The opposite of El Niño is called La Nina, which describes the cold version of El Niño. La Nina usually prevails in the eastern Pacific following El Niño and effects our climate by the intensifying upwelling, enhancing the trans-Pacific SLP gradient (the degree by which the ocean level is different) and by increasing the trade winds. Convection over Australia and Indonesia increases, where as the American coast receives very little rain.

La Nina (cold) ConditionsDecember 1988 Normal ConditionsDecember 1990 El Niño (warm) ConditionsDecember 1997
(Taken from www.pmel.noaa.gov)