[1] An adjoint method is applied to a three-dimensional global ocean biogeochemical cycle model to optimize the ecosystem parameters on the basis of SeaWiFS surface chlorophyll observation. We showed with identical twin experiments that the model simulated chlorophyll concentration is sensitive to perturbation of phytoplankton and zooplankton exudation, herbivore egestion as fecal pellets, zooplankton grazing, and the assimilation efficiency parameters. The assimilation of SeaWiFS chlorophyll data significantly improved the prediction of chlorophyll concentration, especially in the high-latitude regions. Experiments that considered regional variations of parameters yielded a high seasonal variance of ecosystem parameters in the high latitudes, but a low variance in the tropical regions. These experiments indicate that the adjoint model is, despite the many uncertainties, generally capable to optimize sensitive parameters and carbon fluxes in the euphotic zone. The best fit regional parameters predict a global net primary production of 36 Pg C yr À1

Equatorial East Africa suffered severe drought during its 2005 "short rains," centered on OctoberNovember. The circulation mechanisms of such precipitation anomalies are examined, using long-term upper-air and surface datasets, and based on diagnostic findings from earlier empirical investigations. The steep eastward pressure gradient is conducive to fast westerlies over the central-equatorial Indian Ocean, surface manifestation of a powerful zonal circulation cell with subsidence over East Africa, and ascending motion over Indonesia. With fast westerlies, rainfall in East Africa is deficient and they tend to be accompanied by anomalously cold waters in the northwestern and warm anomalies in the southeastern extremity of the equatorial Indian Ocean Basin, without any seesaw between these domains. In October-November 2005, pressure in the west was anomalously high, entailing a steep eastward pressure gradient along the equator, conducive to fast westerlies and, further symptomatic of the zonal circulation cell, subsidence in the west and ascending motion in the east were enhanced. Overall, the chain of causalities can be traced to anomalously high pressure in the west.

Expanding earlier studies on the boreal spring and autumn rainy seasons in equatorial East Africa, pending challenges on the mechanisms of rainfall variability, are investigated. Eastward pressure gradient and slack south Indian Ocean trade winds allow surface equatorial westerlies in spring and autumn. Complementing that, upper-tropospheric easterlies are required for the development of a zonal vertical circulation cell along the Indian Ocean equator. Because of the summer warming and high stand of upper-tropospheric topography over South Asia, strong upper-tropospheric easterlies over the tropical northern and equatorial Indian Ocean persist from summer into autumn, thus allowing the development of a zonal vertical circulation cell. By contrast, the winter cooling entails low stand of upper-tropospheric topography in the north, thus hindering easterlies over the equator. Consequently, an equatorial zonal circulation cell does not develop in boreal spring. The equatorial zonal circulation cell, with subsidence over East Africa, strongly controls the boreal autumn rains, as evidenced in their tight correlation with the equatorial westerlies. In a related vein, rain gauge stations show much shared variance in boreal autumn as compared to spring. Plausibly consistent with this, boreal autumn rather than spring has brought the extreme flood and drought disasters in the course of the past half-century.

[1] Building on earlier work on the interannual variability of the boreal autumn equatorial westerlies (UEQ) over the Indian Ocean and concomitant rainfall anomalies at the coast of East Africa and in Indonesia, the inherent circulation mechanisms are here explored further from long-term surface and upper air data. Fast UEQ and deficient East African rainfall come with positive sea level pressure (P) and negative sea surface temperature (T ) departures in a domain (W) at the northwestern extremity and opposite departures in a domain (E) at the southeastern extremity of the equatorial Indian Ocean. However, there is no seesaw between W and E in either P or T and no indication of local forcing of T on P. The large-scale pressure field, in particular the zonal pressure gradient along the equator and the South Indian Ocean pressure and southern tradewinds, control the evolution of UEQ. Fast UEQ steepens the zonal temperature gradient, thus tightening the inverse relationships between the zonal gradients of pressure and temperature. The rainfall anomalies associated with the interannual variability of UEQ, surface manifestation of a zonal circulation cell along the Indian Ocean equator, are favored by the kinematic and thermodynamic conditions in W and E. Thus, with fast UEQ the domain W features departure lower tropospheric divergence and subsidence and, favored by the cold T and subsidence, reduced precipitable water, all conducive to deficient precipitation. By contrast, E has departure lower tropospheric convergence and ascending motion and, favored by the warm T and ascending motion, enhanced precipitable water, in conjunction conducive to abundant rainfall. The interannual variability of the boreal autumn equatorial westerlies, dominated as it is by the large-scale pressure field, is crucial in the climate dynamics of the equatorial Indian Ocean region. This leads to the question: What controls the pressure pattern over the Indian Ocean basin?

Building on an earlier report on the 2005 drought in equatorial East Africa, this short note examines the circulation mechanisms of the anomalies in the boreal autumn ''short rains'' season in the subsequent three years. Westerlies during this season are the surface manifestation of a powerful zonal-vertical circulation cell along the Indian Ocean equator. The surface equatorial westerlies were fast during the 2005 and 2008 droughts, near average during the near-average 2007 short rains, and slack during the 2006 floods, consistent with the known circulation diagnostics.