JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. C9, PAGES 18,225-18,233, SEPTEMBER 15, 1994 Upper layer temperature structure of the western tropical Atlantic Robert L. Molinari and Elizabeth Johns NOAA Atlantic Oceanographicand Meteorological Laboratory, Miami, Florida Abstract. Mean monthly topographies of the 20øCand 10øC isothermal surfacesare used to describe the vertical displacementsof the upper and lower thermocline in the western tropical Atlantic. The isotherm topographies are generated from expendable bathythermograph data collected between 1966 and 1993. The topographiesconfirm, and extend closer to the coast, earlier findings that demonstrate large spatial and temporal variability in the region. For example, the ridge and trough systems observed previously in the interior are shown, and their extension to the western boundary is described. In particular, it is shown that the ridge associated with the North Equatorial Countercurrent (NECC) extends from the interior northwestward along the western boundary, reaching farther north along the boundary in the upper thermocline than in the lower thermocline. South of the equator the northwestern corner of the countercurrent trough is apparent on the lower surface but not on the upper. The annual and semiannual harmonics of the vertical isotherm displacements account on the average for about 60% of total variance on both surfaces. The horizontal structure of the first harmonic amplitude is similar for both surfaces, showing maximum amplitude along the axis of the NECC ridge. Minimum amplitudes are observed to the north along the axis of the countercurrent trough. These distributions are similar to the pattern of the first-harmonic amplitude of the wind stresscuff, supporting earlier studiesof curl forcing of near-surface current features. Introduction The upper layer temperature structure of the western and central tropical Atlantic Ocean has been characterized pre- viously by large variability in both time and space. Away from the western boundary, Merle [1978] showed that sea- sonal upper layer temperature and dynamic topography distributions are dominated by a series of zona!!y oriented ridges and troughs. These features are shown schematically in Figure 1 superimposed on the upper layer current distri- bution. The equatorial trough is bounded on the north and south by countercurrent ridges, called equatorial ridges by Katz [1981]. These ridges are typically located 3 ø to 5ø away from the equator. Poleward of the ridges in both hemi- spheres are the countercurrent troughs [Katz, 1981]. Katz [1981] found a significant reduction in the dynamic height differences between the ridges and the equatorial trough from late winter to summer. These differences appear as reductions in the meridional temperature gradients in the charts of Merle [1978]. Garzoli and Katz [1983], among others, show that the northern hemisphere countercurrent ridge and trough vary in phasewith the local wind stress curl field in the interior but that the relation breaks down as the boundary is approached. Along the western boundary, eddies, retroflections, and undercurrents have been observed which add considerable structure to the temperature and dynamic height distribu- tions. For instance, Cochrane et al. [1979] described an Amazon Anticyclone in dynamic topography surfaces, with This paper is not subjectto U.S. copyright. Publishedin 1994by the American Geophysical Union. Paper number 94JC01204. its northern boundary at 5øN. They concluded that this feature is an extension of the interior countercurrent ridge and that it has a large annual variability in its intensity. Such variability also is present in numerical models of the region [e.g., Philander and Pacanowski, 1986]. Cochrane et al. [1979] also found evidence for anticy- clones to the north and west of the Amazon Anticyclone but ...... 11 ....... 1 ..... A :-•'•:--'•*'•'• *•"•* thcsc are u•ua.y wca•.ct a.u more variable in 111UI•,eI:•L L•,eU position. Bruce and Kerling [1984], on the other hand, describe another anticyclone, the Demerara Eddy, located to the north of the Amazon Anticyclone at about 8ø to 9øN, as a permanent feature of the temperature and dynamic topography field. The majority of these studies, however, were based on limited amounts of data, and the estimation of meaningful annual cycles is particularly problematic. Expendable bathythermograph (XBT) observations pro- vide a relatively inexpensive way to study the upper ocean temperature structure on large spatial scales. In particular, XBTs have been widely used as a regional survey tool on research vessels and are now deployed routinely on a global basis by Voluntary Observing Ships (VOS; typically mer- chant ships)to study the large-scale ocean thermal structure. Both modes of data collection have been and continue to be used in the western tropical Atlantic Ocean. For example, XBTs were recently used for regional western boundary surveys during the 1990-1991 Western Tropical Atlantic Experiment (WESTRAX) [Brown et al., 1992], while coarser samplingis also available in the region as part of the World Ocean Circulation Experiment (WOCE) and the Tropical Ocean-Global Atmosphere (TOGA) effort. Herein, we use XBT data to generate a monthly climatol- ogy of upper ocean thermal structure in the western tropical 18,225