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Abstract
In this study, 19 simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) have been analyzed to examine how winter cyclones producing extreme near-surface winds are projected to change. Extreme wind thresholds correspond to a top 5 or top 1 cyclone per winter month in the entire Northern Hemisphere (NH). The results show that CMIP5 models project a significant decrease in the number of such cyclones, with a 19-model mean decrease of about 17% for the entire NH toward the end of the twenty-first century, under the high-emission RCP8.5 scenario. The projected decrease is larger in the Atlantic (about 21%). Over the Pacific, apart from an overall decrease (about 13%), there is a northeastward shift in the extreme cyclone activity. Less decrease is found in the frequency of cyclones producing extreme winds at 850 hPa (about 5% hemisphere-wide), with models mainly projecting a northeastward shift in the Pacific. These results suggest that 850-hPa wind changes may not be a good proxy for near-surface wind changes. These results contrast with those for the Southern Hemisphere, in which the frequency of cyclones with extreme winds are projected to significantly increase in all four seasons.
Abstract
In this study, 19 simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) have been analyzed to examine how winter cyclones producing extreme near-surface winds are projected to change. Extreme wind thresholds correspond to a top 5 or top 1 cyclone per winter month in the entire Northern Hemisphere (NH). The results show that CMIP5 models project a significant decrease in the number of such cyclones, with a 19-model mean decrease of about 17% for the entire NH toward the end of the twenty-first century, under the high-emission RCP8.5 scenario. The projected decrease is larger in the Atlantic (about 21%). Over the Pacific, apart from an overall decrease (about 13%), there is a northeastward shift in the extreme cyclone activity. Less decrease is found in the frequency of cyclones producing extreme winds at 850 hPa (about 5% hemisphere-wide), with models mainly projecting a northeastward shift in the Pacific. These results suggest that 850-hPa wind changes may not be a good proxy for near-surface wind changes. These results contrast with those for the Southern Hemisphere, in which the frequency of cyclones with extreme winds are projected to significantly increase in all four seasons.
Abstract
In this paper, ECMWF 40-yr reanalysis data have been examined to study the influence of upper-level wave packets propagating across Asia into the Pacific on surface cyclone development over the Pacific. Previous studies have shown that in winter, wave packets propagate across Asia over two branches—a northern branch over Siberia and a southern branch along the subtropical jet across southern Asia. Results presented here show that subsequent to the presence of wave packets on either branch, the frequency of occurrence of deep cyclones (defined as cyclones with central pressure below 960 hPa), as well as explosively deepening cyclones (those with a deepening rate of 1 Bergeron or more), are significantly enhanced. This enhancement also clearly follows the wave packet eastward as it propagates across the Pacific.
Wave packets from the two branches are found to interfere with each other, such that if wave packets of the appropriate configuration are present on both the northern and southern branch, subsequent surface cyclone development over the western Pacific is further enhanced. Examination of the evolution of the anomalies suggests that these interferences can largely be explained by linear superposition of wave packets from the two branches.
Examination of the evolution of the composite structure of wave packets that are followed by the development of a significant surface cyclone indicates that cyclones that develop as the northern packet propagates into the Pacific are phase locked with the upper-level trough and maintain a favorable westward tilt with height throughout their development, consistent with the hypothesis that cyclogenesis is triggered by the approach of the wave packet. In contrast, significant cyclones whose development are influenced by the southern packets initially develop west of the upper-level trough, and propagate eastward with a phase speed that is much faster than that of the upper-level trough, attaining a westward phase tilt with height only at the mature stage, suggesting that cyclogenesis for these cases is probably not triggered by the wave packet.
Abstract
In this paper, ECMWF 40-yr reanalysis data have been examined to study the influence of upper-level wave packets propagating across Asia into the Pacific on surface cyclone development over the Pacific. Previous studies have shown that in winter, wave packets propagate across Asia over two branches—a northern branch over Siberia and a southern branch along the subtropical jet across southern Asia. Results presented here show that subsequent to the presence of wave packets on either branch, the frequency of occurrence of deep cyclones (defined as cyclones with central pressure below 960 hPa), as well as explosively deepening cyclones (those with a deepening rate of 1 Bergeron or more), are significantly enhanced. This enhancement also clearly follows the wave packet eastward as it propagates across the Pacific.
