Influences of ENSO SST Anomalies and Winter Storm Tracks on the Interannual Variability of Upper-Troposphere Water Vapor over the Northern Hemisphere Extratropics

Abstract

This study examines the interannual variability of winter upper-troposphere water vapor over the Northern Hemisphere using the National Aeronautics and Space Administration Water Vapor Project, the International Satellite Cloud Climatology Project data, and the European Centre for Medium-Range Weather Forecasting reanalysis. The El Niño-Southern Oscillation related tropical sea surface temperature (SST) anomalies dominate the upper-troposphere water vapor anomalies south of the climatological jet. The anomalies of baroclinic instability in the storm track regions, which relate to the Pacific-North American and the North Atlantic oscillation patterns, dominate those north of the climatological jet. The upper-troposphere water vapor increases in the eastern tropical Pacific, the Gulf of Mexico, and some areas of the North Atlantic with warmer tropical SST. It decreases in the subtropical and extratropical northeastern Pacific. Deep convection and vertical moisture fluxes dominate these changes. To the north of the climatological jet, stronger upper-level cyclonic flow dries the upper troposphere when the baroclinicity of the storm tracks is enhanced. Both vertical and meridional moisture transport contribute to these water vapor anomalies in the midlatitudes. High clouds, as a possible source/sink of water vapor, respond to the tropical SST anomalies and extratropical circulation in a pattern similar to the uppertroposphere water vapor, and they consequently positively correlate to the latter. In the Tropics and extratropics where high clouds are relatively abundant, water vapor concentration increases with temperature. Thus, the increase of evaporation or sublimation of high clouds probably contributes to the observed moistening of the upper troposphere, in addition to enhanced vapor transport. Conversely, in the subtropics where high clouds appear infrequently, water vapor concentration decreases with temperature, suggesting that the downward advection of drier air associated with subsidence dominates the drying of the upper troposphere.

A Last Saturation Analysis of ENSO Humidity Variability in the Subtropical Pacific

“…This result is summarized in Fig. 2 and documented elsewhere (Chang 2006;Held et al 1989;Orlanski 2005;Wang and Fu 2000) the winter storm track over the North Pacific is stronger and migrates equatorward during El Niñ o and migrates poleward during La Niñ a. A warm tropical Pacific Ocean corresponds with enhanced extratropical last saturation of the subtropics.…”

“…The humidity of the subtropical troposphere responds to oscillations in the El Niñ o-Southern Oscillation (ENSO) system on interannual time scales (Lau et al 1998;Wang and Fu 2000), providing a test bed for evaluating the processes that impact subtropical aridity. The water vapor content of the subtropical troposphere decreases during El Niñ o (Dessler et al 2008;Lau et al 1998;Peixoto and Oort 1992) and the winter storm track strengthens over the North Pacific, deflecting equatorward over the eastern North Pacific (Chang 2006;Chang et al 2002).…”

The enhanced drying effect of Central-Pacific El Niño on US winter

“…Figures 4(b) and (c) indicate that the jet streams shift southward during both types of El Niño, with large negative wind anomalies in the northern US and large positive wind anomalies in the south. Previous studies have suggested that such an equator-ward shift of the tropospheric jet streams during El Niño events result from El Niño-induced Rossby wave trains and strengthening of the Hadley circulation (e.g., Wang and Fu 2000, Seager et al 2003, Lu et al 2008. As the jet streams shift southward, winter storms shift south with them, leading to a dry-north, wet-south pattern of precipitation anomalies during the El Niño.…”

The ENSO Signal in the Stratosphere

“…The term Southern Oscillation refers to the variation in sea level pressure between the eastern Pacific and the Indian Ocean. ENSO is known to be one of the main sources of variability in the tropical troposphere and it has been related to changes in the jet stream, the strong westerly wind current concentrated at middle latitudes in the upper troposphere 1,2 ; to variations in the Monsoon system over Asia and Australia 3‐5 ; and to changes in cyclone frequency over Europe 6 …”