Mary-Elena Carr

Estimation of potential productivity in eastern boundary currents using remote sensing

Abstract This study provides a satellite-based estimate of potential primary production in the four Eastern Boundary Currents (EBCs), i.e. the California, Humboldt, Canary, and Benguela currents, from the first 24 months of the Sea-Viewing Wide Field of View Sensor, SeaWiFS. Within each EBC, production was estimated for the area of high chlorophyll concentration ( >1 mg m −3 ) or active area, which is likely to determine the production that can be utilized by higher trophic levels. Primary production decreased with latitude within each EBC while the extent of the active area was related to the magnitude of offshore transport. The most productive EBC was the Benguela Current (0.37 Gt C yr −1 ) , followed by the Canary (0.33 Gt C yr −1 ) , Humboldt (0.20 Gt C yr −1 ) , and California (0.04 Gt C yr −1 ) Currents. Interannual differences between 1997, 1998, and 1999 were largest for 1997 (measured by the Ocean Color Temperature Scanner, OCTS), which may be due primarily to the different sensor and algorithm. The Humboldt Current was more productive, and the Canary much less, during 1997 than in the two following years. The El Nino of 1997–1998 led to smaller annual production in 1998 in the Pacific EBCs. The upper bound of sustainable fish yield was estimated assuming a food chain of 2.6 links and an average trophic efficiency of 10%. The resulting values are 4–150 times larger than the observed fish catch from 1990 through 1997. Actual catch data in the Benguela Current were 20 times smaller than in the Humboldt Current. The most likely explanations for the differences in potential and observed fish catch are related to differing trophic structure and spatial accessibility in different EBCs. If the estimated yield is an upper bound that will be decreased to 10% or 20% by environmental accessibility, the small pelagic fishery in all four EBCs is likely to be food-limited.

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.

Chlorophyll variability in eastern boundary currents

The first three years of SeaWiFS data (199% 2000) provide the most complete quantification to date of chlorophyll seasonal variability along the full latitudinal ex- tent of the four major eastern boundary currents (EBCs). Comparisons to previously published chlorophyll seasonal climatologies deduced from the relatively sparse coverage provided by the Coastal Zone Color Scanner (CZCS) show significant differences in both southern hemisphere EBCs, while northern hemisphere regions are qualitatively simi- lar. Comparisons between chlorophyll and cross-shelf Ek- man transport seasonal cycles, calculated from coincident satellite scatterometer data, show seasonal maxima have similar phases over most of the California Current, at higher (> 32oS) latitudes in the Peru-Chile and Benguela Currents (> 30oS) and at lowest latitudes (< 20oN) in the Canary Current. Latitudinal zones within which phases diverge are indicative of alternate and/or more distant forcing.

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.

Physical control of phytoplankton distributions in the Alboran Sea: A numerical and satellite approach

This study aims to identify the controlling processes of phytoplankton distributions in the Alboran Sea (western Mediterranean Sea). A three-dimensional primitive equation model, the Navy Layered Ocean Model, forced by satellite-measured winds from the European Remote Sensing satellites (ERS-1 and 2) and the NASA scatterometer (NSCAT), is used to examine circulation from January 1996 to December 1998 and to simulate the distribution of passive tracers within the basin. We discuss the impact of the basin circulation on the observed patterns of chlorophyll observed by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Ocean Color and Temperature Scanner (OCTS). The upper layer is fertilized with nutrients which are transported by the western anticyclonic gyre from the coastal upwelling sites in the northwest. Nutrients are supplied also by eddy-induced upwelling in the periphery of the gyre path (found to be 20 to 60% of the supply due to advection in the present simulations). Circulation controls the supply of nutrients and the observed phytoplankton biomass in the upper layer during the nonbloom months (late spring, summer, and early fall). During the bloom regime, seasonal destratification is the dominant fertilizing process for the entire basin.

The climatological annual cycle of satellite‐derived phytoplankton pigments in the Alboran Sea

The circulation and upwelling processes (coastal and gyre-induced) that control the phytoplankton distribution in the Alboran sea are examined by analyzing monthly climatological patterns of Coastal Zone Color Scanner (CZCS) pigment concentrations, sea surface temperatures, winds, and seasonal geostrophic fields.

