While modelling studies suggest that mesoscale eddies strengthen the subduction of mode waters, this eddy effect has never been observed in the field. Here we report results from a field campaign from March 2014 that captured the eddy effects on mode-water subduction south of the Kuroshio Extension east of Japan. The experiment deployed 17 Argo floats in an anticyclonic eddy (AC) with enhanced daily sampling. Analysis of over 3,000 hydrographic profiles following the AC reveals that potential vorticity and apparent oxygen utilization distributions are asymmetric outside the AC core, with enhanced subduction near the southeastern rim of the AC. There, the southward eddy flow advects newly ventilated mode water from the north into the main thermocline. Our results show that subduction by eddy lateral advection is comparable in magnitude to that by the mean flow—an effect that needs to be better represented in climate models.

In Spring 2014, two subthermocline eddies (STEs) were observed by rapid‐sampling Argo floats in the subtropical northwestern Pacific (STNWP). The first one is a warm, salty, and oxygen‐poor lens, with its temperature/salinity /dissolved oxygen (T/S/DO) anomalies reaching 1.16°C/0.21 practical salinity unit (psu)/−29.9 µmol/kg, respectively, near the 26.62σ0 surface. The other is a cold, fresh, and oxygen‐rich lens, with its T/S/DO anomalies reaching −1.95°C/−0.34 psu/88.0 µmol/kg, respectively, near the 26.54σ0 surface. The vertical extent of the water mass anomalies in the warm (cold) STE is about 190 m (150 m), and its horizontal length scale is 22 ± 7 km (18 ± 10 km). According to their water mass properties, we speculate that the warm and cold STEs are generated in the North Pacific Subtropical and Subarctic Front region, respectively. The observed STEs may play an important role in modifying the intermediate‐layer water properties in the STNWP, and this needs to be confirmed by more focused observations in the future.

Mesoscale eddy effects on the subduction of North Pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved. The eddy-resolving models produce results closer to observations than the noneddy-resolving model. There are large discrepancies in subduction patterns between eddy-resolving and noneddy-resolving models. In the noneddy-resolving model, subduction on a given isopycnal is limited to the cross point between the mixed layer depth (MLD) front and the outcrop line whereas in eddy-resolving models and observations, subduction takes place in a broader, zonally elongated band within the deep mixed layer region. Mesoscale eddies significantly enhance the total subduction rate, helping create remarkable peaks in the volume histogram that correspond to North Pacific subtropical mode water (STMW) and central mode water (CMW). Eddy-enhanced subduction preferentially occurs south of the winter mean outcrop. With an anticyclonic eddy to the west and a cyclonic eddy to the east, the outcrop line meanders south, and the thermocline/MLD shoals eastward. As eddies propagate westward, the MLD shoals, shielding the water of low potential vorticity from the atmosphere. The southward eddy flow then carries the subducted water mass into the thermocline. The eddy subduction processes revealed here have important implications for designing field observations and improving models.

Decadal variability in the interior subtropical North Pacific is examined in the Geophysical Fluid Dynamics Laboratory coupled model (CM2.1). Superimposed on a broad, classical subtropical gyre is a narrow jet called the subtropical countercurrent (STCC) that flows northeastward against the northeast trade winds. Consistent with observations, the STCC is anchored by mode water characterized by its low potential vorticity (PV). Mode water forms in the deep winter mixed layer of the Kuroshio-Oyashio Extension (KOE) east of Japan and flows southward riding on the subtropical gyre and preserving its low-PV characteristic. As a thick layer of uniform properties, the mode water forces the upper pycnocline to shoal, and the associated eastward shear results in the surface-intensified STCC.On decadal time scales in the central subtropical gyre (158-358N, 1708E-1308W), the dominant mode of sea surface height variability is characterized by the strengthening and weakening of the STCC because of variations in mode water ventilation. The changes in mode water can be further traced upstream to variability in the mixed layer depth and subduction rate in the KOE region. Both the mean and anomalies of STCC induce significant sea surface temperature anomalies via thermal advection. Clear atmospheric response is seen in wind curls, with patterns suggestive of positive coupled feedback.In oceanic and coupled models, northeast-slanted bands often appear in anomalies of temperature and circulation at and beneath the surface. The results of this study show that such slanted bands are characteristic of changes in mode water ventilation. Indeed, this natural mode of STCC variability is excited by global warming, resulting in banded structures in sea surface warming.

