TITLE: Leading modes of synoptic scale convective activity within the ITCZ

ABSTRACT: 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, 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 disturbances can be characterized as 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 in many cases should 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 within the extratropical storm tracks prior to the peak convective activity near the equator. The dynamical basis for such tropical-extratropical interaction will be discussed.

SPEAKER WEBSITE: http://www.esrl.noaa.gov/psd/people/george.kiladis/