THE INTERTROPICAL CONVERGENCE ZONE OF THE EASTERN PACIFIC REVISITED

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 22: (2002) Published online in Wiley InterScience ( DOI: /joc.739 THE INTERTROPICAL CONVERGENCE ZONE OF THE EASTERN PACIFIC REVISITED STEFAN HASTENRATH* Department of Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI, USA Received 8 May 2001 Revised 25 September 2001 Accepted 5 October 2001 ABSTRACT The Intertropical Convergence Zone (ITCZ) complex over the eastern Pacific is re-examined from the NCEP NCAR 40 year reanalysis and other data. Consistent with earlier work, the new analysis yields for July August: a surface position of the near-equatorial wind confluence at 12 N; the cross-equatorial flow changing from divergent to convergent at the latitude of recurvature from southeasterly to southwesterly; strongest convergence in the surface layer to the south of the wind confluence, with this feeding vigorous ascending motion and compensating divergence in the upper troposphere; and the interface between cross-equatorial flow and northeast trades sloping southward at 1 : An easterly speed maximum in the mid troposphere over the equator is consistent with the 4 year rawin record at Galapagos. The new coarse-resolution dataset fails to capture a well-developed Intertropical Divergence Zone. The annual cycle features for the wind confluence and the ITCZ a position closest to the equator in February, a northward migration to June, southward shift in July and August, and a northernmost location in September. This behaviour is paralleled by maxima in the frequency of the Temporal weather systems and rainfall on the Pacific side of Central America in June and September. Regarding the climatic variability, with an anomalously far southerly position of the wind confluence, Central American rainfall tends to be deficient, and such a combination of departures is common in the warm phase of the southern oscillation. The long-term evolution is characterized by a warming trend of equatorial Pacific waters and weakening of cross-equatorial wind, while trends in the latitude position of the wind confluence and in Central American rainfall are not significant. Copyright 2002 Royal Meteorological Society. KEY WORDS: eastern Pacific; wind; sea surface temperature; pattern analysis 1. INTRODUCTION The Intertropical Convergence Zone (ITCZ) of the eastern Pacific and its role in the climate of the Central American equatorial Pacific region has recurrently been the subject of our investigations at the University of Wisconsin since the l960s (Hastenrath, l966, 1967, 1976, 1977a,b; Hastenrath and Lamb, l977, l978a,b; see review in Hastenrath (l995: )). The research was based on long-term surface and upper-air station records and surface ship observations, and has included the construction of atlases of surface circulation and climate and the oceanic heat budget (Hastenrath and Lamb, l977, l978a). More recently, the atmosphere ocean system of the region has attracted the attention of other research groups (Hayes et al., 1989, 1991; Wallace et al., 1989; Umatani and Yamagata, 1991; Mitchell and Wallace, 1992; Köberle and Philander, 1994; Wang, 1994; Waylen et al., 1994, 1996; Dijkstra and Neelin, 1995; World Climate Research Programme, 1995: 38 39), as it is increasingly conjectured that this limited domain may have a major role to play in the functioning of the tropical climate system. * Correspondence to: Stefan Hastenrath, Department of Atmospheric and Oceanic Sciences, University of Wisconsin, 1225 West Dayton Street, Madison, WI, USA; barafu@macc.wisc.edu Copyright 2002 Royal Meteorological Society

