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On the Response of the Global Subduction Rate to Global Warming in Coupled Climate Models


doi: 10.1007/s00376-013-2323-9

  • The response of the global subduction rate to global warming was assessed based on a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models. It was found that the subduction rate of the global ocean could be significantly reduced under a warming climate, as compared to a simulation of the present-day climate. The reduction in the subduction volume was quantitatively estimated at about 40 Sv and was found to be primarily induced by the decreasing of the lateral induction term due to a shallower winter mixed layer depth. The shrinking of the winter mixed layer would result from intensified stratification caused by increased heat input into the ocean under a warming climate. A reduction in subduction associated with the vertical pumping term was estimated at about 5 Sv. Further, in the Southern Ocean, a significant reduction in subduction was estimated at around 24 Sv, indicating a substantial contribution to the weakening of global subduction.
    摘要: The response of the global subduction rate to global warming was assessed based on a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models. It was found that the subduction rate of the global ocean could be significantly reduced under a warming climate, as compared to a simulation of the present-day climate. The reduction in the subduction volume was quantitatively estimated at about 40 Sv and was found to be primarily induced by the decreasing of the lateral induction term due to a shallower winter mixed layer depth. The shrinking of the winter mixed layer would result from intensified stratification caused by increased heat input into the ocean under a warming climate. A reduction in subduction associated with the vertical pumping term was estimated at about 5 Sv. Further, in the Southern Ocean, a significant reduction in subduction was estimated at around 24 Sv, indicating a substantial contribution to the weakening of global subduction.
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Manuscript received: 07 January 2013
Manuscript revised: 08 March 2013
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On the Response of the Global Subduction Rate to Global Warming in Coupled Climate Models

  • 1. School of Marine Sciences, Nanjing University of Information Science & Technology, Nanjing 210044

Abstract: The response of the global subduction rate to global warming was assessed based on a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models. It was found that the subduction rate of the global ocean could be significantly reduced under a warming climate, as compared to a simulation of the present-day climate. The reduction in the subduction volume was quantitatively estimated at about 40 Sv and was found to be primarily induced by the decreasing of the lateral induction term due to a shallower winter mixed layer depth. The shrinking of the winter mixed layer would result from intensified stratification caused by increased heat input into the ocean under a warming climate. A reduction in subduction associated with the vertical pumping term was estimated at about 5 Sv. Further, in the Southern Ocean, a significant reduction in subduction was estimated at around 24 Sv, indicating a substantial contribution to the weakening of global subduction.

摘要: The response of the global subduction rate to global warming was assessed based on a set of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models. It was found that the subduction rate of the global ocean could be significantly reduced under a warming climate, as compared to a simulation of the present-day climate. The reduction in the subduction volume was quantitatively estimated at about 40 Sv and was found to be primarily induced by the decreasing of the lateral induction term due to a shallower winter mixed layer depth. The shrinking of the winter mixed layer would result from intensified stratification caused by increased heat input into the ocean under a warming climate. A reduction in subduction associated with the vertical pumping term was estimated at about 5 Sv. Further, in the Southern Ocean, a significant reduction in subduction was estimated at around 24 Sv, indicating a substantial contribution to the weakening of global subduction.

1 Introduction
  • Ventilation-related water mass formation has been proven to respond to climate change on multiple timescales (e.g., Joyce et al., 2000; Banks et al., 2002; Xie et al., 2002; Oka, 2009; Downes et al., 2009; Liu and Huang, 2011). The changes in subduction rate under a warming climate are of particular concern because of the important “memories” of the climate system that they hold (Saenko et al., 2005; Lee, 2009). Owing to buoyancy loss from the ocean to the atmosphere in winter, the shallow seasonal pycnocline is eroded to form the late winter mixed layer front; and thus, through the large-scale subduction of mixed layer water, the subduction-related water masses detrain into the permanent pycnocline. (Tsubouchi et al., 2007; Holbrook and Maharaj, 2008). Carrying water with distinctly low potential vorticity (PV) from the sea surface to the subsurface interior, both ocean stratification and circulation may be substantially impacted by subduction-related water masses (Xie et al., 2011), especially for mode waters Kwon and Riser, 2004; Qiu et al., 2006;Herraiz-Borreguero and Rintoul, 2011).

