ON THE EVOLUTION - DYNAMICS OF T R O P I C A L OCEAN - ATMOSPIIERE ANNUAL - CYCI . lc VARIAIII 1 . ITY

The structure of ocean-atmosphere annual-cycle variability across the global tropics is extracted from the Comprehensive Ocean-Atmosphere Data Set (COADS) surface winds and SSTS, and oceanic heat-content simulation from a nonlinear shallow water model (forced by COADS wind stress) using the co-variance based rotated principal component analysis technique. ‘l’he coupled annual-cycle variability is compactly describable using Iwo modes that are in temporal quadrature, and whose structures are insensitive to the inclusion/exclusion of oceanic heatcontent in the combined variability analysis. The firs? mode, peaking in July (and January), represents large-scale rnonsoonal flow onto the warmer continents: Indo-China, Central America, and western Africa, ~’he second annual-cycle mode peaks in October (and April) when it represents the extreme phases of SST annual variability in the eastern oceans. An analysis of the associated surface-momentum balance shows the equatorial flow in the cold-tongue’s core region to be dynamically consistent with the ‘SST-gradient driven’ sea-level pressure gradients, and more so, when the impact of near-surface static-stability horizontal-momentum dissipation is factored in. inspection of temporal phasing of variation on the extracted annual-cycle related zonal and meridional winds w.r.t. SSTS furthermore shows easterly-tendency to lead local cooling in the off-coastal ]Ongitudes (westward of 10OOW) by about a month. Taken together, this indicates that the co]d-tongue’s westward expansion in the off-coastal eastern Pacific and Atlantic results from the local interaction of zonal wind and zonal SSrl’-gradient, particularly, at the tongue’s leading edge; the westward expansion of warm-phase SSTS proceeds similarly. A ‘westward expansion hypothesis’ encapsulating these coupled interactions is proposed. The weak SST annual-cycle in the off-coastal northern equatorial Indian Ocean, on the other hand, appears to be driven by modulations of latent-heat flLlx and oceanic mixed-layer stirring that are induced by robust surface wind-speed variability associated with the Asian monsoon, J. Introduction The ocean-atmosphere seasonal cycle in the eastern Pacific and Atlantic is notable for its robustness, and distinctiveness from the seasonal march at other tropical longitudes. ~’he coupled seasonal cycle is also remarkable because its period is twice that of the dominant external forcing Pacific has existed since Horel’s (1982) annual-harmonic analysis of and atmospheric variables, the interest in understanding the dynamics several related of the coupled in these latitudes, While a comprehensive documentation of seasonal-cycle variability in the tropical oceanic ocean-atmosphere seasonal-cycle evolution is rather recent (e.g., Chao, 1990), and, principally, spurred by the need to better understand, model and predict the low-frequency El Nifio Southern Oscillation (ENSO) variability. The latter is connected to aspects of seasonal-cycle variability in the eastern Pacific (Rasmussen and Carpenter, 1982), and known for its global climatic impacts (Ropelewski and }Ialpert, 1987). The mechanism(s) responsible for the unique seasonal evolution of the tropical oceanatmosphere system have been explored by Mitchell and Wallace (1992) and Wang (1992), and from a dynamical instability perspective by Chang and Philander (1 994), Mitchell and Wallace’s (1992) diagnostic analysis reveals the importance of interactions/feedbacks of the eastern ocean basins with the adjoining land-masses, and with insolation via shortwave modulation by the lowIevel stratus decks, windstress changes evolving Central American monsoons) in promoting the westward expansion of the SST coldtongue via an oceanic mixed-layer remote response. Wang’s (1992) multi variate extended empirical orthogonal function analysis of outgoing longwave radiation and Comprehensive Ocean-Atn]osphere I>ata Set’s (COADS) surface winds, Mitchell and Wallace (1 992) argue for a leading role of the mcridiomf at the northern edge of the equatorial waveguide (that are driven by the