Global warming effects on the Arctic and Sub-Arctic Seas

Transcription

1 Global warming effects on the Arctic and Sub-Arctic Seas Jacques C.J. Nihoul Modelenvironment, University of Liège, Sart Tilman, B-4000 Liege, Belgium, Abstract After a rather hydrostatic approach to global warming (mean earth temperature increasing, ice melting, sea level raising) one came to realize that the effects of global warming were more of a hydrodynamic nature and that the ocean dynamics and its modifications in response to global warming constituted an essential factor. Taking into account the effect of global warming on ocean temperature distribution and currents contributed to a large extent to clarify the problem. The next step was obviously to include the effect of global warming on the atmospheric circulation. We would like to illustrate this point by briefly discussing the so-called North Atlantic Oscillation (NAO). The North Atlantic Oscillation (NAO) The NAO dictates much of the Climate variability from the eastern seaboard of North America to Siberia and from the Arctic to the subtropical Atlantic, especially during the boreal winter (Hurrett et al. 2003). NAO refers to a redistribution of atmospheric mass between the Arctic and subtropical Atlantic. The NAO is traditionally described by two weather maps showing the distribution of sea level pressure (SLP) over the North Atlantic in two typical educational situations: NAO + and NAO when the pressure difference between Lisbon and Reykjavik is respectively positive and negative (Figs. 1, 2). The NAO is understood to swing from one phase to another to produce large changes in the mean wind speed and direction over the Atlantic, the heat and moisture transport between the Atlantic and the neighbouring continents and the intensity and number of storms, their paths, and the associated weather. Such variations have a significant impact on the wind and buoyancy-driven ocean circulation as well as on the site and intensity of water mass transformation (Fig. 3). J.C.J. Nihoul and A.G. Kostianoy (eds.), Influence of Climate Change on the Changing Arctic and Sub-Arctic Conditions, Springer Science + Business Media B.V

2 8 J.C.J. Nihoul NAO + Russian rivers father east CHL Warmer Altantic inflow to A.O. Ice flux+ 50 NwAC Narrow Fast +LSW PRODN Small Calanus fin. stock +R AI N Storm centre in Lab-Nordic Seas L FST MAX +Westerl 50 Min. Baltic ice Max. Baltic inflow ies 65Mts NAC Warm H +Sa hara dust +Coastal upwelling +Trades cold Base map: Inst. of Geography, U. Berne Fig. 1. A schematic of the Atlantic Arctic sector under NAO positive conditions (Stenseth et al. 2004). + In NAO winter situations, enhanced westerly flow across the North Atlantic moves warm and moist maritime air over Europe, northerlies over Greenland and northeastern Canada carry cold air southwards, decreasing SST and land temperatures over the North-West Atlantic, the Labrador Sea ice extends further south

3 9 Global warming effects on the Arctic and Sub-Arctic Seas Russian rivers to Eurasian basin. +CHL NAO Ice flux50 50 H NwAC broad slow Sea ice +650k km +GSDW PRODN 40 Large Calanus fin. Stock Warm L Storm centre off US coast +300 k km Baltic Ice FST MIN LC+1 Sv 50Mts NAC +18 W PRODN Westerlies +Rain H Warm-Trades Base map: Inst. of Geography, U. Berne Fig. 2. A schematic of the Atlantic Arctic sector under NAO negative conditions (Stenseth et al. 2004). while the Greenland Sea ice boundary is found to the North of its climatological mean extent. NAO+ winters, associated with chill, dry, northwesterlies across the Labrador Sea are characterized by deep-reaching convective renewal of LSW and widespread distribution of chilled SST across the Northwest Atlantic.

4 10 J.C.J. Nihoul 6 NAO Index (December March) (L n S n ) Fig. 3. Winter (December March) index of the NAO based on the difference of normalized SLP between Lisbon, Portugal, and Stykkisholmur/Reykjavik, Iceland form 1864 through The heavy solid line represents the index smoothed to remove fluctuations with periods less than 4 years (Stenseth et al. 2004). The NAO strange attractors However, examining records of the fluctuations of the NAO index over more than a century, one realizes that the jumps from one value of the index to the next value of the same or different sign occur rather at random, strongly suggesting that the case study situations, although exemplary, do not actually describe two definite states of the atmosphere but two strange attractors: the system attracted by NAO + oscillates in its vicinity until it finds a way out and goes to NAO, oscillates in its vicinity until it finds a way out and goes back to the vicinity of NAO + (Fig. 4). The records show also a definite trend towards the positive NAO phase in recent decades and the increased attractiveness of the positive phase has often been attributed to a global warming effect. One should however be attentive to the fact that, in addition to possible jumps between NAO + and NAO (with a possible predominance of NAO + attributed to global warming) small deviations from the educational weather maps may occur as the system wanders in the vicinity of one or the other strange attractor.

5 Global warming effects on the Arctic and Sub-Arctic Seas 11 Fig. 4. An educational image of trajectones jumping from one strange attractor to the other. For instance, NAO + conditions of the most recent winters have shown a shutdown of Labrador Sea Convection. In just two and three winters, the long-sustained cooling and freshening of LSW has been largely reversed. A comparison of Atlantic SLP anomaly pattern between the period with that for shows a slight east and northeast displacement, in the more recent period, responsible for important differences to the marine climate of the West Greenland Banks and to the convective center of the Labrador Sea. The NAO is reminiscent of the pioneer work of Lorenz (1990) who, with the help of a severely truncated and simplified model of atmospheric dynamics, essentially, a layer of fluid heated from below, showed that for small values of the Rayleigh number (a non-dimensional measure of the temperature difference between the lower layer and the upper layer), heat was transported by conduction; for higher values of the Rayleigh number, convective cells appeared to transport heat, for still higher values of the Rayleigh number, two strange attractors appear and the system may jump from one to the other as described above. Further increase of the Rayleigh number, however sees the strange attractors disappear and a limit cycle appear. Could it be the same bifurcation as the one which we observed in the NAO Index in the last decades and that many authors attributed to global warming? Even if this is stretching the comparison a little far, one should remark that, in Lorenz s model, a further increase of the Rayleigh number makes away with the limit cycle and the strange attractors reappear.

6 12 J.C.J. Nihoul Further increase of the Rayleigh number generates an intermittent succession of periodic regimes and bursts of disorder (Nihoul 2007). In fact, although models like the Lorentz model and the NAO Index representation are helpful to guide one s intuition of possible bifurcations of the atmospheric dynamics they are far too rudimentary to describe the dynamics of the system where the number of spatial modes generated by nonlinear interactions can be the determinant factor (Nihoul 2007). One should thus be extremely careful in interpreting the NAO Index and presumably also in looking in its variations for global warming indications. References Hurrell JW, Kushnir Y, Ottersen G, Visbeck M (2003) The North Atlantic Oscillation. Climatic significance and environmental impact. Geophysical Monograph Series, 134. American Geophysical Union, Washington, DC, 279 pp Lorenz EN (1990) Can chaos and intransitivity lead to internannual variability. Tellus, 42 A, Nihoul JCJ (2007) Chaos, diversity, turbulence and sustainable development. International Journal of Computing Science and Mathematics, 1, 1, Stenseth NCh, Ottersen G, Hirnel JW, Belgrano A (2004) Marine Ecosystems and Climate Variation, Oxford University Press, Oxford, 252 pp