Subduction Of Mode Water Research Articles

Effects of Ocean Chlorophyll on the Mode Water Subduction Rate in the Northwestern and Central Pacific Ocean

The effects of ocean chlorophyll on the mode water subduction rate in the subtropical mode water (STMW) and central mode water (CMW) in Pacific Ocean are investigated by performing two ocean-only experiments, using two different solar radiation penetration schemes, one with and one without chlorophyll effects. The biological impacts on mixed layer depth (MLD), upper ocean temperature and density are analyzed. Results show that the subduction rates of both STMW and CMW are increased with the effects of ocean chlorophyll. The increase in the subduction rate is mainly caused by the increased lateral induction term, which is related to larger MLD gradient in early spring in the chlorophyll experiment.

Quasi-Biennial Variability of Indian Ocean Subtropical Mode Water Subduction Driven by Atmospheric Circulation Modes during the Argo Period

Abstract Argo data were used to investigate interannual variability in the subduction rate of the Indian Ocean Subtropical Mode Water (IOSTMW). The IOSTMW subduction rate in the southwestern part of the southern Indian Ocean subtropical circulation varies substantially from year to year without significant trend during the Argo period 2005–19. The variability of the IOSTMW subduction rate is quasi-biennial and dominated by the lateral induction term associated with the wintertime mixed layer depth (MLD). The contribution of vertical pumping to the change of subduction rate is relatively small. This variability in the subduction rate directly contributes to IOSTMW volume with a quasi-biennial variation. The quasi-biennial variations in the IOSTMW subduction and volume reflect the variability of the overlying atmosphere. The wintertime Mascarene high (MH) modulates the winter MLD in the subduction area through changes in heat fluxes and wind forcing. The quasi-biennial variability in MH is associated with the southern annular mode, but it was disturbed during two El Niño events in 2009 and 2015/16. Variability of the wintertime MH plays a key role in driving the quasi-biennial variability of the IOSTMW subduction during the Argo period.

Physical Mechanisms Driving Oxygen Subduction in the Global Ocean

AbstractFuture changes in subduction are suspected to be critical for the ocean deoxygenation predicted by climate models over the 21st century. However, the drivers of global oxygen subduction have not been fully described or quantified. Here, we address the physical mechanisms responsible for the oxygen transport across the late‐winter mixed layer base and their relation with water mass formation. Up to 70% of the global oxygen uptake takes place during Mode Water subduction mostly in the Southern Ocean and the North Atlantic. Te driving mechanisms are (i) the combination of strong currents with large mixed‐layer‐depth gradients at localized hot spots and (ii) the wind‐driven vertical velocity within the subtropical gyres. Oxygen diffusion, despite being underestimated in this study, is likely to play an important role in the global ocean oxygenation. The physical mass flux dominates the total oxygen subduction while the oxygen solubility plays a minor role in its modulation.

Variability of the Sub‐Antarctic Mode Water Subduction Rate During the Argo Period

AbstractBoth a quasi‐biennial variability and an overall linearly increasing trend are identified in the Sub‐Antarctic Mode Water (SAMW) subduction rate across the Southern Hemisphere ocean, using the Argo data during 2005–2019. The quasi‐biennial variability is mainly due to variability of the mixed layer depth. Variability of wind stress curl in the SAMW formation regions associated with the Southern Annular Mode plays a critical role in generating the quasi‐biennial variability of the mixed layer depth and consequently the SAMW subduction rates. The SAMW subduction rate across the Southern Hemisphere ocean, long‐term mean totaling 56 Sv, has increased at 0.73 ± 0.65 Sv year−1 over the past 15 years. The increase has directly contributed to the observed increase in the total SAMW volume. Much of this increasing trend can be explained by the deepening mixed layers, which in turn are primarily forced by the strengthening westerly winds under an increasing Southern Annular Mode.

