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Abstract
The role of tropical Pacific ocean dynamics in regulating the ocean response to thermodynamic forcing is investigated using an ocean general circulation model (GCM) coupled to a model of the atmospheric mixed layer. It is found that the basin mean sea surface temperature (SST) change is less in the presence of varying ocean heat transport than would be the case if the forcing was everywhere balanced by an equivalent change in the surface heat flux. This occurs because the thermal forcing in the eastern equatorial Pacific is partially compensated by an increase in heat flux divergence associated with the equatorial upwelling. This constitutes a validation of a previously identified “ocean dynamical thermostat.”
A simple two-box model of subtropical–equatorial interaction shows that the SST regulation mechanism crucially depends on spatial variation in the sensitivity of the surface fluxes to SST perturbations. In the GCM, this sensitivity increases with latitude, largely a result of the wind speed dependence of the latent heat flux, so that a uniform forcing can be balanced by a smaller SST change in the subtropics than in equatorial latitudes. The tropical ocean circulation moves heat to where the ocean more readily loses it to the atmosphere. Water that subducts in subtropical latitudes and returns to the equatorial thermocline therefore has a smaller temperature perturbation than the surface equatorial waters. The thermocline temperature adjusts on timescales of decades to the imposed forcing, but the adjustment is insufficient to cancel the thermostat mechanism.
The results imply that an increase in the downward heat flux at the ocean surface, as happens with increasing concentrations of greenhouse gases, should be accompanied by a stronger equatorial SST gradient. This contradicts the results of coupled atmosphere–ocean GCMs. Various explanations are offered. None are conclusive, but the possibility that the discrepancy lies in the low resolution of the ocean GCMs typically used in the study of climate change is discussed.
Abstract
The role of tropical Pacific ocean dynamics in regulating the ocean response to thermodynamic forcing is investigated using an ocean general circulation model (GCM) coupled to a model of the atmospheric mixed layer. It is found that the basin mean sea surface temperature (SST) change is less in the presence of varying ocean heat transport than would be the case if the forcing was everywhere balanced by an equivalent change in the surface heat flux. This occurs because the thermal forcing in the eastern equatorial Pacific is partially compensated by an increase in heat flux divergence associated with the equatorial upwelling. This constitutes a validation of a previously identified “ocean dynamical thermostat.”
A simple two-box model of subtropical–equatorial interaction shows that the SST regulation mechanism crucially depends on spatial variation in the sensitivity of the surface fluxes to SST perturbations. In the GCM, this sensitivity increases with latitude, largely a result of the wind speed dependence of the latent heat flux, so that a uniform forcing can be balanced by a smaller SST change in the subtropics than in equatorial latitudes. The tropical ocean circulation moves heat to where the ocean more readily loses it to the atmosphere. Water that subducts in subtropical latitudes and returns to the equatorial thermocline therefore has a smaller temperature perturbation than the surface equatorial waters. The thermocline temperature adjusts on timescales of decades to the imposed forcing, but the adjustment is insufficient to cancel the thermostat mechanism.
The results imply that an increase in the downward heat flux at the ocean surface, as happens with increasing concentrations of greenhouse gases, should be accompanied by a stronger equatorial SST gradient. This contradicts the results of coupled atmosphere–ocean GCMs. Various explanations are offered. None are conclusive, but the possibility that the discrepancy lies in the low resolution of the ocean GCMs typically used in the study of climate change is discussed.
Abstract
A reduced gravity, primitive equation, ocean GCM with an isopycnal vertical coordinate is developed. A “buffer” layer is introduced to allow the mixed layer to detrain mass at arbitrary densities without the coordinate drift or the heat loss suffered by other isopycnal models. The diapycnal velocity is derived from the thermodynamic equation. Negative layers are removed by a heat- and mass-conserving convective adjustment scheme. The model formulation on a β plane employs an A grid and allows irregular coastlines and local grid stretching.
