Annual Subduction Rate Research Articles

Heat transport into the interior ocean induced by water-mass subduction

The subduction of oceanic water masses provides a crucial pathway for anthropogenic heat to enter the subsurface ocean, thereby shaping deep-reaching warming signatures. Analyzing data from eight ocean and atmosphere reanalysis datasets, we show that the average annual subduction rate of the global ocean (excluding 10° S–10° N) is 312.4 ± 27.9 Sv, resulting in a mean heat transport of 20.2 ± 2.1 PW towards the subsurface ocean. This subduction-driven heat transport has exhibited an increase of 0.09 ± 0.08 PW/decade since 1970. The increase predominantly stems from the overall enhancement of subduction within the latitudes of 30° S–50° S, dictated by intensified westerly winds that lead to the deepening of the local mixed layer depth. Our findings underscore the essence of wind-driven changes in the Southern Ocean subduction, which wield considerable influence over the global climate by regulating the vertical transport of heat and carbon from the sea surface to the deep waters.

Indian Ocean Subtropical Underwater and the Interannual Variability in Its Annual Subduction Rate Associated with the Southern Annular Mode

Abstract In this study, the Indian Ocean Subtropical Underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–20) for the first time. It has mean salinity, potential temperature, and potential density values of 35.54 ± 0.29 psu, 17.91° ± 1.66°C, and 25.56 ± 0.35 kg m−3, respectively, and mainly extends between 10° and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004–19 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv ≡ 106 m3 s−1) with an interannual variability that is closely related to the southern annular mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD are primarily determined by SAM-related air–sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component. Significance Statement The Indian Ocean Subtropical Underwater (IOSTUW) is characterized as a subsurface salinity maximum in the subtropical south Indian Ocean, and it has potential contribution to regional sea level change and interoceanic salinity exchange. The purpose of this work is to give a more precise description of the IOSTUW and to better understand the formation process of this water mass. Our results shine new light on hydrological characteristics of the IOSTUW based on an observational dataset collected during the past two decades. In addition, the formation of the IOSTUW was found to be closely related to the leading mode of variability in the atmospheric circulation of the Southern Hemisphere though air–sea heat flux and surface wind anomaly.

Deviation of the Lagrangian particle tracing method in the evaluation of the Southern Hemisphere annual subduction rate

Deviation of the Lagrangian particle tracing method in the evaluation of the Southern Hemisphere annual subduction rate

Recent Decadal Change in the North Atlantic Subtropical Underwater Associated With the Poleward Expansion of the Surface Salinity Maximum

AbstractYu et al. (2017, https://doi.org/10.1002/2017GL075772) reported that the annual mean sea surface salinity maximum (SSS‐max) in the North Atlantic expanded northward by 0.35 ± 0.11° per decade over the 34‐year data record (1979–2012). The expansion shifted and expanded the ventilation zone northward and increased the production of the Subtropical Underwater (STUW). As a result, the STUW became deeper, thicker, and saltier. In this study, the seasonal characteristics of the poleward expansion of the North Atlantic SSS‐max and their effects on the STUW are examined. The results show that the SSS‐max expansion occurred primarily during boreal spring (April, May, and June) and expanded northward by 0.43 ± 0.21° per decade over the 34‐year period. The annual volume of the STUW increased by 0.21 ± 0.09 1014 m3 per decade over the same period, and the spring (April, May, and June) volume increased by 0.31 ± 0.02 1014 m3 per decade (a relative increase of 48 ± 1%). The characteristics of the decadal changes in STUW were attributable to the increased subduction rate associated with the northward expansion of the SSS‐max. The annual subduction rate increased by 0.29 ± 0.07 Sv per decade over the 34 years, and the greatest increase of 1.73 ± 0.61 Sv per decade occurred in April. The change in subduction associated with the expansion of the SSS‐max appeared to be consistent with the Atlantic Multidecadal Oscillation.

