Research On Physical Processes Of Abyssal Warming And Front Bifurcation In The Nordic Seas

The Nordic seas are the most important oceanic passage connecting the Arctic Ocean with the Atlantic Ocean, particularly in the core area of Arctic Oscillation. Special geographical conditions make it possible for vertical water exchange through cooling convection and mixing, and this process is a key link for the high latitude thermohaline circulation. Specific study of oceanic changes and associated processes can benefit a further understanding of what an important role the Nordic seas play in the regional and global climate system, now days especially under the background of rapid changes in the Arctic climate.Since the 1970s, deep basins of the Nordic seas (the Greenland basin, the Lofton basin and the Norwegian basin) have shown a coincident warming trend. A temperature increase of about 0.37℃ occurs in the Greenland basin, leading to a most obvious warming among the three basins. As for the other two basin, the relative slower rate of warming is due to that the huge residual of old deep waters largely suppresses temperature rising by offsetting the advective input of relatively warm waters. Besides the abyssal warming, deep waters have been more saline and this change leads to a more stable stratification between the intermediate layer and the deep layer. In the past half a century, deep-reaching convection in the Greenland basin has weakened a lot, and nearly ceased in recent 20 years. From then on, the reduced exchanges with the relatively cold and fresh waters in the upper layer could not compensate for the continuing input of relatively warm and saline Arctic deep waters. In the future, the enhanced stratification of deep layer will in turn suppress the deep-reaching convection, leading to a warmer and saltier abyssal plain.In the Greenland basin, waters in the intermediate layer can obtain a relatively cool and fresh character by ventilating with the surface waters. Meanwhile, deep-reaching convection between the intermediate layer and the deep layer could also cool and dilute deep waters. But at present the trend of salinity change from the subsurface to the intermediate layer is the same as that of deep waters. From the year 2003 on, a clear downtrend occurs in the variation of water bulk stability which is identified as the heat loss needed for waters sinking to one certain depth by surface cooling. This change is mainly attributed to a more homogenous and saline intermediate layer resulting from the increased component of recirculated Atlantic waters and ocean internal mixing. Though this change is benefit for the cooling convection between the subsurface layer and the intermediate layer, it appears that the increased salt in upper water column also causes a temperature rising during the period of cooling convection. This process will make the abyssal warming continues, even if the deep-reaching convection recoveries in the future, until the upper layers freshened again by vertical water exchanging.Jan Mayen Ridge-Jan Mayen Fracture Zone-Mohn Ridge is a region connecting all the three deep basins in the Nordic seas and here the Arctic Front bifurcates into a Y structure consisting of shallow and deep branches. This structure implies that it is hard for Greenland upper waters shallower than 300 m to be transported into the other two basins directly, but below this depth where the Arctic Front is missing there might be exchanges between different basins through the intermediate circulation. Along with the temperature and salinity rising, presently the Greenland intermediate waters has been warmer and saltier than those in the other two basins, resulting in an obvious input of saltier waters through the Jan Mayen channel into the Norwegian basin which had been observed in 2014. This more saline water mass which is modified from the recirculated Atlantic waters brings extra salt into the intermediate layer (400 m～1500 m) of the Norwegian basin, raising the salinity of waters there.The spatial and temporal changes of Arctic sea-ice drift are analyzed based on monthly sea ice motion vectors since 1979. We find that Beaufort Gyre+Transpolar Drift, Anticyclonic Drift, Cyclonic Drift and Double Gyre Drift are four primary types in the Arctic Ocean and the contributions of every type to maintaining the perennial sea ice are different. In summer the type of Cyclonic Drift with a large-scale anticlockwise ice drift pattern trends to prevail while the type of Anticyclonic Drift prevails in winter and spring when the Arctic Ocean is under control of the Beaufort High. In the interannual variation, when the Arctic Oscillation (AO) index stays in a high positive (negative) condition, the sea-ice motion in the Arctic Ocean shows a more anticlockwise (clockwise) drifting pattern as a whole. But when the AO index stays in a neutral condition, the sea-ice motion becomes much more complicated and more transitional types trend to occur.The influence depth of Ekman drift motion under the bottom of sea ice shoals with the thickness of mixed layer reducing. Moreover, a thinner mixed layer leads to water column in the mixed layer gains more momentum. Currently according to the mixed layer depth in the Canadian Basin, the momentum caused by the adjustment of Ekman drift structure is estimated to increase by about 10%. Accelerated flow in the upper layer not only results in an intensified Beaufort Gyre which affects regional fresh water balance, but also contributes to a faster sea ice drift by reducing the stress of sea ice and water beneath.The ratio of wind speed to ice speed is identified tas a factor to evaluate the sea ice drift condition and its value from 2003 to 2012 has reduced about 30% compared to the average level before the year 2003. In addition, the annual cycle of which sea ice drifts faster in summer and more sluggish in winter has almost disappeared. This change in the sea ice drift condition implies that the Arctic sea ice, especially during the wintertime, drifts faster under the same wind speed sine the year 2003. On one hand, the accelerated sea ice drift during winter leads to a decrease in the residence time for sea ice to grow and thicken, leading to less sea ice survival from the next summer. On the other hand, it may cause an increase in fresh water flux of the Fram Strait during wintertime, and the variation of fresh water distribution in winter make a significant effect on the progress of cooling convection in the subpolar oceans.