The Atmospheric Bridge Between the Atlantic Subpolar And Tropical Regions: A Perplexing Nonlinear Teleconnection

Abstract:

The cause of and mechanisms governing the Atlantic multidecadal variability (AMV) have been a subject of intense research and debate in the recent decade. This low-frequency phenomenon is distinguished by its time scale and spatial extent. The long time-scale indicates an active ocean participation while the basin-wide spatial pattern is most likely due to the atmosphere. The subpolar gyre is where ocean heat storage and dynamics can display long-term anomalies. We investigated the role of the atmosphere in bridging between the subpolar region and the rest of the Basin by using an atmospheric model coupled to a slab ocean. The slab maintains a climatological annual cycle of SST through the addition of a climatological “flux correction”. In this configuration we perform a long control run and two separate experiments in which the climatological flux is modified to force a colder and warmer than normal subpolar gyre, alternatively. With these two anomalies the model run is started in a September state and then integrated for a full calendar year. Each of these switch-on, anomaly experiments is repeated 60 times to create two 60-member ensembles in which the evolving SST and atmospheric response across the Basin is examined. The results display a perplexing non-linearity, in which in the cold subpolar gyre runs a robust process is initiated immediately, leading to a cooling of SST that spreads southward into the tropical North Atlantic. In the positive subpolar run, SST outside the forced region remains largely unchanged. These contrasting responses are associated with the way the atmosphere balances the change in the surface heat flux over the subpolar region. When SST is cooled there, the atmosphere forms a high-pressure-anomaly over the northern basin, which transports heat from the south toward the subpolar region. This surface high results in the cooling of the SST in the subtropics. within a few months, the effect extends into the tropics through the wind-evaporation-SST mechanism. In the warm subpolar experiment, the atmosphere balances the surface flux by driving cold air from the Arctic toward the subpolar region and thus there is no effect to the south of the warm region. We show that this nonlinearity is also present in fully coupled models.