11:15 AM - 11:30 AM
[ACG30-03] Isopycnal thickness anomalies and their propagation associated with mode water variability in the upper North Pacific
Keywords:isopycnal thickness, mode waters, stratification, North Pacific
The thermohaline variations in oceans can be considered in two mechanisms. One is the density-compensated temperature and salinity anomalies, so-called spiciness anomalies. The other is the vertical displacement of isopycnals, which can be traced by the thickness anomalies of the isopycnal layers. While our investigation of spiciness anomalies has been published in Wang et al. (2022), this presentation will introduce our findings of thickness anomalies and their propagation in the upper North Pacific during 2004-2018 with links to the variability of mode waters: Subtropical Mode Water (STMW), Central Mode Water (CMW), and Eastern Subtropical Mode Water (ESTMW).
The investigation based on Argo observations indicates that the volume of STMW is dominated by a decadal variability, and the thickness anomalies in it have a faster response to the variability of Kuroshio Extension (KE) compared to spiciness anomalies. Its lighter and denser surfaces showed opposite variations in thickness anomalies in some years, along with the variations of potential vorticity, and the mutually offsetting changes of late-winter mixed layer and outcrop area, providing insights into the formation of STMW. The volume of CMW decreased with a quasi-decadal variability. Thickness anomalies in CMW were simultaneous between different latitudes but showed different time scales between west and east sides, inconsistent with the direction of mean flow .The volume of ESTMW showed a quasi-decadal variability with a rapid shrinking in 2010-2011. Its lower part showed thickness anomalies synchronized with those at the upper STMW in most of the years. The two have a similar density range, implying that there might be a large-scale stratification change. Related to this, a preliminary discussion is conducted on the relationships between the anomalies and the KE variability and Pacific Decadal Oscillation.
The propagation of the anomalies of a conserved tracer, potential thickness, is generally consistent with the direction of mean flow in subtropics but showed a propagation speed larger than geostrophic speed in the western subtropics, similar to the propagation of the passive tracer, spiciness anomalies. However, compared to spiciness anomalies, potential thickness anomalies decayed more rapidly and propagated with a larger westward component. Additionally, we estimated the eddy-induced thickness flux using a high-resolution reanalysis dataset and verified the effects of eddies in transporting anomalies.
Overall, this study provide instructive results on the dynamical changes in the upper North Pacific and how they are related to the thermohaline conditions in remote regions.
The investigation based on Argo observations indicates that the volume of STMW is dominated by a decadal variability, and the thickness anomalies in it have a faster response to the variability of Kuroshio Extension (KE) compared to spiciness anomalies. Its lighter and denser surfaces showed opposite variations in thickness anomalies in some years, along with the variations of potential vorticity, and the mutually offsetting changes of late-winter mixed layer and outcrop area, providing insights into the formation of STMW. The volume of CMW decreased with a quasi-decadal variability. Thickness anomalies in CMW were simultaneous between different latitudes but showed different time scales between west and east sides, inconsistent with the direction of mean flow .The volume of ESTMW showed a quasi-decadal variability with a rapid shrinking in 2010-2011. Its lower part showed thickness anomalies synchronized with those at the upper STMW in most of the years. The two have a similar density range, implying that there might be a large-scale stratification change. Related to this, a preliminary discussion is conducted on the relationships between the anomalies and the KE variability and Pacific Decadal Oscillation.
The propagation of the anomalies of a conserved tracer, potential thickness, is generally consistent with the direction of mean flow in subtropics but showed a propagation speed larger than geostrophic speed in the western subtropics, similar to the propagation of the passive tracer, spiciness anomalies. However, compared to spiciness anomalies, potential thickness anomalies decayed more rapidly and propagated with a larger westward component. Additionally, we estimated the eddy-induced thickness flux using a high-resolution reanalysis dataset and verified the effects of eddies in transporting anomalies.
Overall, this study provide instructive results on the dynamical changes in the upper North Pacific and how they are related to the thermohaline conditions in remote regions.