Wave packets from the two branches are found to interfere with each other, such that if wave packets of the appropriate configuration are present on both the northern and southern branch, subsequent surface cyclone development over the western Pacific is further enhanced. Examination of the evolution of the anomalies suggests that these interferences can largely be explained by linear superposition of wave packets from the two branches.
Examination of the evolution of the composite structure of wave packets that are followed by the development of a significant surface cyclone indicates that cyclones that develop as the northern packet propagates into the Pacific are phase locked with the upper-level trough and maintain a favorable westward tilt with height throughout their development, consistent with the hypothesis that cyclogenesis is triggered by the approach of the wave packet. In contrast, significant cyclones whose development are influenced by the southern packets initially develop west of the upper-level trough, and propagate eastward with a phase speed that is much faster than that of the upper-level trough, attaining a westward phase tilt with height only at the mature stage, suggesting that cyclogenesis for these cases is probably not triggered by the wave packet.
Abstract
In this paper, reanalysis data for the Southern Hemisphere summer season of 1984/85, produced by the European Centre for Medium-Range Weather Forecasts, have been analyzed to examine wave packets and life cycles of baroclinic waves. A semiobjective algorithm has been devised to track wave packets. It is found that the perturbations in the upper troposphere are dominated by wave packets. Some of these wave packets can be tracked for several weeks, during which they can make up to two trips around the latitude circle. The energy life cycles of waves that make up several of these wave packets were analyzed, and apart from the incipient waves that started the wave packets, most of the subsequent wave development was found to be dominated by convergence and divergence of energy fluxes rather than due to baroclinic or barotropic conversions, demonstrating that these wave packets are indeed coherent entities that propagate due to downstream development (or dispersion). The energy life cycle of the most significant waves in terms of total energy or height fall during the entire season have also been examined, and the results show that during that season, approximately two-thirds of the most significant wave cases had developed due to downstream development, with the remaining ones developed due to baroclinic or barotropic growth. In addition, during the decay phase of these waves, again about two-thirds of the cases are dominated by downstream development rather than barotropic decay or dissipation. These results show that a majority of the most significant waves in that season were associated with coherent wave packets. In nearly all cases of upper-level trough development associated with these downstream developing wave packets that were studied, a surface cyclone can be identified developing just to the east of the upper-level trough, suggesting that surface cyclogenesis is frequently caused by the approach of an upper-level wave packet.
Abstract
In this paper, reanalysis data for the Southern Hemisphere summer season of 1984/85, produced by the European Centre for Medium-Range Weather Forecasts, have been analyzed to examine wave packets and life cycles of baroclinic waves. A semiobjective algorithm has been devised to track wave packets. It is found that the perturbations in the upper troposphere are dominated by wave packets. Some of these wave packets can be tracked for several weeks, during which they can make up to two trips around the latitude circle. The energy life cycles of waves that make up several of these wave packets were analyzed, and apart from the incipient waves that started the wave packets, most of the subsequent wave development was found to be dominated by convergence and divergence of energy fluxes rather than due to baroclinic or barotropic conversions, demonstrating that these wave packets are indeed coherent entities that propagate due to downstream development (or dispersion). The energy life cycle of the most significant waves in terms of total energy or height fall during the entire season have also been examined, and the results show that during that season, approximately two-thirds of the most significant wave cases had developed due to downstream development, with the remaining ones developed due to baroclinic or barotropic growth. In addition, during the decay phase of these waves, again about two-thirds of the cases are dominated by downstream development rather than barotropic decay or dissipation. These results show that a majority of the most significant waves in that season were associated with coherent wave packets. In nearly all cases of upper-level trough development associated with these downstream developing wave packets that were studied, a surface cyclone can be identified developing just to the east of the upper-level trough, suggesting that surface cyclogenesis is frequently caused by the approach of an upper-level wave packet.
