Feature Articles—September 2009 Issue
North Atlantic SST Impacts Ice Extent in the Kara and Barents SeasData Analysis Shows Slow Oscillation in Ice Extent Followed By Average SST in the North Atlantic Throughout the 20th Century
By Oleg Pokrovsky
Principal Scientist
Main Geophysical Observatory
St. Petersburg, Russia
Many climatologists consider the rapid reduction in the Arctic Ocean ice extent that began in the 1980s as an early sign of global climate change. However, surface air temperatures at Arctic meteorological stations did not exhibit any considerable positive linear trend during the last half century, at least in winter seasons. On the other hand, the heat capacity of the ocean exceeds that of the atmosphere by many hundreds of times, so it is reasonable to assume that the contribution of incoming warm ocean waters is essential to the Arctic ice melting process.
In particular, a considerable amount of warm Atlantic water enters the Arctic Ocean and plays an important role in shaping its circulation regime. Warm Atlantic waters enter the Arctic Ocean through the eastern Fram Strait and the Barents Sea and form an intermediate layer as they move below colder, fresher (less dense) Arctic surface water. These inflows may promote ice melting and discourage ice growth.
Atlantic Multidecadal Oscillation
The climate swings of the Atlantic Multidecadal Oscillation (AMO), most evident in and around the North Atlantic, take roughly 60 years to complete. Most recently, thermometers picked up a swing early in the 20th century from abnormally cold to unusually warm and back. The width of the rings of trees near the Atlantic coast record similar swings in climate, going back several hundred years.
The average temperature of the North Atlantic is subject to slow oscillations. The AMO is an ongoing series of long-duration changes in sea surface temperature (SST), with cool and warm phases that may last for 20 to 40 years at a time with a difference of about 0.5° C between extremes. These changes are natural and have been occurring for at least the last 1,000 years.
The AMO variability is pronounced: Its range (0.49° C) is larger than either the range of interannual to decadal variability (0.46° C) or the integrated trend over the period 1870 to 1999 (0.38° C). The index shows persistently warm (pre-1900 and 1930s to 1950s) and cool (1900s to 1920s and 1960s to 1980s) phases typically lasting a few decades, as well as the onset of a warm phase in the 1990s.
Is the AMO a natural phenomenon, or is it related to global warming? Instruments have observed AMO cycles only for the last 150 years, which is not long enough to conclusively answer this question. However, studies of paleoclimate proxies, such as tree rings and ice cores, have shown that oscillations similar to those observed have been occurring for at least the last millennium. This is clearly longer than modern man has been affecting climate, so the AMO would appear to be a natural climate oscillation. In the 20th century, the climate swings of the AMO have alternately camouflaged and exaggerated warming from greenhouse gases, making the atmospheric causes of global warming more difficult to ascertain.
The AMO index is correlated to air temperatures and rainfall over much of the Northern Hemisphere, in particular North America and Europe. The AMO itself is associated with changes in the frequency of North American droughts and is reflected in the frequency of severe Atlantic hurricanes. It alternately obscures and exaggerates the global increase in temperatures due to human-induced global warming. Recent research suggests that the AMO is related to the occurrence of major droughts in the Midwest and Southwest of the United States. When the AMO is in its warm phase, these droughts tend to be more frequent or prolonged. The opposite occurs during the negative AMO (cool phase).
The U.K. Met Office Hadley Centre for Climate Prediction and Research has developed a climate model that produces a rather realistic AMO with a period of 70 to 120 years. Moreover, the AMO model persists throughout the 1,400-year run. Thus, the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries. Its principal expression is in the SST field and related to variability in the oceanic thermohaline circulation. Judging by the 1,400-year simulation, researchers predict that the conveyor will begin to slow within a decade or so. Subsequent slowing would offset—although only temporarily—a “fairly small fraction” of the greenhouse warming expected in the Northern Hemisphere in the next 30 years. Likewise, researchers predict more drought-prone summers in the central United States in the next few decades.
