Warming and Freshening of Polar Waters
Beyond the surface

The polar oceans warm not only through the surface but also through deeper depths. While surface warming is due to increased radiation absorption, subsurface warming happens when warm water inflows from other oceans. In the Arctic Ocean, subsurface inflow is from the North Atlantic and Pacific Oceans, and in the Southern Ocean, it is through the Circumpolar Deep Water from the Indian and Pacific Oceans. Global warming leads to sea ice and glacier melt, which adds fresh water to the polar oceans. How deep can these warming and freshening effects reach? What changes can they make at deeper depths?

As the ice-free surface water is more exposed to the atmosphere, it is vulnerable to turbulent mixing during storm events. The storms generate near-inertial waves in a stratified water column with a periodicity of Coriolis frequency which is equal to twice the rate of Earth’s rotation multiplied by sine of the latitude. These waves propagate vertically downward and mix the ocean’s interior.

In the North, as the Arctic has been witnessing frequent and intensifying storms in recent decades, an increase in water column mixing by near-inertial waves is being reported. Most Arctic studies focused on the open and deep ocean, but what is the scenario in shallow areas like fjords and coastal regions?

To find out, Subeesh M P and a team from the NCPOR took the case of Kongsfjorden, a shallow Arctic fjord in the west Svalbard archipelago. Fjords formed by glacial calving are like estuaries but connect glaciated land at their head and ocean at their mouth. This setting makes fjords more responsive to changes happening on either side. The researchers investigated storm-driven mixing and near-inertial waves in Kongsfjorden using hydrography and atmospheric observations during 2014-2015 and 2016-2017. They also used model simulations to understand the processes and impact of glacial melting on the mixing.

 The researchers filtered velocities at the near-inertial frequency from the fjord current observations and found that the waves coincided with storms. The waves were stronger in the summer when the water column was stratified. They explained that storm energy was easily transferred to the deeper depths under stratified conditions. The near-inertial waves results in elevated shear in the water column and lead to strong vertical mixing. On the other hand, the team found weak waves in the winter as the water column was well mixed.

The researchers further set out to find implications of the mixing in summer. They observed that the upper water column became more homogenous after a storm event with a one-degree Celsius warmer mixed layer. According to the researchers, the warming was due to more heat from the warm Atlantic water brought upwards by the near-inertial wave induced vertical mixing. Besides, a substantial amount of storm energy was penetrated downward by the near-inertial waves leading to mixing in the deep fjord.

How will the vertical mixing be with the projected increase in the glacial melting? To answer, the team simulated enhanced freshwater conditions in the fjord and found that the mixing will decrease due to enhanced stratification. But, the shear generated by the storms may overpower stratification and mix the water column with near-inertial waves.

Similar to the North, the Southern Ocean is also warming and freshening. A team led by N. Anilkumar, NCPOR, recently investigated the signatures of freshening in the deepest layers of the Indian sector of the Southern Ocean. When sea ice forms, it expels salt into the water below, increasing the salinity. This saline-dense water sinks below 4000 meters to form the Antarctic Bottom Water, which remains undisturbed from surface changes. However, the researchers observed variations in the bottom water characteristics using temperature and salinity measurements.

The researchers observed that the bottom water became fresher by 0.002 and warmer by 0.04 degrees Celsius in 2020 than in 2017-2018. In one of their earlier studies, they reported a more freshening and warming over the period 2006 to 2010 though the salinity rebounded by 2018. From 2006 to 2020, they observed a decrease of 50-120 meters in the thickness of the bottom water layer over the different latitudes of the Indian sector of the Southern Ocean.

What could have led to this reduction? To understand, the team analysed possible factors. One of them is the Circumpolar Deep Water found at around 300 meters. The intrusion of the warm deep water enhances basal melting and calving of the Antarctic ice shelves and increases meltwater flow freshening the bottom water.

The researchers found a 0.3 degree Celsius warming of the deep water from 2016 to 2020. The warming was accompanied by upwelling and poleward intrusion of the deep water due to the weakening of the easterly winds by the positive phase of the Southern Annular mode that shifted strong westerly winds southward. These slowed down the bottom water formation and increased the melting and mixing of the freshwater with the bottom water reducing its thickness, say the researchers.

Besides, between 2016 and 2020, the researchers also saw a decline in the sea ice area. They added that the decline increased shortwave radiation absorption by the water column in the summer, which hindered winter sea ice formation and thus influenced the bottom water formation.

Bottom water formation in the polar regions is a critical process in the global ocean circulation. This water mass ventilates the deepest layers of the ocean by supplying oxygen and acts as a medium of carbon sink through the biological pump. But, the ongoing melting and freshwater pumping are influencing the circulation. With their studies, the NCPOR researchers outlined some interesting changes and processes at the deeper depths.