The Multiyear Variation And Changing Mechanisms Of The Upper And Intermediate Layer Water In The Canada Basin Under The Influence Of Beaufort Gyre

Based on a vast collected hydrographic dataset during 2003 to 2012, we analyzed the upper and intermediate layer water in the Canada Basin in the Beaufort Gyre system. The multiyear variation and the regulating mechanisms were revealed in this study. The Canada Basin is one of the regions that experience the dramatic sea ice decline. The rapid change of sea ice is in close relationship with the upper ocean variability. The heat content and freshwater content of the upper ocean increased gradually in the Canada Basin, as did momentum input. The Beaufort Gyre (BG) was spun up in the last decade which is an important factor in regulating the variation of the upper ocean. Both the geostrophic wind curl and freshwater content could contribute to the spin-up of BG. The center of the BG is shifting to the southwestern Canada Basin. The spin-up BG contributes to the deepening of Pacific Winter Water and more Pacific Summer Water flows to the northern Chukchi Sea that resulting in the redistribution of Pacific Water in the basin.The freshwater content (relative to salinity of 34.8) in the Canada Basin is ～22m in 2003-2006 and increased to ～28m in 2007-2010 and then gradually decreased. The increasing freshwater content indicates the upper ocean becomes more stratified. The dynamic heights (relative to 400 db) in the Canada Basin had changed from the values smaller than 0.65 gpm (geopotential meter) in 2003 to those larger than 0.85 gpm in 2008 which resulted in the increasing of geostrophic current. The downwelling velocity (absolute value) in the BG region had been generally larger than 2cm/day after 2003 (absolute value>4cm/day in 2007 and<1cm/day in 2012). Instead of downwelling, most of the basin was dominated by upwelling in 2012 and there was a strong northward Ekman transport in the basin which would benefit the release of freshwater that had resided in the basin for years.The high value area of upper ocean heat content (above 200m) was only appeared in the southwestern basin. The overall heat content around the basin was 0.6TJ/m2. The heat content was gradually increased in the southwestern basin during 2004-2006 while the variation was rather stable in central and northern basin with the heat content less than 0.7TJ/m2. Since 2007, the high value area of heat content was expanding from the southwestern basin to the central and northern basin. The maximum value of heat content in the central basin could be 0.9TJ/m2. By 2012, the heat content of most area of the upper ocean was higher than 0.9TJ/m2 while it was larger than ITJ/m2 in the southern Beaufort Sea. There are two aspects that contribute to the increasing heat content:(1) More ice-free area means that more solar energy absorbed by the upper ocean. (2) More warmer Pacific Water flowed into the basin.The Atlantic Water (AW) shows a different variation in the intermediate layer comparing to the upper ocean. Its original sources come from the Atlantic Ocean. It contains large heat energy and plays an important role in the exchange of water mass and circulation process. There are two ways for the AW that flows into the Arctic Ocean. One is through the Fram Strait and the other is through the Barent Sea (More of it via the St Anna Trough). The AW is dominated by the warm Fram Strait branch water. And it is the Fram Strait branch of AW that was used to study its flow pathway and distribution. Since the 90s the AW was experienced two major changes. One is in the early 90s and one is in the 2006 with two anomalous warm water higher than the climatic records. The early 90s warm anomaly AW reach the Canada Basin in 2000 and then expanding around the basin. Since 2004 the AW temperature in the basin shows a decreasing trend and this was due to the inflow of cold phase of AW. The heat content of 200-700m layer shows a decreasing trend during 2003-2012 with higher than 5.8TJ/m2 in 2003 to less than 4.2TJ/m2 in 2012.The core depth of AW was increasing as more relative cold AW flowed into the basin. But the deepen AW that induced by the relative cold and dense water was not the whole story. Since 1997, the sea ice experienced a catastrophic retreat in the Canada Basin with stronger sea ice motion and the spin-up BG. The increasing negative surface stress curl resulted in stronger convergence of the upper ocean. The dynamic properties of the upper ocean have changed which contributed to the deepened of AW in a relative stable change of AW temperature in the central BG area. It was shown in this study that the depth of AW core was not only related to its thermodynamic change but also the dynamical change as a result of spin-up BG. Using a reduce gravity model we have shown that the relationship between the surface stress and the depth of AW core. The density versus depth relationship revealed in this study shows that the depth of the maximum AW temperature was mainly controlled by the density structure before 2007. However, the combined effect of density and sea ice retreat that enhanced surface stress curl determined the depth of the AW inside the Beaufort Gyre since 2008. The deepening of the AW core and expanding of the area where the AW deepening occurred had the profound effect on the large-scale circulation in the Arctic Ocean.