Alan Condron

Meltwater routing and the Younger Dryas

The Younger Dryas—the last major cold episode on Earth—is generally considered to have been triggered by a meltwater flood into the North Atlantic. The prevailing hypothesis, proposed by Broecker et al. [1989 Nature 341:318–321] more than two decades ago, suggests that an abrupt rerouting of Lake Agassiz overflow through the Great Lakes and St. Lawrence Valley inhibited deep water formation in the subpolar North Atlantic and weakened the strength of the Atlantic Meridional Overturning Circulation (AMOC). More recently, Tarasov and Peltier [2005 Nature 435:662–665] showed that meltwater could have discharged into the Arctic Ocean via the Mackenzie Valley ∼4,000 km northwest of the St. Lawrence outlet. Here we use a sophisticated, high-resolution, ocean sea-ice model to study the delivery of meltwater from the two drainage outlets to the deep water formation regions in the North Atlantic. Unlike the hypothesis of Broecker et al., freshwater from the St. Lawrence Valley advects into the subtropical gyre ∼3,000 km south of the North Atlantic deep water formation regions and weakens the AMOC by <15%. In contrast, narrow coastal boundary currents efficiently deliver meltwater from the Mackenzie Valley to the deep water formation regions of the subpolar North Atlantic and weaken the AMOC by >30%. We conclude that meltwater discharge from the Arctic, rather than the St. Lawrence Valley, was more likely to have triggered the Younger Dryas cooling.

Polar Mesoscale Cyclones in the Northeast Atlantic: Comparing Climatologies from ERA-40 and Satellite Imagery

Abstract Polar mesoscale cyclones over the subarctic are thought to be an important component of the coupled atmosphere–ocean climate system. However, the relatively small scale of these features presents some concern as to their representation in the meteorological reanalysis datasets that are commonly used to drive ocean models. Here polar mesocyclones are detected in the 40-Year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis dataset (ERA-40) in mean sea level pressure and 500-hPa geopotential height, using an automated cyclone detection algorithm. The results are compared to polar mesocyclones detected in satellite imagery over the northeast Atlantic, for the period October 1993–September 1995. Similar trends in monthly cyclone numbers and a similar spatial distribution are found. However, there is a bias in the size of cyclones detected in the reanalysis. Up to 80% of cyclones larger than 500 km are detected in MSL pressure, but this hit rate decreases, approximately linearly, ...

The impact of polar mesoscale storms on northeast Atlantic Ocean circulation

Every year, thousands of mesoscale storms (termed polar lows) cross the climatically sensitive subpolar North Atlantic Ocean. High-resolution numerical simulations of the ocean circulation, taking into account the effect of these storms on deep-water formation, suggest that polar lows significantly affect the global ocean circulation. Atmospheric processes regulate the formation of deep water in the subpolar North Atlantic Ocean and hence influence the large-scale ocean circulation1. Every year thousands of mesoscale storms, termed polar lows, cross this climatically sensitive region of the ocean. These storms are often either too small or too short-lived to be captured in meteorological reanalyses or numerical models2,3,4. Here we present simulations with a global, eddy-permitting ocean/sea-ice circulation model, run with and without a parameterization of polar lows. The parameterization reproduces the high wind speeds and heat fluxes observed in polar lows as well as their integrated effects, and leads to increases in the simulated depth, frequency and area of deep convection in the Nordic seas, which in turn leads to a larger northward transport of heat into the region, and southward transport of deep water through Denmark Strait. We conclude that polar lows are important for the large-scale ocean circulation and should be accounted for in short-term climate predictions. Recent studies3,4 predict a decrease in the number of polar lows over the northeast Atlantic in the twenty-first century that would imply a reduction in deep convection and a potential weakening of the Atlantic meridional overturning circulation.

Simulated response of the Arctic freshwater budget to extreme NAO wind forcing.

Abstract The authors investigate the response of the Arctic Ocean freshwater budget to changes in the North Atlantic Oscillation (NAO) using a regional-ocean configuration of the Massachusetts Institute of Technology GCM (MITgcm) and carry out several different 10-yr and 30-yr integrations. At 1/6° (∼18 km) resolution the model resolves the major Arctic transport pathways, including Bering Strait and the Canadian Archipelago. Two main calculations are performed by repeating the wind fields of two contrasting NAO years in each run for the extreme negative and positive NAO phases of 1969 and 1989, respectively. These calculations are compared both with a control run and the compiled observationally based freshwater budget estimate of Serreze et al. The results show a clear response in the Arctic freshwater budget to NAO forcing, that is, repeat NAO negative wind forcing results in virtually all freshwater being retained in the Arctic, with the bulk of the freshwater content being pooled in the Beaufort gyre...

Multidecadal North Atlantic climate variability and its effect on North American salmon abundance

[1] Climate variability is now known to play a key role in the abundance of marine fisheries, and must be accounted for to implement sustainable management strategies. We show that North American Atlantic salmon abundance has fluctuated in parallel with the Atlantic Multidecadal Oscillation (AMO); a basin-wide, low frequency climate mode producing cold-warm-cold sea surface temperatures over the last century. During the AMO warm (cool) phase salmon abundance is lower (higher). Changes in sea surface temperature associated with the AMO are most pronounced in the winter season near the Grand Banks of Newfoundland, a known overwintering area for salmon and an important time for determining survival. A moratorium on salmon fishing was established in 1992, but has so far contributed few signs of improvement in stock size. This may be explained by a shift in the AMO to a positive phase, producing persistently warm temperatures in the marine environment. Our findings show that a continued warming near the Grand Banks of Newfoundland will have a detrimental impact on this already depleted stock despite the reduction in commercial fishing. Citation: Condron, A., R. DeConto, R. S. Bradley, and F. Juanes (2005), Multidecadal North Atlantic climate variability and its effect on North American salmon abundance, Geophys. Res. Lett., 32, L23703,