North Atlantic isotherms displacement is not unprecedented and is within the variability range for the last 150 yrs. In the last few decades, however, the frequency and rate of isotherm migration to the north has increased.
Hobson and co-authors report on the analysis of 150 yrs record of North Atlantic data. Although the basin has undeniably undergone a net warming since 1976, this recent warming parallels a similar warming from 1910-1945, and it is hotly debated whether these changes are part of a natural cycle or a long-term trend of progressive ocean warming.
The authors analyzed the International Comprehensive Ocean-Atmosphere Data Set (ICOADS, Worley et al., 2005; http://www.cdc.noaa.gov/). ICOADS is considered the most recently-available and complete historical dataset for global SST values, incorporqting up-to-date, sophisticated statistical methods that allow the reconstruction of SSTs from sparse in-situ ship measurements, drifters and moored buoys, and since 1981, satellite data, over the last 150 years.
Hobson et al. looked at three isotherms, 12ºC, 15ºC and 18ºC, as indicators of the summer SST behaviour across the North Atlantic as they lie in the region of most marked seasonal warming and cooling, and so are sensitive indicators of interannual thermal variability.
The mean summer positions of these isotherms show considerable interannual variability, with as much as 2 deg in a year (1974-1975). Northward displacements occurred in two distinct periods (1930s-1940s and then again at the end of the 20th century) but the rate of displacement distinctly increased 1992-2005 from ~7 km y-1 to ~27 km y-1.
The authors argue that the North Atlantic has not been impacted as severely as some terrestrial systems, and also urge caution in ascribing species northerly displacement solely due to anthropogenic induced climate warming, as other factors also play a role (e.g. fishing impact on predator-prey relationships, natural isotherm migration variability).
Although the most recent 20th century warming in the North Atlantic does not eclipse previous warming events, the rate of northerly isotherm displacement has increased from ~7 km y-1 between 1926 and 1939, to ~27 km y-1 in 1992-2005. The authors also warn of increasing frequency of warmer sea-surface temperatures propagation further north, and more frequent reports of warm-water “exotic” marine species in northern Europe.
Hobson V.J., C.R. McMahon, A. Richardson and G.C. Hays. Ocean surface warming: The North Atlantic remains within the envelope of previous recorded conditions, Deep-Sea Research: Oceanographic Research Papers DOI:10.1016/j.dsr.2007.11.003
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