Simulations with climatological winds and surface heat fluxes based on observed sea surface temperatures (SSTs) are presented for the Atlantic, the Pacific, and the Indian Oceans. The surface mixed layer is modeled as a constant depth layer, and salinity effects are neglected in this version. The surface heat flux parameterization used here leads to errors in model SSTs, which are reasonable in the Tropics but are higher in the western boundary current regions. The seasonal dependence of the currents compare reasonably well with the available observations and other model results, though there are differences in the amplitudes of the currents. The model thermocline reproduces the observed slopes, troughs, and ridges in the Tropics. Neglecting salinity effects and lack of a variable depth mixed layer affect the model simulation of the thermocline at higher latitudes. The cold tongue in the eastern Pacific is also affected by the assumption of a constant-depth mixed layer, but the warm pool in the west and the zonal slope of the thermocline correspond well with the observations. The Gulf Stream and the Kuroshio have reasonable current speeds but separate slightly earlier than observed, in contrast to most models that separate late. Seasonal reversal of the Somali Current in the Indian Ocean and the South Equatorial Current in the Pacific Ocean are reproduced, and the Equatorial Undercurrent is stronger than in most models with comparable grid resolution. Effects underway to improve model performance are listed along the way.
Abstract
A reduced gravity, primitive equation, ocean GCM with an isopycnal vertical coordinate is developed. A “buffer” layer is introduced to allow the mixed layer to detrain mass at arbitrary densities without the coordinate drift or the heat loss suffered by other isopycnal models. The diapycnal velocity is derived from the thermodynamic equation. Negative layers are removed by a heat- and mass-conserving convective adjustment scheme. The model formulation on a β plane employs an A grid and allows irregular coastlines and local grid stretching.
Simulations with climatological winds and surface heat fluxes based on observed sea surface temperatures (SSTs) are presented for the Atlantic, the Pacific, and the Indian Oceans. The surface mixed layer is modeled as a constant depth layer, and salinity effects are neglected in this version. The surface heat flux parameterization used here leads to errors in model SSTs, which are reasonable in the Tropics but are higher in the western boundary current regions. The seasonal dependence of the currents compare reasonably well with the available observations and other model results, though there are differences in the amplitudes of the currents. The model thermocline reproduces the observed slopes, troughs, and ridges in the Tropics. Neglecting salinity effects and lack of a variable depth mixed layer affect the model simulation of the thermocline at higher latitudes. The cold tongue in the eastern Pacific is also affected by the assumption of a constant-depth mixed layer, but the warm pool in the west and the zonal slope of the thermocline correspond well with the observations. The Gulf Stream and the Kuroshio have reasonable current speeds but separate slightly earlier than observed, in contrast to most models that separate late. Seasonal reversal of the Somali Current in the Indian Ocean and the South Equatorial Current in the Pacific Ocean are reproduced, and the Equatorial Undercurrent is stronger than in most models with comparable grid resolution. Effects underway to improve model performance are listed along the way.
Abstract
A reduced gravity, primitive equation, ocean general circulation model (GCM) is coupled to an advective atmospheric mixed-layer (AML) model to demonstrate the importance of a nonlocal atmospheric mixed-layer parameterization for a proper simulation of surface heat fluxes and sea surface temperatures (SST). Seasonal variability of the model SSTs and the circulation are generally in good agreement with the observations in each of the tropical oceans. These results are compared to other simulations that use a local equilibrium mixed-layer model. Inclusion of the advective AML model is demonstrated to lead to a significant improvement in the SST simulation in all three oceans. Advection and diffusion of the air humidity play significant roles in determining SSTs even in the tropical Pacific where the local equilibrium assumption was previously deemed quite accurate. The main, and serious, model flaw is an inadequate representation of the seasonal cycle in the upwelling regions of the eastern Atlantic and Pacific Oceans. The results indicate that the feedback between mixed-layer depths and SSTs can amplify SST errors, implying that increased realism in the modeling of the ocean mixed layer increases the demand for realism in the representation of the surface heat fluxes. The performance of the GCM with a local-equilibrium mixed-layer model in the Atlantic is as poor as previous simple ocean model simulations of the Atlantic. The conclusion of earlier studies that the simple ocean model was at fault may, in fact, not he correct. Instead the local-equilibrium heat flux parameterization appears to have been the major source of error. Accurate SST predictions may, hence, be feasible by coupling the AML model to computationally efficient simple ocean models.
