Search Results
Abstract
The spatial distribution and evolution of variability of near-global SST and SLP data in the quasi-biennial (QB) and 3–7 year low-frequency (LF) period bands are investigated and described. The largest signals in both bands are in the tropics. The near-equatorial characteristics of the QB in the SLP field are those of a quasi-progressive wave while the LF variation in the same field is closer to the standing wave. Both bands show the traditional Southern Oscillation pattern. The SST variability in both bands is essentially that of El Niño.
It is shown that ENSO is partially due to a nonlinear interaction between the two frequency bands. Both bands appear important to the ENSO cycle. The current work could not establish conclusively that if either was the fundamental mode, although there is weak evidence favoring the QB mode.
The QB signal described here is essentially the ENSO signal and does not seem to be simply related to the stratospheric QBO. Calculations suggest the tropospheric QB described here is not due to a consistent interaction of the annual cycle with itself. The current results do not exclude the possibility that the QB is due to forcing processes which regularly switch sign with season.
Abstract
The spatial distribution and evolution of variability of near-global SST and SLP data in the quasi-biennial (QB) and 3–7 year low-frequency (LF) period bands are investigated and described. The largest signals in both bands are in the tropics. The near-equatorial characteristics of the QB in the SLP field are those of a quasi-progressive wave while the LF variation in the same field is closer to the standing wave. Both bands show the traditional Southern Oscillation pattern. The SST variability in both bands is essentially that of El Niño.
It is shown that ENSO is partially due to a nonlinear interaction between the two frequency bands. Both bands appear important to the ENSO cycle. The current work could not establish conclusively that if either was the fundamental mode, although there is weak evidence favoring the QB mode.
The QB signal described here is essentially the ENSO signal and does not seem to be simply related to the stratospheric QBO. Calculations suggest the tropospheric QB described here is not due to a consistent interaction of the annual cycle with itself. The current results do not exclude the possibility that the QB is due to forcing processes which regularly switch sign with season.
Abstract
The common variance between 100-yr-long control runs from 11 coupled global climate models (CGCMs) has been studied by use of common empirical orthogonal functions (EOFs). The results suggest that there is a considerable disparity between the CGCMs estimates of internal variability. About one-half of this difference can be attributed to model drift or other low-frequency variations in several of the models. However, even after accounting for this effect, it was found that the models can easily differ by a factor of 2 or more for the energy levels in different EOF mode (wave) numbers. Comparison with observations showed that no one model consistently reproduced the observed partial eigenvalue spectrum. Again, differences between observed and model energy levels were commonly a factor of 2 or more. It is speculated that at least some of the disagreement is due to the relative coarse resolution of the models used in this study.
Separate analysis of a 1000-yr control run of the Geophysical Fluid Dynamics Laboratory model suggested that intramodel variability is much smaller than intermodel variability. It was also found that an estimate of the anthropogenic signal due to greenhouse gases and aerosols from the Max Planck Institute had strong spatial similarities to the leading modes of the models’ common EOFs. This fact complicates the detection/attribution problem.
Abstract
The common variance between 100-yr-long control runs from 11 coupled global climate models (CGCMs) has been studied by use of common empirical orthogonal functions (EOFs). The results suggest that there is a considerable disparity between the CGCMs estimates of internal variability. About one-half of this difference can be attributed to model drift or other low-frequency variations in several of the models. However, even after accounting for this effect, it was found that the models can easily differ by a factor of 2 or more for the energy levels in different EOF mode (wave) numbers. Comparison with observations showed that no one model consistently reproduced the observed partial eigenvalue spectrum. Again, differences between observed and model energy levels were commonly a factor of 2 or more. It is speculated that at least some of the disagreement is due to the relative coarse resolution of the models used in this study.
Separate analysis of a 1000-yr control run of the Geophysical Fluid Dynamics Laboratory model suggested that intramodel variability is much smaller than intermodel variability. It was also found that an estimate of the anthropogenic signal due to greenhouse gases and aerosols from the Max Planck Institute had strong spatial similarities to the leading modes of the models’ common EOFs. This fact complicates the detection/attribution problem.
Abstract
The dynamics and predictability of decadal climate variability over the North Pacific and North America are investigated by analyzing various observational datasets and the output of a state of the art coupled ocean–atmosphere general circulation model that was integrated for 125 years. Both the observations and model results support the picture that the decadal variability in the region of interest is based on a cycle involving unstable ocean–atmosphere interactions over the North Pacific. The period of this cycle is of the order of a few decades.
