Drivers of marine cold air outbreak intensity along the Gulf Stream and Kuroshio Current in a warmer climate

Marine Cold Air Outbreaks (MCAOs) along the western boundary currents trigger strong air-sea interactions in the entrance region of the storm tracks and act as an important moisture source for cyclones developing within the northern hemispheric storm tracks. Changes in MCAO intensity with climate change, however, are complex to evaluate because of the competing effects of the expected increase in air temperature (which would reduce MCAO intensity) and in sea surface temperatures (which would increase MCAO intensity). This study aims to achieve a detailed understanding of MCAO intensity changes in relation to these opposing effects, by comparing present and future MCAOs along the northern hemispheric western boundary currents as simulated by the Community Earth System Model 2 (CESM2) forced by the SSP3-7.0 radiative forcing scenario. Lagrangian, three-dimensional air parceltrajectories initialized from within the MCAOs are computed directly from the 6-hourly climate model output, allowing to gain insights into the processes responsible for changes in MCAO intensity.

We find that in the considered scenario the increase in air temperature outweighs the increase in SSTs, leading to weakening of future MCAOs along western boundary currents. Backward trajectories initiated from the MCAOs show that the increase in air temperature in the MCAOs results from substantially higher initial potential temperature and slightly weaker diabatic cooling experienced by the air parcels on their way towards the MCAOs. For future MCAOs along the Gulf Stream specifically, the permanently sea ice-free Hudson Bay additionally acts as a new warming source on the trajectories, prior to reaching the Gulf Stream region. Despite the decrease in intensity, future MCAOs are associated with increased net evaporation, suggesting that MCAOs are expected to remain an important contributor to the water cycle of the northern hemispheric storm tracks.