"An important indicator of winter atmospheric conditions over the North Atlantic is the North Atlantic Oscillation (NAO), which is related to the average position of the jet stream over the Atlantic," Pershing explains. "When the NAO is in its 'positive' state, conditions over the Labrador Sea in the northwest Atlantic are colder and stormier, while the northeast Atlantic and northern Europe experience warmer and milder weather. When the NAO is in its 'negative' state, the conditions reverse. After a winter of positive NAO conditions, the deep waters in the Gulf of Maine typically become warmer and saltier -- leading to higher abundances of zooplankton. After negative NAO conditions, the waters become colder and fresher -- not as hospitable for the food that whales need to eat."
For example, Pershing and Greene report right whales suffered through two years of physiological stress and poor reproduction in 1999 and 2000, a setback that can be traced to a dramatic negative "flip" in the NAO in 1996 and a subsequent decline in copepod abundance in 1998. Then a sudden return of the NAO to positive conditions from 1997 to 2000 led to a 10-fold boom in copepod populations. Consequently, the northern right whales produced more calves in 2001 than at any time since records began in 1980.
During the 1980s, the NAO was predominantly positive, leading to warm conditions in the Gulf of Maine as well as pl
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