David J. Lorenz

Eddy-Zonal Flow Feedback in the Southern Hemisphere

Abstract The variability of the zonal-mean zonal wind in the Southern Hemisphere is studied using EOF analysis and momentum budget diagnostics of NCEP reanalysis data (1978–97). The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north–south movement of the midlatitude jet. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes accounts for the unusual persistence of EOF1 and plays an important role in the selection of the leading EOF of midlatitude variability. Further analysis also shows a propagating feedback, common to both EOF1 and EOF2, which is responsible for the poleward drift of wind anomalies with time. The observations support the following feedback mechanism. Anomalous baroclinic wave activity is generated at the latitude of anomalous temperature gradient that, by thermal wind, coincides with the latitude of the anomalous zonal jet. The net propagation of baroclinic...

Eddy–Zonal Flow Feedback in the Northern Hemisphere Winter

The variability of the zonal-mean zonal wind in the Northern Hemisphere winter (December‐March) is studied using EOF analysis and momentum budget diagnostics of NCEP‐NCAR reanalysis data (1976‐2001). The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north‐south movement of the midlatitude westerlies. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes selects the leading EOF of midlatitude variability. Like the Southern Hemisphere, the baroclinic eddies reinforce the zonal wind anomalies while external Rossby waves damp the wind anomalies. In the Northern Hemisphere, the quasi-stationary eddies also reinforce the zonal wind anomalies, but the baroclinic eddies are most important for the positive eddy‐zonal flow feedback. The observations support the following feedback mechanisms. 1) Above-normal baroclinic wave activity is generated in the region of enhanced westerlies. This leads to wave propagation out of the westerlies that is associated with reinforcing eddy momentum fluxes. 2) The westerly jet is a waveguide for external Rossby waves that tend to propagate into the jet and remove momentum from it. 3) The quasi-stationary waves respond to a refractive index anomaly in the high latitudes below the tropopause. During the high (low) index this anomaly is negative (positive) leading to an acceleration (deceleration) of the zonal wind in the high latitudes.

The effect of the MJO on the North American Monsoon

Abstract The effect of the Madden–Julian oscillation (MJO) in the eastern Pacific on the North American monsoon is documented using NCEP–NCAR reanalysis and daily mean precipitation data from 1958 to 2003. It is found that positive zonal wind anomalies in the eastern tropical Pacific lead to above-normal precipitation in northwest Mexico and Arizona from several days to over a week later. This connection between the tropical Pacific and monsoon precipitation appears to be limited to regions influenced by moisture surges from the Gulf of California as a similar connection does not exist for New Mexico precipitation. The evidence suggests that the MJO might affect monsoon precipitation by modulating the strength of low-level easterly waves off the coast of Mexico, which in turn triggers the development of a gulf surge.

The Response of the Extratropical Hydrological Cycle to Global Warming

Abstract The change in the hydrological cycle in the extratropics under global warming is studied using the climate models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. The changes in hydrological quantities are analyzed with respect to the increases expected from the Clausius–Clapeyron (C–C) equation, which describes the rate of increase of a hydrological quantity per temperature increase. The column-integrated water vapor increases at a rate close to the C–C rate, which is expected if relative humidity remains nearly constant. The poleward moisture transport and the precipitation increase with temperature at a rate less than the C–C rate, with the precipitation increasing the least. In addition, the intermodel variance of poleward moisture transport and precipitation is explained significantly better when the zonal-mean zonal wind change as well as the temperature change is taken into account. The percent increase in precipitation per temperature increas...

The Signature of the Annular Modes in the Tropical Troposphere

The extratropical annular modes are coupled with a distinct pattern of climate anomalies that spans the circulation of the tropical troposphere. The signature of the annular modes in the tropical troposphere exhibits a high degree of equatorial symmetry. It is associated with upper-tropospheric zonal wind anomalies centered about the equator, midtropospheric temperature anomalies located ;208N and 208S, and opposing mean meridional circulation anomalies that span the subtropics of both hemispheres. The linkages between the annular modes and the tropical circulation are only evident during the cold season months, and are most robust in association with the Northern Hemisphere annular mode (NAM). The coupling between the annular modes and the circulation of the tropical troposphere is consistent with forcing by waves originating at extratropical latitudes. Both annular modes are characterized by anomalies in the eddy momentum flux convergence at tropical latitudes that act to reinforce the changes in the tropical wind and temperature fields. The most pronounced tropical anomalies lag indices of the annular modes by;2 weeks and are found over the eastern tropical Pacific, where climatological westerlies permit extratropical waves to propagate into the deep Tropics. The linkages between the NAM and the tropical tropospheric circulation are

Twenty-First-Century Projections of Snowfall and Winter Severity across Central-Eastern North America*,+

AbstractStatistically downscaled climate projections from nine global climate models (GCMs) are used to force a snow accumulation and ablation model (SNOW-17) across the central-eastern North American Landscape Conservation Cooperatives (LCCs) to develop high-resolution projections of snowfall, snow depth, and winter severity index (WSI) by the middle and late twenty-first century. Here, projections of a cumulative WSI (CWSI) known to influence autumn–winter waterfowl migration are used to demonstrate the utility of SNOW-17 results. The application of statistically downscaled climate data and a snow model leads to a better representation of lake processes in the Great Lakes basin, topographic effects in the Appalachian Mountains, and spatial patterns of climatological snowfall, compared to the original GCMs. Annual mean snowfall is simulated to decline across the region, particularly in early winter (December–January), leading to a delay in the mean onset of the snow season. Because of a warming-induced a...

