No abstract

Satellite and atmospheric model fields are used to describe the wind forcing, surface ocean circulation, temperature and chlorophyll-a pigment concentrations along the coast of southern Chile in the transition region between 38° and 46°S. Located inshore of the bifurcation of the eastward South Pacific Current into the equatorward Humboldt and the poleward Cape Horn Currents, the region also includes the Chiloé Inner Sea and the northern extent of the complex system of fjords, islands and canals that stretch south from near 42°S. The high resolution satellite data reveal that equatorward currents next to the coast extend as far south as 48°−51°S in spring-summer. They also display detailed distributions of forcing from wind stress and wind stress curl near the coast and within the Inner Sea. Between 38°−46°S, both winds and surface currents during 1993–2016 change directions seasonally from equatorward during summer upwelling to poleward during winter downwelling, with cooler SST and greater surface chlorophyll-a concentrations next to the coast during upwelling, opposite conditions during downwelling. Over interannual time scales during 1993–2016, there is a strong correlation between equatorial El Niño events and sea level and a moderate correlation with alongshore currents. Looking more closely at the 2014–2016 period, we find a marginal El Niño during 2014 and a strong El Niño during 2015 that connect the region to the tropics through the oceanic pathway, with some atmospheric connections through the phenomenon of atmospheric blocking (as noted by others). The period also includes a Harmful Algal Bloom of the dinoflagellate Alexandrium catenella during early-2016 that occurred during a sequence of physical conditions (winds, currents and temperatures) that would favor such a bloom. The most anomalous physical condition during this specific bloom is an extreme case of atmospheric blocking that creates a long period of calm in austral autumn after strong upwelling in austral summer. The blocking is related to the 2015–2016 El Niño and an unusual coincident positive phase of the Southern Annular Mode.

The physical forcing of the upwelling along the subtropical west coasts of the continents encompasses a broad range of time scales which shape both phytoplankton biomass (Chl-a) and primary productivity (carbon fixation) changes over any given time interval. The narrow continental shelf and the steep alongshore orography off north-central Chile provide for a unique combination of year-round, upwelling-favorable winds with quasi-weekly upwelling pulses associated with atmospheric coastal-trapped disturbances (coastal lows). This variability is modulated by intraseasonal oscillations in the depth of the thermo/nutricline, produced by coastal-trapped waves in the ocean, upon which annual (seasons) and interannual (ENSO) cycles are superimposed. During coastal field experiments off Cruz Grande bay (29º S), carried on in November 1987 and 1988 (opposite extremes of the ENSO cycle), mean changes of the phytoplankton-integrated Chl-a (B) and carbon fixation rate (PP) from the active to the relaxed phases of the local upwelling forcing cycle (phytopattern) were characterized. Those data were contrasted against similar ones reported off Punta Lengua de Vaca (Coquimbo, 30º S) and off Mejillones peninsula (Antofagasta, 23º S), encompassing different seasons and phases of intraseasonal and interannual (ENSO) cycles from 1992 to 1997. A "warm" phyto-pattern was schematically characterized by a significant increase in B and a quasi-steady evolution of PP from the active to the relaxed phases of one complete upwelling event. Conversely, relative small changes in B and a significant increase in PP characterized a "cold" phyto-pattern. It is proposed here that the ENSO "cold/warm" signal may be offset by more than one opposite "thermal" condition (seasonal and/or intraseasonal) in defining a "warm" or "cold" phyto-pattern associated with a particular cycle of the local upwelling forcing.Key words: Chile, coastal upwelling, phytoplankton, local and remote upwelling forcing cycles, coastal-trapped waves, ENSO cycle. RESUMENEl forzamiento físico de la surgencia a lo largo de las costas occidentales de los continentes comprende una amplia gama de escalas temporales que configuran los cambios en la biomasa fitoplanctónica (Cl-a) y en la productividad primaria (fijación de carbono) sobre un intervalo de tiempo cualquiera. La estrecha plataforma continental y la fuerte pendiente orográfica a lo largo de la costa en el norte y centro de Chile proporcionan una combinación única de vientos favorables a la surgencia durante todo el año con pulsos de surgencia cuasi-semanales asociados con perturbaciones atmosféricas atrapadas a la costa ("bajas costeras"). Esta variabilidad es modulada por oscilaciones intraestacionales en la profundidad de la termo/nutriclina producida por ondas oceánicas atrapadas a la costa, sobre las cuales se superponen los ciclos anuales (estaciones) e interanuales (ENOS). Durante los experimentos costeros frente a la bahía de Cruz Grande (29,5º S) realizados en noviembre de 1987 y 1988 (extremos opuestos del...

No abstract

, and photosynthetic rates concomitantly doubled from 15 to 35 µg C l -1 d -1 . Water transparency declined, as indicated by the vertical diffusion attenuation coefficient K d for photosynthetically active radiation (PAR), which increased from 0.076 to 0.090 m -1 and decreased the euphotic depth from 60 to 45 m. During this time, a significant increase in silica deposition by the diatoms was also detected. We attribute the mentioned changes in environmental characteristics to wind-generated surface currents. Strong winds (up to 10 m s -1 ) during the measurements enriched the surface layers with unusually high nutrient concentrations within <1 d. Hence, primary production rates were observed at a relatively eutrophic nearshore station (MP) and pelagic station (A 1 , 10 km towards the center of the Gulf). They were compared with rates measured on the subsequent day. Values measured were twice as high as those at the pelagic station during the previous calm day. Routine monitoring programs with monthly or semi-weekly sampling are thus likely to miss brief but significant injections of nutrients, leading to the underestimation of seasonal and annual primary production. Our results demonstrate the impacts of transient events on the function and annual yield of aquatic ecosystems.