Downloaded 10/01/21 04:50 PM UTC

Home , Isthmus of Tehuantepec, Papagayo Jet

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 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.

1. Introduction above 27°C) and relatively weak zonal winds. The eastern 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 conditions, 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

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. The authors conclude sion of the main results and the conclusions of the study. that this seasonal SST fluctuation along with fluctuations in the low-level convergent flow and an intensification 2. Data of the trade winds, which they suggest is part of the Previous studies have shown that scatterometers are dynamical response to the intensity of the convective unique among satellite remote sensors in their ability to forcing of the ITCZ, produce the midsummer drought. measure wind speed and direction over the global sur- Recently, Magaña and Caetano (2005), based on ob- face waters. In particular, QSCAT provides wind mea- servations of a field campaign conducted over the surements with a much greater spatial and temporal Americas warm pool in summer 2001, revealed the coverage than in situ observations or any previous scat- limitations of the Magaña et al. (1999) hypothesis and terometer missions and a significantly higher resolution the necessity of an improved theory. Contrary to what than what is currently provided by numerical weather they expected, solar radiation observations were higher prediction models or reanalysis wind products (Ebuchi during September than in July. They also observed an et al. 2002; Bourassa et al. 2003; Chelton et al. 2004). out-of-phase relationship in precipitable water between QSCAT collects data within a 1600-km-wide swath, the NETP warm pool, off the coast of Central America, covering roughly 93% of the ice-free global oceanic and the Caribbean coast of Central America and the surface in one day with a 25-km spatial resolution, and western side of the NETP warm pool, where a maxi- measures wind speed with an accuracy of Ϯ2msϪ1 and mum is observed during the midsummer drought pe- wind direction with an accuracy of Ϯ20° or better riod. They proposed a combined effect of SST changes (http://winds.jpl.nasa.gov/missions/quikscat). In the and subsidence related to direct circulations as a better present study, daily near-surface vector winds are pro- explanation of the convective activity observed during cessed and analyzed for the period July 1999– summer over the NETP. December 2005 over the region 0°–33°N, 130°–75°W. At present, studies on the variability of the jets in QSCAT winds were downloaded from the Center for boreal summer are scarce, and the influence and pos- Ocean–Atmospheric Prediction Studies Web site sible link between the midsummer Tehuantepec and (http://coaps.fsu.edu/cgi-bin/qscat/wind-swath-ku2001), Papagayo wind jets and the atmospheric circulation and which consists of the Remote Sensing Systems Ku-2001 convection patterns over the NETP still remains to be swath datasets. Winds are referenced to a height of 10 m, completely understood. In this study we discuss the and the data files include wind speed, wind direction, midsummer strengthening of the Tehuantepec and time, latitude, and longitude. Zonal (u) and meridional Papagayo jets and its impact on the low-level circula- (␷) wind components are calculated from the wind tion of the NETP, together with possible causes and speed and direction information and hourly averaged implications. The near-surface wind analysis is per- within 0.5° ϫ 0.5° latitude–longitude boxes. Daily and formed by using the increasingly utilized National monthly mean winds are calculated and long-term

