Large-scale Air-sea Interaction Research Articles

Interdecadal change in the relationship between the boreal winter North Pacific Oscillation and the following summer El Niño

AbstractThe boreal winter North Pacific Oscillation (NPO) has a significant impact on the tropical Pacific large scale air-sea interaction in the following spring and summer. But it is unclear whether the boreal winter NPO is associated with the El Niño development or decaying during following summer. This study confirms the relationship between the winter NPO-like sea level pressure anomalies and following summertime El Niño and reveals an interdecadal change in this relationship. It tends to promote the development of sea surface temperature anomaly in central equatorial Pacific in the following summer when the subtropical lobe of the NPO located west of its average position. But after a significant eastward shift of NPO’s subtropical lobe, it is more conducive to the development of sea surface temperature anomaly in eastern equatorial Pacific. The eastward shift of the NPO’s subtropical lobe altered both the seasonal foortprint mechanism and the trade wind charging mechanism associated with the NPO, thus profoundly influenced the summertime El Niño. The findings in the present study have implications for a better understanding of the relationship between the boreal winter NPO and the El Niño in developing summer.

Rethinking the Characteristic Parameters of Typhoon Activity Frequency in the Western North Pacific

ABSTRACT Typhoons are among the most dangerous weather systems on earth and can cause devastating disasters. Although great improvements have been made in the prediction of typhoon tracks during the past decades, typhoon frequency remains a major challenge in operational forecasting and climate prediction. Typhoon genesis frequency focuses on the frequency of formation of a single typhoon. However, the accumulation of single typhoons cannot effectively represent the anomalous characteristics of the corresponding circulation because of considerable differences among the typhoons, such as life history, track, and intensity. The occurrence of multiple typhoons or a single typhoon lasting for a long time over a particular region is usually closely related to the atmospheric circulation over that region. Therefore, in this study, the frequency of typhoons during a certain period and region is used to measure typhoon activity in the western North Pacific (WNP). It is found that the variation of typhoon existence frequency is different from that of typhoon genesis frequency. Typhoon existence frequency greatly improves the connection between typhoon activity and large-scale air–sea interactions (such as the El Niño–Southern Oscillation) and can better indicate the difference in weather and climate corresponding to typhoon activity anomalies. Further investigation reveals that typhoon existence frequency in the WNP during typhoon season is closely related to the North Pacific tripole pattern of sea surface temperature (SST) anomaly (NPSST) and the SST anomaly along the eastern coast of Australia in the South Pacific (SPSST) in the spring (April). A regression model using the NPSST and SPSST indices in April as predictors proved to have good skill in predicting the existence frequency of typhoons in the WNP during typhoon season.

Morphological evolution of river mouth spits: Wave effects and self-organization patterns

Sand spits as a type of common landform features, associated with wave-dominated deltas, may be influenced by self-organization processes due to shoreline instability at the downdrift flank of the delta, which forms an asymmetric plan morphology for the delta. Here, we examine the spatial-temporal variations and their controlling processes of the sand spit system at the Changhua River mouth, northwest of the South China Sea, by combining 34-year (1986–2019) satellite-derived shoreline data with an ERA5 wave hindcast dataset. The spit system is characterized by extension towards the sea, as well as recurvature towards the land, in response to dominating N-NE waves, whilst, for the sheltered coastal embayment behind the spit, echelon reverse spits have developed due to the dominating S-SW waves. The wave action is governed by a combination of the short period local marine climate of the area (monsoonal regime) and large-scale weather cycles (i.e., El Niño and La Niña), resulting in inter-seasonal and inter-annual variations of the spit neck morphology. Furthermore, the river mouth spit shows self-organization patterns at different temporal-spatial scales, as indicated by the cyclical behavior of the updrift coast, together with the stepwise formation of the echelon reverse spits. Such behavior influences the spit morphology and its rate of change. The geomorphological self-organization patterns contain the information on local dynamic processes, as well as on large scale air-sea interactions as exemplified by the ENSO events. Thus, the analysis presented in this study is valid to describe and estimate the morphological evolution of highly dynamic barrier-spit systems elsewhere, based on wave energy flux estimated by available wave parameters.

