Extratropical Sea Surface Temperature Research Articles

Interference of the East Asian winter monsoon in the impact of ENSO on the East Asian summer monsoon in decaying phases

The variability of the East Asian winter monsoon (EAWM) can be divided into an ENSO-related part (EAWMEN) and an ENSO-unrelated part (EAWMres). The influence of EAWMres on the ENSO-East Asian summer monsoon (EASM) relationship in the decaying stages of ENSO is investigated in the present study. To achieve this, ENSO is divided into four groups based on the EAWMres: (1) weak EAWMres-El Nino (WEAWMres-EN); (2) strong EAWMres-El Nino (SEAWMres- EN); (3) weak EAWMres-La Nina (WEAWMres-LN); (4) strong EAWMres-La Nina (SEAWMres-LN). Composite results demonstrate that the EAWMres may enhance the atmospheric responses over East Asia to ENSO for WEAWMres-EN and SEAWMres-LN. The corresponding low-level anticyclonic (cyclonic) anomalies over the western North Pacific (WNP) associated with El Nino (La Nina) tend to be strong. Importantly, this feature may persist into the following summer, causing abundant rainfall in northern China for WEAWMres-EN cases and in southwestern China for SEAWMres-LN cases. In contrast, for the SEAWMres-EN and WEAWMres-LN groups, the EAWMres tends to weaken the atmospheric circulation anomalies associated with El Nino or La Nina. In these cases, the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes, which results in deficient summer rainfall in northern China for SEAWMres-EN and in southwestern China for WEAWMres-LN. Further study suggests that anomalous EAWMres may have an effect on the extra-tropical sea surface temperature anomaly, which persists into the ensuing summer and may interfere with the influences of ENSO.

An analysis of trends in the boreal winter mean tropospheric circulation during the second half of the 20th century

A relaxation technique applied to the ECMWF model is used to analyse 11, 21 and 31 year trends in the boreal winter mean 500 hPa North Atlantic Oscillation (NAO), Pacific North America pattern (PNA) and Southern Annular Mode (SAM) indices. For the PNA, the results indicate a strong influence from the tropics on all time scales, whereas for the NAO, the stratosphere is important on time scales of 11 and 21 but with an indication of feedback from extratropical sea surface temperature and sea‐ice (SSTSI) anomalies on the 11 year time scale. For the SAM, the tropics emerge as the most important influence. We find an influence from the stratosphere consistent with expectations based on ozone depletion, although no clear role for stratospheric forcing of the SAM is found in these experiments.

On the Linkages between the Tropospheric Isentropic Slope and Eddy Fluxes of Heat during Northern Hemisphere Winter

Abstract Previous studies have demonstrated the key role of baroclinicity and thus the isentropic slope in determining the climatological-mean distribution of the tropospheric eddy fluxes of heat. Here the authors examine the role of variability in the isentropic slope in driving variations in the tropospheric eddy fluxes of heat about their long-term mean during Northern Hemisphere winter. On month-to-month time scales, the lower-tropospheric isentropic slope and eddy fluxes of heat are not significantly correlated when all eddies are included in the analysis. But the isentropic slope and heat fluxes are closely linked when the data are filtered to isolate the fluxes due to synoptic (<10 days) and low-frequency (>10 days) time scale waves. Anomalously steep isentropic slopes are characterized by anomalously poleward heat fluxes by synoptic eddies but anomalously equatorward heat fluxes by low-frequency eddies. Lag regressions based on daily data reveal that 1) variations in the isentropic slope precede by several days variations in the heat fluxes by synoptic eddies and 2) variations in the heat fluxes due to both synoptic and low-frequency eddies precede by several days similarly signed variations in the momentum flux at the tropopause level. The results suggest that seemingly modest changes in the tropospheric isentropic slope drive significant changes in the synoptic eddy heat fluxes and thus in the generation of baroclinic wave activity in the lower troposphere. The linkages have implications for understanding the extratropical tropospheric eddy response to a range of processes, including anthropogenic climate change, stratospheric variability, and extratropical sea surface temperature anomalies.