2 348 S. HASTENRATH In conjunction, these studies have provided a perception of the average structure of the circulation, its annual cycle, and the climatic variability of the region. The novel l National Centers for Environmental Prediction (NCEP) National Center for Atmospheric Research (NCAR) reanalysis invited a reappraisal, especially of the upper-air structure. The aim of the present study is twofold: to complement and compare the findings of earlier work, and to explore further the information value of the new NCEP NCAR upper-air dataset. 2. BACKGROUND The surface pattern and vertical structure of the ITCZ complex over the eastern Pacific has been analysed from long-term ship observations and scant upper-air soundings (Hastenrath, l966, l977a,b; Hastenrath and Lamb, l977, l978a), and also aircraft traverses during WWII (Alpert, l945). The near-equatorial low-pressure trough sits over a band of warm surface waters well into the Northern Hemisphere. Following the meridional pressure gradient, the cross-equatorial surface airflow from the Southern Hemisphere recurves from southeasterly to southwesterly and changes from divergence to convergence at about 5 N. Further downstream, this air current from the Southern Hemisphere meets the northeast trades along a confluence embedded within the near-equatorial low-pressure trough. The characteristics of the surface flow imply a zone of divergence between the equator and flow recurvature the Intertropical Divergence Zone (ITDZ; Hastenrath and Lamb, 1978b; Hastenrath, l995: ; l999) and the ITCZ further north, with maximum convergence to the south of the surface wind confluence. The wind-stress forcing and Ekman pumping by the cross-equatorial surface winds (Philander and Pacanowski, 1981) result in a tongue of cold surface waters immediately to the south of the equator, contrasting with much warmer surface temperature conditions to the north of the equator. Turning to the upper-air conditions, the interface between cross-equatorial flow and northeast trades slopes southward with a slope of about 1 : Thus, the intense cross-equatorial surface current is remarkably shallow. At the equator, the westerly wind component in the lower layers fades out around 850 hpa, the location of the interface, giving way to an easterly component aloft, with a speed maximum at around 600 hpa (Hastenrath, 1999). In accordance with the seasonal variations of the sea surface temperature (SST) and surface pressure fields, the near-equatorial wind confluence and associated band of convergence exhibit a marked annual cycle in latitude position: from a location nearest to the equator in February, the wind confluence and ITCZ migrate poleward until June, recede southward in July and August, move northward again and reach their northernmost position in September; after that the system shifts to its location closest to the equator in February. This marked double peak in the latitude position of wind confluence and ITCZ is paralleled by the annual cycle of climate in the Central American region. Thus, while the rainy season broadly extends from May through October, or the boreal summer half-year, precipitation activity is reduced in July and August. Furthermore, the Temporal weather systems, which originate in the ITCZ over the eastern Pacific and which provide a substantial part of the annual rainfall totals on the Pacific side of Central America, reach a first peak of occurrence in June, are rare in July and August, and are most frequent in September. They are absent during the dry season winter half-year, November April. Though it is recognized that the double peak in the annual cycle of Central American rainfall may result from a combination of mechanisms (Hastenrath, 1967; Magaña et al., 1999), the double-peaked annual cycle in the latitude position of the wind confluence and ITCZ over the eastern Pacific must be regarded as a major factor. Work since the late 1980s recognizes the effect of the oceanic cold tongue on atmospheric stability, and the effect of stability on vertical transfer of momentum (Wallace et al., 1989; Hayes et al., l989), as well as the complexity of processes controlling the temperature of the oceanic mixed layer (Hayes et al., 1991). In particular, the passage of the cross-equatorial surface flow from over colder to over warmer waters would favour enhanced vertical exchange of momentum, and this is thought to lead to acceleration and divergence (Wallace et al., 1989; Hayes et al., 1989). Though such a hypothesized causality sequence would cooperate with the dynamics of surface flow presented before (Hastenrath and Lamb, 1977, 1978b; Hastenrath, l995: ), it should be recalled that the winds aloft are about perpendicular to those at the surface.