    Based on analyses of model results, significant changes in the volumes of subduction-related water masses (e.g., mode waters) have been identified in response to increased concentrations of greenhouse gases (Downes et al., 2009; Saenko et al., 2011). Under a warming climate, in the North Pacific Ocean, the production of mode waters is significantly weakened with a more stratified upper ocean resulting mainly from decreased ocean-to-atmosphere heat loss (Luo et al., 2009). A weakening of the North Pacific countercurrent may arise due to the reduction of North Pacific subtropical mode waters (Xie et al., 2011). In contrast to changes in mode water in the North Pacific, an intensification of the South Pacific Eastern Subtropical Mode Water has been detected due to a deepening of the winter mixed layer depth (MLD), resulting mainly from intensified southeast trade winds in response to global warming (Luo et al., 2011). Diagnosed by a climate model, the Sub-Antarctic Mode Water (SAMW) (Aoki et al., 2007)and Antarctic Intermediate Water (AAIW) are likely to shift to lighter densities under a warming climate, with a reduction of the two water masses due to a shallower MLD resulting from a net buoyancy gain having been predicted (Downes et al., 2009). Based on eight IPCC models, the SAMW and AAIW have also been assessed through a multi-model intercomparison study (Downes et al., 2010), in which it was suggested that these two water masses will warm and freshen as they shift to lighter density classes toward the end of the 21st century under the A2 scenario (atmospheric CO2 at a concentration of 860 ppm by 2100). (Saenko et al., 2011) reported that subduction, transformation, and transportation of mode water in both hemispheres shifted to lighter densities in response to a doubling of CO2 in a coupled climate model; however, the level of production did not change significantly.

    Based on ocean reanalysis data, the interannual and decadal variability of the global subduction rate have been assessed [Liu and Huang, 2011], but the changes in global subduction in response to a warming climate remain unclear. Furthermore, many studies based on coupled climate models under future anthropogenic forcing scenarios have been carried out by focusing on the changes in the production of mode waters in response to global warming (e.g., Luo et al.,2009; Downes et al., 2010; Luo et al., 2011), but the variability of global subduction-related water masses under a warming climate still need to be investigated. In the present study, by focusing on global subduction-related water masses, we assessed the response of the global subduction volume to a warming climate based upon coupled climate models.

2 Data
  • Outputs from 12 IPCC Fourth Assessment Report (AR4) climate models were used to assess the response of global subduction to global warming. Present-day climate was represented by the IPCC 20C3M experiment, in which the atmospheric CO2 concentrations and other input data are based upon historical records of 20th century climate. The other experiment [the A1B scenario of the Special Report on Emissions Scenarios (SRES A1B)] was selected to represent a warmer climate in the future, in which the atmospheric CO2 concentration is set to 720 ppm—a doubling of the present-day level in the year 2100.

    We focused on the annual mean subduction rates averaged over the periods 1951-2000 and 2051-2100 from 20C3M and SRESA1B, respectively. Considering the sufficient outputs available, ensemble means from 12 climate models were examined: (1) CCCMA_CGCM3_1; (2) CCCMA_CGCM3_1_T63; (3) CNRM_CM3; (4) CSIRO_MK3_ 0; (5) CSIRO_MK3_5; (6) IAP_FGOALS1_0_G; (7) INGV_ ECHAM4; (8) IPSL_CM4; (9) MIROC3_2_MEDRES; (10) MPI_ECHAM5; (11) MRI_CGCM2_3_2A; and (12) NCAR_CCSM3_0 (IPCC, 2007).

3 Results
  • In this study, the MLD was defined as the depth at which σθ (potential density) is 0.125 kg m-3 denser than at the sea surface, and the maximum MLD in a given year was used to calculate the global subduction rate [Liu and Huang, 2011]. In the North Pacific, the MLD front is located to the east of Japan, where it tilts northeastward, indicating an eastward advection effect of the Kuroshio Extension.

    In the 20C3M simulation results (Fig. 1a), the MLD is about 300 m deep to the east of Japan, with a maximum of about 350 m at (40°N, 150°E). To the north of the subtropical gyre of the North Atlantic, the equatorward shoaling MLD extends northeastward across the central North Atlantic, with a maximum of 400 m. The deep mixed layer extends into the subpolar region and Labrador Sea, with the depth varying from roughly 600 m in the east to more than 800 m in the west. In the east of the subtropical gyre of the South Pacific, an MLD maximum of about 200 m is centered at (30°S, 100°W). Two zonal northwest slant bands of the deep MLD are found in the Southern Ocean—one is located in the east of the South Indian Ocean [Tsubouchi et al., 2010] and south of New Zealand (30°-60°S), with a maximum of 600 m and extending from 50°-180°E; while the other is located in the Southeast Pacific (45°-60°S), with a maximum of 700 m and extending from about 130°-50°W.