Asian Origin of Interannual Variations of Summer Climate over the Extratropical North Atlantic Ocean

AbstractThe authors have identified an interannual relationship between Asian tropospheric temperature and the North Atlantic Ocean sea surface temperature (SST) during summer (May–September) and discussed the associated features of atmospheric circulation over the Atlantic–Eurasian region. When tropospheric temperature is high (low) over Asia, positive (negative) SST anomalies appear in the extratropical North Atlantic. This relationship is well supported by the changes in background atmospheric circulation and ocean–atmosphere–land thermodynamic processes. When heat transfer from the land surface to the atmosphere over Asia strengthens, local tropospheric temperature increases and positive temperature anomalies propagate westward from Asia to the North Atlantic, leading to an increase in summer tropospheric temperature over the Atlantic–Eurasian region. Accordingly, a deep anomalous ridge occurs over the extratropical North Atlantic Ocean, with low-level southerly anomalies over the western portion of the ocean. Sensitivity experiments with climate models show that the interannual variations of the North Atlantic–Eurasian atmospheric circulation may not be forced by the extratropical Atlantic SST. Instead, experiments with changing Asian land surface heating capture the above observed features of atmospheric circulation anomalies, westward propagation of tropospheric anomalies, and Atlantic SST anomalies. The consistency between the observational and model results indicates a possible impact of Asian land heating on the development of atmospheric circulation and SST anomalies over the Atlantic–Eurasian region.

Season-Dependent Forecast Skill of the Leading Forced Atmospheric Circulation Pattern over the North Pacific and North American Region*

Abstract Multimodel ensemble (MME) seasonal forecasts are analyzed to evaluate numerical model performance in predicting the leading forced atmospheric circulation pattern over the extratropical Northern Hemisphere (NH). Results show that the time evolution of the leading tropical Pacific sea surface temperature (SST)-coupled atmospheric pattern (MCA1), which is obtained by applying a maximum covariance analysis (MCA) between 500-hPa geopotential height (Z500) in the extratropical NH and SST in the tropical Pacific Ocean, can be predicted with a significant skill in March–May (MAM), June–August (JJA), and December–February (DJF) one month ahead. However, most models perform poorly in capturing the time variation of MCA1 in September–November (SON) with 1 August initial condition. Two possible reasons for the models’ low skill in SON are identified. First, the models have the most pronounced errors in the mean state of SST and precipitation along the central equatorial Pacific. Because of the link between the divergent circulation forced by tropical heating and the midlatitude atmospheric circulation, errors in the mean state of tropical SST and precipitation may lead to a degradation of midlatitude forecast skill. Second, examination of the potential predictability of the atmosphere, estimated by the ratio of the total variance to the variance of the model forecasts due to internal dynamics, shows that the atmospheric potential predictability over the North Pacific–North American (NPNA) region is the lowest in SON compared to the other three seasons. The low ratio in SON is due to a low variance associated with external forcing and a high variance related to atmospheric internal processes over this area.

Extratropical Forcing of Tropical Atlantic Variability during Boreal Summer and Fall

Abstract The connection between midlatitude Atlantic sea surface temperature (SST) anomalies and tropical SST variations during boreal summer and fall are investigated using a coupled general circulation model (GCM). This research follows on an observational study that finds that, using linear inverse modeling (LIM), predictions of boreal summer tropical Atlantic Meridional Mode (AMM) variations can be made with skill exceeding persistence with lead times of about one year. The LIM framework identified extratropical Atlantic SST anomalies as important precursors to the AMM variations. The authors have corroborated this finding using a general circulation model coupled to a slab ocean, which represents a completely different physical basis from the LIM. Initializing the GCM with the LIM-derived “optimal” SST anomaly in November results in a steady equatorward propagation of SST anomalies into the subtropics during the following boreal spring. Thereafter, the GCM suggests that two possible feedbacks propagate the SST anomalies farther equatorward and westward with minimal loss of amplitude: the dominant wind–evaporation–SST (WES) thermodynamic feedback and a secondary low-cloud–SST radiative feedback. This study shows that this result has strong seasonal dependence and consists of nonlinear interactions when considering warm and cold “optimal” conditions separately. One main finding is that oceanic dynamics are not essential to understanding extratropical–tropical interaction in the Atlantic basin. The authors also discuss the results of the study in context with previous studies investigating the extratropical forcing of tropical air–sea variability.

