North Pacific Subtropical Mode Water Volume Decrease in 2006–09 Estimated from Argo Observations: Influence of Surface Formation and Basin-Scale Oceanic Variability

Abstract

Analysis of Argo temperature and salinity profiles (gridded at 0.5° × 0.5° resolution for 2005–12) shows a strong North Pacific Subtropical Mode Water (NPSTMW) volume and density decrease during 2006–09. In this time period, upper-ocean temperature, stratification, and potential vorticity (PV) all increased within the region in and around the NPSTMW low-PV pool, contributing to the NPSTMW volume decrease in two ways: (i) the volume of water satisfying the low-PV constraint that is part of the “mode water” definition decreased, and (ii) some water that was initially in the NPSTMW density range σθ = 25.0–25.5 kg m−3 was transformed into lighter water. Both changes in density and in PV in the NPSTMW region were a manifestation of basinwide changes. A positive PV anomaly started to propagate westward from the central Pacific in 2005, followed by a negative density anomaly in 2007, which caused a dramatic NPSTMW volume and density decrease. A Walin estimate of surface formation in the NPSTMW density range accounted better (although not entirely) for the interannual variability of the volume of water in the NPSTMW density range without imposing the PV < 2 × 10−10 m−1 s−1 constraint than did the same estimate with the PV constraint imposed. This underlines the importance of the PV constraint in identifying the mode water. The mode water evolution cannot be fully described from a density budget alone; rather, the PV budget must be considered simultaneously.

Climatology and decadal variations in multicore structure of the North Pacific subtropical mode water

“…STMW forms on the southern flank of the KE system. Its formation, transportation, and dissipation are closely related to the varying KE system [ Qiu et al ., ; Rainville et al ., ; Oka et al ., ; Cerovečki and Giglio , ]. The accumulated Argo float data for 2004–2015 are available to investigate the interannual to decadal variations of STMW.…”

“…The formation and distribution of STMW feature considerable interannual and decadal variations [ Pan and Liu , ; Miyazawa et al ., ; Oka , ; Sugimoto and Hanawa , ; Oka et al ., ; Cerovečki and Giglio , ], in close relationship to the dynamic state of the KE system. When the KE has large (small) meanders, the KE is in unstable (stable) state [ Qiu and Chen , ; Qiu et al ., ].…”

“…The volume and density of STMW decrease during 2006–2009 when the KE system is unstable [ Cerovečki and Giglio , ; Oka et al ., ]. The decrease of the STMW volume is associated with a deepened upper boundary ( = 25.0 ) of STMW and shoaling lower boundary (σ θ = 25.6 ) [ Cerovečki and Giglio , ]. In addition, temperatures of STMW also change with the KE dynamic state.…”

Decadal Variability of North Pacific Eastern Subtropical Mode Water

“…The North Pacific Subtropical Gyre contains three types of mode waters formed in the western, poleward, and eastern parts of the gyre: the North Pacific Subtropical Mode Water (NPSTMW; Masuzawa, ), the North Pacific Central Mode Water (NPCMW; Nakamura, ; Suga et al, ), and the North Pacific Eastern Subtropical Mode Water (NPESTMW; Hautala & Roemmich, ). Previous studies have been devoted to the formation, circulation, dissipation, and interannual to decadal variability of NPSTMW (e.g., Cerovečki & Giglio, ; Oka, ; Qiu & Chen, ; Rainville et al, ) and NPCMW (e.g., Hosoda et al, ; Ladd & Thompson, , ; Oka & Suga, ). Particularly, these studies show remarkable decadal variabilities of NPSTMW and NPCMW involved with the Pacific Decadal Oscillation (PDO; Graham, ; Mantua et al, ; Trenberth, ).…”

“…To answer these questions, long‐term NPESTMW volume from the observations of RG09 and EN4 is analyzed. Based on previous studies (Cerovečki & Giglio, ; Marshall et al, ; Nishikawa et al, ; Rainville et al, ), a diagnosis for the volume budget of the surface NPESTMW density layer in the NPESTMW formation region is further conducted with the ECCO2 output.…”