The Various Aspects of the Large- scale Atmospheric Circulation Response to the Northern Hemispheric Ocean Western Boundary Currents

Semi-idealized Atmospheric General Circulation-Model (AGCM) experiments are used, in order to study the different aspects of the hemisphere-scale wintertime troposphere/stratosphere-coupled circulation that are maintained by the North Atlantic and Pacific Ocean Western Boundary Currents (OWBCs). Here we show that the North Atlantic and Pacific OWBCs jointly maintain and shape the wintertime hemispheric circulation and its leading mode of variability Northern Annular Mode (NAM). The OWBCs energize baroclinic waves that reinforce quasi-annular hemispheric structure in the tropospheric eddy-driven jetstreams and NAM variability. Without the OWBCs, the wintertime NAM variability is much weaker and its impact on the continental and maritime surface climate is largely insignificant. Atmospheric energy redistribution caused by the OWBCs acts to damp the near-surface atmospheric baroclinicity and compensates the associated oceanic meridional energy transport in agreement with the Bjerknes compensation. Furthermore, the OWBCs substantially weaken the wintertime stratospheric polar vortex by enhancing the upward planetary wave propagation, and thereby affecting both stratospheric and tropospheric NAM-annularity. It is shown that the impact of OWBCs on northern hemisphere circulation has significant implication for stratosphere/troposphere dynamical coupling, time-scales on the NAM, frequency of Sudden stratospheric warming and potential formation of polar stratospheric clouds.

 

Reference:

Omrani et al., 2019: Key Role of the ocean Western Boundary currents in shaping the Northern Hemisphere climate, Scientific Reports, https://doi.org/10.1038/s41598-019-39392-y