On the Persistence of Cold-Season SST Anomalies Associated with the Annular Modes

Abstract In this study, a simple stochastic climate model is used to examine the impact of the ocean mixed layer depth, surface turbulent energy fluxes, and Ekman currents on the persistence of cold-season extratropical sea surface temperature (SST) anomalies associated with variability in the annular modes of atmospheric circulation in both hemispheres. Observational analysis reveals that during the cold season, SST anomalies associated with the southern annular mode (SSTSAM) persist considerably longer than those associated with the northern annular mode (SSTNAM). Using the simple model, it is shown that the persistence of the cold-season SSTSAM is consistent with the simple stochastic climate paradigm in which the atmospheric forcing is approximated as white noise, and the persistence of SST anomalies can be largely determined by the thermal inertia of the ocean mixed layer. In the North Atlantic, however, the simple climate model overestimates the persistence of the cold-season SSTNAM. It is thought that this overestimate occurs because the NAM-related heat flux forcing cannot be described purely as white noise but must also include a feedback from the underlying SST anomalies.

Anomalous midsummer rainfall in Yangtze River-Huaihe River valleys and its association with the East Asia westerly jet

In this study, the interannual and interdecadal relationship between midsummer Yangtze River-Huaihe River valley (YHRV) rainfall and the position of the East Asia westerly jet (EAWJ) were investigated. The midsummer YHRV rainfall was found to significantly increase after the 1980s. Moreover, the location of the EAWJ was found abnormally south of the climatic mean during 1980–2008 (ID2) compared to 1951–1979 (ID1). During ID2, associated with the southward movement of the EAWJ, an anomalous upper-level convergence occurred over middle-high latitudes (35°-55°N) and divergence occurred over lower latitudes (∼30°N) of East Asia. Correspondingly, anomalous descending and ascending motion was observed in middle-high and lower latitudes along 90°–130°E, respectively, favoring more precipitation over YHRV. On an interannual time scale, the EAWJ and YHRV rainfall exhibited similar relationships during the two periods. When the EAWJ was centered abnormally southward, rainfall over YHRV tended to increase. However, EAWJrelated circulations were significantly different during the two periods. During ID1, the circulation of the southward-moving EAWJ exhibited alternating positive-negative-positive distributions from low to middle-high latitudes along the East Asian coast; the most significant anomaly appeared west of the Okhotsk Sea. However, during ID2 the EAWJ was more closely correlated with the tropical and subtropical circulations. Significant differences between ID1 and ID2 were also recorded sea surface temperatures (SSTs). During ID1, the EAWJ was influenced by the extratropical SST over the northern Pacific; however, the EAWJ was more significantly affected by the SST of the tropical western Pacific during ID2.

Rossby wave dynamics of the North Pacific extra-tropical response to El Niño: importance of the basic state in coupled GCMs

The extra-tropical response to El Niño in a “low” horizontal resolution coupled climate model, typical of the Intergovernmental Panel on Climate Change fourth assessment report simulations, is shown to have serious systematic errors. A high resolution configuration of the same model has a much improved response that is similar to observations. The errors in the low resolution model are traced to an incorrect representation of the atmospheric teleconnection mechanism that controls the extra-tropical sea surface temperatures (SSTs) during El Niño. This is due to an unrealistic atmospheric mean state, which changes the propagation characteristics of Rossby waves. These erroneous upper tropospheric circulation anomalies then induce erroneous surface circulation features over the North Pacific. The associated surface wind speed and direction errors create erroneous surface flux and upwelling anomalies which finally lead to the incorrect extra-tropical SST response to El Niño in the low resolution model. This highlights the sensitivity of the climate response to a single link in a chain of complex climatic processes. The correct representation of these processes in the high resolution model indicates the importance of horizontal resolution in resolving such processes.

Interannual variations in the total ozone, stratospheric dynamics, extratropical SST anomalies and predictions of abnormal winters in Eurasia

Analysis of the monthly mean interannual variations in the total ozone (TO), zonal stratospheric circulation, three-dimensional Eliassen–Palm (EP) fluxes and their correlations with the surface air temperature (SAT) and sea surface temperature (SST) anomalies in the North Pacific and North Atlantic was carried out. Interannual TO variations are strongly connected with those of the zonal wind in the lower stratosphere during the winter. A strong upward propagation of planetary waves from the troposphere to the stratosphere over northern Eurasia and a weak downward propagation in Labrador and South Greenland regions are indicated. There are large differences in the interaction of the zonal mean circulation and wave processes between early winter (November–December) and mid-to-late winter (January–March). Amplification of the penetration into the stratosphere of planetary waves in December is strongly associated with changes in the stratospheric dynamics in January, creating the ‘preconditions’ for the appearance of stratospheric warming. We show that the ‘stratospheric bridge’, with a strengthening of the downward propagation of the vertical EP flux over the North Atlantic, is being formed for years with a strong polar vortex. Simultaneous correlations of the stratospheric circulation variations with the Southern Oscillation Index show its small linkage with the stratospheric dynamics in the Arctic. However, there are significant correlations of the extratropical SST anomalies in the North Pacific and the North Atlantic with the interannual variations in the stratospheric dynamics. We demonstrate that the TO changes over the UK in January can be a good predictor of the anomalous cold/warm winters in distinct regions of Eurasia in February. The TO and other stratospheric predictors can be used for extended-range forecasting of extreme winters in Eurasia. Possible mechanisms for stratosphere–troposphere coupling and its link to the extratropical SST anomalies are discussed.