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 1AUGUST 2007 ROMERO-CENTENO ET AL. 3771 monthly means for the analyzed period are then esti- elevation mountain gaps of the cordillera in southern mated from the monthly averages and used to analyze Mexico and Central America (see Fig. 1 for location), the seasonal and intraseasonal variations of the wind which has been documented in previous works. The jets and the low-level circulation over the NETP. signal of the across-gap winds over the Gulf of Tehuan- Monthly mean divergence fields, calculated using a cen- tepec (Tehuantepec jet) is noticeable most of the year, tered difference scheme, are also analyzed. being more evident during boreal winter (November– Data flagged as rain contaminated were excluded. February) and showing a prevalent meridional orienta- Heavy rain degrades the quality of surface wind retriev- tion just off the coast (Figs. 2a,b,k,l). The southwest- als from scatterometer observing systems due to the ward-oriented winds over the Gulf of Papagayo (Papa- attenuation and backscatter of the emitted microwave gayo jet) are weakly distinguishable in November and pulse and the backscatter from the ocean surface (Mil- then become stronger reaching their maximum in Janu- liff et al. 2004). Systematic biases in distributions of ary. During winter, the Tehuantepec jet extends far to wind stress curl and divergence fields have been iden- the south, merging with the Papagayo jet and the Pa- tified from scatterometer surface wind retrievals that cific northeast trades and turning westward. The Te- are due to the observations missing from the QSCAT huantepec and Papagayo jets begin weakening in record, mostly due to rain (Milliff et al. 2004). The March (Fig. 2c) and they are barely visible in May (Fig. percentage of flagged data for the area and period of 2e). The January and February monthly averages also study is between 20% and 25% within the ITCZ region show evidence of the Panama crossing winds, which are and up to 40% in small regions of highly divergent north–south-oriented like the Tehuantepec jet, and winds over the Gulf of Tehuantepec (not shown). they are still distinguishable as late as March and April Derived NCEPR2 products (daily and monthly aver- (Figs. 2a–d). In general, the late spring and summer ages from 6-hourly values) for sea level pressure, wind winds are very weak in the eastern region of the NETP components, geopotential height, specific humidity, north of the ITCZ, with the exception of a slight inten- and precipitation rate were provided by the NOAA/ sification of the Tehuantepec and Papagayo jets in July Office of Oceanic and Atmospheric Research/Earth and August, though less intense in the latter. System Research Laboratory Physical Sciences Divi- During winter and spring months (from December sion (OAR/ESRL PSD), Boulder, Colorado, from their through April), when the northerly trades are more Web site at http://www.cdc.noaa.gov/. The NCEPR2 is intense, winds in the NETP are mainly easterlies blow- based on the NCEP–National Center for Atmospheric ing from the coast to the western Pacific (Figs. 2a–d,l). Research (NCAR) reanalysis (NCEPR1) and produces In May, this pattern begins changing: southerly trades analyses of atmospheric fields using data from 1979 to start intensifying and slightly turn to the east around 3° the present. Improvements of the NCEPR2 project on to 7°N and east of ϳ110°W, while light westward winds the NCEPR1 are the fixing of processing problems and cover the area north of ϳ8°N and east of ϳ110°W (Fig. the updates of the forecast model, the data assimilation 2e). In June, while the northerly trades are relatively system, and the parameterizations of the physical pro- weak, the southerly trades keep intensifying and pen- cesses. Surface and pressure level data are available on etrating northward and the light winds to the north a 2.5° ϫ 2.5° grid, and the data period used in this study (around 10°N) change direction, heading east from is from July 1999 to December 2005. Monthly fields ϳ115°W (Fig. 2f). This circulation pattern favors mois- from the NOAA optimum interpolation SST V2 data ture transport into the continent, coinciding with the on a one-degree grid for this same period are also used. first precipitation maximum observed in central- The optimum interpolation V2 analysis uses in situ and southern Mexico and Central America (see Fig. 3b). satellite SSTs plus SSTs simulated by sea ice cover However, in July and, though less intense, in August, (http://www.cdc.noaa.gov/cdc/data.noaa.oisst.v2.html). when a slight strengthening of the Tehuantepec and Papagayo wind jets is observed, the low-level flow in 3. Results the central region of the NETP (within box A in Fig. 1) shows again a westward orientation (Figs. 2g,h). This a. Winds over the NETP westward flow is not restricted to the area near the Long-term monthly mean wind vectors from the 6.5- coast but is observed several hundred kilometers yr QSCAT period from July 1999 through December (ϳ2200 km) offshore over the central NETP. This cir- 2005 show a strong seasonal signal in the NETP, the culation pattern seems to inhibit the northward pen- Gulf of Mexico, and the Caribbean regions (Fig. 2). etration of the southerly trades, mainly in the eastern One of the more outstanding features is the signal as- region of the NETP, despite their strength and east- sociated with the winds crossing through the low- ward orientation, and restricts the low-level moisture

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 3772 JOURNAL OF CLIMATE VOLUME 20

FIG. 2. Long-term monthly mean wind vectors and wind speed color contours from QSCAT data for the period July 1999–December 2005: (a)–(l) Jan–Dec. transport into the continent coinciding with the mid- Fig. 1 for its location) and that of the precipitation rates summer drought observed in the region during July– averaged within box B (10°–18°N, 100°–85°W) (Fig. 3b; August (see Fig. 3b). see Fig. 1 for its location), both computed over the The wind pattern changes abruptly in September QSCAT observations period, summarize the results (Fig. 2i), featuring imperceptible wind jets, the weakest presented in the previous section. It is worth mention- northerly trades, the northernmost penetration of the ing that box A is the region over the central NETP southerly trades, and a low-level eastward flow that where changes in wind direction occur during the sum- favors the moisture transport into the continent. Dur- mer; this region is influenced by the gap winds but it is ing this month, the precipitation increases again in the not much affected by the shift of the zonal wind com- region, when the second maximum is observed. In Oc- ponent of the strong southerly trades during this time of tober, this wind circulation pattern begins weakening, the year. Box B covers part of southern Mexico and giving rise to that observed during winter and spring. Central America, where the midsummer drought is observed. Plots in Fig. 3 show that easterlies are observed b. Zonal winds and precipitation most of the year in box A; the strongest easterlies dur- The annual cycle of the zonal wind component aver- ing the winter–spring months coincide with the lowest aged within box A (10°–15°N, 115°–95°W) (Fig. 3a; see precipitation rates in box B, and the easterlies observed

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC

Fig 2 live 4/C 1AUGUST 2007 ROMERO-CENTENO ET AL. 3773

FIG.2.(Continued) in July–August coincide with the reduction in precipi- c. Moisture fluxes tation during this time of the year. The two precipita- Ϫ1 Ϫ1 tion maxima are observed in June and September, Monthly mean zonal moisture fluxes (kg kg ms ) when westerlies are present in box A. at different atmospheric levels within box A are calcu- The 30-day running means of the zonal wind compo- lated from daily averages of the zonal wind component nent averaged within box A and those of the precipi- and specific humidity data from the NCEPR2 for the tation rates averaged within box B are shown in Fig. 4. period July 1999–December 2005. The annual cycle of The correlation between these two variables is very the zonal moisture fluxes at the 1000-mb level (Fig. 5a) high, with a determination coefficient of r2 ϭ 0.71. Dur- shows westward fluxes from November to May, with ing the rainy season, from June through September, maximum values in December–January and very small precipitation rates show fluctuations that correspond fluxes in May. In June, moisture fluxes reverse direc- with changes in the zonal winds, like the clear corre- tion showing a small eastward component, while in July spondence between the precipitation peaks in June and they reverse again heading westward and remain so in September and the westerlies over the central NETP. August, although they are very small. September is the Between these two peaks, precipitation variations also month with the largest eastward moisture fluxes, re- correspond with zonal wind changes most of the years maining so in October but with a considerable smaller in the 2000–05 time series. value. A similar behavior is observed at the 925-mb

