Pausing of the ENSO Cycle: A Case Study from 1998 to 2002

The heat balance of the surface mixed layer is analyzed at the eastern equatorial Pacific Ocean (0°, 140°W) in order to examine the transition from the 1998 La Niña to the 2002 El Niño. The data used are observations from the Tropical Atmosphere Ocean/Triangle Trans-Ocean Buoy Network (TAO/TRITON). Results show that interannual variation of eddy heat flux due to tropical instability waves slows the transition from La Niña to El Niño. Previous studies have described this slow transition as a pausing period of the ENSO cycle; that is, La Niña lingers and El Niño does not immediately appear despite a deepened thermocline. Heat balance analysis shows that the vertical heat advection anomaly and surface heat flux anomaly warm the mixed layer from 1999 to 2002. These warming anomalies cause the rise of the mixed layer temperature anomaly in the transition from La Niña to El Niño. In contrast, a cooling anomaly of the horizontal heat advection reduces the warming anomaly and slows down the transition from La Niña to El Niño. In horizontal heat advection terms, the eddy heat flux anomaly significantly contributes to the cooling anomaly associated with weakened variability in the 14–50-day-period band, that is, weakened tropical instability waves. During the transition from La Niña to El Niño, the meridional shear between the South Equatorial Current (SEC) and North Equatorial Counter Current is weakened because of the eastward current anomaly at the equator (i.e., weakened SEC) associated with relaxing trade winds. Weakened shear would suppress tropical instability waves. The results presented here suggest that the synoptic-scale processes work effectively at the basin scale to slow down the transition from La Niña to El Niño.