Modulation of Spcz Convection By Equatorial and Extratropical Wave Activity

Abstract:

In this study we examine the leading modes of submonthly variability of the SPCZ. A December-Feburary covariance EOF analysis of OLR is used to identify the leading modes of convective variability on submonthly time scales. An important modulator of the SPCZ is shown be related to equatorial Rossby (ER) modes, which are westward propagating features with a spectral peak in the 10-40 day period range (Wheeler and Kiladis, 1999). It is useful, therefore, to also take advantage space-time filtering techniques, so for instance, we filter for westward moving 10-96 day variability as well as westward and eastward less than 30 day variability for portions of this analysis. This procedure typically generates EOF pairs that represent propagating disturbances. Reanalysis data are projected onto the principal components of each mode at lag to investigate the evolution of large scale dynamical and thermodynamical fields associated with each mode. In all cases using less than 30 day filtering, clear evidence of ER modes are seen, with SPCZ convection occurring from the equator to around 10S associated with “cyclone pair” and associated equatorial westerly wind burst circulations. These features propagate westward at around 5 m/s phase speed. However, when higher frequency data are used instead, such as less than 10 day filtered Tb, mixed Rossby-gravity modes are evident, which propagate much faster and are characterized by antisymmetric convective signals between the SPCZ and the Northern Hemisphere ITCZ at a similar longitude. All of these cases are also shown to have strong associations with extratropical storm track activity. When looking at SPCZ variability poleward of around 15S, the primary modes are also characterized by strong extratropical signals, but in this case the interpretation is more straightforward: convection in the diagonal portion of the SPCZ can be forced by transient activity within the Southern Hemisphere storm track. This forcing is generally characterized by upward motion and cloudiness ahead of upper level troughs, which frequently leads to a northwest-southeast oriented cloud band typical of the poleward side of the SPCZ. In these cases, simple quasi-geostrophic arguments can be applied to explain the dynamical relationship between precipitation and the large scale circulation.