Simulating the impact of typhoons on air‐sea CO2 fluxes on the northern coastal area of the South China Sea

The South China Sea is a typhoon-prone region, and previous studies have shown that typhoons have significant impacts on air-sea CO2 fluxes. However, the effect of typhoons on the northern coastal area of the South China Sea is not well understood owing to limited observational data. In this study, we used a coupled model to simulate the impact of four typhoons (Hato, Mangkhut, Nida, and Merbok) on the partial pressure of CO2 in seawater (pCO2sea) and the CO2 fluxes in this area. Our results show that the coupled model effectively reproduces the spatial pattern of pCO2sea in this region. The response of pCO2sea to typhoons was determined by typhoon-induced vertical mixing and coastal upwelling, along with initial oceanic conditions. Typhoon Nida caused a decrease in pCO2sea with Total Alkalinity and Sea Surface Temperature being the primary factors. However, typhoons Hato, Mangkhut, and Merbok caused an increase in pCO2sea with Dissolved Inorganic Carbon playing a more prominent role. The average CO2 fluxes during the passage were approximately 6–14 times higher than those before typhoon passage. These results enhance our understanding of the effect of typhoons on air-sea CO2 fluxes over the northern coastal area of the South China Sea.


Introduction
Typhoons are tropical cyclones that occur frequently in the northwestern Pacific and South China Sea frequently (Zhan et al 2012, Zhong et al 2019, Liu et al 2020, Hong and Vinh 2022).Previous studies have shown that typhoons have a significant impact on the air-sea CO 2 fluxes (CO 2 fluxes) (Lu et al 2012, Ma et al 2020).Bates et al (1998) demonstrated that typhoons increased the summer CO 2 fluxes in the Kuroshio Extension by nearly 55%.Hood et al (2001) found that high wind speeds during typhoon events led to a sharp decrease in sea surface temperature (SST), a significant increase in the partial pressure of CO 2 in seawater (pCO 2sea ) and nitrate concentration, and doubling of CO 2 fluxes.Nemoto et al (2009) indicated that during three typhoon events in the western subtropical region of the Northwest Pacific, the CO 2 fluxes accounted for 60% of the total summer fluxes.Sun et al (2014) discovered that when a tropical cyclone passed through the low-salinity region of the South China Sea, it resulted in increased pCO 2sea and doubled CO 2 fluxes in low-salinity water areas.Yu et al (2020) found that the average CO 2 fluxes during typhoons was 3.6-5.4times higher than that during the pre-typhoon period.Over the past decade, typhoons have increased annual CO 2 fluxes in the northern South China Sea by 23%-56% (Yu et al 2020).In addition, previous studies have explored the causes of CO 2 fluxes variations during typhoons.Ye et al (2017) demonstrated that when Typhoon Wutip passed through the anticyclonic and cyclonic regions, the cyclonic region became a stronger carbon source, while the anticyclonic region changed from a carbon source to a carbon sink, which was primarily due to changes in vorticity, sea surface salinity (SSS), temperature, and pCO 2sea .Research by Kao et al (2023) indicated that when a typhoon occurs in the southern East China Sea, the factors affecting pCO 2sea in the upwelling area are the temperature effect (38%-40%), net biological activity (−33% to −36%), and the mixing effect (−24%).
The northern coastal area of the South China Sea, located in the northern part of the South China Sea continental shelf, receives freshwater from the Pearl River throughout the year (Cai et al 2004, Ni et al 2008, Li et al 2016, Liu et al 2018, Wei et al 2020).This region is characterized by complex physical and biological environments, which are often influenced by extreme weather events, such as typhoons (Zhao et al 2009, Pan et al 2017, Yin et al 2017, Qiu et al 2019, Qu et al 2021).Unlike in the open ocean, the seawater environment in this area is intricate.Previous studies have demonstrated that the Pearl River plume and seasonal upwelling have significant effects on biogeochemical processes in the nearshore area of the northern South China Sea (Gan et al 2010, Bai et al 2015, Li et al 2020, Geng et al 2021).These processes contribute a substantial amount of nutrients to the northern coastal area of the South China Sea, resulting in a notable decrease in pCO 2sea due to increased primary productivity (Dai et al 2008, Cao et al 2011, Zhao et al 2021).Furthermore, Zhai et al (2009) found that air-sea exchange significantly influences pCO 2sea in the northern coastal area of the South China Sea.
The frequency of typhoons has increased in the past decade owing to global warming (Goldenberg et al 2001, Webster et al 2005, Wu et al 2005, Elsner et al 2008, Balaguru et al 2012, Mei et al 2015, Balaguru et al 2016, Da et al 2021).In future warming scenarios, even more typhoon events will affect the region.However, the impact of typhoons on CO 2 fluxes in the northern coastal area of the South China Sea is not well understood because of the limited observational data on pCO 2sea and CO 2 fluxes.
In order to compare the effects of typhoons with varying intensities on pCO 2sea and CO 2 fluxes in this area, we classify Hato and Mangkhut as stronger typhoons, while Nida and Merbok are considered weaker.This study used a coupled model to analyze changes in pCO 2sea and CO 2 fluxes during the passage of typhoons Hato, Mangkhut, Nida, and Merbok, and explored the underlying mechanisms.Models can offer three-dimensional responses of dynamically consistent variables of interest, thereby improving our comprehension.This study also quantitatively evaluated the impact of typhoons on CO 2 fluxes.This research will significantly improve our understanding of CO 2 fluxes variation in the northern coastal area of the South China Sea during typhoons.
The remainder of this paper is structured as follows.Section 2 provides an overview of the data and analysis methods.Section 3 examines the temporal and spatial changes in the pCO 2sea and CO 2 fluxes in the northern coastal area of the South China Sea during typhoon events.Section 4 investigates and quantifies the factors that influence pCO 2sea and CO 2 fluxes in the study area during typhoons.Finally, the conclusions are presented in section 5.

