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Winter and Summer Structure of the Caribbean Low-Level Jet



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1260 JOURNAL OF CLIMATE VOLUME 21 Winter and Summer Structure of the Caribbean Low Level Jet ERNESTO MU OZ AND ANTONIO J BUSALACCHI Earth System Science Interdisciplinary Center and Department of Atmospheric and Oceanic Science University of Maryland College Park College Park Maryland SUMANT NIGAM Department of Atmospheric and Oceanic Science and Earth System Science Interdisciplinary Center University of Maryland College Park College Park Maryland ALFREDO RUIZ BARRADAS Department of Atmospheric and Oceanic Science University of Maryland College Park College Park Maryland Manuscript received 22 January 2007 in final form 13 July 2007 ABSTRACT The Caribbean region shows maxima in easterly winds greater than 12 m s 1 at 925 hPa in July and February herein referred to as the summer and winter Caribbean low level jet LLJ respectively It is important to understand the controls and influences of the Caribbean LLJ because other LLJs have been observed to be related to precipitation variability The purpose of this study is to identify the mechanisms of the Caribbean LLJ formation and variability and their association to the regional hydroclimate Climatological fields are calculated from the North American Regional Reanalysis and the 40 yr ECMWF Re Analysis from 1979 to 2001 It is observed that the low level 925 hPa zonal wind over the Caribbean basin has a semiannual cycle and an interannual variability with greater standard deviation during boreal summer The semiannual cycle has peaks in February and July which are regional amplifications of the large scale circulation 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 Caribbean LLJ is associated with subsidence over the subtropical North Atlantic 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 through greater moisture flux divergence From May to September the moisture carried by the Caribbean LLJ into the Gulf of Mexico is strongest The summer interannual variability of the Caribbean LLJ is due to the variability of the meridional pressure gradient across the Caribbean basin influenced by tropical Pacific variability during summer 1 Introduction Low level jets LLJs are regional maxima of winds in the lower troposphere Stensrud 1996 Two prominent LLJs in the Americas are the Great Plains LLJ GPLLJ Helfand and Schubert 1995 Ting and Wang 2006 and the South American LLJ SALLJ Berbery and Collini 2000 Vera et al 2006 Whereas a body of Corresponding author address Ernesto Mu oz Cooperative Institute for Marine and Atmospheric Studies University of Miami 4600 Rickenbacker Causeway Miami FL 33149 E mail ernesto munoz noaa gov DOI 10 1175 2007JCLI1855 1 2008 American Meteorological Society research has developed over the past decade regarding the GPLLJ and the SALLJ the easterly LLJ over the Caribbean Sea has remained understudied With a peak during summer the Caribbean LLJ is part of the circulation that flows from the Caribbean Sea through the Gulf of Mexico transporting moisture into the continental United States Rasmusson 1967 Bosilovich and Schubert 2002 A greater understanding of the Caribbean LLJ will improve our understanding of the role of the Caribbean atmospheric circulation on the atmospheric moisture fluxes from the Caribbean Sea Currently only a few studies explicitly document the atmospheric low level jet in the Caribbean and 15 MARCH 2008 MU OZ ET AL some aspects of its annual and interannual variability Stensrud 1996 indicated the western Caribbean Sea to be an area where a low level atmospheric jet is suspected to exist More recently Mo et al 2005 identified a summer maximum of surface scatterometer zonal winds over the region 12 14 N 70 80 W and associated it with the Caribbean LLJ Still much is unknown about the forcings of the Caribbean LLJ either local or remote or about the influences the LLJ has on the regional hydroclimate This study will focus on the seasonal and interannual variability of the easterly LLJ over the Caribbean Sea and its forcings from the regional and larger scale climate Although no single definition applies to all LLJs in the global atmosphere there are a few characteristics that serve to identify an atmospheric LLJ as discussed by Stensrud 1996 The principal criterion is the existence of a maximum in wind speed in a contained area in the lower levels below 700 hPa of the atmosphere that is a narrow band of strong winds The vertical structure of the wind should be one with vertical shear that is a wind vertical profile with weaker winds at the bottom a maximum in wind speed above and a decrease of wind speed at higher levels Another criterion is a horizontal wind structure with horizontal shear that is weaker winds at the edges of the jet These criteria are met by the LLJ in the Caribbean as demonstrated in this study A similarity between the Caribbean LLJ and the GPLLJ and SALLJ is that they flow along mountains The GPLLJ flows along the eastern side of the Sierra Madre and the Rocky Mountains of North America while the SALLJ flows along the eastern side of the Andes of South America The Caribbean Sea over which the Caribbean LLJ flows is bounded to the south by the northern coast of South America which has mountains higher than 1 km The GPLLJ and the SALLJ have been observed to be influenced by the horizontal temperature gradients resulting from the topography of their domain Holton 1967 The potential of a similar influence from the mountains of northern South America on the Caribbean LLJ is presented in this study Other LLJs such as the Great Plains and the South American LLJs have been shown to be important for their potential to transport moisture from remote upwind regions their relation to nighttime convection and their potential to alter the convergent and divergent circulations on interannual time scales Blackadar 1957 Bonner and Paegle 1970 Berbery and Barros 2002 LLJs have an important role in the exchange of atmospheric water vapor from upwind to downwind re 1261 gions Berbery et al 1996 Previous studies Rasmusson 1967 Bosilovich and Schubert 2002 have indicated the Caribbean Sea as a moisture source for the Gulf of Mexico and the continental United States during boreal summer In this study the


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