Abstract:
The role of ocean dynamics in driving air-sea interaction is examined at two contrasting sites
on 125°W in the eastern tropical Pacific Ocean using data from the Pan American Climate
Study (PACS) field program. Analysis based on the PACS data set and satellite observations
of sea surface temperature (SST) reveals marked differences in the role of ocean dynamics in
modulating SST. At a near-equatorial site (3°S), the 1997-1998 El Nino event dominated the
evolution of SST and surface heat fluxes, and it is found that wind-driven southward Ekman
transport was important in the local transition from El Nino to La Nina conditions. At
a 10°N site near the summertime position of the Inter-tropical Convergence Zone, oceanic
mesoscale motions played an important role in modulating SST at intraseasonal (50- to
100-day) timescales, and the buoy observations suggest that there are variations in surface
solar radiation coupled to these mesoscale SST variations. This suggests that the mesoscale
oceanic variability may influence the occurrence of clouds.
The intraseasonal variability in currents, sea surface height, and SST at the northern site
is examined within the broader spatial and temporal context afforded by satellite data. The
oscillations have zonal wavelengths of 550-1650 km and propagate westward in a manner
consistent with the dispersion relation for first baroclinic mode, free Rossby waves in the
presence of a mean westward flow. The hypothesis that the intraseasonal variability and
its annual cycle are associated with baroclinic instability of the North Equatorial Current is
supported by a spatio-temporal correlation between the amplitude of intraseasonal variability
and the occurrence of westward zonal flows meeting an approximate necessary condition for
baroclinic instability.
Focusing on 10°N in the eastern tropical Pacific, the hypothesis that mesoscale oceanic
SST variability can systematically influence cloud properties is investigated using several
satellite data products. A statistically significant relationship between SST and columnar
cloud liquid water (CLW), cloud reflectivity, and surface solar radiation is identified within
the wavenumber-frequency band corresponding to oceanic Rossby waves. Analysis of seven
years of CLW data and 20 years surface solar radiation data indicates that 10-20% of the
variance of these cloud-related properties at intraseasonal periods and wavelengths on the
order of 10° longitude can be ascribed to SST signals driven by oceanic Rossby waves.