Interannual ENSO variability forced through coupled atmosphere‐ocean feedback loops

El Niño/Southern Oscillation (ENSO) events are known to force atmospheric teleconnections that impact extratropical sea surface temperatures and surface winds. In this paper we use focused model experiments to investigate whether this extratropical variability can feedback to, and significantly impact, the Tropics through ocean Rossby waves. We use an atmospheric general circulation model coupled to a reduced gravity Pacific Ocean model to isolate these potential feedback loops and quantify their impact on ENSO variability. We find that anomalous winds and heat fluxes located in regions of maximum mean subduction in the subtropical North Pacific trigger ocean Rossby waves that take approximately four years to reach the equator. Most notably, we demonstrate that this feedback loop causes a primarily 2‐year ENSO, when only the Tropics is coupled, to shift to a more realistic broad 2–5 year range by damping ∼2 year variability and amplifying ∼4 year variability.

Assessment of the dependence of the skill and predictability of seasonal forecasts on boundary conditions of the model

A comparison of estimates of the root-mean-square error (RMSE) and potential predictability index (PPI) is carried out between experiments with observed and “persistent” anomalies of sea surface temperature (SST). The results obtained point to a possible significant bias of seasonal forecasting results in some regions when boundary conditions are introduced by a “persistence” procedure, particularly for summer T850. Indirect evidence of the influence of extratropical SST anomalies points to their possible role in seasonal forecasts, which is more substantial in the summer season. Although the conclusions should rather be regarded as preliminary ones because of a limited size of the sample, it is nonetheless certain that the influence of boundary conditions governing the signal becomes more significant in summer because of a decrease in the instability of the internal atmospheric dynamics.

The 1998–2000 La Niña in the context of historically strong La Niña events

Contrary to the impacts ascribed to strong cold El Niño–Southern Oscillation (ENSO) events (La Niña) during the last half of the twentieth century, the 2‐year‐long La Niña during 1998–2000 was accompanied by abnormally warm winter conditions over most of North America. In this study, climate anomalies associated with the 1998–2000 La Niña are compared with the composite of the previous seven strong La Niña events since 1950. Despite the presence of a moderately strong La Niña, analyses of atmospheric circulation and thermal features show that the Pacific–North American sector climate was associated with a zonally elongated ridge in the extratropical North Pacific and warmer‐than‐normal sea surface temperatures (SSTs) in the western tropical Pacific that extended into the North Pacific midlatitudes during 1998–2000 event. Relative to the composite of the previous seven strong La Niñas, anomalous Walker circulation was considerably stronger and shifted westward with the ascending branch of the cell located near 110°E during the 1998–2000 event. This was associated with stronger convection and hence positive precipitation anomalies in the western Pacific. The relationship between SSTs and 500‐hPa geopotential heights over the North Pacific shows that the differences, in thermal and circulation features, between the 1998–2000 La Niña and the seven strong La Niñas bear a close resemblance to a coupled mode of extratropical ocean‐atmosphere variability as revealed by singular value decomposition analysis. This non‐ENSO coupled mode of variability, which relates the interannual variability in SSTs with 500‐hPa heights, was prominent during the winter of 1998–2000. The processes related to this mode are different from those involved in the establishment of extratropical atmospheric circulation and SST anomalies associated with typical La Niña events.

Tropical Atlantic climate response to low‐latitude and extratropical sea‐surface temperature: A Little Ice Age perspective

Proxy reconstructions and model simulations suggest that steeper interhemispheric sea surface temperature (SST) gradients lead to southerly Intertropical Convergence Zone (ITCZ) migrations during periods of North Atlantic cooling, the most recent of which was the Little Ice Age (LIA; ∼100–450 yBP). Evidence suggesting low‐latitude Atlantic cooling during the LIA was relatively small (<1°C) raises the possibility that the ITCZ may have responded to a hemispheric SST gradient originating in the extratropics. We use an atmospheric general circulation model (AGCM) to investigate the relative influence of low‐latitude and extratropical SSTs on the meridional position of the ITCZ. Our results suggest that the ITCZ responds primarily to local, low‐latitude SST anomalies and that small cool anomalies (<0.5°C) can reproduce the LIA precipitation pattern suggested by paleoclimate proxies. Conversely, even large extratropical cooling does not significantly impact low‐latitude hydrology in the absence of ocean‐atmosphere interaction.