A statistical study on the subthermocline submesoscale eddies in the northwestern Pacific Ocean based on Argo data

AbstractIn this study, statistical characteristics and spatiotemporal variability of the cold‐core subthermocline eddies (STEs) discovered by Zhang et al. (2015) in the northwestern Pacific are investigated with 14 years of Argo temperature/salinity (T/S) data between 2002 and 2015. By applying an objective identification procedure to Argo T/S profiles, a total of 337 lens‐like STEs that trap cold and fresh homogeneous water are obtained. The STEs' cores are primarily located between 26.5 and 26.9 (300–800 m) and their shapes share a uniform Gaussian structure in vertical. Vertical and horizontal scale of the STEs is generally between 100 and 200 m and 5 and 15 km, respectively, suggesting that they are in the category of submesoscale processes. According to their different densities and seasonalities, the STEs are classified into upper and lower types with density interface of 26.7 . The number of upper‐type STEs is much larger in spring and summer than in autumn and winter while that of lower‐type ones show relatively uniform seasonal distributions. Spatially, the STEs are primarily distributed within the region between the Kuroshio Extension (KE) and subarctic front (SAF) in meridional and 140°E–155°E in zonal direction. Based on STEs' spatial distributions and vertical locations, we propose to name these STEs as KE intermediate‐layer eddies (Kiddies). Water property analysis suggests that the Kiddies most likely originated from the SAF region. For the upper‐type Kiddies, they are probably generated through subduction associated with mixed‐layer frontogenesis, which at least contribute to 0.25 Sv annual subduction rate for the layers 26.2–26.7 .

The shallow meridional overturning circulation in the South China Sea and the related internal water movement

The structure of the annual-mean shallow meridional overturning circulation (SMOC) in the South China Sea (SCS) and the related water movement are investigated, using simple ocean data assimilation (SODA) outputs. The distinct clockwise SMOC is present above 400 m in the SCS on the climatologically annual-mean scale, which consists of downwelling in the northern SCS, a southward subsurface branch supplying upwelling at around 10°N and a northward surface flow, with a strength of about 1×106 m3/s. The formation mechanisms of its branches are studied separately. The zonal component of the annual-mean wind stress is predominantly westward and causes northward Ekman transport above 50 m. The annual-mean Ekman transport across 18°N is about 1.2×106 m3/s. An annual-mean subduction rate is calculated by estimating the net volume flux entering the thermocline from the mixed layer in a Lagrangian framework. An annual subduction rate of about 0.66×106 m3/s is obtained between 17° and 20°N, of which 87% is due to vertical pumping and 13% is due to lateral induction. The subduction rate implies that the subdution contributes significantly to the downwelling branch. The pathways of traced parcels released at the base of the February mixed layer show that after subduction water moves southward to as far as 11°N within the western boundary current before returning northward. The velocity field at the base of mixed layer and a meridional velocity section in winter also confirm that the southward flow in the subsurface layer is mainly by strong western boundary currents. Significant upwelling mainly occurs off the Vietnam coast in the southern SCS. An upper bound for the annual-mean net upwelling rate between 10° and 15°N is 0.7×106 m3/s, of which a large portion is contributed by summer upwelling, with both the alongshore component of the southwest wind and its offshore increase causing great upwelling.

North Atlantic Subtropical Underwater and Its Year-to-Year Variability in Annual Subduction Rate during the Argo Period

AbstractSubtropical underwater (STUW) and its year-to-year variability in annual subduction rate are investigated using recently available Argo data in the North Atlantic. For the period of observation (2002–14), the mean annual subduction rate of the STUW is 7.3 ± 1.2 Sv (1 Sv = 106 m3 s−1) within the density range between 25.0 and 26.0 kg m−3. Once subducted, the STUW spreads in the subtropical gyre as a vertical salinity maximum. In the mean, the spatial changes in temperature and salinity of the STUW tend to compensate each other, and the density of the water mass remains rather stable near 25.5 kg m−3 in the southwestern part of the subtropical gyre. The annual subduction rate of the STUW varies from year to year, and most of this variability is due to lateral induction, which in turn is directly linked to the variability of the winter mixed layer depth. Through modulation of surface buoyancy, wind anomalies associated with the North Atlantic Oscillation are primarily responsible for this variability. Sea surface salinity anomalies in the formation region of the STUW are conveyed into the thermocline, but their westward propagation cannot be detected by the present data.

