Equatorial Pacific Subsurface Countercurrents: A Model–Data Comparison in Stream Coordinates*

Kathleen A Donohue,Akio Ishida,G Dail Rowe,Eric Firing,Humio Mitsudera

doi:10.1175/1520-0485(2002)032<1252:epscam>2.0.co;2

Abstract

Abstract An isopycnal stream-coordinate analysis of velocity, transport, and potential vorticity (PV), recently applied to observations of the subsurface countercurrents (SCCs) in the equatorial Pacific Ocean, is applied here to the SCCs in a numerical general ocean circulation model, run by the Japan Marine Science and Technology Center (JAMSTEC). Each observed SCC core separates regions of nearly uniform potential vorticity: low on the equatorward side, high on the poleward side. Similar low-PV pools are found in the model, but the high-PV region poleward of the southern SCC is missing. The potential vorticity gradient in each core is weaker in the model than in observations, and relative vorticity plays only a minor role in the model. Its unusually high vertical resolution, with 55 levels, together with its weak lateral dissipation may be key factors in the JAMSTEC model's ability to simulate SCCs.

Highlights

The Pacific equatorial subsurface countercurrents (SCCs) are narrow eastward jets found a few degrees on either side of the equator, below the sharp equatorial thermocline (Fig. 1) (Tsuchiya 1975)
The ultimate purpose of examining the SCCs in a numerical model and comparing them to observations is to learn something about the SCCs in the ocean; we would like to understand where and how they are forced and dissipated, and how they fit into the general circulation
We cannot provide definitive critiques of these theories, we can point out some strengths and weaknesses that are indicated by observations and by the Japan Marine Science and Technology Center (JAMSTEC) model

Summary

Introduction

The Pacific equatorial subsurface countercurrents (SCCs) are narrow eastward jets found a few degrees on either side of the equator, below the sharp equatorial thermocline (Fig. 1) (Tsuchiya 1975). Johnson and Moore (1997) and Johnson and McPhaden (1999) pro-. Johnson and Moore (1997) modeled them as inertial jets; the observed downstream divergence of the jet cores from the equator is a consequence of the mean tilt of the equatorial thermocline, upward to the east, that forms their upper boundary. Marin et al (2000) suggest that the SCCs are the ocean analog of the atmospheric Hadley cells and associated jet streams Their two-dimensional model produces subpycnocline eastward jets separated by an equatorial region of homogenized low potential vorticity. The purpose of this note is to contribute to the JAMSTEC model analysis with a comparison to observations, using the Rowe et al (2000) methodology

Model and methods

Findings

Discussion