The influence of tropical sea surface temperatures and precipitation on north Pacific atmospheric blocking

Blocking is a major component of the extratropical climate and any changes in it would be a very important aspect of climate change there. Previous studies have shown that mid-latitude variability such as blocking is sensitive to tropical sea surface temperature (SST) anomalies and to variations in tropical precipitation. Climate models exhibit a wide range of skill in representing blocking, with all models having deficiencies in certain respects. In addition, coupled climate models often exhibit significant biases in both tropical precipitation and tropical and extratropical SSTs. This suggests that tropical systematic biases in coupled climate models may influence the representation of blocking and its sensitivity to climate change. We examine the relationship between winter north Pacific blocking and tropical precipitation and tropical SSTs through the use of idealised SST anomaly experiments. We find that interannual variations in convection over the Maritime Continent and eastern equatorial Pacific regions both influence the central and eastern Pacific winter blocking frequency. In addition, systematic underestimation of tropical rainfall over the Maritime Continent region in climate models can lead to underestimation of time-mean winter Pacific blocking. Finally, the sign, magnitude and variability of tropical SST biases in a coupled model, and their associated effects on tropical precipitation, could influence its representation of northern hemisphere blocking, and thus affect its ability to represent this mode of remotely-forced mid-latitude variability. These results have important implications for model development.

Relationships between interannual variations in stratospheric warmings, tropospheric circulation, and sea surface temperature in the Northern Hemisphere

The leading modes of interannual and long-term variations in the stratospheric and tropospheric circulation and total ozone (TOMS data) and their relations to Northern Hemisphere sea surface temperature (SST) anomalies are investigated using the monthly mean NCEP/NCAR reanalysis data for the winter months of 1958–2003. Strong correlations are indicated between the interannual total ozone variations over Labrador and the North Atlantic and changes in the stratospheric polar vortex. The onset of major stratospheric warmings is connected not only with the strengthening of westerlies at the 500-hPa level in the midlatitude Atlantic, but also with the weakening of tropospheric winds over the north of eastern Siberia and strengthening over the Far East. In years with major stratospheric warmings, abnormally cold winters are observed in Eurasia, especially in eastern Siberia and northeastern China. The calculated simultaneous (with no time lags) correlations of the stratospheric circulation changes with El Nino/La Nina events give evidence of low correlations between the tropical Pacific SST anomalies and the stratospheric dynamics in the Arctic. However, there are high correlations of the extratropical Pacific and Atlantic SST anomalies with interannual tropospheric and stratospheric circulation variations, the stratospheric dynamics being more strongly connected with Pacific SST than with Atlantic SST anomalies. The interannual changes in tropospheric circulation are coupled to SST anomalies in both the Pacific and the Atlantic. Mechanisms of long-term changes in the interactive ocean-atmosphere-ozone layer system are discussed.

Observations of Large-Scale Ocean–Atmosphere Interaction in the Southern Hemisphere

Abstract The authors provide a detailed examination of observed ocean–atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM), and the El Niño–Southern Oscillation (ENSO). Results are examined separately for the warm (November–April) and cold (May–October) seasons and for monthly and weekly time scales. It is shown that the signatures of the SAM and ENSO in the SH SST field vary as a function of season, both in terms of their amplitudes and structures. The role of surface turbulent and Ekman heat fluxes in driving seasonal variations in the SAM- and ENSO-related SST anomalies is investigated. Analyses of weekly data reveal that variability in the SAM tends to precede anomalies in the SST field by ∼1 week, and that the e-folding time scale of the SAM-related SST field anomalies is at least 4 months. The persistence of the SAM-related SST anomalies is consistent with the passive thermal response of the Southern Ocean to variations in the SAM, and seasonal variations in the persistence of the SAM-related SST anomalies are consistent with the seasonal cycle in the depth of the ocean mixed layer.