Model settings
As figure 1 shows, this study utilized a coupled model (Luo et al 2023) based on the Coupled Ocean-Atmosphere-Wave-Sediment Transport modeling system.The model consists of a Weather Research Forecasting (WRF) module for the atmosphere and a Regional Ocean Modeling System (ROMS) module for the ocean (Warner et al 2008, Warner et al 2010).The WRF domain covers the Pearl River Watershed and northern part of the South China Sea, with a horizontal resolution of approximately 12 km.The ROMS domain includes the northern part of the continental shelf of the South China Sea and Pearl River Estuary, with a horizontal grid spacing ranging from 2.8 km to 40.7 km and an average resolution of approximately 4 km.The model had 30 vertical layers with a higher resolution near the surface and bottom boundaries.The vertical s-coordinate function was based on Shchepetkin and McWilliams (2009).The physical model used a recursive Multidimensional Positive Definite Advection Transport Algorithm three-dimensional advection scheme for tracers, fourth-order horizontal advection of tracers, thirdorder upwind advection of momentum, and the vertical mixing turbulence closure scheme of Mellor and Yamada (1982).The viscosity/diffusion coefficient for second-harmonic level mixing was set to 10 −5 m 2 s −1 .The numerical integration had an internal mode time step of 900 s and an external mode time step of 60 s, as determined by the Courant-Friedrichs-Lewy criterion.The Model Coupling Toolkit facilitates the interaction between WRF and ROMS.WRF transfers fluxes to ROMS, including variables such as sensible heat fluxes, precipitation fluxes, surface stress components, and surface pressure.ROMS provides SST information to WRF.Furthermore, this model is coupled with a nitrogen cycle model that considers carbonate chemistry (Fennel et al 2006(Fennel et al , 2008(Fennel et al , 2011)).
Figure 1 indicates that in the northern coastal area of the South China Sea, the pCO 2sea in nearshore waters is lower than that in the inner estuary and offshore waters.This difference may be attributed to the intricate biological processes in this area (Song et al 2023).Therefore, we selected the coastal waters of the Pearl River Estuary as our research region.The area enclosed by the black dashed line represents the selected study area, while the red line represents the vertical profile line positioned at the center of the region.

