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Chester F. Ropelewski

Abstract

Monthly estimates of Antarctic sea-ice area for the past decade were extracted from operational churts. Empirical orthogonal function analyses of these satellite-derived data revealed the existence of six distinct ice area sub-regions. Comparison of ice area time series for these sub-regions highlights the substantial differences among them. For example, total sea-ice extent typically reached a maximum in either August or September, while the Ron Sea often exhibited two relative maxima (July and October). The data show considerable year-to-year variability during this short period of record with the minimum sea-ice area varying by more than a factor of two and maximum sea-ice area varying by almost 20%. The large year-to-year variability precludes a reliable identification of longer term trends during the relatively short era of satellite observations.

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Thomas M. Smith and Chester F. Ropelewski

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This paper is an extension of a study by C. Ropelewski and M. Halpert, which examines observed precipitation relationships with the Southern Oscillation. Here, the authors repeat their analysis using atmospheric general circulation model precipitation from the average of a 13-run ensemble. The GCM is the atmospheric component of the coupled model used for seasonal prediction at the National Centers for Environmental Prediction, except that in this study, the observed sea surface temperatures were specified for the ensemble runs. Results are compared and contrasted with the observed Southern Oscillation–related precipitation behavior. These comparisons show that the multiple ensemble simulations compare favorably to the observations for most areas in the Tropics and subtropics. However, outside of the deep Tropics, the model simulations show large shifts or biases in the location of the Southern Oscillation–related anomalies. In particular, anomalies shown by the observations to occur in the southeastern United States are shifted westward in the simulation.

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Chester F. Ropelewski and Evgeney S. Yarosh

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The mean annual cycle of the atmospheric and terrestrial water balance over the central United States is examined through an analysis of observational data over the 20-yr period 1973–92. The mean quantities from this study are expected to serve as a climatology for empirical investigations and a benchmark for numerical model-based water balance computations for the central United States. Monthly means and statistics of atmospheric water balance quantities were computed from twice daily radiosonde data. These data form a monthly“climatology” and 240 month time series of the major water budget components, including the vertically integrated vapor flux divergence, the rate of change of precipitable water, and precipitation minus evapotranspiration, PE. The mean annual cycle of evaporation given estimates of precipitation over the same area is also computed. Through comparison with observed river discharge, estimates are formed of the mean annual cycle of surface and subsurface storage and its interannual variability (as a residual). The mean observed and residual quantities of the historical water budget components are in general agreement with earlier studies based on shorter time series.

The 20-yr mean water budget shows a maximum of PE in March–April with a secondary maximum in November–December. In this analysis, mean evaporation exceeds mean precipitation during the June–September period with largest evaporation values in July and August. Thus, the heartland of the United States acts as mean net moisture source during the summer months. Individual monthly estimates of evaporation, given the gauge-estimated precipitation over the region, show negative evaporation estimates during some cold season months over the 1973–92 period. This suggests that gauge-measured precipitation is underestimated, at least during the cold months, in agreement with several rain gauge intercomparison studies.

The sonde-based budgets also confirm previous studies in showing that the rate of change of precipitable water is a small contributor to the atmospheric water budget through most of the mean annual cycle. However, the relative importance of this term increases during the transition seasons (late spring and early fall) when the magnitude of the vapor flux divergence term in the atmospheric water balance is also quite small.

The mean PE estimates computed from the vertically integrated atmospheric moisture flux were found to average 0.4 mm day−1 low in comparison to the observed total net river discharge. When the mean atmospheric PE is adjusted to the net discharge, the annual cycle of storage shows an amplitude of 14 cm yr−1, consistent with local measurements of soil moisture in Illinois () and also in agreement with earlier studies. The 20-yr time series shows multiyear variations in the storage term with magnitudes of near 45 cm, far in excess of the mean annual cycle. This low-frequency variability in storage is generally consistent with the accumulated precipitation anomaly, an independently estimated quantity, for most of the analysis period.

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Chester F. Ropelewski and Michael S. Halpert

Abstract

A series of earlier studies has identified regions of the world in which precipitation appears to have a consistent relationship with the Southern Oscillation (SO). In this paper, the authors attempt to quantify this relationship based on shifts in the statistical distribution of precipitation amounts with emphasis on shifts in the median, which are associated with the warm (low SO index) and cold (high SO index) phases of the SO. This paper is partially an attempt to provide long-range forecasters with some guidance in making seasonal and multiseasonal predictions. Observed SO-related shifts in the median precipitation amounts, expressed as percentiles with respect to “climatological” conditions, can he used as a simple indication of the “typical” SO response for a given region. In general, the authors find that for many of the large areas identified in previous studies, median precipitation amounts shift on the order of 20 percentile points, that is, from the median to either the 30th percentile or the 70th percentile. The authors also find considerable spatial variations in the typical patterns of SO-related precipitation percentiles in some regions.

This study also provides empirically based estimates of SO-related precipitation anomalies in terms of precipitation rates for use in numerical model studies. For selected areas in the Tropics, the authors find empirically estimated anomalous precipitation rates ranging from 1 to 3.5 mm/day, that is, from 15% to 83% of the climatological median.

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Michael S. Halpert and Chester F. Ropelewski

Abstract

The “typical” global and large-scale regional temperature patterns associated with the low (warm) and high (cold) phases of the Southern Oscillation (SO) are investigated. A total of 12 separate regions were found to have consistent temperature patterns associated with low phase of the SO, while 11 areas were found to have temperature patterns associated with the high phase. Of these areas, 9 have expected temperature patterns during both phases of the SO. In the tropics, temperature anomalies are of the same sign as the SO-related sea surface temperature (SST) anomaly in all land regions except for one area in the west Pacific. Three extratropical responses to the low phase of the SO are found over North America and one is found in Japan. High SO-temperature patterns were found in the extratropies for Japan, western Europe, and northwestern North America. The identified temperature responses are more consistent in tropical regions than in the extratropies. The SO can influence the estimation of global surface temperature anomalies.