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC

Fig 2 live 4/C 3774 JOURNAL OF CLIMATE VOLUME 20

fluxes in the upper levels, especially in the winter months (Figs. 5d–f). From the 700-mb level up, westward fluxes are observed along the whole year, with maximum values in July–August. Therefore, westward moisture fluxes are observed from the surface up to, at least, 500-mb in midsummer. The above analysis reinforces the hypothesis that the strengthening of the wind jets and the associated rever- sal of the low-level circulation over the central NETP during midsummer are closely related with the reduction of precipitation in southern Mexico and Central America by means of a reduction of the moisture transport toward the continent, mainly at the lower levels of the atmosphere.

d. Wind divergence Previous studies reveal that high-resolution QSCAT winds are capable of resolving important details of the wind field, which are undetected by the reanalyses, and improve our understanding of the surface circulation in the Atlantic and east Pacific regions (McNoldy et al. FIG. 3. Long-term monthly means of (a) zonal winds from QSCAT data averaged within box A and (b) precipitation rate 2004; Chelton et al. 2004). For the present study, the from NCEPR2 data averaged within box B. See Fig. 1 for location monthly wind divergence fields derived from the of boxes A and B. QSCAT vector winds are an important tool to analyze the impact of the gap winds over the eastern Pacific level, showing slightly smaller values year round, except ITCZ. These fields show the seasonal fluctuation of the in July–August when westward moisture fluxes show a ITCZ, which shifts to the south during the boreal winter slight increase (Fig. 5b). At the 850-mb level, westward months and to the north in summer (Fig. 6). The extent moisture fluxes are observed in all months but Septem- of the displacement appears more pronounced in the ber when eastward fluxes are still present (Fig. 5c). western region of the NETP. From November to Janu- From December to March, a decrease in the intensity of ary, when the northerly and southerly trades are both the westward fluxes is observed compared with the relatively intense, a band of strong convergence is ob- lower levels. The 850-mb level is where maximum west- served, being slightly stronger and narrower in the eastward fluxes are observed during the midsummer. ern part of the NETP (Figs. 6a,k,l). From November to There is a considerable decrease in the moisture February the wind jets are very strong and their asso-

FIG. 4. The 30-day running means of the zonal winds averaged within box A from QSCAT data (thick line) and those of the precipitation rates averaged within box B from NCEPR2 data (thin line). See Fig. 1 for location of boxes A and B.

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 1AUGUST 2007 ROMERO-CENTENO ET AL. 3775

FIG. 5. Monthly zonal moisture fluxes at different atmospheric levels calculated using zonal winds and specific humidity data from the NCEPR2 (July 1999–December 2005): (a)–(f) 1000–500 mb. ciated divergent winds are clearly observed. During this are very strong, the southerly trades are weak, and the time of the year, when the ITCZ is shifting southward, convergence is weak (Fig. 6c). The northward migra- the wind jets produce large divergence patches over tion of the ITCZ begins in April, when the southerly relatively weak convergence areas just north of the trades begin strengthening (Fig. 6d). band of stronger convergence, interrupting the ITCZ From May onward, the southerly trades gradually zonal continuity (Figs. 6a,b,k,l). In February, the south- intensify and the convergence strengthens and pen- erly trades begin weakening and the convergence is less etrates farther north, mainly over the western half of intense. March is the month with the maximum south- the NETP. In June, scattered convergence areas close ward displacement of the ITCZ: the northerly trades to the continent and a relatively narrow convergence

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 3776 JOURNAL OF CLIMATE VOLUME 20

