North Pacific Eastern Subtropical Mode Water simulation and future projection

The present climate simulation and future projection of the Eastern Subtropical Mode Water （ESTMW） in the North Pacific are investigated based on the Geophysical Fluid Dynamics Laboratory Earth System Model （GFDL-ESM2M）. Spatial patterns of the mixed layer depth （MLD） in the eastern subtropical North Pacific and the ESTMW are well simulated using this model. Compared with historical simulation, the ESTMW is produced at lighter isopycnal surfaces and its total volume is decreased in the RCP8.5 runs, because the subduction rate of the ESTMW decreases by 0.82×10？6 m/s during February–March. In addition, it is found that the lateral induction decreasing is approximately four times more than the Ekman pumping, and thus it plays a dominant role in the decreased subduction rate associated with global warming. Moreover, the MLD during February–March is banded shoaling in response to global warming, extending northeastward from the east of the Hawaii Islands （20°N, 155°W） to the west coast of North America （30°N, 125°W）, with a max-imum shoaling of 50 m, and then leads to the lateral induction reduction. Meanwhile, the increased north-eastward surface warm current to the east of Hawaii helps strengthen of the local upper ocean stratification and induces the banded shoaling MLD under warmer climate. This new finding indicates that the ocean surface currents play an important role in the response of the MLD and the ESTMW to global warming.

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℃/（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）℃, （35.46 ±0.04）, （26.02 ±0.04） ae over the past 60 years.

Contribution of Mesoscale Eddies to the Subduction and Transport of North Pacific Eastern Subtropical Mode Water

This study investigates the contribution of mesoscale eddies to the subduction and transport of North Pacific Eastern Subtropical Mode Water(ESTMW)using the high-frequency output of an eddy-resolved ocean model spanning the period 1994–2010.Results show that the subduction induced by mesoscale eddies accounts for about 31%of the total subduction of ESTMW formation.The volume of ESTMW trapped by anticyclonic eddies is slightly larger than that trapped by cyclonic eddies.The ESTMW trapped by all eddies in May reaches up to about 2.8×1013m3,which is approximately 16%of the total ESTMW volume.The eddy-trapped ESTMW moves primarily westward,with its meridional integration at 18°–30°N reaching about 0.17Sv,which is approximately 18%of the total zonal ESTMW transport in this direction,at 140°W.This study highlights the important role of eddies in carrying ESTMW westward over the northeastern Pacific Ocean.

Storage and redistribution of anthropogenic CO_(2) in the western North Pacific:The role of subtropical mode water transportation

Oceanic uptake and storage of anthropogenic CO_(2)(CANT)are regulated by ocean circulation and ventilation.To decipher the storage and redistribution of CANT in the western North Pacific,where a major CANT sink develops,we investigated the water column carbonate system,dissolved inorganic radiocarbon and ancillary parameters in May and August 2018,spanning the Kuroshio Extension(KE,35-39°N),Kuroshio Recirculation(KR,27-35°N)and subtropical(21-27°N)zones.Water column CANT inventories were estimated to be 40.5±1.1 mol m^(-2) in the KR zone and 37.2±0.9 mol m^(-2) in the subtropical zone.In comparison with historical data obtained in 2005,relatively high rates of increase of the CANT inventory of 1.05±0.20 and 1.03±0.12 mol m^(-2) yr^(-1) in the recent decade were obtained in the KR and subtropical zones,respectively.Our water-mass-based analyses suggest that formation and transport of subtropical mode water dominate the deep penetration,storage,and redistribution of CANT in those two regions.In the KE zone,however,both the water column CANT inventory and the decadal CANT accumulation rate were small and uncertain owing to the dynamic hydrology,where the naturally uplifting isopycnal surfaces make CANT penetration relatively shallow.The findings of this study improve the understanding of the spatiotemporal variations of CANT distribution,storage,and transport in the western North Pacific.

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

The classical Lagrangian particle tracing method is widely used in the evaluation of the ocean annual subduction rate.However,our analysis indicates that in addition to neglecting the effect of mixing,there are two possible deviations in the method:one is an overestimation due to not considering that the amount of subducted water at the source location may be inadequate during the late winter of the first year when the mixed layer becomes shallow;the other one is an underestimation due to the neglect of the effective subduction caused by strong vertical pumping.Quantitative analysis shows that these two deviations mainly exist in the low-latitude subduction areas of the South Pacific and South Atlantic.The two deviations have very similar distribution areas and can partially off set each other.However,the overall deviation is still large,and the maximum relative deviation ratio can reach 50%;therefore,it cannot be ignored.