First, we look at sea-surface height. The gray sheet rises and dips as much as 30 centimeters from normal. Next, we map sea-surface temperature. Finally, black arrows are added to mark sea-surface winds. A weakening of the trade winds in the far western Pacific fuels an eastward-moving wave.
Sea level is raised as the windswept wave progresses.
The wave's arrival at the South American coast reduces the normal upwelling of cold water and warms the surface by as much as 5 degrees Celsius. Intense atmospheric convection associated with equatorial ocean warming causes local winds to converge, abating the trade winds' strength and extending the warm pool of water into the Eastern Pacific. Cyclone Susan barreled through the South Pacific in early January 1998. The Tropical Rainfall Measuring Mission satellite, launched in November 1997, observed rain height and coverage. Red coloring indicates rainfall exceeding 5 centimeters per hour.