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- Author or Editor: David S. Battisti x
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Abstract
Recent theoretical and numerical modeling studies of the coupled tropical atmosphere-ocean system suggest that equatorial ocean wave dynamics may play an important role in the evolution of ENSO (El Niño/Southern Oscillation). These studies emphasize that the oceanic wave signal is confined to within a narrow equatorial band (within 6° of the equator).
In this study we use a coupled atmosphere–ocean model to investigate the role of off-equatorial Rossby waves observed in the western North Pacific Ocean during the ENSO cycle. We find that these off-equatorial Rossby waves (found poleward of 6° from the equator) are formed through both eastern boundary reflection of the equatorial Kelvin wave signal generated in a warm event (El Niño), and changes in the off-equatorial wind stress curl. Our results indicate that, independent of the generation mechanism, off-equatorial Rossby waves should be thought of as the product and not the triggering mechanism for an ENSO event.
Abstract
Recent theoretical and numerical modeling studies of the coupled tropical atmosphere-ocean system suggest that equatorial ocean wave dynamics may play an important role in the evolution of ENSO (El Niño/Southern Oscillation). These studies emphasize that the oceanic wave signal is confined to within a narrow equatorial band (within 6° of the equator).
In this study we use a coupled atmosphere–ocean model to investigate the role of off-equatorial Rossby waves observed in the western North Pacific Ocean during the ENSO cycle. We find that these off-equatorial Rossby waves (found poleward of 6° from the equator) are formed through both eastern boundary reflection of the equatorial Kelvin wave signal generated in a warm event (El Niño), and changes in the off-equatorial wind stress curl. Our results indicate that, independent of the generation mechanism, off-equatorial Rossby waves should be thought of as the product and not the triggering mechanism for an ENSO event.
Abstract
A simple coupled ocean–atmosphere model, similar to that of Zebiak and Cane, is used to examine the dynamic and thermodynamic processes associated with El Niño/Southern Oscillation (ENSO). The model is run for 300 years. The interannual variability which results is regular, with a period of either 3 or 4 years, quantized by the annual cycle. The amplitude (∼1.5 m s−1 wind and 2°C SST anomalies), period and structure of the interannual variability compare well with observations. The model warm event is initiated in the spring prior to the event peak, and is well described as an instability of the coupled system. During instability growth, the sea surface temperature (SST) anomaly is primarily generated by vertical upwelling processes. The SST anomaly can be approximately described by the expression ∂T/∂t = KTh − α*T, where T is the SST anomaly, t time, h the upper layer thickness (pycnocline) perturbation and α* an effective damping time which includes heat loss to the atmosphere. KT parameterizes vertical upwelling and mixed layer processes.
Oceanic wave dynamics determines the fate of the growing instability. The warming of the SST produces westerly wind anomalies in the equator central Pacific, forcing equatorially trapped Rossby waves that propagate freely to the western boundary. These waves reflect at the western boundary, sending upwelling equatorial Kelvin waves back to the central basin. These cooling Kelvin waves act to terminate instability growth and rapidly plunge the coupled system into a cold regime. The western boundary reflection is necessary for event termination. The system returns from a cold regime via reduced heat flux to the atmosphere and, to a lesser extent, by wave induced processes like that which lead to the warm event termination. The interannual variability is not produced by vacillation between two equilibrium states: a cold and a warm state. The growth rate to either the cold or warm state is too slow for the system to achieve equilibrium, even for a basin the size of the Pacific. The model results indicate that shortly after the initial set of gravest mode Rossby reflections on the western boundary, the instability growth is already being substantially moderated by the equatorial wave processes in the ocean. Thus the system is oscillatory around a single basic state.
