Effects of Anticyclonic Eddies on the Multicore Structure of the North Pacific Subtropical Mode Water Based on Argo Observations

Abstract

The North Pacific subtropical mode water (STMW) often displays a “multicore structure” with more than one minimum in the vertical potential vorticity profile. Anticyclonic eddies (AEs) play an important role in STMW formation and transport, but their effect on the multicore structure of the STMW has not been fully studied. Here, we investigated the formation and evolution of STMW multicore structures in AEs using Argo profiles, which followed two AEs in the STMW formation region for approximately 1.5 years from 2013 to 2015. We found that after the two AEs formed east of 150°E, they trapped the local cold and dense STMW and migrated westward. Since sea surface temperatures increased toward the west, warmer and lighter STMWs were formed during the winter ventilation process as the two AEs moved westward. The newly formed STMW rode on the preexisting cold and dense STMW inside the eddy core, forming a multicore structure in the STMW. Given that water mass properties within AEs are much more conservative, the multicore structure of STMW is much more easily formed inside AEs than outside of eddies. There was a larger (smaller) proportion of profiles containing multiple STMW cores inside (outside) the AE cores, indicating that the AEs were “hot spots” for developing the multicore structure of STMW. The occurrence of the multicore structure of STMW in AEs was highest in spring (March–May) but decreased quickly as the season progressed.

Subtropical Mode Water in a recent persisting Kuroshio large-meander period: part I—formation and advection over the entire distribution region

“…This definition differs from that used in our previous studies using Argo profiles, i.e., a layer where Q is lower than 2.0 × 10 -10 m −1 s −1 and θ at the Q minimum (called core θ hereafter) is between 16 and 19.5 °C (Oka 2009;Oka et al 2011Oka et al , 2015. In the new definition, we permit the existence of more than one STMW layer in each profile, which has been frequently observed in mode waters (Taneda et al 2000;Oka et al 2011Oka et al , 2020Gao et al 2016;Liu et al 2017Liu et al , 2019. It should also be noted that the θ range for the STMW definition is different between the 137ºE and Argo float data; the narrower and relatively warm θ range for the more contemporary Argo data reflects long-term warming of STMW (Sugimoto et al 2017;Oka et al 2019).…”

Formation of Central Mode Water based on two zonal hydrographic sections in spring 2013 and 2016

“…Water pycnostads and thought to be formed by interleaving of mode waters with different densities (Oka et al 2011b) or by formation of a lighter mode water over a preexisting denser mode water (Liu et al 2017(Liu et al , 2019. In our cruises in 2013 and 2016, they were observed in 38% of TRMW/D-CMW pycnostads in the 41° N-west section in mid-April, Fig.…”

“…In each pycnostad, the core, defined as a Q minimum, is considered to be the least modified portion since the pycnostad was formed, thus best preserving the water properties at the time of formation while the pycnostad is modified through mixing with surrounding water (Talley and Raymer 1982;Suga et al 1989). Observations of Subtropical Mode Water (Masuzawa 1969) south of the Kuroshio Extension have demonstrated that pycnostads sometimes have two or more cores (Taneda et al 2000;Oka et al 2011b;Gao et al 2016;Liu et al 2017Liu et al , 2019, as observed in those in Eddies A, B, and C (Figs. 2d, e, 4d, e).…”

“…To examine the occurrence of multiple cores in pycnostads throughout the three sections, we followed the previous studies (Oka et al 2011b;Liu et al 2017Liu et al , 2019 to define a core in a pycnostad as the Q minimum being less than 1.5 × 10 -10 m −1 s −1 and having a magnitude of 0.25 × 10 -10 m −1 s −1 between depths of 50 dbar and σ θ = 26.7 kg m −3 , excluding shallow surface mixed layers and intermediate waters that are not outcropped in the open North Pacific (Talley 1988;Suga et al 2004). If more than one core was detected in a CTD/XCTD profile, they were called the 1st, 2nd, … core in order of increasing Q.…”

Impact of Anticyclonic Eddies Under Stormy Weather on the Mixed Layer Variability in April South of the Kuroshio Extension

“…It seems that the Argo float #2901551 left the STMW in April 7, 2014 (Figure 6). The STMW formed during spring storms inside AE14 would be trapped and transported southwestward by AE14 (Liu et al, 2019;Xu et al, 2017).…”