Controlling factors of pCO 2sea and CO 2 fluxes
This study examines the impact of temperature and non-temperature factors on the pCO 2sea during typhoons, based on the research conducted by Takahashi et al (2002): We represented the changes in pCO 2sea caused by non-temperature and temperature effects using the variables npCO 2nt and npCO 2 , respectively.The term pCO 2sea refers to the partial pressure of CO 2 at the ocean surface, whereas SST represents the SST in degrees Celsius.The subscript 'mean' indicates the average value.
During a typhoon, factors such as SST, Dissolved Inorganic Carbon (DIC), Total Alkalinity (TA), and SSS can be altered, leading to changes in the pCO 2sea through vertical mixing (Takahashi et al 2002, 2009, McKinley et al 2006, Ji et al 2022).To determine the factors controlling the changes in pCO 2sea during the typhoon period, the following equation was used (Takahashi et al 1993): where dpCO 2sea is the change in pCO 2sea , and T, ALK, S denote SST, TA, and SSS, respectively.
The effect of each factor on pCO 2sea can be determined by the thermodynamics of seawater CO 2 (Takahashi et al 1993).
Air-sea CO 2 fluxes (F CO2 ) in the model was calculated as follows: where K is the gas transfer velocity and K H is the solubility of CO 2 in seawater (Weiss 1974, Wanninkhof 1992).pCO 2sea and pCO 2air are the partial pressures of CO 2 in the ocean and the atmosphere, respectively.A positive F CO2 indicates fluxes of CO 2 from the ocean to the atmosphere, meaning that the ocean acts as a source of CO 2 to the atmosphere.Conversely, negative F CO2 indicates fluxes of CO 2 from the atmosphere to the ocean, indicating that the ocean acts as a CO 2 sink.

MAPE and weighted average
We employed MAPE to assess the disparity between the model results (y sim ) and inversion data (y i ) within the selected study area: We used the weighted average method to calculate time series: where X i represents the value at each grid point of model outputs, A i is the corresponding area of each grid point, n is the total number of grid points.

Model evaluation
Previous studies have demonstrated a good performance in the physical and ecological simulations of our model (Luo et al 2023).In this study, we compared the simulated typhoon paths with the observed typhoon paths and used inverted ocean pCO 2sea data to further evaluate the simulation ability of the coupled model.The results are shown in figure 2.
The simulated paths of typhoons are in good agreement with the observations (figure 2(j)).Typhoons Hato and Mangkhut mainly pass through the left side of the Pearl River Estuary, whereas typhoons Nida and Merbok mainly pass through the right side of the estuary.Both inverted and simulated data indicate that the pCO 2sea is high in the inner Pearl River Estuary and offshore waters and low in nearshore waters (figures 2(a)-(j)).These results demonstrate that the coupled model can effectively capture the spatial variations of the pCO 2sea .It is worth noting that the simulated pCO 2sea by the coupled model appears to be slightly higher than the inversion data, especially at the boundaries.This discrepancy may be attributed to a deficiency in oceanic boundary conditions.

