin‐scale SST influence associated with El Niño. For the boreal and austral monsoon respectively, regional coupled processes contribute an additional 19% and 10%, leaving about 47% and 16% due to other factors including high frequency transients. Results suggest that in order to improve seasonal‐to‐interannual predictability of the AA‐monsoon, we need to investigate and exploit not only monsoon‐El Niño relationship, but also intrinsic monsoon regional coupled processes.

Influences of Sea Surface Temperature and Ground Wetness onAsian Summer Monsoon

Abstract The authors have conducted a series of experiments with a general circulation model to understand the influences of sea surface temperature (SST) and ground wetness (GW) (measured by snow amount and soil moisture content) on the Asian summer monsoon. The experiments are designed to illustrate the dominant features of monsoon response to SST and GW forcings and to delineate the relative importance of each forcing function in contributing to the variability of the monsoon. Results indicate that ocean basin-scale SST anomalies exert a stronger control on the interannual variability of the monsoon compared to GW anomalies. The impact of SST anomalies on the monsoon appears nonlinear with respect to warm and cold events. The monsoon is weakened during the warm events but changes less noticeably during the cold events. The diminution of monsoon circulation associated with the warm SST anomalies is accompanied by a broad-scale reduction in water vapor convergence and monsoon rainfall. Results also indic...

Relationship between sea surface temperature, vertical dynamics, and the vertical distribution of atmospheric water vapor inferred from TOVS observations

With the aim of better understanding the respective role of sea surface temperature (SST) and vertical dynamics on the vertical distribution of atmospheric water vapor, particularly in the tropics, global scale observations from NOAA 10, covering a 31−month period, have been processed using the improved initialization inversion ((3I) [Chédin and Scott, 1984]) retrieval method and interpreted in terms of tropospheric layered water vapor contents. The method of analysis uses the power law, which expresses the specific humidity q at pressure p as a function of their values at the surface, q0 and p0; q = q0(p/p0)λ. This description is applied independently to three layers giving three values of λ: λ1 for surface‐700 hPa, λ2 for 700–500 hPa, and λ3 for 500–300 hPa. It is shown that λ2 is a good indicator of the large‐scale vertical dynamics and gives results equivalent to those obtained using the vertical velocity at 500 hPa issued from a model. Consequently, the role of enhanced upward motion with increased SST for the “super greenhouse effect” situations is confirmed as well as the contribution of externally forced subsidence on the suppression of the deep convection for cases where SSTs exceed about 303 K. In addition, the influence of SST on the vertical distribution of water vapor is analyzed together with the large‐scale vertical dynamics contribution. The results show that the rate of change of water vapor content in the 700‐ to 500‐hPa and 500‐ to 300‐hPa layers with respect to SST increases with decreasing rate of change of λ2 with respect to SST, that is, with increasing rate of change of upward vertical dynamics with respect to SST.

Southern hemisphere sea-surface temperatures and their contemporary and lag association with New Zealand temperature and precipitation

The influence of sea-surface temperature (SST) anomalies on New Zealand climate variations is examined systematically, using data for the period 1949–1991. New Zealand temperature and precipitation anomalies, both monthly and seasonal, are represented in terms of rotated EOF patterns, the time series of which are then cross-correlated against sea-surface temperature data for an extensive region of the Southern Hemisphere. Relationships with mean sea level pressure anomalies in the Australasian region are examined to clarify significant associations found. Monthly lead and lag cross-correlations suggest that sea temperatures in the immediate vicinity of New Zealand are responding to atmospheric circulation variations, a result that is consistent with many similar studies elsewhere regarding midlatitude sea temperature anomalies. New Zealand temperatures, particularly in the North Island, are also significantly correlated to the previous month's sea-surface temperature anomaly in the western Tasman Sea and south of Australia, but only in autumn months (March, April, May). Subsequent analysis suggests the most likely cause of this is atmospheric forcing in the antecedent month. On the seasonal time scale, a number of SST–New Zealand climate lag relationships are found, many of which appear to be related to ENSO variations. However, tropical Indian Ocean SSTs unrelated to ENSO are prominent in the autumn–winter period. One phase of this teleconnection pattern is for warmer Indian Ocean waters in autumn to be followed by a stronger subtropical ridge just north of North Island in winter, stronger westerly flow over the country, higher South Island temperatures, more rainfall in west South Island districts and drier conditions in other areas of the country exposed to the north and east. © 1998 Royal Meteorological Society

Influence of sea surface temperatures on air temperatures in the tropics

Interannual variations of tropical tropospheric temperatures are closely related to sea surface temperature (SST) changes in the tropical eastern Pacific (TEP). This study investigated the physical mechanisms for such an air-sea interrelationship. SSTs and latent heat flux were analyzed to find the unique properties of their variations during El Nino. A Gill-type model was used to investigate how a local heat source communicates with the entire tropics. Radiative fluxes in the tropics were evaluated to search for the factors limiting air temperature increases when warm SSTs remain in the TEP. We found that interannual variabilities of SST and latent heat flux are dominated by the variations in the TEP region. The SST variations there have three unique properties that allow the ocean to influence the atmosphere effectively: large magnitude, long persistence, and spatial coherence. The Gill-type model shows that a local heat source can warm the entire tropical troposphere when the heat source is near the equator. Released latent heat in the heat source region and forced adiabatic subsidence elsewhere in the tropics warm the atmosphere. As a result, a local heat source warms the entire tropical strip. The forced subsidence depresses clouds, allowing more infrared radiation to leave the atmosphere and preventing further atmospheric warming when warm SSTs remain in the TEP. This finding is verified by reanalysis data from the National Centers for Environmental Prediction.