Influence of El Niño events on sea surface salinity over the central equatorial Indian Ocean

The El Niño event is a major large-scale air-sea interaction phenomenon over the tropical Pacific region. Previous studies classified El Niño into three types — canonical El Niño, El Niño Modoki I, and El Niño Modoki II. This research reveals that different types of El Niño present dramatic effects on the interannual variability of sea surface salinity over the central equatorial Indian Ocean in the boreal autumn. The decreasing (increasing) sea surface salinity during the canonical El Niño and the EI Niño Modoki I (the EI Niño Modoki II) events is identified. The salinity budget analysis is performed to identify the dominant factors responsible for the variability of sea surface salinity over the central Indian Ocean. The results indicate that the wind-driven anomalous zonal advection plays an important role in sea surface salinity variability during the El Niño events associated with the forcing from the anomalous Walker circulation over the equatorial Indian Ocean.

Subsurface ocean biases in climate models and its implications in the simulated interannual variability: A case study for Indian Ocean

Coupled ocean atmosphere general circulation models (CGCMs) are known to have deep ocean biases when simulated over long term climate timescales. Here, an analysis has been done for investigating the impacts of these deep ocean biases, especially those which occur in temperature and salinity, ensuing biases in ocean dynamics and large scale air-sea interactions in state of the art CGCMs. The outputs from historical runs of 20 Coupled Model Intercomparison Project Phase 5 (CMIP-5) models have been analyzed for Indian Ocean. All candidate models develop internal warm and saline biases approximately between a depth range of 100 and 800 m in long term simulations. These internal biases are found to have implications in large scale ocean dynamics via their linkage through baroclinicity of the ocean. The role of internal biases in the ensuing dynamics is correlated via relations between Brunt Väisälä frequency (N2) and baroclinic wave speeds using both Sturm-Loiuville theorem and WKBJ approximation. The CMIP-5 models analyzed here have a higher baroclinic speed compared to that of observations. Annual propagating modes in the ocean reveal higher speeds in most of the models, and that the phase reversal is taking place at lead or lag by a month or more as compared to observations. The study suggests that faster wave propagation in climate models due to subsurface biases in temperature, salinity, N2 and baroclinicity have potential to impact simulated planetary scale events in terms of their life cycle, periodicity and seasonality as analyzed with Indian Ocean Dipole Zonal Mode. A corollary being a cautionary outlook on climate projections made by coupled models as long as the biases are persistent.

Association between mean and interannual equatorial Indian Ocean subsurface temperature bias in a coupled model

In the present study the association between mean and interannual subsurface temperature bias over the equatorial Indian Ocean (EIO) is investigated during boreal summer (June through September; JJAS) in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFSv2) hindcast. Anomalously high subsurface warm bias (greater than 3 °C) over the eastern EIO (EEIO) region is noted in CFSv2 during summer, which is higher compared to other parts of the tropical Indian Ocean. Prominent eastward current bias in the upper 100 m over the EIO region induced by anomalous westerly winds is primarily responsible for subsurface temperature bias. The eastward currents transport warm water to the EEIO and is pushed down to subsurface due to downwelling. Thus biases in both horizontal and vertical currents over the EIO region support subsurface warm bias. The evolution of systematic subsurface warm bias in the model shows strong interannual variability. These maximum subsurface warming episodes over the EEIO are mainly associated with La Nina like forcing. Strong convergence of low level winds over the EEIO and Maritime continent enhanced the westerly wind bias over the EIO during maximum warming years. This low level convergence of wind is induced by the bias in the gradient in the mean sea level pressure with positive bias over western EIO and negative bias over EEIO and parts of western Pacific. Consequently, changes in the atmospheric circulation associated with La Nina like conditions affected the ocean dynamics by modulating the current bias thereby enhancing the subsurface warm bias over the EEIO. It is identified that EEIO subsurface warming is stronger when La Nina co-occurred with negative Indian Ocean Dipole events as compared to La Nina only years in the model. Ocean general circulation model (OGCM) experiments forced with CFSv2 winds clearly support our hypothesis that ocean dynamics influenced by westerly winds bias is primarily responsible for the strong subsurface warm bias over the EEIO. This study advocates the importance of understanding the ability of the models in representing the large scale air–sea interactions over the tropics and their impact on ocean biases for better monsoon forecast.

