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Edmund K. M. Chang

Abstract

In this paper, the mean meridional circulation (MMC) forced by eddy fluxes of heat and momentum is examined using a simple quasigeostrophic, two-level model of the zonal-mean atmosphere. Analytic solutions have been obtained, which show that analyses of the eddy-induced MMC using the Kuo-Eliassen equation are most appropriate for high-frequency fluctuations. For steady-state or low-frequency fluctuations, the eddy fluxes will lead to changes in the zonal-mean zonal wind and temperature. These changes in the zonal-mean state will induce changes in frictional dissipation and diabatic heating, which (together with the eddy fluxes) are constrained to satisfy a generalization of the Eliassen-Palm theorem and will also act as source terms to the Kuo-Eliassen equation. The inclusion of these induced terms usually leads to a significant enhancement in the diagnosed intensity of the MMC. This can explain why previous studies of the MMC found a much weaker eddy-induced Ferrel cell than that observed when the induced frictional and diabatic heating terms were left out and the eddy fluxes only were used as source terms. The relevant timescale separating the high- and low-frequency limits is found to be the radiative timescale in the model.

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Edmund K. M. Chang

Abstract

Recent studies, based largely on analyses of reanalysis datasets, suggest that the Northern Hemisphere winter storm track activity has increased significantly during the second half of the twentieth century. In this study, this increasing trend, in terms of filtered mean sea level pressure (MSLP) variance statistics, is assessed using surface ship observations and a statistical storm track model.

MSLP observations made by ships, archived as part of the reanalysis project conducted by the National Centers for Environmental Prediction–National Center for Atmospheric Research, have been analyzed. Observational errors are estimated by comparing reports of nearly collocated observations. Consistent with previous studies, the observational errors of ship pressure observations are found to be very large during the late 1960s and early 1970s. Without correcting for observational errors, the storm track activity over the Pacific, computed based on ship observations, is found to be decreasing with time, while the upward trend in the Atlantic is much smaller than that found in the reanalysis data. Even after corrections have been made to account for secular changes in observational error statistics, the ship-based trend in the Pacific is still found to be much smaller than that found in the reanalysis, while over the Atlantic, the corrected ship-based trend is consistent with that found in the reanalysis.

The robustness of the results is tested by application of data trimming based on the reanalysis products. Ship observations that are different from the reanalysis by more than a prescribed limit are removed before the statistics are computed. As the prescribed limit is reduced from 30 to 2.5 hPa, the ship-based storm track activity becomes increasingly consistent with that based on the reanalysis. However, even when the smallest limit is used, the trends computed from the ship observations are still smaller than those computed from the reanalysis, strongly suggesting that the trends in the reanalysis are biased high. Nevertheless, the results suggest that decadal-scale variability of the Atlantic storm track activity is not very sensitive to the trimming limit, while results for the Pacific storm track are not as robust.

As an independent corroboration of the ship observation results, a statistical model is used to test whether the storm track trend found in the reanalysis is dynamically consistent with observed mean flow change. Five hundred winters of GCM simulations are used to construct a linear model based on canonical correlation analysis (CCA), using monthly mean distribution of MSLP anomalies as a predictor to hindcast monthly mean MSLP variance. The Atlantic storm track in the CCA model hindcast is well correlated with the storm track in the reanalysis on both interannual and decadal time scales, with the hindcast trend being 82% of that found in the reanalysis. Over the Pacific, the CCA hindcast does not perform as well, and the hindcast trend is only 32% of that found in the reanalysis.

The results of this study suggest that the actual trend in Pacific storm track activity is probably only about 20%–60% of that found in the reanalysis, while over the Atlantic, the actual trend is likely to be about 70%–80% of that found in the reanalysis. Two new basinwide storm track indices, which should contain less bias in the secular trends, have been defined based mainly on ship observations.

