latitude cyclonic anomalies may be closely related to negative SST anomalies in the upstream regions(Palmer and Sun 1985), the projection is calculated over 50-80｡? and 25-45｡? for the North Atlantic and over 170｡?-150｡? and 25-45｡? for the North Pacific (framed areas in Fig. 1). When the p-regions are slightly shifted, the results are not largely affected.
The coupled model is integrated for 150 years with a time step of 2 days, and the result for last 100 years is analyzed. The horizontal resolution is 2° by 2° and domains of integration are the same as in Fig. 1. Although our coupled model is simplified, it is expected that the physics included in the model, especially the oceanic advection and atmosphere-ocean interaction, are crucial to simulate decadal variability in the extratropics.
RESULTS
a. Coupled decadal oscillations
We first describe results of the numerical integration with standard parameters, i.e. γc = γt = γh = 1.0. Continuous oscillations with the periods of about 8.8 and 22.6 yrs occur in the North Atlantic and the North Paciflc, respectively. These periods are roughly consistent with the observed counterparts. The temporal evolutions of the decadal oscillations with standard parameters are represented by the lag regressions of temperature anomalies in the p-region with temperature and stream function anomalies at each point (Figs. 2 and 3). Because of different period in two basins, the lag regression maps are presented for 0-3 years in the North Atlantic while for 0-9 years in the North Pacific. In both oceans, the anomalous stream function is propagative to the west due to the Rossby waveand moreover, the temperature anomalies also have a non-stationary component affected by the ocean dynamic fields. In the maximum phase of temperature anomalies, the spatial patterns are similar to Q* and hence T* except for local differences, indicating the excitation of the temperature anomaly pattern by the heat flux anomalies.
For the North Atlantic (Fig. 2), two maxima of positive and negative temperature anomalies are found along the Gulf Stream for the lag 0 year. Positive one located near the east coast of the US arises from the anomalous meridional heat transport associated with the current anomaly as can be inferred from the contours of stream function anomalies. Another center located off Newfoundland is caused from the advection by the eastward current anomal. While the ocean flow field continuously propagate westward, the phase reversal of the temperature anomaly pattern is relatively rapid during the lag 2-3 years. The warm temperature core in the p-region decays westward, while the rest positive anomalies move toward the east along the Gulf Stream. The cold anomalies penetrate to the p-region from the south due to the advection by the mean currents. This evolution is similar to the observations (Watanabe et al. 1998). While the local temperature maxima are mainly due to the current anomaly as note the heat flux anomalies excite the basin-scale temperature anomaly pattern during the mature phase.
A temperature regression pattern at the lag 0 in the North Pacific (Fig. 3) partly resembles the observed pattern of surface temperature anomalies associated with the decadal shift in 1976/77 (Nakamura et al. 1997). Namely, the warm anomalies in the central basin have two centers at the lag 0; the western center around 160°E and 40°N arises from the current anomaly which is confirmed by the dense ψg contours, while the eastern center around 170°W and 30°N is generated by the net heat flux anomalies.During the lag 3-6 years, positive stream functron anomalies in the midlatitude induce the intensified southward heat transport in the central basin between centers of temperature anomalies. At the Same time, the mean current (Kuroshio) advects cooler tmperature from the western subtropics to the central basin. The combination of those geostrophic advections results in cold anomalies and thus consequently the opposite phase of oscillation.
