Updated: Apr 11, 2020
Sea ice is one of the defining features of a marine polar environment. It’s a massive habitat that supports large numbers of plants and animals, from microscopic algae to krill and all the way up to the top predators, like seals, whales and penguins. In both the Arctic and Antarctic, the sea ice expands from its minimum extent in late summer to its maximum extent in late winter. It is now August 6th, heading towards late winter, and we have not yet had any decent sea ice to speak of here at Palmer Station. It’s not just because we’re too far north, in years past winter-over teams could ski on the sea ice, it was so thick.
In both the Arctic and Antarctic, part of the sea ice remains year-round, this is called multiyear sea ice. The rest of the sea ice follows an annual cycle, growing in the autumn and winter, and then melting back again in the spring and summer. But all of this is changing. The annual growth of sea ice is starting later in some regions (the Arctic and the Antarctic Peninsula) and the annual melt is starting sooner, making the duration of what we refer to as the sea ice season nearly 3 months shorter than it was 30 years ago.
In addition to this, multiyear sea ice (especially that in the Arctic) is now melting with the annual sea ice in the summer. The less multiyear sea ice there is, the more the sun’s energy can penetrate the ocean, warming it up. The warmer the ocean gets, the more the sea ice melts. It’s a vicious cycle known as a positive feedback loop – though it’s not a positive thing, it’s a really, really negative thing. This is a major issue in the Arctic and it’s already wreaking havoc with weather patterns, ecosystems and communities in the northern hemisphere.
Here, along the Western Antarctic Peninsula, there isn’t much multiyear sea ice (though there is to the south of us in the Bellingshausen Sea and to the east of us on the other side of the Peninsula in the Weddell Sea).
Instead, it is the annual sea ice that affects the ecosystem most dramatically. Typically, sea ice begins to form in the autumn when temperatures drop, and winds pick up. The winds cause the surface of the ocean to churn up like a giant washing machine. As the air temperature drops, the ocean rapidly loses heat to the atmosphere. When the roiling seawater reaches -1.8˚C, ice crystals begin to form. These delicate shards of ice are tumbled around in the washing-machine ocean, becoming deformed and jagged. We call these ice crystals frazil ice; you would be frazzled too if you had spent time in a washing machine of an ocean. When the winds subside and the ocean calms, the frazil ice crystals begin to rise to the surface. As they do so, they pass by the remnants of the summer phytoplankton blooms. Phytoplankton are microscopic algae that grow in large densities in the spring and summer when there is plenty of sunlight. As the frazil crystals drift upwards, some of the phytoplankton cells become stuck to the ice (we call this process scavenging...I'm not sure why) and are carried up to the surface with it. At the surface of the ocean, the frazil ice crystals form a slush that quickly solidifies to form what we call pancake ice, because of its appearance.
The phytoplankton cells scavenged by the frazil ice are embedded in the pancake ice and remain there until the ice melts, or until something eats them. In this position, they have good access to sunlight and can continue to grow with the growing sea ice, developing a nice thick, green-brown layer at the bottom of the ice. It’s this green-brown layer that is so important for young Antarctic krill. If the sea ice doesn’t form too late, then there is still phytoplankton in the water for the frazil ice crystals to scavenge and there is still enough daylight for the ice algae to grow. But, in recent years, the sea ice has been forming late, and this is not good. Studies have found connections between sea ice and the survival of young Antarctic krill. It seems that without good sea ice growth, young Antarctic krill do not fare too well.
Sea ice can also form under calm conditions. When this happens, the ice crystals don't become frazzled and, instead of forming pancakes, the ice forms a smooth solid shield on the ocean surface. When the winds aren't blowing, sea ice can form this way in Arthur Harbor, as you can see in the photo below.
So, this brings me back to it being August 6th and there being no sea ice. Well, I take that back, there is some sea ice in Arthur Harbor today, but I’m not convinced it will stay. The seawater temperature has been low enough to form sea ice for about a month now and, occasionally, sea ice has begun to form. But then the wild winds from the North pick up with gusts over 100 knots and it all blows away, as you will see in the video below.
Not only do the North winds blow the sea ice away, but they bring warm temperatures and sometimes even rain. Rain. In Antarctica. In the winter! The warmer air temperatures melt the snow, which quickly freezes over as soon as the winds subside, and the temperatures drop again. This makes walking around more of an adventure than we need it to be.
It’s likely we’re seeing more North winds because of SAM. SAM is the Southern Annular Mode, and it’s a type of climate variability a bit like El Niño. It has positive and negative phases that are determined by the atmospheric pressure at mid- (40˚S) and high- (65˚S) latitudes. If atmospheric pressure is higher than the average (we refer to this as being anomalously high) over Antarctica (i.e. high latitudes), then SAM is in its positive phase, and vice versa. When SAM is positive, the westerly winds that surround Antarctic move in closer to the continent and become stronger. Because the Antarctic Peninsula juts out the way it does, the northern part of it is exposed to warmer conditions and strong northerly winds when SAM is positive. Studies have also shown that positive SAM results in a shorter sea ice season along the Antarctic Peninsula. SAM is currently positive (or at least it was in June, which is when the most recent data is available for), which might explain why we haven’t had decent sea ice yet this winter.
But, I remain hopeful that things might turn around and we'll get to experience the wonder of a completely frozen-solid ocean before we leave.