Abstract
This study examines interannual variations in the seawater CO2 partial pressure (pCO2) for months (August–October) with frequent tropical cyclone (TC) events in the northwestern subtropical Pacific Ocean (22°N–28°N, 135°E–145°E) between 2007 and 2017. The temperature-normalized pCO2 averaged over August–October showed a year-to-year variation ranging from 346 to 359 μatm over the 11 study years, which appeared to be related to the variation in vertical mixing that likely results from the TC activity in these months. Sea surface temperature and wind data consistently supported the association between mixing and TC intensity. Nonetheless, the pCO2 reduction caused by negative sea-surface temperature anomalies found over the TC season (July–October) shifted the study area from a CO2 source to a CO2 sink over these months. In the south (17°N–22°N) of the study area, mixing-driven variations in pCO2 were smaller during the same months, which appeared to be caused by the relatively deeper mixed layer depth and the more homogenous profile of CO2 in this tropical region. These results suggest that more extensive pCO2 measurements are required to fully resolve the effect of TCs on the carbonate system from the regional- to the basin-scale in the western Pacific Ocean, where TC intensity is expected to increase in the future.
Highlights
The measurement of surface seawater CO2 partial pressure allows the estimation of the oceanic uptake of anthropogenic CO2
For the northwestern subtropical Pacific Ocean (NWSP), we explored the effects of tropical cyclone (TC)-driven physical processes on interannual variations in pCO2 in August–October using 11 years of pCO2 observations (pCO2obs) data
While seasonal variations in surface pCO2 were largely controlled by seasonal variations in SST, vertical mixing, and biological activities, during the TC months (August–October), TC events cause a seasonal deviation of SST and pCO2 because TC winds cool the sea surface in warm seasons through Ekman upwelling and vertical turbulent mixing (Price, 1981; D’Asaro et al, 2007; Zhang et al, 2021)
Summary
The measurement of surface seawater CO2 partial pressure (pCO2) allows the estimation of the oceanic uptake of anthropogenic CO2. To maintain the consistency of the pCO2th and pCO2nt values calculated from the various pCO2obs datasets, including SOCAT, GLODAP, and climatological datasets, an of 26.03◦C was used, which was the mean value of the reported SOCAT SST data collected over the 11 years in our study area To identify the dominant factor controlling variations in pCO2nt, we used indicators representing mixing and biological activities, while the effect of air–sea exchange was roughly predicted using the climatological pCO2 data (Landschützer et al, 2020). Daily pCO2 values were predicted from the watercolumn T–pCO2 relationship derived from GLODAP v2 data (Olsen et al, 2016) in the upper 60 m, which was performed to reflect the effect of vertical mixing on the SST drop during the cooling period
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