Of the Rossby waves produced in the central Pacific by the warm event, only the two gravest mode symmetric modes are important in the reflection process, which produce the Kelvin waves that terminate the warm event. In nature, the actual western boundary for the equatorial Pacific wave guide is very ambiguous. Calculations indicate, however, that efficient reflection of the gravest symmetric Rossby waves from a more realistic boundary than the meridional wall in the model is possible. Finally, if the model is indeed simulating the correct processes controlling ENSO events, the nature of the instability mechanism that leads to growth and the wave-induced termination of the model warm event suggests that, for realistic instability growth rates for the coupled equatorial ocean-atmosphere system, interannual variability analogous to ENSO should not be possible in equatorial basins significantly smaller than the Pacific.
Abstract
A simple coupled ocean–atmosphere model, similar to that of Zebiak and Cane, is used to examine the dynamic and thermodynamic processes associated with El Niño/Southern Oscillation (ENSO). The model is run for 300 years. The interannual variability which results is regular, with a period of either 3 or 4 years, quantized by the annual cycle. The amplitude (∼1.5 m s−1 wind and 2°C SST anomalies), period and structure of the interannual variability compare well with observations. The model warm event is initiated in the spring prior to the event peak, and is well described as an instability of the coupled system. During instability growth, the sea surface temperature (SST) anomaly is primarily generated by vertical upwelling processes. The SST anomaly can be approximately described by the expression ∂T/∂t = KTh − α*T, where T is the SST anomaly, t time, h the upper layer thickness (pycnocline) perturbation and α* an effective damping time which includes heat loss to the atmosphere. KT parameterizes vertical upwelling and mixed layer processes.
Oceanic wave dynamics determines the fate of the growing instability. The warming of the SST produces westerly wind anomalies in the equator central Pacific, forcing equatorially trapped Rossby waves that propagate freely to the western boundary. These waves reflect at the western boundary, sending upwelling equatorial Kelvin waves back to the central basin. These cooling Kelvin waves act to terminate instability growth and rapidly plunge the coupled system into a cold regime. The western boundary reflection is necessary for event termination. The system returns from a cold regime via reduced heat flux to the atmosphere and, to a lesser extent, by wave induced processes like that which lead to the warm event termination. The interannual variability is not produced by vacillation between two equilibrium states: a cold and a warm state. The growth rate to either the cold or warm state is too slow for the system to achieve equilibrium, even for a basin the size of the Pacific. The model results indicate that shortly after the initial set of gravest mode Rossby reflections on the western boundary, the instability growth is already being substantially moderated by the equatorial wave processes in the ocean. Thus the system is oscillatory around a single basic state.
Of the Rossby waves produced in the central Pacific by the warm event, only the two gravest mode symmetric modes are important in the reflection process, which produce the Kelvin waves that terminate the warm event. In nature, the actual western boundary for the equatorial Pacific wave guide is very ambiguous. Calculations indicate, however, that efficient reflection of the gravest symmetric Rossby waves from a more realistic boundary than the meridional wall in the model is possible. Finally, if the model is indeed simulating the correct processes controlling ENSO events, the nature of the instability mechanism that leads to growth and the wave-induced termination of the model warm event suggests that, for realistic instability growth rates for the coupled equatorial ocean-atmosphere system, interannual variability analogous to ENSO should not be possible in equatorial basins significantly smaller than the Pacific.
Abstract
The “background” state is commonly removed from synoptic fields by use of either a spatial or temporal filter prior to the application of feature tracking. Commonly used spatial and temporal filters applied to sea level pressure data admit substantially different information to be included in the synoptic fields. The spatial filter retains a time-mean field that has comparable magnitude to a typical synoptic perturbation. In contrast, the temporal filter removes the entire time-mean field. The inclusion of the time-mean spatially filtered field biases the feature tracking statistics toward large cyclone (anticyclone) magnitudes in the regions of climatological lows (highs). The resulting cyclone/anticyclone magnitude asymmetries in each region are found to be inconsistent with the unfiltered data fields and merely result from the spurious inclusion of the time-mean fields in the spatially filtered data. The temporally filtered fields do not suffer from the same problem and produce modest cyclone/anticyclone magnitude asymmetries that are consistent with the unfiltered data. This analysis suggests that the weather forecaster’s assertion that cyclones have larger amplitudes than anticyclones is due to a composite of a small magnitude asymmetry in the synoptic waves and a large contribution from inhomogeneity in the background (stationary) field.
