Sensitivity of asymmetric Oxygen Minimum Zones to remineralization rate and mixing intensity in the tropical Pacific using a basin-scale model (OGCM-DMEC V1.2)

The tropical Pacific Ocean holds the world’s two largest Oxygen Minimum Zones (OMZs), showing a prominent 10 hemispheric asymmetry, with a much stronger and broader OMZ north of the equator. However, many models have difficulties in reproducing the observed asymmetric OMZs in the tropical Pacific. Here, we apply a fully coupled basin-scale model (OGCM-DMEC V1.2) to evaluate the impacts of remineralization rate and the intensity of vertical mixing on the dynamics of OMZs in the tropical Pacific. We first utilize observational data of dissolved oxygen (DO), dissolved organic nitrogen (DON) and oxygen consumption to calibrate and validate the basin-scale model. Our model experiments 15 demonstrate that enhanced vertical mixing combined with reduced remineralization rate can significantly improve our model capability of reproducing the asymmetric OMZs. Our study shows that DO is more sensitive to biological processes over 200-400 m but to physical processes over 400-1000 m. Enhanced vertical mixing not only causes an increase in DO supply at mid-depth, but also results in lower rates of biological consumption in the OMZs, which is associated with redistribution of DON. Our analyses demonstrate that weaker physical supply in the ETNP is the dominant process responsible for the 20 asymmetry of the lower OMZs whereas greater biological consumption to the north plays a larger role in regulating the upper OMZs. This study highlights the complex roles of physical supply and biological consumption in shaping the asymmetric OMZs in the tropical Pacific.


Introduction
Photosynthesis and respiration are important processes in all ecosystems on the Earth, with carbon and oxygen being the two 25 main elements. The carbon cycle has garnered much attentions, which made significant progresses in both the observations and modelling of biological processes (e.g., uptake of CO2 and respiration), and physical/chemical processes (e.g., carbon fluxes between the atmosphere, land and ocean). However, the oxygen cycle has received much less attention despite its large role in the earth system Oschlies et al., 2018). Dissolved oxygen (DO) is a sensitive indicator of physical and biogeochemical processes in the ocean thus a key parameter for understanding the ocean's role in the climate system (Stramma et al., 2010). In addition to photosynthesis and respiration, the distribution of DO in the world's oceans is also regulated by air-sea gas exchange, ocean circulation and ventilation (Bopp et al., 2002;Bettencourt et al., 2015;Levin, 2018). Unlike most dissolved nutrients that display an increase in concentration with depth, DO concentration is generally low at mid-depth of the ocean. The most remarkable feature in the 35 oceanic oxygen dynamics is the so-called Oxygen Minimum Zone (OMZ) that is often present below 200 m in the open oceans (Karstensen et al., 2008;Stramma et al., 2008).
The world's two largest OMZs are observed in the Eastern Tropical North Pacific (ETNP) and South Pacific (ETSP), showing a peculiar asymmetric structure across the equator, i.e., a much larger volume of suboxic water (<20 mmol m -3 ) to 40 the north than to the south (Paulmier and Ruiz-Pino, 2009;Bettencourt et al., 2015). It is known that OMZs are caused by the biological consumption associated with remineralization of organic matter (OM), and weak physical supply of DO due to sluggish subsurface ocean circulation and ventilation (Czeschel et al., 2011;Brandt et al., 2015;Kalvelage et al., 2015).
Although there have been a number of observation based analyses addressing the dynamics of OMZs in the tropical Pacific during the past decade (Stramma et al., 2010;Czeschel et al., 2012;Schmidtko et al., 2017;Garç on et al., 2019), our 45 understanding is uncompleted in terms of the underlying mechanisms that regulate DO dynamics at mid-depth due to the limitation of available data Oschlies et al., 2018).
Large-scale physical-biogeochemical models have become a useful tool to investigate the potential sensitivity of OMZs to climate change (Duteil and Oschlies, 2011;Williams et al., 2014;Ward et al., 2018). However, many models still have some 50 difficulties in reproducing observed asymmetric OMZs in the tropical Pacific (Cabre et al., 2015;Shigemitsu et al., 2017), which may be due to "unresolved ocean transport processes, unaccounted for variations in respiratory oxygen demand, or missing biogeochemical feedbacks" . A common problem is that the two asymmetric OMZs merge into one in most models that often overestimate the volume of OMZs in the tropical Pacific, which may be related to weaker physical supply and/or higher rates of biological consumption (Cabre et al., 2015;Shigemitsu et al., 2017). Recent studies 55 have also indicated that a realistic representation of circulation and ventilation processes with a high-resolution ocean model is critical to predict the asymmetric OMZs in the tropical Pacific (Berthet et al., 2019;Busecke et al., 2019). Apparently, it's necessary to carry out model-data integrative studies to improve model capacity of simulating the dynamics of the tropical OMZs, and to better understand the relative roles of physical and biological processes.