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

Two zonal high-density hydrographic sections along 41° N and 37.5° N east of Japan were occupied in April 2013 and June 2016 to examine the formation of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in relation to fronts and eddies. In the 41° N section traversing the meandering subarctic front, the denser variety of CMW (D-CMW) and TRMW was formed continuously on both sides of the front, except for the part of the section located south of the Kuroshio bifurcation front where the lighter variety of CMW (L-CMW) and D-CMW was formed instead. L-CMW and D-CMW were also formed in the eastern part of the 37.5° N section between the Kuroshio Extension front and the Kuroshio bifurcation front, but were hardly formed in the western part of the section west of the bifurcation point of the two fronts. D-CMW and TRMW pycnostads in the western part of the 41° N section observed in April 2013 tended to exhibit more than one core (vertical minimum of potential vorticity), which might be formed by destruction of deep winter mixed layers. Such multiple-core structure was also observed in L-CMW and D-CMW pycnostads in the eastern part of both the sections south of the Kuroshio bifurcation front in June 2016, being particularly abundant in three anticyclonic eddies. It was likely to be formed by the exchange of low-potential vorticity water among the eddies and the ambient region in association with eddy-to-eddy interaction, suggesting a new mechanism of mode water subduction.

The Sensitivity of Southeast Pacific Heat Distribution to Local and Remote Changes in Ocean Properties

AbstractThe Southern Ocean features ventilation pathways that transport surface waters into the subsurface thermocline on time scales from decades to centuries, sequestering anomalies of heat and carbon away from the atmosphere and thereby regulating the rate of surface warming. Despite its importance for climate sensitivity, the factors that control the distribution of heat along these pathways are not well understood. In this study, we use an observationally constrained, physically consistent global ocean model to examine the sensitivity of heat distribution in the recently ventilated subsurface Pacific (RVP) sector of the Southern Ocean to changes in ocean temperature and salinity. First, we define the RVP using numerical passive tracer release experiments that highlight the ventilation pathways. Next, we use an ensemble of adjoint sensitivity experiments to quantify the sensitivity of the RVP heat content to changes in ocean temperature and salinity. In terms of sensitivities to surface ocean properties, we find that RVP heat content is most sensitive to anomalies along the Antarctic Circumpolar Current (ACC), upstream of the subduction hotspots. In terms of sensitivities to subsurface ocean properties, we find that RVP heat content is most sensitive to basin-scale changes in the subtropical Pacific Ocean, around the same latitudes as the RVP. Despite the localized nature of mode water subduction hotspots, changes in basin-scale density gradients are an important controlling factor on heat distribution in the southeast Pacific.

Heat Distribution in the Southeast Pacific Is Only Weakly Sensitive to High‐Latitude Heat Flux and Wind Stress

AbstractThe Southern Ocean features regionally varying ventilation pathways that transport heat and carbon from the surface ocean to the interior thermocline on timescales of decades to centuries, but the factors that control the distribution of heat along these pathways are not well understood. In this study, we use a global ocean state estimate (ECCOv4) to (1) define the recently ventilated interior Pacific (RVP) using numerical passive tracer experiments over a 10‐year period and (2) use an adjoint approach to calculate the sensitivities of the RVP heat content (RVPh) to changes in net heat flux and wind stress. We find that RVPh is most sensitive to local heat flux and wind stress anomalies north of the sea surface height contours that delineate the Antarctic Circumpolar Current, with especially high sensitivities over the South Pacific Gyre. Surprisingly, RVPh is not especially sensitive to changes at higher latitudes. We perform a set of step response experiments over the South Pacific Gyre, the subduction region, and the high‐latitude Southern Ocean. In consistency with the adjoint sensitivity fields, RVPh is most sensitive to wind stress curl over the subtropical gyre, which alter isopycnal heave, and it is only weakly sensitive to changes at higher latitudes. Our results suggest that despite the localized nature of mode water subduction hot spots, changes in basin‐scale pressure gradients are an important controlling factor on RVPh. Because basin‐scale wind stress is expected to change in the coming decades to centuries, our results may have implications for climate, via the atmosphere/ocean partitioning of heat.