Abstract
The “background” state is commonly removed from synoptic fields by use of either a spatial or temporal filter prior to the application of feature tracking. Commonly used spatial and temporal filters applied to sea level pressure data admit substantially different information to be included in the synoptic fields. The spatial filter retains a time-mean field that has comparable magnitude to a typical synoptic perturbation. In contrast, the temporal filter removes the entire time-mean field. The inclusion of the time-mean spatially filtered field biases the feature tracking statistics toward large cyclone (anticyclone) magnitudes in the regions of climatological lows (highs). The resulting cyclone/anticyclone magnitude asymmetries in each region are found to be inconsistent with the unfiltered data fields and merely result from the spurious inclusion of the time-mean fields in the spatially filtered data. The temporally filtered fields do not suffer from the same problem and produce modest cyclone/anticyclone magnitude asymmetries that are consistent with the unfiltered data. This analysis suggests that the weather forecaster’s assertion that cyclones have larger amplitudes than anticyclones is due to a composite of a small magnitude asymmetry in the synoptic waves and a large contribution from inhomogeneity in the background (stationary) field.
Abstract
The Last Glacial Maximum (LGM), 21 000 yr before present, was the time of maximum land ice extent during the last ice age. A recent simulation of the LGM climate by a state-of-the-art fully coupled global climate model is shown to exhibit strong, steady atmospheric jets and weak transient eddy activity in the Atlantic sector compared to today’s climate. In contrast, previous work based on uncoupled atmospheric model simulations has shown that the LGM jets and eddy activity in the Atlantic sector are similar to those observed today, with the main difference being a northeastward extension of their maxima. The coupled model simulation is shown to agree more with paleoclimate proxy records and thus is taken as the more reliable representation of LGM climate. The existence of this altered atmospheric circulation state during LGM in the model has implications for understanding the stability of glacial climates, for the possibility of multiple atmospheric circulation regimes, and for the interpretation of paleoclimate proxy records.
Abstract
The Last Glacial Maximum (LGM), 21 000 yr before present, was the time of maximum land ice extent during the last ice age. A recent simulation of the LGM climate by a state-of-the-art fully coupled global climate model is shown to exhibit strong, steady atmospheric jets and weak transient eddy activity in the Atlantic sector compared to today’s climate. In contrast, previous work based on uncoupled atmospheric model simulations has shown that the LGM jets and eddy activity in the Atlantic sector are similar to those observed today, with the main difference being a northeastward extension of their maxima. The coupled model simulation is shown to agree more with paleoclimate proxy records and thus is taken as the more reliable representation of LGM climate. The existence of this altered atmospheric circulation state during LGM in the model has implications for understanding the stability of glacial climates, for the possibility of multiple atmospheric circulation regimes, and for the interpretation of paleoclimate proxy records.
Abstract
The question of why the intertropical convergence zone (ITCZ) is generally north of the equator in the tropical Pacific is addressed. Experiments with an atmospheric general circulation model coupled to idealized representations of the ocean show that the presence of the Andes is enough to lower sea surface temperature (SST) off the west coast of South America through evaporation, thus promoting a north–south asymmetry, with the ITCZ north of the equator, which is amplified by interactions between the ocean and the atmosphere. The evaporative cooling results mainly from the subsidence of low specific humidity air, which is due in turn to the mechanical effect of the Andes on the zonal mean flow. The positive feedback from low-level clouds on SST is an important factor for the efficiency of the mechanism described.
West of 120°W, the presence of the Rockies and Himalayas produces a comparable forcing to that of the Andes, but this is not enough to reverse or neutralize the north–south asymmetry set by the Andes. It is shown that the longitudinal offset between the forcings in both hemispheres allows the Andes to preferentially set the north–south asymmetry, which propagates westward into the rest of the Pacific.
