Deciphering The Mechanisms Of Oceanic CO₂ Uptake,Storage And Transport In The Western North Pacific

The global oceans absorb approximately a quater of anthropogenic CO₂(C_ANT)emissions annually to mitigate the anthropogenic CO₂ accumulation and global warming.The western North Pacific is one of the most important continuous ecosystems in the world,and is also the strongest CO₂ sink in the Pacific.However,so far,there are few studies on the oceanic uptake,transport and storage of C_ANT in the western North Pacific and its control mecahnisms remain unclear.Taking the western North Pacific as the object,this paper aims to further explore the mechanisms controlling the oceanic C_ANT uptake,transport and storage in the western North Pacific.It is of great scientific significance to understand the fate of C_ANT under the background of global change and to timely grasp the ecological response of the ocean to global change.Based on the investigated water column carbonate system and ancillary parameters in the western North Pacific（147°E,21°-39°N;136°E,24°-30°N;118°-147°E,21°N）from May to June and August to September 2018,and in the Philippine Sea（130°E,8-18°N）from October to November 2019,the main research contents include:（1）Firstly,the author unveiled the controls of the latitudinal gradient of surface pCO₂ in the Kuroshio Extension and its recirculation regions in late spring.（2）Moreover,the author estimated the latest C_ANT storage and analyzed the regional differences and its control mecahnisms in the western North Pacific,including the Kuroshio Extension（KE,[147°E,35°-39°N]）,Kuroshio Recirculation（KR,[147°E,27°-35°N]and[136°E,25°-30°N]）,subtropical（[147°E,21°-27°N]and[130°-136°E,15°-25°N]）and tropical（130°E,8°-15°N）zones;Incorporating historical GO-SHIP dataset along the 137°E section in 2010 and 149°E section in 2005,the author analyzed the decadal variations of the C_ANT storage in the western North Pacific.（3）Finally,the author revealed the southward transport of C_ANT with the STMW using the dissolved inorganic radiocarbon as a tracer;by comparing and analyzing the C_ANT storage along the 136°/137°E and 147°/149°E sections,the westward transport of C_ANT with the STMW were studied.Some new results related to oceanic uptake,storage,and transport of C_ANT are found as follows.Based on the results of underway pCO₂ and ancillary parameters in the western North Pacific in May 2018,the author found that the sea-to-air pCO₂ difference（ΔpCO₂）was negative（-76±16 μatm in the KR zone,-31±10 μatm in the KR zone and-5± 2μatm in the subtropical zone）.That is,KE and KR zones were atmospheric CO₂ sinks,while the subtropical zone was nearly in air-equilibrium with atmospheric CO₂.The ΔpCO₂ gradually increased southward across the KE,KR and subtropical zones in later spring,and this latitudinal distribution pattern was consistent with the annual net sea-air CO₂ flux in the western North Pacific.We found that northward cooling and vertical mixing were the two major processes governing the latitudinal gradient in surface pCO₂ and ΔpCO₂,while biological influences were relatively minor.In the KE zone,the prolonged air-sea equilibration for CO₂ and relatively short hydraulic retention time jointly led to the low surface pCO₂ and thus strong CO₂ sink in the KE zone in spring.Ultimately,the cooling KE current flows out of the region before it can be re-equilibrated with atmospheric CO₂,inducing that the KE zone continuously absorbs atmospheric CO₂.In the Philippine Sea in 2018/2019,the C_ANT inventory had a maximum value of 38.8±0.6 mol m^-2 at 25°-30°N（KR zone）due to deep penetration of C_ANT and decreased southward to 27.8±2.5 mol m^-2 at 8°-15°N（tropical zone）.In the tropical zone,more than 82%of the C_ANT inventory was input from extra-tropics via ocean circulation rather than through local air-sea CO₂ equilibration.The C_ANT inventory of 30.6±1.2 mol m^-2 in the west of Luzon Strait is lower than that of 35.2±1.2 mol m^-2 in the east of Luzon Strait because the intermediate waters in the west of Luzon Strait mix with the