Subduction/obduction rate in the North Pacific diagnosed by an eddy-resolving model

Ventilation in the North Pacific is examined using data from the eddy-resolving 1/12° global HYbrid Coordinate Ocean Model (HYCOM) and QuikSCAT wind stress data. For the period January 2004 to December 2006 and area 20°–40°N, the total annual subduction rate is estimated at 79 Sv, and the obduction rate 41 Sv. Resolving the small-scale and high-frequency components of the eddy field can increase the subduction rate by 42 Sv, and obduction by 31 Sv. Lateral induction is the dominant contributor to enhancement of subduction/obduction, and temporal change of mixed layer depth has a secondary role. Further analysis indicates that the high-frequency components of the eddy field, especially those with timescale shorter than 10 days, are the most critical factor enhancing subduction/obduction.

A study of Indian Ocean Subtropical Mode Water: subduction rate and water characteristics

The annual subduction rate in the South Indian Ocean was calculated by analyzing Simple Ocean Data Assimilation (SODA) outputs in the period of 1950–2008. The subduction rate census for potential density classes showed a peak corresponding to Indian Ocean subtropical mode water (IOSTMW) in the southwestern part of the South Indian Ocean subtropical gyre. The deeper mixed layer depth, the sharper mixed-layer fronts and the associated relatively faster circulation in the present climatology resulted in a larger lateral induction, which primarily dominants the IOSTMW subduction rate, while with only minor contribution from vertical pumping. Without loss of generality, through careful analysis of the water characteristics in the layer of minimum vertical temperature gradient (LMVTG), the authors suggest that the IOSTMW was identified as a thermostad, with a lateral minimum of low potential vorticity (PV, less than 200×10–12 m–1·s–1) and a low dT⁄dz (less than 1.5°C/(100 m)). The IOSTMW within the South Indian Ocean subtropical gyre distributed in the region approximately from 25° to 50° E and from 30° to 39°S. Additionally, the average characteristics (temperature, salinity, potential density) of the mode water were estimated about (16.38 ± 0.29)°C, (35.46 ± 0.04), (26.02 ± 0.04) σ θ over the past 60 years.

Interannual Variation of Annual Subduction Rate in the North Pacific Estimated from a Gridded Argo Product

AbstractSpatiotemporal variability of the subduction rate in the North Pacific from 2005 to 2012 is examined based on the Argo observational data. The subduction rate in the subtropical North Pacific varies significantly from year to year between 25 and 50 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1), and it is well correlated with the Pacific decadal oscillation. The temporal change of the subduction rate is largely determined by that of the late winter mixed layer depth through the lateral induction term. The increase (decrease) in the subduction rate in the subtropical mode water areas accompanies densification (lightening) of the mode density class of the subducted water. The subduction rate variability in the central mode water and eastern subtropical mode water regions is anticorrelated as found in the previous study using the output from an ocean GCM. The subduction rate in the central mode water density range changes dramatically, which is very large in 2005 and 2010 but almost disappears in 2009. The subduction rate variability in the western subtropical mode water regions seems to be correlated with the Pacific decadal oscillation with a lag of a few years.