Ϫ Ϫ FIG. 6. Long-term monthly mean wind divergence fields (s 1 ϫ 10 5) from QSCAT data: (a)–(l) Jan–Dec. zone in the western region of the NETP are observed winds, moisture fluxes, and divergence analyses show (Fig. 6f). During July and August, the Tehuantepec and that the midsummer drought observed in central- Papagayo wind jets strengthen and the low-level circu- southern Mexico and Central America is closely related lation over the central region of the NETP reverses (see with the strengthening of the Tehuantepec and Papa- Figs. 2g,h), shifting convergence areas west of ϳ107°W gayo wind jets, which induces a westward circulation where the ITCZ widens (Figs. 6g,h). Scarce conver- over the central-eastern NETP that shifts convergence gence areas are observed close to the continent and the areas away from the continent, confines the relatively relatively strong convergence in the easternmost NETP strong convergence in the eastern region of the NETP is confined south of the area of influence of the wind to the south of the wind jets, and reduces the moisture jets. The abrupt change of the circulation pattern in fluxes toward the continent, mainly north of ϳ10°N. September, when wind jets are imperceptible, causes convergence areas to be widely dispersed covering a e. Sea surface temperature large part of the NETP, next to the continent and up to SST gradients, along with surface winds, are very im- ϳ22°N (Fig. 6i). In this month, despite the ITCZ reach- portant elements in determining the structure and vari- ing its maximum northward penetration into the west- ability of the ITCZ. The eastern Pacific warm pool ern NETP, the ITCZ is slightly narrower than in July– (EPWP), as mentioned in the introduction, is a rela- August in the westernmost NETP. tively large area characterized by warm SSTs (mostly These divergence patterns agree with the analyses of above 27°C) that is located off the west coast of south- the low-level circulation and the zonal moisture fluxes ern Mexico and Central America, north of ϳ6°N and over the central NETP presented above. The zonal east of ϳ120°W (Wang and Enfield 2001; Xie et al.

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC

Fig 6 live 4/C 1AUGUST 2007 ROMERO-CENTENO ET AL. 3777

2005). Although the warmest waters are located in the March. The NPH is less intense from October through eastern NETP, next to the continent, the warm waters February, showing an absolute minimum in February extend farther west bounded by cooler water on the (Fig. 9a). The absolute maximum of the ABH intensity north and by a sharp SST front on the south (Fig. 7). in July coincides with the maximum westward displace- This strip of warm waters shows a northeast-to- ment of its center over the subtropical Atlantic, while in southwest tilt; it varies in intensity (SST maximum) and January its center is shifted far to the east (Fig. 9b). The has a meridional displacement along the year, being at center of the NPH also shows its maximum westward its southernmost position in March (Fig. 7). From April displacement over the subtropical Pacific in July, while onward, the strip displaces northward reaching its in winter it is closer to the continent. A meridional northernmost position in September. During July and migration is also observed on the annual course of both August, while the strip of warm waters keeps migrating subtropical anticyclones (Figs. 8 and 9c); the maximum northward, the southerly winds and convergence seem northward shift of the NPH is observed during July– to remain confined south of the area of influence of the September, while in winter its center is shifted south- wind jets and the associated easterlies in the central- ward. The center of the ABH is at its southernmost ϳ eastern region of the NETP (see Figs. 2 and 6), when it position in March when it reaches 28°N, and the rest might be expected that they continue penetrating of the year it moves between 32.5° and 37.5°N, being at northward following the strip of maximum SST. As its northernmost position in February. These SLP mentioned above, this interruption of the northward variations determine, to a large extent, the monthly intrusion of the convergence in the eastern region of course of the circulation patterns over the region. the NETP occurs simultaneously with the intensifica- The forcing mechanisms and three-dimensional tion of the wind jets and the change in the low-level structure of the subtropical anticyclones in the North- circulation over the NETP during midsummer. It is ern Hemisphere are not completely understood. There worth mentioning that the close relationship between are several theories involving processes like upper-level the SST gradients and the surface winds in the NETP planetary waves, monsoonal deep convective heating, has been largely documented by Chelton et al. (2001), midtropospheric subsidence, near-surface land–sea although on higher temporal frequencies. thermal contrasts across the west coasts of the subtropical continents, and topographic features, among others, as important mechanisms determining the structure and f. Pressure and wind relationship annual evolution of the subtropical highs (Chen et al. In this section, the seasonal and intraseasonal varia- 2001; Liu and Wu 2004; Miyasaka and Nakamura 2005). tions of the large-scale pressure systems involved in the As was mentioned previously, the mechanism that generation of the Tehuantepec and Papagayo wind jets generates the Tehuantepec jet is the across-gap pres- are analyzed, both in the lower and upper atmosphere. sure gradient through the Isthmus. The annual cycles of the SLP at 20°N–95°W in the southern Gulf of Mexico (GM) and that at 15°N–95°W in the Gulf of Tehuante- 1) SEA LEVEL PRESSURE AND WIND VARIATIONS pec (GT) (see Romero-Centeno et al. 2003, their Fig. Long-term monthly mean sea level pressures (SLP) 13) show that, on a monthly time scale, the SLP in the for the period July 1999–December 2005 over the re- GM is larger than in the GT along the year and the gion 0°–50°N, 150°–10°W from NCEPR2 data (Fig. 8) pressure differences between them (GM Ϫ GT) are show the two permanent high pressure systems around larger in the winter months (November–February), 30°–35°N: over the subtropical Pacific [the North Pa- reaching ϳ3.5 hPa in January (Fig. 10). The monthly cific high, (NPH)] and the subtropical Atlantic [the mean SLP conditions in winter (see Figs. 8a,b,k,l) show Azores–Bermuda high (ABH)]. Also evident are a a high pressure system over the southeastern United third nonpermanent system over the continental States, which induces a large meridional pressure gra- United States and a belt of low pressures dominating dient over southern Mexico, Central America, and the the tropical areas. The ABH and NPH show variations Caribbean Sea, and the eastern NETP being affected both in intensity and location along the year (Figs. 8 by a relatively weak NPH. This pattern induces a large and 9). Both systems reach their absolute maximum SLP difference between the GM and the GT, which intensity in July, although the ABH shows a relative favors the generation of the Tehuantepec jet. The SLP maximum in January (Fig. 9a). In the monthly mean, over the two gulfs decreases in the spring months as the ABH is less intense by the end of the year (Sep- does the pressure difference between them, being tember–December), with a slight intensification in No- around 0.82 hPa in June (Fig. 10). The SLP slightly vember, although it shows an absolute minimum in increases in July–August at both gulfs, but the increase

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 3778 JOURNAL OF CLIMATE VOLUME 20

FIG. 7. Long-term monthly mean sea surface temperatures (°C) from the NOAA optimum interpolation V2 data (contours are every 1°C; thick contours at 15°,20°,25°, and 30°C).