Asymmetry in the observed ocean heat transports (more heat transport convergence in the Northern Hemisphere) associated with the Kuroshio was found to reinforce the effect of the Andes, although it is not a strong forcing by itself. Sensitivity experiments indicate that the north–south asymmetry of the ITCZ caused (evaporatively) by the Andes is robust to the presence of a strong equatorial cold tongue and to seasonality in insolation.
Abstract
The question of why the intertropical convergence zone (ITCZ) is generally north of the equator in the tropical Pacific is addressed. Experiments with an atmospheric general circulation model coupled to idealized representations of the ocean show that the presence of the Andes is enough to lower sea surface temperature (SST) off the west coast of South America through evaporation, thus promoting a north–south asymmetry, with the ITCZ north of the equator, which is amplified by interactions between the ocean and the atmosphere. The evaporative cooling results mainly from the subsidence of low specific humidity air, which is due in turn to the mechanical effect of the Andes on the zonal mean flow. The positive feedback from low-level clouds on SST is an important factor for the efficiency of the mechanism described.
West of 120°W, the presence of the Rockies and Himalayas produces a comparable forcing to that of the Andes, but this is not enough to reverse or neutralize the north–south asymmetry set by the Andes. It is shown that the longitudinal offset between the forcings in both hemispheres allows the Andes to preferentially set the north–south asymmetry, which propagates westward into the rest of the Pacific.
Asymmetry in the observed ocean heat transports (more heat transport convergence in the Northern Hemisphere) associated with the Kuroshio was found to reinforce the effect of the Andes, although it is not a strong forcing by itself. Sensitivity experiments indicate that the north–south asymmetry of the ITCZ caused (evaporatively) by the Andes is robust to the presence of a strong equatorial cold tongue and to seasonality in insolation.
Abstract
Recent work has identified variability in the Pacific Ocean SST with a structure qualitatively similar to ENSO, but at lower frequencies than ENSO. Zhang et al. have documented the atmospheric circulation anomalies in the Tropics and Northern Hemisphere that are associated with decadal ENSO-like variability and compared these anomalies to those associated with the (interannual) ENSO cycle.
Here the authors extend the study of Zhang et al. to the Southern Hemisphere using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data for 1958–96. Consistent with previous studies, the Southern Hemisphere circulation anomalies associated with ENSO display a teleconnection pattern from the central tropical Pacific into the far southeastern Pacific Ocean. Comparatively larger circulation anomalies are found in the Southern Hemisphere associated with the decadal ENSO-like variability, though aloft the structure of the anomalies emphasizes the polar vortex with an adorning wavenumber-3 anomaly along 60°S. There is a common pattern of SST anomalies throughout the South Pacific associated with the ENSO and the decadal ENSO-like variability, and these anomalies appear to be forced by (inferred) surface heat flux anomalies that should be associated with the changes in the atmospheric circulation. Finally, subtle differences in the tropical circulation anomalies are found to be associated with the two different timescales of variability. Further studies are required to demonstrate whether these differences are responsible for the different structure of the tropospheric mid- and high-latitude circulation anomalies in the Northern and Southern Hemispheres.
Abstract
Recent work has identified variability in the Pacific Ocean SST with a structure qualitatively similar to ENSO, but at lower frequencies than ENSO. Zhang et al. have documented the atmospheric circulation anomalies in the Tropics and Northern Hemisphere that are associated with decadal ENSO-like variability and compared these anomalies to those associated with the (interannual) ENSO cycle.
