Weakening Atlantic Ocean Currents
On November 30, the journal Nature published research from the UK’s National Oceanography Centre in Southampton showing that the volume of the overall circulation has slowed by 30% over the last 50 years. Most of the decrease has occurred since 1998.
According to the news story published by Nature online,
The likely cause is more fresh water flowing into the ocean from rivers, rain and melting ice, and this is thought to be linked to global warming. But climate modelers are worried that the resulting weakening of ocean currents could ultimately lead to substantial cooling of the North Atlantic.
The most immediate effect of this change in circulation would be on the climate of Northern Europe. The UK research findings indicate that the circulation is concentrating more in the southern areas of the Atlantic, which means that the Gulf Stream running along the North American Coast and then across the ocean to Africa would not be affected unless there are additional changes. The part that of the circulation that is slowing down is the return flow to the polar regions that swings up past Great Britain and Scandinavia. According to the Nature article,
the deep-ocean return flow of cooler water has decreased dramatically. This cycle usually returns water to more southerly latitudes from as far north as Greenland and Scandinavia.
Global climate is not typically our concern here at Acadia Partners and SERC. Our focus is more local. But, the slowing down of the cooler water return flow makes me wonder about possible effects on the Labrador current. Can any readers who are more familiar with ocean currents shed light on possible effects?
December 14th, 2005 at 8:15 pm
Bill,
This is an interesting and significant reseearch finding for a number of reasons. First, as you point out, if indeed the North Atlantic Deep Water circulation is slowing down, this will likely have a profound effect on northern european climates. I am always amazed when I look at a map of the nroth Atlantic and realize how far north places like London (51.5 deg N), Dublin (53.3 deg N) and Reykjavik (64 deg N). Compare this to Bar Harbor (44.4 deg N), St. John’s Newfoundland (47.5 deg N), or Cartwright (54 deg N) along the northern Newfoundland coast.
Now compare the (approximate) mean annual temperatures for these eastern Atlantic cities and western Atlantic cities: London (50 deg F), Dublin (48 deg F), Reykjavik (41 deg F) and Bar Harbor (46 deg F), St John’s (41 deg F), Cartwright (31 deg F). Reykjavik is approximately the same average temperature as St John’s Newfoundland, but it is 16 degrees of latitude further north. Dublin is approximately the same latitude as Cartwright, Newfoundland but averages about 17 degrees warmer.
Most of this temperature difference is usually attributed to the northern flow of the warm surface waters which had originated in the Gulf Stream. The primary forcing mechanism which draws these waters up along the northern european coast is the deep water formation which occurs off the coasts of Greenland and Norway in the Arctic ocean. In these regions of the Arctic ocean, the originally warm, salty waters of the Gulf Stream are entrained in a circular flow which allows the waters to cool significantly at these high latitudes. The warm, salty waters become very cold salty waters. This temperature change results in a significant increase in density and this water sinks and eventually flows southward as the North Atlantic Deep Water. This sinking water mass essentially sucks more warm surface waters into the Arctic and the circulation pattern continues.
The findings in the Nature study suggest that the flow of North Atlantic Deep Water is slowing down. This implies that less surface flow is moving north along the northern european coast. If true, and northerly surface flow slows down siignificantly, then temperatures in northern european could potentially cool quite substantially.
The Nature study is significant for a second reason: it provides evidence for a phenomenon which has been predicted by global warming climate models. For at least a decade there have been some predictions from climate models that warming in the nrothern hemisphere could lead to a slow down, or even shut down of deep water formation in the North Atlantic. These models predicted that global warming, and the resulting increase in melting ice and freshwater inputs, could result in significant freshening of the waters in the Arctic. Remember that it is cooling of warm, SALTY water which results in the high density deep water formation. Freshwater is not as dense, nor can it cool to the low temperatures of saltier water (fresher water will freeze at higher temps). The result is that freshening results in decreased densities, and hence a slow down in deep water circulation.
So the Nature article is significant because it would appear to provide some important empirical support for global warming predictions. This is important. It adds to the evidence for global warming and suggests that our global climate models are indeed becoming predictive models.
I suspect someone is looking at this already, but I’d like to know if salinities in the Arctic are indeed freshening. Finding decreased salinities in the Arctic would further strengthen the conclusion that North Atlanitc Deep Water formation is slowing down.