Two Distinct Interdecadal Modes of the Variability in the Pacific as Simulated in the MRI Coupled GCM
Seiji Yukimoto (Climate Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba 305-0052, Japan)
e-mail: yukimoto@mri-jma.go.jp
M. Endoh (Ocean Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima, Yokosuka 237-0061, Japan)
Y. Kitamura (Oceanographic Research Department, Meteorological Research Institute, Tsukuba 305-0052, Japan)
A. Kitoh (Climate Research Department, Meteorological Research Institute, Tsukuba 305-0052, Japan)
T. Motoi (Climate Research Departrnent, Meteorological Research Institute, Tsukuba 305-0052, Japan)
A. Noda (Climate Research Department, Meteorological Research Institute, Tsukuba 305-0052, Japan)
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 coupled 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. Anomaly of the upper thermocline temperature is formed in the eastern Pacific, and its northern subtropical signal propagates westward with getting enhanced at the central basin by a wind forcing. Arrival of the subtropical signal in the western Pacific around 20｡? switches anomaly of subsurface tmperature along the equatorial through anomalous heat transport. Travel time of the trans-Pacific singnal in the subtropics appears to determine the timescale of the first mode.The second mode is characterized by a midlatitude-subtropical dipole pattern of the upper ocean temperture, which rotates clockwise around the North Pacific subtropical gyre. The temperature anomary 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 to the associated wind forcing. Apparent negative feedback processes between atmosphere and ocean cannot be fuund in the second mode. It is suggested that the second mode is excited by timescale. It is proposed that the timescale of the second mode is selected by the ocean which has the interdecadal timecscale associated with the clockwise rotation of the temperature anomaly around the North Pacific subtropical gyre.
The Pacific Ocean has basin scale variablities on wide range of time scales on seasonal. ENSO, decadal to interdecadal and probably longer. What mechanisms can explain spatial and temporal structure of such variabilities ? There are major two hypothesis for decadal to interdecadal variability the Pacific. One is based on ‘Tropicai/Subtropical atmosphere-ocean osphere-ocean (A-O) feedbacks' in which variability is excited by tropical A-O interaction, and its timescale is determined by a travel time of midlatitude surface signal to be subducted and advected to tropical subsurface ocean (e.g. Gu and Philander. 1997). The other hypothesis is based on ‘midlatitude A-O feedbacks'. Interaction between sea surface temperature (SST) around the Kuroshio Extension and the Aleutian Low work as positive feedback and enhances anomalies each other.
