Strikingly stormy weather, science, and (a lack of) stratification

Here on Anvers Island, another stormy spring day is coming to a close. The weather system responsible for this morning’s gale came in from the north around 4 a.m., bringing wind gusts in excess of 50 knots and driving snow that had the feel of sand. The worst of the storm hit us right at 8 a.m. — the same time that Jeff and I had planned to hit the water to collect some samples for our research. We resigned ourselves to (mostly) indoor work for the day as we listened to the wind howl outside. This is what it looked like this morning on the boardwalk between the two main buildings here on station:

Given the other storms I’ve already written about this season (here and here), you might suppose the video clip is a re-post. (I assure you, the above is original footage from my trip to breakfast this morning.) As I wrote earlier this season, stretches of fair weather here on the Peninsula are really just breaks between an endless procession of low-pressure systems that circulate around the Southern Ocean, to our north.

Jeff and I are taking advantage of the “weather day” to catch up on some personal business and complete some lingering lab work from a successful on-water LTER sampling effort yesterday at stations B and E. (Yesterday’s outing was our first “two station” sampling of the season.)

At a basic level, we keep tabs on the weather for its impact on our ability to sample: Safety regulations here prohibit us from heading out onto the water when winds exceed 20 knots. But, the conditions so far this season — particularly the high winds we’ve been seeing — have also been a subject of considerable scientific interest for the members of team C-045/C-019 (see this past post for some introductions).

This has been a stormy past month here at Palmer. In the past 25 days, we’ve been swept over by six low pressure systems with sustained winds that exceeded tropical storm force — that is, greater than 34 knots. This morning’s storm (not on the plot below) makes seven such events:

A stormy spring: Wind speeds and peak gusts recorded at Palmer Station over the past 25 days.

A stormy spring: Wind speeds and peak gusts recorded at Palmer Station over the past 25 days.

I took a quick look at archived Palmer Station weather data from the past six years to get a sense of whether we’ve had an anomalously high number of storm events so far this year. I confined my search to the number of unique storm events with winds ≥ 40 knots, or 46 miles per hour. The data seem to suggest we aren’t crazy in thinking this spring has been worst than most in recent memory. Over the same 25-day period in the past six years, Palmer has borne the brunt of just eight or nine storm events with sustained winds ≥ 40 knots:

Wind speeds recorded at Palmer Station over the past six years, for the period Oct. 28 to Nov. 22. This is the same time interval as in the plot above. The record from each year (2009-2014) is plotted in a different color. Only eight or nine storms with winds ≥ 40 knots were observed in six years over this particular 25-day period.

Wind speeds recorded at Palmer Station over the past six years, for the period Oct. 28 to Nov. 22. This is the same time interval as in the plot above. The record from each year (2009-2014) is plotted in a different color. Only eight or nine storms with winds ≥ 40 knots were observed in six years over this particular 25-day period.

This year, we’ve had seven such events over the same 25-day period. The data I’ve introduced here aren’t conclusive, of course — but they do give the general sense that this spring has been a bit unusual. As Jeff points out, this is consistent with the current positive state of the Southern Annular Mode, or SAM. Jeff has explained quite a bit about the SAM — and its hypothesized influence on the climate and ecosystem of the West Antarctic Peninsula — here and here.

Among the many scientific reasons we — and almost all oceanographers, for one reason or another — are interested in the wind is its effect on the physical structure of the water column. In many locations in the ocean, seasonal or persistent stratification of the water column — that is, separation of less dense water at the surface of the ocean from deeper water that is less dense — is what sets the stage for algae or photosynthetic bacteria to bloom. (Wikipedia has a good definition of bloom, here; I was told my use of the term in a previous post might have been confusing to some non-oceanographers!) The strength and extent of surface wind largely determine how deeply the water column gets mixed; wind, therefore, represents a key constraint on stratification. In general, stronger, more frequent storms can prevent the water column from stratifying, while calm weather allows stratification to take hold.