Abstract
A reduced gravity, primitive equation, ocean general circulation model (GCM) is coupled to an advective atmospheric mixed-layer (AML) model to demonstrate the importance of a nonlocal atmospheric mixed-layer parameterization for a proper simulation of surface heat fluxes and sea surface temperatures (SST). Seasonal variability of the model SSTs and the circulation are generally in good agreement with the observations in each of the tropical oceans. These results are compared to other simulations that use a local equilibrium mixed-layer model. Inclusion of the advective AML model is demonstrated to lead to a significant improvement in the SST simulation in all three oceans. Advection and diffusion of the air humidity play significant roles in determining SSTs even in the tropical Pacific where the local equilibrium assumption was previously deemed quite accurate. The main, and serious, model flaw is an inadequate representation of the seasonal cycle in the upwelling regions of the eastern Atlantic and Pacific Oceans. The results indicate that the feedback between mixed-layer depths and SSTs can amplify SST errors, implying that increased realism in the modeling of the ocean mixed layer increases the demand for realism in the representation of the surface heat fluxes. The performance of the GCM with a local-equilibrium mixed-layer model in the Atlantic is as poor as previous simple ocean model simulations of the Atlantic. The conclusion of earlier studies that the simple ocean model was at fault may, in fact, not he correct. Instead the local-equilibrium heat flux parameterization appears to have been the major source of error. Accurate SST predictions may, hence, be feasible by coupling the AML model to computationally efficient simple ocean models.
Abstract
Interannual variability of the tropical Indian Ocean is studied with a reduced gravity, primitive equation, ocean general circulation model (OGCM). The OGCM is coupled to an atmospheric mixed layer model for surface heat flux computation. The seasonal simulation of sea surface temperatures (SST), current, and thermocline structures are in good agreement with observations and other models. The seasonal cycle of SST along the equator exhibits an eastward propagation with larger variability in the west. The interannual simulations are carried out over 1980–95 with interannual wind stresses and wind speeds but climatological data for solar radiation and cloudiness. The SST anomalies are smaller than 1°C over most of the basin and the leading EOF shows an ENSO-related warming. However, the correlation between the Southern Oscillation index and the time series of the leading EOF is only −0.51 and SST anomalies of similar magnitudes as an El Niño year appear in other years too. ENSO-related equatorial winds determine the SST anomalies along the coast of Sumatra and this anomaly in the eastern southern tropical Indian Ocean (STIO) is typically opposite in sign to the anomaly in the western STIO. The western STIO has some of the largest SSTA because of a shallow thermocline and the entrainment effects associated with wind stress curl anomalies in the region. The quasi-biennial oscillation in the thermocline and the SST gradient in the STIO is correlated with the Somali jet, which in turn is correlated with the Indian summer monsoon. An experiment with climatological wind stresses but interannual wind speeds demonstrates that the wind-driven variations in SST are larger than previously estimated with relaxation type heat fluxes. A parallel experiment with climatological wind speeds but interannual wind stresses shows that there are regions where heat fluxes contribute significantly to SST variability. Another simulation with interannual data for radiation and cloudiness shows that model simulation is affected significantly in some regions by the use of climatological data for solar radiation and cloudiness. A model experiment with an open eastern boundary provides a simplistic illustration of the effects of the Indonesian Throughflow (ITF). The main influence of the ITF is to warm the Indian Ocean and reduce the effect of upwelling on SST.
