An Analysis of the Linkage of Pacific Subtropical Cells with the Recharge–Discharge Processes in ENSO Evolution

Abstract

The Simple Ocean Data Assimilation, version 2.2.4 (SODA 2.2.4), analysis for the period of 1960-2010 is used to study the variability of Pacific subtropical cells (STCs) and its causal relation with tropical climate variability. Results show that the interior STC transport into the equatorial basin through 98S and 98N is well connected with equatorial sea surface temperature (SST) (98S-98N, 1808-908W). The highest correlation at interannual time scales is contributed by the western interior STC transport within 1608E and 1308W. It is known that the ENSO recharge-discharge cycle experiences five stages: the recharging stage, recharged stage, warmest SST stage, discharging stage, and discharged stage. A correlation analysis of interior STC transport convergence, equatorial warm water volume (WWV), wind stress curl, and SST identifies the time intervals between the five stages, which are 8, 10, 2, and 8 months, respectively. A composite analysis for El Niñodeveloping and La Niña-developing events is also performed. The composited ENSO evolutions are in accordance with the recharge-discharge theory and the corresponding time lags between the above denoted five stages are 4-12, 6, 2, and 4 months, respectively. For stronger El Niño events, the discharge due to interior STC transport at 98N terminates earlier than that at 98S because of the southward migration of westerly winds following the El Niño peak phase. This study clarifies subsurface transport processes and their time intervals, which are useful for refinement of theoretical models and for evaluating coupled ocean-atmosphere general circulation model results.

The Distinct Contributions of the Seasonal Footprinting and Charged‐Discharged Mechanisms to ENSO Complexity

“…We first selected the months in which the PC2 (PC3) values are larger than one standard deviation for the composite of the positive SF (CD) mechanisms. It should be noted that previous studies have found that the charging associated with La Niña and the discharging associated with El Niño can be asymmetric in amplitude (Chen et al, 2015;Hu et al, 2017); however, this asymmetry differs from the transition asymmetry discussed here. Figures 2a-2d show the evolutions of the composited SST anomalies along the equatorial Pacific (5°S-5°N) during a period of 12 months before to 12 months after the peak of the two onset mechanisms.…”

“…The evolution shows little asymmetry during ENSO transitions between positive and negative phases. It should be noted that previous studies have found that the charging associated with La Niña and the discharging associated with El Niño can be asymmetric in amplitude (Chen et al, 2015;Hu et al, 2017); however, this asymmetry differs from the transition asymmetry discussed here.…”

Hemispheric Asymmetry of the Pacific Shallow Meridional Overturning Circulation

“…We find a temporal asymmetry of the STC transports in the two hemispheres, whereby the NH STC leads the SH STC by 3 months. As most studies concentrate on the subsurface STC convergence which is proposed to occur in phase across the hemispheres (e.g., Chen et al, ; Cheng et al, ; Farneti et al, ; Izumo, ; Lbbecke et al, ; Schott et al, ), this temporal asymmetry has only briefly been mentioned before by Ishida et al (). In their study, the authors highlight the temporal asymmetry between the NH and SH upper ocean meridional transports.…”

The warm Blob in the northeast Pacific—the bridge leading to the 2015/16 El Niño

“…The surface patterns associated with these VM events did not extend far enough toward the equator compared with those associated with other VM events related to ENSO. To initiate ENSO development, the preconditioned tropical subsurface heat content and its propagation are required to provide favorable tropical ocean conditions at interannual time scales (Chen et al 2015, Levine and McPhaden 2016, Tseng et al 2017. In terms of the 2015/16 El Niño, the warm water content was high from spring 2014 to early 2016 (www.pmel.…”