C_ANT-deficient deep waters originated from the North Pacific deep water.On a decadal scale,the C_ANT inventory increased by 0.59 ± 0.08 mol m^-2 yr^-1 in the tropical zone between 2010（along 137°E）and 2019（along 136°E）,which is similar to the rate during 1994-2007(-0.54 mol m^-2 yr^-1).In the KR zone of the Philippine Sea,however,the C_ANT inventory increased by 0.95 ± 0.10 mol m2 yr^-1 during 2010-2019,which is 40%higher than that during 1994-2007(-0.68 mol m^-2 yr^-1).That is,the C_ANT inventory increased faster in the 2010s than before in the KR zone along 136°/137°E section.In the western North Pacific along 147°E section,the water column C_ANT inventory was estimated at the maximum of 40.5±1.1 mol m^-2 in the KR zone（27°-35°N）.By contrast,relatively low water column C_ANT inventory of 37.2 ±0.9 mol m^-2 in the subtropical zone（21 °-27°N）and the minimum of 24.2±1.8 mol m^-2 in the KE zone（35°-39°N）were estimated in August 2018.In the KR zone along 147°E section,subtropical mode water（STMW）is the largest contributor to the formation of column C_ANT inventory,which supports the idea that the STMW formation is a key process in the natural sequestration of C_ANT.In the KR and subtropical zones,C_ANT inventories have minor seasonal variations,while in the dynamic KE zone,from May to August 2018,the water column C_ANT inventories decreased significantly due to the perturbation of the mesoscale eddy.Therefore,the decadal variations of C_ANT inventories also have large variability in the KE zone.In the KR zone,water column C_ANT inventory increased by 1.05 ± 0.20 mol m^-2 yr^-1 between May 2005（along 149°E）and May 2018（along 147°E）in agreement with the rate in subtropical zone(1.03±0.12 mol m^-2 yr^-1),This rate was 54%higher than that between 1994 and 2007(～0.68 mol m^-2 yr^-1).That is,the C_ANT inventory increased faster in the 2010s than before in the KR zone along 147°/149°E.In terms of the mechanism of C_ANT transport:（1）Firstly,the C_ANT transport with the STMW were revealed using the dissolved inorganic radiocarbon as a tracer.Although the subtropical zone is a week sink of atmospheric CO₂,the C_ANT inventory in the subtropical zone owing to the southward transportation of C_ANT-rich STMW is slightly lower than that in the KR zone;although the KE zone is a strong sink of atmospheric CO₂,the C_ANT inventory in the KE zone owing to the uplifting isopycnal(shallow penetration of C_ANT)is lower than that in the KR zone mainly owing to the STMW formation(deep penetration of C_ANT).（2）In addition,by comparing and analyzing the C_ANT storage in the KR zone along the 137°E and 147°E sections,the author found that the rapid rates of C_ANT accumulation are consistently observed in the entire region across the 137°-147°E sections.The accelerated accumulation of C_ANT inventory in the KR zone along the 137°E and 147°E sections was mainly attributable to the C_ANT increase of STMW,and this acceleration is also higher than that predicted solely on the basis of increased atmospheric CO₂.While the exact mechanism responsible for the accelerated C_ANT accumulation in the STMW in recent decade is still not determined（likely related to decadal decline in wintertime sea surface temperature of STMW formation region）,this study highlights that,the KR zone across the 137°-147°E sections have simultaneous variability of C_ANT accumulation in the STM W,which is regulated by the westward transport of C_ANT with the STMW.Overall,the southwestward transport of C_ANT is revealed with the STMW,which is consistent with the STMW pathway in the mean circulation.On the other hand,those results also imply that the STMW pathway is traced by the C_ANT accumulation.（3）Finally,the rates of C_ANT accumulation in the STMW are consistent with that in the surface water of STMW formation region,indicating that the rate of C_ANT accumulation in the STMW could be traced back to the surface formation waters via STMW formation.This vertical consistency implies the memory function of mode waters in retaining the anthropogenic carbon fingerprint during its formation.