Tropical-Midlatitude Interactions over Asia and the Western Pacific Ocean during the 1983/84 Northern Winter

C-P. Chang Department of Meteorology, Naval Postgraduate School, Monterey, CA 93943

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K. G. Lum Malaysian Meteorological Service, Petalin Jaya, Selangor, Malaysia

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Abstract

Previous studies of cold surges during the northern winter monsoon suggested a short-term midlatitude-tropical interaction such that the variations of the midlatitude jet over East Asia correspond to variations in tropical convection. However, because cold surges occur during periods of intensified baroclinicity, it is possible that the strengthening of the jet may be due entirely to midlatitude baroclinic development rather than the enhanced local Hadley circulation forced by the increase in tropical convection. In this study objectively analyzed 200 mb wind data for the 1983/84 winter are examined to address the problem of determining cause and effect in short-term midlatitude-tropical interactions.

Significant positive correlation between the midlatitude jet acceleration and tropical divergence was found in several regions in the Asia-Pacific-Indian Ocean region. Among six major intensifications of the East Asian jet streak maximum during December 1983, three were related to tropical cyclone activity and two to cold surges. The tropical cyclone cases led to the conclusion that the midlatitude jet can be influenced effectively by tropical convective activity on a day-to-day basis. This result has possible implications for midlatitude weather forecasting. The study also confirmed the downstream propagation of the strengthened jet streaks, which is probably due to self-advection, and the existence of thermally indirect circulations at the exit region of the jet, both in the time-mean and in the transient motion fields.

Abstract

Previous studies of cold surges during the northern winter monsoon suggested a short-term midlatitude-tropical interaction such that the variations of the midlatitude jet over East Asia correspond to variations in tropical convection. However, because cold surges occur during periods of intensified baroclinicity, it is possible that the strengthening of the jet may be due entirely to midlatitude baroclinic development rather than the enhanced local Hadley circulation forced by the increase in tropical convection. In this study objectively analyzed 200 mb wind data for the 1983/84 winter are examined to address the problem of determining cause and effect in short-term midlatitude-tropical interactions.

Significant positive correlation between the midlatitude jet acceleration and tropical divergence was found in several regions in the Asia-Pacific-Indian Ocean region. Among six major intensifications of the East Asian jet streak maximum during December 1983, three were related to tropical cyclone activity and two to cold surges. The tropical cyclone cases led to the conclusion that the midlatitude jet can be influenced effectively by tropical convective activity on a day-to-day basis. This result has possible implications for midlatitude weather forecasting. The study also confirmed the downstream propagation of the strengthened jet streaks, which is probably due to self-advection, and the existence of thermally indirect circulations at the exit region of the jet, both in the time-mean and in the transient motion fields.

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