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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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Edmund K. M. Chang

Abstract

The resonance between a neutral Eady mode and a continuous mode with the same phase speed is used to interpret the growth of neutral modes discussed by Farrell. It is shown that the existence of such a resonance may lead to spurious linear growth of error in a discretized quasigeostrophic (QG) model if one of the levels lies very close to the steering level of the neutral Eady normal mode. For a primitive equation model, this spurious resonance manifests itself as an exponentially growing-decaying pair of short waves. Such waves are similar to those found by Arakawa and Moorthi for the Lorenz grid for the QG equations. However, here it is found that for the primitive equation model, these spurious short waves exist both for the Lorenz grid and the Charney–Phillips (CP) grid, although the growth rate is somewhat smaller for the CP grid.

It is also shown that the growth rate of the nongeostrophic short waves discussed by Stone is erroneously overestimated in vertically discretized models.

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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

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.

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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

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

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

In this study, reanalysis data produced by the European Centre for Medium-Range Weather Forecasts for 14 Southern Hemisphere (SH) summer seasons have been analyzed. All cases of hemispheric transient eddy kinetic energy (TEKE) maxima have been identified, and the evolution of the local energetics and planetary-scale flow anomalies accompanying these TEKE growth/decay episodes are composited.

The longitude–time evolution of the composite energetics shows the clear signature of a wave packet propagating eastward at a group velocity of about 27° longitude per day and undergoing a life cycle of growth and decay, with the energetics within a volume close to the wave packet center dominating the hemispheric mean energetics. When individual cases are examined, 52% are found to resemble the composite and have the energetics life cycle dominated by the evolution of a single wave packet, and an additional 21% are found to be dominated by the evolution of two wave packets having similar amplitudes. Only the remaining 27% can be regarded as having experienced TEKE growth and decay throughout much of the hemisphere.

The zonal mean flow and eddy feedback anomalies (i.e., reduction in the meridional temperature gradient due to the effects of the eddy heat fluxes, as well as increase in the barotropic shear due to a narrowing of the midlatitude jet through the effects of the eddy momentum fluxes) associated with the cases dominated by the evolution of a single wave packet are also found to be dominated by anomalies close to the wave packet center.

The fact that hemispheric wave growth/decay is often dominated by the evolution of a single wave packet has interesting dynamical consequences when the climatological basic flow is not zonally symmetric. When a wave packet propagates over regions of enhanced baroclinicity, it can extract more energy from the mean flow via baroclinic conversion, leading to its preferential growth. On the other hand, when a wave packet propagates over regions of weak baroclinicity, baroclinic conversion is suppressed; hence any packet growth must be due to other processes. By examining the location of wave packet peaks when hemispheric TEKE is at a maximum, it is observed that hemispheric mean TEKE peaks much more frequently when the dominant wave packet is located downstream of the region with strongest baroclinicity. In addition, the growth in TEKE for these cases is usually dominated by an increase in baroclinic conversion. In contrast, for the small number of cases in which the hemispheric mean TEKE maximum occurs when the dominant wave packet is located downstream of the region with weakest baroclinicity, the growth of the hemispheric TEKE is instead dominated by a reduction in barotropic dissipation.

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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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