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Kiladis, G., G. Meehl and K. Weickmann, 1994: The large scale circulation associated with westerly wind bursts and deep convection over the western equatorial Pacific. J. Geophys. Res., 99, 18527-18544.


ABSTRACT

The relationship between deep equatorial convection over the western Pacific and atmospheric circulation during November to February, 1986-1992 is studied using cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses. We focus on intraseasonal convective events on the 6- to 30-day timescale over two regions: just east of Borneo and over the Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) domain to the northeast of New Guinea. Although comparatively little work has been done on this timescale, we show that the bulk of the intraseasonal variability in both convection and low-level circulation over the tropical western Pacific occurs in this period range. As a comparison with these higher-frequency events, the evolution of COARE convection on the 30- to 70-day timescale is also considered. The results of the historical data analysis are compared to a case study of deep convection associated with strong surface westerly winds during November 1989. This analysis is meant to serve as a benchmark for future case studies from the Tropical Ocean-Global Atmosphere COARE experiment and other periods. In general, 6- to 30-day equatorial convection in the regions studied occurs in conjunction with low-level equatorial westerly wind anomalies, paired anomalous cyclonic circulations straddling the equator, and a strengthening of the sea level pressure gradient along the equator. The increase in the pressure gradient appears most often tided to the simultaneous equatorward movement of high-pressure systems originating over southeastern Asia and Australia. A surge of trades over the central Pacific also accompanies the development of convection over both study regions. Following the convective peak, the low-level westerlies slowly decay, as the northern cyclone moves northwestward away from the equator in a track characteristic of a tropical depression. It is also observed that low-level westerly anomalies occur frequently without deep convection. The vertical structure during convective events along the equator on the 30- to 70-day timescale and over Indonesia in the 6- to 30-day band is baroclinic, with easterly anomalies at upper levels overlying low-level westerlies. At 200 mbar a strengthened meridional outflow from the OLR signal into a wave train on the equatorward side of the Asian jet is observed to develop following the convective peak, suggesting forcing of this circulation by the convection. Over the COARE domain there is evidence that 6- to 30-day convection often occurs in conjunction with deep westerly flow. It is suggested that these types of events are frequently characterized by vertical propagation of westerlies from the surface to the upper troposphere, as is shown for a case study from November 1989. This westerly burst episode is shown to have many features in common with the "typical" case as defined from historical data.