3 EASTERN PACIFIC ITCZ DATA The main database for this study is the NCEP NCAR 40 year reanalysis (Kalnay et al., l996) and the comprehensive ocean atmosphere data set (COADS; Woodruff et al., l993), stemming from the same source as used in our atlases in the l970s (Hastenrath and Lamb, l977, l978a). The NCEP NCAR data are in a 2.5 spatial resolution. Elements of interest here are 1000 hpa topography, wind at surface (10 m) and wind and omega vertical velocity at 1000, 925, 850, 700, 600, 500, 400, 300, 200 and 100 hpa. Divergence was calculated from the horizontal wind fields at constant pressure level. From the COADS, SST was analysed. As a measure of the phases of the southern oscillation (SO), the PAC index of the SST anomaly in the equatorial Pacific proposed by Wright (1984) was used (domain encompassing the regions 2 6 N, W; 2 N 6 S, W; 6 10 S, W). Observational coverage along the major ship tracks is optimal in this sensitive domain, and the index has proven informative in earlier work. From this PAC index for July August, the ten most extreme warm years (1963, 65, 69, 72, 76, 82, 83, 87, 91, 97), and the ten most extreme cold years (1961, 64, 70, 71, 73, 75, 78, 85, 85, 88) were identified. Regarding the work reviewed in Section 2, a diversity of data sources, temporal resolutions, and time periods were used that are not to be detailed here. 4. MEAN CIRCULATION From the perspective of the review of earlier work in Section 2, the evaluation from the NCEP NCAR dataset and COADS is presented here. Figure 1 offers maps of pertinent fields for the July August middle of the boreal summer half-year, analysed from the NCEP NCAR dataset and COADS. Figure 1(a) of SST shows the cold water tongue immediately to the south of the equator and the band of warmest surface waters around 15 N. The map in Figure 1(c) of 1000 hpa topography depicts the near-equatorial low-pressure trough located somewhat to the south of the band of warmest surface waters (Figure 1(a)). The map of Figure 1(d) illustrates the cross-equatorial airstream from the Southern Hemisphere recurving from southeasterly to southwesterly around 3 N and meeting the northeast trades along a confluence line around 12 N embedded within the near-equatorial low-pressure trough (Figure 1(c)). Figure 1(b) does not show a proper ITDZ as described in Section 2, but the ITCZ in a band at about 5 10 N, broadly embedded within the near-equatorial low-pressure trough (Figure 1(c)), with the strongest convergence located to the south of the wind confluence (Figure 1(d)). Overall, the maps of Figure 1(a) (d) are compatible with our atlas (Hastenrath and Lamb, l977: charts 56 57, 36 37, 8 9, 20 21). It should be noted, however, that the NCEP NCAR dataset does not resolve the flow recurvature well and fails to capture the ITDZ, presumably due to shortcomings in the data assimilation and the coarse spatial resolution. With the surface pattern of the ITCZ complex illustrated by Figure 1, the upper-air structure may be appreciated with reference to the meridional vertical cross-sections in Figure 2. Before the advent of the NCEP NCAR reanalysis the pertinent upper-air information was scarce. The 4 year s of rawin soundings at Galapagos allow one to retrieve the vertical wind profile in the lower atmosphere over the equator and to discern the upper limit of the cross-equatorial flow, or height of the interface with the northeast trades. An analysis of aircraft observations (Sadler, l975) provided insight into the divergence pattern in the upper troposphere. These sources, together with the more abundant surface information, were used a quarter of a century ago (Hastenrath, l977a) for the assessment of the upper-air structure of the ITCZ complex over the eastern Pacific reproduced in Figure 2(a). The cross-section in Figure 2(a) illustrates: the oceanic cold tongue immediately to the south of the equator; the ITDZ between the equator and about 5 N; the ITCZ over warm surface waters, with strongest convergence and cloudiness to the south of the wind discontinuity; subsidence in the realm of the ITDZ; and an interface between the cross-equatorial flow and the northeast trades sloping southward with a slope of about 1 : Not indicated in Figure 2(a) are isotachs of zonal wind component, but the wind soundings over Galapagos show an easterly speed maximum at about 600 hpa.