Anticyclonic eddies (AEs) trap and transport the North Pacific subtropical mode water (STMW), but the evolution of the STMW trapped in AEs has not been fully studied due to the lack of eddy‐tracking subsurface observations. Here we analyze profiles from special‐designed Argo floats that follow two STMW‐trapping AEs for more than a year. The enhanced daily sampling by these Argo floats swirling around the eddies enables an unprecedented investigation into the structure and evolution of the trapped STMW. In the AEs, the upper (lower) thermocline domes up (concaves downward), and this lens‐shaped double thermocline encompasses the thick STMW within the eddy core. The lighter STMW (25.0 ∼ 25.2 σθ) trapped in AEs dissipates quickly after the formation in winter because of the deepening seasonal thermocline, but the denser STMW (25.2 ∼ 25.4 σθ) remains largely unchanged except when the AE passes across the Izu Ridge. The enhanced diapycnal mixing over the ridge weakens the denser STMW appreciably. While many AEs decay upon hitting the ridge, some pass through a bathymetric gap between the Hachijojima and Bonin Islands, forming a cross‐ridge pathway for STMW transport. By contrast, the North Pacific Intermediate Water (NPIW) underneath is deeper than the eddy trapping depth (600 m), and hence left behind east of the Izu Ridge. In Argo climatology, the shallow STMW (< 400 m) intrudes through the gap westward because of the eddy transport, while the NPIW (800 m) is blocked by the Izu Ridge.

[1] Seventeen coupled general circulation models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed to assess the dynamics and variability of the North Pacific Subtropical Countercurrent (STCC). Consistent with observations, the STCC is anchored by mode water to the north. For the present climate, the STCC tends to be stronger in models than in observations because of too strong a low potential vorticity signature of mode water. There are significant variations in mode water simulation among models, i.e., in volume and core layer density. The northeast slanted bands of sea surface height (SSH) anomalies associated with the STCC variability are caused by variability in mode water among models and the Hawaii islands are represented in some models, where the island-induced wind curls drive the Hawaiian Lee Countercurrent (HLCC) located to the south of STCC. Projected future changes in STCC and mode water under the Representative Concentration Pathways (RCP) 4.5 scenario are also investigated. By combining the historical and RCP 4.5 runs, an empirical orthogonal function analysis for SSH over the central subtropical gyre (160 E-140 W, 15 -30 N) is performed. The dominant mode of SSH change in 17 CMIP5 models is characterized by the weakening of the STCC because of the reduced formation of mode water. The weakened mode water is closely related to the increased stratification of the upper ocean, the latter being one of the most robust changes as climate warms. Thus the weakened STCC and mode water are common to CMIP5 future climate projections.Citation: Xu, L., S.-P. Xie, and Q. Liu (2012), Mode water ventilation and subtropical countercurrent over the North Pacific in CMIP5 simulations and future projections,

Mesoscale eddies play an important role in transporting North Pacific subtropical mode water (STMW). Using eddy samples adopted from a 3‐day and 0.1° ocean model output spanning from 1980 to 2014, this study quantifies the eddy‐trapped STMW volume and transport south of the Kuroshio Extension. Based on the shape of their isopycnals, anticyclonic eddies (AEs) in the region are classified into two types. The first type (AE1) has a lens‐like structure of isopycnals, and the second type (AE2) has downward bending isopycnals throughout the pycnocline. In contrast to AE2, a cyclonic eddy is characterized by upward bending isopycnals throughout the pycnocline. Although all three eddy types can trap STMW, the low potential vorticity water within an AE1 is found to be thicker in the spring and better preserved through the rest of the year. A quantitative estimation finds that the STMW volume trapped by an AE1 is approximately 1.5 and 2.5 times larger than the volumes trapped by an AE2 and a cyclonic eddy, respectively. The eddy‐trapped STMW moves primarily westward, with its meridional integration between 25 and 35°N reaching ~1 Sv at 143°E, approximately 17% of the time‐mean total zonal STMW transport there. This study highlights the important role of eddies (particularly the AE1) in carrying STMW westward and thus modulating North Pacific climate variability.

The formation and transport of the Subantarctic Mode Water (SAMW) are part of the upper branch of the global meridional overturning circulation (Hanawa & Talley, 2001;McCartney, 1997). The SAMW accounted for more than half of the heat gain in the upper 2,000 m of the Southern Ocean during (Roemmich et al., 2015 and contributes significantly to the global ocean heat capacity (Morrison et al., 2016;Rathore et al., 2020). It is also the largest contributor to the uptake and storage of anthropogenic carbon dioxide in the Southern Ocean (Raupach et al., 2007;Sallée et al., 2012). This study investigates the changes in the SAMW via observations and the drivers of that change using models.Prior to the early 2000s, there were few hydrographic sections in the Southern Ocean, thereby limiting the observations of SAMW variability. The SAMW shows warming and freshening trends between the 1960s and the 1990s (Bindoff & Church, 1992;Johnson & Orsi, 1997), which has been linked to warming due to the greenhouse effect (Banks et al., 2002;Bindoff et al., 2007;Downes et al., 2010). However, conflicting views exist. Bryden et al. (2003) observe that, in 2002, the salinity in the Indian Ocean recovered to the values of the 1960s. The temperature and salinity of the SAMW are reported to have interannual to interdecadal variability (e.g., Naveira-Garabato et al., 2009). However, Herraiz-Borreguero and Rintoul (2010) caution that based on infrequently repeated sections, researchers may misinterpret the eddy advection of an anomalous SAMW from another region as interannual variability.The Argo program, which launched in the early 2000s, has dramatically increased the frequency and coverage of global observations of ocean temperature and salinity in the top 2,000 m (Gould et al., 2004). Gao