Effect of Oceanic Stochastic Forcing on Wintertime Atmospheric Decadal Variability Over Midlatitude North Pacific

Previous studies showed that random atmospheric forcing can stimulate oceanic low frequency variability in midlatitudes, since ocean has huge heat capacity that can redden the atmospheric high frequency variability. However, whether the oceanic stochastic forcing can in turn influence atmospheric variability is still unclear. To answer this question, two parallel 63-year numerical experiments are conducted with an atmospheric general circulation model. In the control experiment, the atmosphere is forced by observed global sea surface temperature (SST). After adding stochastic forcings that are constrained by observed standard deviations of mesoscale SST anomalies (SSTAs) over the North Pacific in a sensitivity experiment, large-scale atmospheric variability is greatly increased over the midlatitude northwest Pacific, especially on decadal timescale. Moreover, the anomalous atmospheric circulations associated with the Pacific Decadal Oscillation (PDO), like the enhanced westerly wind in the Kuroshio-Oyashio and its extension (KOE) region, are more consistent with that resolved by a reanalysis model after the stochastic SSTAs are added. Dynamic diagnostics show that under the background of PDO warm phase, stochastic SSTAs can generate larger diabatic heating over the north of KOE region by inducing more moisture condensation in the mid-lower troposphere, which is balanced by anomalous cold low-level air temperature advection. Thus, the atmospheric baroclinicity is increased by the cold temperature advection anomaly, and the transient eddy activity and westerly jet are both enhanced above the KOE region accordingly. Our results suggest that to accurately represent the oceanic effect on large-scale atmospheric variability and anomalies, oceanic stochastic SSTAs should also be considered. Plain Language Summary Midlatitude air-sea interaction is essential for setting climate variability at interannual to decadal timescales, and both atmosphere and ocean have strong small-scale high-frequency disturbances that are usually called "weather." While it is known that the atmospheric weather can be transferred to oceanic low-frequency variability by ocean huge heat capacity, the oceanic weather manifested as mesoscale ocean eddies can influence larger atmospheric variability at longer time scales through its induced systematic atmospheric transient eddy activities, which is the new finding in this study. To verify such idea, stochastic SST forcing is used to represent the statistical effect of mesoscale SST anomalies on the atmosphere, and comparison of two long-term experiments forced by global observed SST with and without the additional stochastic fluctuations in a coarse resolution model confirm that, the oceanic stochastic forcing can induce stronger atmospheric decadal variability over the midlatitude northwest Pacific, and it is conductive to better simulate the Pacific Decadal Oscillation-related atmospheric circulations as well as their associated climate variabilities over Eurasian continent. Our results suggest that oceanic stochastic forcing can mimic the influence of oceanic eddies in SST in coarse resolution models and compensate the impact of large-scale SST anomalies in producing more realistic atmospheric responses.