Here the authors extend the study of Zhang et al. to the Southern Hemisphere using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data for 1958–96. Consistent with previous studies, the Southern Hemisphere circulation anomalies associated with ENSO display a teleconnection pattern from the central tropical Pacific into the far southeastern Pacific Ocean. Comparatively larger circulation anomalies are found in the Southern Hemisphere associated with the decadal ENSO-like variability, though aloft the structure of the anomalies emphasizes the polar vortex with an adorning wavenumber-3 anomaly along 60°S. There is a common pattern of SST anomalies throughout the South Pacific associated with the ENSO and the decadal ENSO-like variability, and these anomalies appear to be forced by (inferred) surface heat flux anomalies that should be associated with the changes in the atmospheric circulation. Finally, subtle differences in the tropical circulation anomalies are found to be associated with the two different timescales of variability. Further studies are required to demonstrate whether these differences are responsible for the different structure of the tropospheric mid- and high-latitude circulation anomalies in the Northern and Southern Hemispheres.
Abstract
The aim of this paper is to determine how an atmosphere with enhanced mean-state baroclinity can support weaker baroclinic wave activity than an atmosphere with weak mean-state baroclinity. As a case study, a Last Glacial Maximum (LGM) model simulation previously documented to have reduced baroclinic storm activity, relative to the modern-day climate (simulated by the same model), despite having an enhanced midlatitude temperature gradient, is considered. Several candidate mechanisms are evaluated to explain this apparent paradox.
A linear stability analysis is first performed on the jet in the modern-day and the LGM simulation; the latter has relatively strong barotropic velocity shear. It was found that the LGM mean state is more unstable to baroclinic disturbances than the modern-day mean state, although the three-dimensional jet structure does stabilize the LGM jet relative to the Eady growth rate. Next, feature tracking was used to assess the storm track seeding and temporal growth of disturbances. It was found that the reduction in LGM eddy activity, relative to the modern-day eddy activity, is due to the smaller magnitude of the upper-level storms entering the North Atlantic domain in the LGM. Although the LGM storms do grow more rapidly in the North Atlantic than their modern-day counterparts, the storminess in the LGM is reduced because storms seeding the region of enhanced baroclinity are weaker.
Abstract
The aim of this paper is to determine how an atmosphere with enhanced mean-state baroclinity can support weaker baroclinic wave activity than an atmosphere with weak mean-state baroclinity. As a case study, a Last Glacial Maximum (LGM) model simulation previously documented to have reduced baroclinic storm activity, relative to the modern-day climate (simulated by the same model), despite having an enhanced midlatitude temperature gradient, is considered. Several candidate mechanisms are evaluated to explain this apparent paradox.
A linear stability analysis is first performed on the jet in the modern-day and the LGM simulation; the latter has relatively strong barotropic velocity shear. It was found that the LGM mean state is more unstable to baroclinic disturbances than the modern-day mean state, although the three-dimensional jet structure does stabilize the LGM jet relative to the Eady growth rate. Next, feature tracking was used to assess the storm track seeding and temporal growth of disturbances. It was found that the reduction in LGM eddy activity, relative to the modern-day eddy activity, is due to the smaller magnitude of the upper-level storms entering the North Atlantic domain in the LGM. Although the LGM storms do grow more rapidly in the North Atlantic than their modern-day counterparts, the storminess in the LGM is reduced because storms seeding the region of enhanced baroclinity are weaker.
Abstract
The δ 18O of calcite (δ 18O c ) in speleothems from South America is fairly well correlated with austral summer [December–February (DJF)] insolation, indicating the role of orbitally paced changes in insolation in changing the climate of South America. Using an isotope-enabled atmospheric general circulation model (ECHAM4.6) coupled to a slab ocean model, the authors study how orbitally paced variations in insolation change climate and the isotopic composition of precipitation (δ 18O p ) of South America. Compared with times of high summertime insolation, times of low insolation feature (i) a decrease in precipitation inland of tropical South America as a result of an anomalous cooling of the South American continent and hence a weakening of the South American summer monsoon and (ii) an increase in precipitation in eastern Brazil that is associated with the intensification and southward movement of the Atlantic intertropical convergence zone, which is caused by the strengthening of African winter monsoon that is induced by the anomalous cooling of northern Africa. Finally, reduced DJF insolation over southern Africa causes cooling and the generation of a tropically trapped Rossby wave that intensifies and shifts the South Atlantic convergence zone northward. In times of low insolation, δ 18O p increases in the northern Andes and decreases in northeastern Brazil, consistent with the pattern of δ 18O c changes seen in speleothems. Further analysis shows that the decrease in δ 18O p in northeastern Brazil is due to change in the intensity of precipitation, while the increase in the northern Andes reflects a change in the seasonality of precipitation and in the isotopic composition of vapor that forms the condensates.
