Dependence of equatorial Rossby waves on the background flow and extratropical forcing

This observational study focuses on atmospheric Equatorial Rossby (ER) waves propagating over the warm pool.  We investigate the background conditions and potential extra-tropical precursors that are favorable for ER waves to develop. ER waves are isolated through an Empirical Orthogonal Function (EOF) analysis performed season by season from 30ºS to 30ºN on westward filtered NOAA 2.5ºx2.5º Outgoing Long-wave Radiation (OLR) data and CLAUS brightness temperature data (both used as proxies for tropical deep convection). Dynamical variables from NCEP reanalysis, raw OLR, and brightness temperature are then projected onto the principal components (PCs) for different time lags. The wave structure and propagation characteristics are analyzed as well as the basic-state horizontal and vertical shear. The potential forcing by extra-tropical transient activity is also examined through E-vector, Eddy Kinetic Energy, and measures of extratropical Rossby wave breaking east of the Asian jet exit region. ER waves with symmetric structures across the equator are common both in conjunction with and independent of the Madden Julian Oscillation (MJO). They are particularly important in modulating convective activity within the South Pacific Convergence Zone (SPCZ) and in the ITCZ over the warm pool of the western Pacific and Indian Ocean. During northern winter, enhanced extra-tropical Rossby wave activity is seen penetrating into the tropics in the East Pacific sector prior to the development of ER waves over the central Pacific. The PC time-series can be used to look at changes in ER occurrences over time, and this analysis reveals substantial intra-seasonal variations in ER wave activity that can be related to a modulation of the background flow by the MJO, as well as extra-tropical impacts.