Sub-decadal North Atlantic Oscillation variability in observations and the Kiel Climate Model

The North Atlantic Oscillation (NAO) is the dominant mode of winter climate variability in the North Atlantic sector. The corresponding index varies on a wide range of timescales, from days and months to decades and beyond. Sub-decadal NAO variability has been well documented, but the underlying mechanism is still under discussion. Other indices of North Atlantic sector climate variability such as indices of sea surface and surface air temperature or Arctic sea ice extent also exhibit pronounced sub-decadal variability. Here, we use sea surface temperature and sea level pressure observations, and the Kiel Climate Model to investigate the dynamics of the sub-decadal NAO variability. The sub-decadal NAO variability is suggested to originate from dynamical large-scale air-sea interactions. The adjustment of the Atlantic Meridional Overturning Circulation to previous surface heat flux variability provides the memory of the coupled mode. The results stress the role of coupled feedbacks in generating sub-decadal North Atlantic sector climate variability, which is important to multiyear climate predictability in that region.

North Atlantic Ocean control on surface heat flux on multidecadal timescales

Nearly 50 years ago Bjerknes suggested that the character of large-scale air-sea interaction over the mid-latitude North Atlantic Ocean differs with timescales: the atmosphere was thought to drive directly most short-term--interannual--sea surface temperature (SST) variability, and the ocean to contribute significantly to long-term--multidecadal--SST and potentially atmospheric variability. Although the conjecture for short timescales is well accepted, understanding Atlantic multidecadal variability (AMV) of SST remains a challenge as a result of limited ocean observations. AMV is nonetheless of major socio-economic importance because it is linked to important climate phenomena such as Atlantic hurricane activity and Sahel rainfall, and it hinders the detection of anthropogenic signals in the North Atlantic sector. Direct evidence of the oceanic influence of AMV can only be provided by surface heat fluxes, the language of ocean-atmosphere communication. Here we provide observational evidence that in the mid-latitude North Atlantic and on timescales longer than 10 years, surface turbulent heat fluxes are indeed driven by the ocean and may force the atmosphere, whereas on shorter timescales the converse is true, thereby confirming the Bjerknes conjecture. This result, although strongest in boreal winter, is found in all seasons. Our findings suggest that the predictability of mid-latitude North Atlantic air-sea interaction could extend beyond the ocean to the climate of surrounding continents.

Tropical Pacific influences on the North Atlantic Thermohaline Circulation

Most global climate models simulate a weakening of the North Atlantic thermohaline circulation (THC) in response to enhanced greenhouse warming. Both surface warming and freshening in high latitudes, the so-called sinking region, contribute to the weakening of the THC. Some models simulate even a complete breakdown of the THC at sufficiently strong forcing. Here results from a state-of-the-art global climate model are presented that does not simulate a weakening of the THC in response to greenhouse warming. Large-scale air-sea interactions in the tropics, similar to those operating during present-day El Ninos, lead to anomalously high salinities in the tropical Atlantic. These are advected into the sinking region, thereby increasing the surface density and compensating the effects of the local warming and freshening. The results of the model study are corroborated by the analysis of observations.

Two modes of dipole events in tropical Indian Ocean

By analyzing the distributions of subsurface temperature and the surface wind stress anomalies in the tropical Pacific and Indian Oceans during the Indian Ocean Dipole (IOD) events, two major modes of the IOD and their formation mechanisms are revealed. (1) The subsurface temperature anomaly (STA) in the tropical Indian Ocean during the IOD events can be described as a “<” -shaped and west-east-oriented dipole pattern; in the east side of the “<” pattern, a notable tongue-like STA extends westward along the equator in the tropical eastern Indian Ocean; while in the west side of the “<” pattern, the STA has opposite sign with two centers (the southern one is stronger than the northern one in intensity) being of rough symmetry about the equator in the tropical mid-western Indian Ocean. (2) The IOD events are composed of two modes, which have similar spatial pattern but different temporal variabilities due to the large scale air-sea interactions within two independent systems. The first mode of the IOD event originates from the air-sea interaction on a scale of the tropical Pacific-Indian Ocean and coexists with ENSO. The second mode originates from the air-sea interaction on a scale of the tropical Indian Ocean and is closely associated with changes in the position and intensity of the Mascarene high pressure. The strong IOD event occurs when the two modes are in phase, and the IOD event weakens or disappears when the two modes are out of phase. Besides, the IOD events are normally strong when either of the two modes is strong. (3) The IOD event is caused by the abnormal wind stress forcing over the tropical Indian Ocean, which results in vertical transports, leading to the upwelling and pileup of seawater. This is the main dynamic processes resulting in the STA. When the anomalous easterly exists over the equatorial Indian Ocean, the cold waters upwell in the tropical eastern Indian Ocean while the warm waters pileup in the tropical western Indian Ocean, hence the thermocline in the tropical Indian Ocean is shallowed in the east and deepened in the west. The off-equator component due to the Coriolis force in the equatorial area causes the upwelling of cold waters and the shallowing of the equatorial India Ocean thermocline. On the other hand, the anomalous anticyclonic circulations and their curl fields located on both sides of the equator, cause the pileup of warm waters in the central area of their curl fields and the deepening of the equatorial Indian Ocean thermocline off the equator. The above three factors lead to the occurrence of positive phase IOD events. When anomalous westerly dominates over the tropical Indian Ocean, the dynamic processes are reversed, and the negative-phase IOD event occurs.

