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Climate Change and Coastal Upwelling
January 20, 2016 By CIRCulator Editorial
Staff
Areas of upwelling around the world.
(Photo: NOAA, this image is in the public
domain.)
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 yearto-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.