The cycle involves the two major circulation regimes in the North Pacific climate system, the subtropical ocean gyre, and the Aleutian low. When, for instance, the subtropical ocean gyre is anomalously strong, more warm tropical waters are transported poleward by the Kuroshio and its extension, leading to a positive SST anomaly in the North Pacific. The atmospheric response to this SST anomaly involves a weakened Aleutian low, and the associated fluxes at the air–sea interface reinforce the initial SST anomaly, so that ocean and atmosphere act as a positive feedback system. The anomalous heat flux, reduced ocean mixing in response to a weakened storm track, and anonmalous Ekman heat transport contribute to this positive feedback.
The atmospheric response, however, consists also of a wind stress curl anomaly that spins down the subtropical ocean gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The ocean adjusts with some time lag to the change in the wind stress curl, and it is this transient ocean response that allows continuous oscillations. The transient response can be expressed in terms of baroclinic planetary waves, and the decadal timescale of the oscillation is therefore determined to first order by wave timescales. Advection by the mean currents, however, is not negligible.
The existence of such a cycle provides the basis of long-range climate forecasting over North America at decadal timescales. At a minimum, knowledge of the present phase of the decadal mode should allow a “now-cast” of expected climate “bias” over North America, which is equivalent to a climate forecast several years ahead.
Abstract
The dynamics and predictability of decadal climate variability over the North Pacific and North America are investigated by analyzing various observational datasets and the output of a state of the art coupled ocean–atmosphere general circulation model that was integrated for 125 years. Both the observations and model results support the picture that the decadal variability in the region of interest is based on a cycle involving unstable ocean–atmosphere interactions over the North Pacific. The period of this cycle is of the order of a few decades.
The cycle involves the two major circulation regimes in the North Pacific climate system, the subtropical ocean gyre, and the Aleutian low. When, for instance, the subtropical ocean gyre is anomalously strong, more warm tropical waters are transported poleward by the Kuroshio and its extension, leading to a positive SST anomaly in the North Pacific. The atmospheric response to this SST anomaly involves a weakened Aleutian low, and the associated fluxes at the air–sea interface reinforce the initial SST anomaly, so that ocean and atmosphere act as a positive feedback system. The anomalous heat flux, reduced ocean mixing in response to a weakened storm track, and anonmalous Ekman heat transport contribute to this positive feedback.
The atmospheric response, however, consists also of a wind stress curl anomaly that spins down the subtropical ocean gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The ocean adjusts with some time lag to the change in the wind stress curl, and it is this transient ocean response that allows continuous oscillations. The transient response can be expressed in terms of baroclinic planetary waves, and the decadal timescale of the oscillation is therefore determined to first order by wave timescales. Advection by the mean currents, however, is not negligible.
The existence of such a cycle provides the basis of long-range climate forecasting over North America at decadal timescales. At a minimum, knowledge of the present phase of the decadal mode should allow a “now-cast” of expected climate “bias” over North America, which is equivalent to a climate forecast several years ahead.
Abstract
Long-range sea surface temperature forecasts from two different coupled ocean-atmosphere models of the tropical Pacific are used in conjunction with statistical models relating winter Northern Hemisphere 700-mb height and tropical SST to forecast the former field at a lead time of two seasons in advance. The forecasts show considerable skill over large areas, with a regional distribution of predictive performance that is consistent with the observed contemporaneous relation between the two fields. Comparable skills for lead time of a year or more in advance seem likely.
Abstract
Long-range sea surface temperature forecasts from two different coupled ocean-atmosphere models of the tropical Pacific are used in conjunction with statistical models relating winter Northern Hemisphere 700-mb height and tropical SST to forecast the former field at a lead time of two seasons in advance. The forecasts show considerable skill over large areas, with a regional distribution of predictive performance that is consistent with the observed contemporaneous relation between the two fields. Comparable skills for lead time of a year or more in advance seem likely.
