The Asymmetric Atmospheric Response to the Midlatitude North Pacific SST Anomalies

Forced by Pacific Decadal Oscillation-related sea surface temperature (SST) anomalies with the same pattern but opposite signs in the western-central North Pacific, nonlinear wintertime atmospheric responses are produced by a state-of-the-art atmospheric general circulation model (GFDL AM2.1); that is, an obvious equivalent barotropic geopotential low appears over the cold SST forcing ("CSST"), whereas a weak baroclinic structure shows up corresponding to the warm SST forcing ("WSST"), and both of them have similar characteristics in the lower troposphere. Specifically, because of the relatively dry environment in the central North Pacific, nonlinear responses of moisture process including latent heat flux and low-level atmosphere moisture advection induce asymmetric diabatic heating (Q(d)): in WSST, Q(d) tends to increase in the middle-lower troposphere but decrease in the middle-upper level, whereas it always increases in the whole troposphere in CSST. Thus, Q(d) has the same low-level positive vertical gradient in both CSST and WSST, which produces similar atmospheric circulation anomalies in the lower troposphere. In turn, the asymmetric responses of low-level temperature advection further modify air temperature meridional gradient as well as atmospheric baroclinicity in the lower troposphere, significantly shifting the transient eddy activities southward in CSST and greatly weakening their intensity in WSST, respectively. Accordingly, the transient eddy vorticity forcing primarily determines the upper-level atmospheric responses in CSST, but it has unsystematic effects in WSST that are overtaken by Q(d). Therefore, the dominance of diabatic heating in WSST and transient eddy forcing in CSST over the central North Pacific lead to the asymmetric atmospheric responses among which the asymmetry of moisture plays an essential role.