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Equatorward spreading of a passive tracer with application to North Pacific interdecadal temperature variations

 

Masami Nonaka (Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan)

nona@lowtem.hokudai.ac.jp

Shang-Ping Xie (Graduate School of Environmental Earth Science. Hokkaido University, Sapporo 060-0810, Japan)

xie@ees.hokudai.ac,jp

Kensuke Takeuchi (Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan)

takeuchi@lowtem.hokudai.ac.jp

 

ABSTRACT

 

A simulation is conducted with a realistic ocean general circulation model to investigate the three dimensional spreading of a passive tracer prescribed at the sea surface with the same distribution as the interdecadal sea surface temperature (SST) anomalies observed in the North Pacific. The tracers reaching the equator have the same sign as the major oval-shaped SST anomaly Pattern in the North Pacific but with a much reduced magnitude less than 10% of the mid-latitude SST anomaly. The mixing with the water containing SST anomalies of an opposite sign off the west coast of North America, and with the Southern Hemisphere thermocline water both contribute to the reduced equatorial amplitude. On the way to the equator in the southwestern part of the subtropical gyre, the subducted water is replenished by tracers leaking from the recirculation region to the north. The simulated passive tracer field in the subsurface layers agrees with the observed interdecadal temperature anomalies, suggesting the relevance of the processes shown here to the thermocline variability in the real North Pacific.

 

1. INTRODUCTION

 

Decadal/interdecadal variations in the North Pacific sea surface temperature (SST) and global atmosphere have been revealed by many studies (Nitta and Yamada 1989; Tanimoto et al. 1993: Trenberth and Hurrell 1994; Nakamura et al. 1997, among others). Recently, Gu and Philander (1997) and K. Hanawa (1995, personal communication) proposed a “delayed action oscillation” theory for these variations, to which the exchange of the sea, water between the North Pacific and the equatorial region is a key process. In this theory, the water subducted from the North Pacific with anomalous temperatures is transported to the equatorial region on the subsurface isopycnals and upwells to the sea surface (McCreary and Lu 1994; Liu 1994). The resultant equatorial SST anomalies induce anomalous atmospheric circulation that changes the sign of SST anomalies over the North Pacific. The time scale of this subtropical-Tropical water exchange decides the time scale of this “delayed action oscillation.” However, the arrival of the North Pacific temperature anomalies at the equator via ventilation has not been demonstrated.

 

 

 

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