Recent studies indicate that the rates of sea level rise (SLR) along the U.S. mid-Atlantic coast ... more Recent studies indicate that the rates of sea level rise (SLR) along the U.S. mid-Atlantic coast have accelerated in recent decades, possibly due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and its upper branch, the Gulf Stream (GS). We analyzed the GS elevation gradient obtained from altimeter data, the Florida Current transport obtained from cable measurements, the North Atlantic Oscillation (NAO) index, and coastal sea level obtained from 10 tide gauge stations in the Chesapeake Bay and the mid-Atlantic coast. An Empirical Mode Decomposition/Hilbert-Huang Transformation (EMD/HHT) method was used to separate long-term trends from oscillating modes. The coastal sea level variations were found to be strongly influenced by variations in the GS on timescales ranging from a few months to decades. It appears that the GS has shifted from a 6-8 year oscillation cycle to a continuous weakening trend since about 2004 and that this trend may be responsible for recent acceleration in local SLR. The correlation between long-term changes in the coastal sea level and changes in the GS strength was extremely high (R = À0.85 with more than 99.99% confidence that the correlation is not zero). The impact of the GS on SLR rates over the past decade seems to be larger in the southern portion of the mid-Atlantic Bight near Cape Hatteras and is reduced northward along the coast. The study suggests that regional coastal sea level rise projections due to climate change must take into account the impact of spatial changes in ocean dynamics.
Three CODAR 25 MHz RADAR antennas have been installed in the lower Chesapeake Bay to observe surf... more Three CODAR 25 MHz RADAR antennas have been installed in the lower Chesapeake Bay to observe surface currents. Over the past year, data have been produced that will be valuable for researchers, commercial and recreational interests. In this paper we give a description of the antenna sites, examples of data products, a comparison of CODAR data with Acoustic Doppler Current
This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) lar... more This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) larvae off northern Chile (18°-24°S), within the coastal zone (out to 100 nm) and their relationship to the oceanographic conditions in the top layer (0-200 m depth). The data were derived from surveys undertaken from 1990 to 1995, during each winter (July-September), the season of the main spawning and higher larval abundance. During the 1995 survey, ichthyoplankton samples were also taken at shallower depths (<100 m), to assess the influence of low dissolved oxygen concentration upon the vertical distribution of the anchovy larvae. Both surface temperature and surface salinity distributions revealed that the period of this study covered a sequence from a relatively cold condition (1990) to warmer weak El Niño condition (1992), and back to cooler conditions (1995) in the area. A deepening of the base of the thermocline (15°C isotherm) occurred during the warmer period along the whole of the coast. During cooler conditions, the upper boundary of the low dissolved oxygen layer (ca. 1 .0 ml 1-') was found at shallower depths (<50 m) in most of the northern part of the area (Arica-Iquique) and also the waters above the thermocline were significantly more oxygenated, though variations with depth were greater, than during the warmer periods. Mean integrated abundances of anchovy larvae varied significantly between the winter periods of 1990 to 1995 (range : 230-5300 larvae in 10 m 2), with the highest mean values observed during 1993 and 1994. The highest abundances occurred at the most nearshore stations (1-5 nm) during all the cruises, though high values were also found at some offshore stations, mainly during the 1993-95 period. The comparison of the mean density of larvae (number m-) over different depth ranges (0-25, 0-50, and/or 0-100 m), suggested the occurrence of a barrier to their vertical distribution, located in the vicinity of the 0.75 ml O,1-' concentration. Overall, the warmer conditions initiated during 1992 appeared to have provided a more suitable environment for the survival and feeding of anchovy larvae in the area, whereas changes in the distribution of low oxygen concentrations did not appear to have a direct influence on larval abundance and distribution .
Different papers have been written comparing the long term changes in abundance among the differe... more Different papers have been written comparing the long term changes in abundance among the different eastern margins of the ocean, such as the California, Humboldt, Benguela and Canary currents and the Kuroshio Current. The focus has been on the simultaneous and ...
The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) b... more The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) between 1996 and 1998 (including the 1997-1998 El Niño) is presented using hydrographic measurements acquired on quarterly cruises of the Chilean Fisheries Institute, with sea surface temperature (SST), sea level, and wind speeds from Arica (18.5°S), Iquique (20.5°S), and Antofagasta (23.5°S) and a time series of vertical temperature profiles off Iquique.
