José Luis Blanco

Seasonal climatology of hydrographic conditions in the upwelling region off northern Chile

Over 30 years of hydrographic data from the northern Chile (18oS-24oS) upwelling region are used to calculate the surface and subsurface seasonal climatology extending 400 km offshore. The data are interpolated to a grid with sufficient spatial resolution to preserve cross- shelf gradients and then presented as means within four seasons: austral winter (July- September), spring (October-December), summer (January-March), and fall (April-June). Climatological monthly wind forcing, surface temperature, and sea level from three coastal stations indicate equatorward (upwelling favorable) winds throughout the year, weakest in the north. Seasonal maximum alongshore wind stress is in late spring and summer (December- March). Major water masses of the region are identified in climatological T-S plots and their sources and implied circulation discussed. Surface fields and vertical transects of temperature and salinity confirm that upwelling occurs year-round, strongest in summer and weakest in winter, bringing relatively fresh water to the surface nearshore. Surface geostrophic flow nearshore is equatorward throughout the year. During summer, an anticyclonic circulation feature in the north which extends to at least 200 rn depth is evident in geopotential anomaly and in both temperature and geopotential variance fields. Subsurface fields indicate generally poleward flow throughout the year, strongest in an undercurrent near the coast. This undercurrent is strongest in summer and most persistent and organized in the south (south of 21oS). A subsurface oxygen minimum, centered at ~250 m, is strongest at lower latitudes. Low-salinity subsurface water intrudes into the study area near 100 m, predominantly in offshore regions, strongest during summer and fall and in the southernmost portion of the region. The climatological fields are compared to features off Baja within the somewhat analogous California Current and to measurements from higher latitudes within the Chile-Peru Current system.

Interannual variability in the mesoscale distribution of the depth of the upper boundary of the oxygen minimum layer off northern Chile (18–24S): Implications for the pelagic system and biogeochemical cycling

The low oxygen concentration (<2 ml L -1 ) at relatively shallow depths (<100 m) in the coastal upwelling zone and in the adjacent oceanic area is a distinct feature of the eastern boundary Humboldt Current System (HCS) off Peru and northern Chile; it affects the distribution of pelagic organisms and is associated with an important denitrification regime in the water column. Nevertheless, little information is available about the spatial and temporal variability in the distribution of the Oxygen Minimum Layer (OML) present in the HCS and the impact of its variability upon the pelagic system and biogeochemical cycles in the region. The present study reviews the oceanographic data obtained for the area off northern Chile (ca. 18 to 24S, out to 370 km), between 1980 and 1997, with the aim of characterizing the depth distribution of the upper boundary of the OML (1 ml L -1 iso-oxyline, representing also the oxycline) and investigating the association of its interannual variability with changes in coastal sea level and in the equatorial and local thermoclines. The depth of the upper boundary of the OML undergoes pronounced deepening during the occurrence of warm ENSO (El Nino Southern Oscillation) events over the whole area of study, and this, in turn, determines a condition of higher oxygen concentrations in the top 100 m layer. These changes follow closely the patterns of interannual variability in coastal sea level and depth of the thermocline in the area of study during the 1980-1997 period. Most of this variability can be accounted for, as expected from previous studies, by remote forcing originating in the equatorial zone of the Pacific Ocean, as evidenced from the significant correlation between the above local parameters and the interannual variability in the depth of the equatorial thermocline. The 1982-83 and 1997-98 El Nino events seemed to have been the most important ones in terms of their effects upon sea level and depth of the thermocline and oxycline off northern Chile. The potential impacts of the interannual changes in the depth of the upper boundary of the OML upon the pelagic system and biogeochemical cycling in the region are discussed.

Satellite-measured chlorophyll and temperature variability off northern Chile during the 1996-1998 La Nina and El Niño

Time series of satellite measurements are used to describe patterns of surface temperature and chlorophyll associated with the 1996 cold La Nina phase and the 1997–1998 warm El Nino phase of the El Nino-Southern Oscillation cycle in the upwelling region off northern Chile. Surface temperature data are available through the entire study period. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data first became available in September 1997 during a relaxation in El Nino conditions identified by in situ hydrographic data. Over the time period of coincident satellite data, chlorophyll patterns closely track surface temperature patterns. Increases both in nearshore chlorophyll concentration and in cross-shelf extension of elevated concentrations are associated with decreased coastal temperatures during both the relaxation in El Nino conditions in September-November 1997 and the recovery from El Nino conditions after March 1998. Between these two periods during austral summer (December 1997 to March 1998) and maximum El Nino temperature anomalies, temperature patterns normally associated with upwelling were absent and chlorophyll concentrations were minimal. Cross-shelf chlorophyll distributions appear to be modulated by surface temperature frontal zones and are positively correlated with a satellite-derived upwelling index. Frontal zone patterns and the upwelling index in 1996 imply an austral summer nearshore chlorophyll maximum, consistent with SeaWiFS data from 1998–1999, after the El Nino. SeaWiFS retrievals in the data set used here are higher than in situ measurements by a factor of 2–4; however, consistency in the offset suggests relative patterns are valid.