Diagnosis of water mass transformation and formation rates in a high‐resolution GCM of the North Pacific

AbstractWe studied the relationship between assessments of water mass formation estimated by thermodynamic and kinematic methods by applying both methods to the Subtropical Mode Water (STMW) and Central Mode Water (CMW) formation regions of the North Pacific. We used long‐term outputs of a high‐resolution general circulation model under climatological normal year forcings to calculate the water mass transformation rate and its density derivative (the net formation rate) and to examine the roles of mixing, shortwave penetration, and mesoscale eddies. Diagnosis of the thermodynamic relationship indicated that the transformation rate associated with surface density flux largely explained the net transformation rate. The difference of these rates, the total mixing effect, was relatively small compared with previous estimates. Explicit and implicit mixing largely explained the total mixing effect. Diagnosis of the kinematic relationship indicated that the annual subduction rate could be calculated from the net transformation rate and the open boundary volume flux over the analysis region. Comparison of both methods implied that the source of subducted STMW was determined mainly by the transformation associated with surface density flux, but this conclusion was not true for CMW. The modeled effects of internal mixing and volume flux from the north (Oyashio Current) also became important in the formation of CMW. Omission of shortwave penetration led to an error in the air‐sea transformation and formation rates in the STMW and CMW regions. We also found the important roles of mesoscale eddies in the transformation and formation of mode waters.

Enhancement of subduction/obduction due to hurricane-induced mixed layer deepening

Ventilation, including subduction and obduction, in the North Pacific is re-examined, based on SODA outputs and the Eulerian definition. The annual subduction rate averaged from 2001 to 2004 is estimated at 49.8 Sv, whereas the annual obduction rate is 26.7 Sv. Furthermore, the annual subduction/obduction rate enhancement induced by tropical cyclones in the North Pacific, defined as the difference between the annual subduction/obduction rate for the cases including the mixed layer depth perturbations induced by tropical cyclones and that for the cases without the perturbations, is estimated. Based on SODA outputs and the mixed layer deepening obtained from a hurricane–ocean coupled model, the annual tropical cyclone-induced subduction rate enhancement averaged from 2001 to 2004 is estimated at 4.4 Sv and the obduction rate enhancement 5.2 Sv, and such enhancement is mainly concentrated in the latitudinal band from 10°N to 30°N.

Ventilation of the North Pacific subtropical pycnocline and mode water formation

The annual subduction rate of the North Pacific was calculated based on isopycnally averaged hydrographic climatology (HydroBase), high-resolution winter mixed-layer climatology (NWMLC), and various wind stress climatologies from ship reports, numerical weather prediction products, and satellite products. The calculation was performed using Lagrangian coordinates in the same manner as in previous works, except a less smoothed oceanic climatology (HydroBase and NWMLC) was used instead of a World Ocean Atlas. Differences in the wind stress climatologies have very little effect on subduction rate estimates. The subduction rate census for density classes showed peaks corresponding to subtropical mode water (STMW), central mode water (CMW), and eastern subtropical mode water (ESTMW). The deeper mixed layer and the associated sharper mixed-layer fronts in the present climatology resulted in a larger lateral induction, which boosted the subduction rate, especially for the potential density anomaly ( σ θ ) range of the lighter STMW (25.0 < σ θ < 25.2 kg m −3) and lighter CMW (26.0 < σ θ < 26.2 kg m −3), compared to previous estimates. The renewal time of permanent pycnocline water was estimated as the volume of water divided by the subduction rate for each σ θ class: 2–4 years for ESTMW (24.5 < σ θ < 25.2 kg m −3), 2 years for the lighter STMW (25.0 < σ θ < 25.3 kg m −3), 5–9 years for the denser STMW (25.3 < σ θ < 25.6 kg m −3), 10–20 years for the lighter CMW (26.0 < σ θ < 26.2 kg m −3), 20–30 years for the middle CMW (26.2 < σ θ < 26.3 kg m −3), and 60 years or longer for the denser CMW (26.3 < σ θ < 26.6 kg m −3). A comparison of the water volume and subduction rate in potential temperature–salinity ( θ– S) space indicated that the upper permanent pycnocline water (25.0 < σ θ < 26.2 kg m −3) was directly maintained by nondiffusive subduction of winter surface water, including STMW and lighter CMW. The lower permanent pycnocline water (26.2 < σ θ < 26.6 kg m −3) may be maintained through the subduction of fresher and colder water from the subarctic–subtropical transition region and subsequent mixing with saltier and warmer water. Diagnosis of the potential vorticity (PV) of the subducted water demonstrated that the low PV of STMW was mainly due to the large subduction rate, whereas that of both ESTMW and CMW was due mainly to the small density advection rate (cross-isopycnal flow). Additionally, a relatively large subduction rate probably contributes to the low PV of part of the lighter CMW (ESTMW) formed in the region around 38°N and 170°W (28°N and 145°W), which is characterized by a relatively thick winter mixed layer and an associated mixed-layer front, causing a large lateral induction rate.