Impacts of SST anomalies on the North Atlantic atmospheric circulation: a case study for the northern winter 1995/1996

The present paper selects the northern winter of December 1995–February 1996 for a case study on the impact of sea surface temperature (SST) anomalies on the atmospheric circulation over the North Atlantic and Western Europe. In the Atlantic, the selected winter was characterized by positive SST anomalies over the northern subtropics and east of Newfoundland, and negative anomalies along the US coast. A weak La Nina event developed in the Pacific. The North Atlantic Oscillation (NAO) index was low, precipitation over the Iberian Peninsula and northern Africa was anomalously high, and precipitation over northern Europe was anomalously low. The method of study consists of assessing the sensitivity of ensemble simulations by the UCLA atmospheric general circulation model (UCLA AGCM) to SST anomalies from the observation, which are prescribed either in the World Oceans, the Atlantic Ocean only, or the subtropical North Atlantic only. The results obtained are compared with a control run that uses global, time-varying climatological SST. The ensemble simulations with global and Atlantic-only SST anomalies both produce results that resemble the observations over the North Atlantic and Western Europe. It is suggested that the anomalous behavior of the atmosphere in the selected winter over those regions, therefore, was primarily determined by conditions within the Atlantic basin. The simulated fields in the tropical North Atlantic show anomalous upward motion and lower (upper) level convergence (divergence) in the atmosphere overlying the positive SST anomalies. Consistently, the subtropical jet intensifies and its core moves equatorward, and precipitation increases over northern Africa and southern Europe. The results also suggest that the SST anomalies in the tropical North Atlantic only do not suffice to produce the atmospheric anomalies observed in the basin during the selected winter. The extratropical SST anomalies would provide a key contribution through increased transient eddy activity, which causes an extension of the subtropical jet eastward from the coast of North America.

Extratropical control of recent tropical Pacific decadal climate variability: a relay teleconnection

Observations indicate that recent tropical Pacific decadal climate variability tends to be associated with the extratropical North Pacific through a relay teleconnection of a fast coupled ocean-atmosphere bridge and a slow oceanic tunnel. A coupled ocean-atmosphere model, forced by the observed decadal wind in the extratropical North Pacific, explicitly demonstrates that extratropical decadal sea surface temperature (SST) anomalies may propagate to the tropics through a coupled wind-evaporative-SST (WES) feedback. The WES feedback cannot only lead to a nearly synchronous change of tropical SST, but also force a delayed adjustment of the meridional overturning circulation in the upper ocean to further sustain the tropical SST change. The study further suggests that the extratropical–tropical teleconnection provides a positive feedback to sustain the decadal changes in both the tropical and extratropical North Pacific.

Effects of Ekman Transport on the NAO Response to a Tropical Atlantic SST Anomaly

Abstract A recent study showed that a tropical Atlantic sea surface temperature (SST) anomaly induces a significant coupled response in late winter [February–April (FMA)] in a coupled model, in which an atmospheric general circulation model is coupled to a slab mixed layer ocean model (AGCM_ML). The coupled response comprises a dipole in the geopotential height, like the North Atlantic Oscillation (NAO), and a North Atlantic tripole in the SST. The simulated NAO response developed 1 or 2 months later in the model than in observations. To determine the possible effects of Ekman heat transport on the development of the coupled response to the tropical forcing, an extended coupled model (AGCM_EML), including Ekman transport in the slab mixed layer ocean, is now used. Large ensembles of AGCM_EML experiments are performed, parallel to the previous AGCM_ML experiments, with the model forced by the same tropical Atlantic SST anomaly over the boreal winter months (September–April). The inclusion of Ekman heat transport is found to result in an earlier development of the coupled NAO–SST tripole response in the AGCM_EML, compared to that in the AGCM_ML. The mutual reinforcement between the anomalous Ekman transport and the surface heat flux causes the tropical forcing to induce an extratropical SST response in November–January (NDJ) in the AGCM_EML that is twice as strong as that in the AGCM_ML. The feedback of this stronger extratropical SST response on the atmosphere in turn drives the development of the NAO response in NDJ. In FMA, the sign of the anomalous surface heat flux is reversed in the Gulf Stream region such that it opposes the anomalous Ekman transport. The resulting equilibrium NAO response in the AGCM_EML is similar to that in the AGCM_ML, but it is reached 1–2 months sooner in the AGCM_EML. Hence, the presence of Ekman transport causes a seasonal shift in the evolution of the coupled response. The faster development of the NAO response in the AGCM_EML suggests that tropical Atlantic SST anomalies should be able to influence the NAO, in nature, on the seasonal time scale, and that efficient interactions with the extratropical ocean play a significant role in determining the coupled response.