The statistical characteristics of the winds at the Isthmus of Tehuantepec and their seasonal and interannual variability are studied through the analysis of several datasets and a reconstruction of the winds for a 31-yr period. Observations show that the long-term monthly mean wind speeds and frequency of occurrence of northerly winds have a strong seasonal signal, with maximum values during December-January, minimum during May-June, and a relative maximum in July. The frequency distribution of wind speed is bimodal, a feature that is closely related to the wind direction, with northerly winds being stronger. Based on these results and the close relationship between the across-Isthmus pressure differences and the local winds, a statistical model is developed to get a reconstruction of 12-hourly winds through the Isthmus of Tehuantepec for 1964-95. The model reproduces fairly well the main characteristics of the observed winds: the bimodal distribution of the wind speed and the seasonal signal in the wind speed and frequency of occurrence of northerly winds. Reconstructed winds show that the high frequency of northerly winds in July is associated with weaker winds than those observed in winter. The summer maximum seems to be related with the westward displacement and strengthening of the Bermuda high during this time of the year. Based on the model results, the long-term monthly mean wind speeds show larger values during El Niño years compared with La Niña years. During La Niña years winds are significantly weaker than in neutral years for February-March, June-September, and November, and the percentage of occurrence of northerly winds is significantly lower than in neutral years from June to November. The larger occurrence of northerly winds during El Niño years compared with neutral years is statistically significant only for May and September.

RESUMENLa dinámica del Golfo de México (GoM) se asocia frecuentemente con la Corriente del Lazo y los remolinos que se desprenden de ésta, los cuales son altamente energéticos y ocasionan corrientes intensas que pueden penetrar varios cientos de metros en la columna de agua. Sin embargo, hay regiones en el GoM y periodos de tiempo en que el forzamiento atmosférico local juega un papel importante en su dinámica y termodinámica. La circulación en las plataformas, particularmente en la interna, es generada principalmente por viento y tiene estacionalidad, ya que cambia de dirección a lo largo del año y tiene algunos periodos de condiciones favorables para el afloramiento/hundimiento. La circulación generada por el viento se vincula con el transporte de aguas con diferentes características de temperatura y salinidad. La variabilidad interanual de la circulación en las plataformas está asociada con la variabilidad interanual de la circulación atmosférica. Se ha identificado que la variabilidad intraestacional de los patrones de viento modifica considerablemente la ocurrencia de afloramientos y hundimientos. Debido a la forma y tamaño del GoM, cuando la circulación en una plataforma ocurre en sentido horario, puede darse en sentido opuesto en otras plataformas. El ancho de las plataformas en el GoM es variable: la plataforma oeste de Florida, la de Texas-Louisiana y el Banco de Campeche miden más de 200 km, en tanto que las más angostas son las de Veracruz y Tabasco. Otra consecuencia de la fisiografía del GoM y el forzamiento del viento es el desarrollo de transportes perpendiculares a la plataforma en el sur de la Bahía de Campeche, el sur de la plataforma de Texas y al sureste del Mississippi, los cuales a su vez varían a lo largo del año. El GoM es afectado durante el otoño-invierno (desde septiembre hasta abril) por frentes fríos provenientes del noroeste de Estados Unidos, los cuales tienen asociados vientos fuertes, secos y fríos que mezclan sus aguas y generan intensos flujos de calor sensible y latente del mar hacia la atmósfera. Estos vientos también enfrían el GoM debido a la mezcla de aguas cálidas de la superficie con aguas subsuperficiales de menor temperatura. Durante el verano, los ciclones tropicales que cruzan el GoM afectan su circulación y los afloramientos costeros. ABSTRACTThe Loop Current and its shed eddies dominate the circulation and dynamics of the Gulf of Mexico (GoM) basin. Those eddies are strongly energetic and are the cause of intense currents that may penetrate several hundred meters deep. However, there are regions in the GoM and periods of time in which the local 318 J. Zavala-Hidalgo et al.atmospheric forcing plays an important role in its dynamics and thermodynamics. The circulation on the shelves, and particularly on the inner shelf, is mainly wind-driven with seasonality, changing direction during the year with periods of favorable upwelling/downwelling conditions. The wind-driven circulation is associated with the transport of waters with different temperature and salinity chara...