Abstract
Gridded 300-hPa meridional wind data produced by the ECMWF reanalysis project were analyzed to document the seasonal and hemispheric variations in the properties of upper-tropospheric wave packets. The properties of the wave packets are mainly illustrated using time-lagged one-point correlation maps performed on υ′. Based on indices that show the coherence of wave propagation, as well as examination of correlation maps, schematic waveguides were constructed for the summer and winter seasons of both hemispheres along which waves preferentially propagate with greatest coherence. In the summers, the waveguides basically follow the position of the midlatitude jets. In the Northern Hemisphere winter, the primary waveguide follows the subtropical jet over southern Asia into the Pacific, but there is a secondary branch running across Russia, joining the primary waveguide near the entrance to the Pacific storm track. Over the Atlantic, the waveguide passes east-southeastward toward North Africa, then back to southern Asia. During the Southern Hemisphere winter, the primary waveguide splits in two around 70°E, with the primary (more coherent) branch deviating equatorward to join up with the subtropical waveguide, and a secondary branch spiraling poleward along with the subpolar jet and storm track maxima. Wave packet envelopes were also defined and group velocities of wave packets were computed based on correlations performed on packet envelopes. These group velocities were found to agree qualitatively with those defined previously based on wave activity fluxes.
By examining the wave coherence indices, as well as individual correlation maps and Hovmöller diagrams of correlations computed along the primary waveguides, it was concluded that wave propagation is least coherent in Northern Hemisphere summer, and that waves in Southern Hemisphere summer are not necessarily more coherent than those in Southern Hemisphere winter. Data from a GCM experiment were also analyzed and showed that wave packets in the GCM also display such a seasonal variation in coherence. Results from experiments using an idealized model suggest that coherence of wave packets depends not only on the baroclinicity of the large-scale flow, but also on the intensity of the Hadley circulation, which acts to tighten the upper-tropospheric potential vorticity gradient.
Abstract
Gridded 300-hPa meridional wind data produced by the ECMWF reanalysis project were analyzed to document the seasonal and hemispheric variations in the properties of upper-tropospheric wave packets. The properties of the wave packets are mainly illustrated using time-lagged one-point correlation maps performed on υ′. Based on indices that show the coherence of wave propagation, as well as examination of correlation maps, schematic waveguides were constructed for the summer and winter seasons of both hemispheres along which waves preferentially propagate with greatest coherence. In the summers, the waveguides basically follow the position of the midlatitude jets. In the Northern Hemisphere winter, the primary waveguide follows the subtropical jet over southern Asia into the Pacific, but there is a secondary branch running across Russia, joining the primary waveguide near the entrance to the Pacific storm track. Over the Atlantic, the waveguide passes east-southeastward toward North Africa, then back to southern Asia. During the Southern Hemisphere winter, the primary waveguide splits in two around 70°E, with the primary (more coherent) branch deviating equatorward to join up with the subtropical waveguide, and a secondary branch spiraling poleward along with the subpolar jet and storm track maxima. Wave packet envelopes were also defined and group velocities of wave packets were computed based on correlations performed on packet envelopes. These group velocities were found to agree qualitatively with those defined previously based on wave activity fluxes.
By examining the wave coherence indices, as well as individual correlation maps and Hovmöller diagrams of correlations computed along the primary waveguides, it was concluded that wave propagation is least coherent in Northern Hemisphere summer, and that waves in Southern Hemisphere summer are not necessarily more coherent than those in Southern Hemisphere winter. Data from a GCM experiment were also analyzed and showed that wave packets in the GCM also display such a seasonal variation in coherence. Results from experiments using an idealized model suggest that coherence of wave packets depends not only on the baroclinicity of the large-scale flow, but also on the intensity of the Hadley circulation, which acts to tighten the upper-tropospheric potential vorticity gradient.
Abstract
The eddy–zonal flow feedback in the Southern Hemisphere (SH) winter and summer is investigated in this study. The persistence time scale of the leading principal components (PCs) of the zonal-mean zonal flow shows substantial seasonal variation. In the SH summer, the persistence time scale of PC1 is significantly longer than that of PC2, while the persistence time scales of the two PCs are quite similar in the SH winter. A storm-track modeling approach is applied to demonstrate that seasonal variations of eddy–zonal flow feedback for PC1 and PC2 account for the seasonal variations of the persistence time scale. The eddy feedback time scale estimated from a storm-track model simulation and a wave-response model diagnostic shows that PC1 in June–August (JJA) and December–February (DJF), and PC2 in JJA, have significant positive eddy–mean flow feedback, while PC2 in DJF has no positive feedback. The consistency between the persistence and eddy feedback time scales for each PC suggests that the positive feedback increases the persistence of the corresponding PC, with stronger (weaker) positive feedback giving rise to a longer (shorter) persistence time scale.
