oktober 5, 2006

Fler bud om Golfströmmens framtid

Filed under: Havet,Klimatdata,Oceancirkulation — by Daniel @ 19:11

Ett antal gånger har det kommit bud på hur golfströmmen kommer reagera på framtidens klimat (se här, här och här) och fler lär komma. Då jag råkade snubbla över ytterligare ett bud känner jag mig manad att uppmärksamma detta. Det är publicerat av forskare från Kiel i Tyskland (Mojib Latif som förstaförfattare) i Journal of Climate; Is the Thermohaline Circulation changing?. Tänker inte gå in på några detaljer, istället låter jag dig få läsa abstraktet och slutsatserna.

Abstraktet lyder som följer:

Analyses of ocean observations and model simulations suggest considerable changes in the thermohaline circulation (THC) during the last century. These changes are likely to be the result of natural multidecadal climate variability and driven by low-frequency variations of the North Atlantic Oscillation (NAO) through changes in Labrador Sea convection. Indications of a sustained THC weakening are not seen during the last few decades. Instead a strengthening since the 1980s is observed. The combined assessment of ocean hydrography data and model results indicates that the expected anthropogenic weakening of the THC will remain within the range of natural variability during the next several decades.

Diskussionen och artikelns sammanfattning lyder såhär:

We have investigated the multidecadal variability of the Atlantic MOC during the 20th century by making use of a characteristic relationship between MOC and SST found in global climate models. The simulated Atlantic SST response to multidecadal changes in the MOC is the interhemispheric dipole pattern, with opposite changes in the North and South Atlantic. An index of the MOC during the 20th century was derived by computing the observed SST difference between the North and South Atlantic. This index exhibits pronounced multidecadal variability during the 20th century. We found evidence that these multidecadal MOC variations can be understood as the lagged response to the multidecadal variations in the NAO and the associated variations in Labrador Sea convection. Finally, we estimated from the analysis of ocean model simulations that the observed density change over the period 1970-2000 in the region of Denmark Straight translates into an MOC change of about 1Sv, which is well within the range of the natural multidecadal variability that we estimated to amount to about ±1.5-3 Sv.

While the combined evidence from surface observations, the hydrographic data and model simulations suggests that the variations in the MOC over the last decades can primarily be regarded as a response to the NAO variability, a continuing freshening (or warming) trend in the Nordic Seas must be considered as a key additional factor for the future evolution of the MOC in view of anthropogenic climate change. A recent assessment of global warming simulations by Schweckendiek and Willebrand, 2005 suggests that 21st-century projections of the MOC are mainly tied to the evolution of hydrographic conditions in the Nordic Seas. Most global climate model projections for the 21st century suggest a gradual anthropogenic weakening of the MOC of up to 40% (IPCC, 2001; Gregory et al., 2005). As described above, the level of internal multidecadal varability has been estimated to about 1.5-3 Sv. Thus, such a weakening will not exceed the range of multidecadal variability within the next several decades.

Our analysis does not provide indications for a sustained weakening of the MOC during the last few decades, which is consistent with the study of Knight et al. 2005. Recently , however, Bryden et al. 2005 found some evidence of such a weakening of the MOC by analyzing a transatlantic section at 25°N that has been occupied five times since 1957. Thus, the current state of the MOC is discussed controversially. As a cautionary remark it is noted that the possibility of increased melting of the Greenland ice sheet, which could substantially change the freshwater budget in the Nordic Seas, is not included in present models. In this regard a monitoring of the tendencies in the hydrographic properties of the overflow should constitute a key element of a long-term observing system for the MOC in the Atlantic Ocean.

En ‘pre-print’-kopia finns att läsa här. Frågor rörande metoder eller resultat bör ställas till de berörda författarna.

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