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Winter and Summer Structure of the Caribbean Low-Level Jet

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Winter and Summer Structure of the Caribbean Low-Level JetERNESTO MUÑOZ AND ANTONIO J. BUSALACCHIEarth System Science Interdisciplinary Center, and Department of Atmospheric and Oceanic Science, University of Maryland,College Park, College Park, MarylandSUMANT NIGAMDepartment of Atmospheric and Oceanic Science, and Earth System Science Interdisciplinary Center, University of Maryland,College Park, College Park, MarylandALFREDO RUIZ-BARRADASDepartment of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland(Manuscript received 22 January 2007, in final form 13 July 2007)ABSTRACTThe Caribbean region shows maxima in easterly winds greater than 12 m s⫺1at 925 hPa in July andFebruary, herein referred to as the summer and winter Caribbean low-level jet (LLJ), respectively. It isimportant to understand the controls and influences of the Caribbean LLJ because other LLJs have beenobserved to be related to precipitation variability. The purpose of this study is to identify the mechanismsof the Caribbean LLJ formation and variability and their association to the regional hydroclimate. Clima-tological fields are calculated from the North American Regional Reanalysis and the 40-yr ECMWFRe-Analysis from 1979 to 2001. It is observed that the low-level (925 hPa) zonal wind over the Caribbeanbasin has a semiannual cycle and an interannual variability, with greater standard deviation during borealsummer. The semiannual cycle has peaks in February and July, which are regional amplifications of thelarge-scale circulation. High mountains to the south of the Caribbean Sea influence the air temperaturemeridional gradient, providing a baroclinic structure that favors a stronger easterly wind. The borealsummer strengthening of the Caribbean LLJ is associated with subsidence over the subtropical NorthAtlantic from the May-to-July shift of the ITCZ and the evolution of the Central American monsoon.Additionally, the midsummer minimum of Caribbean precipitation is related to the Caribbean LLJ throughgreater moisture flux divergence. From May to September the moisture carried by the Caribbean LLJ intothe Gulf of Mexico is strongest. The summer interannual variability of the Caribbean LLJ is due to thevariability of the meridional pressure gradient across the Caribbean basin, influenced by tropical Pacificvariability during summer.1. IntroductionLow-level jets (LLJs) are regional maxima of windsin the lower troposphere (Stensrud 1996). Two promi-nent LLJs in the Americas are the Great Plains LLJ(GPLLJ; Helfand and Schubert 1995; Ting and Wang2006) and the South American LLJ (SALLJ; Berberyand Collini 2000; Vera et al. 2006). Whereas a body ofresearch has developed over the past decade regardingthe GPLLJ and the SALLJ, the easterly LLJ over theCaribbean Sea has remained understudied. With a peakduring summer, the Caribbean LLJ is part of the circu-lation that flows from the Caribbean Sea through theGulf of Mexico, transporting moisture into the conti-nental United States (Rasmusson 1967; Bosilovich andSchubert 2002). A greater understanding of the Carib-bean LLJ will improve our understanding of the role ofthe Caribbean atmospheric circulation on the atmo-spheric moisture fluxes from the Caribbean Sea.Currently, only a few studies explicitly documentthe atmospheric low-level jet in the Caribbean andCorresponding author address: Ernesto Muñoz, Cooperative In-stitute for Marine and Atmospheric Studies, University of Miami,4600 Rickenbacker Causeway, Miami, FL 33149.E-mail: [email protected] JOURNAL OF CLIMATE VOLUME 21DOI: 10.1175/2007JCLI1855.1© 2008 American Meteorological SocietyJCLI1855some aspects of its annual and interannual variability.Stensrud (1996) indicated the western Caribbean Sea tobe an area where a low-level atmospheric jet is sus-pected to exist. More recently, Mo et al. (2005) identi-fied a summer maximum of surface (scatterometer)zonal winds over the region (12°–14°N, 70°–80°W), andassociated it with the Caribbean LLJ. Still, much is un-known about the forcings of the Caribbean LLJ (eitherlocal or remote) or about the influences the LLJ has onthe regional hydroclimate. This study will focus on theseasonal and interannual variability of the easterly LLJover the Caribbean Sea and its forcings from the re-gional- and larger-scale climate.Although no single definition applies to all LLJs inthe global atmosphere, there are a few characteristicsthat serve to identify an atmospheric LLJ as discussedby Stensrud (1996). The principal criterion is the exis-tence of a maximum in wind speed in a contained areain the lower levels (below 700 hPa) of the atmosphere,that is, a narrow band of strong winds. The verticalstructure of the wind should be one with vertical shear,that is, a wind vertical profile with weaker winds at thebottom, a maximum in wind speed above, and a de-crease of wind speed at higher levels. Another criterionis a horizontal wind structure with horizontal shear,that is, weaker winds at the edges of the jet. Thesecriteria are met by the LLJ in the Caribbean, as dem-onstrated in this study.A similarity between the Caribbean LLJ and theGPLLJ and SALLJ is that they flow along mountains.The GPLLJ flows along the eastern side of the SierraMadre and the Rocky Mountains of North Americawhile the SALLJ flows along the eastern side of theAndes of South America. The Caribbean Sea, overwhich the Caribbean LLJ flows, is bounded to the southby the northern coast of South America, which hasmountains higher than 1 km. The GPLLJ and theSALLJ have been observed to be influenced by thehorizontal temperature gradients resulting from the to-pography of their domain (Holton 1967). The potentialof a similar influence from the mountains of northernSouth America on the Caribbean LLJ is presented inthis study.Other LLJs, such as the Great Plains and the SouthAmerican LLJs, have been shown to be important fortheir potential to transport moisture from remote up-wind regions, their relation to nighttime convection,and their potential to alter the convergent and diver-gent circulations on interannual time scales (Blackadar1957; Bonner and Paegle 1970; Berbery and Barros2002). LLJs have an important role in the exchange ofatmospheric water vapor from upwind to downwind re-gions (Berbery et al. 1996). Previous studies (Rasmus-son 1967; Bosilovich and Schubert 2002) have indicatedthe Caribbean Sea as a moisture source for the Gulf ofMexico and the


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