The low-level seasonal and intraseasonal wind variability over the northeastern tropical Pacific (NETP), its relationship with other variables, and the connection with large- and middle-scale atmospheric patterns are analyzed using a suite of datasets. Quick Scatterometer (QuikSCAT) wind data show that the low-level circulation over the NETP is mainly affected by the northerly trades, the southerly trades, and the wind jets crossing through the Tehuantepec, Papagayo, and Panama mountain gaps. The seasonal and intraseasonal evolution of these wind systems determines the circulation patterns over the NETP, showing predominant easterly winds in winter and early spring and wind direction reversals in summer over the central region of the NETP. During summer, when southerly trades are the strongest and reach their maximum northward penetration, weak westerlies are observed in June, easterlies in July–August, despite that strong southerlies tend to turn eastward, and again westerlies in September–October. This circulation pattern appears to be related to the Tehuantepec and Papagayo jets, which slightly strengthen during midsummer favored by the westward elongation and intensification of the Azores–Bermuda high (ABH). This ABH evolution induces an across-gap pressure gradient over the Isthmus of Tehuantepec favoring the generation of the jet and a meridional sea level pressure (SLP) gradient in the western Caribbean that favors the funneling of the trade winds through the Papagayo gap. The SLP pattern causing the gap winds in winter is different than in midsummer, being the southeastward intrusion of high pressure systems coming from the northwest, the main cause of the large meridional SLP gradients in Tehuantepec and the western Caribbean. The westward low-level circulation observed over the central-eastern region of the NETP during midsummer induces westward moisture fluxes in the lower layers of the atmosphere, displaces convergence areas away from the coasts, and confines the relatively strong convergence in the easternmost NETP to the south of the area of influence of the wind jets and associated easterlies, contributing to the development of the midsummer drought observed in southern Mexico and Central America.

In many regions of Mexico, precipitation occurs in a very well defined annual cycle, with peaks in May–June and September–October and a relative minimum in the middle. This minimum in the middle of the rainy season is known as the midsummer drought (MSD) and impacts agriculture and industry. However, in Mexico there are large areas with either sparse meteorological station coverage or where time series of historical observations have many missing data, which make it difficult to study and analyse the precipitation variability at different scales of space and time. Therefore, the most important objective of this study is to evaluate the performance of the recently available Climate Hazards Group InfraRed Precipitation with Station (CHIRPS) product in estimating the duration and intensity of the MSD in Mexico, taking advantage of its very fine spatial resolution (0.05°) and continuous coverage to improve on current understanding of the MSD. In order to achieve this, MSD duration and intensity are calculated from the CHIRPS data and then compared to gauge data for the 1981–2010 period. In addition, two new indices for estimating the intensity of the MSD are defined, and these new indices provide complementary information to that obtained with more traditional methods. Results show that CHIRPS overestimates (underestimates) precipitation in Mexico during summer and autumn (winter and spring) seasons by up to 30%. Most importantly, by using CHIRPS and the two new indices proposed, the most detailed spatial representation ever of the MSD in Mexico has been obtained through the elimination of spatial and temporal coverage gaps.

Regions in the Gulf of Mexico are determined based on the statistical behavior of the long-term monthly means of chlorophyll-a concentration from SeaWiFS satellite estimations. An analysis based on the four largest modes of an empirical orthogonal decomposition, which account for 84.9% of the variance, results in nine spatial patterns with different statistical behavior representing 14 connected regions. The time evolution (or principal component) of the first two modes resemble the annual cycle, but each one with a different phase; the third mode represents a semiannual period and the fourth mode shows three maxima and minima. A map of the resulting regions is obtained and the oceanographic processes taking place in each region are discussed. The largest region covers most of the deep Gulf and the continental slope. Other regions in the deep Gulf are located southeast of the Mississippi River mouth and off-shelf of southern Texas and Tabasco, all associated with seasonal offshore cross-shelf transports. The shelves are associated with specific regions, but in wide shelves the inner and outer continental platforms are separated. Among the causes that determine different regions are topographic characteristics and the seasonal variability of physical processes, mainly entrainment caused by heat and momentum fluxes, upwelling, river plumes, and cross-shelf transports associated with the convergence of the along-coast currents.