Sea Ice South (2): Another Brick in the Wall

Sea Ice South (2): Another Brick in the Wall

My previous entry was setting the foundation for understanding the differences between sea ice in the northern and southern hemispheres. It focused on the physical geography of the Earth. Specifically, the distribution of land and ocean are different at the two poles; hence, there is no reason to expect one pole to behave like the other pole - beyond perhaps, they both get very cold in the winter.

This entry will focus on the basics of the physical climate needed to understand sea ice. As summarized in Spencer Weart’s excellent history, we have known for a long time that water vapor and carbon dioxide are ingredients of the atmosphere that are important to our ability to live on the planet. Specifically, based only on the amount of energy coming from the Sun, the temperature of the surface of the Earth should be about zero degrees Fahrenheit. It is the presence of greenhouse gases in the atmosphere that holds heat close to the surface for a while, leading to an average temperature closer to, say 60 degrees Fahrenheit.

When thinking about the climate, it is important to remember that the Earth is always cooling to get rid of the energy that comes from the Sun. A good way to experience this cooling and the effects of greenhouse gases is to spend a summer night in Death Valley, CA, and another summer night in the Everglades, FL. Because of the lack of water after sunset it cools down much faster in Death Valley. I like to think of this tendency to cool as a thermal spring always pulling the Earth towards zero degrees.

We have to remember another fact of the Earth, which is the tilt of the axis of rotation that is responsible for the seasons. As a result of this tilt, the solar energy that is directly received at the poles goes through huge cycles every year. In winter it is dark, and there is no direct solar heating of the pole. In the summer there is continuous light, but the heating is weak because the Sun is low in the sky. As a result of this tilt, far more energy comes into the Earth in the tropics than at the poles.

Interestingly, when we look at the energy leaving the Earth, on an annual average basis, there is not a huge difference between the poles and the tropics. What that means is that the “excess” of energy entering the Earth in the tropics is moved towards the poles, where there is a net loss of energy to space. This energy is carried from the tropics to the poles by the oceans and the atmosphere. Without transport of energy to the poles, in winter, when the Sun is not present at the poles, the temperature would drop to 100s of degrees below zero. That does not happen, but it still gets cold – cold enough to make ice.

That’s what the oceans and the atmosphere do. They are fluids that move to even out the distribution of energy – or effectively, heat. Therefore, the role of the atmosphere and ocean is pretty straightforward; they are not random and chaotic and unconstrained. They respond to heating and cooling through well understood physical mechanisms – like gravity and pressure. Another important force is due to the rotation of the Earth (see link at bottom).

One of the interesting things about transport is that it occurs in, let’s say, events or features. A useful metaphor on people’s minds this week is the Mississippi-Atchafalaya River Basin. The Mississippi and Atchafalaya Rivers carry a LOT of water in a channel to the Gulf of Mexico, where it immediately spreads out of the channel. You might say that it fans out, but it really doesn’t just diffuse into the Gulf. It moves as distinct features, as seen in this 2001 figure of sediment from NASA’s Earth Observatory.



Figure 1: Sediments in the Gulf of Mexico from Mississippi and Atchafalaya Rivers.


The water is channeled by the river basins; it is not like a shallow film of water spread out between Brownsville, TX and Homestead, FL. The water is channeled, and big events, like the spring runoff are responsible for a large portion of the transport. The atmosphere and oceans behave in the same way - heat is transported, preferentially, in certain places, for example in ocean currents such as the Gulf Stream and atmospheric storm tracks.

Let’s focus on the ocean. If a current like the Gulf Stream brings a lot of warm water to Greenland, then what keeps all of that water from piling up in the Arctic? There has to be a return flow, and that return flow takes cold water back towards the tropics. The Earth’s weather is just part of mixing warm and cold.

Okay, it’s time to pull together this information. The temperature at the poles, especially in the winter, is largely determined by oceanic and atmospheric transport of heat. Alternatively, the heating and cooling at the winter pole is not day-to-day determined by the radiative energy from the Sun and greenhouse gas concentrations. The heat transport occurs in preferential locations, and return flow takes cold air and water back towards the tropics in preferential locations. Fluctuations in the preferential locations mean that warm and cold regions move around. Given the information from the previous blog, the North and South Poles are different. Hence sea ice behavior is different.

Sea ice – I am setting the foundation for sea ice. From two blogs ago, one on the Northern Hemisphere, there was a number associated with the melting of the Arctic sea ice. That number is 1 watt per square meter. The melting of the Arctic sea ice that has been observed over a certain amount of time, say a decade or two, is consistent with a sustained, change in the energy balance of 1 watt for every square meter – that’s about a square yard – 3 feet by 3 feet. How much energy does this represent? Let’s go to an iconic figure of the radiative balance the Earth updated by Trenberth et al. in 2009.



Figure 2: The global annual mean Earth’s energy budget for the Mar 2000 to May 2004 period (W m–2). The broad arrows indicate the schematic flow of energy in proportion to their importance. (from Trenberth et al. , 2009)

I am mainly interested in sizes. The amount of energy at the top of the atmosphere from the Sun is about 341 watts per square meter. Ultimately, that is also just about the amount of energy that goes back to space. In the various ways that energy is absorbed and reflected and transported there are numbers in the figures that are 10s watts per square meter. Down at the very bottom of the figure is the amount absorbed by the Earth – in this figure 0.9 watt per square meter. (Very close to 1, I note.)

If we look at the energy that is transported to and away from the poles as well as that associated with energy from the Sun and emitted back to space, then there are several paths that deliver or take away 10s of watts per square meter. A change of 1 watt per square meter can be realized in several ways. And if we get right down to sea ice it gets more complicated. What happens if there is more fresh water in the ocean because of more rain, more snow, more melting of ice sheets? Fresh water freezes at a higher temperature than salt water. So could it in fact get warmer and freeze more ice in the ocean because the water is less salty? Plausible, I assert – it would become a matter of measurements, and numbers, and untangling the many different paths that energy is provided to and taken away from the surface of the sea.

Next time I will get a little more specific about the southern ocean and its sea ice.

r


(If you want to see cool movies that show how rotation organizes flow go to MIT and look at these movies.)


Useful links
Recent sea ice trends
Sea ice data
Rood’s Blogs on Ice