Water Masses and Circulation in the Tropical Pacific off Central Mexico and Surrounding Areas

13 páginas, 8 figuras, 2 tablasThe seasonal variations and the interactions of the water masses in the tropical Pacific off central Mexico (TPCM) and four surrounding areas were examined based on an extensive new hydrographic database. The regional water masses were redefined in terms of absolute salinity (SA) and conservative temperature (Θ) according to the Thermodynamic Equation of Seawater 2010 (TEOS-10). Hydrographic data and the evaporation minus (precipitation + runoff) balance were used to investigate the origin and seasonality of two salinity minima in the area. The shallow (50–100 m) salinity minimum originates with the California Current System and becomes saltier as it extends southeastward and mixes with tropical subsurface waters while the surface salinity minimum extends farther north in the TPCM in summer and fall because of the northward advection of tropical surface waters. The interactions between water masses allow a characterization of the seasonal pattern of circulation of the Mexican Coastal Current (MCC), the tropical branch of the California Current, and the flows through the entrance of the Gulf of California. The seasonality of the MCC inferred from the distribution of the water masses largely coincides with the geostrophic circulation forced by an annual Rossby waveThis is a product of the project CONACyT (SEP2011–168034-T), with collaboration from the following sources: CONACyT Projects 168034-T, T-9201, 4271P-T, 38797-T, 26653-T, 1076-T9201, 4271PT9601, C01–25343; 38834-T, C02-44870F,G34601-S, and 103898; Naval Postgraduate School; NOC-US; NOAA (GC04– 219); and the regular UABC budget through Projects 4009, 4015, 0324, 0333, and 0352. Funding came from CONACyT, México through the Grant 1329234 for the Ph.D. studies of Esther PortelaPeer reviewe

Seasonal and interannual variability of satellite-derived chlorophyll pigment, surface height, and temperature off Baja California

Mean fields, seasonal cycles, and interannual variability are examined for fields of satellite-derived chlorophyll pigment concentrations (CHL), sea surface height (SSH), and sea surface temperature (SST) during 1997–2002. The analyses help to identify three dynamic regions: an upwelling zone next to the coast, the Ensenada Front in the north, and regions of repeated meanders and/or eddy variability west and southwest of Point Eugenia. High values of CHL are found in the upwelling zone, diminishing offshore. The exception is the area north of 31°N (the Ensenada Front), where higher CHL are found about 150 km offshore. South of 31°N, the long-term mean dynamic topography decreases next to the coast, creating isopleths of height parallel to the coastline, consistent with southward geostrophic flow. North of 31°N the mean flow is toward the east, consistent with the presence of the Ensenada Front. The mean SST reveals a more north-south gradient, reflecting latitudinal differences in surface heating due to solar radiation. Harmonic analyses and EOFs reveal the seasonal and interannual patterns, including the region of repeated eddy activity to the west and southwest of Point Eugenia. A maximum CHL occurs in spring in most of the inshore regions, reflecting the growth of phytoplankton in response to the seasonal maximum in upwelling-favorable winds. SST and SSH anomalies are negative in the coastal upwelling zone in spring, also consistent with a response to the seasonal maximum in upwelling. When the seasonal cycle is removed, the strongest signal in the EOF time series is the response to the strong 1997– 1998 El Niño, with a weaker signal representing La Niña (1998–1999) conditions. El Niño conditions consist of low chlorophyll, high SSH, and high SST, with opposite conditions during La Niña

Effects of mesoscale processes on phytoplankton chlorophyll off Baja California

Using satellite sea surface height (SSH) and chlorophyll (CHL), the year 2000 is analyzed to characterize the effects of mesoscale circulation patterns on phytoplankton spatial variability in the California Current (CC) off Baja California. Satellite data are combined with and compared to in situ field measurements (chlorophyll-a and hydrographic variables) along vertical alongshore sections located similar to 130 km offshore between similar to 24.5 degrees -33 degrees N. Monthly average maps of SSH and surface geostrophic velocities depict the characteristics of mesoscale meanders and eddies, which correspond well with the subsurface hydrographic and velocity fields. Satellite-derived pigment (CHL) represent in situ fields in the upper 0-20 m (overall r = 0.53; p < 0.05), but their representation of peak values in Deep Chlorophyll Maxima (DCM) at similar to 50 m depth are inaccurate. DCM are traced in all three seasons (January-July), descending from near the surface (north of 31 degrees N) to 50 m over a large extent of the transect to the south, approximately following the 24.7-25.1 isopycnals as they and the isotherms deepen to the south. In January, phytoplankton chlorophyll concentrations in the DCM are relatively uniform, originating during upwelling events that occur farther north, then following the equatorward flow of the CC. During April and July, the discrete maxima in the DCM occur at the centers of cyclonic meanders and the chlorophyll concentrations inside these maxima are enhanced as a result of local coastal upwelling off Baja California. Phytoplankton blooms created by coastal upwelling spread offshore and subduct along the 24.7-25.1 isopycnals, creating the DCM along the inner part of the meandering jet.KEYWORDS: Current system, Satellite, Pigment, State, Temperature, El Nino, Salinity, Evolution, Interannual variability, Coastal transition zon

