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Leonard M. Druyan, Patrick Lonergan, and Judah Cohen

Abstract

African wave disturbances (AWDs), an important trigger of Sahel summer rainfall, are studied using ECMWF gridded datasets for July and August 1987 and 1988. Power spectra of time series of 700-mb meridional winds near Niamey taken from analyses at both 2° × 2.5° and 4° × 5° horizontal resolution are compared to spectra based on Niamey station data. In addition, spatial distributions of meteorological fields at both resolutions are discussed for three case studies, including the synoptic features of several AWDs. Additional examples are presented from GCM simulations at comparable horizontal resolutions. While vertical motion and divergence centers were more extreme at 2° × 2.5°, many of the deduced characteristics of an AWD were similar at both resolutions. The higher-resolution analyses and simulation show a sharp transition across wave troughs between lower-tropospheric convergence (uplift) on the west and divergence (subsidence) on the east for several AWDs. For the two more southerly AWDs analyzed here, uplift associated with the convergence ahead of the trough appears to be displaced to the southwest at midtropospheric altitudes. Twice-daily July–September precipitation at Niamey is weakly, but significantly, correlated with corresponding time series of ECMWF analyzed vertical motion at nearby grid points.

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Kazuyuki Saito, Judah Cohen, and Dara Entekhabi

Abstract

Recently it has been shown that the area extent of Eurasian snow cover during September–October–November (SON) and the principal component of the leading mode of extratropical Northern Hemisphere (NH) climate variability in the following winter are statistically correlated. In this paper, physical linkages between SON Eurasian snow cover and the wintertime climate variability in the NH atmosphere are postulated. And in order to test the proposed hypotheses, comprehensive analyses of satellite-based observations for snow cover and reanalysis data for geopotential heights and sea level pressure are used.

The magnitude of the correlation between snow cover and climate variability is found to be inversely proportional to the height suggesting that snow cover may act as a lower boundary forcing to the tropospheric circulation. Conversely, however, an index constructed to capture the downward propagating circulation anomaly from the lower stratosphere to the middle troposphere is shown to be as highly correlated with snow cover variability as the Arctic oscillation derived from sea level pressure.

A mechanism involving the vertical propagation of Rossby waves is proposed to explain this apparent contradiction. Anomalous fall snow cover variability not only alters near-surface temperatures but also impacts upward propagating Rossby waves. Changes forced in the stratosphere by anomalous snow cover are not realized until later in the winter season when the troposphere and stratosphere are actively coupled.

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