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A basin-scale ocean general circulation model coupled with a dynamic marine ecosystem-carbon model (OGCM-DMEC) was developed for the tropical Pacific (Wang et al., 2008;Wang et al., 2009b;Wang et al., 2015), which showed capability of reproducing observed spatial and temporal variations of physical, nutrient and carbon fields in the upper ocean. In this study, we conduct model sensitivity experiments and evaluation on responses of mid-depth DO to parameterizations of two https://doi.org/10.5194/gmd-2020-431 Preprint. Discussion started: 4 March 2021 c Author(s) 2021. CC BY 4.0 License. relevant processes (i.e., remineralization and vertical mixing). We first carry out model calibration and validation using 65 observational data of DO and consumption rate to improve the simulation of OMZs in the tropical Pacific. Then, we use the improved model to evaluate how biological consumption and physical supply regulate the dynamics of mid-depth DO. The objective of this study is to advance our model capacity to simulate the oceanic oxygen cycle, and to identify the mechanisms driving the asymmetric OMZs in the tropical Pacific.

Ocean physical model
The basin-scale OGCM, a reduced-gravity, primitive-equation, sigma-coordinate model, is coupled to an advective atmospheric model (Murtugudde et al., 1996). There are 20 layers with variable thicknesses in the OGCM. The mixed layer (the upper-most layer) depth is determined by the Chen mixing scheme (Chen et al., 1994), which varies from 10 m to 50 m on the equator. The remaining layers in the euphotic zone are approximately 10 m in thickness. The model domain is 75 between 30º S and 30º N, and zonal resolution is 1º . Meridional resolution varies between 0.3º and 0.6º over 15º S-15º N (1/3° over 10°S-10ºN ), and increases to 2º in the southern and northern "sponge layers" (the 25º -30º bands) where temperature, salinity, and nitrate are gradually relaxed back towards the observed climatological seasonal means from the World Ocean Atlas, 2013 (WOA2013: http://www.nodc.noaa.gov/OC5/woa13/pubwoa13.html).

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The model is forced by atmospheric conditions: climatological monthly means of solar radiation and cloudiness, and interannual 6-day means of precipitation and surface wind stress. Precipitation is from ftp://ftp.cdc.noaa.gov/Datasets/gpcp. Wind stresses are from the National Centers for Environmental Prediction (NCEP) reanalysis (Kalnay et al., 1996). Air temperature and humidity above the ocean surface are computed by the atmospheric mixed layer model. Initial conditions were obtained from outputs of an interannual hindcast simulation over 1948-1978, which itself is initialized from a 85 climatological run with a 30-year spin up. The initial conditions for the climatological spin up are specified from the WOA2013. We carry out an interannual simulation for the period of 1978-2018, and analyse model output for the period of 1981-2000.

Ocean biogeochemical model
where 6.625 is the C:N ratio, µ the rate of phytoplankton growth, r the rate of zooplankton respiration, c the rates of detritus decomposition and DON remineralization. The equations for biogeochemical processes and model parameters are described in Appendix A and B. There have been changes in some parameters comparing with those in Wang et al. (2008), which were based on our model calibration and validation for chlorophyll (Wang et al., 2009a), nitrogen cycle (Wang et al., 2009b) and carbon cycle (Wang et al., 2015). 100

Computation of oxygen sources and sinks
The time evolution of DO is regulated by physical, biological and chemical processes: where u, v, and w are zonal, meridional, and vertical velocity, respectively. Omix is the vertical mixing term that is calculated by three subroutines. Briefly, the first one computes convection to remove instabilities in the water column, and the second 105 one determines the mixed layer depth. The third one computes partial vertical mixing (Kz) between two adjacent layers to relieve gradient Richardson (Ri) number instability, which is calculated as follows: where the mixing parameter λ is set to 1. 110 The biological source/sink term Obio is computed as follows: where 1.3 is the O:C Redfield ratio. Below the euphotic zone, DO concentration is determined by physical supply and biological consumption that results from detritus decomposition and DON remineralization, in which DON remineralization 115 is dominant because DON poor is several times greater than detritus (Wang et al., 2008).
The flux of O2 from the atmosphere to the surface ocean is computed as: where Osat is the O2 saturation, a function of temperature and salinity (Weiss, 1970), and K0 the gas transfer velocity that is a 120 function of wind speed (us) and SST according to Wanninkhof (1992): where Sc and Sc20 are the Schmidt number at SST and 20ºC, respectively:

