Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: J. M. Straus x
  • Refine by Access: All Content x
Clear All Modify Search
David M. Straus and J. Shukla

Abstract

The wavenumber-frequency spectra of geopotential height have been computed from a winter simulation of a GLAS general circulation model, and are compared to the spectra obtained from 15 winters of observed analyses. The variances in several broadly defined space-time categories are presented, and their distribution with latitude and height discussed.

The low-frequency planetary waves (wavenumbers 1–4) in the model have substantially less variance than is observed and their latitudinal structure also differs from observations. The same is true for the medium-frequency planetary waves. The synoptic-scale waves (wavenumbers 5–10) with both low and medium frequencies are quite well reproduced by the model, with the total variances and their latitude-height structure comparing well to the observations. The net propagation tendency is examined for low-frequency planetary waves and the medium-frequency synoptic-scale waves. The model results show too much tendency for eastward propagation, the discrepancy being much larger in the case of the planetary waves. The model stationary waves were also examined. Their variance is considerably less than is observed, and their latitudinal structure incorrect. These findings are compared to the limited statistics available for the NCAR and GFDL models. The GLAS model's simulation of the synoptic-scale waves (with both low and medium frequencies) is apparently more realistic than that of the other models. The opposite is true for the stationary waves. All the models underpredict the low-frequency planetary-wave variance.

The differences between model and observed spectra are compared to (i) the range of interannual variability, and (ii) the estimated statistical uncertainties of the spectra. For the synoptic-scale waves the differences are generally smaller than either (i) or (ii), whereas for the planetary waves the differences are larger.

Possible causes for the GLAS model's underprediction of the stationary planetary wave variance are briefly discussed. It is suggested that the diabatic heating due to latent beat of condensation may play an important role. Further, evidence is presented indicating that the lack of variance in the model low-frequency planetary waves is closely related to the lack of variance in the stationary waves.

Full access
David M. Straus and J. Shukla

Abstract

The primary effect of El Niño–Southern Oscillation (ENSO) sea surface temperature (SST) anomalies is to force distinct midlatitude patterns, and not only to modify the probability of the internal variability patterns [such as the Pacific–North American (PNA) pattern of Wallace and Gutzler]. Both the spatial structure and probability distribution of the external ENSO pattern are distinct from the PNA pattern. Ensemble general circulation model (GCM) integrations for 30 winters have been analyzed in the Pacific–North America region. These winters span the recent period of 1981/82 through 1998/99, plus 12 earlier winters; the entire dataset includes six El Niño (warm) and seven La Niña (cold) events. The ensemble size is nine simulations. Empirical orthogonal function (EOF) analysis is carried out for all GCMs and observed seasonal means, for the GCM ensemble means, and for the GCM deviations about the ensemble means. EOF-1 of the GCM 200-hPa height (Z) ensemble mean agrees well with EOF-1 of all GCM seasonal means, and with the pattern that optimally filters out the internal variability. This ENSO pattern agrees with EOF-1 of reanalysis data, although the latter is modified in the Atlantic sector by the presence of the North Atlantic Oscillation pattern. An internal pattern closely resembling the PNA pattern is obtained here from reanalyses as EOF-2 of Z for 37 normal winters (winters that are neither warm nor cold). The GCM version of the PNA pattern can be seen in EOF-2 of the deviation of the GCM means about the ensemble mean, EOF-3 of all GCM seasonal means, and EOF-2 of a GCM integration made with climatologically varying SST.

Projections of all GCM seasonal means in a low-dimensional space indicate a segregation of the warm winter seasonal means from those of normal winters along the axis representing the external pattern. There is no support for the hypothesis that the probability distribution functions (pdfs) of internal and external variability are similar.

There is clear evidence of shifts in the probability of occurrence of internal patterns in warm and cold winters compared to normal winters. This has been analyzed in terms of only one set of characteristic patterns based on all winters. A more detailed investigation of the dependence of internal variability on SST forcing requires a larger ensemble size.

Intraseasonal variability is examined by projecting all individual pentad means on both the ENSO pattern and the PNA pattern (represented by the EOF-1 and EOF-3 of all seasonal means). For five out of the six warm winters, the pdfs of the time series of the coefficient of the ENSO pattern are shifted significantly toward more positive values, with very little probability of having a negative coefficient. For the cold events, the ENSO pattern has greater variability than the PNA pattern. The pdf of the projections of pentads on the seasonal mean anomaly for the same year are sharply peaked.