    In the SRES A1B runs (Fig. 1b), the spatial pattern of the MLD is similar to that in 20C3M; however, the MLD becomes shallower in most parts of the global ocean, particularly in the Southern Ocean. This significant shoaling of the MLD results from a more stratified upper layer due to greater ocean warming near the surface. In comparison with 20C3M, the MLD is about 100 m shallower to the southeast of Japan and about 50 m shallower along the path of the Gulf Stream. In the Southern Ocean, the deep MLD areas are also seen to shrink significantly under the warming climate, being about 100 m shallower to the south of the New Zealand and in the east of the South Indian Ocean. In the southeast of the South Pacific, the MLD is 250 m shallower than in the 20C3M results.

    Figure 2 shows the distribution of maximum MLD in the World Ocean Atlas 2009 (WOA09) and, as can be seen, the pattern is similar to that derived from the 20C3M experiment. However, the MLD in the subtropical gyre of the Northern Hemisphere is roughly 50 m deeper in the results of the IPCC AR4 models than in WOA09, and in the former the mixed layer maximum occupies a larger region. In the Antarctic Circumpolar Current (ACC) region, the MLD in the IPCC model results is relatively shallower and shifts poleward in comparison to WOA09 (Fig. 2), and this may result from the different MLD distributions in the ACC region in the IPCC AR4 models.

    The annual subduction rate in the global ocean was calculated for both the 20C3M and the SRES A1B scenarios. From the kinematic perspective, by following a particle released at the base of the mixed layer, the Lagrangian subduction rate is calculated at the time when the effective detrainment starts throughout the entire seasonal cycle (Huang and Qiu, 1994). The annual subduction rate (Sann) is defined as follows:

    where hm denotes the mixed layer depth and wnb is the vertical velocity of a water parcel at the base of the mixed layer. By integrating from the end of the first winter (t1) to that of the second winter (t2) over one year (T), the Sann could be obtained. On the right-hand side of Eq. (1), the first term denotes the contribution of vertical pumping at the base of the mixed layer. The second term, which is called lateral induction, represents the contribution due to the slope of the mixed layer base. Compared to previous studies (Huang and Qiu, 1994), the Ekman pumping velocity and a reduction term due to the meridional velocity in the surface layer is replaced by the vertical velocity.

    Figure 1.  The climatological maximum MLD in the corresponding year from (a) 1951-2000 of the 20C3M runs and (b) 2051-2100 of the SRES A1B runs (units: m). The bottom of the mixed layer is defined as the depth whereσθ first exceeds its surface value by 0.125 kg m-3.

    Figure 2.  The climatological maximum MLD derived from WOA09. The bottom of the mixed layer is defined as the depth where σθ first exceeds its surface value by 0.125 kg m-3.

    As mentioned above, regions of high subduction rate are generally characterized by thick mixed layers. In the Southern Hemisphere in the 20C3M runs (Fig. 3a), strong subduction occurs around the northern region of the Southern Ocean in the Pacific and eastern Indian sectors, with maximums of more than 300 m yr-1 and 250 m yr-1, respectively. In the subtropical region of both the Pacific and Atlantic, the subduction rates are roughly 100 m yr-1, corresponding to local MLD fronts. In the subpolar region of the North Atlantic, there is also a maximum subduction rate of about 250 m yr-1 at around (55°N, 40°W) corresponding to the local deep MLD [Marshall et al., 1993].

    Similar to the response of the MLD to the SRES A1B global warming scenario, the spatial pattern of global subduction rate does not change (Fig. 3b), but there are reductions of subduction rates in most parts of the global ocean. In the subpolar region of the North Atlantic, the subduction rate is around 75 m yr-1, which is 25 m yr-1 less than in 20C3M. A decrease in subduction can also be identified in the subtropical region of both hemispheres, especially in the North Pacific, as found by Luo et al., (2009). Compared to the subtropical region, there is noticeable weakening of subduction under the SRES A1B scenario in the northern region of the Southern Ocean in the Pacific sector, with a maximum of about 250 m yr-1, and in the eastern Indian sector, with a decrease of about 50 m yr-1 (Fig. 3b).