Time series of satellite measurements are used to describe patterns of surface temperature and ch... more Time series of satellite measurements are used to describe patterns of surface temperature and chlorophyll associated with the 1996 cold La Nifia phase and the 1997-1998 warm El Niflo phase of the El Niflo-Southern Oscillation cycle in the upwelling region off northern Chile. Surface temperature data are available through the entire study period. Seaviewing Wide Field-of-view Sensor (SeaWiFS) data first became available in September 1997 during a relaxation in El Niflo 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 crossshelf extension of elevated concentrations are associated with decreased coastal temperatures during both the relaxation in El Niflo conditions in September-November 1997 and the recovery from El Niflo conditions after March 1998. Between these two periods during austral summer (December 1997 to March 1998) and maximum El Niño 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 satellitederived 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 Niflo. 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.
Over 30 years of hydrographic data from the northern Chile (18°S-24°S) upwelling region are used ... more Over 30 years of hydrographic data from the northern Chile (18°S-24°S) 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 crossshelf 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 m 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 21°S). 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.
The physical oceanography of the biologically productive coastal waters of central Chile (36°to 4... more The physical oceanography of the biologically productive coastal waters of central Chile (36°to 40°S) is relatively unknown. In December 1998 we made a short exploratory cruise between Valdivia (40°S) and Concepción (37.8°S) taking temperature, salinity, oxygen, and current velocity profiles. Coincident sea surface temperature and color measurements were obtained by satellite. The results showed an area dominated by windinduced coastal upwelling, river runoff, intrusion of offshore eddies, mixing, and heating. Upwelling centers were found over the shelf at three locations: inshore of Mocha Island, off Valdivia, and off Lavapie Point. At these centers, equatorial subsurface water (ESSW) intrudes into the coastal waters, sometimes affecting the surface waters. Since ESSW has characteristically low-oxygen and high-salinity values, it is easily detected. Off Valdivia, runoff imparts stratification, while farther north, solar heating and reduced mixing may facilitate stratification. In some areas, even strong winds would not destroy the stratification imparted by the advection of buoyancy that occurs during the upwelling process. Strong equatorward currents (>1 m s À1) in the form of an upwelling jet were found off Lavapie Point. This is also the location of an intruding anticyclone. Elsewhere, currents were mainly northward but highly variable because of intrusions from offshore eddies. The sea surface temperature and ocean color images show a complex field of onshore and offshore intrusions combined with the effects of mixing on chlorophyll concentrations. The residence time of upwelled water on the shelf is estimated to be less than 1 week.
A three-dimensional (3D) numerical model using the finite element method was used to diagnose coa... more A three-dimensional (3D) numerical model using the finite element method was used to diagnose coastal currents off Antofagasta, in the upwelling region of northern Chile. The diagnosis was emphasized in the nearshore zone off Peninsula Mejillones (23 S), a well-known upwelling site off northern Chile. Water density fields, available from seasonal climatologies of the region, were used to predict currents for the austral winter, and summer conditions. Steady-state conditions were simulated under absence of wind, with a constant southwest wind of 10 knots (5.14 m s À1), which is the dominant upwelling-favorable wind in the zone, and with a constant north wind of 10 knots. Spatial variability of winds was examined by using historical wind fields for the region. A 3D particle tracking program was further used to diagnose expected particle trajectories over a 72 h steady-state period. Coastal circulation was dominated by alongshore currents, and presence of reversal currents and coastal eddies. Wind forcing accelerated the flow of the nearsurface layer in most cases, but did not considerably alter the circulation pattern of the average water column. Simulation of passive particle drift showed that the whole nearshore zone might act as an efficient retention area. It was concluded that, as a result of the interactions among distinct water masses, upwelling plumes, and alongshore currents, physical barriers may develop and give rise to semi-enclosed habitats. Physical structuring may have important ecological implications for the upwelling ecosystem.