Satellite-derived variability in chlorophyll, wind stress, sea surface height, and temperature in the northern California Current System

Satellite-derived data provide the temporal means and seasonal and nonseasonal variability of four physical and biological parameters off Oregon and Washington (41°–48.5°N). Eight years of data (1998–2005) are available for surface chlorophyll concentrations, sea surface temperature (SST), and sea surface height, while six years of data (2000–2005) are available for surface wind stress. Strong cross-shelf and alongshore variability is apparent in the temporal mean and seasonal climatology of all four variables. Two latitudinal regions are identified and separated at 44°–46°N, where the coastal ocean experiences a change in the direction of the mean alongshore wind stress, is influenced by topographic features, and has differing exposure to the Columbia River Plume. All these factors may play a part in defining the distinct regimes in the northern and southern regions. Nonseasonal signals account for ∼60–75% of the dynamical variables. An empirical orthogonal function analysis shows stronger intra-annual variability for alongshore wind, coastal SST, and surface chlorophyll, with stronger interannual variability for surface height. Interannual variability can be caused by distant forcing from equatorial and basin-scale changes in circulation, or by more localized changes in regional winds, all of which can be found in the time series. Correlations are mostly as expected for upwelling systems on intra-annual timescales. Correlations of the interannual timescales are complicated by residual quasi-annual signals created by changes in the timing and strength of the seasonal cycles. Examination of the interannual time series, however, provides a convincing picture of the covariability of chlorophyll, surface temperature, and surface height, with some evidence of regional wind forcing

SST, thermohaline structure, and circulation in the southern Gulf of California in June 2004 during the North American Monsoon Experiment

The article of record as published may be located at http://dx.doi.org/10.1029/2008JC004896The thermohaline structure, circulation, and heat fluxes in the Gulf of California entrance during June 2004 are described based on conductivity-temperature-depth and Lowering Acoustic Doppler Current Profiler data collected in a 14-day survey, supported by satellite data. The AVHRR images show extensive mesoscale structures in the region, the most striking being (1) a cool filament extending from the California Current domain and (2) a warm intrusion along the mainland shelf. On the warm side of the thermal front created by the cool filament there was a strong current flowing into the Gulf, with speeds up to 0.70 ms!1 in the surface; this current, which the SST images suggest was associated with a decaying eddy, carried 6 Sv into the Gulf. Associated with the second structure, there was an ingoing coastal current on the mainland shelf, with weak surface currents but with speeds "0.25 ms!1 at its core, between 70 and 200 m; this coastal current transported 2 Sv into the Gulf. The two ingoing currents appear to join inside the Gulf, forming a very strong (speeds 0.40–0.80 ms!1) narrow ("30 km) coastal current between the surface and 500 m depth. Changes in the thermohaline structure of the upper layers observed by repeat sampling of three cross sections were dominated by advection. However, it was found that the advective heat flux is very variable in space and time. For the period of observation it was estimated that the lateral heat input was 4.8 ± 3.0 # 105 Wm!2 as estimated with LADCP currents and 5.7 ± 2.20 X 10^5 Wm!2 with geostrophic velocities.This is a product of project ‘‘The Role of Oceanic Processes on the Gulf of California SST Evolution during the North American Monsoon Experiment,’’ which is part of the North American Monsoon Experiment (NOAA contract GC04– 219, P.I. Michael Douglas). This work was also supported by CONACyT (Mexico) projects D41881-F (P.I., MFL) and C01–25343 (P.I., RC), by UABC projects (P-0324 and P-0352) and by CICESE. VMG held a CONACYT scholarship. MFL was at SIO-UCSD as recipient of a UCMEXUSCONACYT sabbatical scholarship, hosted by P. Niiler, while working on this article