The GCM tropical heating maps show similar patterns for cold and normal winters, while the warm winter pattern is clearly different, especially along the equator. The heating anomalies have larger magnitude in warm than in cold winters. Linearity of the leading ensemble mean principal component (PC) with respect to the Niño-3 index of SST is seen only for positive SST anomalies. When Niño-3 is replaced by heating averaged over the central Pacific, there is a consistent relationship between the ENSO PCs and both positive and negative heating. Niño-3 depends on heating in a linear way for positive heating anomalies, but is independent of heating for negative anomalies.

Full access
David M. Straus and J. Shukla

Abstract

The winter response of the atmosphere to El Niño events in the Pacific is studied both from a 14-year integration of the Center for Ocean–Land–Atmosphere GCM using observed SSTs from January 1979 to February 1993 and from the corresponding analyses of ECMWF. Emphasis is put on the shift in the high-frequency transients that define the Pacific storm track during warm events. Warm and normal ensembles are defined on the basis of the GCM’s diabatic heating field in the tropical Pacific, which falls in one of two states. During the winters of 1982/83, 1986/87, and 1991/92, the heating averaged between 6°S and 6°N lies in the range of 100–200 W m−2 all the way across the basin. The remaining 10 “normal” years all show no large tropical diabatic heating anomalies in the mid or eastern Pacific.

The difference between warm and normal ensembles for the mean fields of zonal wind u and height z indicates an eastward and equatorward extension of the midlatitude Pacific jet, associated with a similar extension of the transient feedback on the mean flow as measured by the convergence of vorticity flux. The increase in high-frequency (periods of ∼2 to 10 days) transient kinetic energy in the eastern portion of the Pacific during El Niño extends across Mexico. The high-frequency transient vertical and meridional sensible heat fluxes, and the low-level diabatic heating and baroclinicity of the mean state (measured by Ri−1/2) also indicate an eastward and equatorward shift of the entire storm track complex in the GCM and the analyses. The shift is consistent with the increased midlatitude shear during El Niño that accompanies the overall tropical warming.

While the GCM storm track shift has strong similarities to that in the analyses, the GCM’s large systematic errors in the mid Pacific (eastward extension of the Pacific jet and the storm track) lead to an underestimation of the response to El Niño, which has a very similar form. However, the GCM also shows a spurious tendency to move the storm tracks equatorward in the far western Pacific, as seen in the large positive anomalies in all dynamical indicators of the storm tracks at latitudes 20°–30°N.

Full access
Y. T. Chiu and J. M. Straus

Abstract

Invoking recent satellite observations of the planetary-scale variations of auroral forms, as well as direct satellite observation of polar upper atmospheric winds during magnetic storms, we suggest that substorms may be a source of planetary waves of frequency intermediate between the Rossby and acoustic-gravity regimes. The zonal wavenumber of such waves is approximately 3–6; therefore, their propagation is strongly affected by the Coriolis force. Interpretation of worldwide traveling ionospheric disturbances in terms of meridional propagation of these waves is discussed.

Full access
D. A. Paolino, Q. Yang, B. Doty, J. L. Kinter III, J. Shukla, and David M. Straus

Results are presented from a retrospective analysis of 19 months (May 1982–November 1983) of global atmospheric observations. The National Meteorological Center Global Data Assimilation System was used in tandem with the atmospheric general circulation model of the Center for Ocean–Land–Atmosphere Studies to produce four-times-daily representations of the global atmosphere. Statistics were compiled regarding the use of data by the analysis and the decisions of the quality control procedures. Comparison of the reanalyses with both observation and the archived contemporaneous analyses showed substantial improvements in the representation of the global atmospheric circulation, possibly excepting the Southern Hemisphere south of 60°S. A list of data products from the reanalysis is given in an appendix.

