Sea surface and upper ocean temperatures are controlled by the heat flux through the ocean surface, namely insolation, outgoing and incoming infrared radiation, sensible heat flux, latent heat flux and advection, including upwelling. While sea surface temperature warming was observed in most of the Atlantic and Indian Oceans, and the Western Pacific Ocean, the Central Tropical and the Eastern Pacific Ocean exhibited a cooling trend.

A recent study presents the results of direct observations of seawater temperature and salinity over the continental shelf off central-south Chile that shows an unprecedented cooling of the entire water column, and an increase in upper layer salinity during 2002 to 2013. It provides evidence that this phenomenon is related to the intensification but mostly to a recent southward displacement of the South Pacific High (SPH) over the same period, from 2007 on. The SPH resides at its northern position (26~30°S) during later austral fall and winter when it is also situated at its most coastal location (85~95°W). The shelf offshore central -south Chile (30~43°S) is the least impacted by the SPH during these seasons; thus, alongshore southerly winds are weakest. Equatorward winds are strongest during austral spring and summer when the SPH has moved southwest, located between 33-36°S and 100-108°W. This seasonal wind pattern in turn has accelerated alongshore, equatorward, subtropical coastal upwelling favourable winds, particularly during winter, injecting colder water from below into the upper water column.

Fig. 1. The South Pacific High. (a) A typical austral summer/spring position of the SPH with alongshore winds off-shore central-south Chile. (b) A typical austral fall/winter position of the SPH with on-shore winds off-shore central-south Chile. The orange square marks the location of Station 18.

Consequently, the environmental conditions on the shelf off central-south Chile shifted from a warmer (fresher) to a cooler (saltier) phase; water column temperature dropped from 11.7 °C (2003–2006) to 11.3 °C (2007–2012) and upper layer salinity rose by 0.25; water column stratification gradually decreased.

Fig. 2. Temperature, salinity and stratification. (a) Mean vertical temperature profiles; (b) Mean vertical salinity profiles; (c) Potential Energy Anomalies as a measure for water column stratification of the upper 50 m. All three panels for the first 4 years (October 2002 to September 2006, red) and the last 6 years (October 2007 to September 2013, red) of time series observations at Station 18. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Offshore central-south Chile, like in all the other eastern boundary current systems, wind-driven upwelling enriches the sunlit upper water column with nutrients thus stimulating primary and higher level biological productions. The biological impacts of such abrupt cooling are apparently already happening in this coastal ecosystem, as recent evidence shows substantial changes in the plankton community and negative trends in zooplankton biomass over the same period.

The major conclusions from this paper are:

• The lower water column temperature off-shore central Chile is proportional to the meridional position of the SPH and the upper layer salinity indirect proportional.
• The SPH strengthened by 2 hPa between 2002 and 2013.
• From 2007 on, the SPH resided during winter on average 2° of latitude poleward of its winter-time mean position (1979–2013).
• As a result, equator ward winds accelerated offshore central Chile during fall and winter, coastal upwelling intensified, thus cooling the entire shelf water column by 0.4 °C.
• Further, precipitation reduced giving rise to higher upper layer salinities.

Summary written by Jing Li

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Water-column cooling and sea surface salinity increase in the upwelling region off central-south Chile driven by a poleward displacement of the South Pacific High (Progress in Oceanography)

Wolfgang Schneider ^a,b,*, David Donoso ^c, José Garcés-Vargas ^d, Rubén Escribano ^a,b

1. Departamento de Oceanografía, Universidad de Concepción, Campus Concepción, Víctor Lamas 1290, Casilla 160-C, código postal: 4070043 Concepción, Chile
2. Millennium Institute of Oceanography (IMO), University of Concepcion, P.O. Box 1313, Concepcion 3, Chile
3. Departamento de Geofísica, Universidad de Concepción, Campus Concepción, Víctor Lamas 1290, Casilla 160-C, código postal: 4070043 Concepción, Chile
4. Instituto de Ciencias Marinas y Limnológicas, Facultad de Ciencias, y Centro FONDAP de Investigación en Dinámica de Ecosistemas Marinos de Altas Latitudes (IDEAL), Universidad Austral de Chile, Avenida Rector Eduardo Morales, Edificio Emilio Pugín, Valdivia, código postal 5090000, Chile

http://dx.doi.org/10.1016/j.pocean.2016.11.004