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC

Fig 7 live 4/C 1AUGUST 2007 ROMERO-CENTENO ET AL. 3779

FIG. 8. Long-term monthly mean sea level pressures (contour interval 1 mb) from NCEPR2 data (July 1999–December 2005): (a)–(l) Jan–Dec.

is larger over the GM than over the GT, enhancing the ever, in winter the main cause of the large SLP differ- across-gap pressure difference (ϳ2.0 hPa in July) and ence between the GM and the GT is the intrusion into inducing the strengthening of the Tehuantepec jet. The the GM of high pressure systems coming from the increase of SLP over the GM during midsummer is north, while during midsummer it is the penetration of associated with the intensification and westward extent the ABH into the GM. of the ABH, while the GT seems to be affected by a The annual evolution of the zonal wind component combined effect of the intensification of both subtropi- averaged over the Papagayo jet area (10°–13°N, 85°– cal highs, though the center of the NPH is shifted to the 88°W) is very similar to that of the meridional wind northwest (see Figs. 8g,h and 9b,c). component, with the zonal winds slightly predominat- The annual cycle of the GM Ϫ GT SLP differences, ing over the meridional winds most of the year (not from NCEPR2 data, and that of the meridional wind shown). Zonal winds are westward most of the time component averaged over the GT (14°–16°N, 94.5°– except during September–October, and meridional 95.5°W), from the QSCAT data, have a very high cor- winds are southward all year-round except in Septem- relation (r ϭϪ0.93, r2 ϭ 0.87) (Fig. 10). The strength- ber. This annual evolution of the Papagayo winds is ening of the offshore winds in midsummer corresponds mainly related with the meridional SLP differences in with the increase in SLP difference between the GM the western Caribbean, calculated by the SLP differ- and the GT in this period. Consequently, the main ences between 15°N, 80°W (CBN) and 10°N, 80°W physical mechanism that produces the intensification of (CBS) (Fig. 11; see Fig. 1 for sites’ location), while it is the Tehuantepec jet in July–August is the same as that less influenced by the across-gap pressure differences in winter: a large across-gap pressure gradient. How- estimated by the zonal SLP differences between the

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 3780 JOURNAL OF CLIMATE VOLUME 20

FIG. 9. Annual evolution of the (a) intensity, (b) zonal location, and (c) meridional location of the ABH (solid line) and the NPH (dashed line). The values were obtained from 0.5-hPa contour plots. western Caribbean (12.5°N, 80°W) and the easternmost bean trade winds is a mechanism for the generation of NETP (12.5°N, 90°W) (not shown). The correlation be- the Papagayo jet that was previously proposed by Chel- tween the annual cycle of the zonal wind component ton et al. (2000a). averaged over the Papagayo jet area and that of the meridional SLP differences in the western Caribbean 2) UPPER-LEVEL PRESSURE AND WIND (CBN Ϫ CBS) is very high (r ϭϪ0.93, r2 ϭ 0.86), in VARIATIONS contrast with the very low correlation shown with the The midsummer strengthening and westward elonga- annual cycle of the across-gap pressure differences (r ϭ tion of the ABH center is also observed at upper levels. Ϫ0.24, r2 ϭ 0.06). In the monthly mean, the largest Long-term monthly averages of geopotential height meridional SLP differences in the western Caribbean and vector winds from the NCEPR2 data for the 850-, match with strong offshore Papagayo winds in winter 700-, 600-, and 500-mb surfaces over the region 0°– and with relatively strong offshore winds in July– 50°N, 150°–10°W show these ABH features during August, while the lowest meridional SLP differences in July–August (Fig. 12). At lower levels, up to ϳ850 mb, the western Caribbean match with onshore Papagayo the distribution and location of the Atlantic and Pacific winds during September–October (Fig. 11). subtropical highs, separated by low pressures over the These results indicate that the monthly mean condi- continent, establish relatively strong zonal pressure gra- tions favoring the generation of the Papagayo wind jet dients north of ϳ15°N and relatively strong meridional are associated with large-scale pressure systems that pressure gradients between ϳ12° and 18°N. In the affect the tropical Atlantic and the Caribbean Sea, lower atmosphere, the pressure patterns determine which induce a large meridional SLP gradient in the complex wind fields that are also affected by the SST western Caribbean and the funneling of the trade winds gradients and the topography. It has been proposed through the cordillera gap. The funneling of the Carib- that the separated high pressure cells structure suggests

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 1AUGUST 2007 ROMERO-CENTENO ET AL. 3781

FIG. 10. Annual cycle of the SLP differences between 20°N, 95°W in the southern GM and 15°N, 95°W in the GT from NCEPR2 data (dashed line) and that of the meridional wind component averaged over the GT from the QSCAT data (solid line).