Velocity profiles and time-series data were combined with conductivity-temperature-depth (CTD) ca... more Velocity profiles and time-series data were combined with conductivity-temperature-depth (CTD) casts to describe the general circulation at tidal and subtidal scales in a bay of semi-arid climate, Coquimbo Bay ($308S), Chile. This was the first study that used a towed acoustic Doppler current profiler (ADCP) in coastal Chilean waters and is one of the very few in semiarid bays. The ADCP was towed for two semi-diurnal tidal cycles in early austral autumn, between March 23 and 24, 1997 along a triangular trajectory that covered most of the bay. Additional data consisted of moored current meters and CTD casts. The observations indicated the presence of a surface layer, above the pycnocline, that showed predominantly diurnal variability forced by the breeze regime and by tides. The tidal circulation in the surface layer featured amplitudes of 10 cm/s within an anticyclonic gyre that occupied most of the bay. The subtidal circulation in the surface was characterized by a pair of counter-rotating gyres. The northernmost three-fourths of the bay showed an anticyclonic gyre, and the observations over the southern fourth implied a cyclonic gyre. The subtidal anticyclonic gyre had a counterpart rotating in opposite direction within a lower layer, underneath the pycnocline. The lower layer showed semidiurnal variability in addition to diurnal variability and was insulated by the pycnocline from heat and momentum fluxes through the air-water interface. Circulations that resemble estuarine and anti-estuarine patterns were found associated with the subtidal gyres. A horizontal divergence related to a 10 cm/s near-surface outflow around Point Tortuga, to the south of the bay entrance, allowed the development of upward motion off the Point, as evidenced by the tilt of the isopycnals at the entrance to the bay.
The along-fjord variability of nitrate and dissolved silicate was studied in a silled fjord, Seno... more The along-fjord variability of nitrate and dissolved silicate was studied in a silled fjord, Seno Ballena, in the Strait of Magellan during flood and ebb tidal phases in December 2007. The spatial and temporal variability of both nitrate and dissolved silicate were consistent with the dynamics of a tidal intrusion front previously described for this fjord by Valle-Levinson et al. (2006). During flood, maximum nitrate values were found seaward and close to the sill due to the upwelling of dense, nutrient-rich water by means of Bernoulli aspiration. Conversely, a sharp drop in surface nitrate landward of the sill was consistent with the sinking of saltier, nitrate-rich, dissolved silicate-poor upwelled waters under relatively less dense, nitrate-poor, dissolved silicate-rich surface waters after flowing landward over the shallow sill. The waters flooding over the sill were particularly enriched in nitrate but poor in dissolved silicate, (NO À 3 :Si(OH) 4 ratio $ 5). The ratio tended to decrease landward of the sill, particularly during the ebb tide (NO À 3 :Si(OH) 4 ratio o 2), but the absolute Si(OH) 4 concentration values were still very low, normally ranging between 2 and 3 mM Si(OH) 4. Consistent with the low availability of dissolved silicate (DSi), only dinoflagellates (in the micro-phytoplankton size range) bloomed in nitrate-enriched (10 mM NO À 3) waters at the base of a shallow halocline. Thus, it is likely that the variability of surface water nutrients and DSi in Seno Ballena results mainly from the interplay among tidally forced fertilization (the aspiration of subpycnocline water with a high NO À 3 :Si(OH) 4 ratio), the input of continental waters (low NO À 3 :Si(OH) 4 ratio), and local productivity. The combination of these factors explains the microphytoplankton composition observed landward of the sill on this and previous expeditions to the study area.
The physical oceanography of the biologically productive coastal waters of central Chile (36°to 4... more The physical oceanography of the biologically productive coastal waters of central Chile (36°to 40°S) is relatively unknown. In December 1998 we made a short exploratory cruise between Valdivia (40°S) and Concepción (37.8°S) taking temperature, salinity, oxygen, and current velocity profiles. Coincident sea surface temperature and color measurements were obtained by satellite. The results showed an area dominated by windinduced coastal upwelling, river runoff, intrusion of offshore eddies, mixing, and heating. Upwelling centers were found over the shelf at three locations: inshore of Mocha Island, off Valdivia, and off Lavapie Point. At these centers, equatorial subsurface water (ESSW) intrudes into the coastal waters, sometimes affecting the surface waters. Since ESSW has characteristically low-oxygen and high-salinity values, it is easily detected. Off Valdivia, runoff imparts stratification, while farther north, solar heating and reduced mixing may facilitate stratification. In some areas, even strong winds would not destroy the stratification imparted by the advection of buoyancy that occurs during the upwelling process. Strong equatorward currents (>1 m s À1) in the form of an upwelling jet were found off Lavapie Point. This is also the location of an intruding anticyclone. Elsewhere, currents were mainly northward but highly variable because of intrusions from offshore eddies. The sea surface temperature and ocean color images show a complex field of onshore and offshore intrusions combined with the effects of mixing on chlorophyll concentrations. The residence time of upwelled water on the shelf is estimated to be less than 1 week.