Evaluation of DO distribution from the reference run
We first evaluate simulated DO for the tropical Pacific Ocean using the outputs from OGCM-DMEC V1.2 (hereafter reference run). We focus on model-data comparisons over 200-400 m, 400-700 m and 700-1000 m that broadly represent the upper OMZ, lower OMZ and beneath OMZ, respectively. The WOA2013 data shows a much larger area of suboxic waters

Sensitivity experiments
Given that the mid-depth DO concentration is influenced by physical supply and biological consumption, and remineralization of DON is the dominant process for oxygen consumption, the underestimated DO at mid-depth would be a result of overestimation of consumption associated with DON remineralization and/or underestimation of supply. Indeed, the 140 reference run over-estimates biological consumption over 100-400 m (Figure 3). Thus, we apply a reduced DON remineralization constant (50% of the reference run), which leads to a remarkable improvement in simulated DON and consumption. The reference run applies a zero value for background diffusion. However, a previous modelling study has demonstrated that vertical background diffusion is an important process for DO supply at mid-depth (Duteil and Oschlies, 2011). Accordingly, we conduct a few more simulations (Table 1)
We also compare the sizes of suboxic water and hypoxic water between model and WOA2013 (Table 3). Based on WOA2013, we estimate that the sizes of suboxic water and hypoxic water are 5.97x10 15 m 3 and 19.98x10 15 m 3 in the north, and 1.43×10 15 m 3 and 7.12x10 15 m 3 in the south, respectively. While the Cd0.5 run (with a reduced remineralization rate) 170 results in an improvement in simulated OMZ volume, significant improvements are obtained with the combination of reduced remineralization and enhanced vertical mixing (i.e., with background diffusion). Overall, the best performance for reproducing OMZ volume is Cd0.5Kb0.5 simulation that predicts similar volumes for the suboxic water (6.61x10 15 m 3 to the north and 1.56x10 15 m 3 to the south) and the hypoxic water (19.62x10 15 m 3 and 7.13x10 15 m 3 ). We then use cruise data to further validate modelled DO from the best run (Cd0.5Kb0.5). Figure 6 shows that the model can generally reproduce the 175 vertical-zonal distribution of DO along 10°N and 17°S, spanning from 1989 to 2009, particularly in the eastern tropical Pacific.

Model evaluation and discussions
In this section, we further compare the improved model simulations (Cd0.5 and Cd0.5Kb0.5) with reference run to diagnose the relative contributions of biological consumption and physical supply to the asymmetric OMZs in the tropical Pacific, 180 aiming to identify the underlying mechanisms regulating the dynamics of mid-depth DO. impacts on the mean state of DO distributions at mid-depth (Duteil and Oschlies, 2011;Gnanadesikan et al., 2013). 190 We also assess the response of mid-depth DO to the combination of reduced remineralization rate and enhanced vertical mixing (Cd0.5Kb0.5 minus reference run). Overall, the increase of DO is greater over 200-400 m (~10-24 mmol m -3 ) than over 400-700 m (~8-18 mmol m -3 ) and 700-1000 m (~6-12 mmol m -3 ) (Figure 7c, 7f & 7j). The spatial pattern and magnitude of increased DO resulted from the combined changes of remineralization rate and vertical mixing have a large 195 similarity to those caused by reduced remineralization rate for the 200-400 m layer (Figure 7a), but are similar to those due to enhanced mixing below 400 m (Figure 7d & 7h). Our analyses indicate that DO dynamics is regulated by biological processes above 400 m, but by physical processes over 400-1000 m. The larger biological influence on the upper OMZ is attributable to the greater rate of DO consumption (Karstensen et al., 2008) whereas the greater physical impact on the lower OMZ reflects the relatively larger role of supply than consumption. 200

