The Connection between Trends of Mean Temperature and Circulation at the Surface: Part IV. Comparison of the Surface Changes in the Northern Hemisphere with the Upper Air and with the Antarctic in Winter

Harry van Loon National Center for Atmospheric Research, Boulder, Colo. 80307

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Jill Williams National Center for Atmospheric Research, Boulder, Colo. 80307

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Abstract

During 24 years when the 700 mb winter mean temperature dropped over most of the hemisphere north of 20°N, the biggest falls were in the belt of strongest baroclinity and were simultaneous with a southward movement and strengthening of the peak in total meridional eddy transport of sensible heat. These changes were accompanied by a southward displacement of the region of most frequent storm tracks at the surface and by compatible trends in surface mean temperature and sea level pressure. At middle latitudes the layer between surface and 700 mb destabilized, while in the arctic it stabilized as the surface temperature over a large part of the polar cap fell more than the 700 mb temperature.

A comparison with the Southern Hemisphere showed that local temperature trends in the antarctic also take place on the scale of long waves, that they are as large as those in the Northern Hemisphere and that a zonally averaged trend is not necessary the same in summer and winter. The net transport of sensible heat by stationary waves is much smaller in the Southern than in the Northern Hemisphere, and changes in stationary wave transport in the Southern Hemisphere are therefore not likely to contribute much to large changes in the net poleward transport of sensible heat by waves. This is connected with the observation that the stationary waves in temperature and pressure are nearly in phase over the almost continuous water surface in southern temperate latitudes.

Abstract

During 24 years when the 700 mb winter mean temperature dropped over most of the hemisphere north of 20°N, the biggest falls were in the belt of strongest baroclinity and were simultaneous with a southward movement and strengthening of the peak in total meridional eddy transport of sensible heat. These changes were accompanied by a southward displacement of the region of most frequent storm tracks at the surface and by compatible trends in surface mean temperature and sea level pressure. At middle latitudes the layer between surface and 700 mb destabilized, while in the arctic it stabilized as the surface temperature over a large part of the polar cap fell more than the 700 mb temperature.

A comparison with the Southern Hemisphere showed that local temperature trends in the antarctic also take place on the scale of long waves, that they are as large as those in the Northern Hemisphere and that a zonally averaged trend is not necessary the same in summer and winter. The net transport of sensible heat by stationary waves is much smaller in the Southern than in the Northern Hemisphere, and changes in stationary wave transport in the Southern Hemisphere are therefore not likely to contribute much to large changes in the net poleward transport of sensible heat by waves. This is connected with the observation that the stationary waves in temperature and pressure are nearly in phase over the almost continuous water surface in southern temperate latitudes.

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