The polar regions are covered by a thin sheet of sea ice – frozen water that forms out of the same ocean water it floats on. Often, portrayals of Earth’s sea ice cover show it as a great, white, sheet. Looking more closely, however reveals the sea ice cover to be a varied and jumbled collection of floating pieces of ice, known as floes. The distribution and size of these floes is vitally important for understanding how the sea ice will interact with its environment in the future. [Read More]
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!
- 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
One of the most dramatic signals of Earth’s recent warming has been the precipitous decline of the Arctic sea ice. While the sea-ice decline is in response to warming ocean and atmosphere, it also has an important feed-back on the climate itself.
Solar radiation and albedo
Earth’s main energy source is solar radiation. This solar radiation is either absorbed in the atmosphere or at the surface of the planet, or it is reflected back into space. The measure of how reflective a surface is is called its albedo. Sea ice, being white, has an extremely high albedo, compared to the ocean. Therefore when the sea ice declines, more solar radiation is absorbed by the planet, leading to more warming.
Shrinking sea ice decreases the albedo of the Arctic
Our image of the week shows a figure based on data presented in Pistone et al (2014), produced by the NASA/GSFC Scientific Visualization Studio. It shows how the declining Arctic sea ice has decreased the average albedo in the Arctic, with darker colors indicating a declining albedo and therefore warming of the Arctic Ocean. Based on satellite estimates of the sea-ice extent since 1979, the authors were able to constrain that the impact of the sea-ice retreat alone has led to an amount of global warming that is more than 1/4 as strong as the effect due to increased C02 in the atmosphere.
An animation of the annual Arctic sea ice minimum with a graph overlay showing the area of the minimum sea ice in millions of square kilometres.(Credit: NASA/GSFC Scientific Visualization Studio)