Subduction of South Pacific waters

Using existing high‐resolution CTD observations, complemented by a large amount of recently available Argo floating data, this study provides a detailed description of the subduction of South Pacific waters. With a significantly improved climatological dataset on the mixed layer properties, we obtain an annual subduction rate of 48.8 Sv (1 Sv = 106 m3 s−1) from 10°S to 60°S in the South Pacific. Two peaks stand out in this subduction rate sorted by winter mixed layer density: one corresponds to the formation of eastern Subtropical Mode Water (STMW) and part of the Subtropical Underwater (SUW) between 25.0 and 25.5 σθ, and the other has a density range between 26.6 and 27.1 σθ, representing the formation of Sub‐Antarctic Mode Water and Antarctic Intermediate Water (SAMW/AAIW). The subduction in eastern STMW/SUW range is 12.7 Sv, only slightly smaller than that (14.6 Sv) associated with the SAMW/AAIW. Sandwiched between these two peaks, roughly in the density range between 26.0 and 26.5 σθ, is the southwestern STMW, with a relatively small annual subduction of 5.6 Sv. Uncertainties of these estimates are discussed.

A high‐resolution simulation of the ocean during the POMME experiment: Mesoscale variability and near surface processes

This article is the second part of a modeling study of upper ocean mesoscale physical processes in an area of the northeast Atlantic that was extensively sampled during the Programme Océan Multidisciplinaire Méso‐Echelle (POMME experiment, October 2000–September 2001). The model is a high‐resolution regional version of the ocean general circulation model OPA integrated from February to May 2001. The mesoscale upper ocean dynamics has been carefully validated in a first paper (Paci et al., 2005). In the present article, the simulation is used to analyze the contribution of mesoscale eddies to the mixed layer processes during the period that controls the annual subduction rate and the characteristics of the subducted water. Mesoscale eddies account for a significant part of the mixed layer heat, salt, and water budgets through mesoscale and submesoscale structures acting on the horizontal advective terms. They represent respectively 52% and 66% of the horizontal advection of temperature and salinity and 70% of the lateral induction. They also induce an earlier and more intense restratification. The domain‐averaged detrainment appears to be reduced by more than 15% by mesoscale eddies and reaches about 0.8 Sv. However, these eddies increase the density of the detrained water. Some dense water, likely to be involved in the formation of subpolar mode water, is detrained by them in a way that maximizes its chance to be effectively subducted in the area.

Investigation of variability of the North Atlantic Subtropical Mode Water using profiling float data and numerical model output

The basic characteristics of the North Atlantic Subtropical Mode Waters (STMW) are well documented in the literature from one-time hydrographic sections or from long-term measurements at one location. From July 1997 to March 1998, 71 vertical profiling floats were deployed in the Western Subtropical North Atlantic region to provide a broader spatial and temporal coverage of STMW. In this study, the STMW properties are estimated from a total of 5352 float temperature profiles in an area covering 30°W–80°W and 20°N–45°N, from January 1998 to December 2000. These STMW properties are compared to those derived from numerical simulation output using the Miami Isopycnic Coordinate Ocean Model (MICOM). The model is configured to the North Atlantic Ocean with a 1-deg horizontal resolution and 20 vertical layers. The model temperature profiles are collocated with the observed float profiles for a direct comparison. The values of the mean STMW temperature derived from both the float data and model output are roughly equal (18.1 °C for floats and 18.0 °C for the model). The model is found to capture well the observed annual cycle of the STMW volume. Given good agreement between float and model profiles, we use a longer MICOM simulation to investigate decadal variability of the STMW renewal rate in terms of the annual subduction rate and its relationship to the large-scale atmospheric circulation pattern changes measured by the North Atlantic Oscillation index. Good correlation is found between the model annual subduction rate anomaly at the Panulirus Station and the NAO winter index anomaly on decadal time scales with NAO leading by 2–3 years.

