Role of sea surface physical processes in mixed-layer temperature
changes during summer marine heat waves in the Chile-Peru Current System
Abstract
We identified anomalously warm sea surface temperature (SST) events
during 1980–2019 near the major upwelling center at Punta Lavapié in
the central Chile-Peru Current System, using the European Centre for
Medium-Range Weather Forecasts reanalysis and focusing on time scales of
10 days to 6 months. Extreme warm SST anomalies on these time scales
mostly occurred in the austral summer, December through February, and
had spatial scales of 1000s of km. By compositing over the 37 most
extreme warm events, we estimated terms in a heat budget for the ocean
surface mixed layer at the times of strongest warming preceding the
events. The net surface heat flux anomaly is too small to explain the
anomalous warming, even when allowing for uncertainty in mixed-layer
depth. The composite mean anomaly of wind stress, from satellite ocean
vector wind swath data, during the 37 anomalous warming periods has a
spatial pattern similar to the resulting warm SST anomalies, analogous
to previous studies in the California Current System. The weakened
surface wind stress suggests reduced entrainment of cold water from
below the mixed layer. Within 100-200 km of the coast, the typical
upwelling-favorable wind stress curl decreases, suggesting reduced
upwelling of cold water. In a 1000-km area of anomalous warming
offshore, the typical downwelling-favorable wind stress curl also
decreases, implying reduced downward Ekman pumping, which would allow
mixed-layer shoaling and amplify the effect of the positive
climatological summertime net surface heat flux.