    Figure 3.  Subduction rate (units: m yr-1) derived from (a) 20C3M and (b) the response (SRES A1B minus 20C3M) (downward is positive). Black rectangles mark for the key regions for mode water formation.

    Figure 4.  Subduction volume (units: Sv) integrated in the North Pacific (NP), North Atlantic (NA) and South Pacific (SP) for each model of the 20C3M experiment.

    In order to evaluate the uncertainties of the model results, we integrated the subduction volume in the North Pacific (20°-50°N, 120°-240°E), North Atlantic (20°-50°N, 80°-0°W) and South Pacific (10°-60°S, 140°-290°E) for each model, respectively (Fig. 4)—regions for which there are observational estimates. The model mean subduction volume is 44.2 Sv, 25.5 Sv and 65.3 Sv in the North Pacific, North Atlantic and South Pacific, respectively. Although these values are slightly different from the 35.2 Sv and 27.0 Sv reported by [Qiu and Huang, 1995] for the North Pacific and North Atlantic, respectively, and the 48.8 Sv reported by [Qu et al., 2008] for the South Pacific, all of which were derived from observational data, the multi-model mean results still capture the major subduction process in the main subduction regions. Figure 5 presents the subduction volume of the global ocean in the 20C3M and SRES A1B experiments separately for each model. As can be seen, all of the models present a substantial decrease of the subduction rate under a warming climate (Fig. 5a), and most of them also depict a decrease in subduction in the Southern Ocean, except for Cnrm_cm3 and Mri_cgcm2_3_2a (Fig. 5b). The standard deviation for the change in subduction volume in the global ocean and Southern Ocean is 15.2 Sv and 18.2 Sv respectively, reflecting the fact that this decrease in subduction is significant. We also checked the spatial pattern of the response of the subduction process in SRES A1B for each model, and the results revealed that all the models produce a similar decreasing spatial pattern, except for CNRM_CM3 and MRI_CGCM2_3_2a in the Southern Ocean, with an increase of subduction (not shown).

    Figure 6 shows that the contribution of lateral induction, which is closely associated with mixed layer fronts (Xie et al., 2000), also has a decreasing over the global ocean, especially at around 45°-60°S in the Southern Ocean. The lateral induction terms for the 20C3M runs (Fig. 6a) present a similar spatial pattern as the subduction rate, suggesting that lateral induction may play a dominant role in subduction in most parts of the global ocean, except in the tropical region. In contrast to lateral induction, no significant differences can be seen in the vertical pumping terms between the 20C3M (Fig. 7a) and SRES A1B (Fig. 7b) results. In the Northern Hemisphere, the subduction associated with vertical pumping presents a northwest slant in the subtropical area, while in the Southern Hemisphere it presents a southeast slant in the subtropical area. As suggested by (Huang and Qiu, 1994), the vertical pumping term is predominantly controlled by the wind stress curl. A negative value in the tropical region of the Pacific and Indian oceans is apparently induced by wind-driven upwelling, which may imply that subduction would be inhibited by the vertical pumping term in the tropical area.

    Figure 5.  Subduction volume (units: Sv) integrated in the (a) global ocean and (b) Southern Ocean for each model in the order the models are introduced in section 2. The blue stars are derived from 20C3M, and the red plus symbols are derived from SRES A1B.

    Figure 6.  Lateral induction (units: m yr-1) of the subduction rate for (a) 20C3M and (b) the response (SRES A1B minus 20C3M) (downward is positive).

    Figure 7.  Subduction rate associated with vertical pumping (units: m yr-1) for (a) 20C3M and (b) the response (SRES A1B minus 20C3M) (downward is positive).

    Figure 8.  (a) Time series of the global subduction volume (units: Sv) derived from the output of 20C3M runs (black) and SRES A1B runs (gray). (b) Time series of the lateral induction term of the global subduction volume (units: Sv) for 20C3M (black) and for SRES A1B (gray). (c) Time series of the vertical pumping term of the global subduction volume (units: Sv) for 20C3M (black) and SRES A1B (gray). (d) Time series of the subduction volume of the Southern Ocean (units: Sv) for 20C3M (black) and SRES A1B (gray).