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Edmund K. M. Chang

Abstract

In this paper, the three-dimensional structure of baroclinic wave packets is studied using composites from ECMWF reanalysis data. Eddy variances and covariances associated with wave packets are examined in terms of the eddy energy budget and wave activity and fluxes, and the large-scale, phase-independent flow component associated with wave packets is studied using composites of the low zonal wavenumber component of the flow. The results suggest that the sources of upper-level eddy energy–wave activity associated with the wave packets are located over the central and upstream portion of the wave packet, and the downstream part of the wave packet is entirely maintained by radiation of energy–wave activity fluxes from the upstream end. The results are entirely consistent with previous studies of the energetics of individual wave evolution cases. The composites also show that the basic flow response to eddy transports associated with the wave packets consists mainly of a poleward shift of the jet near the center of the wave packet.

One of the goals of this study is to find out how the structure of wave packets is maintained against the linear tendency of dispersion. In the 10 Southern Hemisphere summer seasons examined, between two and six coherent wave packets that basically maintain their structure for at least 7 days are found for each season. However, previous hypotheses that the upstream ends of wave packets are stabilized by enhanced barotropic dissipation due to the formation of a barotropic jet, or that wave packets are maintained by nonlinear self-focusing of wave activity, are not supported by the data analyses. The results presented here suggest that alternative mechanisms will be needed to explain the zonal confinement of wave packets against the linear tendency of lateral spreading.

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Edmund K. M. Chang

Abstract

Extratropical cyclones are responsible for much of the extreme weather in the midlatitudes; thus, how these cyclones may change under increasing greenhouse gas forcing is of much general interest. Previous studies have suggested a poleward shift in the location of these cyclones, but how the intensity may change remains uncertain, especially in terms of maximum wind speed. In this study, projected changes in extreme cyclones in the Southern Hemisphere, based on 26 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), are presented. Multiple definitions of extreme cyclones have been examined, including intensity exceeding constant thresholds of sea level pressure perturbations, 850-hPa vorticity, and 850-hPa winds, as well as variable thresholds corresponding to a top-5 or top-1 cyclone per winter month in these three parameters and the near-surface winds. Results presented show that CMIP5 models project a significant increase in the frequency of extreme cyclones in all four seasons regardless of the definition, with over 88% of the models projecting an increase. Spatial patterns of increase are also consistent, with the largest increase projected between 45° and 60°S, extending from the South Atlantic across the south Indian Ocean into the Pacific. The projected increases in cyclone statistics are consistent with those in Eulerian statistics, such as sea level pressure (SLP) variance. However, while the projected increase in SLP variance can be linked to increase in the mean available potential energy (MAPE), the increases in cyclone statistics are not well correlated with those in MAPE.

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Edmund K. M. Chang

Abstract

In this paper, a nonlinear dry model, forced by fixed radiative forcing alone, has been constructed to simulate the Northern Hemisphere winter storm tracks. A procedure has been devised to iterate the radiative equilibrium temperature profile such that at the end of the iterations the model climate closely resembles the desired target climate.

This iterative approach is applied to simulate the climatological storm tracks in January. It is found that, when the three-dimensional temperature distribution in the model resembles the observed distribution, the model storm tracks are much too weak. It is hypothesized that this is due to the fact that eddy development is suppressed in a dry atmosphere, owing to the lack of latent heat release in the ascending warm air. To obtain storm tracks with realistic amplitudes, the static stability of the target climate is reduced to simulate the enhancement in baroclinic energy conversion due to latent heat release. With this modification, the storm tracks in the model simulation closely resemble those observed except that the strength of the Atlantic storm track is slightly weaker than observed. The model, when used as a forecast model, also gives high-quality forecasts of the evolution of observed eddies.

The iterative approach is applied to force the model to simulate climate anomalies associated with ENSO and the interannual variations of the winter Pacific jet stream/storm tracks. The results show that the model not only succeeds in simulating the climatology of storm tracks, but also produces realistic simulations of storm track anomalies when the model climate is forced to resemble observed climate anomalies.

An extended run of the control experiment is conducted to generate monthly mean flow and storm track statistics. These statistics are used to build a linear statistical model relating storm track anomalies to mean flow anomalies. This model performs well when used to hindcast observed storm track anomalies based on observed mean flow anomalies, showing that the storm track/mean flow covariability in the model is realistic and that storm track distribution is not sensitive to the exact form of the applied forcings.

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Edmund Kar-Man Chang

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.

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Edmund K. M. Chang

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.