Spatial variation of pCO 2sea and CO 2 fluxes in northern coastal area of the South China Sea during typhoons passage
As shown in figure 3, three time periods were determined for the analysis: when the typhoon entered the study area, when the typhoon was in the study area, and when the typhoon left the study area.
When Typhoon Hato entered the study area, the distribution of CO 2 fluxes was uniform, and the study area was a weak CO 2 source (figure 3(b)).When Typhoon Hato was in the study area, the entire study area became a stronger source of CO 2 (figure 3(f)), and became a weak CO 2 source when Typhoon Hato left the study area (figure 3(j)).Throughout all three periods, the pCO 2sea values were high near Lingdingyang Bay (figures 3(c), (g) and (k)), possibly because of the effect of carbon-rich water from the Pearl River Estuary.When Typhoon Hato was in the study area, there was an obvious increase in pCO 2sea and CO 2 fluxes.During this period, the temperature of nearshore waters drops more significantly than that of offshore waters.
When typhoon Mangkhut entered the study area, the distribution of sea-air CO 2 fluxes was uniform, and the study area was a weak CO 2 source overall (figure 4(b)).When the typhoon was in the study area, with a lower wind speed and corresponding smaller CO 2 fluxes at the center of the typhoon, the entire study area became a strong CO 2 source (figure 4(f)).When the typhoon left the study area, it became a weak CO 2 source.When typhoon Mangkhut entered the study area, the pCO 2sea in coastal waters was lower than that in offshore waters.During the typhoon's presence, there was a uniform increase in the pCO 2sea , accompanied by an even drop in temperature, which is different from typhoon Hato.
As shown in figure 5, when Typhoon Nida was in the study area, the entire area was a strong CO 2 source.The eastern part of the study area exhibits a more pronounced transfer of CO 2 from the ocean to the atmosphere.When the typhoon left the study area, it became a weak CO 2 source.During this period, pCO 2sea increased in the eastern part of the study area and slightly decreased in the western part; the decrease in SST was more significant in the eastern part of the nearshore waters.This could be because the typhoon mainly passed through the eastern side of the study area, resulting in stronger upwelling of deep water in the eastern part.
When Typhoon Merbok entered the study area, the study area was a weak CO 2 source overall, became a stronger CO 2 source when the typhoon was in the study area, and then recovered to a weak CO 2 source when the typhoon left (figure 6).Notably, the CO 2 fluxes were lower in the center of the typhoon, where the wind was weak.When Typhoon Merbok entered the study area, the pCO 2sea in the nearshore waters was lower than that in the offshore waters.When the typhoon left, there was a more significant increase in the pCO 2sea on the eastern side of the nearshore waters.In addition, more significant changes in SST were observed in the eastern part of the study area.

Temporal changes in pCO 2sea and CO 2 fluxes in in northern coastal area of the South China Sea during typhoons passage
Before the passage of the typhoon, there was a positive correlation between temperature and pCO 2sea for Hato, Nida, and Merbok, but the situation was different for Typhoon Mangkhut (figure 7).One possible reason for this difference is that the pCO 2sea of the open ocean is higher than that in the study area.Before the passage of Typhoon Mangkhut, high pCO 2sea seawater from the open ocean flowed into the study area, causing a dilution of chla and a reduction in carbon consumption in the surface seawater, which resulted in a slight increase in pCO 2sea when the SST decreased.However, the specific reasons for this phenomenon require further investigation.In addition, U10 varied from 5 to 10 m s −1 , and the CO 2 fluxes ranged from 0 to 10 mmol −1 m −2 d −1 .Generally, this region was a weak source of CO 2 during this period.
During the passage of typhoons, npCO 2nt increased, while npCO 2 decreased (figure 8), which means that pCO 2sea driven by temperature increased, while pCO 2sea driven by non-temperature factors decreased.
It is worth noting that, as the wind speeds increased and decreased, the typhoons seemed to have a two-step impact on the pCO 2sea and CO 2 fluxes (figure 8).In the first phase, during the initial passage of typhoons, the region experienced lower CO 2 content and higher SST.As U10 increased, the CO 2 fluxes increased.During this period, pCO 2sea increased for Hato, Mangkhut, and Merbok, whereas pCO 2sea decreased with fluctuations in typhoon Nida.In the second step, the concentration of CO 2 on the sea surface increased, the SST had already cooled to some extent, and the U10 and CO 2 fluxes also decreased.The growth of pCO 2sea slowed down for typhoons Mangkhut and Merbok, while the pCO 2sea showed a slight decrease for typhoon Hato.However, Typhoon Nida consistently caused a smooth decrease in pCO 2sea .Overall, the influence of the nontemperature factors on the change in pCO 2sea in the first step was more significant than that in the second step.
Notably, the pCO 2sea and npCO 2nt levels initially decreased and then increased for typhoon Merbok.Additionally, when npCO 2nt increased, pCO 2sea continued to decrease for a short period.This suggests that SST played a primary role in pCO 2sea during the initial limited time period of Typhoon Merbok.Furthermore, there was no significant slowdown in the temperature-induced pCO 2sea relative to non-temperature factors during the second step of Typhoon Nida (figure 8), indicating that SST may not only be affected by deeper water but may also be influenced by other factors such as heavy rainfall, which could cause a decrease in SST (Deng et al 2019, Feng et al 2021).
Considering the entire process, except for typhoon Nida, pCO 2sea increased, with nontemperature factors having a more significant impact on pCO 2sea than temperature factors.In addition, the region acts as a significant CO 2 source during typhoons.
After the passage of typhoons, pCO 2sea increased and showed a positive correlation with SST, indicating that SST was the main factor in the change in pCO 2sea .The U10 and CO 2 fluxes decreased to a lower positive value, indicating that the study area was restored to a weak CO 2 source.However, neither pCO 2sea nor SST fully recovered within 36 h of the typhoon's passage.

