Heat Balance in the Northwest Atlantic Coastal Ocean: the Role of Atmospheric Forcing versus Ocean Advection during an Extreme Warming

In the coastal ocean of the Northwest Atlantic, the sea surface temperature (SST) in the first half of 2012 was the highest on the record for the past roughly 150 years of recorded observations. This resulted in major impacts on the marine ecosystem and commercial fisheries. The underlying dynamical processes responsible for this extreme event are examined using observations and a numerical model. Analyses based on both observations and realistic numerical modeling show that the warming event was primarily driven by the anomalous air-sea heat flux, while the smaller contribution by the ocean advection worked against this flux by acting to cool the shelf. The anomalous air-sea heat flux exhibited a shelf-wide coherence, consistent with the shelf-wide warming pattern, while the ocean advective heat flux was dominated by localized, relatively smaller scale processes. Further investigation of the linkages to basin-scale forcing suggests that the anomalous atmospheric jet stream position induced smaller heat loss from the ocean and caused a much slower cooling rate in late autumn and early winter of 2011–2012. Strong jet stream intraseasonal oscillations in the first half of 2012 systematically increased the warm anomalies over the continental shelf. On the other hand, the anomalous cooling due to ocean advection primarily resulted from the along-shelf heat flux divergence in the Gulf of Maine, while in the Middle Atlantic Bight the advective contribution from the along-shelf and cross-shelf heat flux divergences was comparable. The integrated analysis of observations and numerical modeling concludes that the changes in the large-scale atmospheric circulation in the winter of 2011–2012 primarily caused the extreme warm anomaly in the spring of 2012. The effect of along-shelf or cross-shelf ocean advection on the warm anomalies from either the Scotian Shelf or adjacent continental slope was secondary. Better understanding of this unprecedented event provides insights into the interannual variability of temperature in the Northwest Atlantic coastal ocean and linkages between the large-scale atmospheric/oceanic forcing and the coastal processes.