E. Linacre and B. Geerts
The South Pacific high
The colony of small penguins on the Galapagos Islands (0°S) suffer in El Niño years, as the sea surface temperature (SST) is 2-6 K higher than normal (1). Upwelling is suppressed both along the Peruvian coast and along the equator, and the Humboldt ocean current is weaker than normal off Peru. More surprisingly, the population of larger penguins of southern Chile (40-50°S) also declines in some El Niño years, because of a weaker Humboldt Current. Both the coastal upwelling and the Humboldt Current are driven by the low-latitude anticyclonic winds over the southeastern Pacific.
Thus there is another factor to be considered in assessing the strength of El Niño’s: the Central Pressure of the Anticyclone (CPA) over the southeastern Pacific. The stronger the CPA, the colder the ocean current. This high will be stronger when the SST is lower (boundary layer temperatures matter, as for the Siberian high in winter). Between April and October 1997 (a strong El Niño year) the CPA in the southern Pacific fell from 1024 hPa to 1015 hPa. In the same year, the highest pressure anomaly shifted southwards from 17°S in March to 27°S in May, to 40°S in June, all at about 77°W.
Ocean currents and SSTs
On average, the surface of the Humboldt Current (which recurves along the equator to become the Equatorial Current) warms from 15°C at 40°S to 29°C at 140°W, 19,000 km downstream. At a point on the equator, normally at about 150°E, the westwards surface current dives beneath the well-known ‘warm pool’, to a depth of 150 - 300 m. This slope in the thermocline is known as the Equatorial Front in the ocean. The front shifts eastward during an El Niño event, e.g. to 140°W, which is equivalent to an enlargement of the warm pool.
The surface currents in the equatorial central Pacific change during an El Niño. In a La Niña year, there is a strong westward North Equatorial Current at about 10°N, a weak and narrow eastward Counter Current at 5°N, and a moderately powerful westward Equatorial Current at the equator (extending to 5°S). At about 110 - 150 m beneath the Equatorial Current, there is an appreciable Equatorial Under Current, flowing eastward. This countercurrent is best defined in the central Pacific, east of the Equatorial Front. In an El Niño year all currents are weaker, especially the Equatorial Current. This is because the Trade winds have slackened in response to a weaker east-west pressure gradient, and also a lower CPA.
The Equatorial Current and the upwelling off Peru lead to a difference between west and east surface temperatures which is normally about 12 K. But it was only 7 K during the super-El Niño in August 1997, on account of the warmth in the central and eastern equatorial Pacific, where the Trade winds were weaker. Simultaneously there was a cooling of temperatures in the equatorial Atlantic up to 3.5 K, and a smaller cooling in the Indian Ocean.
Sea-surface temperatures (SST) off Peru indicate El Niño’s in 1951, 53, 57, 65, 69, 72, 76, 82-3, 87, 92, 94, 1997, i.e. twelve in 48 years. SST’s over 48 years show that there may be two peaks within a particular El Niño. There is a well-defined seasonal cycle of SST off Peru, with a maximum in March and a minimum in August. An El Niño is usually first felt in December (although it may be earlier). The highest SST anomalies during both an El Niño and a La Niña usually occur in May or June. La Niña SST anomalies usually last longer and they are more widespread, whereas SST anomalies in El Niño years are more concentrated to the Peruvian coast and the central equatorial Pacific.
There are no real precursors but the following clues may indicate an El Niño’s onset -
(1) Fujita, T.T. and S. Fujita 1998. Mystery of El Niño and hurricanes; overview of present, distant past and future. Paper 249 (Wind Res. Lab. , Univ. Chicago) 73pp.