Investigating the effect of ENSO upon oceanic rainfall estimates

With a lack of ground observations over the oceans, much of our knowledge of oceanic rainfall is largely obtained from microwave precipitation retrievals using space-borne satellite measurements. In order to understand the variability of global and regional rainfall, or infer global rainfall variability given an altered climate, a consensus must exist in rainfall observations. While there is general agreement, discrepancies are found between active and passive microwave oceanic precipitation retrievals, particularly during different phases of ENSO where passive precipitation retrievals exhibit a systematic shift in precipitation seemingly correlated with ENSO phase. To investigate the causality of this relationship, this paper focuses, both spatially and temporally, on the evolution of precipitation organization over a period of El Niño, La Niña, and neutral conditions and their impacts on active and passive retrieved precipitation over a tropical belt covering ±30 degrees of latitude. The foremost findings link the anomalous shifts in passive oceanic rainfall intensity to organized oceanic precipitating systems. Shifts toward more organization of precipitating systems during El Niño conditions bring about an increase in precipitation for passive retrievals that active systems do not detect. While a higher percentage of tropical precipitating systems are classified as isolated convection, the majority of tropical rainfall originates from organized precipitating systems (i.e. mesoscale convective systems), and variability in organized precipitating systems regulates the primary changes observed in total precipitation over the tropical belt. Variability in organization leads to changes in the global fraction of rainfall produced from stratiform regions and the biases between active and passive retrievals of the rain intensity in stratiform regions appear to be the likely cause for the discrepancies found in total oceanic rainfall between active and passive retrievals. Data from the recently launched Global Precipitation Mission are used to further investigate these findings along with ground validation studies using the Kwajalein radar site.