Abstract
The δ 18O of calcite (δ 18O c ) in speleothems from South America is fairly well correlated with austral summer [December–February (DJF)] insolation, indicating the role of orbitally paced changes in insolation in changing the climate of South America. Using an isotope-enabled atmospheric general circulation model (ECHAM4.6) coupled to a slab ocean model, the authors study how orbitally paced variations in insolation change climate and the isotopic composition of precipitation (δ 18O p ) of South America. Compared with times of high summertime insolation, times of low insolation feature (i) a decrease in precipitation inland of tropical South America as a result of an anomalous cooling of the South American continent and hence a weakening of the South American summer monsoon and (ii) an increase in precipitation in eastern Brazil that is associated with the intensification and southward movement of the Atlantic intertropical convergence zone, which is caused by the strengthening of African winter monsoon that is induced by the anomalous cooling of northern Africa. Finally, reduced DJF insolation over southern Africa causes cooling and the generation of a tropically trapped Rossby wave that intensifies and shifts the South Atlantic convergence zone northward. In times of low insolation, δ 18O p increases in the northern Andes and decreases in northeastern Brazil, consistent with the pattern of δ 18O c changes seen in speleothems. Further analysis shows that the decrease in δ 18O p in northeastern Brazil is due to change in the intensity of precipitation, while the increase in the northern Andes reflects a change in the seasonality of precipitation and in the isotopic composition of vapor that forms the condensates.
Abstract
The nature of the South Pacific convergence zone (SPCZ) is addressed by focusing on the dry (and cool) zone bounded by it and the coast of South America through numerical experiments. As shown in a companion paper, this dry zone is due, to a large extent, to orographically forced subsidence. Here it is shown that the northwestward expansion of this dry zone can be explained by advection of low moist static energy by the trade winds. These results provide an explanation of the geometry of the western edge of the dry zone and, therefore, of the eastern edge of the adjacent SPCZ. Sea surface temperature underneath the SPCZ is enhanced by relatively high near-surface humidity through evaporative processes, which feeds back into its organization. However, in this model, this feedback is not critical for the existence of the SPCZ. The subsidence associated with the ITCZ in the North Hemisphere negatively affects the precipitation rate in the SPCZ. It was also found that the sensitivity of the forced response is largest for peak orographic heights below 3000 m, which indicates that the exact representation of the Andes in numerical models might not be as critical as that of lower orography such as that in southern Africa.
Abstract
The nature of the South Pacific convergence zone (SPCZ) is addressed by focusing on the dry (and cool) zone bounded by it and the coast of South America through numerical experiments. As shown in a companion paper, this dry zone is due, to a large extent, to orographically forced subsidence. Here it is shown that the northwestward expansion of this dry zone can be explained by advection of low moist static energy by the trade winds. These results provide an explanation of the geometry of the western edge of the dry zone and, therefore, of the eastern edge of the adjacent SPCZ. Sea surface temperature underneath the SPCZ is enhanced by relatively high near-surface humidity through evaporative processes, which feeds back into its organization. However, in this model, this feedback is not critical for the existence of the SPCZ. The subsidence associated with the ITCZ in the North Hemisphere negatively affects the precipitation rate in the SPCZ. It was also found that the sensitivity of the forced response is largest for peak orographic heights below 3000 m, which indicates that the exact representation of the Andes in numerical models might not be as critical as that of lower orography such as that in southern Africa.