Abstract
Interannual variability of the tropical Indian Ocean is studied with a reduced gravity, primitive equation, ocean general circulation model (OGCM). The OGCM is coupled to an atmospheric mixed layer model for surface heat flux computation. The seasonal simulation of sea surface temperatures (SST), current, and thermocline structures are in good agreement with observations and other models. The seasonal cycle of SST along the equator exhibits an eastward propagation with larger variability in the west. The interannual simulations are carried out over 1980–95 with interannual wind stresses and wind speeds but climatological data for solar radiation and cloudiness. The SST anomalies are smaller than 1°C over most of the basin and the leading EOF shows an ENSO-related warming. However, the correlation between the Southern Oscillation index and the time series of the leading EOF is only −0.51 and SST anomalies of similar magnitudes as an El Niño year appear in other years too. ENSO-related equatorial winds determine the SST anomalies along the coast of Sumatra and this anomaly in the eastern southern tropical Indian Ocean (STIO) is typically opposite in sign to the anomaly in the western STIO. The western STIO has some of the largest SSTA because of a shallow thermocline and the entrainment effects associated with wind stress curl anomalies in the region. The quasi-biennial oscillation in the thermocline and the SST gradient in the STIO is correlated with the Somali jet, which in turn is correlated with the Indian summer monsoon. An experiment with climatological wind stresses but interannual wind speeds demonstrates that the wind-driven variations in SST are larger than previously estimated with relaxation type heat fluxes. A parallel experiment with climatological wind speeds but interannual wind stresses shows that there are regions where heat fluxes contribute significantly to SST variability. Another simulation with interannual data for radiation and cloudiness shows that model simulation is affected significantly in some regions by the use of climatological data for solar radiation and cloudiness. A model experiment with an open eastern boundary provides a simplistic illustration of the effects of the Indonesian Throughflow (ITF). The main influence of the ITF is to warm the Indian Ocean and reduce the effect of upwelling on SST.
Abstract
A reduced-order Kalman filter is used to assimilate observed fields of the surface wind stress, sea surface temperature, and sea level into the coupled ocean–atmosphere model of Zebiak and Cane. The method projects the Kalman filter equations onto a subspace defined by the eigenvalue decomposition of the error forecast matrix, allowing its application to high-dimensional systems.
The Zebiak and Cane model couples a linear, reduced-gravity ocean model with a single, vertical-mode atmospheric model. The compatibility between the simplified physics of the model and each observed variable is studied separately and together. The results show the ability of the empirical orthogonal functions (EOFs) of the model to represent the simultaneous value of the wind stress, SST, and sea level, when the fields are limited to the latitude band 10°S–10°N, and when the number of EOFs is greater than the number of statistically significant modes.
In this first application of the Kalman filter to a coupled ocean–atmosphere prediction model, the sea level fields are assimilated in terms of the Kelvin and Rossby modes of the thermocline depth anomaly. An estimation of the error of these modes is derived from the projection of an estimation of the sea level error over such modes.
The ability of the method to reconstruct the state of the equatorial Pacific and to predict its time evolution is shown. The method is quite robust for predictions up to 6 months, and able to predict the onset of the 1997 warm event 15 months before its occurrence.
Abstract
A reduced-order Kalman filter is used to assimilate observed fields of the surface wind stress, sea surface temperature, and sea level into the coupled ocean–atmosphere model of Zebiak and Cane. The method projects the Kalman filter equations onto a subspace defined by the eigenvalue decomposition of the error forecast matrix, allowing its application to high-dimensional systems.
The Zebiak and Cane model couples a linear, reduced-gravity ocean model with a single, vertical-mode atmospheric model. The compatibility between the simplified physics of the model and each observed variable is studied separately and together. The results show the ability of the empirical orthogonal functions (EOFs) of the model to represent the simultaneous value of the wind stress, SST, and sea level, when the fields are limited to the latitude band 10°S–10°N, and when the number of EOFs is greater than the number of statistically significant modes.
