Changes in Tropical Pacific Thermocline Depth and Their Relationship to ENSO after 1999

Abstract The characteristics of El Niño–Southern Oscillation (ENSO) variability have experienced notable changes since the late 1990s, including a breakdown of the zonal mean upper-ocean heat content as a precursor for ENSO. These changes also initiated a debate on the role of thermocline variations on the development of ENSO events since the beginning of the twenty-first century. In this study, the connection between thermocline variations and El Niño and La Niña events is examined separately for the 1980–98 and 1999–2012 periods. The analysis highlights the important role of thermocline variations in modulating ENSO evolutions in both periods. It is found that thermocline variation averaged in the central tropical Pacific, including both equatorial and off-equatorial regions, is a good precursor for ENSO evolutions before and after 1999, while the traditional basinwide mean of equatorial thermocline variation is a good precursor only before 1999. The new precursor, including both high-frequency variability in equatorial regions and low-frequency variability in off-equatorial regions, is found to be indicative of multiyear persistent warm and cold conditions in the tropical Pacific. Further, it is found that the strength of the subtropical cells (STCs) interior mass transport in both hemispheres increased rapidly around the late 1990s. It is proposed that the strengthened STC interior transports provide a pathway for the enhanced influence of off-equatorial thermocline variations on the development of ENSO events after 1999.