Abstract
The authors have investigated the interactions of the tropical oceans on interannual timescales by conducting a series of uncoupled atmospheric and oceanic general circulation experiments and hybrid-coupled model simulations. The results illustrate the key role of the El Niño/Southern Oscillation phenomenon in generating interannual variability in all three tropical ocean basins. Sea surface temperature anomalies in the tropical Pacific force SST anomalies of the same sign in the Indian Ocean and SST anomalies of the opposite sign in the Atlantic via a changed atmospheric circulation. However, although air-sea interactions in the Indian and Atlantic Oceans are much weaker than those in the Pacific, they contribute significantly to the variability in these two regions. The role of these air-sea interactions is mainly that of an amplifier by which the ENSO-induced signals are enhanced in the ocean and atmosphere. This process is particularly important in the tropical Atlantic region.
The authors investigated, also, whether ENSO is part of a zonally propagating “wave,” which travels around the globe with a timescale of several years. Consistent with observations, the upper-ocean heat content in the various numerical simulators seems to propagate slowly around the globe. SST anomalies in the Pacific Ocean introduce a global atmospheric response, which in turn forces variations in the other tropical oceans. Since the different oceans exhibit different response characteristics to low-frequency wind changes, the individual tropical ocean responses can add up coincidentally to look like a global wave, and that appears to be the situation. In particular, no evidence is found that the Indian Ocean can significantly affect the ENSO cycle in the Pacific. Finally, the potential for climate forecasts in the Indian and Atlantic Oceans appears to be enhanced if one includes, in a coupled way, remote influences from the Pacific.
Abstract
The authors have investigated the interactions of the tropical oceans on interannual timescales by conducting a series of uncoupled atmospheric and oceanic general circulation experiments and hybrid-coupled model simulations. The results illustrate the key role of the El Niño/Southern Oscillation phenomenon in generating interannual variability in all three tropical ocean basins. Sea surface temperature anomalies in the tropical Pacific force SST anomalies of the same sign in the Indian Ocean and SST anomalies of the opposite sign in the Atlantic via a changed atmospheric circulation. However, although air-sea interactions in the Indian and Atlantic Oceans are much weaker than those in the Pacific, they contribute significantly to the variability in these two regions. The role of these air-sea interactions is mainly that of an amplifier by which the ENSO-induced signals are enhanced in the ocean and atmosphere. This process is particularly important in the tropical Atlantic region.
The authors investigated, also, whether ENSO is part of a zonally propagating “wave,” which travels around the globe with a timescale of several years. Consistent with observations, the upper-ocean heat content in the various numerical simulators seems to propagate slowly around the globe. SST anomalies in the Pacific Ocean introduce a global atmospheric response, which in turn forces variations in the other tropical oceans. Since the different oceans exhibit different response characteristics to low-frequency wind changes, the individual tropical ocean responses can add up coincidentally to look like a global wave, and that appears to be the situation. In particular, no evidence is found that the Indian Ocean can significantly affect the ENSO cycle in the Pacific. Finally, the potential for climate forecasts in the Indian and Atlantic Oceans appears to be enhanced if one includes, in a coupled way, remote influences from the Pacific.
Abstract
In this paper a decadal climate cycle in the North Atlantic that was derived from an extended-range integration with a coupled ocean–atmosphere general circulation model is described. The decadal mode shares many features with the observed decadal variability in the North Atlantic. The period of the simulated oscillation, however, is somewhat longer than that estimated from observations. While the observations indicate a period of about 12 yr, the coupled model simulation yields a period of about 17 yr. The cyclic nature of the decadal variability implies some inherent predictability at these timescales.
The decadal mode is based on unstable air–sea interactions and must be therefore regarded as an inherently coupled mode. It involves the subtropical gyre and the North Atlantic oscillation. The memory of the coupled system, however, resides in the ocean and is related to horizontal advection and to the oceanic adjustment to low-frequency wind stress curl variations. In particular, it is found that variations in the intensity of the Gulf Stream and its extension are crucial to the oscillation. Although differing in details, the North Atlantic decadal mode and the North Pacific mode described by M. Latif and T. P. Barnett are based on the same fundamental mechanism: a feedback loop between the wind driven subtropical gyre and the extratropical atmospheric circulation.
Abstract
In this paper a decadal climate cycle in the North Atlantic that was derived from an extended-range integration with a coupled ocean–atmosphere general circulation model is described. The decadal mode shares many features with the observed decadal variability in the North Atlantic. The period of the simulated oscillation, however, is somewhat longer than that estimated from observations. While the observations indicate a period of about 12 yr, the coupled model simulation yields a period of about 17 yr. The cyclic nature of the decadal variability implies some inherent predictability at these timescales.
