Sunday, February 5, 2012

climate findings

R. Jaiser, K. Dethloff, D. Handorf, A. Rinke, and J. Cohen have found out how global warming causes colder winters.

Their "Impact of  sea ice cover changes on the Northern Hemisphere atmospheric winter circulation" is now in Tellus A 64 (2012): 11595.  The abstract is here.  The full text is here.

Here are excerpts, with the gist in parentheses: 

"The observed decrease in Arctic summer sea ice cover over recent decades is likely due to a combination of decadalscale variability in the coupled ice-ocean-atmosphere-land system and radiative greenhouse gas forcing ... [others] analysed the Arctic sea ice cover changes in the Fourth IPCC Assessment Report model simulations and demonstrated that the observed sea ice retreat is much faster than in the model mean. Their results suggest that Arctic sea ice influences the formation of mid-latitude teleconnection patterns and especially the NAO mode." (p.1-2)

(... which means that the ice melts because of our emissions plus the usual climate wave over the arctic,which is in the warm phase, so it's double trouble; that the melt seen by 2011 is faster than the melt we thought in 2007 we would see; and that there's a link between polar ice melt and weather down south.)
"We study the connection between atmospheric planetary waves and baroclinic cyclonic systems in winter, both influenced by Arctic heat anomalies in autumn following low sea ice concentrations and their impact on large-scale circulation changes. [Colleagues] found a temperature amplification above the surface and concluded that changes in meridional atmospheric heat transport may be an important driver for the recent Arctic temperature amplification. By diagnosing the non-linear connections between the Arctic sea ice cover, planetary waves and synoptic storm tracks during winter, the influence of sea ice concentration changes on atmospheric circulation changes has been identified." (p.3)

(... others measured that arctic temps are getting more extreme and blamed heat arriving from down south.  We looked at the polar melt, the long-term seesaw, and storms, and saw that polar melt informs winter weather.)

"For our sensitivity studies, we selected two consecutive winter periods 1990-2000 and 2001-2010. The first 11-year period, 1990-2000, is chosen as a time slice with significantly larger sea ice concentration in the preceding late summer (with a mean value in the Siberian domain of 0.57), referred to as the high ice phase. The second 10-year period, 2001-2010, represents less sea ice concentration (mean value of 0.45), referred to as the low ice phase." (p.3)

(... we looked at the winter weather over the past two decades; the difference between the 1990s and the 2000s is that now there's far less ice up north than there used to be.) 

"A maximum covariance analysis (MCA; von Storch and Zwiers, 1999, see Appendix for details) is used to describe the relation between the fields of averaged August/September sea ice concentration and mean sea level pressure or 500 hPa geopotential heights in the consecutive autumn or winter over the whole time period 1989_2010. The results of this analysis method are pairs of patterns and associated time series for each climatic field, which are coupled through a maximised co-variance of their associated time series. The figures show the pairs of patterns that are expected to occur simultaneously." (p.3)

(... and we found a pattern.)

"The winter 500 hPa geopotential differences between low and high ice concentration ... exhibit pronounced changes over high and middle latitudes. The lower/higher values of geopotential heights over the Arctic Ocean are associated with higher/lower sea ice concentration during the analysed period. In contrast, positive/negative geopotential anomalies are observed over the Atlantic and Pacific Oceans during the high/low ice phase. Similar changes in the mean sea level pressure fields between both periods are shown in Fig. 2b consistent with the more frequent occurrence of negative AO pattern in the later period." (p.3)

(... which is this: lows in the arctic happen when there's more ice, arctic highs happen when there's less ice.  So now there are more high pressure zones in the arctic air because of the melt.  When there's more ice, there are more highs down south, and when there's less ice, there are more lows down south.  BTW: "hPa" is hectopascal, which measures air pressure.  And "geopotential height" refers to how high in altitude some air pressure level is, as measured from mean sea level up.)

"Comparing both figures, a decrease in atmospheric stability during the low ice period is visible. The difference plot ... shows this reduction in the middle and lower troposphere in autumn continuing until December. Eady growth rates are larger in autumn compared to winter as seen ... Furthermore, they are increased in the low ice period ... This is partly due to decreased atmospheric stability and therefore baroclinicity rises in the later period. [Another figure] shows this increase beginning in late September continuing to November. Meridional baroclinic heat fluxes are enhanced around the beginning of October ... Additional oceanic heat uptake during summer is rapidly returned to the atmosphere during the following autumn. This heat release to the atmosphere in autumn is connected to an earlier onset of baroclinic instability because of static stability and Eady growth rate changes during the low ice phase. Low sea ice concentration is associated with higher temperatures in the lower troposphere in the polar region (658_808 N) peaking at 758 N (indicated by a negative correlation). The temperature increase … reduces the vertical static stability of the lower Arctic atmosphere. ... the Arctic atmosphere remembers the summer sea ice concentration reduction through a warming and de-stabilisation of the lower troposphere. The positive correlation between vertical static stability and sea ice concentration in the Siberian domain ... demonstrates that reduced vertical stability is connected with less sea ice concentration. Because the onset of baroclinic instability is proportional to the strength of the vertical static stability of the atmosphere, reduced stability leads to an earlier onset of unstable baroclinic systems in the Arctic troposphere. To examine the impact on baroclinic systems in more detail, we plot ... the correlation between winter Eady growth rate and late summer sea ice concentration in the Siberian domain. The effect of the changed meridional temperature gradient dominates. The correlation indicates an enhanced baroclinicity north of 758 N in winter for less sea ice in summer, whereas the Eady growth rate between 608 and 708 N is decreased. This may indicate a shift in the occurrence of storms and cyclones to the north. [Colleagues] analysed the JRA-25 atmospheric dataset and detected a shift towards stronger and more frequently occurring cyclones in the Atlantic sector of the Arctic." (p.5-6)

