Many thanks to Rachael Mueller (College of Oceanic and Atmospherice Sciences, Oregon State University) and Jenny Rock ( School of Biological Sciences, Bangor University) for writing this piece about the Physical Ocean. Thanks to the Association of Polar Early Career Scientists, APECS, for connecting us all.
The dramatic landscape of the polar regions has been captured in breath-taking images that show a unique combination of mountains, ice and surreal lighting. More difficult to see, however, is the complexity of the surrounding oceanic environment. The reward of looking through a portal into the polar oceans is a discovery of an environment that is just as exotic and intoxicating as the surrounding landscape above water.
Oceanographic data shows ice crystals forming at depths, temperature and salinity variations causing the oceanographic "winds", and water temperatures that can be colder than your freezer's ice cubes due to the effects of salt and pressure. In this environment, the light may be diffused by ice on the surface and the sounds are enlivened by the noises of crackling ice, which comes in many forms from newly forming ice on the surface (Shuga, Frazil, or Pancake to name a few forms of sea ice) to ice that has been forming over thousands of years to create 1-2 km thick shelves that spill out over the oceans (continental ice) and cap the surface waters.
The interplay between temperature and salinity in the polar oceans has global consequences to climate through its effects on ocean circulation. Similar to the oceanographic phenomenon known as El Nino and La Nina, changes in surface temperature and salinity in the polar oceans has non-local consequences. For example, paleoclimate evidence suggests that rapid discharge of glacial ice or water in the North Atlantic has induced abrupt, global climate change (1). Although the influence of polar oceans on regulating deep water formation is thought to be a significant climate control, these feedbacks are still poorly understood.
We are currently living the experiment of feedbacks between polar ocean circulation and global climate. What is evident is that the polar oceans are changing.
Photo from Plates & Gates IPY project
Significant and sustained increases in surface water temperature have now been documented in multiple oceanic regions, but polar seas are undoubtedly experiencing the most significant change. The most dramatic thermal shifts have been recorded in the Southern Ocean, including regions of the Scotia and Weddell Seas(2-4). In the last 50 years, summertime shelf water temperatures of the western Antarctic Peninsula (WAP) have increased by ~1.3ºC, and at South Georgia the increase is as much as 2.3ºC. Even deep waters of 700-1000m within the Antarctic Circumpolar Current are warming at a rate much higher than the global average.
Figure 1: Trends in summer sea temperature during 1955-1998. Four different depths are shown, with areas of no data left in white (from Meredith & King 2005).
The mechanisms responsible for the Southern Ocean warming are not yet fully understood, however, most theories relate this warming to the strengthening of the circumpolar westerly winds, associated with the upward trend in the Southern Annular Mode (SAM). This trend is known to be strongly driven by anthropogenic influences, including greenhouse gas emissions and ozone depletion, and there is thus general agreement that the Southern Ocean warming is due at least in part to human activities. The current ocean warming trend is driven by, and expected to progress in concert with, continued atmospheric warming and sea ice retreat. such that summer surface waters over the WAP shelf are predicted to continue their significant warming in the next few decades. While the Southern Ocean has historically experienced fluctuations in environmental temperature, the present speed of atmospheric warming is unprecedented across recent geological time-scales.
Change in ocean temperature has a direct influence on society through its affect on sea level rise.
Beneath the sea ice and the continental ice in the oceans of the Arctic and Antarctic, a turbulent environment mixes water of different temperatures and salinity towards or away from the ice to introduce either melt or freeze conditions. As ocean currents change course, similar to a change in wind, cold(warm) waters may become warm(cold) and introduce more melt(freeze).
Plate-ice, Qaanaaq, Greenland, Lars Poort
Ocean-caused melting impacts the speed at which glaciers run towards the sea: More melt and thinner ice shelves leads to faster ice streams (5-8). It is estimated from observations that for every 1º increase in ocean temperature, the melt rate under the ice-shelf increases by 10 m/yr (9), and more melt leads to thinner and less stable ice shelves. As a result, the warming oceans have a significant influence on the stability of Antarctic ice shelves.
The last time an ice shelf collapsed along the Antarctic Peninsula, the upstream ice flowed 2-6x more quickly into the oceans (7); thus increasing by 200-600% the rate at which this region contributes to sea level rise. This nonlinear feedback is so poorly understood that the Intergovernmental Panel on Climate Change (IPCC) could not include these kinds of responses in their most recent assessment of future sea level rise (10), rendering their estimates conservative. If Antarctica and Greenland continue to loose mass at their current rates, they alone will contribute 2 meters of sea level rise over the next 100 years (11). The caveat is that we don’t know how Greenland and Antarctica will change over this time period and extrapolating a current trend in this way is prone to error. Recent observations in Greenland have demonstrated that mass loss fluctuates as some ice streams have slowed, causing less mass loss; however, these recent decreases in mass loss do not reverse the 50 year trend of shrinking Greenland (11, 12) and the reversal of this trend is unlikely considering the forcing mechanisms are in support of further, accelerated mass loss. Considering these various factors, it is reasonable to anticipate that the Antarctic and Greenland Ice Sheets will continue to shrink and that non-linear feedbacks will likely cause larger changes in sea level than is reported by the IPCC assessment.
Clearly, the polar regions are changing rapidly in response to both atmospheric and oceanic changes, and we may have an opportunity to learn more about feedback mechanisms through direct observations. Some of the dramatic changes that we are experiencing include: decline in the maximum sea ice extent and thickness in the Arctic; collapse of ice shelves in the Antarctic that have existed over the oceans for thousands of years, disappearing over the course of five weeks or less (See Figure 2); and a southern migration of ecosystems along the Antarctic Peninsula as sea ice is diminishing and species are being forced south. We are living through the experiment of how these changes will affect society.
Figure 2: Collapse of the Larsen-B ice shelf, over a course of approximately 5 weeks in 2002. Images from http://nsidc.org
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