Cold air outbreaks over the Gulf Stream and implications for the North Atlantic Ocean

During Cold Air Outbreak (CAO) events over North America during winter, cold dry air is often advected over the ocean offshore of the East Coast. A motivating hypothesis of this study is that the air-sea fluxes associated with CAO events will be manifested in offshore ocean variations that will be advected poleward by the Gulf Stream, possibly impacting the subpolar regions where oceanic deep convection drives the thermohaline circulation. This study uses an observational data analysis and model experiments with CESM’s Parallel Ocean Processor to examine CAO intensive winters alongside contrasting years of winter warm events to quantify impacts on the North Atlantic Ocean, including the subpolar seas. The main findings of this study do not confirm the original hypotheses about downstream (subpolar) oceanic impacts of cold air outbreaks. They do, however, provide information relevant to the variability of the subpolar North Atlantic. In particular, the observational analysis shows that CAOs over the Gulf Stream produce large upward sensible and latent heat fluxes from the ocean to the atmosphere. These fluxes remove heat from the ocean’s mixed layer, altering temperatures in the upper ocean. The model experiments show similar results off of the East Coast of the US, with colder, denser waters over the Gulf Stream in CAO years and associated variations of density patterns in the subpolar North Atlantic. Wind forcing from regions of warm air advection further north dominates downstream impacts of CAOs over the Gulf Stream, as the model experiments show regions of warmer, less dense water in the geographic region of North Atlantic Deep Water formation. This bipolarity of the wind forcing and its oceanic impacts are manifestations of the North Atlantic Oscillation. The results indicate that the NAO may well be the key to ocean variations that drive the global thermohaline circulation