The decadal mode is based on unstable air–sea interactions and must be therefore regarded as an inherently coupled mode. It involves the subtropical gyre and the North Atlantic oscillation. The memory of the coupled system, however, resides in the ocean and is related to horizontal advection and to the oceanic adjustment to low-frequency wind stress curl variations. In particular, it is found that variations in the intensity of the Gulf Stream and its extension are crucial to the oscillation. Although differing in details, the North Atlantic decadal mode and the North Pacific mode described by M. Latif and T. P. Barnett are based on the same fundamental mechanism: a feedback loop between the wind driven subtropical gyre and the extratropical atmospheric circulation.
Abstract
A phenomenon called the Antarctic Circumpolar Wave (ACW), suggested earlier from fragmentary observational evidence, has been simulated realistically in an extended integration of a Max Planck Institute coupled general circulation model. The ACW both in the observations and in the model constitutes a mode of the coupled ocean–atmosphere–sea-ice system that inhabits the high latitudes of the Southern Hemisphere. It is characterized by anomalies of such climate variables as sea surface temperature, sea level pressure, meridional wind, and sea ice that exhibit intricate and evolving spatial phase relations to each other.
The simulated ACW signal in the ocean propagates eastward over most of the high-latitude Southern Ocean, mainly advected along in the Antarctic Circumpolar Current. On average, it completes a circuit entirely around the Southern Ocean but is strongly dissipated in the South Atlantic and in the southern Indian Ocean, just marginally maintaining statistical significance in these areas until it reaches the South Pacific where it is reenergized. In extreme cases, the complete circumpolar propagation is more clear, requiring about 12–16 yr to complete the circuit. This, coupled with the dominant zonal wavenumber 3 pattern of the ACW, results in the local reappearance of energy peaks about every 4–5 yr.
The oceanic component of the mode is forced by the atmosphere via fluxes of heat. The overlying atmosphere establishes patterns of sea level pressure that mainly consist of a standing wave and are associated with the Pacific–South American (PSA) oscillation described in earlier works. The PSA, like its counterpart in the North Pacific, appears to be a natural mode of the high southern latitudes. There is some ENSO-related signal in the ACW forced by anomalous latent heat release associated with precipitation anomalies in the central and western tropical Pacific. However, ENSO-related forcing explains at most 30% of the ACW variance and, generally, much less.
It is hypothesized that the ACW as an entity represents the net result of moving oceanic climate anomalies interacting with a spatially fixed atmospheric forcing pattern. As the SST moves into and out of phase with the resonant background pattern it is selectively amplified or dissipated, an idea supported by several independent analyses. A simplified ocean heat budget model seems to also support this idea.
Abstract
A phenomenon called the Antarctic Circumpolar Wave (ACW), suggested earlier from fragmentary observational evidence, has been simulated realistically in an extended integration of a Max Planck Institute coupled general circulation model. The ACW both in the observations and in the model constitutes a mode of the coupled ocean–atmosphere–sea-ice system that inhabits the high latitudes of the Southern Hemisphere. It is characterized by anomalies of such climate variables as sea surface temperature, sea level pressure, meridional wind, and sea ice that exhibit intricate and evolving spatial phase relations to each other.
The simulated ACW signal in the ocean propagates eastward over most of the high-latitude Southern Ocean, mainly advected along in the Antarctic Circumpolar Current. On average, it completes a circuit entirely around the Southern Ocean but is strongly dissipated in the South Atlantic and in the southern Indian Ocean, just marginally maintaining statistical significance in these areas until it reaches the South Pacific where it is reenergized. In extreme cases, the complete circumpolar propagation is more clear, requiring about 12–16 yr to complete the circuit. This, coupled with the dominant zonal wavenumber 3 pattern of the ACW, results in the local reappearance of energy peaks about every 4–5 yr.
The oceanic component of the mode is forced by the atmosphere via fluxes of heat. The overlying atmosphere establishes patterns of sea level pressure that mainly consist of a standing wave and are associated with the Pacific–South American (PSA) oscillation described in earlier works. The PSA, like its counterpart in the North Pacific, appears to be a natural mode of the high southern latitudes. There is some ENSO-related signal in the ACW forced by anomalous latent heat release associated with precipitation anomalies in the central and western tropical Pacific. However, ENSO-related forcing explains at most 30% of the ACW variance and, generally, much less.