Diagnoses of simulated water-mass subduction/formation/transformation in the Japan/East Sea (JES)

Abstract The impacts of surface atmospheric forcing of different time–space scales on the simulation of water-mass formation and spreading of formed water are investigated by quantifying water-mass subduction/formation/transformation for the Japan/East Sea (JES). The Princeton Ocean Model (POM) was implemented for the JES (JES-POM) to simulate interannual, seasonal, and mesoscale variations in velocity and mass fields. Three sets of atmospheric flux data were used; (1) 6-h fluxes calculated from 6-h atmospheric variables (syn), (2) monthly means of 6-h fluxes (empm), and (3) fluxes calculated from monthly averaged atmospheric variables (mont). The mass exchange between the mixed layer and interior in the JES was diagnosed in terms of annual subduction rate (Sann). Three areas of local maximum Sann (>500 m/yr) occurred: area V (41–43°N west of 137°E), K (36–39°N west of 132°E), and KB (near Korea Bay). Area V corresponds to the “flux center” (i.e., maximum heat and momentum fluxes) described by Kawamura and Wu [1998. Formation mechanism of Japan Sea Proper Water in the flux center off Vladivostok. J. Geophys. Res. 103 (C10), 21611–21622] the subduction region suggested by Senjyu and Sudo [1994. The upper portion of the Japan Sea proper water: its source and circulation as deduced from isopycnal analysis. J. Oceanogr. 50, 663–690; 1996. Interannual variation of the upper portion of the Japan proper water and its probable cause. J. Oceanogr. 52, 72–42] and Yoshikawa et al. [1999. Formation and circulation processes of intermediate water in the Japan Sea. J. Phys. Oceanogr. 29 (8), 1701–1722], and the wintertime convection location identified by Seung and Yoon [1995. Some features of winter convection in the Japan Sea. J. Oceanogr. 51, 61–73]. With monthly forcing (mont), there is no localized maximum value off Vladivostok, while with forcing influenced by synoptic events (monthly; empm and 6-h; syn), either one or two localized areas with a high subduction rate occur with year-to-year variations. The presence of simulated dense surface water ( > σ θ = 26.8 ) in the northern JES is due to the effects of synoptic events during winter months when large heat loss and strong wind stress associated with cold-air outbreaks and extratropical cyclones occur. The convection magnitudes as a result of air–sea interaction are 3.0, 4.0, and 6.0 Sv for mont, empm, and syn, respectively. A net flux of mass from the interior to the mixed layer (entrainment) occurs in the density range between σ θ = 24.0 and σ θ = 26.2 for both empm (3.5 Sv) and syn (3.0 Sv), while it occurs in the density range between σ θ = 24.0 and σ θ = 25.8 for mont (2.8 Sv). The diffusive fluxes across the winter mixed layer base are about 0.8, 1.0, and 1.4 Sv for mont, empm, and syn, respectively. An undiagnosed eddy-induced turbulent mixing (the residual of the balance after removing discretization error) for syn is almost twice that for the other two cases (mont and empm). These results indicate that without synoptic atmospheric forcing, the diagnostics using the numerical circulation model may significantly underestimate buoyancy loss at the surface, and, hence, water-mass formation, as well as mixing and spreading of the formed water mass.

