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Two distinct Interdecadal modes of the variabllity in the Pacific as simulated in the MRI coupled GCM

 

Seiji Yukimoto, Masahiro Endoh*, Yoshiteru Kitamura, Akio Kitoh, Tatsuo Motoi and Akira Noda

Meteorological Research Institute

* Present affiliation : Japan Marine Science and Technology Center

 

Two distinct interdecadal modes of the coupled atmosphere-upper ocean temperature variability are found in the Pacific Ocean in a 150-year-run of a coup led atmosphere-ocean general circulation model (GCM). The modes have spatio-temporal structures coherent to the observed SST variability. The first mode which accompanies an ENSO-like spatial pattern for the SST and the surface wind behaves like a delayed oscillator in ENSO. Westward propagating trans-Pacific signal in the upper thermocline temperature in the extratropics appears to determine the delay time for the first mode. The second mode is characterized by a midlatitude-subtropical dipole pattern of the upper ocean temperature, which rotates clockwise around the North Pacific subtropical gyre. The temperature anomaly at the sea surface, which is reinforced by the atmospheric forcing around the Kuroshio Extension region, is subducted into subtropical subsurface layer along isopycnal surface, and is reinforced again in the southern branch of the North Pacific subtropical gyre in response tothe associated wind forcing. Apparent negative feedback processes between atmosphere and ocean cannot be found in the second mode. It is suggested that the second mode is excited by the internal variability of the atmosphere which has no characteristic interdecadal timescale. It is proposed that the timescale of the second mode is selected by the ocean which has the interdecadal timescale associated with the clockwise rotation of the temperature anomaly around the North Pacific subtropical gyre.

 

 

 

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