Analytical prototypes for ocean-atmosphere interaction at midlatitudes. Part I: Coupled feedbacks as a sea surface temperature dependent stochastic process

Effects of ocean-atmosphere feedback processes and large-scale atmospheric stochastic forcing on the interdecadal climate variability in the North Atl antic and North Pacific Oceans are examined in a simple midlatitude ocean-a tmosphere model. In the ocean, the authors consider a linearized perturbati on system with quasigeostrophic shallow-water ocean dynamics and a sea surf ace temperature (SST) equation for a surface mixed layer. The atmosphere is represented as stochastic wind stress and heat flux forcing. This includes a noise component that depends on SST, as well as an additive component th at is independent of SST. Coupling is represented by the SST dependent stoc hastic process, in which SST influences the probability density function of the atmospheric noise both in shifting the mean and affecting the variance . It thus includes a multiplicative noise component. The model results in b oth oceans indicate that large-scale additive atmospheric stochastic forcin g alone (the uncoupled case) can give coherent spatial patterns in the ocea n and sometimes even a weak power spectral peak at interdecadal periods. Co upling due to the SST dependent stochastic process can produce a more disti nct power-spectral peak relative to the uncoupled ocean. Moreover, the time and spatial scales of the interdecadal mode are insensitive to the standar d deviation of the multiplicative noise. Thus a deterministic feedback limi t can be used to simplify the coupled model for further investigation of th e physical mechanisms of the interdecadal mode. In both uncoupled and coupled cases, the period of the interdecadal oscilla tion is determined by the zonal length scale of atmospheric wind stress and oceanic Rossby wave dynamics. The atmospheric spatial pattern sets the len gth scale of large-scale wave motion in the ocean. This wave propagates to the west due to oceanic Rossby wave dynamics and is dissipated at the weste rn boundary. However, in the coupled case, the SST anomalies generated by g eostrophic current can feed back to the atmosphere, which in turn brings so me information back to the east and reexcites oceanic waves there. Although the magnitude of the feedback of SST on the atmosphere is much smaller tha n atmospheric internal variability, its effects are significant.