Weakened overturning and tide control the properties of Oyashio Intermediate Water, a key water mass in the North Pacific

Using eddy‐resolving Community Earth System Model (CESM) simulations, this study investigates mesoscale eddy energetics and the role of air‐sea interaction for both anticyclonic and cyclonic eddies (AEs and CEs) in the Kuroshio Extension (KE) region. Based on energy budget analysis and eddy tracking algorithm, it is found that eddy energy balance depicts similar characteristics for AEs and CEs. The eddy kinetic energy (EKE) is generated through barotropic instability, vertical buoyancy flux, as well as transported from the upper stream Kuroshio. In addition, the temperature variance, which is directly related to eddy potential energy, is maintained by baroclinic instability. Air‐sea heat flux and wind stress act as eddy killers to move energy from oceanic eddies. For both AEs and CEs, heat exchange between atmosphere and oceanic eddies dominates the dissipation of temperature variance and accounts for more than 60% and 72% of the total dissipation, respectively. In comparison, the role of wind power in damping the EKE is relatively small. Only 14% (5%) of EKE dissipation in AEs (CEs) is attributed to eddy wind power.

Section: Data and Validationmentioning

confidence: 98%

Role of Air‐Sea Interaction in the Energy Balance of Anticyclonic and Cyclonic Eddies in the Kuroshio Extension

Cai,

Li,

Yang

et al. 2024

“…The North Pacific Intermediate Water (NPIW) and the Okhotsk Sea Intermediate Water (OSIW) are characterized by a potential density of around σ θ = 26.8 kg m 3 (e.g., Mensah & Ohshima, 2021;Yasuda, 1997). The Oyashio Intermediate Water, which is a source of the NPIW, had a warming trend on an isopycnal surface of σ θ = 26.8 kg m 3 or larger during 1990-2020, and this trend resulted from the warming and reduction of the OSIW (Mensah & Ohshima, 2021). Meanwhile, Nakanowatari et al (2015) indicated using a numerical model that the northward advection of subtropical water was related to the warming of intermediate water in the northwestern subarctic NP.…”

Section: Isopycnal Advectionmentioning

confidence: 99%

“…Salinity in this isopycnal layer clearly increased west of 170°W in the subarctic region (Figure 13). The North Pacific Intermediate Water (NPIW) and the Okhotsk Sea Intermediate Water (OSIW) are characterized by a potential density of around σ θ = 26.8 kg m −3 (e.g., Mensah & Ohshima, 2021; Yasuda, 1997). The Oyashio Intermediate Water, which is a source of the NPIW, had a warming trend on an isopycnal surface of σ θ = 26.8 kg m −3 or larger during 1990–2020, and this trend resulted from the warming and reduction of the OSIW (Mensah & Ohshima, 2021).…”

Section: Reasons For the Salinity Changesmentioning

confidence: 99%

Spatiotemporal Variations in Upper‐Ocean Salinity Over the North Pacific in 2004–2021

Kawai,

Katsura,

Hosoda

2024

Interannual variations in upper ocean salinity in the North Pacific Ocean (NP) were examined comprehensively over a period of 18 years (2004–2021) with gridded data sets considering vertical profiles of salinity. The salinity in nearly half of the basin varied in phase with a slight lag, and the rest of the basin exhibited the opposite phase on a decadal scale. The average salinity above 400 dbar prominently increased in the eastern and western mid‐latitude regions from approximately 2010 to the latter half of the 2010s, and decreased in the central subarctic and subtropics during the same period. This spatial pattern was similar to that of temperature change. While the freshening in the subtropics was remarkably above 100 dbar, salinity changes in the subarctic were large even in the subsurface. The first mode of the complex empirical orthogonal function analysis indicated the eastward (westward) propagation of salinity anomalies in the subarctic (subtropics), that is, a clockwise shift. Salinity budget analysis revealed that the changes in the 2010s were mainly caused by geostrophic advection in zonal regions around 40°N and 20°N, and the eastern NP. Precipitation also contributed to freshening in the subtropics, but not in the central subarctic. The meridional migration of salinity fronts in the subarctic and subtropics induced by wind stress, which is meridional contraction of the subtropical gyre, contributed to freshening in the central NP. It is unclear whether salinification in the eastern and western regions inevitably synchronizes and should be further examined.

“…The properties of WSAW, dSCW, and DSW have been defined by Itoh et al (2003) but may have changed significantly between the periods 1930-1990 and 1990-2022. Such is the case, for example, for WSAW (Andreev & Baturina, 2006;Mensah & Ohshima, 2021). We will therefore update the definitions of these water masses for each of the two periods studied in this paper and their combined period .…”

Section: Water Masses Definitionmentioning

confidence: 99%

“…Variability in the sea ice production could also affect OSIW properties as Kashiwase et al (2014) found a significant relationship between sea ice production in the northern Sea of Okhotsk and OSIW temperature. Besides the interannual variabilities, periodic bi-decadal variations of the OSIW or Oyashio water properties have been observed in several studies (Andreev & Baturina, 2006;Itoh, 2007;Mensah & Ohshima, 2021;Ono et al, 2001;Osafune & Yasuda, 2006) and attributed to the 18.6-year cycle of the diurnal tide.…”

mentioning

confidence: 94%

Evaluation of the Water Mass Composition in the Sea of Okhotsk and Its Long‐Term Change Using an Advanced Mapping Technique

Mensah,

Ohshima,

Drucker

et al. 2024

The Sea of Okhotsk is a marginal sea that plays a major role in the ventilation of the North Pacific, being the key location where Dense Shelf Water (DSW) forms at the surface and sinks to the intermediate layer. The Okhotsk Sea Intermediate Water (OSIW) is a key water mass because it includes large amounts of DSW, outflows to the North Pacific, and supplies the ocean with the micronutrient iron. OSIW has been warming over the past few decades, which is attributed to a decreasing trend in DSW production. The acquisition of numerous hydrographic data after 2000 in the Kuril Basin, especially dissolved oxygen from profiling floats, offers an opportunity to better quantify the water mass composition of OSIW, and the changes in OSIW properties and DSW volume. Here, we used all available hydrographic records and a mapping technique specially adapted to polar and sub‐polar regions to revisit the Sea of Okhotsk water properties and document their long‐term changes. Our analysis revealed that the volume of heavier DSW (potential density above 26.9 kg.m−3) has decreased over the past three decades by 3,600 km3, or 15% of the volume present before 1990. This decline is nearly entirely compensated for by an increase in lighter DSW. This shift toward lighter DSW is possibly a sign of the weakening of the intermediate overturning circulation starting in the Okhotsk Sea. Additionally, we found that dense Soya Current Water only accounts for about 1% of OSIW, against the 5% previously estimated.