ESRL/PSD Seminar Series

Abstract

---

Convectively coupled equatorial waves (CCEWs) are responsible for a large portion of the convective variability within the ITCZ and monsoon regions. This talk will first review the statistical structure of these waves and compare that structure with that predicted by Matsuno's classical shallow water theory on an equatorial beta plane. An Empirical Orthogonal Function (EOF) analysis is undertaken of global tropical (20S-20N) brightness temperature data filtered to retain fluctuations on various synoptic (<10 day) to submonthly (<30 day) time scales. The analysis can also be further restricted to westward or eastward propagating modes using space-time filtering techniques. The leading modes broadly correspond to known CCEW disturbances, although in some cases the analysis reveals a mixture of such waves. In general many of the modes also have strong extratropical signals associated with them, and extratropical forcing of the equatorial wave activity is unambiguous based on their lead-lag relationships. A common manifestation of this type of interaction involves the initiation of convectively coupled Kelvin waves within the western Pacific ITCZ, which are often triggered by Rossby wave activity propagating into the Australasian region from the South Indian Ocean storm track. The resulting waves frequently propagate eastward across the entire Pacific, and continue uninhibited across the Andes into South America and beyond. At other times, Kelvin waves are seen to originate along the eastern slope of the Andes. In the latter case the initial forcing is sometimes due to a low-level "pressure surge," initiated by wave activity propagating equatorward from the South Pacific storm track. In yet other cases, such as over Africa, the forcing appears to be related to wave activity in the extratropical storm track, which is not necessarily propagating into low latitudes, but appears to "project" onto the Kelvin structure, in line with past theoretical and modeling studies. Observational evidence for such interactions will be presented, along with a review of some recent theoretical work aimed at explaining their dynamical causes.

---