A seesaw in the South Pacific western and eastern subtropical mode waters

The South Pacific western subtropical mode water (WSTMW) and eastern subtropical mode water (ESTMW) are important components of the subtropical meridional overturning cells. By synthesizing observations, reanalysis products, and eddy-resolving ocean model results, we find an interannual volume seesaw in the WSTMW and ESTMW during 1980–2020. Further analysis of the data and model outputs indicates that this volume seesaw is closely related to the El Niño–Southern Oscillation through its modulation of the Ekman pumping velocity, buoyancy fluxes, and mixed layer depth over the formation regions of the two water masses. This negative correlation between the WSTMW and ESTMW modulates the depth of the permanent pycnocline and directly contributes to the east–west contrast between the two water masses in their core layer properties, especially, during negative phase of the Interdecadal Pacific Oscillation since 2000.