A three-dimensional (3D) numerical model using the finite element method was used to diagnose coa... more A three-dimensional (3D) numerical model using the finite element method was used to diagnose coastal currents off Antofagasta, in the upwelling region of northern Chile. The diagnosis was emphasized in the nearshore zone off Peninsula Mejillones (23 S), a well-known upwelling site off northern Chile. Water density fields, available from seasonal climatologies of the region, were used to predict currents for the austral winter, and summer conditions. Steady-state conditions were simulated under absence of wind, with a constant southwest wind of 10 knots (5.14 m s À1), which is the dominant upwelling-favorable wind in the zone, and with a constant north wind of 10 knots. Spatial variability of winds was examined by using historical wind fields for the region. A 3D particle tracking program was further used to diagnose expected particle trajectories over a 72 h steady-state period. Coastal circulation was dominated by alongshore currents, and presence of reversal currents and coastal eddies. Wind forcing accelerated the flow of the nearsurface layer in most cases, but did not considerably alter the circulation pattern of the average water column. Simulation of passive particle drift showed that the whole nearshore zone might act as an efficient retention area. It was concluded that, as a result of the interactions among distinct water masses, upwelling plumes, and alongshore currents, physical barriers may develop and give rise to semi-enclosed habitats. Physical structuring may have important ecological implications for the upwelling ecosystem.
The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) b... more The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) between 1996 and 1998 (including the 1997-1998 El Niño) is presented using hydrographic measurements acquired on quarterly cruises of the Chilean Fisheries Institute, with sea surface temperature (SST), sea level, and wind speeds from Arica (18.5°S), Iquique (20.5°S), and Antofagasta (23.5°S) and a time series of vertical temperature profiles off Iquique.
This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) lar... more This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) larvae off northern Chile (18°-24°S), within the coastal zone (out to 100 nm) and their relationship to the oceanographic conditions in the top layer (0-200 m depth). The data were derived from surveys undertaken from 1990 to 1995, during each winter (July-September), the season of the main spawning and higher larval abundance. During the 1995 survey, ichthyoplankton samples were also taken at shallower depths (<100 m), to assess the influence of low dissolved oxygen concentration upon the vertical distribution of the anchovy larvae. Both surface temperature and surface salinity distributions revealed that the period of this study covered a sequence from a relatively cold condition (1990) to warmer weak El Niño condition (1992), and back to cooler conditions (1995) in the area. A deepening of the base of the thermocline (15°C isotherm) occurred during the warmer period along the whole of the coast. During cooler conditions, the upper boundary of the low dissolved oxygen layer (ca. 1 .0 ml 1-') was found at shallower depths (<50 m) in most of the northern part of the area (Arica-Iquique) and also the waters above the thermocline were significantly more oxygenated, though variations with depth were greater, than during the warmer periods. Mean integrated abundances of anchovy larvae varied significantly between the winter periods of 1990 to 1995 (range : 230-5300 larvae in 10 m 2), with the highest mean values observed during 1993 and 1994. The highest abundances occurred at the most nearshore stations (1-5 nm) during all the cruises, though high values were also found at some offshore stations, mainly during the 1993-95 period. The comparison of the mean density of larvae (number m-) over different depth ranges (0-25, 0-50, and/or 0-100 m), suggested the occurrence of a barrier to their vertical distribution, located in the vicinity of the 0.75 ml O,1-' concentration. Overall, the warmer conditions initiated during 1992 appeared to have provided a more suitable environment for the survival and feeding of anchovy larvae in the area, whereas changes in the distribution of low oxygen concentrations did not appear to have a direct influence on larval abundance and distribution .
