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Mean Three-Dimensional Circulation in the Northeast Tropical Pacific*

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Mean Three-Dimensional Circulation in the Northeast Tropical Pacific* SEPTEMBER 2002 KESSLER 2457 Mean Three-Dimensional Circulation in the Northeast Tropical Paci®c* WILLIAM S. KESSLER NOAA/Paci®c Marine Environmental Laboratory, Seattle, Washington (Manuscript received 5 July 2001, in ®nal form 26 December 2002) ABSTRACT Historical XBT data are used to construct a mean climatology of the three-dimensional geostrophic circulation in the northeast tropical Paci®c (southwest of Mexico and Central America) and are diagnosed based on linear dynamics forced with satellite scatterometer winds. Unlike the familiar central tropical Paci®c, where the zonal scales are very large and the wind forcing nearly a function of latitude alone, the North Paci®c east of about 1208W is strongly in¯uenced by wind jets blowing through gaps in the Central American cordillera. The curl imposed by these wind jets imprints on the ocean, producing a distinctive pattern of thermocline topography and geostrophic currents that are consistent with the Sverdrup balance. Notably, the weakening of the North Equatorial Countercurrent near 1108W is due to the wind forcing. Given the observed strati®cation and wind stress curl, planetary vorticity conservation also determines the distribution of vertical velocity in the region, with about 3.5 Sv (Sv [ 106 m3 s21) of upwelling through the base of the thermocline under the Costa Rica Dome. This upwelling is associated with stretching of the water column under the dome, which thereby causes the northern ``Subsurface Counter Current'' (SSCC or Tsuchiya Jet) to turn away from the equator; about half the transport of the SSCC upwells through the thermocline via this mechanism. This may be part of the process by which intermediate-depth water, ¯owing into the Paci®c from the south, is brought to the surface and into the Northern Hemisphere. 1. Introduction thermal ®eld, which permits the vertical structure of the geostrophic currents in the region to be estimated. This Although the northeastern tropical Paci®c is close to structure is diagnosed based on linear dynamics forced a well-populated coast, is crossed by often-traveled by highly spatially resolved satellite scatterometer shipping lanes, and is the long-term site of major ®sh- eries, there has been relatively little attention paid to winds. The mean signature of the mountain-gap winds the dynamics that control the mean circulation in the is found to be the crucial forcing element, as others region since the pioneering investigations done at the have found for the short-timescale episodes. However, Scripps Institution of Oceanography more than 30 years it is argued here that the mean ocean response can be ago (Cromwell 1958; Wyrtki 1964, 1966, 1967). Much largely understood without reference to the nonlinear more work has studied the ocean responses to the some- mechanisms that are invoked for the high-frequency ed- times-spectacular Central American mountain-gap wind dies. While it is likely that these nonlinear processes events that can generate intense mixing and eddy ac- rectify into both the mean and the annual cycle, the ®rst tivity over periods of a few days (Roden 1961; Clarke step in understanding the effect of the winds on the 1988; McCreary et al. 1988; Barton et al. 1993; Giese ocean is to quantify the linear effect of the mean forcing, et al. 1994; TrasvinÄa 1995). Others have looked at the which is the aim of the present study. effects of propagating, nonlinear eddies produced by The importance of the mountain-gap winds empha- strong currents near the coast (Umatani and Yamagata sizes the difference between the eastern Paci®c and the 1991; Hansen and Maul 1991). wide open spaces of the central tropical Paci®c, where In the present paper, historical expendable bathy- the familiar zonal currents (the westward North and thermograph (XBT) data are used to de®ne the mean South Equatorial Currents, and the eastward North Equatorial Countercurrent) that enter and leave this re- gion have been well studied (Wyrtki and Kilonsky 1984; * Paci®c Marine Environmental Laboratory Contribution Number 2351. Johnson et al. 1988; Taft and Kessler 1991; Picaut and Tournier 1991; Reverdin et al. 1994; Lagerloef et al. 1999; Johnson et al. 2002). In the central Paci®c, mean Corresponding author address: William S. Kessler, NOAA/Paci®c winds are nearly a function of latitude alone, and a Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115. useful diagnosis of the ocean response can be gained E-mail: [email protected] by interpretation based on linear, Sverdrupian dynamics Unauthenticated | Downloaded 09/28/21 02:49 AM UTC 2458 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 32 FIG. 1. Distribution of XBT data, strati®ed by season. The number in parenthesis