In 2 and a half months, and unless there’s another government shutdown, I’m heading down to Palmer Station to collect a key set of samples for one of my projects. The idea is to time this sampling effort with the spring diatom bloom. This bloom is a critical pulse of fixed carbon into the ecosystem after the dark (sub)polar winter, and is followed by a series of blooms by lesser phytoplankton players – cryptophytes, dinoflagellates, and Phaeocystis. The problem with this plan is that it is pretty much impossible to guess when the spring bloom will happen. Ecologically speaking it happens as soon as the ice retreats (or rather, as the ice is retreating), but this can vary by many weeks from one season to the next. The problem is compounded by the sampling strategy at Palmer Station. Scientists at the station rely on zodiacs for sampling; those ubiquitous inflatable craft that, while surprisingly durable, are pretty useless in even very light ice conditions.
So one thing we would very much like to know is what the ice conditions will be like in mid October when we arrive on station. A good place to start a discussion of likely ice conditions is, surprisingly, the tropical Pacific. The tropical Pacific is a mess right now. There’s a tremendous amount of heat in the surface ocean and cyclones have been pinging around the South Pacific for the last few weeks like a bunch of bumper cars. This is the result of a strong El Nino taking shape, possibly the strongest we’ve seen in over a decade.
The El Nino Southern Oscillation (ENSO) is a tropical phenomenon with global consequences. One of these is reduced sea ice extent along the West Antarctic Peninsula (WAP). During an El Nino the polar jet (the analogous to the northern hemisphere’s jet stream) is weakened and there is less transport of heat from the subtropical Pacific to the WAP. During La Niña the opposite happens; the jet strengthens, driving warm, wet storms south across the Southern Ocean to the WAP. The strong winds break up the ice and the heavy snow and rain has a pretty bad effect on Adélie penguin chicks, occasionally causing total breeding failures.
ENSO’s not the only pattern of climate variability with an impact on Antarctic sea ice, however. The Southern Annual Mode (SAM), also called the Antarctic Oscillation (AAO), has a major impact on sea ice extent. Unlike ENSO, which is the result of complex dynamics between the atmosphere and the ocean, SAM is primarily an atmospheric phenomenon linked to the magnitude of the north/south pressure gradient across the Southern Ocean. This differential controls the strength of westerly winds that help deliver subtropical heat to the West Antarctic. SAM has two phases; positive and negative, and can hold one phase for weeks or months, then suddenly shift. During its negative phase SAM is correlated with reduced westerly winds and increased sea ice along the WAP (and happy penguins).
Right now SAM is in a positive phase, and has been for some time. But we’ve also got an uber El Nino. So what does that mean? I asked Sharon Stammerjohn, a physical oceanographer with the Palmer LTER project, what happens in these situation. Sharon and several colleagues wrote a paper in 2008 exploring the impact of SAM and ENSO on Antarctic sea ice extent. It’s pretty clear what happens if a La Niña lines up with a positive SAM (low ice year), or an El Nino coincides with a negative SAM (high ice year). But what about a positive SAM and a strong El Nino? In that case it can, apparently, go either way. Either the strong subtropical storm effect will overcome the weakened polar jet or it won’t. To get a sense of which is winning so far this year we can take a look at the the NSIDC’s current map of Antarctic sea ice extent.
Ouch, take a look at the northern tip of the WAP. The pink line is, as the figure indicates, the median sea ice edge. Clearly El Nino is winning; most of Antarctica is normalish, but the WAP region has some extraordinary ice cover right now. Sea ice has been more or less on the decline there for the last couple of decades, it will be very interesting to see what kind of impact this has on the ice-dependent WAP ecosystem. Of course sea ice that far north is pretty fickle; the SAM could switch modes, or the westerlies could increase, and the sea ice could breakup and move out before spring. Otherwise it looks like we might be sampling from the Palmer dock…