大気海洋相互作用からみた気候変動

In the tropics, the El Nino-Southern Oscillation (ENSO) occupies a major part of the interannual air-sea interactive system. The ENSO plays a vital role in triggering the occurrence of extraordinary anomalous climates and weather not only in the tropics but also in the extratropical regions. In the South Asian monsoon region, the ENSO can influence the interannual variability of the monsoon system through at least two different impacts. During the decay phase of ENSO (from winter to summer) its delayed impact operates through large-scale air-sea interaction in the tropical Indian Ocean and land-surface hydrological processes over the Asian Continent, eventually bringing about change in summer monsoon activity in June and July. During the ENSO growth phase (from summer to winter), in contrast, its direct impact is considered to account for monsoon interannual variability especially in August and September. East Asian monsoon variability is also significantly affected by ENSO-related tropical forcing. Especially in early winter, ENSO-related anomalous convection can give rise to a change in the East Asian winter monsoon system through stationary Rossby wave propagation along the South Asian waveguide, but the remote response depends on the geographical configuration of the anomalous tropical convection. In summer, the ENSO's delayed impact is associated with excitation of an extratropical teleconnection, which causes anomalous weather in northeastern Asia. Midlatitude air-sea interactions and their potential impact on large-scale atmospheric circulations are also discussed. The coexistence of the East Asian winter monsoon flow and western boundary current makes air-sea heat exchanges in the Kuroshio extension very active. Due to enhanced baroclinicity and surface heat fluxes from the ocean, a number of extratropical cyclones tend to develop explosively in the vicinity of Japan. The activity of these extratropical cyclones contributes to the downstream development of upper-level teleconnections.

Origin of Pacific Multidecadal Variability in Community Climate System Model, Version 3 (CCSM3): A Combined Statistical and Dynamical Assessment

Abstract Observations indicate that Pacific multidecadal variability (PMV) is a basinwide phenomenon with robust tropical–extratropical linkage, though its genesis remains the topic of much debate. In this study, the PMV in the Community Climate System Model, version 3 (CCSM3) is investigated with a combined statistical and dynamical approach. In agreement with observations, the modeled North Pacific climate system undergoes coherent multidecadal atmospheric and oceanic variability of a characteristic quasi-50-yr time scale, with apparent connections to the tropical Indo-Pacific. The statistical assessment based on the CCSM3 control integration cannot exclusively identify the origin of the modeled multidecadal linkage, while confirming the two-way interactions between the tropical and extratropical Pacific. Two sensitivity experiments are performed to further investigate the origin of the PMV. With the atmosphere decoupled from the tropical ocean, multidecadal variability in the North Pacific climate remains outstanding. In contrast, without midlatitude oceanic feedback to atmosphere, an experiment shows much reduced multidecadal power in both extratropical atmosphere and surface ocean; moreover, the tropical multidecadal variability seen in the CCSM3 control run virtually disappears. The combined statistical and dynamical assessment supports a midlatitude coupled origin for the PMV, which can be described as follows: extratropical large-scale air–sea interaction gives rise to multidecadal variability in the North Pacific region; this extratropical signal then imprints itself in the tropical Indo–Pacific climate system, through a robust tropical–extratropical teleconnection. This study highlights a midlatitude origin of multidecadal tropical–extratropical linkage in the Pacific in the CCSM3.