Eliassen–Palm flux diagnostics have been performed to demonstrate the dynamics governing the positive feedback between eddies and anomalous zonal flow. The mechanism of the positive feedback, for PC1 in JJA and DJF and PC2 in JJA, is as follows: an enhanced baroclinic wave source (heat fluxes) at a low level in the region of positive wind anomalies propagates upward and then equatorward from the wave source, thus giving momentum fluxes that reinforce the wind anomalies. The difference of PC2 between DJF and JJA is because of the zonal asymmetry of the climatological flow in JJA. For PC2 in DJF, wind anomalies reinforce the climatological jet, thus increasing the barotropic shear of the jet flow. The “barotropic governor” plays an important role in suppressing eddy generations for PC2 in DJF and thus inhibiting the positive eddy–zonal flow feedback.
Abstract
The eddy–zonal flow feedback in the Southern Hemisphere (SH) winter and summer is investigated in this study. The persistence time scale of the leading principal components (PCs) of the zonal-mean zonal flow shows substantial seasonal variation. In the SH summer, the persistence time scale of PC1 is significantly longer than that of PC2, while the persistence time scales of the two PCs are quite similar in the SH winter. A storm-track modeling approach is applied to demonstrate that seasonal variations of eddy–zonal flow feedback for PC1 and PC2 account for the seasonal variations of the persistence time scale. The eddy feedback time scale estimated from a storm-track model simulation and a wave-response model diagnostic shows that PC1 in June–August (JJA) and December–February (DJF), and PC2 in JJA, have significant positive eddy–mean flow feedback, while PC2 in DJF has no positive feedback. The consistency between the persistence and eddy feedback time scales for each PC suggests that the positive feedback increases the persistence of the corresponding PC, with stronger (weaker) positive feedback giving rise to a longer (shorter) persistence time scale.
Eliassen–Palm flux diagnostics have been performed to demonstrate the dynamics governing the positive feedback between eddies and anomalous zonal flow. The mechanism of the positive feedback, for PC1 in JJA and DJF and PC2 in JJA, is as follows: an enhanced baroclinic wave source (heat fluxes) at a low level in the region of positive wind anomalies propagates upward and then equatorward from the wave source, thus giving momentum fluxes that reinforce the wind anomalies. The difference of PC2 between DJF and JJA is because of the zonal asymmetry of the climatological flow in JJA. For PC2 in DJF, wind anomalies reinforce the climatological jet, thus increasing the barotropic shear of the jet flow. The “barotropic governor” plays an important role in suppressing eddy generations for PC2 in DJF and thus inhibiting the positive eddy–zonal flow feedback.
Abstract
The leading mode of interannual variability of the midwinter Pacific storm track is such that the storm track is weaker during the winters when the Pacific jet is strong, and stronger when the jet is weak. In this paper, experiments are conducted using a stationary wave model as well as an idealized global circulation model to explore the roles of anomalous tropical heating and eddy fluxes in forcing the observed Pacific jet anomalies.
It is found that enhanced tropical heating over the region 60°E to the date line, 25°S to 25°N, acts to force a stronger and narrower Pacific jet. On average, tropical heating may account for about one-third of the strong jet anomaly, but there is significant year-to-year variability. Moreover, tropical heating does not appear to contribute to the weak jet anomaly. Much of the Pacific jet anomalies are forced by anomalous eddy fluxes. By examining the regional contributions from the Pacific, the Atlantic, and Asia, it is found that local eddy feedback over the Pacific only acts to force part of the stationary anomaly, while much of the signal is forced by remote eddy forcings from the Atlantic and Asia. Since significant parts of the jet anomalies are forced by anomalous tropical heating and remote eddy fluxes, it is concluded that the observed Pacific jet/storm-track variability is not a pure local wave–mean flow interaction mode internal to the Pacific basin.
Both stationary wave model diagnostics and idealized global circulation model experiments suggest that stronger eddy activity over the Atlantic may force a weaker Pacific jet and stronger Pacific eddies. On the other hand, changes in eddy activity over the Pacific may also act to force changes in the Atlantic storm track. There are also indications that tropical heating anomalies may force a simultaneous weakening of both storm tracks. These possibilities may be some of the factors behind the observed significant correlation between the Pacific and Atlantic storm tracks and should be further explored in more realistic GCM experiments.
Abstract
The leading mode of interannual variability of the midwinter Pacific storm track is such that the storm track is weaker during the winters when the Pacific jet is strong, and stronger when the jet is weak. In this paper, experiments are conducted using a stationary wave model as well as an idealized global circulation model to explore the roles of anomalous tropical heating and eddy fluxes in forcing the observed Pacific jet anomalies.