Recent studies indicate that the rates of sea level rise (SLR) along the U.S. mid-Atlantic coast ... more Recent studies indicate that the rates of sea level rise (SLR) along the U.S. mid-Atlantic coast have accelerated in recent decades, possibly due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and its upper branch, the Gulf Stream (GS). We analyzed the GS elevation gradient obtained from altimeter data, the Florida Current transport obtained from cable measurements, the North Atlantic Oscillation (NAO) index, and coastal sea level obtained from 10 tide gauge stations in the Chesapeake Bay and the mid-Atlantic coast. An Empirical Mode Decomposition/Hilbert-Huang Transformation (EMD/HHT) method was used to separate long-term trends from oscillating modes. The coastal sea level variations were found to be strongly influenced by variations in the GS on timescales ranging from a few months to decades. It appears that the GS has shifted from a 6-8 year oscillation cycle to a continuous weakening trend since about 2004 and that this trend may be responsible for recent acceleration in local SLR. The correlation between long-term changes in the coastal sea level and changes in the GS strength was extremely high (R = À0.85 with more than 99.99% confidence that the correlation is not zero). The impact of the GS on SLR rates over the past decade seems to be larger in the southern portion of the mid-Atlantic Bight near Cape Hatteras and is reduced northward along the coast. The study suggests that regional coastal sea level rise projections due to climate change must take into account the impact of spatial changes in ocean dynamics.
Three CODAR 25 MHz RADAR antennas have been installed in the lower Chesapeake Bay to observe surf... more Three CODAR 25 MHz RADAR antennas have been installed in the lower Chesapeake Bay to observe surface currents. Over the past year, data have been produced that will be valuable for researchers, commercial and recreational interests. In this paper we give a description of the antenna sites, examples of data products, a comparison of CODAR data with Acoustic Doppler Current
This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) lar... more This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) larvae off northern Chile (18°-24°S), within the coastal zone (out to 100 nm) and their relationship to the oceanographic conditions in the top layer (0-200 m depth). The data were derived from surveys undertaken from 1990 to 1995, during each winter (July-September), the season of the main spawning and higher larval abundance. During the 1995 survey, ichthyoplankton samples were also taken at shallower depths (<100 m), to assess the influence of low dissolved oxygen concentration upon the vertical distribution of the anchovy larvae. Both surface temperature and surface salinity distributions revealed that the period of this study covered a sequence from a relatively cold condition (1990) to warmer weak El Niño condition (1992), and back to cooler conditions (1995) in the area. A deepening of the base of the thermocline (15°C isotherm) occurred during the warmer period along the whole of the coast. During cooler conditions, the upper boundary of the low dissolved oxygen layer (ca. 1 .0 ml 1-') was found at shallower depths (<50 m) in most of the northern part of the area (Arica-Iquique) and also the waters above the thermocline were significantly more oxygenated, though variations with depth were greater, than during the warmer periods. Mean integrated abundances of anchovy larvae varied significantly between the winter periods of 1990 to 1995 (range : 230-5300 larvae in 10 m 2), with the highest mean values observed during 1993 and 1994. The highest abundances occurred at the most nearshore stations (1-5 nm) during all the cruises, though high values were also found at some offshore stations, mainly during the 1993-95 period. The comparison of the mean density of larvae (number m-) over different depth ranges (0-25, 0-50, and/or 0-100 m), suggested the occurrence of a barrier to their vertical distribution, located in the vicinity of the 0.75 ml O,1-' concentration. Overall, the warmer conditions initiated during 1992 appeared to have provided a more suitable environment for the survival and feeding of anchovy larvae in the area, whereas changes in the distribution of low oxygen concentrations did not appear to have a direct influence on larval abundance and distribution .
Different papers have been written comparing the long term changes in abundance among the differe... more Different papers have been written comparing the long term changes in abundance among the different eastern margins of the ocean, such as the California, Humboldt, Benguela and Canary currents and the Kuroshio Current. The focus has been on the simultaneous and ...
The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) b... more The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) between 1996 and 1998 (including the 1997-1998 El Niño) is presented using hydrographic measurements acquired on quarterly cruises of the Chilean Fisheries Institute, with sea surface temperature (SST), sea level, and wind speeds from Arica (18.5°S), Iquique (20.5°S), and Antofagasta (23.5°S) and a time series of vertical temperature profiles off Iquique.
