The Role of Local and Regional Atmospheric Forcing for Convection in the Subpolar North Atlantic

Variabilities in upper ocean hydrography in the subpolar North Atlantic and Nordic Seas can have large implications for deep water formation, often seen as the downward branch of the Atlantic meridional overturning circulation. These variabilities encompass a wide range of spatial and temporal scales and their causes may vary between different years and basins. Specific causes on short time scales of the order of days include local air-sea heat fluxes during intense wind events and transient eddies. Freshwater fluxes associated with discharges from land and sea ice and vertical transports related to the regional wind stress curl can cause variations in stratification on time scales of the order of weeks to months. On even longer time scales, changes in ocean currents, subsurface water mass anomalies and large-scale climate variability may alter the upper ocean hydrography.

In this presentation, we focus on relatively short time scales of the order of days up to several weeks. First, we will characterize the atmospheric forcing on these time scales over the convection centers in the subpolar North Atlantic with a reanalysis product, remote sensing data and observations from nearby weather stations. Using observations from moored instruments in the subpolar North Atlantic, we will then trace the spectral characteristics of the local heat and freshwater fluxes, as well as of the regional momentum fluxes, in salinity and temperature variabilities in the upper ocean. Finally, we will explore the coherence among the different fluxes, relate them to specific forcing mechanisms, such as cyclones, polar lows and blocking anti-cyclones, and evaluate their contribution to convection.