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Edmund K. M. Chang

Abstract

Projections of storm-track changes over the continental United States and southern Canada made by 23 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) have been compared to changes projected by 11 models from phase 3 of CMIP (CMIP3). Overall, under representative concentration pathway 8.5 (RCP8.5) forcing, CMIP5 models project much more significant decreases in North American storm-track activity than CMIP3 models under the Special Report on Emission Scenarios (SRES) A2 scenario, with the largest decrease in summer and the smallest decrease in spring. The decrease is found both in temporal variance and cyclone statistics, with the frequency of strong cyclones projected to decrease by 15.9%, 6.6%, 32.6%, and 16.9% for winter, spring, summer, and fall, respectively. There is a strong consensus among the 23 models regarding the sign of the projected change, with less than 20% of the models projecting changes in the opposite sign in any of the storm-track parameters examined. Nevertheless, there are also significant model-to-model differences in the magnitude of the projected changes.

Projected changes in mean flow baroclinicity have also been examined. Model-to-model differences in the projected storm-track change are found to correlate significantly with model-to-model differences in the projected change in a locally defined mean available potential energy (MAPE) across the ensemble of 34 CMIP5 and CMIP3 models, suggesting that the differences in the projected change in local MAPE can partly account for not only the model-to-model differences but also the differences between CMIP5 and CMIP3 projections. Examination of projected precipitation change suggests that models projecting larger decrease in North American storm-track activity also project a farther northward intrusion of the decrease in subtropical precipitation.

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Edmund K. M. Chang

Abstract

Previous work has found that the Pacific storm track intensity during the cool season is negatively correlated with the upper-tropospheric jet strength. In the seasonal march, such a variation manifests itself as the midwinter suppression of the storm track intensity, while in interannual variations the storm track intensity during midwinter is found to be weaker during years in which the Pacific jet is particularly strong. In this paper, GCM simulations and observational data have been analyzed to shed light on the physical mechanisms responsible for such variations.

By examining the eddy energy budget and eddy structure, two different mechanisms have been found to contribute to the reduction in storm track activity associated with increases in jet intensity and baroclinicity in midwinter. For the seasonal variations, it was found that the major difference between fall and midwinter lies in the changing role of diabatic heating. During fall and spring, diabatic heating acts to generate eddy potential energy, while during midwinter its effect is to dissipate eddy energy. In the GCM simulations, this changing role is found to be due largely to changes in contributions from condensational heating, but NCEP–NCAR reanalysis data suggest that seasonal variations in surface sensible heat fluxes may also play a role.

For the interannual variations, the analyses suggest that the reduction in storm track intensity associated with increasing jet intensity during midwinter is due to changes in eddy structure. Eddies are found to be less efficient in converting mean potential energy to eddy potential energy due to being trapped closer to the tropopause and having relatively weak low-level circulations during the strong jet months. Such a change in eddy structure is also consistent with the finding that the group velocity of eddy propagation is much enhanced during those months. Both the decrease in baroclinic generation and increase in group velocity contribute to the slower downstream growth in eddy energy during the strong jet months despite higher baroclinicity.

For both cases, there is no evidence that changes in barotropic dissipation contributes to the suppression of storm track activity.

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Edmund K. M. Chang

Abstract

Unassimilated observational data, from the years 1979 to 1993, are analyzed and compared to the NCEP–NCAR reanalysis data to confirm the magnitude of the seasonal cycle in Pacific storm track activity. Such a comparison is necessary since recent publications have indicated that the reanalysis data may contain biases and spurious jumps that may affect climate signals, and eddy variance/covariance statistics are particularly sensitive to model biases, as well as changes in data coverage and quality.

High-pass-filtered variance in the 250-hPa meridional wind component is taken to be the indicator of upper-tropospheric storm track intensity. Rawinsonde observations located mainly over the land masses, and aircraft observations over the oceans, both display seasonal cycles very similar to that seen in the reanalysis data. Both the midwinter suppression in Pacific storm track activity, as well as the more recent finding that the Pacific storm track is significantly stronger during the late 1980s and early 1990s than during the early 1980s, are confirmed.

Complications involved in computing variance statistics from raw aircraft observations, as well as preliminary comparisons between aircraft and rawinsonde observations, are discussed in the appendixes.

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