Environmental Science and Engineering Seminar

On average most of the tropical precipitation over the open ocean in the Pacific occurs within the Intertropical Convergence Zone (ITCZ) north of the equator and the South Pacific Convergence Zone (SPCZ) south of the equator. This "double ITCZ" region hosts a variety of equatorial waves coupled to convection that propagate parallel to the equator over a wide range of time scales. In particular, Mixed Rossby-gravity (MRG) waves are primary drivers of synoptic (2-6 day) variability. MRGs were the first waves predicted by Matsuno's equatorial shallow water theory to be observed in the atmosphere. Hundreds of papers have been written on this important mode, starting with the pioneering work of Yanai and collaborators in the 1960's. MRGs comprise the westward propagating branch of the inertio-gravity wave continuum from Matsuno's theory, and their signals in space-time spectra of equatorial cloudiness and vorticity merge with those of the eastward inertio-gravity (EIG) mode. Although the spectral signals for convection associated with both MRG and EIG waves are very strong in the time mean, there are virtually no studies in the literature on convectively coupled EIG waves. This essentially results from the fact that, for a given amount of divergent flow tied to convection, overall MRG circulations are much stronger than those for EIGs, thus EIGs are much more difficult to observe in the wind field. This talk will discuss evidence that, despite the fact that MRG circulations are dominant in the tropics, the convective activity associated with the EIG branch should not be considered to be independent from MRG activity for much of the time. It turns out that the resulting interference between the MRG and EIG waves results in strong anomalies in convection of opposite sign across the equator within the ITCZ and SPCZ that are standing in the zonal direction, as first noted by Wallace and Holton almost 50 years ago. We show that these these out of phase convective oscillations about the equator actually propagate poleward on a 3-6 day time scale and are unique to the west Pacific warm pool, the only region of the globe with a true double ITCZ in the time mean.