On intergyre dynamic communication

Using a continuously stratified three-dimensional ocean model and the wave packet theory, we analytically investigate intergyre dynamical interactions in an idealized three-gyre ocean with both the horizontal and vertical shears between the tropical, the subtropical and subpolar regions. It is found that dynamically the intergyre interaction is determined by the meridional gradient of the mean potential vorticity. For a westward (eastward) wave packet, the interaction takes place down (up) the gradient. Between the subtropical and subpolar regions, the direction of the intergyre interaction is determined by direction toward which the wave packet is tilted. However, there is a critical longitude along the boundary between the subtropical and tropical regions. West of this longitude, a westward (eastward) tilted wave packet penetrates from the tropical (subtropical) gyre into the subtropical (tropical) gyre. East of this longitude, a westward (eastward) wave packet penetrates from the subtropical (tropical) gyre into the tropical (subtropical) gyre.

For long planetary wave packets, there is a critical vertical scale, called (Non-Doppler-Shift) NDS-Mode scale. For a long planetary wave packet with a vertical scale larger than the NDS-Mode scale, the substantial intergyre interaction takes place between the subtropical and subpolar regions with the westward tilted wave packet penetrating from the subpolar gyre into the subtropical gyre. For a long planetary wave packet with a vertical scale smaller than the NDS-Mode scale, the substantial intergyre interaction appears between the subtropical and tropical gyres with westward tilted wave packets penetrating from the subtropical gyre into the tropical gyre on decadal time scale. The former may be due to the zonally symmetric atmospheric wind forcing and ocean topographic forcing, accomplished by barotropic-like or low baroclinic mode perturbations whereas the latter may be due to the zonally asymmetric atmospheric wind forcing and surface heating or cooling, accomplished by relatively high baroclinic mode perturbations. Implications of these results in climate variability are discussed, in particular on interannual to decadal time scale in the North Pacific. It is suggested that long planetary wave packets may penetrate into the tropical ocean from the mid-latitude ocean interior, thus modulating the El Nino events on decadal time scale.