Ocean Climate Change
Igor Yashayaev (Bedford Institute of
Oceanography, Dartmouth, NS )
During the last several decades, the Arctic Ocean has experienced striking large-scale changes in oceanic and sea-ice properties. Recent studies suggest that this region is a highly variable system undergoing major regime shifts at time scales from several years to decades. The ongoing research tends to link these climate shifts to the changing atmospheric dynamics and/or to variations in Arctic-Subarctic oceanic and sea-ice fluxes. Key signatures of Arctic climate change include an increased heat flux into the Arctic Ocean and a significant reduction in thickness and extent of the Arctic perennial sea-ice cover. Some global climate models predict an 80% decrease of the Arctic ice pack within the next 50 years or so.
On the other hand, analyses based on both observations and models suggest at least two regimes in the arctic atmospheric circulation (cyclonic versus anticyclonic) directly influencing sea-ice conditions and the distribution and fluxes of freshwater and Atlantic Water. Whether these regimes result primarily from an oscillatory mode (i.e. Arctic Oscillation) typical of the North Polar Vortex, or from a coupling with lower latitudes (i.e. NAO) similar to the southern hemisphere ENSO, or are they part of a trend related to global changes, is yet to be determined.
The subpolar gyre of the North Atlantic has also undergone dramatic changes during the past five decades. In particular, between the early 1970s and the mid 1990s the entire water column of the subpolar gyre experienced significant freshening and cooling. The magnitude of this freshening was an equivalent to mixing an extra 4 meters of fresh water over the whole gyre. A significant fraction of this change arose from an extreme development of the winter convection in the Labrador Sea in the early 1990s. In addition, based on hydrographic data and direct current measurements, important changes have been reported regarding the North Atlantic Current flow in the subpolar basins and into the Nordic Seas as well as the southward flux of fresh and cold waters across the Greenland-Scotland Ridge.
The observed changes in the heat and freshwater content in the subpolar North Atlantic can be considered a result of the combined local atmospheric forcing and remote advective forcing traced back to its source in the Arctic Ocean and the Nordic Seas. In particular, changes in the ocean intermediate layers were triggered by high wintertime surface heat losses and subsequent exceptionally deep convection in the Labrador Sea. This mixing happened soon after the arrival to the Labrador Sea of anomalously large volume of fresh water from the Arctic Ocean (during the 1980s and early 1990s). The combination of strong local forcing and advective supply of fresh water resulted in one of the largest change ever seen in the ocean - freshening of the subpolar gyre.
Somewhat different tendency of long term temperature and salinity variability can be seen in the subtropical regions, where the upper ocean has recently become warmer and saltier. The warming of the upper layers accompanied with increasing evaporation (causing in its turn in increasing salinity) provides more favourable conditions for hurricane development and intensification. The higher evaporation in the subtropics was also likely a reason of higher transport of moisture to the higher latitudes, resulted in extreme precipitation in the Atlantic Canada.
Although some of these statements are still hypothetical, the ocean studies conducted in the recent years have already shown how important is the knowledge of the ocean circulation and variability for the understanding of the climate change on the entire Planet.
Igor Yashayaev has graduated from Geography faculty of the Moscow State University in 1988. Between 1988 and 1995, he worked and obtained PhD in State Oceanographic Institute (Moscow, Russia) and P.P.Shirshov Institute of Oceanology (Moscow, Russia). Since 1995, he is associated with Bedford Institute of Oceanography (as a visitor, post-doctoral fellow, contractor, and presently, research scientist). Igor has participated in and led numerous oceanographic expeditions. He's first steps in the ocean sciences (PhD) dealt with sea surface temperature variability in the North Atlantic on the weekly-to-decadal scales, with participation in scientific expeditions he's scientific interests became wider and until present he is involved in collection, analyses and interpretation of hydrographic (temperature, salinity, density, flow field) measurements. The recent major works (two Nature papers) reveal some immense changes in salinity in the North Atlantic suggesting significant shifts in the planetary climate system. As a part of he's organizing activity, he convene and chair sessions at the large geosciences conferences (EGU, AGU) and initiated a special issue in Progress Oceanography on the oceanic changes in heat and fresh water budget and transports.