Responses of consumption and supply to reduced remineralization and enhanced mixing
We then evaluate the changes of biological consumption and physical supply of DO due to reduced remineralization and/or enhanced mixing. Reducing remineralization rate by 50% (Cd0.5 minus reference) leads to large decrease (~1.5-2.0 mmol m -3 yr -1 ) over 200-400 m, modest decrease (~0.2-0.5 mmol m -3 yr -1 ) over 400-700 m and small decrease (~0.1-0.2 mmol m -3 yr -1 ) 205 over 700-1000 m (Figure 8a, 8d and 8h). On the other hand, enhanced vertical mixing causes much greater increase of supply over 400-1000 m than over 200-400 m. Numerous studies have indicated that physical mixing is the only source of DO for the tropical OMZs (Czeschel et al., 2012;Brandt et al., 2015;Talley et al., 2016). For example, turbulent background diffusion accounts for 89% of the net DO supply for the core OMZ layer of south tropical Pacific (Llanillo et al., 2018). Remineralization rate of DOM in the ocean is determined by the size of DOM pool and temperature (Wang et al., 2008;Brewer and Peltzer, 2016). Given that there is little difference (<10 -5 °C) in seawater temperature between different model experiments, the reduced consumption rates due to DOM remineralization would be a result of a smaller amount of DOM. 225 Here, we evaluate the zonal and meridional distributions of DON together with remineralization rate. As shown in Figure 9a-9d, modelled consumption decreases from ~8 mmol m -3 yr -1 in the euphotic zone to ~1-2 mmol m -3 yr -1 below 400 m, and modelled DON decreases from 5-8 mmol N m -3 near the surface to 1-4 mmol N m -3 over 400-1000 m. Limited field studies reported that surface DON concentration was ~5-7 mmol N m -3 in the ETSP (Loginova et al., 2019), and consumption rate ranged from 8.3 mmol m -3 yr -1 at ~200 m to <3.1 mmol m -3 yr -1 below 500 m in the subtropical North Pacific (Sonnerup et 230 al., 2013), which are comparable to our model results. Our model simulations indicate that enhanced vertical mixing leads to a redistribution of DON below 200 m, with a decrease in DON concentration (0.06-0.12 mmol N m -3 ) over 600-900 m, but an increase (<0.04 mmol N m -3 ) below 1000 m in the eastern tropical Pacific (Figure 9h, 9i and 9j).

Impacts of biological consumption and physical supply on asymmetry of OMZs 235
Previous studies have demonstrated meridional asymmetric features in many physical and biological fields in the tropical Pacific, e.g., temperature and salinity (Fiedler and Talley, 2006), circulation and ventilation (Kessler, 2006;Kuntz and Schrag, 2018), nitrogen and carbon cycles (Libby and Wheeler, 1997;Wang et al., 2009b), which may be largely associated with the asymmetries in water mass exchange between the equatorial and off-equator Pacific Ocean (Kug et al., 2003).
Accordingly, one may assume that the hemisphere asymmetry of OMZs could be related to the differences in physical 240 supply and biological consumption between the ETNP and ETSP.
There is evidence that the size of tropical OMZ is largely influenced by biological processes, such as organic matter export and oxygen consumption (Keller et al., 2016;Cavan et al., 2017). Figure 10a illustrates that DO is increased in both ETNP and ETSP over 200-1000 m when remineralization rate decreases by 50%. The increase of DO is generally greater in the 245 ETSP than in ETNP, except in the core OMZ (~300-500 m). Earlier field studies have revealed that DON concentration is much higher to the north than to the south in the central-eastern tropical Pacific (Libby and Wheeler, 1997;Raimbault et al., 1999). Later studies showed that rates of DOM remineralization and/or oxygen consumption are also greater at mid-depth in the ETNP than in the ETSP (Feely et al., 2004;Tiano et al., 2014;Kalvelage et al., 2015), indicating that biological processes play a big role in determining the asymmetry of upper OMZs. 250 Recent studies also emphasized the role of changes in physical processes for the observed asymmetric OMZs in the tropical oceans. For instance, there is evidence that larger-scale mass transport related to circulation and ventilation in the southern hemisphere is more efficient than in the northern hemisphere (Kuntz and Schrag, 2018), and the transit time from the surface to the OMZ is much longer in the ETNP than in the ETSP (Sonnerup et al., 2013;Fu et al., 2018). Clearly, our model 255 https://doi.org/10.5194/gmd-2020-431 Preprint. Discussion started: 4 March 2021 c Author(s) 2021. CC BY 4.0 License. experiment shows that enhanced vertical mixing leads to a significant increase in DO concentration below 200 m ( Figure   10b). The increase of DO is similar below 1000 m in the ETNP and ETSP, but differs largely between the two regions, with much greater values over 200-1000 m in the ETSP. Our analysis indicates that enhanced vertical mixing increases the physical supply of DO over most of the water column, except over 300-500 m in the ETNP showing a small decrease ( Figure   10c). The increase of supply is greater over 200-1000 m in the ETSP than in the ETNP, and significant increases (>0.2 mmol 260 m -3 yr -1 ) are below 600 m (500 m) in the ETNP (ETSP). These analyses indicate that physical transport may be largely responsible for the asymmetry of lower OMZs.