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Weickmann, K. M., and S. J. S. Khalsa, 1990: The shift of convection from the Indian Ocean to the western Pacific Ocean during a 30-60 day oscillation. Mon. Wea. Rev., 118, 964-978.


ABSTRACT

A strong 30-60 day oscillation during November/December 1981 is studied using analyzed winds from the National Meteorological Center, outgoing longwave radiation, and satellite low-level moisture data. The outgoing longwave radiation and 150 mb velocity potential for five adjacent events are also examined. Consistent features include the slow (5 m s-1) propagation of convective energy over the Indian Ocean-western Pacific Ocean and the fast (15 m s-1) propagation of 150 mb tropical velocity potential in regions remote from the oceanic warm pool. The individual events have characteristics of the seasonal and anomalous base state within which they are embedded.

For the November/December 1981 event, we concentrate on the details of the shift of convection from the eastern Indian Ocean to the western Pacific Ocean. An intense cold surge and accompanying convective increase near 110°E precede the shift cast by about 12 days. The regional 150 mb circulation responds to the enhanced convection with an intensified anticyclone over southeast China and a downstream trough over the subtropical western Pacific. At low levels, the 850 mb subtropical high over the North Pacific strengthens and amplifies as a wave train emanates from the region of the downstream upper-level trough. A surge in central Pacific Ocean trades accompanies these developments. In equatorial regions a component of convection moves east accompanied by low-level westerly winds to near 170°E. Strong 850 mb convergence occurs between the trade surge and the westerly winds favoring low-level moisture increases over the western Pacific. The convection then intensifies near 150°E and weakens near 110°E, eventually forcing a transition in the regional anomalous circulation (i.e., a 150 mb cyclone over southeast China and anticyclone over the western Pacific).

An important feature of the transition appears to be the initiation of convection near the node of the existing anomalous 150 mb circulation by an eastward moving equatorial transient. Otherwise, convection occurs primarily in regions of upper-level easterlies associated with subtropical anticyclones. Such structures appear to represent quasi-stationary regimes that can last for 10-20 days. These regimes, in conjunction with the rapid eastward shift of convection, give rise to a large standing component in the outgoing longwave radiation and 150 mb streamfunction during the November/December 1981 event and in other 30-60 day oscillation events.