The role of atmospheric forcing versus ocean advection during the extreme warming of the Northeast U.S. continental shelf in 2012

Abstract

In the coastal ocean off the Northeast U.S., 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. The underlying dynamical processes responsible for this extreme event are examined using a numerical model, and the relative contributions of air-sea heat flux versus lateral ocean advective heat flux are quantified. The model accurately reproduces the observed vertical structure and the spatiotemporal characteristics of the thermohaline condition of the Gulf of Maine and the Middle Atlantic Bight waters during the anomalous warming period. Analysis of the model results 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 shelfwide warming pattern, while the ocean advective heat flux was dominated by localized, relatively smallerscale processes. The anomalous cooling due to 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 modeling results confirm the conclusion of the recent analysis of in situ data by Chen et al. (2014a) 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.

Seasonal trends and phenology shifts in sea surface temperature on the North American northeastern continental shelf

“…The extent to which this weakening is due to heat flux, water column mixed layer depth changes and/or advective forcing processes acting during fall or due to the increased water column heat left over from the strongly warming summer is best addressed with future model studies. Chen et al (2015Chen et al ( , 2014 suggest that, at least for the extreme warming event of 2012, changes in atmospherically-driven heat flux that began the previous fall and extended into winter and spring drove warm anomalies the following spring-summer. An important factor that could influence a seasonal bias in SST trends is mixed layer depth.…”

“…Galbraith et al (2012) show that the trends in SST in spring-summer-fall over the Gulf of St. Lawrence, immediately to the north of our study area, have a strong correlation to changes in local surface air temperature. Recent work by Chen et al (2014Chen et al ( , 2015 shows that the extreme 2012 SST anomalies over the entire study area were not phase-lagged, and their model simulations suggest that the synchronous anomalies were driven by anomalous heat flux linked to changes in atmospheric patterns that began the preceding fall. They show that fall-winter advective processes work against these heat flux anomalies to cool the water column, but that air-sea heat flux associated with shifts in jet stream position in both fall and winter-spring overwhelmed the advective processes and resulted in an anomalously warmer water column leading to the strong 2012 SST anomalies.…”

“…However, changes in the proximity of the Gulf Stream to the shelf also impact shelf temperatures, at least in the MAB region (e.g. Gawarkiewicz et al, 2012;Zhang and Gawarkiewicz, 2015) and Gulf of Maine (Pershing et al, 2015), and interannual variability in equatorward flow along the Scotian Shelf appears to be modulated by local wind forcing (Li et al, 2014). Galbraith et al (2012) show that the trends in SST in spring-summer-fall over the Gulf of St. Lawrence, immediately to the north of our study area, have a strong correlation to changes in local surface air temperature.…”

Frequency of marine heatwaves in the North Atlantic and North Pacific since 1950

“…These events have been referred to as marine heatwaves and have been described as regions of large-scale and persistent positive sea surface temperature (SST) anomalies [Pearce et al, 2011]. Well-known marine heatwaves have occurred in the Mediterranean Sea [Black et al, 2004;Olita et al, 2007], off Western Australia [Pearce and Feng, 2013], in the northwest Atlantic [Mills et al, 2013;Chen et al, 2014Chen et al, , 2015, and in the northeast Pacific [Bond et al, 2015;Hartmann, 2015]. Like heatwaves on land, marine heatwaves are likely to become more frequent and intense under continued anthropogenic warming assuming fixed temperature thresholds [Solomon et al, 2007].…”

Projected sea surface temperatures over the 21st century: Changes in the mean, variability and extremes for large marine ecosystem regions of Northern Oceans

“…Extreme conditions, however, also occur in the world oceans (Hobday et al, 2016) and there is a growing appreciation that extremes strongly influence population dynamics and biogeography of many organisms (Portner et al, 2001;Lynch et al, 2014). Recent studies have explored periods with very warm SSTs or "ocean heat waves" in the northwest Atlantic (Mills et al, 2013;Chen et al, 2014Chen et al, , 2015, Northern Hemisphere oceans (Scannell et al, 2016), Mediterranean Sea (Black et al, 2004;Olita et al, 2007), off the coast of western Australia (Pearce and Feng, 2013;Wernberg et al, 2013), and over global coastal regions (Lima and Wethey, 2012). Significant negative effects on living marine resources and marine ecosystems were observed during some of these extreme periods (Mills et al, 2013;Wernberg et al, 2013;Pershing et al, 2015;Caputi et al, 2016).…”