It is found that enhanced tropical heating over the region 60°E to the date line, 25°S to 25°N, acts to force a stronger and narrower Pacific jet. On average, tropical heating may account for about one-third of the strong jet anomaly, but there is significant year-to-year variability. Moreover, tropical heating does not appear to contribute to the weak jet anomaly. Much of the Pacific jet anomalies are forced by anomalous eddy fluxes. By examining the regional contributions from the Pacific, the Atlantic, and Asia, it is found that local eddy feedback over the Pacific only acts to force part of the stationary anomaly, while much of the signal is forced by remote eddy forcings from the Atlantic and Asia. Since significant parts of the jet anomalies are forced by anomalous tropical heating and remote eddy fluxes, it is concluded that the observed Pacific jet/storm-track variability is not a pure local wave–mean flow interaction mode internal to the Pacific basin.
Both stationary wave model diagnostics and idealized global circulation model experiments suggest that stronger eddy activity over the Atlantic may force a weaker Pacific jet and stronger Pacific eddies. On the other hand, changes in eddy activity over the Pacific may also act to force changes in the Atlantic storm track. There are also indications that tropical heating anomalies may force a simultaneous weakening of both storm tracks. These possibilities may be some of the factors behind the observed significant correlation between the Pacific and Atlantic storm tracks and should be further explored in more realistic GCM experiments.
Abstract
The effects of variations in jet width on the downstream growth of baroclinic waves are studied, using a simple quasigeostrophic model with a vertically varying basic state and variable channel width, as well as a simplified primitive equation model with a basic state that varies in latitude and height. This study is motivated by observations that in midwinter in the Pacific the storm track is weaker and the jet is narrower during years when the jet is strong.
The linear models are able to reproduce the observed decrease of spatial growth rate with shear, if the narrowing of the jet is accounted for by assuming it decreases the meridional wavelength of the perturbations, which hampers their growth. A common suggestion has been that perturbations are weaker when the jet is strong because they move faster out of the unstable storm track region. The authors find that one needs to take into account that the jet narrows when it strengthens; otherwise, the increase of growth rate is strong enough to counteract the effect of increased advection speed.
It is also found that, when the model basic state is Eady-like (small or zero meridional potential vorticity gradients in the troposphere), the short-wave cutoff for instability moves to large-scale waves as shear is increased, due to the accompanying increase in meridional wavenumber. This results in a transition from a regime where upper-level perturbations spin up a surface circulation very rapidly, and normal-mode growth ensues, to a regime where the initial perturbations take a very long time to excite growth. Since waves slow down when a surface perturbation develops, this can explain the observations that the storm track perturbations are more “upper level” during strong jet years and their group velocities increase faster than linearly with shear.
Abstract
The effects of variations in jet width on the downstream growth of baroclinic waves are studied, using a simple quasigeostrophic model with a vertically varying basic state and variable channel width, as well as a simplified primitive equation model with a basic state that varies in latitude and height. This study is motivated by observations that in midwinter in the Pacific the storm track is weaker and the jet is narrower during years when the jet is strong.
The linear models are able to reproduce the observed decrease of spatial growth rate with shear, if the narrowing of the jet is accounted for by assuming it decreases the meridional wavelength of the perturbations, which hampers their growth. A common suggestion has been that perturbations are weaker when the jet is strong because they move faster out of the unstable storm track region. The authors find that one needs to take into account that the jet narrows when it strengthens; otherwise, the increase of growth rate is strong enough to counteract the effect of increased advection speed.
It is also found that, when the model basic state is Eady-like (small or zero meridional potential vorticity gradients in the troposphere), the short-wave cutoff for instability moves to large-scale waves as shear is increased, due to the accompanying increase in meridional wavenumber. This results in a transition from a regime where upper-level perturbations spin up a surface circulation very rapidly, and normal-mode growth ensues, to a regime where the initial perturbations take a very long time to excite growth. Since waves slow down when a surface perturbation develops, this can explain the observations that the storm track perturbations are more “upper level” during strong jet years and their group velocities increase faster than linearly with shear.
