Chapter 3 - Changes in Ocean Circulation Patterns

Abstract

The Arctic Ocean is getting fresher. In other words, more freshwater is entering the Arctic Basin than had entered it previously. The sources of this freshwater are twofold. One is simply increased precipitation over the Arctic Ocean, both in the form of snow in winter and as rain in the summer. These changes and precipitation are mainly due to global warming which is causing more storminess in the high latitudes. The other source of additional freshwater entering the Arctic Basin is runoff from streams flowing north into the Arctic Basin from the adjacent continents. While the Arctic Ocean represents only about 2% of the global ocean in terms of volume and surface area, it receives a disproportionate amount of freshwater from rivers. In fact, the Arctic Ocean collects over 11% of global river discharge. Some of this additional runoff is coming from increased precipitation over the land, and some of it is due to the melting of permafrost in these regions, which is releasing moisture into the rivers that flow into the Arctic Ocean and adjacent seas. The winds over the Arctic Ocean are getting stronger. In the past, the force of the winds over the surface waters of the Arctic Ocean was weakened by thick layers of pack ice in and around the gyre. Indeed, the seasonal sea-ice cover, blanketing the entire Arctic Ocean, has been one key to the remarkable quietness of the Arctic Ocean. The ice dampens surface and internal waves and decreases the transfer of wind momentum to the water. But as the ice thins and the margins retreat toward the North Pole, the effect of the winds is increasing. For instance, prior to the past 20 years of diminishing sea-ice cover, the Beaufort Gyre in the Western Arctic Ocean stored about 20,000 km3 of freshwater in the upper few hundred meters of the water column. However, global change has ushered in a new regime of increased freshwater storage in this region, accompanied by increasing wind speeds. The changes in mean monthly wind speeds before and after 2007 are also shown.