4 350 S. HASTENRATH Figure 1. July August mean maps. (a) SST, with isoline spacing of 1 C; heavy solid line denoting the wind confluence is transposed from map (d). (b) Surface divergence, with isoline spacing of s 1 and negative values meaning convergence; heavy solid line denoting the wind confluence is transposed from map (d). (c) 1000 hpa topography, with isoline spacing of 10 gpm, and heavy solid line denoting the surface wind confluence. Straight dotted lines denote the location of the meridional vertical cross-sections in Figure 2, heavy dots the location of rainfall stations plotted in Figure 3, namely from north to south Guatemala (GUA), Acajutla (ACA) in El Salvador, and San Jose (SJ) in Costa Rica, and dashed lines denote the quadrangle (4 N 4 S, W) for which the indices V of meridional wind component, T of SST, and Z of 1000 hpa topography (Figures 3 and 5 and Table I) were compiled. (d) Surface (10 m) wind field, with isotach spacing of 2 m s 1, arrows indicating the direction of flow, and heavy solid line denoting the confluence line (zero meridional wind component) between cross-equatorial flow and northeast trades. Sources are NCEP NCAR, except for COADS in (a) For comparison with Figure 2(a), the cross-section in Figure 2(b) displays the diagnostics from the NCEP NCAR dataset and COADS, with spatial continuity in the vertical. The oceanic cold tongue to the south of the equator and the warmest waters in the Northern Hemisphere correspond to the depiction in Figure 1(a), as does the position of the surface wind confluence over the warm waters. Complementing Figure 2(a), Figure 2(b) shows the amount and meridional and vertical extent of divergence and convergence. Most prominent is the strong convergence in the surface layer in the realm of the ITCZ, with largest values to the south of the wind confluence. This feeds vigorous ascending motion through the lower and mid troposphere, and divergence in the upper troposphere. The near-surface cross-equatorial flow changes from divergent to convergent at the latitude of recurvature from southeasterly to southwesterly. An easterly speed maximum is indicated around 700 hpa over the equator. All these characteristics are consistent with and

5 EASTERN PACIFIC ITCZ mb a m 1000 NE 0 C SW SE SE ATM WARM COLD Ga OCEAN 200 b NCEP/NCAR hpa s 1 5 m s C C 20 N S Figure 2. July August long-term mean meridional vertical cross-sections. (a) Reproduced by permission of Springer-Verlag from an earlier study (Hastenrath, 1977a); NE, SE, SW denote wind directions, heavy dashed line wind discontinuity, hatched area divergence, dot raster convergence, C convergence maximum, arrows motion in meridional vertical plane, Ga Galapagos; vertical exaggeration approximately 1 : 550. (b) Based on NCEP NCAR data and COADS. SST is indicated at base of section by vertical tick marks with numbers in full centigrade. Dashed line represents wind discontinuity (zero meridional wind component) between cross-equatorial flow and northeast trades. Solid thin lines indicate divergence at spacing of s 1, with shading denoting convergence. Arrows represent direction of meridional and vertical motion, with 2 m s 1 and hpa s 1 scaled at 1 latitude. Dotted line is isotach of 10 m s 1 easterly wind component

6 352 S. HASTENRATH complement the earlier findings reviewed in Section 2. By contrast, the NCEP NCAR reanalysis fails to capture a well-developed ITDZ. 5. ANNUAL CYCLE Figure 3 compares the annual cycle of pertinent indicators. The rainfall at three long-term rain gauge stations on the Central American Isthmus, from north to south, Guatemala (Observatory), Acajutla (El Salvador), and San Jose (Costa Rica), all indicate the rainy season spanning the boreal summer half-year, May to October, and particularly the double peaks in June and September, with the intermediate reduction of precipitation activity in July August (Figure 3(a)). As indicated in Section 2, the causes for this bimodality and midsummer drought may be complex, but the processes over the eastern Pacific appear an obvious factor. The annual cycle in the frequency of Temporales (Figure 3(b)), a major rain-bearing weather system for the Pacific side of the Central American Isthmus, shows a minor peak in June and its main maximum in September. Figure 3(c) displays the latitude