In this first application of the Kalman filter to a coupled ocean–atmosphere prediction model, the sea level fields are assimilated in terms of the Kelvin and Rossby modes of the thermocline depth anomaly. An estimation of the error of these modes is derived from the projection of an estimation of the sea level error over such modes.
The ability of the method to reconstruct the state of the equatorial Pacific and to predict its time evolution is shown. The method is quite robust for predictions up to 6 months, and able to predict the onset of the 1997 warm event 15 months before its occurrence.
Abstract
At all longitudes oceanic evaporation rates are lower on the equator than at latitudes to the north and south. Over the oceanic cold tongues this is related to the presence of cold water and divergence of heat by the ocean circulation. Herein is investigated why there is also a minimum over the Indo-Pacific warm pool. Model results confirm the recent suggestion of Sobel that deep convective clouds over the warm pool reduce the amount of solar radiation coming into the ocean that the evaporation has to balance. The results also confirm that this is only a partial explanation. Less evaporation over the warm pool than in the trade wind regions is also caused by an interaction between the ocean heat transport and the distribution of surface wind speeds. Low wind speeds over the warm pool reduce the latent heat flux and increase the SST, and stronger wind speeds in the off-equatorial regions of the Tropics increase the latent heat flux and cool the SST. Consequently, the wind speed distribution increases the meridional temperature gradient and increases the poleward ocean heat transport. Low latent heat fluxes over the warm pool can be sustained because the incoming solar radiation is partially offset by ocean heat flux divergence. Large values under the trade winds are sustained by ocean heat flux convergence. Climate models are used to show that, in equilibrium, wind speeds can only influence the latent heat flux distribution through their coupling to the ocean heat transport. In the presence of ocean heat transport, advection of moisture in the atmospheric boundary layer from the subtropics to the equator also increases the evaporation under the trade winds, but this has a much smaller effect than the wind speed or the cloud–radiation interactions.
Abstract
At all longitudes oceanic evaporation rates are lower on the equator than at latitudes to the north and south. Over the oceanic cold tongues this is related to the presence of cold water and divergence of heat by the ocean circulation. Herein is investigated why there is also a minimum over the Indo-Pacific warm pool. Model results confirm the recent suggestion of Sobel that deep convective clouds over the warm pool reduce the amount of solar radiation coming into the ocean that the evaporation has to balance. The results also confirm that this is only a partial explanation. Less evaporation over the warm pool than in the trade wind regions is also caused by an interaction between the ocean heat transport and the distribution of surface wind speeds. Low wind speeds over the warm pool reduce the latent heat flux and increase the SST, and stronger wind speeds in the off-equatorial regions of the Tropics increase the latent heat flux and cool the SST. Consequently, the wind speed distribution increases the meridional temperature gradient and increases the poleward ocean heat transport. Low latent heat fluxes over the warm pool can be sustained because the incoming solar radiation is partially offset by ocean heat flux divergence. Large values under the trade winds are sustained by ocean heat flux convergence. Climate models are used to show that, in equilibrium, wind speeds can only influence the latent heat flux distribution through their coupling to the ocean heat transport. In the presence of ocean heat transport, advection of moisture in the atmospheric boundary layer from the subtropics to the equator also increases the evaporation under the trade winds, but this has a much smaller effect than the wind speed or the cloud–radiation interactions.
