W. J. Sydeman, M. García-Reyes, D. S. Schoeman, R. R. Rykaczewski, S. A. Thompson, B. A. Black, S. J. Bograd. Climate change and wind intensification in coastal upwelling ecosystems. Science 345, 77-80 (2014). DOI: 10.1126/science.1251635
Upwelling in a coastal setting is a wind-driven process in which surface water is pushed away from the coastline, allowing deeper water to rise up to replace the displaced surface water. The upwelling of deeper water towards the surface typically carries nutrients required for primary production and thus ecosystem health can be greatly influenced by the strength and spatial extent of upwelling. It is well understood that global temperatures have been increasing over the past century due to human activity induced climate change, however, the response of upwelling favorable winds has received less attention. It was hypothesized in 1990 by Andrew Bakun that warming temperatures and the associated change in pressure gradients would lead to the intensification of alongshore winds that drive upwelling. More specifically, Bakun hypothesized that intensification of upwelling favorable winds would occur during warm seasons. The highlighted study explores this hypothesis analyzing historical wind trends in the four major upwelling systems.
The study focused on four eastern boundary current systems (EBCSs). Eastern boundary currents are broad, shallow and generally slow moving ocean currents found on the eastern edge of ocean basins (or west coasts of continents (e.g. California) for more land-centric thinkers). The four systems flow towards the equator. The California and Humboldt currents are respectively located in the North and South Pacific Oceans, whereas the Iberian/Canary and Benguela currents are respectively located in the North and South Atlantic Oceans (Figure 1).
The data used in this study are from direct observations and model reanalysis. Direct observation consisted of approximately four times more seasonal data than annually averaged data. When considering only direct observation data sets, the California, Humboldt and Canary currents all experienced historically intensifying, upwelling favorable winds. Conversely, model reanalysis data featured more than 3 times the amount of annually averaged data. When considering only model reanalysis data the Benguela and California currents show wind intensification trends, whereas the Iberian and Canary currents show weakening wind data over the past several decades (Figure 2).
Additionally, there was a latitudinal variation in wind patterns. The Benguela current experienced intensification poleward of 20° South. Similarly, the California current strengthened poleward of 32.5° North and the Humboldt strengthened poleward of 14° South. Conversely, the Iberian current showed weakening nearly the entire sampled length, while the Canary current weakened equatorward of 23° North (Figure 3).
The results of this study are largely dependent on how the wind data from each study was reported and the type of data collected. Generally, the California and Humboldt currents experienced wind intensification during the warm seasons, as is consistent with Bakun’s hypothesis. For these same current systems, annually averaged data were less likely to show wind intensification. The Canary observational data also suggests wind intensity to be increasing. Contradictory to Bakun’s hypothesis, the Iberian system exhibits a weakening trend. In general, annual data did not support intensification, suggesting that warm season (upwelling) wind data should be considered as suggested by Bakun’s hypothesis. The Benguela current data is comprised of only annually averaged data, which suggests wind intensification. Sources of error or bias include changes in instrumentation, the period covered and climate variability over the decades of collected wind data.
This study suggests that higher latitudes show a higher probability of wind intensification, perhaps due to disproportionately higher amounts of global warming felt towards the poles. Additionally, the Iberian system may be experiencing weakening due to the climate variability caused by the North Atlantic Oscillation (NAO).
Understanding how climate change is impacting upwelling in EBCSs is very important due to these regions being productive fisheries. It is unclear if the marine ecosystems will benefit from increased nutrients and primary production as a result of intensifying upwelling favorable winds. Enhanced nutrient supply may establish a favorable scenario for increased food production in fisheries. Contrary, increased upwelling may transport planktonic organisms away from the shelf, increase acidity and hypoxic conditions, which have the potential to devastate marine ecosystems. It is also possible that increasing wind intensity may be countered by an increasingly stratified water column. Warming surface waters would be much more buoyant than colder, denser deep water which would restrict upwelling. Increasing wind intensity and upwelling have numerous implications, and future studies will be needed to understand exactly how ecosystems have responded to these changes in each Eastern boundary current system.