UPDATE - The source of global sea ice information cited in this posting was NOAA's National Snow and Ice Data Center (NSIDC). They are now reporting issues with the satellite data used to produce these images and this information was not known at the time of the writing of this article. Specifically, the vertically polarized 37 GHz channel (37V) of the Special Sensor Microwave Imager and Sounder (SSMIS) on the Defense Meteorological Satellite Program (DMSP) F-17 satellite that provides passive microwave brightness temperatures is evidently providing spurious data. As a result, the sea ice measurements and any related conclusions in this article are in question. We'll continue to monitor the situation and update as more details emerge. For now, NSIDC has suspended daily sea ice extent updates until further notice.
El Nino strengthened significantly during 2015 and peaked in December as one of the strongest such episodes in the past fifty years. Even though El Nino is a phenomenon characterized by unusually warm water in the equatorial Pacific Ocean, it can have ramifications around the world. In fact, global temperatures spiked during the last half of 2015 as a result of the strong El Nino and were still at very high levels relative-to-normal as recently as last month. In addition, global sea ice appeared to be impacted by El Nino as it took a steep dive during much of 2015 and remained at well below-normal levels going into this year. In the past couple of months, however, El Nino has begun to collapse and will likely flip to a moderate or strong La Nina (colder-than-normal water) by later this year. In rather quick and surprising fashion, global temperatures have seemingly responded to the unfolding collapse of El Nino and global sea ice has actually rebounded in recent weeks to near normal levels.
Global temperatures and global sea ice
The plot above shows global temperature anomalies from the latter part of 2014 through April 11, 2016 using NOAA’s Climate Data Assimilation System (CDAS). As El Nino began to strengthen dramatically during 2015, global temperature anomalies (black line) climbed significantly and that climb lasted right into early 2016. In recent weeks, however, as the collapse of El Nino has begun, global temperature anomalies have apparently responded by dropping rather sharply (indicated on plot above with arrow).
Meanwhile, global sea ice which had hovered relatively close-to-normal from 2013 into 2015, dropped sharply during the second half of last year to well below-normal levels as El Nino strengthened and global temperatures spiked (as indicated by left arrow on above plot). The plot of daily global sea ice anomalies (red) shows that in recent weeks global sea ice has surged back to near normal as the collapse of El Nino has unfolded [right arrow on above plot; source: University of Illinois "cryosphere", NOAA's National Snow and Ice Data Center (NSIDC)].
Looking on a longer-term horizon, the plot above shows global sea ice area over the period of 1974-2015 using the "Hadley Centre Sea Ice and Sea Surface Temperature Dataset" (HadISST). 1974 is a reasonable starting date for showing global sea ice levels as it is the first year with complete data for both polar regions. Data for the Antarctic was not available before the advent of satellite-based imagery in 1973. To understand the variations in the global sea ice area, the signal was “decomposed” by removing the repeating seasonal component of the data. It was determined that the trend was equal to -0.02+/- 0.19 million square kilometers per decade (p-value=0.367) which led to the conclusion that “there is no significant trend at all in the 40+ years of satellite data”. In fact, in an interesting twist, the recent analysis found that the global ice area remained stable throughout the 1980s and the 1990s, while temperatures climbed suggesting “the global sea ice area is not particularly a function of the global average surface temperature.” [Source: Willis Eschenbach/"Watts Up With That" web site]
Northern Hemisphere sea ice
The northern hemisphere sea ice areal extent is still below-normal relative to all years going back to 1979 although it is well above the lowest point set during 2012 and even above levels seen earlier this year. The northern hemisphere sea ice areal extent is currently around 1 million square kilometers below-normal using the base period of 1979-2008 for comparison. The directional shift in the sea ice areal extent trendline that developed during the mid 1990’s in the northern hemisphere correlates quite well with a northern Atlantic Ocean sea surface temperature cycle that is tracked by meteorologists through an index called the Atlantic Multidecadal Oscillation (AMO). Indeed, Atlantic Ocean sea surface temperature anomalies play a critical role in the overall northern hemisphere sea ice areal extent. The AMO index flipped in phase during the mid 1990’s (indicated by arrow on above plot) from negative (cold) to positive (warm) and the sea ice areal extent trendline changed direction right around that point in time. In the time period before the mid 1990’s, the sea ice areal extent was generally above-normal dating back to 1979.
Resiliency of Arctic sea ice
In the last ten years, the Arctic sea ice has actually shown great resiliency albeit at below normal levels. The Arctic sea ice extent has been on a sideways trend (above) in the last ten years or so.
In addition to sea ice extent, an important climate indicator to monitor is sea ice volume as it depends on both ice thickness and extent. Arctic sea ice volume cannot currently be observed on a continuous basis as observations from satellites, submarines and field measurements are all limited in space and time. As a result, one of the best ways to estimate sea ice volume is through the usage of numerical models which utilize all available observations. One such computer model from the University of Washington is called the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) and it is showing an upward trend in Arctic sea ice volume since the low point was reached in 2012 following a long downtrend (circled area above).
Southern Hemisphere sea ice
The rather remarkable period of consistently higher-than-normal sea ice areal extent in this part of the world ended in 2015 most likely due at least in part to the fact that El Nino warmed the globe. Back in 2011, the southern hemisphere sea ice areal extent was still at below-normal levels, but it then surged into 2015 reaching record high levels. However, during 2015 as El Nino strengthened in the tropical Pacific, the southern hemisphere sea ice extent actually dropped sharply into below-normal territory. As with global sea ice, there has been a surge in recent weeks and the Southern Hemisphere sea ice is now actually back into above-normal territory.
Meteorologist Paul Dorian