The Role Of Synoptic Eddy Forcing In Pacific Decadal Variability

As we know, there is significant decadal climate variability in middle latitudes. The two dominant decadal signal in the north Pacific, the Pacific Decadal Oscillation (short for PDO) and the North Pacific Gyre Oscillation (NPGO) is associated with the Aleutian Low mode (AL) and the North Pacific Oscillation (NPO) in the atmosphere. It demonstrates that the generation and persisting of pacific decadal variability is due to ocean-atmosphere interaction. It is the two method how the surface sea temperature (SST) anomaly makes an impact on the atmosphere that the SST anomaly lead to anomaly diabatic heating (including sensible heating and latent heating) to the atmosphere and that the SST anomaly influence the synoptic eddy with its leverage of the positon or strength of the oceanic front zone. The later may be more important.In this paper we have diagnosed the eddy flow interaction associated with PDO and NPGO with the NCEP reanalysis dataset of atmosphere in the Northern Hemisphere cold season (Dec, Jan and Feb) during the period from 1948 to 2010. It is discussed what the role the synoptic eddy play in the PDO and NPGO signal. A simple idealized atmosphere-ocean coupled model is applied to demonstrate the detail process occurring the atmosphere-ocean interaction. Finally, we get some conclusions as followThere are two oceanic fronts in the Northern Pacific, namely the subpolar front (SPF) and the subtropical front (STF). The SST is cold anomaly in the north Pacific basin in PDO warm phase. It means the meridional SST gradient is a dipole pattern, so the SPF is equator-ward displaced meanwhile the STF is enhanced. As a consequence the Pacific storm track is enhanced and equator-ward displaced, the jet is strengthened and the Aleutian Low is enhanced and southeast displaced.It is a dipole pattern that the SST is cold anomaly northern 40°N and warm anomaly southern it in the NPGO warm phase. So the SPF is strengthened while the STF is pole-ward displaced. The storm track is enhanced locally, so is the Aleutian Low and the subtropical high. The jet is northern displaced.The response of atmosphere pressure to the SST anomaly is equivalent barotropic low pressure on cold SST and high on warm SST in both PDO and NPGO signal. And the impact on temperature field is equivalent baroclinic.The solution of the ideal model is the oceanic Rossby wave forced by the ocean-atmosphere interaction. The most instable mode is with the wave length of about 10000km and period of decadal scale, so is the PDO and NPGO.The dominant process in the adjustment of SST is the meridional advection due to the oceanic current and climatology of the SST gradient. The advection makes the oceanic Rossby wave over all the spectrum, however the entrainment destabilize the long oceanic Rossby wave and spin down the short wave.