Low-Latitude Freshwater Influence on Centennial Variability of the Atlantic Thermohaline Circulation

Abstract Variability of the thermohaline circulation (THC) has been analyzed in a long control simulation by the Met Office's Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It is shown that internal THC variability in the coupled climate system is concentrated at interannual and centennial time scales, with the centennial mode being dominant. Centennial oscillations of the THC can impact surface climate via an interhemispheric SST contrast of 0.1°C in the Tropics and more than 0.5°C in mid- and high latitudes. A mechanism is proposed based on detailed process analysis involving large-scale air–sea interaction on multidecadal time scales. Anomalous northward ocean heat transport associated with a strong phase of the Atlantic THC generates a cross-equatorial SST gradient. This causes the ITCZ to move to a more northerly position with increased strength. The extra rainfall resulting from the anomalous ITCZ imposes a freshwater flux and produces a salinity anomaly in the tropical North Atlantic. Such sustained salinity anomalies slowly propagate toward the subpolar North Atlantic at a lag of 5–6 decades. The accumulated low-salinity water lowers upper-ocean density, which causes the THC to slow down. The oscillation then enters the opposite phase.

Progress of large-scale air-sea interaction studies in China

This paper summarizes the progress of large-scale air-sea interaction studies that has been achieved in China in the four-year period from July 1998 to July 2002, including seven aspects in the area of the air-sea interaction, namely air-sea interaction related to the tropical Pacific Ocean, monsoon-related air-sea interaction, air-sea interaction in the north Pacific Ocean, air-sea interaction in the Indian Ocean, air-sea interactions in the global oceans, field experiments, and oceanic cruise surveys. However more attention has been paid to the first and the second aspects because a large number of papers in the reference literature for preparing and organizing this paper are concentrated in the tropical Pacific Ocean, such as the ENSO process with its climatic effects and dynamics, and the monsoon-related air-sea interaction. The literature also involves various phenomena with their different time and spatial scales such as intraseasonal, annual, interannual, and interdecadal variabilities in the atmosphere/ocean interaction system, reflecting the contemporary themes in the four-year period at the beginning of an era from the post-TOGA to CLIVAR studies. Apparently, it is a difficult task to summarize the great progress in this area, as it is extracted from a large quantity of literature, although the authors tried very hard.

Response of the equatorial Indian Ocean to an unusual wind event during 1994

Unusual westward wind anomalies were observed in the Indian Ocean during 1994. The response of the ocean to these anomalies is studied using an ocean general circulation model forced with interannual winds. Model results show that the eastward equatorial jets in the Indian Ocean were weak during 1994. Consequently, the sea level in the eastern Indian Ocean was unusually low, the thermocline and the mixed layer were shallow, and the subsurface flow remained eastward throughout the year. Anomalies of sea surface temperature (SST) and sea level in the eastern Indian Ocean were enhanced by upwelling favorable winds along the coast of Indonesia. The model results agree with observations and TOPEX/POSEIDON data. We suggest that the dynamical response of the ocean to wind anomalies played a role in generating the SST anomalies during 1994 and enhanced the large‐scale air‐sea interaction, independent of the El Niño Southern Oscillation.

Extratropical large-scale air-sea interaction in a coupled and uncoupled ocean-atmosphere model

Extratropical large-scale air-sea interaction in a coupled and uncoupled ocean-atmosphere model

Extratropical large-scale air-sea interaction in a coupled and uncoupled ocean-atmosphere model

Spatial patterns of mid-latitude large-scale ocean-atmosphere interaction on monthly to seasonal time scales have been observed to exhibit a similar structure in both the North Pacific and North Atlantic basins. These patterns have been interpreted as a generic oceanic response to surface wind anomalies, whereby the anomalous winds give rise to corresponding anomalous regions of surface heat flux and consequent oceanic cooling. This mechanistic concept is investigated in this study using numerical models of a global atmosphere and a mid-latitude ocean basin (nominally the Atlantic). The models were run in both coupled and uncoupled mode. Model output was used to generate multi-year time series of monthly mean fields. Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses were then used to obtain the principal patterns of variability in heat flux, air temperature, wind speed, and sea surface temperature (SST), and to determine the relationships among these variables. SVD analysis indicates that the turbulent heat flux from the ocean to the atmosphere is primarily controlled by the surface scalar wind speed, and to a lesser extent by air temperature and SST. The principal patterns of air-sea interaction are closely analogous to those found in observational data. In the atmosphere, the pattern consists of a simultaneous strengthening (or weakening) of the mid-latitude westerlies and the easterly trades. In the ocean there is cooling (warming) under the anomalously strong (weak) westerlies and trade winds, with a weaker warming (cooling) in the region separating the westerly and easterly wind regimes. These patterns occur in both coupled and uncoupled models and the primary influence of the coupling is in localizing the interaction patterns. The oceanic patterns can be explained by the principal patterns of surface heat flux and the attendant warming or cooling of the ocean mixed layer.

