U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Direct measurements of upper ocean currents and water properties across the tropical Pacific during the 1990's

Gregory C. Johnson, Bernadette M. Sloyan1, William S. Kessler, Kristene E. McTaggart

NOAA/Pacific Marine Environmental Laboratory, Seattle, WA 98115-6349, USA
1Present address: Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1050, USA

Prog. Oceanogr. , 52 (1), 31-36, 2002.
Copyright 2002 Elsevier Science Ltd. Further electronic distribution is not allowed.

1. Introduction

There have been many studies of the tropical Pacific Ocean circulation, but only a few have provided sufficient data to describe the mean temperature, salinity, and current field and estimate current transports from contemporaneous observations. The Hawaii-to-Tahiti Shuttle experiment provided a picture of the mean geostrophic currents and water masses in the central tropical Pacific (Wyrtki & Kilonsky, 1984) that is still a standard. This study used Conductivity–Temperature–Depth (CTD) and bottle data taken along 43 meridional sections from 15 cruises between April 1979 and August 1980. The shipboard Acoustic Doppler Current Profiler (ADCP) data from these cruises, which were only available over a very limited depth range and with low-quality navigational data by modern standards, have also been thoroughly exploited to examine energy and momentum balances (Johnson & Luther, 1994). In the western Pacific, CTD and ADCP or profiler data (when available) from several observational programs between 1984 and 1991 (27 sections at 165°E and nine at 142°E) were used in a dynamical interpretation of the local mean circulation (Gouriou & Toole, 1993). Earlier, 21 of these sections had been used to look at the 1986–1987 El Niño and the subsequent La Niña (Delcroix, Eldin, Radenac, Toole, & Firing, 1992). In the central and eastern Pacific, meridional CTD/ADCP sections have been used primarily to diagnose horizontal divergence and infer upwelling, with some discussion of the mean zonal currents (Johnson, McPhaden, & Firing, 2001). Variations in the salinity maximum and zonal advection in the southern branch of the South Equa­torial Current (SEC) have been analyzed using CTD data (Kessler, 1999). The termination of the Equatorial Under-Current (EUC) has also been examined with historical hydrographic data in the eastern Pacific (Lukas, 1986). Finally, the structure of the Subsurface Counter Currents (SCCs) has also recently been examined across the Pacific with meridional CTD/ADCP sections (Rowe, Firing, & Johnson, 2000).

Systematic studies of seasonal and interannual variability of tropical Pacific Ocean current transports have been mostly limited to expendable bathythermograph (XBT) data (Donguy & Meyers, 1996; Kessler & Taft, 1987; Taft & Kessler, 1991; Picaut & Tournier, 1991). These studies relied on regional potential temperature–salinity (S) relations and the geostrophic balance. This reliance means that they have usually been limited to assuming a zero-velocity surface at 400 m. The wind-driven component of the zonal velocity and other ageostrophic terms have been neglected so, although they can be significant near the equator (Bryden & Brady, 1985; Joyce, Lukas, & Firing, 1988). In addition, application of geostrophy on the equator is not a trivial exercise. The small meridional density gradients associated with geostrophic zonal velocity on the equator are easily aliased by internal waves and other transients (Hayes, 1982; Lukas & Firing, 1985; Moum, Chereskin, Park, & Regier, 1987; Picaut, Hayes, & McPhaden, 1989; Picaut & Tournier, 1991; Cornuelle, Morris, & Roemmich, 1993; Lagerloef, Mitchum, Lukas, & Niiler, 1999). Thus careful temporal and spatial smoothing is required. Often studies based on XBTs omit current transport estimates near the equator altogether.

In this study upper ocean currents and water-masses are analyzed at 10 longitudes from 143°E to 95°W, spanning most of the zonal extent of the equatorial Pacific. Zonal velocity, potential temperature, and salinity data are presented. The mean fields, as well as their seasonal cycle and linear correlation with the Southern Oscillation Index (SOI, which refers hereafter to the 5-month running mean of normalized monthly values) are estimated. This work uses 172 synoptic meridional CTD/ADCP sections collected in the tropical Pacific. These data were taken mostly in the 1990s, the majority during the maintenance of the Tropical Ocean-Atmosphere (TAO) and Triangle Trans-Ocean Buoy Network (TRITON) moorings (Hayes, Mangum, Picaut, Sumi, & Takeuchi, 1991; McPhaden et al., 1998).

The paper is structured as follows. The data and their processing are discussed in Section 2. In Section 3 the zonal currents, temperature, and salinity are described. First the means are discussed at all longitudes and along the equator, then the seasonal cycle and the response to the SOI are discussed at western, central, and eastern longitudes as well as along the equator. In Section 4, aspects of the major near-equatorial zonal currents are presented quantitatively through analyses of current transports, positions, and water properties estimated from the synoptic meridional sections. Section 5 concludes the paper.


Return to Abstract or go to next section

PMEL Outstanding Papers

PMEL Publications Search

PMEL Homepage