El Nino, as everyone must know by now, is an occasional warming of the surface waters of the Pacific over a broad band in the low latitudes straddling the equator. It results from a periodic weakening of the trade winds, which in turn causes variations from the normal patterns of surface currents and convective mixing in that ocean.
This warmer water over the vast expanse of the tropical Pacific might be compared to shovelling extra coal into the great firebox of the global atmospheric engine. As we learn to understand it better, it is becoming clear that El Nino not only brings unusual, even freakish, weather to many subtropical places on the Pacific rim, but has implications for more northerly and more distant regions, in particular the southern states of the US, and perhaps even for us here in Europe.
But El Nino has a sister who, although shy and more retiring and receiving less publicity, brings her own pattern of disturbed behaviour. La Nina is the name given to episodes when the surface waters of the tropical Pacific are colder than they ought to be, and it imprints its own particular signature - almost a mirror image of El Nino - on the global weather. Places which suffer droughts during an El Nino experience prolonged torrential downpours when La Nina comes along - and vice versa.
El Nino means "the Boy Child" and is borrowed from the traditional name given to a warm ocean current that used to flow, and indeed still does, southwards near Peru and Ecuador around Christmas time each year; it recalls the coming of the infant Jesus. The term was borrowed by meteorologists to describe a much more widespread ocean-warming phenomenon and then, when it was discovered that there was an almost equal counterpart, "the Girl Child", or La Nina, seemed appropriate.
El Nino and La Nina alternate in a climatic pas de deux. El Nino events took place in 1969, 1972, 1977, 1983, and 1987; there was a long episode from 1991 to 1995, and the present strong El Nino began last year. There were pronounced La Nina episodes in 1955, from 1974 to 1976, and again in 1988.
An anomaly which concentrates the minds of climatologists is that from the 1950s until the late 1970s, La Nina and El Nino years were roughly balanced, but since then there have been more El Nino episodes, and not as many of La Nina. They will be watching closely to see if a La Nina develops later this year after the present El Nino has subsided.