日本財団 図書館


Coupled Ocen-Atmosphere Modes in the Pacific and Atlantic

 

Shang-Ping Xie, Hideyuki Noguchi

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

E-mail: xie@ees.hokudai.ac.jp

Taroh Matsuno, Youichi Tanimoto

(Frontier Reseach System for Global Change, Tokyo 105-6791, Japan)

Shinji Matsumura

(National Research Institute for Earth Science and Disaster Prevention Tsukuba 305-0004, Japan)

 

Abstract.

 

How to characterize the tropical Atlantic sea surface temperature (SST) variability has been highly controversial. Now Tanimoto presents by far the strongest observational evidence in favor of a dipole-monopole decomposition (this volume). Here we provide the physical basis for such an empirical decomposition and address an important question of why such a latitudinally antisymmetric dipole mode is most pronounced in the Atlantic but apparently not in the Pacific.

For this purpose we develop a coupled ocean-atmosphere model that includes both the feedbacks by ocean dynamics and by surface heat flux. Consistent with observations, an interannual equatorial and a decadal dipole mode emerge from the model and are supported by the Bjerknes and a wind-evaporation-SST feedbacks, respectively. The equatorial and the dipole modes exhibit distinctive growth curves, with the former peaking at the size of the Pacific Ocean while the latter decreasing monotonously with wavenumber. It thus follows that the equatorial mode will dominate the Pacific whereas it will co-exist with a zonally uniform dipole mode, a theoretical outcome that explains observations.

Having emphasized the difference between the tropical oceans, we will present evidence for interaction between extratropical North Atlantic and North Pacific, an inter-oceanic link that is particularly pronounced on the quasi-decadal time scale.

 

1. Introduction

 

The tropical Pacific and Atlantic Oceans share many common climatological features: Easterly trade winds, eastward shoaling thermocline, eastern equatorial cold tongue and strong equatorial asymmetries in the ITCZ and SST are just a few such examples. These similarities in the two climatologies might make one think that the climatic variability in the two oceans is similar. As will be briefly reviewed here, comparison between the Pacific and Atlantic variability is better characterized by their differences than similaritles.

To facilitate the comparison, we define a broad-scale tropical Pacific SST index by averaging within 20S-20N and 180-100W (Tpac hereafter). Similarly defined is a tropical Atlantic SST index (Tatl): 50-20W, 0-20N and 30W-0 20-0S. Figures 1a and 1b shows the SST regressions onto Tpac, and Tatl, respectively, for the 40-year period of 1954-1993. The Tpac, regression has so strong an equatorial signal that one knows where the equator is even if the land-sea distribution and latitude marks are not provided. In contrast, the Tatl, regression has a nearly spatially uniform distribution with no partrcularly pronounced features to distinguish the equator.

 

 

 

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