CR
Cryospheric Sciences

Oceanic Circulation

Image of the week – Our salty seas and how this affects sea ice growth

Image  of the week – Our salty seas and how this affects sea ice growth

Earth’s oceans are not simply just water, they are a complicated multi-component fluid consisting of water and dissolved salts (ask anyone who has tried to drink it!). The existence of these salts has a significant impact on global ocean circulation. Nowhere is this more significant than in the polar oceans where it is one of the key factors influencing sea ice formation. In this week’s image of the week we are going to show you how freezing ocean water is a little more complicated than you may think!


The salinity and temperature of ocean water affect its density; essentially how much it weighs. Typical ocean densities are around 1000 kg/m3  and, depending on the temperature and salinity may vary by up to 1 %. This seems tiny, but these small changes in density are what drive the thermohaline circulation, the dominant large-scale ocean circulation. The density of sea water, as a function of temperature and salinity, is expressed in terms of the equation of state  (a mathematical way of describing the density of sea water in relation its temperature and salinity). Contours of the equation of state of seawater are shown in this week’s Image of the Week. The figure is from a recent paper by Mary-Louise Timmermans and Steven Jayne, who try to understand how changes in Arctic temperature will influence the density, and therefore the circulation, in the Arctic Ocean. The y-axis is temperature, and the x-axis is salinity. The black lines are density contours. The dashed line plots the freezing point of water.

Sea Ice Formation

Sea ice begins to form when ocean water is brought to this freezing point. If one was to put a cup of tap water into a freezer, ice would begin to form at 0 °C. But talk to a group of polar ocean modellers, and they will tell you the freezing point of water is about -1.8 °C. How can this be?

Let’s got back to our figure for some clues. Looking at the dashed line representing freezing point of ocean water you will notice that as the salinity increases, the freezing point decreases. So an increase in salinity of sea-water suppresses its freezing point. Just like how salt is used to melt ice in winter, it prevents the water from reaching its freezing point until the water reaches roughly -2 °C.

How does this all link together?

When the ocean gets cold, the influence of temperature on density changes, affecting how rapidly sea ice can form. Take a look at the bending of the black contours as the temperature is reduced to zero and below. Whereas in “normal”, warm contexts, a decrease in temperature leads to an increase in density, this changes as the temperature approaches 0 °C. As the ocean cools, the top-most, coldest water typically sinks, and is replaced by warmer water from below, driving ocean circulation convection. It therefore can take a long time to bring the surface of the ocean to near 0 °C. Since there is salt in the ocean, the water can reach colders temperatures where something very different happens. As the water continues to cool, the coldest water no longer sinks, and may even float, with sea-ice formation happening rapidly.

The formation process of sea ice, and its relationship to the ocean it forms out of is an extremely complicated and rich phenomenon, and it all depends on salt!

Further Reading

  • Mary-Louise Timmermans and Steven R. Jayne, 2016: The Arctic Ocean Spices Up. J. Phys. Oceanogr. 46, 1277–1284, doi: 10.1175/JPO-D-16-0027.1.
  • For more on sea ice check the National Snow and Ice Data Center (NSIDC) website – All About Sea Ice!

Edited by Emma Smith


Image of the week – The winds of summer (and surface fluxes of winter)

Image of the week – The winds of summer (and surface fluxes of winter)

Antarctica is separated from the deep Southern Ocean by a shallow continental shelf. Waters are exchanged between the deep ocean and the shallow shelf, forming the Antarctic cross-shelf circulation:

  • Very dense waters leave the shelf as Antarctic Bottom Water (AABW) that will then flow at the bottom of all oceans.
  • Meanwhile, relatively warm water from the Southern Ocean, Modified Circumpolar Deep Water (MCDW*) comes on the continental shelf and brings heat to the ice shelves.

That is, Antarctic cross-shelf circulation influences the water mass that transports heat, carbon and nutrient all around the globe in very large volumes (Purkey and Johnson 2013), and the basal melting of Antarctic floating ice (Hellmer et al. 2012), hence the stability of the whole Antarctic ice sheet.

Although critical for both the ocean and the cryosphere, very little is known about the mechanisms behind cross-shelf circulation. We know that the mechanisms that control it vary on a seasonal time scale (Snow et al. 2016b). However, most hydrographic observations around Antarctica are taken in summer, when there is less sea ice and when the Southern Ocean is the least stormy. This means that we have very few measurements of the seasonal variations of the cross-shelf circulation itself.

Why does it matter that the cross-shelf circulation varies between summer and winter?

Three words: sea level rise.
Nearly half of the world’s population lives in coastal areas (
UN report). Antarctica contains enough ice to raise the sea level by 60 m, and although a total melting is very unlikely, current rates could raise the sea level by 1m by 2100 (read more about it on AntarcticGlaciers.org). To project future sea level rise and design relevant coastal defences, we need models to predict when and where the Antarctic ice will melt.

However, models are only as good as the observations that were used to constrain them. Having only summer observations in an area of Antarctica that has notable differences between summer and winter ocean circulation means that until now, models could not represent accurately the transfer of heat from the ocean to the ice shelves

A better observation strategy is needed if we want our models to correctly represent Antarctic basal melting and the global ocean circulation.

Antarctic cross-shelf circulation: summer vs winter

In summer, the circulation is mostly controlled by the strong katabatic winds blowing from the interior of the Antarctic continent towards the ocean. All the surface water masses go in the same direction, simply following the Antarctic coastal current. Nothing really happens at depth.

In winter, the circulation is also controlled by buoyancy forcings, that is changes in temperature or salinity at the surface of the ocean. Here, these mostly occur in a polynya (a hole in the sea ice cover) where the “warm” ocean is cooled by the very cold atmosphere, and where the surface becomes very salty as sea ice reforms (a process called “brine rejection”: salt is expelled from the new ice as water freezes). These buoyancy forcings form dense water (DSW), which sinks to the abyss and off the shelf as AABW. Mass conservation means that something else (here MCDW*) needs to come to the shelf to compensate for that outflow. You can notice that MCDW now flows in the opposite direction than it did in summer.

Take home message

Summer data is better than no data. But always be aware of the limitations of your model if you don’t have the datasets to test it– you may have a surprise when you do!

Reference

Snow, K., B. M. Sloyan, S. R. Rintoul, A. McC. Hogg, and S. M. Downes (2016), Controls on circulation, cross-shelf exchange, and dense water formation in an Antarctic polynya, Geophys. Res. Lett., 43, doi:10.1002/2016GL069479.

Edited by Sophie Berger and Emma Smith