Observations of wind influence on exchange flows in a strait of the Chilean Inland Sea

A 100-day time series of velocity profiles, sea level and wind velocity at a strait in the Chilean Inland Sea was analyzed to examine the effects of wind forcing on the mean two-layer exchange. Measurements took place in the Meninea Constriction of the Moraleda Channel during a period dominated by northerly winds. The mean flow in the strait, and in general in the Moraleda Channel, showed net surface northward outflow and net bottom southward inflow that likely resulted from the dynamical balance between pressure gradient and friction. The influence of tidal mixing on mean exchange flows was further suggested by isopycnals intersecting the bottom. The same momentum balance between pressure gradient and friction, applied with temporally varying sea level slopes, satisfactorily described the subtidal modifications to the mean exchange flows produced by wind forcing. The use of sea level slopes to explain the subtidal variability of velocity profiles at the Meninea Constriction was justified by the strong correlation between sea level slopes and wind stresses (0.84). In fact, the vertically integrated dynamics was essentially explained by the balance between wind stress and barotropic pressure gradient for northerly winds. Addition of bottom stress improved the dynamical explanation during periods of weak or southerly winds. This dynamical response was confirmed by the first two empirical orthogonal functions of the record. Two-layer exchange flows were weakened by northerly winds as depicted by the first empirical function mode, which was unidirectional throughout the water column in response to depth-integrated dynamics. The second empirical function mode was related to the depth-dependent response that followed the wind-induced sea level set-up.

Depth-dependent Overtides from Internal Tide Reflection in a Glacial Fjord

Observations of current velocity profiles and hydrography over and near a tall sill in a Chilean glacial fjord are used to illustrate the interactions between barotropic and baroclinic tides. The character of the barotropic tide in the glacial fjord is mixed with semidiurnal dominance. The ratio of sill height to water column depth at the study site is ca. 0.95. Water column stratification appeared only in the upper 5 m of the water column. Current velocity variations in the stratified surface layer were quite different to those underneath. Below the pycnocline, nonlinear interactions between semidiurnal M2 and diurnal K1 oscillations yielded a third-diurnal distortion MK3. Most interesting, surface layer currents were distortedby the superposition of semidiurnal M2 and sixthdiurnal M6 oscillations. The oscillations with M6 variability were identified, through wave superposition approaches, as reflected internal tides linked to M2 tidal variations. This was confirmed by theoretical results of stratified barotropic tidal flows interacting with abrupt bathymetry. Under the predominantly tidally mixed regime of the study area, the distortion to surface currents caused by the reflected wave was nearly symmetric during the large tidal ranges of the diurnal cycle. Nearly symmetric distortions resulted as the phase lag between incident and reflected wave-inducted currents was small (reflected currents developing a few minutes after maximum tidal flows). During the small ranges of the diurnal cycle, distortions were asymmetrical because of the relatively larger phase lags of the reflected signal (reflected currents developing tens of minutes after maximum tidal flows).

Surface Current Mapping in the Lower Chesapeake

Two CODAR 25 MHz radars with co-located transmit/receive antennas have been installed in the lower Chesapeake Bay. Antenna patterns have been measured at both sites and initial quality control tests have included comparisons of measured and ideal baseline data.

Bio-physical Interactions in Ocean Margin Ecosystems (BIOME): understanding coastal dynamics in the Southern Mid-Atlantic Bight

Coastal regions within the Mid-Atlantic Bight (MAB) are significantly influenced by regional freshwater fluxes emanating from several large bay systems, most notably the Hudson-Raritan river systems, the Delaware and Chesapeake Bays. The outflows from these bays have high sediment loads and high levels of nutrients, particulate and dissolved organic matter (POM and DOM) associated with them which strongly influence the adjacent coastal margin ecosystems. Our research and observational effort includes the development and deployment of an observing system aimed at characterizing and monitoring the influence of the Chesapeake Bay on the adjacent coastal ocean margin ecosystem. A primary focus of this effort is to develop and apply state-of-the-art technologies and methodologies to support research, observation, monitoring, and management applications in the coastal ocean. We have developed a program that addresses coastal spatial and temporal scales. Our observing system consists of four components including 1) High Frequency (HF) Radar: 3 long-range HF Radar systems for mapping coastal ocean surface currents along Delmarva and 2 standard-range systems for the Chesapeake Bay mouth region, 2) a Coastal Ocean Bio-optical buoY (COBY), which will be deployed southeast of the Maryland-Virginia border at about 40m water depth, and will be instrumented with an above-water spectroradiometer (19 channel UV-VIS-NIR, 10nm BW, sea- and sky-viewing radiometers plus solar reference) on a robotic arm to measure water-leaving radiances, meteorological package, in-water mooring instruments with ADCP, CTD, nitrate sensor, ac-9, ac-s, and fluorometers for chlorophyll, colored dissolved organic matter (CDOM) and phycoerythrin detection; 3) seasonal cruises in the Mid-Atlantic Bight collecting optical, biological, and chemical data; and 4) the Ocean-Atmosphere Sensor Integration System (OASIS), which is comprised of a fleet (6-12) of solar-powered surface autonomous vehicles deployed offshore to measure surface ocean currents, meteorological measurements, surface ocean salinity and temperature, air-sea CO2 fluxes, water-leaving radiances, chlorophyll and CDOM fluorescence, HAB detection, etc. The HF radars, COBY and OASIS are being deployed this summer, with plans for a fully deployed observing system by the end of 2005 or early 2006.