    The total volumetric subduction rate integrated over the global ocean and averaged from 1951-2000 is estimated at 308.6 Sv from the 20C3M runs, which is about 40 Sv more than that (268.8 Sv) averaged from 2051-2100 from the SRES A1B runs (Fig. 8a), with the standard deviation of the subduction response being 15.1 Sv. The integrated subduction associated with the lateral induction term over the global ocean and averaged from 1951-2000 is estimated at 294.1 Sv from the 20C3M runs, which is about 50 Sv more than that (244.4 Sv) averaged from 2051-2100 from the SRES A1B runs (Fig. 8b). Note that, according to the calculation suggested by Liu and Huang(2011), the negative value was set to zero, so that the differences in lateral induction between the 20C3M and SRES A1B runs may be larger than those of net subduction volume due to the absence of cancellation between the lateral induction and vertical pumping. The integrated subduction associated with the vertical pumping term is estimated at 134.0 Sv from the 20C3M runs, which is only about 5 Sv more than that (128.8 Sv) averaged from 2051-2100 from the SRES A1B runs (Fig. 8c). This indicates that the significant decrease in the global subduction volume is dominated by the lateral induction process, and the vertical pumping term plays a secondary role in this reduction. Figure 8d depicts the subduction volume of the Southern Ocean in different runs. The volumetric subduction rate integrated over the Southern Ocean averaged from 1951-2000 is estimated at 163.0 Sv from the 20C3M runs, which is about 24 Sv more than that (139.0 Sv) averaged from 2051-2100 from the SRES A1B runs (Fig. 8d), with the standard deviation of the subduction response being 18.2 Sv. These results show that subduction of the Southern Ocean plays the most important role in the subduction of the global ocean, and thus the reduction of subduction in the Southern Ocean plays a key role in the weakening of global subduction.

    As noted previously, subduction consists of a vertical pumping term and a lateral induction term. However, lateral induction, through which mixed layer waters are swept into the permanent thermocline, may play the dominant role in reduction of subduction under global warming. Since lateral induction of subduction is typically associated with MLD horizontal gradients, the change of subduction under a warming climate may be closely related to the weakened MLD front. Furthermore, this may indicate that the winter MLD front could make a significant contribution to the evolution of subduction. Under a global warming scenario, wintertime convection can be inhibited by a more stratified upper ocean, resulting in a shallower MLD and thus a reduction of lateral induction.

    The stratification of the upper ocean is influenced by the buoyancy flux (Bnet) (Downes et al., 2009; Downes et al.,2010), which is defined as follows:

    In Eq. (2), a negative Bnet indicates buoyancy loss (i.e., making the density of surface waters weighted) and a positive value indicates a buoyancy gain. The variableρis sea water density at the surface, and the gravitational force is given by g. The first term in Eq. (2) is the buoyancy flux induced by surface heat flux (H), with Cw being the heat capacity of water andαbeing the thermal expansion coefficient. The second term is the buoyancy flux induced by the surface freshwater flux (W), with S being the mixed layer salinity and β being the haline contraction coefficient. The third term represents the buoyancy gain or loss amassed by the Ekman drift, where k is the unit vertical vector, τis the wind stress, and f is the Coriolis parameter.

    Since MLD reaches a maximum in late winter, we diagnosed each term in Eq. (2) at that time. Figure 9 depicts the three terms of the buoyancy flux in late winter from the results of the 20C3M experiment. In the subtropical region of both hemispheres, the term induced by surface heat flux shows a minimum <-10×10-5 N m-2 s-1. In the ACC region, it also reveals a buoyancy loss >-5×10-5 N m-2 s-1 through heat loss from ocean to atmosphere. The term induced by freshwater flux (Fig. 9b) tends to make the surface water lighter in the equatorial region, with a maximum >1.5×10-5 N m-2 s-1, while in the subtropical region more evaporation than precipitation tends to make the surface water denser, with a minimum <-1×10 N m-2 s-1. The term associated with Ekman drift (Fig. 9c) contributes less outside of the equatorial region, while the term induced by surface heat flux may play a dominant role in the global ocean in later winter.

    Figure 10 displays the response of each term in Eq. (2) under a warming climate. In the subtropical region of both hemispheres, the term induced by surface heat flux tends to make the surface water lighter, with a maximum >1× 10-5 N m-2 s-1 (Fig. 10a). In the ACC region and the subpolar region of the North Atlantic, it also reveals a buoyancy gain, with a maximum >1.5×10-5 N m-2 s-1. The term induced by freshwater flux (Fig. 10b) and Ekman drift (Fig. 10c) all contribute a positive buoyancy flux in the equatorial region, with maximums of roughly 5× 10-6 N m-2 s-1 for the freshwater flux and 2× 10-6 N m-2 s-1 for the Ekman drift. The term induced by freshwater flux also makes a positive contribution in the Southern Ocean through more precipitation than evaporation. However, both the term induced by freshwater flux and that by Ekman drift contribute much less to the buoyancy flux than the term induced by surface heat flux (Fig. 10a). This implies that surface heat flux may play a dominant role in the intensification of the stratification of the surface layer (Fig. 10a).