position of the wind confluence as analysed from the NCEP NCAR dataset, and of the ITCZ as captured from aircraft traverses in WWII (Alpert, 1945). As familiar from earlier work (Hastenrath and Lamb, 1977, 1978b; Hastenrath, l995: ), the ITCZ sits somewhat to the south of the wind confluence (Figure 3(c)). The plots both show a migration from a position closest to the equator around February to more poleward locations in the boreal summer half-year, with extrema in June and September, and a southward shift in July and August. In the context of the annual cycle, with a more northerly position of wind confluence and ITCZ, the Temporal weather systems are more frequent and Central American rainfall more abundant. Figure 3(d) and (e) illustrate the annual cycle of wind stress and SST. The southerly wind component increases from boreal winter to summer, consistent with which the wind confluence (Figure 3(c)) displaces northward. The increased southerly wind component is, to the south of the equator, also conducive to westward Ekman transport and cold water advection, the essential factors for the generation of the near-equatorial cold tongue (Philander and Pacanowski, l981; Hastenrath, l999). Consistent with this, the waters of the equatorial Pacific become coldest in boreal summer. 6. CLIMATIC VARIABILITY The SO is widely regarded as a major contributor to the interannual climate variability. In that vein, Figure 4 presents maps of the differences of fields of an ensemble of ten extremely warm minus ten extremely cold July Augusts in the equatorial Pacific. The difference maps of Figure 4(a) and (b) should be appreciated with reference to the corresponding long-term mean maps in Figure 1(a) and (b). Note that the maps of the fields of the warm minus the cold ensemble broadly reflect the characteristics of the departures during the warm years. Figure 4(b) indicates for the warm years a southward displaced wind confluence, significantly weaker convergence in the realm of the long-term mean ITCZ position (Figure 1(b)) and a significant change to convergence in the equatorial region. Consistent with this, Wallace et al. (1989) noted for the cold years a southerly wind component weakened at the equator and enhanced farther north. Figure 4(a) illustrates the significantly warmer conditions, especially in the realm of the near-equatorial cold tongue (Figure 1(a)). Complementing Figure 4, Table I presents, for the boreal summer half-year,a matrix of correlations between pertinent indicators, including the PAC index used in the stratification for Figure 4. Concerning the SO, Table I shows that during the warm phase the southerly wind component over the equatorial eastern Pacific tends to be weaker, the wind confluence is farther south, and the rainfall at the Pacific side of the Central American Isthmus tends to be deficient; all of these correlations being statistically significant. Regarding the associations between the regional indicators, with a weaker meridional wind component over the equatorial Pacific the equatorial surface waters are significantly warmer, and the wind confluence is located farther south. Furthermore, with a more southerly location of the wind confluence, rainfall in Central America is significantly reduced. It may be recalled that, over the distant North America, enhanced precipitation has

7 EASTERN PACIFIC ITCZ 353 Figure 3. Mean annual cycle of pertinent indicators (period , except for (b) and (c)). (a) Rainfall at Guatemala solid, Acajutla (El Salvador) dashed, and San Jose (Costa Rica) dotted line (for location of stations see Figure 1(d)). (b) Number of Temporal weather systems in El Salvador during the 41 year period (c) Latitude position of wind confluence ( mean) at W solid line, and latitude of ITCZ from aircraft traverses during WWII (Alpert, l945) dashed line. (d) Meridional wind component. (e) SST in domain 4 N 4 S, W indicated in Figure 1(d) been noted during the warm SO phase (Ropelewski and Halpert, l986). Overall, Table I bears out the close relationships between the wind field, SST, and latitude of wind confluence line over the eastern Pacific, and the precipitation activity in Central America; it also shows the interannual variations in regional circulation and climate to be, to some extent, in tune with the phases of the SO.

