Semidiurnal Tides Observed in the Western Equatorial Pacific during TOGA COARE

Ming Feng{1,5}, Mark A. Merrifield{1},

Robert Pinkel{2}, Peter Hacker{1}, Albert J. Plueddemann{3}

Eric Firing{1}, Roger Lukas{1}, Charles Eriksen{4}

{1} School of Ocean and Earth Science and Technology, University of Hawaii
Honolulu, Hawaii, USA 96822
{2} Scripps Institution of Oceanography, University of California
San Diego, California, USA 92093-0213
{3} Physical Oceanography Department, Woods Hole Oceanographic Institution
Woods Hole, MA, USA 02543
{4} School of Oceanography, University of Washington
Seattle, Washington, USA 98195
{5} Institute of Oceanography, Chinese Academy of Sciences
Qingdao, Shandong, China, 266071

Abstract

In order to obtain a description of the predictable component of the baroclinic tide and to remove this variability, as well as the barotropic tide, from the mixed temporal-spatial shipboard observations, the semidiurnal tide within a 100 km square region of the western equatorial Pacific warm pool centered at 1.8S, 156.1E in the Intensive Flux Array (IFA) is examined using shipboard survey and mooring data collected during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) (Figure 1).

Baroclinic and barotropic tidal amplitudes and phases are estimated from the survey and mooring observations in the upper 300 m of the 1800 m deep ocean by specifying their horizontal structures, as well as their vertical structures.

The barotropic tide is assumed to have zero horizontal wavenumber over the domain, while a component of the baroclinic tide that is phase-locked to the barotropic tide is determined by a searching method using plane wave fits to both the isopycnal and velocity data (Figure 2).

The estimated barotropic tidal current, which is most energetic in the zonal direction for both the M2 and S2 constituents, is in good agreement with tide models derived from TOPEX/POSEIDON observations (Figure 3). The plane wave analysis for the baroclinic tidal component shows horizontal and vertical structure consistent with a dominant mode one baroclinic wave propagating towards the northeast (Figure 4).

The second vertical mode can also be detected. Given the phase differences between the M2 and S2 constituents in the barotropic and baroclinic tides, the source of the baroclinic tide is determined to be about 320 km southwest of the observing region, at a series of islands and shallow ridges northeast of New Ireland (Figure 1).

The combined estimates of the barotropic and baroclinic tides typically account for only 40-60% of the observed semidiurnal band current variance in the mooring data (Figure 5), indicating the high degree of temporal and spatial variability of the baroclinic tide in this region (Figure 6). The results of this study suggest, however, that coherent barotropic and baroclinic tidal signals can be successfully distinguished in the deep ocean using shipboard survey data, even when the data are limited to the upper 300 m.

Reference:

Feng, M., M. Merrifield, R. Pinkel, P. Hacker and A. Pluddemann, E. Firing, R. Lukas and C. Eriksen, Semidiurnal tides observed in the western equatorial Pacific during TOGA COARE. J. Geophys. Res., accepted. 1997.

Last modified: October 27, 1997