under each season indicator gives the total number of pro®les in the region during that period. (Sverdrup 1947; Meyers 1980; Kessler and Taft 1987; layer. If this is borne out by subsequent analyses, it will McPhaden and Fine 1988; Landsteiner et al. 1990). By be signi®cant because the tropical upwelling usually contrast, in the east, the effect of the Central American considered important (that in the equatorial cold tongue) cordillera funneling winds through narrow passes makes arises primarily from within the thermocline and above. the wind ®eld much more complicated, and likewise so Thus the regional circulation studied here may play a are the ocean thermal and current structures, but we role in basin-scale water mass exchanges. argue here that the wind-driven imprint on the ocean is still controlled by similar dynamical mechanisms. A larger goal, not achieved here, is to ``close'' the 2. Data circulation in the eastern tropical Paci®c by understand- a. XBT data ing the interconnections among the major central Paci®c currents at the eastern boundary. A convincing closure The principal dataset used in this study is the historical will probably include the use of tracer ®elds to establish XBT compilation of Donoso et al. (1994). These pro®les the pathways of ¯ows that are too slow, too deep, too originated from earlier compilations produced principally intermittent, too close to the coast, or are the result of by the National Ocean Data Center, the Joint Environ- mixing and thus are dif®cult to diagnose based on geos- mental Data Center at Scripps, and the Global Subsurface trophy as done here. However, the geostrophic dynamics Data Center in Brest, France. A total of 21 204 pro®les studied here strongly imply that these interconnections were available in the study region 08±258N and from are three-dimensional, in particular that the off-equa- 1208W to the American coast. Figure 1 shows that the torial northeastern tropical Paci®c is a region of signif- pro®les were primarily organized along the major ship- icant upwelling of subthermocline water into the surface ping routes. The main concentration of pro®les were tak- Unauthenticated | Downloaded 09/28/21 02:49 AM UTC SEPTEMBER 2002 KESSLER 2459 en during 1979 through 1992, with smaller numbers after can resolve the wind jets better than any in situ or present- that date. About one third of the pro®les were made generation reanalysis wind product (Freilich and Vanhoff during September±November, but any potential bias in- 2001, unpublished manuscript; Chelton and Freilich 2001, troduced thereby in the mean is alleviated by taking the unpublished manuscript). mean of the average annual cycle, as described below. The Quikscat satellite carrying a SeaWinds scatter- Donoso et al. (1994) describe the objective and subjective ometer (referred to here as Quikscat) was launched in quality control procedures used to ¯ag questionable ob- June 1999 and its operational products became available servations and produce a research-quality dataset. Only in late July 1999. The satellite circles the earth in a 4- data points that they found to be acceptable were used day repeat, sun-synchronous orbit with a spacing of 6.38 in this study. at the equator, sampling a swath 1600 km wide centered The irregularly distributed pro®les were mapped to a on its ground track. Because of the complex way the regular 18 lat by 18 lon by 10 m by climatological-month north- and south-going swaths overlap over time, tem- grid in two steps. First, the temperature data in each in- poral sampling at any location becomes uneven, leading dividual pro®le were linearly interpolated to 10-m depth to an inhomogeneous pattern of sampling errors that intervals from the surface to 450 m (on average, each tend to be largest at latitudes between 208 and 308 pro®le had about 41 samples in the upper 450 m). Second, (Schlax et al. 2001). The swath stress components and in a separate calculation at each 10-m level z0, the irreg- scalar wind speed were gridded according to the method ularly spaced temperatures were mapped onto the (x, y, of Schlax et al. (2001), using a loess smoothing algo- z0, t) grid using a Gaussian-weighted three-dimensional rithm which is analogous to the ®ltering properties of mapping (Kessler and McCreary 1993), with scales of 28 a28 lon, 0.58 lat and 15-day block average. The average lon, 18 lat, and 2 months. The region of in¯uence of each of the ®rst year of these smoothed ®elds (15 August observation was limited to where its weighting was greater 1999±9 August 2000) was taken to represent the long- 24 than e , that is, to twice the scale distance from the center term mean. of each grid box. Only the month was considered in the Fortuitously, winds during this single year in this region gridding, not the year, to produce a climatological average were reasonably typical of the decade-long climatology, annual cycle.
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