Mesoscale and Submesoscale Processes in the Southeast Atlantic and Their Impact on the Regional Thermohaline Structure

AbstractThe turbulent processes in the Cape Basin, the southeasternmost gate of the Atlantic Ocean, play a key role in the transport and mixing of upper to intermediate water masses entering the area from the Indian Ocean, making them especially relevant for the Indo‐Atlantic transfer of heat and salt. In this paper, two numerical simulations at different horizontal resolutions are used to study mesoscale and submesoscale dynamics, their phenomenology, their evolution, and their impact on the local water masses. Submesoscale processes seasonally affect both, the upper and intermediate layers, but there are clear dynamical differences between the two layers. Several types of instabilities underline this spatial and temporal variability. Near the surface, mixed layer instabilities occur during winter, while mesoscale‐driven instabilities, as the symmetric type, prevail in summer. The connection between these two seasonal regimes is ensured, in anticyclonic eddies and within the mixed layers, by Charney baroclinic instabilities, involved in the local formation and subduction of mode water, that we have dubbed as Agulhas Rings mode water. Intermediate depths are instead characterized by mesoscale mechanisms of density compensation and lateral stirring of the tracer variance, triggering a significant filamentogenesis whose vertical scales are comparable to those mentioned in previous studies. This leads to a particularly efficient mixing of Antarctic Intermediate Waters of Indian and Atlantic origins. Lagrangian estimates highlight the new and significant role of fine scale structures in setting the water masses properties of upper and lower thermocline waters materializing the Indo‐Atlantic exchange and therefore potentially affecting the global ocean circulation.

Interannual variability of Indian Ocean subtropical mode water subduction rate

The interannual variation of Indian Ocean subtropical mode water (IOSTMW) subduction rate in the Southwest Indian Ocean from 1980 to 2007 is investigated in this paper based on Simple Ocean Data Assimilation (SODA) outputs. Climatology of subduction rate exceeds 75 m/year in the IOSTMW formation area. The renewal time of permanent pycnocline water mass based on the subduction rate is calculated for each density class: 3–6 years for IOSTMW (25.8 < σ θ < 26.2 kg m−3). Subduction rate in the Southwest Indian Ocean subtropical gyre exhibits a great year-to-year variability. This interannual variations of the IOSTMW subduction rate is primarily dominated by the lateral induction term, associated with the interannual variations of strong meridional gradient of winter mixed layer depth (MLD). The slope of the mixed layer depth in the mode water is closely linked to the large variations of deep late winter MLD in the mid-latitudes and negligible variations of shallow winter MLD in lower latitudes. It is further identified that the interannual variation of late winter MLD in this area is largely controlled by the latent and sensible heat flux components. The water volume of the permanent pycnocline in the IOSTMW distribution area is also found to show a significant interannual variability, and it is well correlated with the interannual variation of subduction rate.

Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry

Temperature and salinity data from Argo profiling floats during 2005–2014 were analyzed to examine the decadal variability of the North Pacific Subtropical Mode Water (STMW) in relation to that of the Kuroshio Extension (KE) system. The formation volume of STMW in the southern recirculation gyre of KE in the cooling season was larger during the stable KE period after 2010 than the unstable KE period of 2006–2009 by 50 %. As a result, the volume and spatial extent of STMW increased (decreased) in the formation region during the stable (unstable) KE period, as well as in the southern, downstream region with a time lag of 1–2 years. The decadal expansion and contraction of STMW were also detected by shipboard observations conducted routinely in the most downstream region near the western boundary, in terms of not only physical, but also biogeochemical parameters. After 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon, among which changes of dissolved inorganic carbon, pH, and aragonite saturation state were against their long-term trends. These results indicate a new mechanism consisting of westward sea surface height anomaly propagation, the KE state transition, and the STMW formation and subduction, by which the climate variability affects physical and biogeochemical structures in the ocean’s interior and potentially impacts the surface ocean acidification trend and biological production.