Variabilidad estacional e interanual océano-atmósfera en la cuenca colombia

La investigación sobre la variabilidad océano-atmósfera en el Caribe se ha concentrado en la mesoescala y no analiza en detalle la zona del Caribe colombiano. Para entender mejor la variabilidad estacional de la cuenca Colombia asociada con El Niño/Oscilación del Sur (ENSO), se analizaron campos de vientos del reanálisis NCEP/NCAR, anomalías del nivel del mar de AVISO, y salidas de nivel del mar y velocidad del modelo de circulación global POCM-4C. Se analizaron los ciclos anuales medios y de años ENSO distribuidos espacialmente, un ajuste estacional basado en cuadrados mínimos para las anomalías del nivel del mar y vientos, y la estructura vertical del océano en ocho perfiles a lo largo de la costa colombiana. La variabilidad estacional del viento está dominada por la acción del Chorro Tropical Superficial del Caribe (ChTSC). Durante El Niño, de diciembre a mayo la velocidad del núcleo del chorro es menor, mientras que de junio a noviembre es mayor. El viento sobre la cuenca Colombia genera una dinámica superficial dada por la Corriente del Caribe (CC) y el Giro Panamá-Colombia (GPC). Se encontró que, aunque en su valor medio el GPC es de circulación ciclónica, las anomalías estacionales de esta circulación son ciclónicas entre julio y octubre y anticiclónicas entre enero y mayo. El análisis indicó que al sur de 14°N, la cuenca Colombia está dominada por la estacionalidad, y al norte por variabilidad de otras frecuencias como los remolinos y la actividad de mesoescala. La estructura vertical cerca de la costa suramericana muestra que la Contracorriente Panamá-Colombia (CPC) puede llegar hasta Barranquilla (75°W) donde una parte continúa al este como la Corriente Subsuperficial Costera del Caribe (CSCC). Durante El Niño, las corrientes costeras en dirección este (CPC y CSCC) se debilitan todo el año 57 excepto en JJA cuando se intensifican, mientras la CC en la Guajira se intensifica todo el año. Durante La Niña, las corrientes en dirección este se debilitan incluso en JJA, mientras que la CC en la Guajira permanece cercana a su media trimestral, excepto en JJA cuando se intensifica

Morphological and biochemical differentiation in Antarctic krill

During the February 1981 cruise FIBEX MD-25 between 30-50 degrees E and 61-64 degrees S, hydrography showed the presence of two gyres, confirmed by the geostrophic circulation relative to 1000 m from Levitus climatology, at the borders of these gyres concentrations of highly morphologically differentiated krill were found. Gaussian component analysis of krill samples, pooled by sectors, showed three cohorts of Euphausia superba in the western sector and one in the eastern sector. Across the sampling area, Thysanoessa mactura and E. superba occurred at separate stations. Analysis of cohorts in T macrura separated two size groups in both the western and the eastern sectors. The use of a Differentiation Index (D.I.) [Farber-Lorda, J., 1990. Somatic length relationships and ontogenetic morphometric differentiation of Euphausia superba and Thysanoessa macrura of the southwest Indian Ocean during summer (February 1981). Deep-Sea Res. 37,1135-1143.], based on somatic lengths, allows studying certain morphological differences within the populations sampled. Morphologically different and bigger males 11 (D.I. from 2.8 to 3.5) were present only in the southern transect while smaller males I (D.I. from 3.5 to 5.0) were present over the entire area. Biochemical composition of both species showed significant differences among stations for protein, lipids, and carbohydrates. A significant difference in lipid content was found between males I, and males II. For T macrura, percentage of lipid content in mature animals was much higher than that in E. superba. The D.I. size distribution showed that when populations of E. superba were highly differentiated (corresponding to mature animals) in morphology, lipid content was high, and they were located near a gyre. Differences in morphometry can influence distribution of the species, because different developing stages have different swimming capacities. It is shown that, together with hydrography and trophic conditions, lipid content and morphometry of krill populations, are different but complementary aspects that help to understand krill ecology and distribution. (C) 2009 Elsevier B.V. All rights reserved

Mesoscale eddies in the northeastern Pacific tropical-subtropical transition zone: Statistical characterization from satellite altimetry

International audienceMesoscale eddies in the northeastern Pacific tropical-subtropical transition zone (16°N-30°N; 130°W-102°W) are analyzed using nearly 18 years of satellite altimetry and an automated eddy-identification algorithm. Eddies that lasted more than 10 weeks are described based on the analysis of 465 anticyclonic and 529 cyclonic eddy trajectories. We found three near-coastal eddy-prolific areas: (1) Punta Eugenia, (2) Cabo San Lucas, and (3) Cabo Corrientes. These three areas are located in places where the coastal morphology changes abruptly and strong surface current intensification occurs at some phase of the seasonal cycle. Although mesoscale eddies in these areas have been previously reported, this study provides their first statistically supported characterization. Punta Eugenia showed the highest eddy production (with more cyclones generated), followed by Cabo Corrientes (also with more cyclones) and Cabo San Lucas (with more anticyclones). Cabo Corrientes eddies showed the highest mean values in propagation speed, swirling speed and eddy kinetic energy, whereas Punta Eugenia eddies showed the lowest values. Cyclonic eddies increased their distance traveled and duration from south to north; in contrast anticyclonic eddies increased from north to south. In average, anticyclones tend to travel faster than cyclones in all the subregions. These long-lived eddies were mainly nonlinear and therefore can redistribute coastal waters relatively far into the open ocean. The peaks in the seasonal signal of eddy generation can be associated with the peaks in the strength of the offshore currents and/or in the Coastal Upwelling Index. No clear relationship could be established between El Niño events and eddy generation