Time series of satellite measurements are used to describe patterns of surface temperature and ch... more Time series of satellite measurements are used to describe patterns of surface temperature and chlorophyll associated with the 1996 cold La Nifia phase and the 1997-1998 warm El Niflo phase of the El Niflo-Southern Oscillation cycle in the upwelling region off northern Chile. Surface temperature data are available through the entire study period. Seaviewing Wide Field-of-view Sensor (SeaWiFS) data first became available in September 1997 during a relaxation in El Niflo 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 crossshelf extension of elevated concentrations are associated with decreased coastal temperatures during both the relaxation in El Niflo conditions in September-November 1997 and the recovery from El Niflo conditions after March 1998. Between these two periods during austral summer (December 1997 to March 1998) and maximum El Niño 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 satellitederived 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 Niflo. 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.
Over 30 years of hydrographic data from the northern Chile (18°S-24°S) upwelling region are used ... more Over 30 years of hydrographic data from the northern Chile (18°S-24°S) 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 crossshelf 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 m 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 21°S). 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.
The physical oceanography of the biologically productive coastal waters of central Chile (36°to 4... more The physical oceanography of the biologically productive coastal waters of central Chile (36°to 40°S) is relatively unknown. In December 1998 we made a short exploratory cruise between Valdivia (40°S) and Concepción (37.8°S) taking temperature, salinity, oxygen, and current velocity profiles. Coincident sea surface temperature and color measurements were obtained by satellite. The results showed an area dominated by windinduced coastal upwelling, river runoff, intrusion of offshore eddies, mixing, and heating. Upwelling centers were found over the shelf at three locations: inshore of Mocha Island, off Valdivia, and off Lavapie Point. At these centers, equatorial subsurface water (ESSW) intrudes into the coastal waters, sometimes affecting the surface waters. Since ESSW has characteristically low-oxygen and high-salinity values, it is easily detected. Off Valdivia, runoff imparts stratification, while farther north, solar heating and reduced mixing may facilitate stratification. In some areas, even strong winds would not destroy the stratification imparted by the advection of buoyancy that occurs during the upwelling process. Strong equatorward currents (>1 m s À1) in the form of an upwelling jet were found off Lavapie Point. This is also the location of an intruding anticyclone. Elsewhere, currents were mainly northward but highly variable because of intrusions from offshore eddies. The sea surface temperature and ocean color images show a complex field of onshore and offshore intrusions combined with the effects of mixing on chlorophyll concentrations. The residence time of upwelled water on the shelf is estimated to be less than 1 week.
A three-dimensional (3D) numerical model using the finite element method was used to diagnose coa... more A three-dimensional (3D) numerical model using the finite element method was used to diagnose coastal currents off Antofagasta, in the upwelling region of northern Chile. The diagnosis was emphasized in the nearshore zone off Peninsula Mejillones (23 S), a well-known upwelling site off northern Chile. Water density fields, available from seasonal climatologies of the region, were used to predict currents for the austral winter, and summer conditions. Steady-state conditions were simulated under absence of wind, with a constant southwest wind of 10 knots (5.14 m s À1), which is the dominant upwelling-favorable wind in the zone, and with a constant north wind of 10 knots. Spatial variability of winds was examined by using historical wind fields for the region. A 3D particle tracking program was further used to diagnose expected particle trajectories over a 72 h steady-state period. Coastal circulation was dominated by alongshore currents, and presence of reversal currents and coastal eddies. Wind forcing accelerated the flow of the nearsurface layer in most cases, but did not considerably alter the circulation pattern of the average water column. Simulation of passive particle drift showed that the whole nearshore zone might act as an efficient retention area. It was concluded that, as a result of the interactions among distinct water masses, upwelling plumes, and alongshore currents, physical barriers may develop and give rise to semi-enclosed habitats. Physical structuring may have important ecological implications for the upwelling ecosystem.