Hydrographic patterns and vertical mixing in the equatorial Pacific along 150°W

The WEC88 cruise sampled along a meridional transect from 15°N to 15°S along 150°W from February 17 to March 18, 1988, with a 6-day time series at the equator. The large-scale hydrographic patterns were typical for boreal spring. Equatorial maxima in dissipation of turbulent kinetic energy ∈, and of thermal variance χ, were found between 2°N and 2°S for the top 60 m. The equatorial time series coincided with a shift from southward to northward velocity, which returned the zonal current system to the equator. This led to a decrease in temperature, and increases in salinity, nutrient, and chlorophyll concentrations in the surface layer. Vertical diffusivity as well as ∈ and χ increased with the observed intensification of the Equatorial Undercurrent. Maximum values of ∈ and χ were observed at around 55 m, and the temporal trends occurred first at depth. Turbulent heat flux out of the mixed layer was the same order of magnitude as the penetrative irradiance at that depth. Maximum vertical heat flux occurred at depth in response to large diffusivity coefficients. The Richardson number was useful in predicting the regions of enhanced mixing in the meridional transect. However, for the equatorial time series, where the Ri was less than 0.45, intensity of dissipation was not proportional to Richardson number.

Pathways of the North Atlantic Current from surface drifters and subsurface floats

The North Atlantic east of the Grand Banks presents a thermal front from 40° to 50°N between the warm North Atlantic Current (NAC) and the cold Labrador Current. Here we use the trajectories of 84 isopycnal RAFOS floats deployed on the 27.2 and 27.5 σT surface in 1993–1995 and 218 surface drifters deployed by the International Ice Patrol and the Institut fur Meereskunde (Kiel, Germany) between 1978 and 1993 to understand the pathways of the NAC. The mean flow pattern, kinetic energy, directional stability, and most probable path through 1° by 1° boxes are derived for each of the three surfaces. Speed and kinetic energy decrease with depth, although directional stability is comparable at all levels. The axis of the NAC, which follows the 4000 m isobath along the western boundary, is described by mean kinetic energy (MKE) values surpassing 100 cm2 s−2 at the surface or 50 cm2 s−2 on the float surfaces and correspond to regions with directional stabilities in excess of 60%. Maximum values of eddy kinetic energy are found just offshore of the maximum MKE associated with the NAC main pathway and decrease rapidly to the east. Two cyclonic meanders are identified at 44°N, 45°W near the Newfoundland Seamounts and at 46°N, 42°W near Flemish Cap. The main difference between the patterns derived for the surface drifters and floats was the greater eastward extent of the cyclonic trough at 44°N and intensity of the Mann Eddy (42°N, 44°W) during the float sampling period (1993–1995).

Remote Measurements of Horizontal Eddy Diffusivity

Abstract The rate of horizontal diffusivity or lateral dispersion is key to understanding the dispersion of tracers and contaminants in the ocean, and it is an elusive, yet crucial, parameter in numerical models of circulation. However, the difficulty of parameterizing horizontal mixing is exacerbated in the shallow coastal ocean, which points to the need for more direct measurements. Here, a novel and inexpensive approach to remotely measure the rate of horizontal diffusivity is proposed. Current shipboard measurement techniques require repeated surveys and are thus time consuming and labor intensive. Furthermore, intensive in situ sampling is generally impractical for routine coastal management or for rapid assessment in the case of emergencies. A remote approach is particularly useful in shallow coastal regions or those with complex bathymetry. A time series of images from a dye-release experiment was obtained with a standard three-megapixel digital camera from a helicopter that hovered over the study ...

Vertical mixing in the equatorial pacific along 150°W in March 1988

Abstract A meridional transect along 150°W in March 1988 revealed a statistically significant maximum in turbulent kinetic energy dissipation, ϵ at 1°S–0° between 40-m and 60-m depth. The wind stress and buoyancy flux were poorly correlated with the observed mixed layer dissipation measured below 10 m. Dissipation modeled using similarity scaling was larger than the observed mixed layer dissipation (below 10 m) away from the equator and smaller than observed at 0°. The ratio of observed and modeled dissipation at the equator was highly correlated to the vertical velocity shear at the base of the mixed layer. The turbulent stress divergence computed as the residual of annual mean terms between 0 and 60 m for the 41/2-day time series was close to the sum of the annual mean terms of the zonal momentum balance of Bryden and Brady. For 21/2 days at 0° the change in heat content of the top 15 m was consistent with the observed one-dimensional fluxes within the uncertainty of the measurements. The 4-day average ...