The Role of Orography in Determining the Sahelian Climate

We have examined numerical modelling evidence based on the NASA GEOS-1 general circulation model in order to investigate the role of orography in determining the West African summer climate anomalies. Orographic climatic control and the influence of sea-surface temperature (SST) anomaly forcing were investigated by systematic suppression or inclusion of these effects in the numerical experiments. Our main finding is that the orography of Africa plays a major role in determining the climate of the Sahel and the coastal region of West Africa during the northern hemispheric summer season. These orographic effects are comprised of: (i) a quasi-stationary orographic ridge–trough dipole generated by the passage of the low-level prevailing easterlies over the Atlas–Ahaggar mountain complex over northern Africa, and (ii) an elongated zonal windward orographic ridge generated as the cross-equatorial summer monsoons from the South Atlantic Ocean basin ascends over the elevated landmass of West Africa. Over the Sahel, the moist south-westerly flow emanating from the Atlas–Ahaggar orographic trough over the Atlantic Ocean and the dry hot north-easterlies originating from the corresponding orographic windward ridge tend to converge along a zonal axis thus reinforcing the Inter-Tropical Convergence Zone. This results in wetter conditions over the Sahel than would have been the case in the absence of orography. The model simulations show that further to the south along the coastal region of West Africa, the zonal orographic ridge results in drier conditions than in the model runs when orography is suppressed. The net result is a permanent orographic-induced rainfall dipole pattern over West Africa. Further model simulations were conducted in which the 1973 SST anomaly conditions were described with or without the orography of Africa. The 1973 SST anomaly pattern is characterized by positive anomalies in the Southern Hemisphere and negative anomalies in the Northern Hemisphere. The model results show a weaker orographic-induced rainfall dipole pattern across West Africa in response to the 1973 SST anomaly pattern. The net result is wetter conditions along the coastal region and rainfall deficits over the Sahelian zonal strip. © 1997 by the Royal Meteorological Society.

The Influence of Sea Surface Temperature on the Tropical Intraseasonal Oscillation: A Numerical Study

Abstract The development and movement of the tropical intraseasonal system (TIS) exhibit remarkable annual variations. It was hypothesized that spatial and temporal variation in sea surface temperature (SST) is one of the primary climatic factors that are responsible for the annual variation of TISs. This paper examines possible influences of SST on the TIS through numerical experiments with a 2.5-layer atmospheric model on an equatorial β plane, in which SST affects atmospheric heating via control of the horizontal distribution of moist static energy and the degree of convective instability. The gradient of the antisymmetric (with respect to the equator) component of SST causes a southward propagation of the model TIS toward northern Australia in boreal winter and a northward propagation over the Indian and western Pacific Oceans in boreal summer. The phase speed of the meridional propagation increases with the magnitude, of antisymmetric SST gradients. The poleward propagation of the equatorial disturba...

Influence of Ocean Surface Conditions on Atmospheric Vertical Thermodynamic Structure and Deep Convection

The authors analyze the influence of Sea Surface Temperature (SST) and surface wind divergence on atmospheric thermodynamic structure and the resulting effects on the occurrence of deep convection using National Meteorological Center radiosonde data and International Satellite Cloud Climatology Program data for July 1983-July 1985. The onset of deep convection requires not only the existence of positive convective available potential energy (CAPE), but also an unstable planetary boundary layer (PBL). A stable PBL is observed to suppress deep convection even when CAPE is positive. Variations of SST have a major effect on CAPE, but surface wind divergence can also affect deep convection by changing the lapse rate in the lower troposphere and humidity in the PBL. Specifically, when SST is greater than or equal to 28 C, CAPE is always positive, and surface wind divergence does not qualitatively change the buoyancy profile above the PBL. Strong surface wind divergence, however, stabilizes the PBL so as to suppress the initiation of deep convection. In warm SST regions, CAPE is greater than 0 regardless of assumptions about condensate loading, although the pseudoadiabatic limit is more consistent with the observed deep convection than the reversible moist-adiabatic limit under these circumstances. When SST is less than 27 C, CAPE is usually negative and inhibits convection, but strong surface wind convergence can destabilize the inversion layer and moisten the PBL enough to make the atmosphere neutrally stable in the mean. As a result, deep convection is generally enhanced either when SST is greater than or equal to 28 C in the absence of strong surface wind divergence or when strong surface wind convergence occurs even if SST is less than 27 C. The anomalous suppression of deep convection in the warm area of the equatorial west Pacific lying between the intertropical convergence zone (ITCZ) and south Pacific convergence zone (SPCZ) is probably caused by dryness in the PBL and an inversion in that area. The seasonal cycles of deep convection and surface wind divergence are in phase with the maximum solar radiation and lead SST for one to three months in the central Pacific. The change of PBL relative humidity plays a critical role in the changeover to convective instability in this case. The seasonal change of deep convection and associated clouds seems not to have important effects on the seasonal change of local SST in the central Pacific.