Velocity profiles and time-series data were combined with conductivity-temperature-depth (CTD) ca... more Velocity profiles and time-series data were combined with conductivity-temperature-depth (CTD) casts to describe the general circulation at tidal and subtidal scales in a bay of semi-arid climate, Coquimbo Bay ($308S), Chile. This was the first study that used a towed acoustic Doppler current profiler (ADCP) in coastal Chilean waters and is one of the very few in semiarid bays. The ADCP was towed for two semi-diurnal tidal cycles in early austral autumn, between March 23 and 24, 1997 along a triangular trajectory that covered most of the bay. Additional data consisted of moored current meters and CTD casts. The observations indicated the presence of a surface layer, above the pycnocline, that showed predominantly diurnal variability forced by the breeze regime and by tides. The tidal circulation in the surface layer featured amplitudes of 10 cm/s within an anticyclonic gyre that occupied most of the bay. The subtidal circulation in the surface was characterized by a pair of counter-rotating gyres. The northernmost three-fourths of the bay showed an anticyclonic gyre, and the observations over the southern fourth implied a cyclonic gyre. The subtidal anticyclonic gyre had a counterpart rotating in opposite direction within a lower layer, underneath the pycnocline. The lower layer showed semidiurnal variability in addition to diurnal variability and was insulated by the pycnocline from heat and momentum fluxes through the air-water interface. Circulations that resemble estuarine and anti-estuarine patterns were found associated with the subtidal gyres. A horizontal divergence related to a 10 cm/s near-surface outflow around Point Tortuga, to the south of the bay entrance, allowed the development of upward motion off the Point, as evidenced by the tilt of the isopycnals at the entrance to the bay.
The along-fjord variability of nitrate and dissolved silicate was studied in a silled fjord, Seno... more The along-fjord variability of nitrate and dissolved silicate was studied in a silled fjord, Seno Ballena, in the Strait of Magellan during flood and ebb tidal phases in December 2007. The spatial and temporal variability of both nitrate and dissolved silicate were consistent with the dynamics of a tidal intrusion front previously described for this fjord by Valle-Levinson et al. (2006). During flood, maximum nitrate values were found seaward and close to the sill due to the upwelling of dense, nutrient-rich water by means of Bernoulli aspiration. Conversely, a sharp drop in surface nitrate landward of the sill was consistent with the sinking of saltier, nitrate-rich, dissolved silicate-poor upwelled waters under relatively less dense, nitrate-poor, dissolved silicate-rich surface waters after flowing landward over the shallow sill. The waters flooding over the sill were particularly enriched in nitrate but poor in dissolved silicate, (NO À 3 :Si(OH) 4 ratio $ 5). The ratio tended to decrease landward of the sill, particularly during the ebb tide (NO À 3 :Si(OH) 4 ratio o 2), but the absolute Si(OH) 4 concentration values were still very low, normally ranging between 2 and 3 mM Si(OH) 4. Consistent with the low availability of dissolved silicate (DSi), only dinoflagellates (in the micro-phytoplankton size range) bloomed in nitrate-enriched (10 mM NO À 3) waters at the base of a shallow halocline. Thus, it is likely that the variability of surface water nutrients and DSi in Seno Ballena results mainly from the interplay among tidally forced fertilization (the aspiration of subpycnocline water with a high NO À 3 :Si(OH) 4 ratio), the input of continental waters (low NO À 3 :Si(OH) 4 ratio), and local productivity. The combination of these factors explains the microphytoplankton composition observed landward of the sill on this and previous expeditions to the study area.
The physical oceanography of the biologically productive coastal waters of central Chile (36°to 4... more The physical oceanography of the biologically productive coastal waters of central Chile (36°to 40°S) is relatively unknown. In December 1998 we made a short exploratory cruise between Valdivia (40°S) and Concepción (37.8°S) taking temperature, salinity, oxygen, and current velocity profiles. Coincident sea surface temperature and color measurements were obtained by satellite. The results showed an area dominated by windinduced coastal upwelling, river runoff, intrusion of offshore eddies, mixing, and heating. Upwelling centers were found over the shelf at three locations: inshore of Mocha Island, off Valdivia, and off Lavapie Point. At these centers, equatorial subsurface water (ESSW) intrudes into the coastal waters, sometimes affecting the surface waters. Since ESSW has characteristically low-oxygen and high-salinity values, it is easily detected. Off Valdivia, runoff imparts stratification, while farther north, solar heating and reduced mixing may facilitate stratification. In some areas, even strong winds would not destroy the stratification imparted by the advection of buoyancy that occurs during the upwelling process. Strong equatorward currents (>1 m s À1) in the form of an upwelling jet were found off Lavapie Point. This is also the location of an intruding anticyclone. Elsewhere, currents were mainly northward but highly variable because of intrusions from offshore eddies. The sea surface temperature and ocean color images show a complex field of onshore and offshore intrusions combined with the effects of mixing on chlorophyll concentrations. The residence time of upwelled water on the shelf is estimated to be less than 1 week.
