Canary current upwelling: More or less?

Abstract

International audienceIt 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 (View the MathML sourceU37K′) 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 View the MathML sourceU37K′ 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

Under Pressure: Climate Change, Upwelling, and Eastern Boundary Upwelling Ecosystems

“…While there have been a number of studies on this topic (Diffenbaugh et al, 2004;Di Lorenzo et al, 2005;Auad et al, 2006;Bakun et al, 2015), the Intergovernmental Panel on Climate Change 5th Assessment Report (IPCC AR5) has concluded that there is low confidence in projections regarding upwelling (Stocker et al, 2013;Barros et al, 2014). In part, this is a result of the substantial spatial heterogeneity in physical characteristics within EBUS, which emphasizes the role of local processes and resultant local biotic effects (Nelson and Hutchings, 1983;Dorman and Winant, 1995;Barton et al, 2013;Bakun et al, 2015). However, the low confidence in predictions regarding the ecological future of EBUS is driven more heavily by uncertainty surrounding the potentially counteracting effects of intensifying winds on the one hand, and increasing thermal stratification on the other.…”

“…Importantly, there is stronger agreement that significant trends of upwelling intensification are evident at higher latitude for all EBUS. Among studies excluded from the meta-analysis, either because they did not meet the strict selection criteria or because they were published after the analysis was completed, most (Di Lorenzo et al, 2005;Alves and Miranda, 2013;Barton et al, 2013;Bylhouwer et al, 2013;Stocker et al, 2013;Cropper et al, 2014;deCastro et al, 2014;Jacox et al, 2014;Sydeman et al, 2014b;Varela et al, 2015) show results consistent with the findings of Sydeman et al (2014a). Those containing seemingly contradictory results (Demarcq, 2009;Dewitte et al, 2012) add uncertainty, but do not refute the findings of the metaanalysis, as they focus on local regions, use short time series and do not add uncertainty.…”

“…There is substantial but conflicting evidence on SST trends in EBUS (Mendelssohn and Schwing, 2002;Arístegui et al, 2009;Belkin, 2009;Demarcq, 2009;Lebassi et al, 2009;García-Reyes and Largier, 2010;Rouault et al, 2010;Gutiérrez et al, 2011;Seo et al, 2012;Barton et al, 2013;Barros et al, 2014;Sydeman et al, 2014b;Salvanes et al, 2015). Agreement among studies is low for two reasons: (1) the varying spatio-temporal resolutions of SST data capture different aspects of upwelling processes, and (2) trends are obscured by the interannual to multi-decadal variability that result from regional/local ocean and atmospheric processes.…”

“…Indeed, many of the observed trends in recent decades might be associated with these oscillations (Santos et al, 2012;Barton et al, 2013). Though predictions are controversial, some of these climate oscillations are expected to increase in amplitude or variance with climate change (Timmermann et al, 1999;Kuzmina et al, 2005;Sydeman et al, 2013;Cai et al, 2015), which would further mask monotonic trends.…”

Changes in the partial pressure of carbon dioxide in the Mauritanian–Cap Vert upwelling region between 2005 and 2012

“…Wind observations and reanalysis products are controversial regarding the Bakun intensification hypothesis (Bakun, 1990). Using different wind databases for the Canary region, Barton et al (2013) concluded that there was no evidence for a general increase in the upwelling intensity off northwest Africa. Marcello et al (2011) found an intensification of the upwelling system in the same area during a 20-year period, while the alongshore wind stress remained almost stable.…”