North Atlantic and Arctic SST trends

Positive trend of SST in many areas of North Atlantic and its modulation In Arctic Ocean for last 60 years was discussed in (Pokrovsky and Timokhov 2005). This paper demonstrated a positive trend of the monthly SST averaged over the whole North Atlantic basin. It is necessary to note that there is the Atlantic Multi-Decadal Oscillation (AMO) of the North Atlantic SST (Enfield et al. 2001). Smoothing technique (Pokrovsky et al. 2004) based on a cross-validation criterion allows us to filter out high frequency oscillation. When applied to the climate series it permits to reveal a natural oscillation of the spatially averaged SST in the North Atlantic for winter months (Fig. 1). 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 1870-1999 (0.38°C). The index shows persistent warm (pre-1900, 1930s-1950s) and cool (1900s-1920s, 1960s-1980s) phases typically lasting a few decades, as well as the onset of a warm phase in the 1990s. Here "alpha" is a parameter providing a specific smoothing rate (Tikhonov 1963). Above oscillation has a time scale of 64 year. Meanwhile, a linear trend demonstrates the CO2 greenhouse effect. Therefore, superposition of the greenhouse effect and a positive phase of Atlantic SST provide rapid water warming during last 3 decades. Is the AMO a natural phenomenon, or is it related to global warming? Instruments have observed AMO cycles only for the last 150 years, 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 instrumentally have been occurring for at least the last millennium (Gray et al. 2004). This is clearly longer than modern man has been affecting climate, so the AMO is probably a natural climate oscillation. In the 20th century, the climate swings of the AMO have alternately camouflaged and exaggerated the greenhouse warming, and made attribution of global warming more difficult to ascertain. Thus, the AMO is a hypothetical mode of natural variability occurring in the North Atlantic Ocean and which has its principle expression in the SST field. Knight et al. (2005) informed that the Hadley Centre model produces a rather realistic AMO with a period of 70-120 years. And the model AMO persists throughout the 1,400-year run, they note, suggesting that the real-world AMO goes back much further than the past century of observations does. Thus, the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation (THC). Judging by the 1,400-year simulation's AMO, Knight and colleagues 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, Sutton and Hodson (2005) predict more drought-prone summers in the central United States in the next few decades. The aim of this paper is to investigate a linkage between the AMO and the Arctic ice extent during last century. Unfortunately, models are not reliable enough to reproduce ice concentration in Arctic Ocean with appropriate accuracy. Therefore, our approach is to implement sophisticated methods for climate time series analysis. These are: (1) a new smoothing algorithm based on Wahba's cross-validation, Cleveland's local polynomial approximation and Tikhonov's regularization; (2) Morlet's wavelet analysis (Cleveland 1979; Goupillaud et al. 1984; Tikhonov 1963; Wahba 1985).

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