Altimeter observations of the Peru-Chile countercurrent

Data from Geosat and TOPEX altimeters are used to infer the structure of the Peru-Chile Countercurrent, a jet that flows from at least as far north as 10ºS (historical data suggests 7ºS) to 35º–40ºS, maintaining its position between approximately 100–300 km offshore. Although the annual mean current cannot be determined from altimeter observations, the nearly antisymmetric patterns in spring and fall, combined with historical observations, suggest that the countercurrent is poleward at most times and is maximum in spring and minimum in fall. Previous studies have linked the offshore countercurrent at 7ºS to the Equatorial Undercurrent west of the Galapagos Islands, suggesting that the countercurrent is part of a continuous flow that extends from the western equatorial Pacific to the region off Chile between 35º–40ºS

Dynamical analysis of the upwelling circulation off central Chile

In this article we analyze the momentum and vorticity balances of a numerical simulation of the upwelling circulation off central Chile (34° –40°S) and its response to interannual local wind changes. Our analysis indicates that the path of the upwelling jet is strongly controlled by the bottom topography. This topographic steering causes the jet to separate from the coast at the Punta Lavapie cape (~37°S). Although the zeroth-order momentum balance is dominated by the geostrophic terms, the circulation is also affected by nonlinear processes, which lead to the formation of large meanders and the shedding of cyclonic eddies north of Punta Lavapie during periods of wind relaxation. The relative contributions of the zeroth-order vorticity balance and the advective terms are also strongly controlled by changes in the coastline geometry and the bottom topography. Vorticity is created along the current axes and transported toward the coast and the Peru-Chile Trench, where it dissipates. South of Punta Lavapie the across-shelf transports are weaker with equatorward flows that are more stable than in the north. Additional numerical simulations indicate that during periods with El Niño conditions, the area is affected with a general weakening of the currents and upwelling activity, although the northern region still shows the formation of eddies. During years with relatively stronger winds, in contrast, the upwelling activity and across-shelf transport processes are significantly increased. The results show that the Punta Lavapie cape has a large effect on the spatial and temporal variability of the coastal currents in the region off central Chile

An Inversion Method to Determine Ocean Surface Currents Using Irregularly Sampled Satellite Altimetry Data

An inversion method is presented that determines mesoscale sea surface currents using satellite altimeter data. The method directly uses geostrophic cross-track velocity components, expands the unknown velocity components with spline functions, and includes weighted constraints for divergence and kinetic energy. The success of this method is measured by the misfit between input and output velocities and depends on the weights of the constraints, the irregular space-time data distribution of the altimeter data, and the nonstationary character of mesoscale oceanic features. Negative effects of the irregular spatial distribution of the altimeter data are overcome by partitioning the larger regions of interest into smaller subareas to make independent inversions. The relatively weak effects of subtrack temporal separations in the presence of slowing evolving fields are addressed by temporal interpolations, which provide the more important benefit of filling in missing data. The procedure is evaluated for the Geosat, Topex-Poseidon, and ERS-1 altimeter sampling geometries determining their “oceanic” resolutions by using simulated stationary and nonstationary velocity fields. As expected for stationary fields the best spatial resolutions correspond to altimeters with the densest data distributions, that is, the longest repeat periods. The smallest scale resolved is approximately 100 km for Geosat (17-day repeat) and ERS-1 (35-day repeat) and is approximately 150 km for Topex-Poseidon (10-day repeat). Tests with simulated nonstationary “eddies” show that the propagation at typical Rossby wave phase speeds (<0.04 m s⁻¹) degrades the solutions only slightly if scales are greater than approximately 200 km for Topex-Poseidon and 150 km for Geosat and ERS-1 17-day periods, even without temporal interpolation. Combination of Topex-Poseidon and ERS-1 data over 17 days provides the best resolution, allowing eddy spatial scales of 100 km to be resolved at the same propagation speeds. Applications of the inverse method to data from the Geosat and Topex-Poseidon missions indicate that application of the method results in spatial resolution similar to the synthetic experiments. Qualitative comparisons to acoustic Doppler current profilers, satellite surface temperature, and dynamic height fields show that application of the method to Geosat data resolves the larger mesoscale structures of a meandering jet in the California Current. Use of temporal interpolation provides little benefit, due to the slow propagation times of the 100–200-km scale features resolved by the method, but it does help fill in missing data for Geosat. Application of the method to Topex-Poseidon data is not as successful but still resolves the largest-scale structure. The results of application to the synthetic data suggest that it is possible to achieve greater resolution by combining two or more satellite altimeter datasets such as Topex-Poseidon and ERS-1

A numerical study of the upwelling circulation off central Chile

We present a modeling study of the upwelling ocean circulation off central Chile (34°–40°S). Using a primitive equation model, we make a numerical simulation of the ocean circulation for summer of 1993, a year characterized by moderate but persistent equatorward winds. The results indicate the formation of an eastern boundary current system that shows classical Ekman and geostrophic dynamics. A strong equatorward coastal jet develops during intense upwelling activity, induced by the prevailing equatorward winds. The spatial and temporal variability of the upwelling‐favorable winds strongly controls the variability in the oceanic circulation. Upwelling activity in the coastal areas occurs during periods of growth, peak, and decay. The circulation in the region is divided into two distinct dynamical areas that extend north and south of a prominent cape (Punta Lavapie), located in the center of the model domain. During the peak upwelling stage, the equatorward jet separates from the coast at Punta Lavapie and remains separated in the northern region. This detachment is followed by the formation of cyclonic eddies and decreased upwelling during periods of wind relaxation. The northern area is affected by advection processes and weaker local winds. In contrast, the equatorward flows in the southern region are persistently attached to the coastline and are more coherent with the stronger local winds