Abstract
Zonal index variations, or north–south shifts of the midlatitude jet, are the dominant mode of zonal wind variability in the Southern Hemisphere. Previous studies have shown that synoptic-time-scale eddy momentum flux provides a positive feedback and acts to increase the persistence and low-frequency variance of the zonal index. However, the impact of diabatic heating due to the precipitation associated with these eddies has not been investigated. In this study, regression analyses have been conducted to demonstrate that a robust precipitation anomaly can be found to accompany the jet and eddy momentum flux anomalies associated with a poleward shift of the jet, with enhanced precipitation on the poleward flank of the jet and reduced precipitation on the equatorward flank. Diabatic heating associated with such a precipitation anomaly is expected to reduce the temperature gradient across the jet anomaly, thus decreasing eddy generation and damping the anomaly. This expectation is confirmed by three sets of mechanistic model experiments, using three different ways to mimic the impact of moist heating in a dry model. Results of this study suggest that diabatic heating provides a negative feedback to zonal index variations, partially offsetting the positive feedback provided by eddy momentum flux. These results could partially explain why zonal index variations have been found to be very persistent in dry mechanistic model experiments since this negative diabatic feedback is absent in dry models. These results suggest that these models may be overly sensitive to climate forcings that produce a jet shift response.
Abstract
Zonal index variations, or north–south shifts of the midlatitude jet, are the dominant mode of zonal wind variability in the Southern Hemisphere. Previous studies have shown that synoptic-time-scale eddy momentum flux provides a positive feedback and acts to increase the persistence and low-frequency variance of the zonal index. However, the impact of diabatic heating due to the precipitation associated with these eddies has not been investigated. In this study, regression analyses have been conducted to demonstrate that a robust precipitation anomaly can be found to accompany the jet and eddy momentum flux anomalies associated with a poleward shift of the jet, with enhanced precipitation on the poleward flank of the jet and reduced precipitation on the equatorward flank. Diabatic heating associated with such a precipitation anomaly is expected to reduce the temperature gradient across the jet anomaly, thus decreasing eddy generation and damping the anomaly. This expectation is confirmed by three sets of mechanistic model experiments, using three different ways to mimic the impact of moist heating in a dry model. Results of this study suggest that diabatic heating provides a negative feedback to zonal index variations, partially offsetting the positive feedback provided by eddy momentum flux. These results could partially explain why zonal index variations have been found to be very persistent in dry mechanistic model experiments since this negative diabatic feedback is absent in dry models. These results suggest that these models may be overly sensitive to climate forcings that produce a jet shift response.
Abstract
A new split-jet index is defined in this study, and composites based on this index show that the split-flow regime is characterized by a cold–warm–cold tripolar temperature anomaly in the South Pacific that extends equatorward from the Southern Hemisphere (SH) high latitudes, while nonsplit flow occurs when the phase of the tripolar temperature anomaly is reversed. Analyses of the heat budget reveal that the temperature anomalies associated with the split/nonsplit flow are mainly forced by mean flow advection instead of local diabatic heating or convergence of eddy heat fluxes. Localized Eliassen–Palm (E–P) flux diagnostics suggest that the zonal wind anomalies are maintained by the eddy vorticity flux anomalies.
These diagnostic results are confirmed by numerical experiments conducted using a stationary wave model forced by observed eddy forcings and diabatic heating anomalies. The model results show that the effects of the vorticity flux dominates over those of the heat flux, which tend to dampen the flow anomalies, and that tropical diabatic heating anomalies are not important in maintaining the split-/nonsplit-flow anomalies.
The organization of high-frequency eddies by the low-frequency split/nonsplit jet is also studied. Two sets of experiments using a linear storm-track model initialized with random initial perturbations superposed upon the split- and nonsplit-jet basic state, respectively, have been conducted. Model results show that the storm-track anomalies that are organized by the split/nonsplit jet are consistent with observed storm-track anomalies, thus demonstrating that the low-frequency split/nonsplit jet acts to organize the high-frequency eddies.
The results of this paper directly establish that there is a two-way reinforcement between eddies and mean flow anomalies in the low-frequency variability of the SH winter split jet.
Abstract
A new split-jet index is defined in this study, and composites based on this index show that the split-flow regime is characterized by a cold–warm–cold tripolar temperature anomaly in the South Pacific that extends equatorward from the Southern Hemisphere (SH) high latitudes, while nonsplit flow occurs when the phase of the tripolar temperature anomaly is reversed. Analyses of the heat budget reveal that the temperature anomalies associated with the split/nonsplit flow are mainly forced by mean flow advection instead of local diabatic heating or convergence of eddy heat fluxes. Localized Eliassen–Palm (E–P) flux diagnostics suggest that the zonal wind anomalies are maintained by the eddy vorticity flux anomalies.