(... as the polar melt goes on, the atmosphere up there gets unruly, which is worst in fall.  In the summer the ice-less arctic ocean soaks up heat, and as soon as it gets cold in fall, the ocean radiates this heat back into the atmosphere.  The less summer ice there is to reflect heat back during summer, the more heat is being soaked up, and the more of this soaked-up heat radiates when fall temps drop, the sooner the fall atmosphere gets unruly.  When winter comes, there are then more storms.)

"[The concluding para of the penultimate section:] We showed that the initial response of the atmosphere to reduced sea ice concentration in late summer is baroclinic in autumn, which changes to barotropic in winter and triggers changes in the large-scale planetary wave trains over the Pacific. These findings are supported by Honda et al. (2009) who demonstrated in an atmosphere-only model that zonally propagating cold atmospheric anomalies from Europe to the Far East in late winter are correlated with the Arctic sea ice cover decrease in the preceding summer-to-autumn seasons." (p.9)

(The winter storms up north bring cold weather down south.)

And the conclusion:

"We showed that Arctic heating anomalies due to low sea ice concentrations in late summer (August/September) trigger changes in baroclinic systems in autumn because of an earlier onset of baroclinic instability that influences the structure of large-scale planetary waves in the following
winter. The baroclinic structure of the direct response in autumn is linked to different patterns of pressure anomalies at the surface and in the mid-troposphere, which are related to the decrease in sea ice concentration. Decreased static stability and changed meridional temperature gradients
induce an earlier onset of baroclinicity north of 758 N with greater amplitude. Winter heat fluxes on baroclinic scales are increased in the whole Arctic troposphere, whereas a non-linear adjustment leads to decreased heat fluxes associated with planetary waves. Arctic EP fluxes due to planetary waves during winter are enhanced between 700 and 200 hPa in the latitudinal belt north of 658 N during the low sea ice phase. The barotropic structure of the atmospheric response in winter is connected to similar patterns of pressure anomalies at the surface and the mid-troposphere. The pattern over the North Atlantic relates the sea ice decline in late summer to a negative NAO phase in winter. The barotropic pattern over the Pacific due to changes in Arctic sea ice concentration is connected to a distinguished planetary wave train over the region. These results deliver a dynamical background for understanding the role of Arctic sea ice decline on the Arctic temperature amplification and its impact on mid-latitudes contributing to the recent shift to the negative NAO phase. The reduced sea ice concentration at the end of the Arctic summer has the potential to change the largescale circulation in the following winter that could feed back on the sea ice concentration. This sea ice_atmosphere relationship suggests a potential for use in operational Northern Hemisphere seasonal forecasts. Sea ice cover loss has the potential to preferentially shift the probability density function of the AO/NAO to the negative phase, in agreement with the investigations by Overland and Wang (2010). The results of the present study showed the large influence of enhanced baroclinicity on planetary waves similar to the connection between snow cover anomalies and the large-scale atmospheric circulation as shown by Cohen et al. (2007). Further investigation is needed to examine the impact of enhanced baroclinic systems on snow anomalies in the Siberian region. It is plausible that both processes are closely related." (p.9-10)

Jaiser et al refer to Honda (2009); the research is Honda, M., Inoue, J. and Yamane, S. 2009. Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters. Geophys.
Res. Lett. 36, L08707. 

They also refer to Francis (2009); the research is Francis, J. A., Chan, W., Leathers, D. J., Miller, J. R. and Veron, D. E. 2009. Winter Northern Hemisphere weather patterns remember summer Arctic sea-ice extent. Geophys. Res. Lett. 36, L07503.

I didn't see Jaiser et al. talking much about Petoukov/Semenov (2010)'s awesome paper, but the bibliography includes it: Petoukhov, V. and Semenov, V. A. 2010. A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents. J. Geophys. Res. 115, D21111.  An earlier blistered orb post on that particular paper is here.

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