8 354 S. HASTENRATH Figure 4. July August maps of the difference of the ensembles of ten extremely warm minus ten extremely cold years in the equatorial Pacific: (a) SST with isoline spacing of 1 C; (b) surface divergence, with isoline spacing of s 1. In both maps the position of the wind confluence is shown by dotted line for the ensemble of ten extremely cold years, by solid line for the mean, and by dashed line for the ensemble of ten extremely warm years. Shading indicates domains reaching statistical significance at the 5% level Table I. Matrix of correlations (in hundredths) between pertinent indicators, for half-year May through October and period V is the meridional wind component; T is the SST; Z is the 1000 hpa topography in the domain 4 N 4 S, W (see Figure 1(c)); PAC is the index of SST anomalies in the equatorial Pacific; LAT is the latitude position of the surface wind confluence; GUA, ACA, and SJ denote rainfall at Guatemala, Acajutla, and San Jose (see Figure 1(c)) respectively. One and two asterisks indicate significance at the 5% and 1% levels respectively V T Z PAC LAT GUA ACA V T 65 Z PAC LAT GUA ACA SJ The time series in Table I were examined for long-term evolutions. There is a weak warming trend of equatorial Pacific waters (PAC and T) accompanying a significant weakening of cross-equatorial wind (V), but there is no indication of significant trends in the latitude position of the wind confluence (LAT), the SST and 1000 hpa topography in the limited equatorial domain (T and Z), or Central America rainfall (GUA, ACA, SJ). The frequency of Temporal weather systems was reduced in the 1970s. Given the absence of significant trends in most elements, the significant correlations in Table I are primarily associated with the interannual variability. The trend in PAC and T accompanies long-term evolutions in the greater Pacific basin (Curtis and Hastenrath, 1999).

9 EASTERN PACIFIC ITCZ CONCLUSIONS The recent NCEP NCAR 40 year reanalysis invited a reappraisal of the earlier work that had to rely on limited upper-air information. Application to a subject studied in earlier work also offers an opportunity to explore the information value of the new dataset. With such motivation, the present study examined the surface and upper-air structure of the ITCZ complex over the eastern equatorial Pacific at the height of the boreal summer, the annual cycle of circulation, and the climatic variability of the region. Broadly in accordance with the earlier work, the new analysis shows for the July August core of boreal summer: a near-equatorial surface wind confluence at 12 N; divergence giving way to convergence at the recurvature latitude of the cross-equatorial flow; maximum convergence in the surface layer to the south of the wind confluence, and along with this a strong upward motion and divergence in the upper troposphere; and the interface between cross-equatorial flow and northeast trades sloping southward at 1 : A mid-tropospheric easterly speed maximum over the equator is supported by the scarce upper-air soundings over Galapagos. An unavoidable drawback is the coarse spatial resolution of 2.5 in the NCEP NCAR dataset; as a consequence, the recurvature of the cross-equatorial flow from southeasterly to southwesterly is less conspicuous and appears at a somewhat lower latitude than in the fine-resolution long-term ship data (Hastenrath and Lamb, 1977: charts 20 21), and along with this the ITDZ is not captured well by the NCEP NCAR dataset. The annual cycle of the surface circulation is characterized by a latitude position of the wind confluence and ITCZ closest to the equator in February, a northward shift till June, a southward displacement in July and August, and a northernmost position in September. In tune with these annual cycle variations of the quasipermanent circulation systems over the eastern Pacific are the June and September maxima of Temporales and of rainfall totals on the Pacific side of the Central American Isthmus. The variability of climate in the region shows some association with the SO. An anomalously far southerly position of the wind confluence is accompanied by deficient rainfall on the Pacific side of Central America. Such a combination of departures is common in the warm phase of the SO. As noted before, the equatorial Pacific waters show a warming trend; this is accompanied by a weakening of cross-equatorial wind, but an overall interannual variability, rather than a long-term trend, dominates in the climate of the eastern Pacific Central American region. ACKNOWLEDGEMENTS This study was supported by NSF grant ATM At the University of Wisconsin, Dierk Polzin assisted me with the computations and