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Anthony G. Barnston and Chester F. Ropelewski

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Canonical correlation analysis (CCA) is explored as a multivariate linear statistical methodology with which to forecast fluctuations of the El Niño/Southern Oscillation (ENSO) in real time. CCA is capable of identifying critical sequence of predictor patterns that tend to evolve into subsequent patterns that can be used to form a forecast.

The CCA model is used to forecast the 3-month mean sea surface temperature (SST) in several regions of the tropical Pacific and Indian oceans for projection times of 0 to 4 seasons beyond the immediately forthcoming season. The predictor variables, representing the climate situation in the four consecutive 3-month periods ending at the time of the forecast, are 1) quasi-global seasonal mean sea level pressure (SLP) and 2) SST in the predictand regions themselves. Forecast skill is estimated using cross-validation, and persistence is used as the primary skill control measure.

Results indicate that a large region in the eastern equatorial Pacific (120°−170°W longitude) has the highest overall predictability, with excellent skill realized for winter forecasts made at the end of summer. CCA outperforms persistence in this region under most conditions, and does noticeably better with the SST included as a predictor in addition to the SLP.

It is demonstrated that better forecast performance at the longer lead times would be obtained if some significantly earlier (i.e., up to 4 years) predictor data were included, because the ability to predict the lower-frequency ENSO phase changes would increase. The good performance of the current system at shorter lead times appears to be based largely on the ability to predict ENSO evolution for events already in progress.

The forecasting of the eastern tropical Pacific SST using CCA is now done routinely on a monthly basis for a 0-, 1-, and 2-season lead at the Climate Analysis Center. Further refinements, and expected associated increases in skill, are planned for the coming several years.

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Chester F. Ropelewski, Peter J. Lamb, and Diane H. Portis

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Robert W. Reeves, Chester F. Ropelewski, and Michael D. Hudlow

Abstract

Upper air and surface data from the GARP Atlantic Tropical Experiment (GATE) are used to examine the interrelationships between convective-scale precipitation and the larger scale wind field. The upper air winds from the inner (B) and outer (A/B) hexagonal observational arrays are fit with second-order polynomials to provide smooth estimates of the vorticity, divergence and vertical motion in the observational array. In these analyses we examined archived validated data from all three phases of the experiment and we formed averages based on the radar-estimated precipitation rates.

Mean profiles for 19-day periods during each of the three observational phases establish the basic similarity of the kinematics during each phase. Strong boundary-layer convergence balanced, for the most part, by upper tropospheric divergence, is common to all three phases.

Radar-estimated precipitation rates are used to define suppressed (precipitation rates <0.1 mm h−1) and highly disturbed (precipitation rates >0.5 mm h−1) states over the observational array. Mean profiles for the disturbed states in each phase show weaker easterly winds and much larger upward vertical velocities than do the mean profiles for the suppressed states. The mean vorticity profiles for each state do not show such clear-cut differences.

Time series of 12 h averages indicate that the precipitation events in Phase III corresponded very closely to the cyclonic maxima of the 700 mb relative vorticity, reflecting the influence of the easterly waves described by Reed et al. (1977). During Phases I and II, when easterly waves were poorly organized, the precipitation events did not correspond closely to the cyclonic vorticity maxima. On the other hand, precipitation events showed good correspondence with the large-scale (A/B) 700 mb upward vertical velocity maxima and surface meridional convergence ∂v/∂y during all three phases. This shows that the precipitation is clearly related to events on a larger scale.

The effects of convective activity on the large-scale flow are examined through the vorticity budget. The vorticity budget residual profiles were similar from phase to phase with cyclonic production maxima in the mid and upper troposphere. The upper tropospheric residual maximum is as strong during the suppressed state as it is during the highly disturbed side. At the surface, individual values of the residual are almost always opposite in sign to the vorticity. The mean vorticity budget for the A/B array shows the tipping term to have magnitudes comparable to other terms in the vorticity budget.

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Alan N. Basist, Chester F. Ropelewski, and Norman C. Grody

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The Microwave Sounding Units (MSU) aboard the NOAA series of polar-orbiting satellites (TIROS-N to NOAA-12) have provided stable and precise measurements of vertically integrated atmospheric temperature since December 1978. Comparisons are made between the MSU channel measurements and temperatures derived from the global data assimilation system (GDAS) at the National Meteorological Center (NMC) for the period 1979–1990. The largest correlations occur at high to midlatitudes, where the troposphere exhibits large monthly anomaly fields, and where radiosondes provide ample coverage for the GDAS. Intermonthly differences from each dataset had global correlations above 0.97. However, poor correlations with MSU were noted over areas of high terrain and tropical landmasses. These poorer correlations can be attributed to temporal changes and data limitations in the GDAS analysis. Comparisons between the GDAS and MSU temperature anomaly fields indicate that frequent model changes mask the climate signal in the GDAS analysis. Nonetheless, the study suggests that both GDAS- and MSU-derived temperature anomalies detect similar spatial and temporal variability over regions where the GDAS is data rich and the signal is large, that is, the El Niño-Southern Oscillations. This study suggests that the NMC reanalysis, using a fixed assimilation model, will produce a stable dataset of tropospheric temperatures. Therefore, the 35 years of reanalyzed NMC model data can he used in conjunction with satellite data to improve the suite of tools used in climate monitoring.

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