In the last post I speculated that the 2015 melt season could be another record breaker for minimum sea-ice extent across the Arctic Basin. That possibility is looking ever more likely as strong ridging patterns continue to bring anomalously warm airflow northward into the Arctic from over Alaska and Siberia (Figure 1). Particularly warm circulation across western North America is coupled to strong El Niño conditions that are also facilitating forest fires and record heat in the region (see this Wunderground blog post).
But what will September bring?
Check out the decline in sea-ice thickness 2012-2015 from the Arctic HYCOM model run by the U.S. Navy (Figure 2). The total area covered by multi-year ice (generally > 2.5 meters thick) has shrunk each year since 2012 when minium ice extent reached an unprecedented historical low. If HYCOM estimates are correct, then ice across most of the Arctic Basin is thinner now (as forecasted for July 6) than on the same day for any of the past 3 years. Conditions look dire over the Beaufort Sea where the current 7-day forecast shows a broad area of multi-year ice thinning out drastically.
In short, Arctic sea-ice cover in September at the end of the 2015 melt season ought to be interesting.
There are three notable climate events so far this year that I would like to comment on:
The first is the emergence over several months of what could become a very strong El Niño across the Pacific Ocean (Figure 1; see also this Jan 2012 - Mar 2015 animation of SST anomalies), holding the promise of weather extremes. This past winter, expansive warm surface temperature anomalies across the Pacific were linked to strong ridging over the western half of North America, facilitating continued drought in California, and record high temperatures over parts of Alaska. Downstream of the ridge, a persistent trough of cold Canadian air delivered one of the coldest winters to the northeastern U.S. in many years. February of this year set all-time cold records in parts of New England, including in Maine where the average monthly temperature registered as the coldest recorded since 1895, besting even the cold January of 1994 (see my April post on the Maine Climate News). Very warm Pacific surface waters also contributed to the May 2014 - April 2015 combined global land and ocean temperatures ranking as the warmest such 12-month period on record since 1880. Moreover, a warm Pacific has already facilitated the formation of two Category 5 typhoons, Noul (May 2-12) and Dolphin (May 6-20) in what is a very early start to the Pacific storm season.
The second notable event of this year is discussed in detail in the NOAA 2015 Atlantic Hurricane Season Outlook (see also Wunderground). In short, 2015 is the second year in a row of a predicted weak hurricane season due to cooling of North Atlantic surface waters, and other factors associated with the buildup of El Niño in Pacific that tend to inhibit development of strong Atlantic hurricanes. The conditions across the North Atlantic are so unusual that NOAA has posed an important question as to whether the warm phase of the Atlantic Multidecadal Oscillation (AMO) has come to a close. The AMO is a mode of sea-surface temperature variability with time wavelength of 60-80 years (Figure 2). AMO phases carry coupled response in the atmosphere, and a shift from warm to cool circulation regime affects global climate. The most recent warm phase of the AMO began ca. 1995, following the relatively cool decades of the 1970s and 1980s. The warm surface boundary conditions have likely played a role in driving atmospheric warming and extreme weather events observed particularly since ca. 2000. Thus, it will be a very big deal if the AMO is indeed slipping into a cool mode of operation.
This is not to downplay climate impacts from human stressors. Increasing atmospheric CO2 concentration from fossil fuel burning, depleting stratospheric ozone from emission of CFCs, and making continent-scale alterations in land surface albedo from industry and agriculture all change the radiative balance of Earth's atmosphere, and therefore cause the climate system to seek a new equilibrium. But natural patterns of variability, such as the AMO, are real, and their climate signatures are readily apparent in temperature records across the U.S. (again, see Figure 2) and worldwide. Identifying the precise origin of the AMO, and its Pacific counterpart, the PDO, remains an active area of scientific investigation with relevance to better prediction of climate evolution over the coming decades. The AMO/PDO patterns may arise in dynamical response to solar variability (by way of changes UV flux and subsequent changes in stratospheric ozone production and circulation of the middle latitude westerly winds), volcanic eruptions (ash injections into the stratosphere reflect sunlight back into space and cause a net cooling), and human modification of the chemical and physical composition of the atmosphere. See for example Ottera et al. (2010), Knudsen et al. (2014), and discussion about climate oscillations on RealClimate.org.
The third notable condition of this year that I would like to mention is the potential for record-breaking minimum Arctic sea ice extent by the end of the 2015 melt season. Here I am conjecturing. My prediction is probably wrong, but I will say it anyway.
My thinking draws from a couple observations. First, this past winter saw frequent warm-air ridging over both Alaska and Europe, bringing very warm airflow into the Arctic Basin from the Pacific and Atlantic sectors (Figure 3). An unusually warm Arctic winter leads to relatively thin ice come spring. Second, a pressure dipole, characterized by a high situated over the Beaufort Sea and a low situated over Franz Josef Land in the Barents Sea, has developed in the last few days providing ideal meteorology for ice melt: sunshine and ablation across the Beaufort, and wind-driven ice export through Fram Strait off the east Greenland (Figure 4). This dipole pattern is reminiscent of the meteorological conditions that dominated the 2007 melt season, leading to a partial collapse of the sea-ice cap, and likewise record minimum ice extent with no known historical analog. (View daily reanalysis weather maps for the Arctic 1979-present right here on climate Reanalyzer). The current 7-day GFS model forecast shows a breakdown of the pressure dipole as the low over Franz Josef Land strengthens and moves poleward (Figure 5a). This storm is being steered by a pronounced loop in the jet stream that has brought an early-season heat wave to Siberia (Figure 5b,c). The storm is also reminiscent of a major Arctic cyclone in early August 2012 that broke up thin late-season ice, and likely contributed record minimum ice extent attained a month later (e.g., see Neven's Arctic Sea Ice Blog).
This is early June, not early August; the ice remains consolidated, and the impending storm is unlikely to have much direct impact. But the appearance of a major storm is rare during the Arctic summer. And preceding appearance of a defined pressure dipole between the Beaufort Sea and Franz Josef Land is a bit unsettling. And let's not forget the extreme opposing conditions across the Pacific and North Atlantic this year. And the record warm global average temperature for May 2014 - April 2015. Regardless of whether the AMO has begun descending into a cool phase, this summer could see record melt across the Arctic. I hope that I am wrong — we'll known come September.