Abstract
Using the Roemmich‐Gilson Argo data set, this study investigates variability of the Subtropical Underwater (STUW) and eastern Subtropical Mode Water (ESTMW) in the South Pacific during 2004–2020. The STUW volume decreased during 2004–2013 and increased during 2013–2020, while the volume of the ESTMW shows the opposite phase. On interannual time scales, there is also a significant negative correlation in volume between the STUW and ESTMW. This anti‐phase relationship is attributed to changes in their volumetric subduction rates, which are in turn closely related to variability in the mixed layer depth (MLD). ENSO directly contributes to variability of the subduction rates by modifying the MLD. Equatorward propagation of spiciness anomalies is identified along isopycnal surfaces of the STUW and ESTMW cores. These spiciness anomalies in the downstream region are correlated with changes in volume of both water masses, and significant spiciness anomalies can reach the tropical Pacific.
Plain Language Summary
The Subtropical Underwater (STUW) and eastern Subtropical Mode Water (ESTMW) are two important water masses and have experienced significant changes in the upper South Pacific during the past decades. The formation of the two water masses is part of the subtropical overturning cells and is believed to play a role in transferring properties from subtropical region to the equatorial thermocline where water properties can affect the atmosphere. We found that the STUW volume decreased during 2004–2013 and increased after 2013, while the ESTMW volume changed oppositely. This anti‐phase relationship between the two water masses is due to changes in their volumetric subduction rates, which in turn mainly result from the varying mixed layer depth (MLD). Further, El Niño‐Southern Oscillation (ENSO) is found to play a role in modifying the MLD in the South Pacific Ocean. Causes and effects of these variabilities are discussed.
Key Points
The STUW volume decreased during 2004–2013 and increased after 2013, while the ESTMW shows an opposite phase
The anti‐phase relationship is attributed to changes in subduction rate which are closely related to variability in the mixed layer depth
Both the STUW and ESTMW volume anomalies are positively correlated with downstream spiciness anomalies