Understanding Midlatitude Jet Variability and Change Using Rossby Wave Chromatography: Poleward-Shifted Jets in Response to External Forcing

Rossby wave chromatography (RWC) is implemented in a linearized barotropic model as a tool to understand the response of the midlatitude jet to external forcing. Given the background zonal-mean flow and the space‐time structure of the baroclinic wave activity source, RWC calculates the space‐time structure of the upper-tropospheric eddy momentum fluxes. RWC is used to diagnose and understand the poleward shift of the jet in an idealized GCM using the convergence of the vertical EP flux in the upper troposphere as the wave activity source. The poleward-shifted jet is maintained via a selective ‘‘reflecting level’’ on the poleward flank of jet: for a given wavenumber, low phase speed waves are reflected but high phase speed waves are absorbed at the critical level on the polewardflank of jet. When the zonal-mean zonal wind increases on the polewardflank of the jet, a wider range of poleward-propagating waves encounter a reflecting level instead of a critical level on the poleward flank. The increased wave reflection leads to increased equatorward-propagating waves (and, therefore, poleward momentum flux) across the jet. Increases in wave phase speeds directly oppose the poleward shift because, in addition to the well-recognized effect of phase speed on wave dissipation in the subtropics, increased phase speeds imply more wave dissipation rather than reflection on the poleward flank via the selective reflecting level.

Characterizing midlatitude jet variability : Lessons from a simple GCM

Fluctuations in the tropospheric zonal jet are often characterized using anomaly patterns, or empirical orthogonal functions, representing deviations of the zonal-mean flow from climatology. In previous studies the leading anomaly pattern has been interpreted as representing north–south jet movements, while the second anomaly pattern has been interpreted as representing independent fluctuations in jet strength and width. Here it is shown that these leading anomaly patterns are in fact dependent and together represent north–south movements of the jet. Fluctuations in jet strength, which are approximately inversely proportional to jet width, superimpose upon these dominant north–south meanderings. The distinction between the usual anomaly pattern perspective and this new perspective may have important implications in the interpretation of tropospheric zonal jet variability.

Diagnosing Northern Hemisphere Jet Portrayal in 17 CMIP3 Global Climate Models: Twenty-First-Century Projections

The anthropogenic climate change impacts on the eddy‐jet system include an intensified midlatitude jet stream and an elevated tropopause, as well as a poleward-shifted jet. While both responses are evident in phase 3 of the Coupled Model Intercomparison Project (CMIP3) ensemble mean twenty-first-century projections, uncertainty in the poleward shift response is large enough that even the sign of the shift is not consistent among all models, especially in the Northern Hemisphere. The present analysis finds that twentyfirst-century projections of the ensemble mean zonal wind change at 300 hPa predict a weakening and poleward expansion of the Pacific jet and an overall expansion of the Atlantic jet. In contrast with the direct zonal mean climate change signal of increasing midlatitude upper-level winds, zonal winds are projected to decreaseinthecoreofthePacificandAtlanticjets,withincreasingzonalwindslocatedprimarilyinthejetexit regions and the meridional flanks of the jets. Uncertainties in SST changes from the twentieth century to the twenty-first century between models are shown to impact modeled Northern Hemisphere jet stream changes.

Diagnosing Northern Hemisphere Jet Portrayal in 17 CMIP3 Global Climate Models: Twentieth-Century Intermodel Variability

AbstractThe present study focuses on diagnosing the intermodel variability of nonzonally averaged NH winter jet stream portrayal in 17 global climate models (GCMs) from phase three of the Coupled Model Intercomparison Project (CMIP3). Relative to the reanalysis, the ensemble-mean 300-hPa Atlantic jet is too zonally extended and located too far equatorward in GCMs. The Pacific jet varies significantly between modeling groups, with large biases in the vicinity of the jet exit region that cancel in the ensemble mean. After seeking relationships between twentieth-century model wind biases and 1) the internal modes of jet variability or 2) tropical sea surface temperatures (SSTs), it is found that biases in upper-level winds are strongly related to an ENSO-like pattern in winter-mean tropical Pacific Ocean SST biases. The spatial structure of the leading modes of variability of the upper-level jet in the twentieth century is found to be accurately modeled in all 17 GCMs. Also, it is shown that Pacific model bi...