Analysis Methods and Data Sets
This article aims to investigate a link between the AMO and the Arctic ice extent during the previous century. It is necessary to keep in mind that the ice extent in the Russian margin seas is a very sensitive characteristic with regard to total Arctic ice extent value and Atlantic water temperatures. Models are not reliable enough to reproduce ice concentration in the Arctic Ocean with appropriate accuracy. Therefore, this research implemented sophisticated methods for climate time series analysis, including a new smoothing algorithm based on Wahba’s cross-validation, Cleveland’s local polynomial approximation, Tikhonov’s regularization and Morlet’s wavelet analysis.
This study used the following data sets: the AMO series for 1856 to 2007, prepared by the NOAA Atlantic Oceanographic and Meteorological Laboratory, and ice extent in the Russian marginal seas for 1900 to 1999, prepared by the Arctic and Antarctic Research Institute.
Systematic aircraft and ship observations of sea ice from the Kara Sea to the Chukchi Sea only began in 1932, when the Northern Sea Route was created, and there were information gaps from 1942 to 1945 (during World War II). The missing data have been reconstructed using statistical regression models relating atmospheric processes (sea level atmospheric pressure gradients and surface air temperature) to ice extent. Aircraft ice-edge observations continued until 1979, when the satellite era began.
Conclusions
Other researchers have presented a smoothed AMO index (derived as the 10-year running mean of detrended Atlantic SST anomalies north of the equator) for the last 150 years, but this smoothing technique allows the remainder of the high-frequency components to be filtered out, revealing a 60-year AMO cycle in a more transparent mode. The AMO wavelet power spectrum demonstrates a very strong anomaly area corresponding to a cycle of about 60 years, confirming the result based on the AMO series smoothing. All other local maximums are much weaker. Moreover, an analysis of statistical significance showed the 60-year cycle to be a significant phenomenon at the five percent level. Similar wavelet analysis was carried out with the paleo-AMO series, permitting the conclusion that the 60-year cycle has been an inherent AMO phenomenon at the five percent statistical significance level during the last 1,400 years.
A smoothed ice extent curve for the Barents and Kara seas in September from 1900 to 2000 demonstrates slow multidecadal oscillations, similar to the AMO. Similar studies were carried out based on a more comprehensive smoothing technique, which revealed multidecadal oscillations in a more transparent mode.
In fact, the monthly ice extent shows persistent low (pre-1910 and 1940s to 1950s) and high (1920s to 1930s and 1960s to 1970s) phases, typically lasting for more than a decade. It is in agreement with the AMO smoothed curve. Periods of high AMO index magnitudes correspond to low ice extent values and vice versa.
Ice extent anomalies show a delay of a few years in relation to the AMO fluctuations. Therefore, it can be inferred that those Atlantic water temperature slow oscillations modulate corresponding Kara Sea ice extent fluctuations. The ice extent wavelet power spectrum and its comparison with the AMO wavelet spectrum confirm this important conclusion. Thus, it is the North Atlantic water temperature that is a major regulator of the ice extent in the Kara Sea. A similar result was obtained for the Barents Sea, but the ice extent values there were much lower, and respective variability was, in contrast, higher.
Natural variability, such as that associated with the AMO, the North Atlantic Oscillation and other circulation patterns, has and will continue to have strong impacts on the Arctic sea ice cover. Links between altered ocean heat transport and observed ice loss remain to be resolved, as does the attribution of these transport changes, but pulses such as those currently poised to enter the Arctic Ocean from the Atlantic could provide a trigger for a rapid transition.
Scientists are not yet capable of predicting in any deterministic sense exactly when the AMO will switch. Computer models, such as those that predict El Niño, are far from being able to do this. What it is possible to do at present is to calculate the probability that a change in the AMO will occur within a given future time frame. Probabilistic projections of this kind may prove very useful for long-term planning in climate-sensitive applications. In this respect, the wavelet tools might be useful to trace short-term climate fluctuations of various scales.
Acknowledgments
This article is based on a paper originally published in the International Climate Variability and Predictability Project Office’s newsletter, Exchanges.
References
For a full list of references, please contact Oleg Pokrovsky at pokrov_06@mail.ru.
Oleg Pokrovsky is the principal scientist at the Russian Federation’s Main Geophysical Observatory. Born in 1945, he received his B.A., M.A., Ph.D., Sci. Doctor and professor degrees from St. Petersburg State University. He has authored five monographs and more than 150 papers.
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