These diagnostic results are confirmed by numerical experiments conducted using a stationary wave model forced by observed eddy forcings and diabatic heating anomalies. The model results show that the effects of the vorticity flux dominates over those of the heat flux, which tend to dampen the flow anomalies, and that tropical diabatic heating anomalies are not important in maintaining the split-/nonsplit-flow anomalies.
The organization of high-frequency eddies by the low-frequency split/nonsplit jet is also studied. Two sets of experiments using a linear storm-track model initialized with random initial perturbations superposed upon the split- and nonsplit-jet basic state, respectively, have been conducted. Model results show that the storm-track anomalies that are organized by the split/nonsplit jet are consistent with observed storm-track anomalies, thus demonstrating that the low-frequency split/nonsplit jet acts to organize the high-frequency eddies.
The results of this paper directly establish that there is a two-way reinforcement between eddies and mean flow anomalies in the low-frequency variability of the SH winter split jet.
Abstract
The seasonal cycles in the distribution of precipitation around the western North Pacific and Atlantic cyclones have been examined by compositing quantitative estimates of the precipitation rate relative to cyclone centers. The precipitation data sources considered include estimates produced by the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) project, the satellite-based daily precipitation estimates produced by the Global Precipitation Climatology Project, and estimates derived based on observed weather reports contained in the Comprehensive Ocean–Atmosphere Data Set (COADS).
Results from all three datasets suggest that for Pacific cyclones, substantially more precipitation is found in the warm sector in fall than in winter and less precipitation is found behind the cold front in spring and summer than in winter. The seasonal cycle for Atlantic cyclones is found to be distinctly different. The distribution in precipitation around cyclones in fall and winter are not very different, while in spring and summer less precipitation is found over much of the cyclone.
The implications for the observed seasonal cycles are discussed. The seasonal cycle for Pacific cyclones suggests that diabatic contributions to the generation of eddy available potential energy (APE) due to latent heat release should be maximal in fall with a relative minimum in midwinter, while for Atlantic cyclones diabatic generation of eddy APE in fall and winter is nearly the same. This is suggested to be one of the factors that can contribute to the observed midwinter minimum in the Pacific storm track, and the absence of such a minimum in the Atlantic.
Possible reasons contributing to the differences in the seasonal cycle between the two basins are discussed. Preliminary analyses suggest that differences in static stability, availability of moisture, as well as dynamical forcing may all be contributing factors.
Issues on estimating rates of precipitation based on ship reports are addressed in . It is argued that it may be a good time to recalibrate existing schemes.
Abstract
The seasonal cycles in the distribution of precipitation around the western North Pacific and Atlantic cyclones have been examined by compositing quantitative estimates of the precipitation rate relative to cyclone centers. The precipitation data sources considered include estimates produced by the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) project, the satellite-based daily precipitation estimates produced by the Global Precipitation Climatology Project, and estimates derived based on observed weather reports contained in the Comprehensive Ocean–Atmosphere Data Set (COADS).
Results from all three datasets suggest that for Pacific cyclones, substantially more precipitation is found in the warm sector in fall than in winter and less precipitation is found behind the cold front in spring and summer than in winter. The seasonal cycle for Atlantic cyclones is found to be distinctly different. The distribution in precipitation around cyclones in fall and winter are not very different, while in spring and summer less precipitation is found over much of the cyclone.
The implications for the observed seasonal cycles are discussed. The seasonal cycle for Pacific cyclones suggests that diabatic contributions to the generation of eddy available potential energy (APE) due to latent heat release should be maximal in fall with a relative minimum in midwinter, while for Atlantic cyclones diabatic generation of eddy APE in fall and winter is nearly the same. This is suggested to be one of the factors that can contribute to the observed midwinter minimum in the Pacific storm track, and the absence of such a minimum in the Atlantic.
Possible reasons contributing to the differences in the seasonal cycle between the two basins are discussed. Preliminary analyses suggest that differences in static stability, availability of moisture, as well as dynamical forcing may all be contributing factors.
Issues on estimating rates of precipitation based on ship reports are addressed in . It is argued that it may be a good time to recalibrate existing schemes.
