Canary current upwelling: More or less?
Abstract
It has been hypothesized that coastal upwelling in the four major eastern boundary current systems might be intensified as global warming could result in a greater land-sea temperature gradient and hence strengthen alongshore winds. Recent research has suggested a substantial increase of upwelling intensity off Northwest Africa. Evidence there is based on the derivation of a proxy for upper ocean temperatures from the alkenone unsaturation index (U37K) derived from two sediment cores recovered off Cape Ghir, Morocco. An accelerating decrease of over 1 °C during the last century was concluded for near surface temperature near the Cape. Support for this conclusion was found in an increase in Bakun’s upwelling index for the same area. The evidence for a general intensification of upwelling within the whole Canary current upwelling system is examined here. Using available estimates of wind from PFEL, NCAR/NCEP, ECMWF, ICOADS and WASWind plus measured wind data from coastal meteorological stations, no evidence of a coherent intensification in winds at the regional scale off Northwest Africa is found. Moreover, sea surface temperature records from ships-of-opportunity (ICOADS data set) and also from the Pathfinder satellite AVHRR data set show a significant and correlated increase at all latitudes in the region, including in the area around Cape Ghir. It is concluded that there is no evidence for a general increase in upwelling intensity off Northwest Africa or Iberia. The apparent lowering of SST off Cape Ghir indicated by the alkenone unsaturation index can be explained by coccolithophorids (phytoplankton from which the U37K signal is derived) living deeper in the water column. The distribution of most phytoplankton (including coccolithophorids) will deepen and have less overlap with mixed layer temperatures as the ocean warms, resulting in a near surface temperature estimate that is increasingly biased by subsurface temperatures and lower than the actual SST.
sea surface temperature is increasing near the coast at all latitudes throughout the region at a rate >0.01 °C y-1; wind estimates from different data bases can differ in trends and variability, but WASWind estimates appear to agree well with the few available coastal stations; no statistically significant change in meridional (upwelling favorable) wind component is found, except off Iberia, where winds are becoming slightly less upwelling favorable; there is no evidence for a general intensification of upwelling in this large marine ecosystem, contrary to the hypothesis of Bakun (1990) and the conclusions of McGregor et al. (2007). Consideration of the factors influencing the alkenone unsaturation index U37K derived from coccolithophorids and other phytoplankton leads to the conclusions that: the use of the alkenone unsaturation index as a proxy for sea surface temperature is subject to considerable uncertainty, and should not be used uncritically; further examination of the mechanisms causing deviations in the alkenone unsaturation index-sea surface temperature relationship is required to understand paleo records (particularly those where SST changes are of lower magnitude); use of the index should preferably be made in conjunction with the examination of other proxies that are sensitive to changes in the structure of the water column (e.g. microfossils). In general, caution should be exercised in the use of proxy temperature estimates and their extrapolation to regional scales from a single proxy and/or sediment site. Multiple proxies and sediment cores are clearly needed in oceanographic reconstructions.- Publication:
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Progress in Oceanography
- Pub Date:
- September 2013
- DOI:
- 10.1016/j.pocean.2013.07.007
- Bibcode:
- 2013PrOce.116..167B