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    The Midsummer Drought over Mexico and Central America

    Source: Journal of Climate:;1999:;volume( 012 ):;issue: 006::page 1577
    Author:
    Magaña, Victor
    ,
    Amador, Jorge A.
    ,
    Medina, Socorro
    DOI: 10.1175/1520-0442(1999)012<1577:TMDOMA>2.0.CO;2
    Publisher: American Meteorological Society
    Abstract: The annual cycle of precipitation over the southern part of Mexico and Central America exhibits a bimodal distribution with maxima during June and September?October and a relative minimum during July and August, known as the midsummer drought (MSD). The MSD is not associated with the meridional migration of the intertropical convergence zone (ITCZ) and its double crossing over Central America but rather with fluctuations in the intensity and location of the eastern Pacific ITCZ. During the transition from intense to weak (weak to intense) convective activity, the trade winds over the Caribbean strengthen (weaken). Such acceleration in the trade winds is part of the dynamic response of the low-level atmosphere to the magnitude of the convective forcing in the ITCZ. The intensification of the trade winds during July and August and the orographic forcing of the mountains over most of Central America result in maximum precipitation along the Caribbean coast and minimum precipitation along the Pacific coast of Central America. Changes in the divergent (convergent) low-level winds over the ?warm pool? off the west coast of southern Mexico and Central America determine the evolution of the MSD. Maximum deep convective activity over the northern equatorial eastern Pacific, during the onset of the summer rainy season, is reached when sea surface temperatures exceed 29°C (around May). After this, the SSTs over the eastern Pacific warm pool decrease around 1°C due to diminished downwelling solar radiation and stronger easterly winds (during July and August). Such SST changes near 28°C result in an substantial decrease in deep convective activity, associated with the nonlinear interaction between SST and deep tropical convection. Decreased deep tropical convection allows increased downwelling solar radiation and a slight increase in SSTs, which reach a second maximum (?28.5°C) by the end of August and early September. This increase in SST results once again in stronger low-level convergence, enhanced deep convection, and, consequently, in a second maximum in precipitation. The MSD signal can also be detected in other variables such as minimum and maximum surface temperature and even in tropical cyclone activity over the eastern Pacific.
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      The Midsummer Drought over Mexico and Central America

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    contributor authorMagaña, Victor
    contributor authorAmador, Jorge A.
    contributor authorMedina, Socorro
    date accessioned2017-06-09T15:44:26Z
    date available2017-06-09T15:44:26Z
    date copyright1999/06/01
    date issued1999
    identifier issn0894-8755
    identifier otherams-5218.pdf
    identifier urihttp://onlinelibrary.yabesh.ir/handle/yetl/4191934
    description abstractThe annual cycle of precipitation over the southern part of Mexico and Central America exhibits a bimodal distribution with maxima during June and September?October and a relative minimum during July and August, known as the midsummer drought (MSD). The MSD is not associated with the meridional migration of the intertropical convergence zone (ITCZ) and its double crossing over Central America but rather with fluctuations in the intensity and location of the eastern Pacific ITCZ. During the transition from intense to weak (weak to intense) convective activity, the trade winds over the Caribbean strengthen (weaken). Such acceleration in the trade winds is part of the dynamic response of the low-level atmosphere to the magnitude of the convective forcing in the ITCZ. The intensification of the trade winds during July and August and the orographic forcing of the mountains over most of Central America result in maximum precipitation along the Caribbean coast and minimum precipitation along the Pacific coast of Central America. Changes in the divergent (convergent) low-level winds over the ?warm pool? off the west coast of southern Mexico and Central America determine the evolution of the MSD. Maximum deep convective activity over the northern equatorial eastern Pacific, during the onset of the summer rainy season, is reached when sea surface temperatures exceed 29°C (around May). After this, the SSTs over the eastern Pacific warm pool decrease around 1°C due to diminished downwelling solar radiation and stronger easterly winds (during July and August). Such SST changes near 28°C result in an substantial decrease in deep convective activity, associated with the nonlinear interaction between SST and deep tropical convection. Decreased deep tropical convection allows increased downwelling solar radiation and a slight increase in SSTs, which reach a second maximum (?28.5°C) by the end of August and early September. This increase in SST results once again in stronger low-level convergence, enhanced deep convection, and, consequently, in a second maximum in precipitation. The MSD signal can also be detected in other variables such as minimum and maximum surface temperature and even in tropical cyclone activity over the eastern Pacific.
    publisherAmerican Meteorological Society
    titleThe Midsummer Drought over Mexico and Central America
    typeJournal Paper
    journal volume12
    journal issue6
    journal titleJournal of Climate
    identifier doi10.1175/1520-0442(1999)012<1577:TMDOMA>2.0.CO;2
    journal fristpage1577
    journal lastpage1588
    treeJournal of Climate:;1999:;volume( 012 ):;issue: 006
    contenttypeFulltext
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