The controlling factors of pCO 2sea during typhoons passage
Changes in pCO 2sea are primarily influenced by temperature, DIC, TA, and salinity (Takahashi et al 1993).DIC, TA, and salinity increased with depth, whereas temperature decreased.During the passage of typhoons, vertical mixing and coastal upwelling occurred (figure 9), which could transport CO 2 -rich and cool deep water to the surface.Coastal upwelling could bring lower temperatures deeper water to the nearshore (figure 9).Coastal upwelling was more pronounced during typhoons Hato, Nida, and Merbok, while vertical mixing was more pronounced during Mangkhut.This explains why temperature changes along the coast were higher than those in the offshore waters during Typhoons Hato, Nida, and Merbok, while SST changes were more uniform during Typhoon Mangkhut (figures 3-6).
For typhoon Hato, the cooling of surface seawater resulted in a decrease of 35 µatm in the surface pCO 2sea .The changes in pCO 2sea caused by DIC and TA were +120 and −100 µatm, respectively.SSS caused an increase of 30 µatm in the pCO 2sea (figures 10(a)-(d)).The ratios of the changes in pCO 2sea caused by DIC, TA, and SSS to those caused by SST were 3.4, 2.9, and 0.9, respectively.
For Typhoon Mangkhut, the cooling of surface seawater caused a decrease in pCO 2sea of 37 µatm.Surface DIC and TA contributed to a +72 and −37 µatm change in pCO 2sea , respectively, whereas salinity caused a +11 µatm change inpCO 2 (figures 10(e)-(h)).Furthermore, the ratios of changes in pCO 2sea caused by DIC, TA, and salinity to those caused by SST were 1.9, 1.0, and 0.3, respectively.
For Typhoon Nida, the cooling of the surface seawater led to a decrease in pCO 2sea by 30 µatm.The changes in pCO 2sea by surface DIC and TA were 43 and −34 µatm, respectively.Additionally, SSS caused a change of 8 µatm in the pCO 2sea (figures 10(i)-(l)).Furthermore, the ratios of the changes in pCO 2sea caused by DIC, TA, and salinity to those caused by SST were 1.4, 1.1, and 0.3, respectively.
For Typhoon Merbok, the cooling of the surface seawater led to a decrease in pCO 2sea by 14 µatm.Surface DIC and TA contributed to +25 and −12 µatm change in pCO 2sea , respectively.Salinity caused a +4.5 µatm change in pCO 2sea (figures 10(m)-(p)).The ratios of the changes in pCO 2sea caused by DIC, TA, and salinity to those caused by SST were 1.8, 0.9, and 0.3, respectively.
Overall, during the passage of typhoons, pCO 2sea driven by DIC and TA continuously increased, whereas pCO 2sea driven by TA and SST continuously decreased.Specifically, typhoon Nida caused a decrease in pCO 2sea (figure 7), and the increased amplitude of pCO 2sea driven by SST was similar to that driven by TA, suggesting that both TA and SST were important factors in the change in pCO 2sea .Conversely, for typhoons Hato, Mangkhut, and Merbok, pCO 2sea increased continuously (figure 7), and the changes in pCO 2sea driven by DIC were significantly greater than those driven by SSS, which indicates that DIC played a more prominent role in the changes in pCO 2sea .Interestingly, for typhoon Hato, the ratio of the variation amplitude of pCO 2sea caused by DIC, TA, and SSS to the variation amplitude of pCO 2sea caused by SST was significantly higher than that of typhoon Mangkhut.This suggests that these non-temperature factors played more significant roles during Typhoon Hato than during Typhoon Mangkhut.Except for the initial oceanic conditions, this difference is also possibly due to the stronger vertical mixing during the passage of typhoon Mangkhut, whereas coastal upwelling was more pronounced during the passage of typhoon Hato (figures 3, 4 and 9).Coastal upwelling initially intensified and then weakened, with a vertical displacement of deep coastal waters, contrasting the vertical mixing of surface and deep waters.Furthermore, in terms of vertical distribution, DIC, TA, and SSS were higher along the coast than offshore at the same depth (figure 11).Coastal upwelling brought higher DIC, TA, and salinity to the nearshore ocean surface than vertical mixing.

