H53C-01
Midsummer Drought in Mexico and Central America and its Relationship With the Eastern Pacific Gap Winds
The low-level circulation in the Northeastern Tropical Pacific (NETP) is affected by three main wind systems: the northerly and southerly trade winds and the wind jets coming through the Tehuantepec, Papagayo and Panama mountain gaps. The seasonal and intraseasonal evolution of these wind systems determines the low-level circulation over the NETP on these timescales. QSCAT wind data (July 1999-December 2006) show that the low- level circulation over the central region of the NETP is mainly directed westward from November to May and undergoes wind direction changes during summer, from weak westerlies in June to easterlies in July and August and changing back to westerlies in September-October. This change in the circulation pattern during midsummer is associated with the Tehuantepec and Papagayo wind jets, which slightly strengthen favored by the westward elongation and intensification of the Azores-Bermuda high. There is a high correlation, in the seasonal, monthly and synoptic timescales, among the zonal winds over the central region of the NETP, the Tehuantepec and Papagayo wind jets, the meridional pressure gradients in the Isthmus of Tehuantepec and the Caribbean Sea, and the precipitation rates in central-southern Mexico and Central America, where the midsummer drought occurs. The westward low-level circulation observed over the central-eastern region of the NETP during midsummer, that occurs simultaneously with the strengthening of the wind jets, induces westward moisture fluxes in the lower layers of the atmosphere, displaces convergence areas away from the coasts, and causes the relatively strong convergence in the easternmost NETP to remain confined south of the area of influence of the wind jets and associated westward winds over the central NETP. These factors determine, to a large extent, the midsummer drought in central-southern Mexico and Central America.
H53C-02
Regional Variations of the Caribbean Mid-Summer Drought
Pentad satellite-based precipitation estimates were input into a wavelet analysis to quantify the length, timing, and strength of the mid-summer drought (MSD) for the Caribbean Sea and surrounding regions. For most of the Caribbean the time between the first and second summer precipitation maxima is 98 to 177 days (3 to 6 months). The MSD appears in early-June over Puerto Rico and Hispaniola, and develops progressively later in the summer season towards the west, finally occurring in early-October over the Gulf of Mexico. The MSD is most intense in the eastern Pacific, strong and significant in the western Caribbean, and almost nonexistent in the eastern Caribbean. Forcing mechanisms are examined to help explain the regional variability in the Caribbean. A July increase in surface pressure and surface divergence, caused by the changing wind field, appears to contribute to a strong concurrent MSD over the waters bounded by Jamaica, Cuba, and the Yucatan peninsula. Finally, the island of Jamaica itself appears to block the flow of the tradewinds as they migrate northward and intensify into mid-summer, thus enhancing the divergence, and in turn MSD, immediately to the west.
H53C-03
The Structure and Summer Intensification of the Caribbean Low-Level Jet
The Caribbean region shows maxima in easterly winds greater than 12 m/s at 925 hPa in July and February. These maxima in February and July are referred to as the winter and summer Caribbean Low-Level Jet (LLJ), respectively. The purpose of this study is to identify the mechanisms of the Caribbean LLJ formation. Another objective is to identify the influence of the Caribbean LLJ on the hydroclimate of the Intra-Americas Sea region and on the Mid-Summer Drought. Climatological fields are calculated from the North American Regional Reanalysis and the ECMWF 40-Year Reanalysis. In February and July the Caribbean LLJ is a regional amplification 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 flow. The boreal summer strengthening of the Caribbean LLJ responds to subsidence over the subtropical North Atlantic from the May-to- July northward shift of the Atlantic Marine ITCZ and to the east-west gradient of diabatic heating along the Caribbean Sea.
H53C-04
The Size, Strength, and Rainfall Production of Convective Systems Over Southern Mexico, Central America, and the East Pacific Warm Pool During Active Versus Break Rainfall Periods
The annual cycle of rainfall over much of southern Mexico, Central America, and parts of the east Pacific warm pool features maxima in June and September-October. The relative break in rainfall, usually during July and August, is known as the Mid Summer Drought. This study focuses on the variations in the properties of the individual convective systems between active and break periods, which may be influencing the rainfall variations. The 3B42 product is used to determine when the active and break periods occur each year for 1998-2006. Then, the University of Utah TRMM Precipitation Feature Database is used to determine the size, intensity, and rainfall production of the individual convective systems during active and break periods. The statistics are compared for the active versus the break periods for each subregion. Preliminary results show that, similar to the Indian and Australian monsoons, active periods are characterized by large but relatively weak convective systems and break periods by smaller but more intense systems.
H53C-05
Mayan Historical drought trends
In this work we present a more objective and general cycle-length determination of catalogues of past drought data for the Yucatan Peninsula between 1502 and 1900 coming from historical written documentation. We use the wavelet transformation based on the Morlet wavelet, and found that the most prominent frequencies of the historical drought series are ~ 3,4, 7, 12, 20, 43 and 70 years. We studied the relation between historical droughts and several large-scale climate phenomena represented by the Atlantic Multidecadal Oscillation (AMO) and the Southern Oscillation Index (SOI). Our results indicate that historical droughts and the cold phase of the AMO coincide, while the influence of the SOI is less clear. The strongest coincidences between historical droughts and AMO occurred around periodicities of 40 yrs. A further study of wavelet coherence shows that there are also common signal along time between droughts and various solar activity phenomena, in particular Be10, a proxy of cosmic rays. Comparing natural terrestrial and solar phenomena, we realize that the most sustained and strongest modulation of historical drought occurrence is at ~ 60-64 yrs and is between droughts and the solar proxy Be10.