FIG. 11. Annual cycle of the SLP differences between 15°N, 80°W (CBN) and 10°N, 80°W (CBS) in the western Caribbean (dashed line) (see Fig. 1 for sites’ location) and that of the zonal winds averaged over the Papagayo wind jet area (solid line). SLP data are from NCEPR2 and winds are from QSCAT.

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 3782 JOURNAL OF CLIMATE VOLUME 20

FIG. 12. Monthly mean wind vectors and geopotential height (color contours in meters ϫ 103) from June to September for the 850-, 700-, 600-, and 500-mb surfaces from NCEPR2 data. that the dynamics of the subtropical highs is related to gion south of ϳ20°N (Fig. 12). In the 600- and 500-mb thermally forced planetary waves, rather than being geopotential height maps, the midsummer strengthen- part of a zonally symmetric circulation (Chen et al. ing of the ABH is still evident and the anticyclonic 2001; Liu and Wu 2004; Miyasaka and Nakamura 2005). circulation associated with the heating over the conti- Studies on the formation of the subtropical anticyclones nental United States is clearly shown in July and Au- state that, near the surface, the strong alongshore gust. At the 500-mb level the cell structure is more northerlies over the eastern North Pacific and Atlantic diffuse showing the merging of the two high pressure basins, associated with the subtropical highs, keep cool systems. SSTs locally by enhancing surface evaporation and coastal upwelling, which, in consequence, increase the 4. Discussion and conclusions land–sea thermal contrast during the summertime. In this context, the shallow continental heating and mari- The low-level circulation in the NETP is affected by time cooling play major roles in the formation of the three main wind systems: the northerly and southerly surface subtropical highs, which intensify from early to trade winds and the wind jets coming through the Te- midsummer through local feedback processes (Miyasaka huantepec, Papagayo, and Panama mountain gaps. The and Nakamura 2005 and references within). seasonal and intraseasonal evolution of these wind sys- In the upper atmosphere the large-scale pressure systems determines the low-level circulation over the tems dominate the scene. At the 700-mb level and up- NETP on these time scales. QSCAT wind data (July ward, the strong meridional component of the south- 1999–December 2005) show that the low-level circula- erly winds vanishes; instead, easterlies dominate the re- tion over the central region of the NETP is mainly di-

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC

Fig 12 live 4/C 1AUGUST 2007 ROMERO-CENTENO ET AL. 3783 rected westward from November to May and under- associated westward winds (Fig. 6). This circulation pat- goes wind direction changes during summer, from weak tern mostly determines the precipitation distribution in westerlies in June to easterlies in July and August and central-southern Mexico and Central America, where a changing back to westerlies in September–October reduction in precipitation rates is observed at the (Fig. 2). This change in the circulation pattern during middle of the rainy season, known as the midsummer midsummer is associated with the strengthening of the drought (Figs. 3 and 4). Time series for the July 1999– Tehuantepec and Papagayo wind jets. December 2005 period of the zonal winds averaged The wind jets are very intense during winter and, within box A and the precipitation rates averaged although much weaker, the Tehuantepec and Papagayo within box B (see Fig. 1 for their location) show that the jets are also observed in July and August. The monthly two precipitation maxima that occur in June and Sep- mean SLP conditions in winter are characterized by the tember have a clear correspondence with eastward presence of a high pressure system over the southeast- winds over the central-eastern NETP and that precipi- ern United States that induces a large meridional pres- tation variations observed between these two peaks sure gradient over southern Mexico, Central America, also correspond with zonal wind changes over that re- and the Caribbean Sea (Figs. 8a,b,k,l). These conditions gion. favor the generation of the Tehuantepec wind jet by Near-surface winds depend, among others factors, on increasing the SLP difference between the GM and the SST gradients. The seasonal cycle of the SST over the GT (Fig. 10) (i.e., by increasing the across-gap pressure NETP is closely related with the seasonal migration of gradient) and the generation of the Papagayo wind jet the southerly trade winds and the ITCZ. The strip of by increasing the meridional SLP difference over the warm waters, that is linked to the eastern Pacific warm western Caribbean (Fig. 11), which induces the funnel- pool and extends toward the west, intensifies and dis- ing of the trade winds through the mountain gap. On places northward during the summer, as the southerly the other hand, the monthly mean SLP conditions in trades do (Figs. 2 and 7). In this season, southerly trades midsummer are characterized by strong subtropical show a strong meridional component and the intensi- fied SST gradients induce southerlies to turn eastward, highs in the North Pacific (NPH) and Atlantic (ABH) mainly in the eastern region of the NETP. This pattern basins, both shifted to the west (Figs. 8g,h). These mean has associated moisture transports toward the conti- SLP conditions in July–August increase the pressure nent, but the northward penetration of the southerly difference between the GM and the GT due to the winds and the ITCZ over the eastern NETP is re- westward elongation of the ABH over the GM, favor- stricted in July–August by the strengthening of the ing the generation of the Tehuantepec jet (Fig. 10), and wind jets and the associated westward low-level circu- strengthen the meridional pressure difference in the lation over the central-eastern NETP, despite that the western Caribbean, favoring the generation of the strip of warm waters continues its northward migration Papagayo jet (Fig. 11). The very low correlation be- (Fig. 7). tween the Papagayo winds and the across-gap pressure In conclusion, large-scale processes in the North At- difference, estimated by the zonal SLP differences be- lantic, like the ABH displacement and intensification, tween the western Caribbean and the easternmost have a major contribution to the NETP seasonal and NETP, shows that the main mechanism of generation intraseasonal variability, affecting the gap winds, the of the Tehuantepec and Papagayo wind jets is different, zonal wind component over the NETP, the low-level in accordance with Chelton et al. (2000a,b) results. moisture transports, and the precipitation in the region. Results show that there is a high correlation among Therefore, these processes contribute to the develop- the Tehuantepec and Papagayo wind jets, the zonal ment of the midsummer drought observed in southern winds and moisture fluxes over the central region of the Mexico and Central America. The relationship of these NETP, and the precipitation rates in central-southern large-scale processes with lower-scale processes, like lo- Mexico and Central America. The westward low-level cal air–sea interactions and thermally direct circula- circulation observed over the central-eastern region of tions, has to be studied with more detail. the NETP during midsummer, which occurs simultaneously with the strengthening of the wind jets, induces Acknowledgments. The authors appreciate the sug- westward moisture fluxes in the lower layers of the at- gestions and comments of three anonymous reviewers mosphere (Fig. 5), displaces convergence areas away whose thorough review of the manuscript led to impor- from the coasts (Fig. 6), and causes the relatively strong tant changes and deeper data analysis that greatly im- convergence in the easternmost NETP to remain con- proved the manuscript. This research was funded by fined south of the area of influence of the wind jets and CONACYT and UNAM.