Curious penguins and clear skies greeted us on Nov. 15 as we ventured out to our ice station in Arthur Harbor. Abnormally stormy weather has prevented the water column from stratifying, but still has not blown away all the sea ice to the north of Palmer Station. We hope to sample from the sea ice again in the next few days.

Curious penguins and clear skies greeted us on Nov. 15 as we ventured out to our ice station in Arthur Harbor. Abnormally stormy weather has prevented the water column from stratifying, but still has not blown away all the sea ice to the north of Palmer Station. We hope to sample from the sea ice again in the next few days.

A brief attempt to explain what I’m talking about:

Water density in the ocean is determined by salinity and temperature. If water at the surface of the ocean is allowed to heat up enough that it can become a distinct, separate layer “floating” atop the rest of the ocean, it can provide an incubator of sorts for the phytoplankton who are lucky enough to find themselves “stuck” there. If there are sufficient supplies of certain nutrients like phosphorus and nitrogen in this layer and there continues to be enough, phytoplankton living there can grow. When this growth is rapid, we call it a bloom.

We are waiting for things to set up like this in the waters adjacent to Palmer Station. We’ve been hoping to collect samples during a bloom here, both because we’re interested in tracking the bloom itself and because several experiments we have planned require water samples rich in algae.

The water at the surface of the ocean here doesn’t ever get “warm” by human standards, but it can heat to 5 or 6 degrees above the temperature of the rest of the water. This is more than enough to set the stage for growth.

To a certain extent, throwing a little wind into the picture isn’t a bad thing. Sure, it can temporarily break down the separation between this distinct surface later and the rest of the water below. If you’re one of the plankton unlucky enough to be driven down into the deeper ocean by the mixing of the wind during such an event, you’re likely done for. But, this mixing can bring critical nutrients up from the deep after concentrations have been depleted by all the plankton feasting in the warm surface water.

However, we’ve been wondering whether there’s been too much wind this season here at Palmer. Our hunch was that all this wind might be mixing the water too frequently and too severely, thus preventing the water column from stratifying to the extent necessary for a big bloom. Jeff decided to ask Nicole, our resident physical oceanographer, whether this could be true. (I introduced Nicole in an earlier post, here.) Sure enough, after some digging, Nicole reported that storms of the sort we’ve been seeing are capable of mixing and homogenizing the water all the way down to 50 meters. That’s 164 feet!

With a new storm every 4-5 days, it’s difficult to imagine the water will remain undisturbed for long enough for the necessary warming to take place.

An elephant seal blocked traffic on station on Nov. 15.

An elephant seal blocked traffic on station on Nov. 15.

That doesn’t mean there isn’t still lots of interesting biology and chemistry (and physics — sorry Nicole!) going on right now, but it means we’re not yet witnessing the even for which we have, in large part at least, been waiting. For my own project, I’ve been hoping to witness such a bloom coincide with the abnormally high doses of ultraviolet radiation we’ve been receiving here this spring. It could still very well happen in the next month, and I’ll be sure to report back when and if it does.

Meanwhile, the seals, penguins, and other birds we’ve had here on station this season do not seem to be preoccupied with the lack of stratification. Perhaps this seal’s choice of resting place, which put a halt to traffic here on station a few afternoons ago, was an expression of displeasure.

There were other some perplexing interspecies encounters here on station this past week. These Gentoo penguins (and one Adélie — see if you can spot it) took some time out of the water to conduct full, bumper-to-bumper inspection of a boat trailer and utility vehicle here by the boat ramp:

Not sure if we passed muster. The persistent presence of this group of penguins here on station over the past three weeks has me convinced that we’ll be receiving a follow-up visit in the very near future.

Lastly, Saturday, marked the 100th anniversary of the date on which Ernest Shackleton’s ship Endurance was finally crushed by the ice in which it had been beset for nearly a year. Jeff has some perspective here.

A few other images from the past 10 days on and around station:

Penguins on the sea ice in Arthur Harbor.

Penguins on the sea ice in Arthur Harbor.

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Strikingly stormy weather, science, and (a lack of) stratification — 1 Comment

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