The El Nino-Southern Oscillation (ENSO) phenomenon and variability in the subtropical North Atlantic high sea level pressure (SLP) are known to affect rainfall in the Caribbean region. An El Nino event is associated with drier-than-average conditions during the boreal summer of year (0), and wetter-than-average conditions during the spring of year (+1). Dry conditions during the summer of year (0) of an El Nino are associated with the locally divergent surface circulation engendered by the eastward shift of deep convection in the Pacific Ocean. Wet conditions during the spring of year (+1) of an El Nino are associated with the lagged warming of the tropical North Atlantic Ocean. Variability in the strength of the North Atlantic high is governed mainly by the North Atlantic oscillation (NAO) with a positive NAO phase implying a stronger than normal high and vice versa. The NAO is negatively correlated with Caribbean rainfall indirectly via anomalous sea surface temperatures (SST) associated with anomalies in the surface wind speed at tropical latitudes and directly via anomalous subsidence. The combined effect of the two phenomena is found to be strongest when the two signals interfere constructively:During the summer following winters characterized by the positive phase of the NAO, the dryness associated with a developing warm ENSO event adds to the dryness associated with a positive SLP anomaly in the subtropical North Atlantic (7 out of 11 El Ninos between 1949 and 1999 fall in this category).During the spring following winters characterized by the negative phase of the NAO, the wetness that follows a warm ENSO event is augmented by the wetness associated with the warmer-than-average tropical North Atlantic SSTs (5 out of 11 El Ninos between 1949 and 1999 fall in this category).The coincidence in the recurrence of a positive phase of the NAO during the winters coinciding with peak warm ENSO conditions has increased in the last 20 years in comparison with the previous few decades. This partially explains the noticeable consistent dry signal over the Caribbean during the summer of year (0) of a warm ENSO event and the disappearance of the wet signal during the spring of year (+1) in the recent record.
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