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC 3784 JOURNAL OF CLIMATE VOLUME 20

REFERENCES Lluch-Cota, S. E., S. Alvarez-Borrego, E. M. Santamaría del An- gel, F. E. Muller-Karger, and S. Hernández-Vázquez, 1997: Alvarez, L. G., A. Badan-Dangon, and A. Valle, 1989: On coastal The Gulf of Tehuantepec and adjacent areas: Spatial and currents off Tehuantepec. Estuarine Coastal Shelf Sci., 29, temporal variation of satellite-derived photosynthetic pig- 89–96. ments. Cienc. Mar., 23, 329–340. Amador, J. A., E. J. Alfaro, O. G. Lizano, and V. O. Magaña, Magaña, V., and E. Caetano, 2005: Temporal evolution of sum- 2006: Atmospheric forcing of the eastern tropical Pacific: A mer convective activity over the Americas warm pools. Geo- review. Prog. Oceanogr., 69, 101–142. phys. Res. Lett., 32, L02803, doi:10.1029/2004GL021033. Barton, E. D., and Coauthors, 1993: Supersquirt: Dynamics of the ——, J. Amador, and S. Medina, 1999: The midsummer drought Gulf of Tehuantepec, Mexico. Oceanography, 6, 23–30. over Mexico and Central America. J. Climate, 12, 1577–1588. Bordoni, S., P. E. Ciesielski, R. H. Johnson, B. D. McNoldy, and McCreary, J. P., H. S. Lee, and D. B. Enfield, 1989: The response B. Stevens, 2004: The low-level circulation of the North of the coastal ocean to strong offshore winds: With applica- American Monsoon as revealed by QuikSCAT. Geophys. tion to circulations in the Gulfs of Tehuantepec and Papa- Res. Lett., 31, L10109, doi:10.1029/2004GL020009. gayo. J. Mar. Res., 47, 81–109. Bourassa, M. A., L. Zamudio, and J. O’Brien, 1999: Noninertial McNoldy, B. D., P. E. Ciesielski, W. H. Schubert, and R. H. flow in NSCAT observations of Tehuantepec winds. J. Geo- Johnson, 2004: Surface winds, divergence, and vorticity in phys. Res., 104 (C5), 311–319. stratocumulus regions using QuikSCAT and reanalysis ——, D. M. Legler, J. J. O’Brien, and S. R. Smith, 2003: SeaWinds winds. Geophys. Res. Lett., 31, L08105, doi:10.1029/ validation with research vessels. J. Geophys. Res., 108, 3019, 2004GL019768. doi:10.1029/2001JC001028. Milliff, R. F., J. Morzel, D. B. Chelton, and M. H. Freilich, 2004: Chelton, D. B., M. H. Freilich, and S. K. Esbensen, 2000a: Satel- Wind stress curl and wind stress divergence biases from rain lite observations of the wind jets off the Pacific coast of Cen- effects on QSCAT surface wind retrievals. J. Atmos. Oceanic tral America. Part I: Case studies and statistical characteris- Technol., 21, 1216–1231. tics. Mon. Wea. Rev., 128, 1993–2018. Miyasaka, T., and H. Nakamura, 2005: Structure and formation ——, ——, and ——, 2000b: Satellite observations of the wind jets mechanisms of the Northern Hemisphere summertime sub- off the Pacific coast of Central America. Part II: Regional tropical highs. J. Climate, 18, 5046–5065. relationships and dynamical considerations. Mon. Wea. Rev., Muller-Karger, F. E., and C. Fuentes-Yaco, 2000: Characteristics 128, 2019–2043. of wind-generated rings in the eastern tropical Pacific Ocean. ——, S. K. Esbensen, M. G. Schlax, N. Thum, and M. H. Freilich, J. Geophys. Res., 105, 1271–1284. 