Response of the North Pacific subtropical countercurrent and its variability to global warming

Lixiao Xu,Shang-Ping Xie,Fumiaki Kobashi,Qinyu Liu

doi:10.1007/s10872-011-0031-6

Abstract

Response of the North Pacific subtropical countercurrent (STCC) and its variability to global warming is examined in a state-of-the-art coupled model that is forced by increasing greenhouse gas concentrations. Compared with the present climate, the upper ocean is more stratified, and the mixed layer depth (MLD) shoals in warmer climate. The maximum change of winter MLD appears in the Kuroshio–Oyashio extension (KOE) region, where the mean MLD is the deepest in the North Pacific. This weakens the MLD front and reduces lateral induction. As a result of the reduced subduction rate and a decrease in sea surface density in KOE, mode waters form on lighter isopycnals with reduced thickness. Advected southward, the weakened mode waters decelerate the STCC. On decadal timescales, the dominant mode of sea surface height in the central subtropical gyre represents STCC variability. This STCC mode decays as CO2 concentrations double in the twenty-first century, owing both to weakened mode waters in the mean state and to reduced variability in mode waters. The reduced mode-water variability can be traced upstream to reduced variations in winter MLD front and subduction in the KOE region where mode water forms.

Highlights

The Subtropical Countercurrent (STCC) is a narrow eastward jet in the central North Pacific (20ο-30οN) where the Sverdrup theory predicts a broad westward flow (Uda and Hasunuma 1969; Yoshida and Kidokoro 1967)
We have examined the response of the mean STCC and its variability to global warming in the Geophysical Fluid Dynamics Laboratory (GFDL) coupled model CM2.1, which is forced by increased greenhouse gas concentrations
Such a change in the heat flux contributes to an ocean warming that is large near the surface and decreases with depth, causing the mixed layer depth (MLD) to shoal

Summary

Introduction

The Subtropical Countercurrent (STCC) is a narrow eastward jet in the central North Pacific (20ο-30οN) where the Sverdrup theory predicts a broad westward flow (Uda and Hasunuma 1969; Yoshida and Kidokoro 1967). Clear atmospheric response is seen in wind stress curls, with patterns suggestive of positive coupled feedback This natural mode of STCC variability is excited by global warming, resulting in banded structures in sea surface warming that slant in a northeast direction. In an analysis of a CM 2.1 future climate projection under the A1B scenario of atmosphere trace composition change, Xie et al (2010) noted banded structures in SST warming over both the subtropical North Pacific and Atlantic. These bands slant in a northeast direction, which Xie et al suggest is indicative of the changing mode waters.

Model and Simulations

The STCC Mean State Response to Global Warming

Response of STCC Variability to Global Warming

Findings

Summary