The northern hemisphere sea ice areal extent has increased significantly from last year’s record low amount and is above four other recent years (2007, 2008, 2010, 2011), but it remains below normal for this time of year. Meanwhile, the southern hemisphere continues at above normal levels and recent measurements suggest it has reached the highest level in decades. On a global basis, sea ice areal extent is currently relatively close to normal with the northern hemisphere near its low point for the year (winter approaching) and southern hemisphere near its high (summer approaching). Oceanic cycles play a major role in weather and climate and can also have a significant impact on sea ice in the polar regions and we’ll focus in on that in this discussion with respect to the Arctic region.
(Courtesy University of Illinois "cryosphere")
Two distinct trend lines can be seen in the northern hemisphere sea ice areal extent dating back to the beginning of satellite era observations in 1979 and there seems to be a direct connection with an oceanic cycle in the north Atlantic involving sea surface temperatures. The Arctic region north of the Atlantic Ocean is open to the warmer waters from the south because of the way the ocean currents flow. These warmer waters can flow into the Arctic and prevent sea ice from forming in the North Atlantic. The northern Atlantic Ocean sea surface temperatures are tracked by meteorologists through an index called the Atlantic Multidecadal Oscillation (AMO) and the two distinct trend lines for northern hemisphere sea ice areal extent are quite well correlated with a flip in phases of the AMO in the mid 1990’s from negative (colder water) to positive (warmer water). Specifically, the northern hemisphere sea ice areal extent featured a general sideways trend at above normal levels from 1979 to the mid 1990’s and then, following that flip in the AMO, there has been an overall downward trend to the current below normal values. These oceanic cycles can last for two or three decades and I believe that when the northern Atlantic sea surface temperatures flip back to cooler-than-normal values – perhaps 5 or 10 years from now - the northern hemisphere sea ice areal extent will return to normal or above normal levels - much like it was before the mid 1990’s while the AMO was negative.
In addition to “areal extent”, “volume” is an important parameter to monitor with respect to sea ice to determine possible long-term climatic trends, but it is harder to measure. In fact, there are no Arctic-wide or Antarctic-wide measurements of the volume of sea ice, but it can be estimated for the Arctic using the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) developed at the University of Washington Applied Physics Laboratory/Polar Science Center. PIOMAS blends satellite-observed sea ice concentrations into model calculations to estimate sea ice thickness and volume and comparisons with submarine, mooring, and satellite observations help to increase the confidence of the model results. In general, the overall trends seen for the modeled sea ice volume in the northern hemisphere seem to correlate pretty well with the observed trends seen for areal extent dating back to the beginning of the satellite era. Namely, similar to sea ice areal extent, the volume features a general sideways trend at above normal levels from 1979 to the middle 1990’s and then a downward trend began at that time when the AMO flipped to a positive phase. We’ll continue to monitor all of this at thesiweather.com over the weeks and months to come as we head into the northern hemisphere winter season.