    The stratification of the upper ocean is mainly influenced by changes in atmosphere-ocean heat flux under global warming, which can be seen to have notable differences in the simulation results of 20C3M and SRES A1B. The climatological heat flux in 20C3M (Fig. 11a) has a similar spatial pattern to that in SRES A1B (not shown). However, the areas of strong ocean-to-atmosphere heat loss shrink significantly as a response to global warming. Large ocean-to-atmosphere heat loss is seen in the west of the subtropical gyre of the North Pacific and the North Atlantic due to the influence of local winter monsoon. In response to the global warming scenario, the heat loss in the SRES A1B runs is about 50 W m-2 less than that in the 20C3M runs in those regions (Fig. 11b). In the subpolar region of the North Atlantic, the heat loss in the SRES A1B runs is about 100 W m-2 less than that in the 20C3M runs. In the Southern Ocean, the heat loss areas also shrink significantly under the warming climate. In the Southern Indian Ocean, the heat loss in the SRES A1B runs is about 40 W m-2 less than in 20C3M, and about 50 W m-2 less in the southern Pacific Ocean around 60°S.

    Figure 9.  Buoyancy flux derived from 20C3M (units: N m-2 s-1 )for (a) the heat flux term; (b) the freshwater flux term; and (c) the Ekman drift term (downward is positive). Note the difference of the colored bars.

    Figure 10.  The response of buoyancy flux to a warming climate (SRES A1B minus 20C3M) (units: N m-2 s-1 for (a) the heat flux term; (b) the freshwater flux term; and (c) the Ekman drift term (downward is positive). Note the difference of the colored bars.

    Figure 11.  Ocean-to-atmosphere heat flux (units: W m-2 corresponding to the maximal MLD for (a) the 20C3M runs and (b) the response (SRES A1B minus 20C3M) (downward is positive).

4 Discussion and concluding remarks
  • The simulated subduction rates in 12 IPCC AR4 coupled climate models were quantitatively estimated. Considerable discrepancies in the simulated intensity and pattern of subduction were found (not shown); nevertheless, the results of the ensemble model mean were consistent with subduction rates estimated based on observational data (Qiu and Huang, 1995; Huang and Qiu, 1998; Qu et al., 2008), as discussed in section 2.

    It has previously been suggested that the production of mode waters in the subtropical gyre of the North Pacific could decrease significantly in response to global warming due to a shallower MLD (Luo et al., 2009), and our results were basically consistent with those findings. Although different responses on different basin scales have been found, a significant decreasing of global subduction in response to global warming scenarios was identified. We further found that the Southern Ocean plays a key role in terms of its volumetric contribution to global subduction, and is a critically important site of water mass formation, as supported by previous studies (e.g., Liu2011). In the Southern Ocean, the SAMW and AAIW are largely responsible for subduction and water mass formation in the global ocean (e.g., Sallee et al., 2010; Sloyan et al., 2010).

    Further analysis revealed that the lateral induction term plays a dominant role in the decreasing of global subduction, which is mainly influenced by a reduced MLD gradient. It is well known that ocean-to-atmosphere heat flux in late winter critically controls the deepening of the MLD. In sharp contrast to the results of the 20C3M runs, the deep MLD was considerably shallower in the SRES A1B experiment (i.e., under a warming climate) due to the reduced heat loss from ocean to atmosphere, and thus resulted in the reduction of the lateral induction process.

    Our study focused on quantifying global subduction rates and their response to global warming; however, there are several shortcomings. Since the datasets used in the study were not eddy-resolving and had a rather low horizontal resolution, critical components of global water mass cycling may not have been accurately represented in the IPCC AR4 models. Subduction may differ in high-resolution models due to the effect of mesoscale eddies (e.g., Qu et al., 2002; Nishikawaet al., 2010; Davis et al., 2011). Investigating the ventilation of the global ocean and its response to climate change with the aid of an eddy-resolving model will be a focus of future work.

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