It is hypothesized that the ACW as an entity represents the net result of moving oceanic climate anomalies interacting with a spatially fixed atmospheric forcing pattern. As the SST moves into and out of phase with the resonant background pattern it is selectively amplified or dissipated, an idea supported by several independent analyses. A simplified ocean heat budget model seems to also support this idea.
Abstract
In this study, a hybrid coupled model (HCM) is used to investigate the physics of decadal variability in the North Pacific. This aids in an understanding of the inherent properties of the coupled ocean–atmosphere system in the absence of stochastic forcing by noncoupled variability. It is shown that the HCM simulates a self-sustained decadal oscillation with a period of about 20 yr, similar to that found in both the observations and coupled GCMs.
Sensitivity experiments are carried out to determine the relative importance of wind stresses, net surface heat flux, and freshwater flux on the initiation and maintenance of the decadal oscillation in the North Pacific. It is found that decadal variability is a mode of the coupled system and involves interaction of sea surface temperature, upper-ocean heat content, and wind stress. This interaction is mainly controlled by the wind stress but can be strongly modified by the surface heat flux. The effect of the salinity is relatively small and is not necessary to generate the model decadal oscillation in the North Pacific.
There are some limitations with this study. First, the effect of a stochastic forcing is not included. Second, a weak negative feedback is needed to run the control experiment for a longer time period. These two areas will be addressed in a future investigation.
Abstract
In this study, a hybrid coupled model (HCM) is used to investigate the physics of decadal variability in the North Pacific. This aids in an understanding of the inherent properties of the coupled ocean–atmosphere system in the absence of stochastic forcing by noncoupled variability. It is shown that the HCM simulates a self-sustained decadal oscillation with a period of about 20 yr, similar to that found in both the observations and coupled GCMs.
Sensitivity experiments are carried out to determine the relative importance of wind stresses, net surface heat flux, and freshwater flux on the initiation and maintenance of the decadal oscillation in the North Pacific. It is found that decadal variability is a mode of the coupled system and involves interaction of sea surface temperature, upper-ocean heat content, and wind stress. This interaction is mainly controlled by the wind stress but can be strongly modified by the surface heat flux. The effect of the salinity is relatively small and is not necessary to generate the model decadal oscillation in the North Pacific.
There are some limitations with this study. First, the effect of a stochastic forcing is not included. Second, a weak negative feedback is needed to run the control experiment for a longer time period. These two areas will be addressed in a future investigation.
Abstract
The characteristic space–time scales of surface solar radiation fields measured by the 111-instrument MESONET in Oklahoma are estimated after removal of the diurnal cycle. These estimates of “within-day” variability are used to deduce the representativeness of surface solar radiation measurements made at the central ARM measurement site as a function of time-averaging interval. Nomograms of the relation between point measurements and area averages are given for different space–time-averaging intervals. Examples from the nomograms show, for instance, that under conditions of low mean radiation (cloudy days), the central site point measurements are representative of a spatial area the size of a T42 GCM grid box (280 km × 280 km) if one uses hourly averages and is willing to accept a correlation of 0.45 between area average and point measurement. The point data represent a 60 km × 60 km region at a 0.90 correlation level if a 5-min time average is used. The characteristic timescale for the within-day radiation variability was roughly 60 min. Estimates of scale lengths for days when the mean background radiation conditions are high are also given in the nomographs.
Abstract
The characteristic space–time scales of surface solar radiation fields measured by the 111-instrument MESONET in Oklahoma are estimated after removal of the diurnal cycle. These estimates of “within-day” variability are used to deduce the representativeness of surface solar radiation measurements made at the central ARM measurement site as a function of time-averaging interval. Nomograms of the relation between point measurements and area averages are given for different space–time-averaging intervals. Examples from the nomograms show, for instance, that under conditions of low mean radiation (cloudy days), the central site point measurements are representative of a spatial area the size of a T42 GCM grid box (280 km × 280 km) if one uses hourly averages and is willing to accept a correlation of 0.45 between area average and point measurement. The point data represent a 60 km × 60 km region at a 0.90 correlation level if a 5-min time average is used. The characteristic timescale for the within-day radiation variability was roughly 60 min. Estimates of scale lengths for days when the mean background radiation conditions are high are also given in the nomographs.
Abstract
Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period.
The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation.
The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient.
The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.
Abstract
Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period.
The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation.
The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient.
The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.