A three-dimensional (3D) numerical model using the finite element method was used to diagnose coa... more A three-dimensional (3D) numerical model using the finite element method was used to diagnose coastal currents off Antofagasta, in the upwelling region of northern Chile. The diagnosis was emphasized in the nearshore zone off Peninsula Mejillones (23 S), a well-known upwelling site off northern Chile. Water density fields, available from seasonal climatologies of the region, were used to predict currents for the austral winter, and summer conditions. Steady-state conditions were simulated under absence of wind, with a constant southwest wind of 10 knots (5.14 m s À1), which is the dominant upwelling-favorable wind in the zone, and with a constant north wind of 10 knots. Spatial variability of winds was examined by using historical wind fields for the region. A 3D particle tracking program was further used to diagnose expected particle trajectories over a 72 h steady-state period. Coastal circulation was dominated by alongshore currents, and presence of reversal currents and coastal eddies. Wind forcing accelerated the flow of the nearsurface layer in most cases, but did not considerably alter the circulation pattern of the average water column. Simulation of passive particle drift showed that the whole nearshore zone might act as an efficient retention area. It was concluded that, as a result of the interactions among distinct water masses, upwelling plumes, and alongshore currents, physical barriers may develop and give rise to semi-enclosed habitats. Physical structuring may have important ecological implications for the upwelling ecosystem.
The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) b... more The evolution of oceanographic conditions in the upwelling region off northern Chile (18°-24°S) between 1996 and 1998 (including the 1997-1998 El Niño) is presented using hydrographic measurements acquired on quarterly cruises of the Chilean Fisheries Institute, with sea surface temperature (SST), sea level, and wind speeds from Arica (18.5°S), Iquique (20.5°S), and Antofagasta (23.5°S) and a time series of vertical temperature profiles off Iquique.
This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) lar... more This study analyses the patterns of distribution and abundance of anchovy (Engraulis ringers) larvae off northern Chile (18°-24°S), within the coastal zone (out to 100 nm) and their relationship to the oceanographic conditions in the top layer (0-200 m depth). The data were derived from surveys undertaken from 1990 to 1995, during each winter (July-September), the season of the main spawning and higher larval abundance. During the 1995 survey, ichthyoplankton samples were also taken at shallower depths (<100 m), to assess the influence of low dissolved oxygen concentration upon the vertical distribution of the anchovy larvae. Both surface temperature and surface salinity distributions revealed that the period of this study covered a sequence from a relatively cold condition (1990) to warmer weak El Niño condition (1992), and back to cooler conditions (1995) in the area. A deepening of the base of the thermocline (15°C isotherm) occurred during the warmer period along the whole of the coast. During cooler conditions, the upper boundary of the low dissolved oxygen layer (ca. 1 .0 ml 1-') was found at shallower depths (<50 m) in most of the northern part of the area (Arica-Iquique) and also the waters above the thermocline were significantly more oxygenated, though variations with depth were greater, than during the warmer periods. Mean integrated abundances of anchovy larvae varied significantly between the winter periods of 1990 to 1995 (range : 230-5300 larvae in 10 m 2), with the highest mean values observed during 1993 and 1994. The highest abundances occurred at the most nearshore stations (1-5 nm) during all the cruises, though high values were also found at some offshore stations, mainly during the 1993-95 period. The comparison of the mean density of larvae (number m-) over different depth ranges (0-25, 0-50, and/or 0-100 m), suggested the occurrence of a barrier to their vertical distribution, located in the vicinity of the 0.75 ml O,1-' concentration. Overall, the warmer conditions initiated during 1992 appeared to have provided a more suitable environment for the survival and feeding of anchovy larvae in the area, whereas changes in the distribution of low oxygen concentrations did not appear to have a direct influence on larval abundance and distribution .
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