2.1. The Coupled Ocean-‐Land-‐Atmosphere System in the Eastern Tropical Pacific descending branch of the Hadley-‐Walker circulation. The large static stabilities associated with cold SSTs and atmospheric subsidence result in extensive marine stratus cloud deck. Although these clouds reflect much of the incoming solar radiation, they interfere very little with the loss of energy via thermal radiation, resulting in less radiative heating of the cold water region than in the warm water area (Ma et al., 1996). The overall effect of the distribution of evaporation and radiative heating and cooling is thus reinforcement of the preexisting thermal contrast illustrated in Figure 2.2.
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Figure 2. 1 Schematic diagram of surface water masses and currents in the eastern tropical Pacific Ocean (modified
from Fiedler and Talley, 2006). (a) Mean surface temperature, and (b) mean surface salinity of the eastern tropical Pacific. The EECT extends out from the west coast of South America westward along, and south of, the equator. The eastern Pacific warm pool is centered along the coast of southwestern Mexico and Guatemala. TSW is characterized by low salinity and high temperature (S<34 p.s.u, T>25°C). ESW properties (S>34 p.s.u, T<25°C) are determined by the seasonal advection of cooler and saltier water from the Peru Current and by equatorial upwelling.
The EECT and the coastal ocean off Peru and Chile are coupled to the atmosphere through surface wind stress and heat exchange. The winds in this region are typically southeasterly.
However, the strength and spatial distribution of the surface winds have shown to be closely tied to the frontal location and strength. The surface wind stress in this southeasterly cross-‐
equatorial flow decreases by more than a factor of 4 over the cold tongue and then increases by almost the same amount to the north of the cold tongue (Chelton et al., 2001). The modulation of the surface wind field by the SST provides a potential mechanism for two-‐way air–sea coupling since the surface interactions are coupled to the atmospheric circulation through horizontal gradients of latent heat release in the vicinity of ITCZ convection, as well as radiative and sensible heating gradients, especially in the atmospheric boundary layer (Chelton et al., 2001). The upper ocean heat budget is strongly coupled to radiative effects of the
Study area: The Eastern tropical and subtropical Pacific.
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extensive decks of boundary layer clouds in the southeast tradewinds, off the Peru and Chilean coasts, and their extension into the equatorial zone (Raymond et al., 2004). Temperature of the EECT decreases towards the east as progressively cooler waters are upwelled from the Equatorial Undercurrent that shoals as it flows from west to east (Fiedler and Talley, 2006).
The intensity and spatial extent of the EECT and associated fronts change seasonally (E.g.
Mitchell and Wallace, 1992) and interannually (E.g. Deser and Wallace, 1990). During the equatorial warm season (March through June) SST colder than 25°C are confined to the upwelling region near the South American coast; the equatorial front is weak or nonexistent (Hays et al., 1989). During the cold season (July through November) the equatorial cold tongue is well developed and SST cooler than 25°C extends westward to 130°W (Figure 2.3a). The EECT seasonal amplitude is ±1–3°C, with coldest temperatures during the Southern Hemisphere winter-‐spring (August-‐September). Surface winds vary in concert with the changes in the SST distribution, meaning that southeasterly winds are strongest during the cold season (Figures 2.3a, c). During El Niño years, SST in the cold tongue is elevated throughout the year;
and there is a southward penetration of warm water, and an abnormal displacement of the front towards the west while, in the east the equatorial front almost disappears (Hays et al., 1989).
Figure 2. 2 Idealized cross-‐sections through the ITCZ–cold tongue complex at approx. 95°W in the east Pacific
showing the atmospheric meridional circulation, atmospheric boundary layer depth, and the oceanic thermal structure, from Raymond et al., (2004). SEC=South Equatorial Current, NECC = North Equatorial Countercurrent, and the EUC = Equatorial Undercurrent. Southeasterly and northeasterly trade winds in the planetary boundary layer (PBL) converge onto the ITCZ (heavy clouds), located over the warmest SST. Encircled x’s (dots) denote westward (eastward) flowing winds or currents.
The rainfall climatology of the eastern tropical Pacific is dominated by the ITCZ. The northeasterly and southeasterly tradewind belts, which occupy most of the eastern tropical and subtropical Pacific, are noted for fair weather and a large excess of evaporation over precipitation, while narrow ITCZ that separates them is marked by heavy and persistent rainfall.
The annual cycle of the surface circulation is characterized by a latitude position of the wind confluence and ITCZ closest to the equator in March-‐April, a northward shift till June, a southward displacement in July and August, and a northernmost position in September (Figures 2.3b and c; Hastenrath, 2002).
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Figure 2. 3 Seasonal climatologies of (a) SST (°C, colors) and surface currents (m/s, vectors), (b) sea surface salinity
(PSU, colors) and rainfall rates (mm/day, contour lines which are associated to the ITCZ) and (c) convergence (positive values) and divergence (negative values) of surface winds (*10-‐5 s-‐1, colors, black lines represents a value of zero) and surface winds (m/s, vectors). Taken from Garcés (2005).
In addition to the ITCZ, another convergence zone exists but in the Southern Hemisphere (SITCZ), occurring at 8°S–2°S, 130°W–90°W in the southeast Pacific (Halpern and Hung, 2001).
The positions and intensities of the ITCZs are highly sensitive to the underlying sea-‐surface temperature distribution. For instance, only in March and April, when SSTs south of the equator approach those to the north, it is possible to observe both intertropical convergence zones, with convection north and south of the equator (Halpern and Hung, 2001). Five features
Study area: The Eastern tropical and subtropical Pacific.
42 with increasing depth (Fiedler and Talley, 2006). This thermocline/productivity linkage occurs because the thermocline almost invariably coincides with the nutricline, defined as that photosynthetic carbon uptake of an elevated phytoplankton biomass supported by upwelled macronutrients (nitrate, phosphate and silicic acid) and micronutrients. Although seasonal variability in phytoplankton assemblages occurs in Peruvian waters, large diatoms tend to dominate the biomass in phytoplankton blooms that develop in these coastal upwelling regimes in all seasons (E.g. de Mendiola, 1981; Abrantes et al., 2007), and it has been argued that diatom-‐driven new production efficiently fuels the food chains leading to fish production (Smetacek, 1998). With this rich productivity, it is surprising that high-‐nitrate, lower than expected chlorophyll (HNLC) conditions have been reported for the Peru upwelling regime.