What Is Enso

The El Niño-Southern Oscillation (ENSO) is a coupled atmospheric-oceanic phenomenon that has global manifestations and recurs approximately every 2 to 10 years. The atmospheric component of ENSO is the Southern Oscillation, an interannual seesawing of sea-level atmospheric pressure anomalies between northern Australia (Darwin) and the southeast Pacific (near Tahiti). There is both a "warm phase" (El Niño) and a "cold phase" (La Niña). The warm phase involves an extensive warming of the upper ocean along the central and eastern equatorial Pacific and a depression of the thermocline (the boundary that separates the warmer mixed upper layer of the ocean from the cold abyss) in the eastern tropical Pacific. The cold phase involves a cooling of the upper ocean and a rise of the thermocline toward the ocean's surface in the eastern tropical Pacific.

During an ENSO warm phase, as the thermocline deepens, wind-driven coastal upwelling off the shores of South America carries warmer water than usual to the surface. Coastal seasurface temperature anomalies as high as 10°C have been recorded off Peru (Sharp and McLain, 1993).

The Southern Oscillation leads to a cyclic increase and decrease in the strength of the Southern Hemisphere (southeast) trade winds. These winds are strongest during the oceanic cold phase, when sea-level atmospheric pressure in northern Australia, normally low, is anomalously lower, while that in the southeast Pacific, normally high, is anomalously higher. During the oceanic warm phases, when the sea-level


pressure in northern Australia is anomalously high and that in the southeast Pacific is anomalously low, the trade winds weaken, and in extreme cases even blow westerly. There is positive interaction (e.g., feedback) between the ocean and atmosphere, as increased sea surface temperatures increase atmospheric pressure (Philander, 1990). For a comprehensive overview of the observations and mechanisms of the 1997— 1998 ENSO see McPhaden (1999).

Dramatic shifts of flora and fauna in waters off southern Colombia and Ecuador to Peru and northern Chile are linked to ENSO events (Arntz et al., 1985). In severe events, the increased ocean temperatures and reduced concentrations of phytoplankton negatively impact some pelagic species, such as the commercially important anchovy and sardines (see Figure 1). Tropical species of fish, however, may extend their ranges to the south and closer to shore, as warmer waters appear along the Peruvian coast. For an overview of the 1997 to 1998 event on biogeo-chemical cycles and on the use of satellite technology during this event, see Chavez et al. (1998), McPhaden (1999), and Carr and Broad (2000).

The workings of ENSO were first investigated in the mid-1960s by a researcher named Jacob Bjerknes (1966). Bjerknes linked the oceanic process off the Peruvian coast with the seesaw in atmospheric pressure between the western and central equatorial Pacific (i.e., the Southern Oscillation). A growing interest in the possible global connection of climatic events led to the establishment of the World Climate

Yearly Small Pelagic Catch: 1950-2000

70's Anchovy Decline

('72-73 ENSO? Overfishing? Interdecadal Fluctuation? Combination of factors?)

'97-98 ENSO

ENSO / \

A Industry \ Debt ($)

/VI Nationalization of Industry f\ / | Period of Sardine Abundance /

f \ 1.8 billion

\ /

California / sardine / decline /

I '82-83 K \ , ENSO / V

I 1

Peru industrial / fishery begins /

\ I \ /\ A ( Anchovy increase 11 V \ / Privatization of industry

----— —1—

' V (labor law shifts, heavy Investment in industry)





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  • antonietta
    What is ENSO in hydrology?
    4 months ago

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