The effect of typhoons on CO 2 fluxes
The impacts of the four typhoons on the CO 2 fluxes were significant.Before the passage of typhoons, the fluxes were approximately 5 mmol −1 m −2 d −1 , increased to a range of approximately 23-54 mmol −1 m −2 d −1 during the passage of typhoons, and then decreased to approximately 6.0 mmol −1 m −2 d −1 after typhoons passage.The maximum average CO 2 flux occurred during typhoon passage and increased approximately 6-14 times than that before typhoon passage (figure 12(a)).However, the change in ∆pCO 2 (pCO 2sea -pCO 2air ) during the typhoon passage did not always increase (figure 12(b)).For example, ∆pCO 2 increased during the passage of typhoons Hato and Mangkhut, but slightly decreased during Typhoons Nida and Merbok.However, U10 always increased during the passage of typhoons (figure 12(c)).Therefore, the increase in CO 2 flux during the passage of typhoons is primarily due to the increase in wind speed (U10).Overall, although each typhoon has its own characteristics and may have different impacts on the pCO 2sea and CO 2 fluxes, these four typhoons significantly increased the amount of CO 2 emissions.

Conclusion
This study is the first to examined the impacts of typhoons (Hato, Mangkhut, Nida, and Merbok) on the pCO 2sea and CO 2 fluxes in the relatively   nutrient-rich northern coastal region of the South China Sea.Due to limited observational data, a threedimensional coupled model was employed for analysis.The results demonstrate that the model effectively replicates the spatial distribution of pCO 2sea in this area.The main findings for the study region are as follows: 1.The response of pCO 2sea to typhoons is influenced not only by typhoon-induced vertical mixing and initial ocean conditions but also by typhooninduced coastal upwelling.Coastal upwelling is more pronounced during typhoon Hato passages, while vertical mixing is more prominent during typhoon Mangkhut passages.Coastal upwelling can transport a greater amount of DIC, TA, Salt, and cold deep-water to the nearshore ocean surface compared to vertical mixing, potentially resulting in higher pCO 2sea changes compared to offshore waters.2. Before the passage of typhoons, the daily pCO 2sea cycle closely followed the daily temperature cycle, suggesting that the temperature played a primary role.During typhoon Nida, pCO 2sea decreased with TA and SST being the primary factors.However, for typhoons Hato, Mangkhut, and Merbok, pCO 2sea increased as DIC played a more prominent role.Overall, non-temperature factors are more likely the main drivers of pCO 2sea variations in the northern coastal waters during typhoons passage.After the passage of typhoons, temperature generally plays a primary role in controlling the pCO 2sea , including its daily cycle.3. The model results suggest that during the passage of typhoons, increased wind speeds significantly affected CO 2 fluxes, resulting in increased CO 2 emissions.The average CO 2 fluxes during the passage were approximately 6-14 times higher than those before typhoon passage.
The current research lacks sufficient shortterm data on pCO 2sea and CO 2 fluxes, one solution is to estimation them using machine learning.Additionally, it is important to study the specific impacts of different typhoons on air-sea feedback and ecological changes in the northern South China Sea.

Figure 1 .
Figure 1.The average distribution of summer pCO2sea (shading), the research area (black box), the vertical section (red line) chosen in this study and the spatial extent covered by WRF and ROMS models.

Figure 8 .
Figure 8.During typhoon Hato (a), typhoon Mangkhut (b), typhoon Nida (c) and typhoon Merbok (d), time series of pCO2sea changes driven by temperature (npCO2) and non-temperature factors (npCO2nt).Red dashed lines mark the period of typhoon passes the study area.