Dispatch 22: Water Column Structure in the Canada Basin     Print

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Mengnan Zhao

October 9, 2015

Today was my busiest day of the entire cruise – we arrived at the Barrow Line in the Barrow Canyon region along 150W. Due to the complex shelf bathymetry here, and the strong influence from Pacific Ocean inflows, the ocean flows are complicated and we wanted to capture these. This meant we had close station spacing and only about one hour between stations, and I was going non-stop! Investigating the freshwater content change of this region and the physical processes responsible (and how this relates to a decreasing ice cover) is a major goal of this cruise. So it is a good idea for me to introduce the water column structure in the Canada Basin. These are the wiggles in temperature and salinity I examine on the computer screen in the CTD shack as the CTD profiles the water column.

Basically, we can consider the Beaufort Gyre water column as composed of an upper relatively fresh layer of water overlying a deep relatively salty layer. Because of the low temperature, the water density in the Arctic is nearly entirely set by its salinity. In the Canada Basin, there are multiple water layers with different temperature and salinity interleaving in the upper water column. First there is a surface mixed layer which is about 20 m deep, followed by a warm peak in temperature called the Near Surface Temperature Maximum (NSTM) layer. This layer is formed every summer when the sun warms the surface ocean and then a layer of fresh water from sea-ice melt is formed over the solar-warmed water. Below the NSTM layer is a temperature minimum called the remnant Winter Mixed Layer that is a left-over from the surface mixed layer of the previous winter. These water layers take up about the top 50 m in depth in the Beaufort Gyre. Coming to the deep relatively salty layer, at around 300 m depth, we find the warmest water in the Arctic, which comes from the Atlantic Ocean. This water is called the Atlantic Water (AW) layer. Even though it’s only about 1^oC, the heat contained in this water layer is enough to melt nearly all of the sea-ice cover in the Arctic! Luckily, however, it is insulated from escaping to the surface by a stratification “barrier” called the halocline between the fresh upper and deep salty layers; from about 50 to 250 m, the salinity increases significantly, and prevents the heat from the AW from being easily mixed up (by winds, tides or eddies) to the surface. To add to the complexity, even inside the halocline, there are two major water masses – cool Pacific Winter Water (PWW) and warm Pacific Summer Water (PSW), which have origins in the Pacific Ocean. So, the wiggles I look at in the CTD shack can be quite complicated.

We have obtained water-column measurements for over a decade now (since 2003) providing information on all of these water layers. This is invaluable for understanding the long-time evolution of the heat balance, the freshwater content, and their influence to ice cover and Arctic climate.