日本財団 図書館


Mean circulation and variability in the global high-resolution GCM

― the Equatorial Currents System in the Pacific Ocean ―

 

Akio Ishida, Yuji Kashino (Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan)

e-mail: ishidaa@jamstec.go.jp,kashinoy@jamstec.go.jp

Humio Mitsudera (International Pacific Research Center, University of Hawaii, Honolulu, 96822, USA)

e-mail: humiom@soest.hawaii.edu

Teruaki Kadokura (Fuji Research Institute Corporation, Pier City Shibaura Bld. 3-18-1 Kaigan Minato-ku, Tokyo 108)

e-mail: kadokura@star.fuji-ric.co.jp

 

Abstract

 

Mean and variability of ocean circulation are investigated by using a high-resolution general circulation model (GCM). The model covers global domain except the Arctic Ocean with 1/4 degrees horizontal grid spacing and 55 vertical levels. The model has been spun-up by the annual averaged climatologies in the first 2 years, and then forced by monthly averaged for 18 years. The quasi-equilibrium state in the final year of the simulation shows many realistic features of ocean circulation including the separation and variability of western boundary currents especially of the Kuroshio, and the westward propagation of mesoscale eddies in the interior ocean. The model also well simulates the vertical structure of the equatorial currents system in the Paciflc Ocean. Especially the Subsurface Countercurrents (SCCs), which have not been well reproduced in previous high-resolution GCMs, are well simulated. The model SCCs have similar structures compared with the observed, i.e., (a) the SCCs are associated with the pycnostad between 26.0 and 26.8δθ, (b) deep and week in the west and shallow and strong in the east, (c) the densities in the SCCs' cores show decreases to the east. Reynolds stress analysis represents that eastward momentum is transferred from the equator to the latitude of the North SCC by eddy activity. This suggests the importance of eddy activity to the maintenance mechanism of the SCCs.

 

1. Introduction

 

There are the eastward currents in about 150m to 300m on either side of the equator between 3° and 6° latitudes in the equatorial Paciflc Ocean. These currents are called the Subsurface Countercurrents (SCCs) or the Tsuchiya jets (Tsuchiya; 1972, 1975). The SCCs are steady and extend across the entire Pacific Ocean and have potential vorticity fronts with a pycnostad between them, and the jets' cores shift poleward from west to east.

The SCCs are distinctly visible in the mean hydrographic section in the Hawaii-Tahiti Shuttle Experiment. Wyrtki and Kilonsky (1984) shows the geostrophic currents referred to 1000db computed from the annual averaged hydrography along 158°W, 153°W, and 150°W. Their plots shows the SCCs in 250m to 300m depth and at about ±4° from the equator, distinct from the Equatorial Undercurrent (EUC) and the North Equatorial Countercurrent (NECC). The mean velocity and tranport are 6.7cm s-1, 8.9 Sv (1 Sv = 106m3s-1) for the North SCC (NSCC) and 4.8cm s-1, 4.3 Sv for the South SCC (SSCC). They also report that there are multiple cores for the SSCC; the equator side jet flows along 4°S and the poleward side at 7°S.

 

 

 

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