Climate Change and Coastal Upwelling

CLIMATE CHANGE threatens some of the most productive marine ecosystems in the world: the coastal upwelling systems that border the eastern boundary of the Pacific and Atlantic Oceans.

In a comprehensive review paper published in June in a new journal called Current Climate Change Reports, Andrew Bakun and colleagues outlined the expected changes to the four major eastern boundary upwelling systems of the world, including the California Current System (CCS) running from British Columbia to Baja, California.

Coastal upwelling along the California Current System occurs seasonally beginning in early spring, when the dominant alongshore wind direction shifts southward and interacts with the Earth’s rotation to push surface waters offshore. This allows the cold, nutrient-rich waters at depth to well up onto the continental shelf where they are close enough to the surface for photosynthesis to occur. The upwelling season draws to a close with a northward shift in wind direction in late summer or fall. Upwelling spurs phytoplankton blooms, which feed the tiniest of ocean animals (zooplankton) and small fish, such as anchovies and sardines. In turn, these zooplankton and small fish provide sustenance for commercial fisheries, seabirds, and marine mammals.

With climate change, the study’s authors note, coastal upwelling–favorable winds are expected to intensify, as they have in recent decades. Upwelling winds are controlled by ocean high-pressure systems, which may strengthen with global warming–induced shifts in the global atmospheric circulation. Major modes of climate variability, such as the El Niño Southern Oscillation, also exert an appreciable effect on upwelling variability from year to year. Another mechanism for future increases in coastal upwelling as the climate warms is greater heating over land than over ocean, which could produce upwelling-favorable winds, at least locally. Much remains to be studied with respect to the mechanism, but the best estimate to-date is that upwelling will likely intensify.

The waters that feed the California Current System could also change. The ocean current running perpendicular to the southward running California Current System at its most poleward section could move farther north thereby providing even more nutrient-rich headwaters. The offshore bottom waters that well up may also have more nutrients but a lower pH—from ocean acidification and less oxygen. While more upwelling of cooler waters could potentially counteract the effects of habitat warming and more nutrients could make for more productive waters (more food = more fish), the authors note, upwelling of more acidic and hypoxic (low oxygen) water would negatively affect the ecosystem.

Changes in marine habitat and food web characteristics driven by changes in upwelling could be one of the most critical impacts, according to the authors. The timing and location of sufficient food resources is crucial for the survival of many larval and juvenile fish. Enhanced coastal upwelling may lead to more nutrients available for photosynthesis. However, the authors note, the stronger winds may push the resulting phytoplankton or zooplankton offshore, creating a mismatch in normal habitat and food supply. This, say the authors, could lead to a redistribution of populations as habitats and food supplies change.

The timing of food supply is also crucial: any delay in the upwelling season, as conjectured with increasing CO2, could reduce survival of young fish if, for example, young Coho salmon arrive to the sea before upwelling produces the appropriate food, say the authors.

Seabirds, seals, and other animals higher on the marine food web within upwelling systems would likely experience climate change impacts through changes in habitat or food supply. Up-the-chain reductions in anchovy, for example, have been linked with lower seabird populations. Seabirds and seals both require specific habitat for nesting and rearing and may face challenges if food resources shift away from such sites, the authors say.

Eastern boundary currents such as the California Current System are highly variable, driven by large-scale modes of climate variability (such as the El Niño Southern Oscillation and the Pacific Decadal Oscillation), which will continue to have a strong impact on year-to-year upwelling conditions and marine species abundance and productivity. Without other stressors (such as over fishing and pollution), this intrinsic variability may make eastern boundary currents resilient ecosystems to such changes in upwelling characteristics, say the authors. At least until future changes surpass the historical variability.

Citation: A. Bakun, B. A. Black, S. J. Bograd, M. García-Reyes, A. J. Miller, R. R. Rykaczewski, W. J. Sydeman, “Anticipated effects of climate change on coastal upwelling ecosystems,” Current Climate Change Reports 2015 1 85-93; published ahead of print March 7, 2015, doi: 10.1007/s40641-015-0008-4.

Photo: Areas of upwelling around the world. (Photo Credit: NOAA, this image is in the public domain.)

At OCCRI since 2011, Meghan Dalton is a trained climate researcher with a BA in Mathematics from Linfield College and an MS in Atmospheric Science from Oregon State University. Meghan has worked closely with several Northwest communities working on Community Adaptation, including the water provider Seattle Public Utilities on the PUMA project. Meghan has worked as the lead on several regional climate assessments, including “Climate Change in the Northwest: Implications for Our Landscapes, Waters, and Communities” and “The Third Oregon Climate Assessment Report.”

Stay up to date on the latest climate science news for the Northwest, subscribe to the CIRCulator.