Abstract
The current coarse-resolution version of the Community Climate System Model is used to assess the impact of phytoplankton on El Niño–Southern Oscillation (ENSO). The experimental setup allows for the separation of the effects of climatological annual cycle of chlorophyll distribution from its interannually varying part. The main finding is that the chlorophyll production by phytoplankton is important beyond modifying the mean and seasonal cycle of shortwave absorption; interannual modifications to the absorption have an impact as well, and they dampen ENSO variability by 9%. The magnitude of damping is the same in the experiment with smaller-than-observed, and in the experiment with larger-than-observed, chlorophyll distribution. This result suggests that to accurately represent ENSO in GCMs, it is not sufficient to use a prescribed chlorophyll climatology. Instead, some form of an ecosystem model will be necessary to capture the effects of phytoplankton coupling and feedback.
Abstract
The current coarse-resolution version of the Community Climate System Model is used to assess the impact of phytoplankton on El Niño–Southern Oscillation (ENSO). The experimental setup allows for the separation of the effects of climatological annual cycle of chlorophyll distribution from its interannually varying part. The main finding is that the chlorophyll production by phytoplankton is important beyond modifying the mean and seasonal cycle of shortwave absorption; interannual modifications to the absorption have an impact as well, and they dampen ENSO variability by 9%. The magnitude of damping is the same in the experiment with smaller-than-observed, and in the experiment with larger-than-observed, chlorophyll distribution. This result suggests that to accurately represent ENSO in GCMs, it is not sufficient to use a prescribed chlorophyll climatology. Instead, some form of an ecosystem model will be necessary to capture the effects of phytoplankton coupling and feedback.
Abstract
This study explores the influence of phytoplankton on the tropical Pacific heat budget. A hybrid coupled model for the tropical Pacific that is based on a primitive equation reduced-gravity multilayer ocean model, a dynamic ocean mixed layer, an atmospheric mixed layer, and a statistical atmosphere is used. The statistical atmosphere relates deviations of the sea surface temperature from its mean to wind stress anomalies and allows for the rectification of the annual cycle and the El Niño–Southern Oscillation (ENSO) phenomenon through the positive Bjerknes feedback. Furthermore, a nine-component ecosystem model is coupled to the physical variables of the ocean. The simulated chlorophyll concentrations can feed back onto the ocean heat budget by their optical properties, which modify solar light absorption in the surface layers. It is shown that both the surface layer concentration as well as the vertical profile of chlorophyll have a significant effect on the simulated mean state, the tropical annual cycle, and ENSO. This study supports a previously suggested hypothesis (Timmermann and Jin) that predicts an influence of phytoplankton concentration of the tropical Pacific climate mean state and its variability. The bioclimate feedback diagnosed here works as follows: Maxima in the subsurface chlorophyll concentrations lead to an enhanced subsurface warming due to the absorption of photosynthetically available shortwave radiation. This warming triggers a deepening of the mixed layer in the eastern equatorial Pacific and eventually a reduction of the surface ocean currents (Murtugudde et al.). The weakened south-equatorial current generates an eastern Pacific surface warming, which is strongly enhanced by the Bjerknes feedback. Because of the deepening of the mixed layer, the strength of the simulated annual cycle is also diminished. This in turn leads to an increase in ENSO variability.
Abstract
This study explores the influence of phytoplankton on the tropical Pacific heat budget. A hybrid coupled model for the tropical Pacific that is based on a primitive equation reduced-gravity multilayer ocean model, a dynamic ocean mixed layer, an atmospheric mixed layer, and a statistical atmosphere is used. The statistical atmosphere relates deviations of the sea surface temperature from its mean to wind stress anomalies and allows for the rectification of the annual cycle and the El Niño–Southern Oscillation (ENSO) phenomenon through the positive Bjerknes feedback. Furthermore, a nine-component ecosystem model is coupled to the physical variables of the ocean. The simulated chlorophyll concentrations can feed back onto the ocean heat budget by their optical properties, which modify solar light absorption in the surface layers. It is shown that both the surface layer concentration as well as the vertical profile of chlorophyll have a significant effect on the simulated mean state, the tropical annual cycle, and ENSO. This study supports a previously suggested hypothesis (Timmermann and Jin) that predicts an influence of phytoplankton concentration of the tropical Pacific climate mean state and its variability. The bioclimate feedback diagnosed here works as follows: Maxima in the subsurface chlorophyll concentrations lead to an enhanced subsurface warming due to the absorption of photosynthetically available shortwave radiation. This warming triggers a deepening of the mixed layer in the eastern equatorial Pacific and eventually a reduction of the surface ocean currents (Murtugudde et al.). The weakened south-equatorial current generates an eastern Pacific surface warming, which is strongly enhanced by the Bjerknes feedback. Because of the deepening of the mixed layer, the strength of the simulated annual cycle is also diminished. This in turn leads to an increase in ENSO variability.
