Pronounced dipolar SST oscillation appears in this forced coupled model simulation (Xie and Tanimoto 1998). Most remarkably, the simulated ISG anomalis follow the observed curve, and so are interhemispheric zonal wind differences (Fig. 5). Note the model curves are based on data taken from the forcing-free tropical regions only and result from the dipole's response to extratropical forcing. This strongly suggests that the quasi-decadal ISG oscillation is not some fortuitous coincidence but owe its existence to the WES feedback. Xie and Tanimoto (1998) suggest that the NAO, with its southern SLP center of action extending into the tropical easterly wind regime, might be the forcing behind the quasi-decadal ISG oscillation.
5. High-latitude link between the Atlantic and Pacific
The Atlantic Ocean is about halfway on the other side of the globe. So why should we in Japan worry about the Atlantic? Here we suggest that there is a good reason for this.
The uppermost curves in Fig. 6 show the North Atlantic Oscillation index, which is the sea level pressure difference between Iceland and the Azores. The thin curve is the winter (Dec-Feb) average and the thick curve is the three-year running mean. We can see that the NAO started a very pronounced quasi-decadal oscillation in late 1960s. The curves below are winter-average surface air temperature at Wakkanai, the northernmost Japanese radiosonde station. We do not need any filter to see a quasi-decadal oscillation at Wakkanai, which is highly correlated with the NAO since late 1960s.This quasi-decadal oscillation does not stop in Japan. This is the sea level pressure in the Aleutian Low. There is quite strong interannual variability, but we can still see a decadal oscillation in phase with both the NAO and the Wakkanai temperature. The last curves are the oscillation Nakamura et al. (1997; also in this volume) Pacific subarctic front. It also fits well with the other curves. So now we seem to have an inter-oceanic decadal oscillation.
To get a global perspective, we computed the linear regression onto the band-passed North Atlantic Oscillation index by subtracting both 3-year and 11-year running means (Fig. 7). The SLP pattern is familiar, showing a north-south seesaw between Iceland and the Azores Islands. In the 850mb temperature field, we see the famous seesaw between Canada and northern Europe. To the south, there is another seesaw between the US and North Affrica. The northern European temperature pattern extends eastward to cover the Siberia, passes over northern Japan and extends all the way to the North Pacific. There is a major center of action in the SLP field over the North Pacific. The assocrated wind anornalies maintain SST anomalies over the North Pacific. With a unique Japanese Fisheries Agency dataset, Watanabe (1998, this volume) detected subsurface temperature variability in the western subarctic gyre that is highly correlated with the quasi-decadal oscillations in the North Atlantic and Wakkanai air temperature.
Near surface air temperature anomalies over continents associated with this quasi-decadal oscillation can be explained by the advection of mean temperature gradient by anomalous winds (Xie et al.1998). At 500mb, the oscillation is associated with the so-called zonal index cycle, a seesaw between subtropical and high-latitude westerly winds, much like the Arctic Oscillation (Thompson and Wallace 1998). Why should there be such a strong linkage between the North Atlantic and Pacific on the quasi-decadal time scale in high latitudes needs further investigations.
