By Jodi Di Menna
The climate systems around the Atlantic — from the southern United States up to Greenland and from northwestern Africa and Europe into northern Asia — in large part depend on the North Atlantic Oscillation. In simple terms, the North Atlantic Oscillation index refers to the pressure difference between the low-pressure air mass over Iceland and the high-pressure air mass over the Azores, a cluster of nine islands off the coast of Portugal.
When there is a large contrast between the two air masses, the index is calculated to be positive. When the pressure difference is about average, the index is zero. And when it’s below average, the index is negative. A positive index (or a large contrast between the air masses) results in colder, drier conditions over the northwestern Atlantic and Mediterranean regions, but warmer and wetter weather in northern Europe, the eastern United States and parts of Scandinavia.
In fact, the trend in the North Atlantic Oscillation index has been toward mostly positive values over the past 30 years. Scientists link this trend to everything from milder winters in Europe to changes in sea-ice cover around Labrador and Greenland to changes in ocean circulation and fish and zooplankton distribution.
For the black-legged kittiwakes nesting on the steep rocky cliffs around Witless Bay, N.L., a global-scale shift in the atmosphere is personal: Their family planning depends on it. Canadian Wildlife Service scientists have found that the success of kittiwake reproduction depends in large part on the North Atlantic Oscillation — the massive circulation pattern that rules much of the climate of the North Atlantic Ocean. It’s a worrisome find, since climate experts predict climate change will throw the oscillation out of whack.
John Chardine and his colleagues have been measuring the breeding success of the kittiwakes on the islands in Witless Bay since the early 1990s. During his observations, Chardine has seen the colonies shrink in overall size and vary tremendously from year to year in reproductive success.
In a good year, an average of between one and two chicks per nest would fledge. In the bad years, almost all the nests failed. “In a survey of a thousand nests, you might find 10 chicks. I had whole plots that I was studying where not a single chick would be fledged,” Chardine says.
Suspecting that the win-some, lose-some pattern was linked to the kittiwakes’ favoured meal — caplin, a small oily fish — Chardine began to examine the ocean conditions during the low reproduction years.
He found was that when the surface water was colder than normal, the birds failed. “We know that caplin distribution in the water is temperature related. So we think the mechanism that causes this failure is temperature changes in the water from year to year, which affect the distribution of caplin.”
But the larger question remained: What was causing the big fluctuations in water temperature? “I had known that the North Atlantic Oscillation affected water temperatures south of Greenland and into the east coast of Newfoundland,” recalls Chardine, “so I started looking at the numbers.”
In particular, he examined the index that is calculated each year for the North Atlantic Oscillation and compared it to the kittiwakes’ breeding success. As the index went up (which indicates colder conditions around the island of Newfoundland; see “Measuring Up,” below), the birds did poorly. When the index was negative (indicating warmer conditions), the birds did well. That finding doesn’t bode well for the birds, given that most climate models predict an increase in the North Atlantic Oscillation index over the next 100 years as a result of human-induced climate change.
Chardine suspects the link between the index and breeding is the caplins’ dislike of cold surface water. That means the fish aren’t readily available when the birds need them most.
“When you decrease the water temperatures, caplin delay their spawning. So we think one problem simply could be timing, in that the kittiwakes couldn’t get the caplin when their chicks were hatching.”
The importance of Chardine’s findings extends beyond the fate of the kittiwake population. “The birds are a very useful indicator for marine environmental quality from one year to the next,” he says.
Furthermore, the upward trend in the North Atlantic Oscillation index could have a wide-ranging influence. “It doesn’t just have an effect on kittiwakes. It affects cod and lobster and a bunch of other species,” explains Chardine. “It is an important number.”