A note on Bjerkne's hypothesis for North Atlantic variability

On decadal time-scales the historical surface temperature record over land in the Northern Hemisphere is dominated by polar amplified variations. These variations are coherent with SST anomalies concentrated in the Northwest Atlantic, but extending with lesser amplitude into the North Pacific as well. Bierknes suggested that multi-year SST anomalies in the subpolar North Atlantic were due to irregular changes in the intensity of the thermohaline circulation. In support of the Bjerknes hypothesis there is evidence that winter overturning in the Labrador Sea was suppressed for a brief period from 1967–1969 by a cap of relative fresh water at the surface. Cause and effect are unclear, but this event was associated with a marked cooling of the entire Northern Hemisphere. The difference in SST averaged over the Northern Hemisphere oceans and SST averaged over the Southern Hemisphere oceans from the equator to 40°S is coherent with Sahel summer rainfall on decadal time scales. Empirical evidence is supported by numerical experiments with the British Meteorological Office atmospheric climate model which simulate augmented monsoonal rainfall in the Sahel region of Africa in response to realistic warm SST anomalies in the Northwest Atlantic. A coupled ocean-atmosphere global model exhibits two equilibrium climate states. One has an active thermohaline circulation in the North Atlantic and the other does not. The two climate states provide an extreme example which illustrates the type of large scale air sea interaction Bjerknes visualized as a mechanism for North Atlantic climate variability on decadal time-scales.

The Effect of Eurasian Snow Cover on Regional and Global Climate Variations

The sensitivity of the global climate system to interannual variability of he Eurasian snow cover has been investigated with numerical models. It was found that heavier than normal Eurasian snow cover in spring leads to a “poor” monsoon over Southeast Asia thereby verifying an idea over 100 years old. The poor monsoon was characterized by reduced rainfall over India and Burma, reduced wind stress over the Indian Ocean, lower than normal temperatures on the Asian land mass and in the overlying atmospheric column, reduced tropical jet, increased soil moisture, and other features associated with poor monsoons. Lighter than normal snow cover led to a “good” monsoon with atmospheric anomalies like those described above but of opposite sign. Remote responses from the snow field perturbation include readjustment of the Northern Hemispheric mass field in midlatitude, an equatorially symmetric response of the tropical geopotential height and temperature field and weak, but significant, perturbations in the surface wind stress and heat flux in the tropical Pacific. The physics responsible for the regional response involves all elements of both the surface heat budget and heat budget of the full atmospheric column. In essence, the snow, soil and atmospheric moisture all act to keep the land and overlying atmospheric column colder than normal during a heavy snow simulation thus reducing the land–ocean temperature contrast needed to initiate the monsoon. The remote responses are driven by heating anomalies associated with both large scale air-sea interactions and precipitation events. The model winds from the heavy snow experiment were used to drive an ocean model. The SST field in that model developed a weak El Niño in the equatorial Pacific. A coupled ocean-atmosphere model simulation perturbed only by anomalous Eurasian snow cover was also run and it developed a much stranger El Niño in the Pacific. The coupled system clearly amplified the wind stress anomaly associated with the poor monsoon. These results show the important role of an evolving (not specified) sea surface temperature in numerical experiments and the real climate system. Our general results also demonstrate the importance of land processes in global climate dynamics and their possible role as one of the factors that could trigger ENSO events.

The Role of Sea Surface Temperature in Large-Scale Air-Sea Interaction

Data from the 1972–73 El Niño event are used to study relationships between sea surface temperature (SST), surface wind, surface heat fluxes and rainfall in the tropical Pacific cast of the dateline. It is found that the maximum in SST anomaly is not correlated spatially or temporally in any consistent manner with the sum of latent and sensible heat fluxes (QE + QS) or rainfall as estimated from highly reflective cloud. Varying anomalies in wind speed and sea-air temperature and humidity differences all influence the QE + QS anomaly in ways which may be unrelated to the SST anomaly. Factors influencing SST, namely zonal wind stress and net surface heating, are found to vary with longitude. The position of the rainfall maximum and the time variation of rainfall in the near-equatorial convergence zone are more closely related to moisture convergence than to SST. Implications to modeling efforts which equate SST anomalies with diabatic heating anomalies are discussed.