2001: Observations of coupling between surface wind stress Parmenter, F. C., 1970: Picture of the month: A “Tehuantepecer.” and sea surface temperature in the eastern tropical Pacific. J. Mon. Wea. Rev., 98, 479. Climate, 14, 1479–1498. Roden, G. I., 1961: On the wind driven circulation in the Gulf of ——, M. G. Schlax, M. H. Freilich, and R. F. Milliff, 2004: Satellite Tehuantepec and its effect upon surface temperatures. measurements reveal persistent small-scale features in ocean Geofis. Int., 1, 55–76. winds. Science, 303, 978–983. Romero-Centeno, R., J. Zavala-Hidalgo, A. Gallegos, and J. J. Chen, P., M. P. Hoerling, and R. M. Dole, 2001: The origin of the O’Brien, 2003: Isthmus of Tehuantepec wind climatology and subtropical anticyclones. J. Atmos. Sci., 58, 1827–1835. ENSO signal. J. Climate, 16, 2628–2639. Clarke, A. J., 1988: Inertial wind path and sea surface temperature Schultz, D. M., W. E. Bracken, L. F. Bosart, G. J. Hakim, M. A. patterns near the Gulf of Tehuantepec and Gulf of Papagayo. Bedrick, M. J. Dickinson, and K. R. Tyle, 1997: The 1993 J. Geophys. Res., 93, 491–501. superstorm cold surge: Frontal structure, gap flow, and tropi- Curtis, S., 2002: Interannual variability of the bimodal distribution cal impact. Mon. Wea. Rev., 125, 5–39; Corrigendum, 125, 662. of summertime rainfall over Central America and tropical Steenburgh, W. J., D. M. Schultz, and B. A. Colle, 1998: The storm activity in the far-eastern Pacific. Climate Res., 22, 141– structure and evolution of gap outflow over the Gulf of Te- 146. huantepec, Mexico. Mon. Wea. Rev., 126, 2673–2691. ——, 2004: Diurnal cycle of rainfall and surface winds and the Stumpf, H. G., 1975: Satellite detection of upwelling in the Gulf of mid-summer drought of Mexico/Central America. Climate Tehuantepec, Mexico. J. Phys. Oceanogr., 5, 383–388. Res., 27, 1–8. ——, and R. V. Legeckis, 1977: Satellite observations of meso- Ebuchi, N., H. C. Graber, and M. J. Caruso, 2002: Evaluation of scale eddy dynamics in the eastern tropical Pacific Ocean. J. wind vectors observed by QuikSCAT/SeaWinds using ocean Phys. Oceanogr., 7, 648–658. buoy data. J. Atmos. Oceanic Technol., 19, 2049–2062. Trasviña, A., E. D. Barton, J. Brown, H. S. Velez, P. M. Kosro, Inoue, M., I. C. Handoh, and G. R. Bigg, 2002: Bimodal distribu- and R. L. Smith, 1995: Offshore wind forcing in the Gulf of tion of tropical cyclogenesis in the Caribbean: Characteristics Tehuantepec, Mexico: The asymmetric circulation. J. Geo- and environmental factors. J. Climate, 15, 2897–2905. phys. Res., 100, 20 649–20 663. Kanamitsu, M., W. Ebisuzaki, J. Woollen, S. Yang, J. J. Hnilo, M. Wang, C., and D. Enfield, 2001: The tropical Western Hemisphere Fiorino, and G. L. Potter, 2002: NCEP–DOE AMIP-II Re- warm pool. Geophys. Res. Lett., 28, 1635–1638. analysis (R-2). Bull. Amer. Meteor. Soc., 83, 1631–1643. Xie, S.-P., H. Xu, W. S. Kessler, and M. Nonaka, 2005: Air–sea Kessler, W. S., 2002: Mean three-dimensional circulation in the interaction over the eastern Pacific warm pool: Gap winds, northeast tropical Pacific. J. Phys. Oceanogr., 32, 2457–2471. thermocline dome, and atmospheric convection. J. Climate, Lavín, M. F., and Coauthors, 1992: Física del Golfo de Tehuante- 18, 5–20. pec. Cienc. Desarrollo, 17, 97–108. Zamudio, L., H. E. Hurlburt, E. J. Metzger, S. L. Morey, J. J. Liu, Y., and G. Wu, 2004: Progress in the study on the formation O’Brien, C. E. Tilburg, and J. Zavala-Hidalgo, 2006: Interan- of the summertime subtropical anticyclones. Adv. Atmos. nual variability of Tehuantepec eddies. J. Geophys. Res., 111, Sci., 21, 322–342. C05001, doi:10.1029/2005JC003182.

Unauthenticated | Downloaded 10/01/21 04:50 PM UTC