Extended Abstract (676K)16B.6
Extratropical forcing of convectively coupled Kelvin waves
Katherine H. Straub, NOAA/CIRES, Boulder, CO; and G. N. Kiladis
Evidence is presented that equatorward-propagating disturbances originating in the Southern Hemisphere subtropical jet during austral winter (JJA) are associated with the initiation of convectively coupled, equatorially trapped Kelvin waves in the tropical western Pacific. The climatological jet is located in the vicinity of Australia during JJA, centered at approximately 30S and 200 hPa. Wave packets that originate over the extratropical South Indian Ocean occasionally split into two branches along the jet axis, the stronger branch propagating northeastward toward the tropics and the weaker branch remaining in the extratropics (Chang 1999). Baroclinic development over Australia associated with the northeastward-propagating wavetrain generates a shallow, low-level pressure surge that propagates northward from the Australian continent. When this pressure surge reaches the equator, the low-level pressure gradient produces westerly anomalies, along with an equatorial region of zonal wind convergence. The abundance of low-level moisture over the warm pool allows this low-level moisture convergence to significantly increase CAPE over a large region, providing the low-level forcing for the initiation of deep tropical convection. At the same time, strong northerly winds in the upper troposphere, to the west of a high pressure cell in the equatorward-propagating wavetrain, are associated with large-scale divergence above the low-level convergence, inducing upward motion in the upper troposphere. The combination of lower- and upper-level forcing enhances the probability that deep convection will be initiated in this large-scale region. Once convection is initiated, it moves eastward at approximately 15 m/s, at approximately the same speed as the upper-level disturbances in the subtropical wave packet. As the convection increases in intensity, it drives temperature and wind anomalies in the troposphere and lower stratosphere that are consistent with a convectively coupled Kelvin wave as described by Wheeler et al. (2000). It is postulated that once initiated, the Kelvin wave becomes self-sustaining through a moisture feedback mechanism. The equivalent depth implied by the phase speed of these Kelvin waves (about 25 m) is consistent with the implied depths of other convectively coupled equatorial waves such as mixed Rossby-gravity and n=1 Rossby waves (Wheeler and Kiladis 1999).
An analogous mechanism during boreal winter (DJF) can be used to explain the initiation of convectively coupled Kelvin waves over eastern Africa. These waves reach their peak intensity in the central Indian Ocean.
Session 16B, Inraseasonal Variability II (Parallel with Sessions 16A and 16C)
Friday, 3 May 2002, 9:00 AM-10:30 AM
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