An intensification trend of South Pacific Mode Water subduction rates over the 20th century

In this paper, the trend of the South Pacific Mode Water subduction rate is assessed based on recently developed ocean and atmosphere reanalysis products for the 20th century. It is found that the subduction volume of the mode water is intensified in the 20th century. The intensification of the mode water in the subtropical gyre of the South Pacific is primarily due to the strengthening of the vertical pumping velocity dominated by the enhanced wind stress curl over the past century. In contrast, the intensification of the South Pacific Subantarctic Mode Water (SAMW) is mainly caused by a deepening of the mixed layer depths and a strengthening of the advection in warm climate. As a result, the total subduction volume of the mode water over the South Pacific is increased in the 20th century.

Mesoscale and diel to monthly variability of CO2 and carbon fluxes at the ocean surface in the northeastern Atlantic

Four CARIOCA Lagrangian buoys were deployed in the northeast Atlantic Ocean as part of the Programme Océan Multidisciplinaire Méso Echelle (POMME) dedicated to the study of the role of mesoscale eddies in biological production, the carbon budget, and the subduction of mode waters. An extensive set of hourly surface measurements of temperature, salinity, and carbon dioxide fugacity (fCO2) was collected from February to August 2001. The high‐frequency spatial and temporal variability observed in surface fCO2 and in the derived dissolved inorganic carbon (DIC) concentrations suggests that abrupt changes of the carbon variables are being generated along frontal patterns and filaments by submesoscale and mesoscale eddy‐eddy interactions, especially in winter. On the basis of a 1‐D model of the diurnal mixed layer along the buoy trajectory, we show that under certain conditions the diel cycle of DIC is driven by the daily cycle of photosynthesis and metabolic CO2 release at the ocean surface. This information is quantitatively used to derive in‐situ primary production, carbon gross, and net community production. The analysis of 107 observed diel cycles of DIC shows that episodic biological production processes are triggered by the mesoscale activity of surface eddies. Over the sampled period, the POMME area is a sink for atmospheric CO2 with an estimated flux of −4 mmol m−2 day−1. The calculated amount of anthropogenic carbon transported into the ocean interior by subduction of subpolar mode water is equal to 2.8 1013 gC yr−1.

Major multidisciplinary ocean project seeks proposals

A major multidisciplinary oceanographic program in the North Atlantic is being put together by the French oceanographic community, and scientists who would like to participate are being asked to submit proposals immediately Field work will begin in September 2000. Known as Programme Océanographique Multidisciplinaire Méso‐Echelle (POMME),the Meso‐scale Multidisciplinary Oceanographic Program, it is supported by several French scientific agencies and will take place between the Azores and the Iberian Peninsula.The program will focus on subduction of mode water in the region, its effect on biological production and the carbon budget in the area, and the fate of organic matter after subduction. More detailed information on the program's framework, rationale, and organization can be found on the Web (http://www.insu.cnrs‐dir.fr).

Shallow Pycnoclines and Mode Water Subduction in the Eastern North Atlantic

Abstract A Lagrangian one-dimensional model is used to study the subduction of Subpolar Mode Water in the eastern North Atlantic and to analyze recently observed hydrological features related to this process. Considering a southward moving column of mode water, subduction occurs when the upper part of the column starts gaining buoyancy in an annual budget. The seasonal pycnocline on top of the column can no longer be completely eroded in winter, and it is shown how its remnant forms a shallow “secondary” pycnocline at about 200 m depth, which isolates the lower part of the fluid column from the atmosphere. This mechanism for the subduction of Subpolar Mode Water induces a strong meridional gradient in the winter mixed layer depth. The various components of the buoyancy input are thoroughly studied. Horizontal advection in the shallow Ekman layer and vertical advection along the column are shown to modify significantly both the thermal and haline contents of the column, with magnitudes comparable to the ai...