The Caribbean low-level jet: Its structure and interannual variability

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The Caribbean region shows maxima in easterly winds greater than 12 m/s at 925 hPa in July and February referred herein as the summer and winter Caribbean low-level jet (CBNLLJ), respectively. The purpose of this study is to identify the mechanisms of the CBNLLJ formation and variability and their association to the regional hydroclimate. To study the climatological aspects of the CBNLLJ, climatological fields are calculated from 1979 to 2001. It is observed that the low-level (925-hPa) zonal wind over the Caribbean basin has a semi-annual cycle. The semi-annual cycle has peaks in February and July that are regional amplifications of the large-scale circulation by means of a meridional pressure gradient. High mountains to the south of the Caribbean Sea influence the air temperature meridional gradient providing a baroclinic structure that favors a stronger easterly wind. The boreal summer strengthening of the CBNLLJ is associated with subsidence over the subtropical North Atlantic from the May-to-July evolution of the Central American monsoon. Additionally, the mid-summer minimum of Caribbean precipitation is related to the Caribbean LLJ through greater moisture flux divergence.

The CBNLLJ has interannual variability with greater standard deviation during boreal summer. The summer interannual variability of the CBNLLJ is due to variability of the meridional pressure gradient across the Caribbean basin influenced by sea surface temperature (SST) gradients between the tropical Pacific and the tropical Atlantic. To determine the inter-decadal changes statistical diagnostics of summer CBNLLJ anomalies are analyzed for different periods, in particular 1958-1978 and 1979-2001. Also analyzed were inter-basin SLP and SST gradient indices. The CBNLLJ showed more intense events and greater persistence during 1979-2001 than during 1958-1978 as a result of more extreme SLP and SST inter-basin gradients during the more recent decades. These are due to changes in the relation between the Pacific and the Atlantic after the late 1970's. The results of the present study are important in the context of understanding an intrinsic component of the Caribbean climate, i.e., the Caribbean low-level jet.