Tuesday, 14 May 2002
Evaporated moisture from Tropical North Atlantic Ocean: Where does it fall?
The Tropical North Atlantic Ocean supplies precipitation to both North and South America. It is important to understand the fate of moisture evaporated from this vast, virtually unlimited source. To address the question, we perform forward-trajectory analysis for evaporated moisture from the Tropical North Atlantic using the NCEP reanalyses and COLA's global hybrid precipitation data. Covering a 19-year period, from September 1979 to August 1998, this study provides seasonal maps of precipitation sinks of evaporated moisture from this ocean region. The NCEP reanalyses provide fields that are necessary for the dynamics in the trajectory analysis.
As expected, seasonal climatology of the sinks is driven by the seasonal dynamics of the ITCZ and is constrained by the orography of the continents. Recycled precipitation over the ocean ranges from 17% (DJF) to 33% (SON) of the total evaporated moisture. The Amazon River basin is the most important terrestrial sink during SON, DJF, and MAM. Surprisingly, the Africa continent precipitates slightly more moisture from this source than the Amazon during JJA. The seasonal swing of the inter-tropical convergence zone (ITCZ) may play an important role in the establishment of two climatic zones over the Amazon: North Amazon (NA) and South Amazon (SA), divided at 5oS. Over the 19-year period, the interannual variability of the Amazon precipitation sink has a strong negative correlation with the Niņo 3.4 index. The correlation suggests that the precipitation contributed by Tropical North Atlantic in the Amazon is reduced (enhanced) during the El Niņo (La Niņa) event. Trend analysis of the sinks and moisture transport suggests a reduction in local rain-producing conditions associated with the negative trend in total NA precipitation during JJA over the 19-year period. Trend analysis also suggests a possible northward shift of the North American polar front allowing more moisture to be transported to the Mississippi River basin by transient eddies, resulting in a positive trend for the share of Tropical North Atlantic moisture to that region during DJF over the 19-year period.
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