Abstract
The causes of the seasonal cycles of the subtropical anticyclones, and the associated zonal asymmetries of sea surface temperature (SST) across the subtropical oceans, are examined. In all basins the cool waters in the east and warm waters in the west are sustained by a mix of atmosphere and ocean processes. When the anticyclones are best developed, during local summer, subsidence and equatorward advection on the eastern flanks of the anticyclones cool SSTs, while poleward flow on the western flanks warms SSTs. During local winter the SST asymmetry across the subtropical North Atlantic and North Pacific is maintained by warm water advection in the western boundary currents that offsets the large extraction of heat by advection of cold, dry air of the continents and by transient eddies. In the Southern Hemisphere ocean processes are equally important in cooling the eastern oceans by upwelling and advection during local winter. Ocean dynamics are important in amplifying the SST asymmetry, as experiments with general circulation models show. This amplification has little impact on the seasonal cycle of the anticyclones in the Northern Hemisphere, strengthens the anticyclones in the Southern Hemisphere, and helps position the anticyclones over the eastern basins in both hemispheres. Experiments with an idealized model are used to suggest that the subtropical anticyclones arise fundamentally as a response to monsoonal heating over land but need further amplification to bring them up to observed strength. The amplification is provided by local air–sea interaction. The SST asymmetry, generated through local air–sea interaction by the weak anticyclones forced by heating over land, stabilizes the atmosphere to deep convection in the east and destabilizes it in the west. Convection spreads from the land regions to the adjacent regions of the western subtropical oceans, and the enhanced zonal asymmetry of atmospheric heating strengthens the subtropical anticyclones.
Abstract
The causes of the seasonal cycles of the subtropical anticyclones, and the associated zonal asymmetries of sea surface temperature (SST) across the subtropical oceans, are examined. In all basins the cool waters in the east and warm waters in the west are sustained by a mix of atmosphere and ocean processes. When the anticyclones are best developed, during local summer, subsidence and equatorward advection on the eastern flanks of the anticyclones cool SSTs, while poleward flow on the western flanks warms SSTs. During local winter the SST asymmetry across the subtropical North Atlantic and North Pacific is maintained by warm water advection in the western boundary currents that offsets the large extraction of heat by advection of cold, dry air of the continents and by transient eddies. In the Southern Hemisphere ocean processes are equally important in cooling the eastern oceans by upwelling and advection during local winter. Ocean dynamics are important in amplifying the SST asymmetry, as experiments with general circulation models show. This amplification has little impact on the seasonal cycle of the anticyclones in the Northern Hemisphere, strengthens the anticyclones in the Southern Hemisphere, and helps position the anticyclones over the eastern basins in both hemispheres. Experiments with an idealized model are used to suggest that the subtropical anticyclones arise fundamentally as a response to monsoonal heating over land but need further amplification to bring them up to observed strength. The amplification is provided by local air–sea interaction. The SST asymmetry, generated through local air–sea interaction by the weak anticyclones forced by heating over land, stabilizes the atmosphere to deep convection in the east and destabilizes it in the west. Convection spreads from the land regions to the adjacent regions of the western subtropical oceans, and the enhanced zonal asymmetry of atmospheric heating strengthens the subtropical anticyclones.
