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.

Spatial distribution and the interdecadal change of leading modes of heat budget of the mixed-layer in the tropical Pacific and the association with ENSO

“…11e, f). Again, the variance changes confirm the weakening of ocean variability at the ENSO time scales in the equatorial Pacific mixed layer since 2000s (McPhaden 2012;Hu et al 2013a;Kumar and Hu 2014;Wen et al 2014).…”

The relationship between thermocline depth and SST anomalies in the eastern equatorial Pacific: Seasonality and decadal variations

“…Moreover, the much longer delay in the central Pacific, in addition to the local advection/mixing, may also be associated with more basin-wide adjustment to thermocline perturbations [e.g., Philander, 1981]. A recent study by Wen et al [2014] also suggested that the characteristics of the lagged relationship underwent a change after 1999 (see their Figure 1). …”

“…For example, the studies so far [Harrison and Vecchi, 2001;Zelle et al, 2004;Wen et al, 2014] did not consider the role of seasonality in modulating the thermocline-SST relation. In fact, both the SST and the zonal slope of the equatorial thermocline experience significant annual cycle.…”

Thermocline Fluctuations in the Equatorial Pacific Related to the Two Types of El Niño Events

“…For instance, Ren and Jin (2013) suggested that thermocline feedback plays a dominant role in the growth and phase transitions of CP El Niño events, whereas the zonal advective feedback contributes mainly to their phase transitions. Wen et al (2014) proposed that the enhanced off-equatorial thermocline plays an important role in the development of El Niño events after 1999. Nevertheless, all of the findings suggest that equatorial Pacific thermocline dynamics play a dominant role during the life cycle of the CP El Niño.…”

“…Based on the interannual variability of upperoceanic heat content, Ren and Jin (2013) noted that the life cycles of both CP and EP ENSO events could be explained by the recharge-discharge process associated with the thermocline feedback, which is a dominant process in the regulation of the development and phase transitions of both types of El Niño events. Therefore, the thermocline variations in the tropical Pacific may be a precursor of the evolution of the two El Niño types (Wen et al 2014). However, Kao and Yu (2009) presented an opposing viewpoint that equatorial Pacific thermocline variations are not crucial in the development of CP El Niño events.…”