Atmospheric conditions and coastal upwelling

Since we do not focus on the atmospheric forcing in this PhD work, only a brief presentation of the atmospheric conditions of the eastern South Pacific is made here. The main components of the atmospheric circulation in the eastern South Pacific region are schematized on figure 1.9:

- the Subtropical High, also called South Pacific High or south-eastern Pacific Subtropical Anticyclone: it is the main driver of atmospheric circulation in this region. As for

other eastern oceanic basins (North Pacific, North and South Atlantic), this high-pressure system drives equatorward trade winds due to its anticyclonic rotation (counter-clockwise in the southern hemisphere). The eastern branch of the anticyclone is limited to the East by the Andes mountains which force the trade winds to follow the coastline direction, generating persistent coastal upwelling off Peru and Chile. As in other EBUS’es, stronger trade winds offshore generate significant Ekman pumping in the 50-200km next to the coast (Bakun and Nelson, 1991). The strength of the upwelling-favorable winds is modulated by the seasonality in the position of the anticyclone (fig. 1.10).

Fig. 1.9: Schematic of the main atmospheric systems in the eastern South Pacific: the Inter-Tropical Convergence Zone (ITCZ), the StratoCumulus cloud deck (Stratus Deck), the Subtropical High (H), and Coastal Jets (low-level jet). The full-line arrow represents equatorward trade winds, and the dashed-line arrow represents the Ekman transport associated to coastal upwelling. The background colors for the ocean schematize annual mean SST (image courtesy Rob Wood, University of Washington).

During the austral summer, the center of the high-pressure system is located near (95°W, 32°S) and favours coastal upwelling from 40°S to the equator; during the austral winter, it moves northeastward to about (85°W, 27°S): as a consequence, winds off central Chile are weaker or even downwelling-favorable in winter, whereas they are stronger off Peru (Bakun and Nelson, 1991). For this reason, coastal upwelling is permanent off Peru but seasonal off central Chile. Off the northern Chile coast between 18°S and 23°S, winds are weaker all year long, and so is upwelling: as a result, SSTs are significantly warmer there than off the rest of Peru and Chile (fig. 1.11).

(a) (b)

Fig. 1.10: Climatological winds (a) and currents (b) during southern hemisphere (S.H.) winter (June-August) and summer (December-February). Winds are from the NCEP reanalysis at 1000 mb (Kalnay et al., 1996).

Regions of heavy precipitation are hatched. ITCZ is for Inter-Tropical Convergence Zone. H is for Subtropical High. EF is for Equatorial Front. Schematic vectors (b) indicate the location and relative strengths of the main currents: the West Wind Drift (WWD), the Equatorial Undercurrent (EUC), the South Equatorial Current (SEC), the North Equatorial Countercurrent (NECC), the Colombia Current (CC), the Annual El Niño Current (AENC), the Peru Current (PC), the Peru-Chile Countercurrent (PCCC), the Poleward Undercurrent (PUC), the Peru Coastal Current (PCC), the Chile Coastal Current (CCC) and the Cape Horn Current (CHC). Adapted from Strub et al. (1998).

- Coastal Jets: these are episodes of very strong alongshore winds (up to 15 m/s) blowing off central Chile between 30°S and 35°S in austral spring and summer (Garreaud and Muñoz, 2005; Muñoz and Garreaud, 2005; Renault et al., 2009). Some are also found off central Peru in the Pisco/San Juan area (15°S) and northern Peru near Paita (4°S), with maximum velocities (~7-8 m/s) in austral fall and winter (Renault, 2008). They have direct consequences on the strength of coastal upwelling due to Ekman transport and pumping (Shaffer et al., 1999).

- the Inter-Tropical Convergence Zone (ITCZ), which limits the eastern South Pacific High at its northern edge, is the zone of convergence of northern and southern hemisphere trade winds, and is present around the planet in the tropical zone. It is characterized by enhanced deep convection and heavy rainfall. In the eastern tropical Pacific, it is located near 10°N in the austral winter and near 5°N in summer: although it has a strong impact on the regional climate of the countries surrounding the Panama Bight (Ecuador,

Colombia, Central American countries), its influence does not go southwards beyond 5°S, so it is generally not considered when studying the dynamics of the HCS.

Fig. 1.11: Climatological mean SST (°C) in the eastern South Pacific from AVHRR Pathfinder satellite data (Vazquez et al., 1995).

- the StratoCumulus cloud deck (SCu deck) is the widest persistent stratocumulus cloud cover in the world (Klein and Hartmann, 1993). Its presence in the eastern South Pacific limits solar penetration and thus radiative forcing of the upper ocean: combined with upwelling, it makes ocean temperatures off Peru the coldest in the tropical band (Yu and Mechoso, 1999). Moreover, its contribution to the heat budget of the upper ocean is significant not only at the regional scale (Takahashi, 2005) but also at the global scale.

However, the fine-scale air-sea couplings that drive its formation, structure and variability have not been very well documented so far. Coarse-resolution global climate models are not able to reproduce the SCu deck: as a result, the simulated ocean temperatures are too warm in the region, inducing important biases in the zonal gradient of SST in the tropical Pacific and thus in the Walker circulation, which is one of the main mechanisms of tropical climate variability at the global scale. This is an important issue because such biases introduce large uncertainties in climate change predictions. Improving our understanding of climate dynamics in this region is a priority for the scientific community working on climate change and has led to the development of the VOCALS project (http://www.eol.ucar.edu/projects/vocals/) which comprises both extensive oceanographic and atmospheric field measurements off Peru and

Chile and a modelling framework. I have had the chance to participate to the VOCALS-Rex Peru oceanographic survey conducted aboard the R/V Jose Olaya (IMARPE, Peru) off the Pisco-San Juan area (around 15°S) in October 2008 (fig. 1.12). Field measurements allowed characterizing the atmospherical (radiosonde launchings), oceanographical (CTD, ADCP, surface drifters and a glider), biogeochemical (oxygen, fluorescence, chlorophyll-a…) and biological conditions (Hensen net, hydroacoustics) during an episode of cloud clearing due to the presence of a strong coastal jet. Understanding the complex interactions between the atmosphere, the ocean and the marine ecosystem is the topic of current research conducted at IRD (Institut de Recherche pour le Développement), IMARPE (the Peruvian Marine Research Institute) and IGP (the Peruvian Institute of Geophysics).

Fig. 1.12: Original track and stations of the VOCALS-Rex Peru cruise conducted aboard the R/V Jose Olaya (IMARPE) from 10/02/08 to 10/17/08 off the Pisco-San Juan area in southern Peru. The background image is for climatological cloud clearing rate (%) for October derived from SeaWIFS data.