Full access
Abdullah A. Fahad, Natalie J. Burls, Erik T. Swenson, and David M. Straus

Abstract

Subtropical anticyclones and midlatitude storm tracks are key components of the large-scale atmospheric circulation. Focusing on the Southern Hemisphere, the seasonality of the three dominant subtropical anticyclones, situated over the South Pacific, South Atlantic, and south Indian Ocean basins, has a large influence on local weather and climate within South America, southern Africa, and Australia, respectively. Generally speaking, sea level pressure within the Southern Hemisphere subtropics reaches its seasonal maximum during the winter season when the Southern Hemisphere Hadley cell is at its strongest. One exception to this is the seasonal evolution of the South Pacific subtropical anticyclone. While winter maxima are seen in the South Atlantic and south Indian subtropical anticyclones, the South Pacific subtropical anticyclone reaches its seasonal maximum during local spring with elevated values extending into summer. In this study, we investigate the hypothesis that the strength of the austral summer South Pacific subtropical anticyclone is largely due to heating over the South Pacific convergence zone. Using added-cooling and added-heating atmospheric general circulation model experiments to artificially change the strength of austral summer diabatic heating over the South Pacific convergence zone, our results show that increased heating, through increased upper-level divergence, triggers a Rossby wave train that extends into the Southern Hemisphere midlatitudes. This propagating Rossby wave train creates a high and low sea level pressure pattern that projects onto the center of the South Pacific subtropical anticyclone to intensify its area and strength.

Open access
J. Shukla, J. Anderson, D. Baumhefner, C. Brankovic, Y. Chang, E. Kalnay, L. Marx, T. Palmer, D. Paolino, J. Ploshay, S. Schubert, D. Straus, M. Suarez, and J. Tribbia

Dynamical Seasonal Prediction (DSP) is an informally coordinated multi-institution research project to investigate the predictability of seasonal mean atmospheric circulation and rainfall. The basic idea is to test the feasibility of extending the technology of routine numerical weather prediction beyond the inherent limit of deterministic predictability of weather to produce numerical climate predictions using state-of-the-art global atmospheric models. Atmospheric general circulation models (AGCMs) either forced by predicted sea surface temperature (SST) or as part of a coupled forecast system have shown in the past that certain regions of the extratropics, in particular, the Pacific–North America (PNA) region during Northern Hemisphere winter, can be predicted with significant skill especially during years of large tropical SST anomalies. However, there is still a great deal of uncertainty about how much the details of various AGCMs impact conclusions about extratropical seasonal prediction and predictability.

DSP is designed to compare seasonal simulation and prediction results from five state-of-the-art U.S. modeling groups (NCAR, COLA, GSFC, GFDL, NCEP) in order to assess which aspects of the results are robust and which are model dependent. The initial emphasis is on the predictability of seasonal anomalies over the PNA region. This paper also includes results from the ECMWF model, and historical forecast skill over both the PNA region and the European region is presented for all six models.

It is found that with specified SST boundary conditions, all models show that the winter season mean circulation anomalies over the Pacific–North American region are highly predictable during years of large tropical sea surface temperature anomalies. The influence of large anomalous boundary conditions is so strong and so reproducible that the seasonal mean forecasts can be given with a high degree of confidence. However, the degree of reproducibility is highly variable from one model to the other, and quantities such as the PNA region signal to noise ratio are found to vary significantly between the different AGCMs. It would not be possible to make reliable estimates of predictability of the seasonal mean atmosphere circulation unless causes for such large differences among models are understood.

Full access
M.J. Fennessy, J.L. Kinter III, B. Kirtman, L. Marx, S. Nigam, E. Schneider, J. Shukla, D. Straus, A. Vernekar, Y. Xue, and J. Zhou

Abstract

A series of sensitivity experiments are conducted in an attempt to understand and correct deficiencies in the simulation of the seasonal mean Indian monsoon with a global atmospheric general circulation model. The seasonal mean precipitation is less than half that observed. This poor simulation in seasonal integrations is independent of the choice of initial conditions and global sea surface temperature data used. Experiments are performed to test the sensitivity of the Indian monsoon simulation to changes in orography, vegetation, soil wetness, and cloudiness.

The authors find that the deficiency of the model precipitation simulation may be attributed to the use of an enhanced orography in the integrations. Replacement of this orography with a mean orography results in a much more realistic simulation of Indian monsoon circulation and rainfall. Experiments with a linear primitive equation model on the sphere suggest that this striking improvement is due to modulations of the orographically forced waves in the lower troposphere. This improvement in the monsoon simulation is due to the kinematic and dynamical effects of changing the topography, rather than the thermal effects, which were minimal.

The magnitude of the impact on the Indian monsoon of the other sensitivity experiments varied considerably, but was consistently less than the impact of using the mean orography. However, results from the soil moisture sensitivity experiments suggest a possibly important role for soil moisture in simulating tropical precipitation, including that associated with the Indian monsoon.

Full access