Leading modes of synoptic scale convective activity within the ITCZ

An Empirical Orthogonal Function (EOF) analysis is undertaken of global tropical (20S-20N) brightness temperature (Tb) data filtered to retain fluctuations on synoptic (<6 day) time scales. A covariance matrix of high resolution CLAUS Tb is used for this purpose, which is representative of deep tropical convection. The analysis can also be further restricted to westward or eastward propagating modes using space-time filtering techniques. The leading mode of <6 day westward propagating Tb effectively isolates antisymmetric (out of phase) variations in convection on either side of the equator within the west Pacific ITCZ and SPCZ propagating westward in both hemispheres over time. Projection of dynamical fields from reanalysis onto the principal component time series of the leading EOF shows that these waves can be characterized by mixed Rossby-gravity (Yanai) waves that originate near the dateline, which then transform into easterly waves north of the equator as they approach the region of New Guinea. In contrast, the leading mode of <6 day filtered Tb data without regard to propagation direction reveals a similar antisymmetric pattern in the same region, but in this case the convective signal propagates poleward in both hemispheres over time, with little zonal propagation evident. Projection of the circulation data onto this pattern reveals a combined Yanai and eastward inertio gravity (EIG) mode. Spectral analysis of the EOF Tb signal also supports this interpretation, yielding a peak signal along the dispersion relationship of the Yanai wave on the westward side and the EIG wave on the eastward side of the spectrum. This result suggests that Yanai and EIG waves form a continuum, as suggested by space-time spectra of long time series of Tb, which should in many cases not necessarily be separated into westward versus eastward components. The combined Yanai-EIG mode also has strong extratropical signals associated with them. Extratropical forcing of this equatorial wave activity is unambiguous based on their lead-lag relationships, with Rossby wavetrains appearing over the Indian Ocean several days prior to the maximum wave activity over the equatorial west Pacific. The principal component time series generated from the EOFs allows the analysis of temporal fluctuations in "wave activity", and the associated basic state changes accompanying these changes. While the Madden-Julian Oscillation (MJO) is prominent in modulating the Yanai-EIG wave activity in many cases, there are other more localized intraseasonal signals in especially the zonal wind that also appear to be important.