3768 JOURNAL OF CLIMATE VOLUME 20 Midsummer Gap Winds and Low-Level Circulation over the Eastern Tropical Pacific ROSARIO ROMERO-CENTENO,JORGE ZAVALA-HIDALGO, AND G. B. RAGA Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico (Manuscript received 12 October 2005, in final form 14 November 2006) ABSTRACT 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 mid- summer 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. 1. Introduction above 27°C) and relatively weak zonal winds. The east- ern Pacific warm pool is separated from the equatorial The northeastern tropical Pacific (NETP), between cold tongue by a sharp SST front and is an important 0° and 25°N and east of 120°W, is characterized by tropical cyclone development region. Another impor- oceanic and atmospheric conditions leading to air–sea tant feature is the intertropical convergence zone interaction processes in a wide range of spatial and tem- (ITCZ), a region of strong atmospheric convection and poral scales. Among them, those worth mentioning are heavy precipitation where the southerly and northerly the equatorial cold tongue, a region characterized by a trades converge. Each one of these features has re- sea surface temperature (SST) minimum that extends ceived special attention and research in a wide number westward from the South American coast into the cen- of publications (e.g., Amador et al. 2006 and references tral Pacific, and the eastern Pacific warm pool, located within). off the west coast of southern Mexico and Central Besides the above-mentioned features of the NETP, America and characterized by warm SSTs (mostly the special orography of the continents on its eastern boundary, combined with the meteorological condi- tions, produces strong winds through the low-elevation Corresponding author address: Rosario Romero-Centeno, Cen- gaps of the Sierra Madre del Sur in southern Mexico tro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Circuito Exterior s/n, Ciudad Universitaria Coyoacan and the Central American cordillera (Fig. 1). The three 04510, Mexico City, D. F., Mexico. major mountain gaps are located at the Isthmus of E-mail: [email protected] Tehuantepec, the lowlands of central Nicaragua, and DOI: 10.1175/JCLI4220.1 © 2007 American Meteorological Society Unauthenticated | Downloaded 10/01/21 04:50 PM UTC JCLI4220 1AUGUST 2007 ROMERO-CENTENO ET AL. 3769 FIG. 1. Location of the major low-elevation mountain gaps in southern Mexico and Central American cordillera. Box A marks the region where low-level circulation and moisture fluxes are analyzed, and box B marks the region where precipitation rates are averaged (see section 3). CBN and CBS locate the sites used to calculate meridional pressure differences in the western Caribbean. Gray shading represents elevation in meters according to the shading bar. Panama, and the strong offshore winds in the lee of to the south, the Tehuantepec jet is followed a few days these mountain gaps are known as the Tehuantepec, later by the Papagayo and Panama jets. However, Papagayo, and Panama jets, respectively. The influence sometimes the Papagayo and Panama jets are influ- of these wind jets on local processes over the adjacent enced by trade wind fluctuations and tropical circula- Pacific waters, such as the generation of large warm tions that have little or no effect on the Tehuantepec jet oceanic eddies, intense offshore currents, increase in (Chelton et al. 2000a). Also, there are differences turbulent heat fluxes, considerable drop of the SST by among the three jets in intensity, orientation, time upwelling and entrainment of subsurface water, and in- scales, and seasonality, which result in different oceanic crease in biological activity, has motivated many stud- and atmospheric responses (Chelton et al. 2000a,b; ies, which have especially focused on the Gulf of Te- Kessler 2002; Xie et al. 2005). Chelton et al. (2000a) huantepec (e.g., Roden 1961; Stumpf 1975; Stumpf and establish that the characteristic time scale of the highly Legeckis 1977; Alvarez et al. 1989; McCreary et al. energetic Tehuantepec jet is about 2 days and it is more 1989; Lavín et al. 1992; Barton et al. 1993; Trasviñaet variable and intense than the other two jets. They sug- al. 1995; Lluch-Cota et al. 1997; Schultz et al. 1997; gest that the Papagayo and Panama jets are predomi- Steenburgh et al. 1998; Bourassa et al. 1999; Muller- nantly controlled by a different mechanism than the Karger and Fuentes-Yaco 2000; Zamudio et al. 2006). across-gap pressure gradients associated with high pres- The generation mechanism of the Tehuantepec jet sure systems of midlatitude origin, being more likely to during boreal winter, when it is strongest and more be coupled to variations in the Caribbean trades that frequent, has been widely documented: large sea level are funneled through the Papagayo and Panama gaps. pressure differences between the Gulf of Mexico and At seasonal scale, the three jets weaken in late spring the eastern Pacific, caused by the southeastward migra- and summer, but there is a slight strengthening of the tion of high pressure systems associated with cold-air Tehuantepec and Papagayo jets during July–August outbreaks coming from the northwestern United whose origin, at least in the case of the Tehuantepec jet, States, generate airflows that are blocked by the cor- has been attributed to a sea level pressure difference dillera and then channeled through the mountain gap between the western Atlantic and the eastern Pacific (e.g., Roden 1961; Parmenter 1970; Clarke 1988; Lavín that might be caused by the midsummer westward ex- et al. 1992; Schultz et al. 1997; Steenburgh et al. 1998). tension and intensification of the Azores–Bermuda Frequently, as the high pressure system penetrates far high (Romero-Centeno et al. 2003). This strengthening Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 3770 JOURNAL OF CLIMATE VOLUME 20 of the jets is in phase with the reduction of the tropical Aeronautics and Space Administration’s Quick Scatter- storm activity in the Caribbean and the far-eastern Pa- ometer (QuikSCAT)/SeaWinds scatterometer (QSCAT) cific (Inoue et al. 2002; Curtis 2002) and with the mid- winds. Recent studies have demonstrated that QSCAT summer drought in southern Mexico and Central winds represent the currently available product that America, a climatological phenomenon unique to the best resolves the wind jets in different space–time scales Western Hemisphere, where rainfall amounts reduce and is an excellent tool to characterize the dynamical by roughly 40% in late July and early August compared state of the lower troposphere with a high degree of to June and September (Magaña et al. 1999; Curtis accuracy (Chelton et al. 2004; Bordoni et al. 2004; Mc- 2004). Magaña et al. (1999) proposed that local air–sea Noldy et al. 2004). The vertical structure of moisture interactions in the NETP lead to changes in the con- fluxes, wind, and pressure fields is analyzed using data vection pattern. They suggest that the increase in from the National Centers for Environmental Predic- cloudiness and reduction in solar radiation in June lead tion–Department of Energy Atmospheric Model Inter- to a drop in SST over the eastern Pacific warm pool that comparison Project II (NCEP–DOE AMIP-II) reanaly- inhibits rainfall in July–August. Cloudiness reduction in sis (NCEPR2; Kanamitsu et al. 2002). Precipitation rate July–August allows a solar radiation increase reaching data from the NCEPR2 and SST data from the Na- the surface that raises the SST and causes the return of tional Oceanic and Atmospheric Administration precipitation by late August and September, establish- (NOAA) are also used. ing the essential role of solar radiation feedbacks in The paper is organized as follows. The following sec- explaining the bimodal distribution in precipitation tion describes the datasets used; section 3 details the re- over central-southern Mexico, most of Central Amer- sults of the data analysis, and section 4 includes a discus- ica, and parts of the Caribbean.