Effects of the Mixed Layer Time Variability on Kinematic Subduction Rate Diagnostics

Abstract An eddy-resolving primitive equation general circulation model is used to estimate water-mass subduction rates in the North Atlantic Ocean subtropical gyre. The diagnostics are based on the instantaneous kinematic approach, which allows the calculation of the annual rate of water-mass subduction at a given density range, following isopycnal outcrop positions over the annual cycle. It is shown that water-mass subduction is effected rapidly (∼1–2 months) as the mixed layer depth decreases in spring, consistent with Stommel’s hypothesis, and occurs mostly over the area of deep late-winter mixed layers (≥150 m) across the central North Atlantic in the density range 26 ≤ σ ≤ 27.2. Annual subduction rates O(100–200 m yr–1) are found south and east of the Gulf Stream extension in the density range of subtropical mode waters from roughly 26.2 to 26.6. In the northeastern part of the subtropical gyre, annual subduction rates are somewhat larger, O(250 m yr–1), from a density of about 26.9 east of the North Atlantic Current to 27.4 (upper cutoff in this study). The overall basin-integrated subduction rate for subtropical mode waters (26.2 ≤ σ ≤ 26.6) is about 12.2 Sv (Sv ≡ 106 m3 s−1), comparable to the total formation rate inferred from the surface density forcing applied in the model of roughly 11 Sv in this density range. In contrast, basin-integrated rates for denser central water (26.8 ≤ σ ≤ 27.2) provide a vanishingly small net subduction. In this range, eddy correlations (&amp;lt;30 days) between the surface outcrop area and the local subduction rate counteract the net subduction by the mean flow (deduced from monthly averaged model fields). Comparison with estimates of the annual subduction rate based on the annual mean velocity and late-winter mixed layer properties alone, as is usual in climatological and coarse-resolution model analyses, indicates a mismatch of at least 8 Sv in the density range where the model forms subtropical mode water. This mismatch is primarily due to time-varying mixed layer processes rather than small-scale mixing not resolved explicitly by the model. Our diagnostics based on the instantaneous kinematic approach provide a more complete picture of the water-mass formation process than diagnostics based only on air–sea flux or late-winter mixed layer model data. They reveal the crucial importance of both the seasonal mixed layer cycle and mesoscale eddies to the overall formation rate and provide thus a valuable tool for the analysis of water-mass formation rates in eddy-resolving numerical simulations at basin scale.

Subduction of the North Pacific Mode Waters in a Global High-Resolution GCM*

Abstract The annual subduction rate in the North Pacific is estimated using five-day outputs from a high-resolution general circulation model (GCM). Two maxima (>200 m yr−1) are found in the western North Pacific: one is responsible for the formation of the subtropical mode water (STMW) and the other for the formation of the central mode water (CMW). A local maximum (>75 m yr−1) is also found in the formation region of the eastern subtropical mode water (ESMW). These results are compared with a calculation using the winter mixed layer depth and annual mean velocity fields to examine the effect of mesoscale eddies. Although the mesoscale eddies do not markedly affect the general subduction pattern, they enhance the annual subduction rate by up to 100 m yr−1 in the formation region of the STMW/CMW, a 34% increase in a regional average (30°–44°N, 140°E–170°W). Further analysis shows that the effects of the mean seasonal cycle and smaller-scale (<30 days) eddies are generally small. The authors suggest that t...

The Structure of the Wind-Driven Circulation in the Subtropical South Pacific Ocean*

The structure of the wind-driven circulation in the subtropical South Pacific is studied using simple diagnostic and analytical models. The diagnostic calculation is based on the Levitus climatology. The analytical model is forced by observed winter mixed layer density and depth calculated from the Levitus climatology and by the surface wind stress data from the Hellerman and Rosenstein climatology. The wind-driven gyre in the South Pacific is relatively deep, reaching 2.4 km along the southern edge of the gyre. The gross feature of subduction obtained from both the data analysis and the analytical model is similar, with an annual ventilation rate of 21.6 Sv (Sv ≡ 106 m3 s−1), including 18.1 Sv from vertical pumping and 3.5 Sv from lateral induction. Although the annual subduction rate in the South Pacific is comparable to that in the North Atlantic, lack of localized subduction leads to relatively weak mode water formation in the region where the East Australian Current separates from its western boundary. In addition, results from the analytical model indicate the existence of an isopycnal slope reversal in the southeastern Pacific.