graphics. For the updated rainfall series of Central American stations I thank Eddy Hardy Sanchez Benett in Guatemala, Juan Carlos Fallas and Vilma Castro in Costa Rica, and Luis Garcia Guirola in El Savador. Finally, I fondly recall exchanges of thought with Albert Pallmann during our work in the Servicio Meteorologico Nacional de El Salvador 40 years ago. REFERENCES Alpert L The intertropical convergence zone of the eastern Pacific region. I. Bulletin of the American Meteorological Society 26: Curtis S, Hastenrath S Long-term trends and forcing mechanisms of circulation and climate in the equatorial Pacific. Journal of Climate 12: Dijkstra JD, Neelin HA Ocean atmosphere interaction and the tropical climatology, II. Why the Pacific cold tongue is in the east. Journal of Climate 8: Hastenrath S On general circulation and energy budget in the area of the Central American seas. Journal of the Atmospheric Sciences 23: Hastenrath S Rainfall distribution and regime in Central America. Archiv für Meteorologie, Geophysik und Bioklimatologie, Serie B 15: Hastenrath S Variations in low-latitude circulation and extreme climatic events in the tropical Americas. Journal of the Atmospheric Sciences 33: Hastenrath S. 1977a. On the upper-air circulation over the equatorial Americas. Archivfür Meteorologie, Geophysik und Bioklimatologie, Serie. A 25: Hastenrath S. 1977b. Hemispheric asymmetry of oceanic heat budget in the equatorial Atlantic and eastern Pacific. Tellus 29:

10 356 S. HASTENRATH Hastenrath S Climate Dynamics of the Tropics, 2nd printing. Kluwer: Dordrecht, Boston, London; 488 pp. Hastenrath S Equatorial mid-tropospheric easterly jet over the eastern Pacific. Journal of the Meteorological Society of Japan 77: Hastenrath S, Lamb P Climatic Atlas of the Tropical Atlantic and Eastern Pacific Oceans. University of Wisconsin Press: Madison; 112 pp. Hastenrath S, Lamb PJ. 1978a. Heat Budget atlas of the Tropical Atlantic and Eastern Pacific Oceans. University of Wisconsin Press: Madison; 103 pp. Hastenrath S, Lamb PJ. 1978b. On the dynamics and climatology of surface flow over the equatorial oceans. Tellus 30: Hayes SP, McPhaden MJ, Wallace JM The influence of sea surface temperature on surface wind in the eastern equatorial Pacific: weekly to monthly variability. Journal of Climate 2: Hayes SP, Chang P, McPhaden M Variability in sea surface temperature in the eastern equatorial Pacific. Journal of Geophysical Research Oceans 96(C6): Kalnay E, Kanamitsu M, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woollen J, Zhu Y, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo KC, Ropelewski C, Wang J, Leetmaa A, Reynolds R, Jenne R, Joseph D The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77: Köberle C, Philander G On the processes that control seasonal variations of sea surface temperatures in the tropical Pacific Ocean. Tellus 46: Magaña V, Amador J, Medina S The mid-summer drought over Mexico and Central America. Journal of Climate 12: Mitchell TP, Wallace JM On the annual cycle in equatorial convection and sea surface temperature. Journal of Climate 5: Philander SGH, Pacanowski RC The oceanic response to cross-equatorial winds (with application to coastal upwelling in low latitudes). Tellus 33: Ropelewski CF, Halpert MS. l986. North American precipitation patterns associated with the El Niño/southern oscillation (ENSO). Monthly Weather Review 114: Sadler JC The upper-tropospheric circulation over the global tropics. Department of Meteorology, University of Hawaii, UHMET ; 35 pp. Umatani S, Yamagata T Response of the eastern tropical Pacific to meridional migration of the ITCZ: the generation of the Costa Rica dome. Journal of Physical Oceanography 21: Wallace JM, Mitchell TP, Deser C The influence of sea surface temperature on surface wind in the eastern equatorial Pacific: seasonal and interannual variability. Journal of Climate 2: Wang X The coupling of the annual cycle and ENSO over the tropical Pacific. Journal of the Atmospheric Sciences 51: Waylen PR, Quesada ME, Caviedes CN The effects of El Niño-southern oscillation on precipitation in San José, Costa Rica. International Journal of Climatology 14: Waylen PR, Caviedes CN, Quesada ME Interannual variability of monthly precipitation in Costa Rica. Journal of Climate 9: Woodruff S, Lubker S, Wolter K, Worley S, Elms J Comprehensive ocean atmosphere data set (COADS) release 1a: Earth System Monitor 4(1): 1 8. World Climate Research Programme CLIVAR, a study of climate variability and predictability. WCRP-89, WMO/TD No World Meteorological Organization: Geneva, Switzerland; 157 pp. Wright PB Relationships between indices of the southern oscillation. Monthly Weather Review 112: