Intensified surface chlorophyll responses to the Indian Ocean Dipole under greenhouse warming

The Indian Ocean Dipole (IOD) has been proposed to be a key driver of biological processes in the Indian Ocean (IO) in the present climate. Given the expected influence of global warming on both the properties of the IOD and the biogeochemistry within the IO, a key question arises: How will the relationship between the IOD and surface chlorophyll evolve in a warming climate? Here, utilizing simulations from the Coupled Model Intercomparison Project Phase 6 Earth System models, our findings reveal a notable intensification in the IOD-chlorophyll relationship under greenhouse warming. This intensification is linked to an increase in surface chlorophyll during the June to November period of positive IOD years in the southeastern IO (SEIO). Interestingly, our analysis indicates a substantial rise in IOD-related chlorophyll levels in a warming climate, despite a marked decrease in IOD-induced upwelling in the SEIO. The shallower thermocline leads to an increase in the mean nutrient concentration in the subsurface layer, thereby facilitating an enhanced anomalous nutrient supply to the surface layer, which contributes to increased surface chlorophyll. Our study highlights the consequential effects of IOD on chlorophyll dynamics and underscores the need for improved coupled models to advance our understanding of biophysical interactions in the IO in response to global warming


Introduction
Marine phytoplankton and microscopic photosynthetic organisms (e.g.algae and cyanobacteria) are of key importance to global ecosystems as they play a central role in the Earth's biogeochemical cycles (Falkowski 2012).The Indian Ocean (IO), with its vast expanse and diverse ecosystems, relies heavily on the contribution of phytoplankton to maintain ecological balance and support marine life (Qasim 1977).The nutrient-rich currents and upwelling zones in the IO create conditions beneficial to phytoplankton growth ( establishing it as a primary food source for various marine organisms.The complex interactions between phytoplankton and zooplankton form the basis of the region's fisheries, which support the livelihoods of coastal communities (Stocker 2015, Watson et al 2015).Furthermore, the key role of IO phytoplankton in mitigating climate change is underlined by its substantial carbon sequestration capacity (Currie et al 2013, Valsala andMurtugudde 2015).Marine phytoplankton in the IO is therefore vital not only to the region's fragile ecosystems but also to global climate regulation and the livelihoods of millions of people who depend on its marine resources (Pörtner et al 2022).
The increasing influence of global warming on marine phytoplankton in the IO represents a concerning ecological development with extensive and significant repercussions (Keith et al 2018, Bryndum-Buchholz et al 2019, Roxy et al 2020, Dalpadado et al 2021, 2024, Tian and Zhang 2023, Zhan et al 2023).Increased ocean temperature has been observed to have a significant impact on the distribution and abundance of phytoplankton in this region (Bopp et al 2013, Roxy et al 2016, Zhan et al 2023).Such changes in phytoplankton can disrupt the delicate balance of marine ecosystems, affecting the entire food web (Das et al 2020, Chakraborty et al 2023).In addition, increased Sea Surface Temperatures (SSTs) contribute to ocean stratification, which limits nutrient upwelling and may lead to nutrient depletion affecting the growth of phytoplankton (Behrenfeld et al 2006, Roxy et al 2016).As a result, these changes have the potential to cascade through the marine food web, affecting higher trophic levels, including commercially important fish species (Roxy et al 2016, Taylor et al 2019, Sridevi et al 2023).A comprehensive understanding of the complex interactions between global warming and marine phytoplankton in the IO is therefore essential to predict and mitigate the broader environmental and socio-economic impacts of climate change.
On the other hand, Indian Ocean Dipole (IOD), an interannual air-sea coupled climate mode in the IO (Saji et al 1999), is known to modulate oceanic conditions that affect nutrient availability and upwelling patterns in the IO.However, as the IO is undergoing a positive IOD-like warming pattern (with faster warming in the western than in the eastern IO) that favors a southeasterly wind trend and a shoaling thermocline in the eastern IO due to anthropogenic warming (Zhen et al 2010, Cai et al 2013, Zhen et al 2013), frequent occurrences of extreme IOD events are expected in the future climate (Cai et al 2021).In addition to the significant effects of global warming on IOD and primary productivity, surface chlorophyll-a (hereafter chlorophyll, a proxy for phytoplankton biomass) in the IO The IOD is a major factor in modulating surface chlorophyll during summer and autumn in the tropical IO (Currie et al 2013).Given the importance of biophysical interactions, several recent studies have investigated the IOD-induced changes in surface chlorophyll in the tropical IO (Park and Kug 2014, Shi and Wang 2021, 2022), and even introduced a new index, the biological dipole index, to explain the robust response of surface chlorophyll changes to IOD (Pathirana et al 2023).Specifically, as reported by Pathirana et al (2023), a dipole-like pattern of surface chlorophyll concentrations emerges, with an increase in the SEIO contrasting with a decrease in the south-southwestern region of India during the positive phase of IOD.This response is closely linked to the IOD-induced southeasterly anomalies, which enhance nutrient upwelling in the SEIO and suppress it in the south-southwest region of India, thereby influencing surface chlorophyll blooms.This scientific understanding of the impact of the IOD on surface chlorophyll in the IO is an important contribution to our understanding of regional marine ecosystems, fisheries dynamics, and wider climate impacts.On the other hand, as global warming is likely to modulate both IOD properties (Zhen et al 2010, Cai et al 2013Cai et al , 2021) ) and biogeochemistry (e.g.Kwiatkowski et al 2020) in the IO, it raises a question of how the IOD-chlorophyll relationship will change in a warming climate.Therefore, understanding the dynamics of these interactions in response to global warming has important implications for the region.Thus, in the present study, we investigate the changes in the biological responses to IOD in a warming climate.

Data and methods
To investigate the effect of IOD on surface chlorophyll, we used the historical simulations and a high emissions scenario of the shared socioeconomic pathway (SSP) 5-8.5 in the Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6) multimodel ensemble.We used data from 13 CMIP6 models, including chlorophyll, SST, low-level (850 hPa) winds, subsurface temperature, and nutrients (nitrate and phosphate).It should be noted that only these 13 models (table S1, O'Neill et al 2016) provide complete 3D ocean biogeochemistry datasets.In addition, monthly data for the last 40 years time period (from 1975 to 2014) are used for the historical simulation and from 2060 to 2099 for the global warming period.Furthermore, we have used only one ensemble member run ('r1i1p1') for each model, and all datasets are interpolated to a 1 × 1 • grid for consistency.
However, many of the CMIP models have a serious bias in simulating observed spatial variability of mean chlorophyll concentrations and interannual variability in the IO (Roxy et al 2016, Pathirana et al 2023).In order to select better models for investigating the biophysical response to IOD, we used monthly chlorophyll (from 1998-01 to 2019-12)  simulations of the CMIP6 models (figure 1).In addition, the dipole mode index (DMI, also referred to as the IOD index) is calculated as the difference between the averaged SST anomaly in the western IO (10 • S-10 • N, 50 • E-70 • E) and the eastern IO (10 Here, all anomalies are calculated by removing the seasonal cycle and the long-term linear trend.We also calculated the strength of coastal upwelling using an upwelling index (UI EK = T y / ρf ) based on the offshore Ekman transport (Bakun 1973, Pathirana et al 2023).T y is the alongshore component of the wind stress, ρ is the density of seawater and is the Coriolis parameter.Here, the regressed meridional wind stress anomalies on the DMI for is used to examine the IOD-induced changes in coastal upwelling.In addition, the details of the definitions and acronyms used in the study are given in table 1.

IOD-chlorophyll relationship in the Indian Ocean
To investigate IOD-induced changes in surface chlorophyll in the IO, we selected 4 CMIP6 models (table S1: MIROC-ES2L, MPI-ESM1-2-LR, NorESM2-LM, NorESM2-MM) based on pattern correlation coefficients (Good models shown in figure 1).The selected models are relatively good at capturing the June to November surface chlorophyll climatology and the interannual variability of the tropical IO.Another important question is to what extent the selected models can accurately simulate the changes in biophysical parameters and how comparable they are to observations.Given this importance, we have examined the IOD-induced changes in SST, low-level wind and surface chlorophyll during the period 1998-01 to 2019-12 using multiple linear regression (figure S1).It is clear that the SEIO is cooler and the western IO is warmer, and the southeasterly winds are enhanced in response to IOD in the observations, and the selected models also show similar responses, suggesting their ability to reproduce the air-sea interactions in the tropical IO.It is also clear that the IOD-induced changes in surface chlorophyll are in good agreement between the observations and the four selected models, suggesting their validity for the use in the present study.
First, we examined the IOD-induced changes in surface chlorophyll in the present and future climate, as shown in figure 2 ).This suggests that despite the weakening of the wind response, surface chlorophyll anomalies in response to IOD are likely to increase significantly in the SEIO region during the boreal summer to autumn.As the regression analysis alone is not sufficient to understand the different effects of positive and negative IOD, we performed a composite analysis to investigate the asymmetric effects of IOD on surface chlorophyll (figure S2).It can be seen that during both positive and negative IOD phases, IOD-related chlorophyll levels are significantly increased under global warming (figures S2(c) and (f)).The composite difference (SSP585 minus historical) also shows that the IODrelated surface chlorophyll anomalies from June to November are stronger during the positive than the negative IOD events (figure S2(i)), indicating the asymmetric effect of IOD on IO surface chlorophyll.Interestingly, the asymmetric responses are strongest in the SEIO region.Given these diverse responses of chlorophyll to IOD, it is important to investigate the underlying mechanisms.
To elucidate the underlying dynamics, a further analysis was performed to assess the thermocline and surface nutrient responses to the IOD and their expected changes in the future.Consistent with the surface chlorophyll response (figure 2(a)), the thermocline in the SEIO region shows a shallowing tendency in response to the IOD in the present climate (figure 2(c)).However, under the greenhouse warming, the IOD-induced thermocline shoaling in the SEIO shows a reduced intensity (figure 2(d)), with a weakened upwelling in the region.Thus, the present climate exhibits an IOD-induced thermocline intensity of which decreases to −9 ± 3 m • C −1 under greenhouse warming in the SEIO.Interestingly, despite the weaker upwelling and thermocline responses, IOD-induced surface nutrient concentrations are enhanced in the SEIO region.Therefore, we further examined the nutrients such as nitrate (figure 2(e)) and phosphate (figure S3(a)).It is clear that the nutrients responses to IOD are an increase in SEIO in the present climate (e.g.nitrate: 0.36 ± 0.20 mmolm −3 • C −1 ), and this pattern is further enhanced under global warming (e.g.nitrate: 0.64 ± 0.44 mmolm −3 • C −1 , figures 2(f) and S3(b)).Hence, it is hypothesized that this increase in nutrient availability has played a key role in the significant increase in IOD-induced surface chlorophyll anomalies in the SEIO under greenhouse warming.
As the IOD-induced changes in surface chlorophyll are robust in the SEIO, we examined the 31 year running correlation between DMI and chlorophyll, nitrate, and phosphate anomalies to understand the time evolution of their relationships (figure 2(g)).
In the present climate, the correlation between DMI and nutrients is higher than the correlation between DMI and chlorophyll, but both correlations are positive.Interestingly, the correlation between DMI and chlorophyll increases strongly under greenhouse warming in all good models.We also examined the temporal evolution of annual mean DMI and chlorophyll in the SEIO region.It can be seen that all models show significant positive correlations, consistent with the main results (figure S4).Given that the IOD-induced southeasterly winds weaken with global warming, a question arises as to what controls the DMI-chlorophyll relationship in the SEIO region in the future.

Role of changes in the Indian Ocean mean state on the relationship between IOD and chlorophyll
To address what controls the IOD-chlorophyll relationship in the SEIO region in the future, we first examined the IOD-induced changes in coastal upwelling.Coastal upwelling is a dominant factor in influencing primary productivity in the SEIO region, as it regulates the essential nutrient supply from the deep to the surface waters (Shi and Wang 2021, Pathirana et al 2023, Zhan et al 2023).Therefore, we calculated the IOD-induced coastal upwelling and found that the UI EK in the SEIO region is 1.09 ± 0.18 m 2 s −1 • C −1 in the present climate, and it is reduced to 0.83 ± 0.14 m 2 s −1 • C −1 under global warming.Thus, the change in IOD-induced upwelling intensity in the SEIO is consistent with the changes in IOD-induced low-level winds shown in figure 2(b).It, therefore, suggests that the other factors contribute to intensifying surface chlorophyll responses to IOD in the SEIO under global warming.
Next, we examined the changes in the mean states of the coupled atmosphere-ocean system in the IO region (e.g.SST, precipitation, low-level winds, and nutrients such as nitrate and phosphate, figure 3).As shown in figures 3(a), a positive IOD-like SST pattern is clearly evident in the IO under global warming (Zhen et al 2010, Cai et al 2013, 2021, Zheng 2019).Changes in mean precipitation also show a dipolelike pattern with enhanced (suppressed) precipitation over the western (eastern) IO.Correspondingly, the southeasterly wind is significantly enhanced in the equatorial IO, leading to a shallower thermocline in the SEIO region under global warming (figure 3(a)).On the other hand, the mean states of nutrients also show an increase in the SEIO region (figure 3(b)), indicating the existence of favorable conditions for surface chlorophyll blooming in this region in response to global warming.Furthermore, the mean state changes in surface conditions are well reflected in subsurface conditions.Although there is no change in thermocline depth in the western equatorial IO between the present and future climate, the thermocline in the eastern equatorial IO becomes shallower in the future (figure 3(c)).Therefore, a subsurface cooling is seen in the SEIO region during the global warming period.
The shallower thermocline allows nutrient-rich water to rise, helping to supply nutrients into the surface layer.Consistent with the shallowing mean thermocline, the mean nutricline, defined as the depth of the maximum vertical gradient of nutrients, also shoals in the future climate, resulting in increased subsurface nutrients.It has been previously reported that surface nutrients are depleted in a warming climate due to intensified ocean stratification (Bopp et al 2013, Moore et al 2018, Kwiatkowski et al 2020).However, despite the mean nutrient depletion in the western equatorial IO (figure S7), nutrients are enhanced in the SEIO region (figures 3(d) and (e)) under global warming.Thus, changes in the mean thermocline are likely to favor the enrichment of subsurface nutrients in the SEIO.Such mean state enhancements of nutrients are likely to favor an increase in surface chlorophyll blooms in the SEIO in future climate (figure S8(a)).On the other hand, it suggests that despite the weakening of IOD-induced upwelling intensity in the warming climate, surface chlorophyll may still be strengthened due to mean state increases of nutrients in the SEIO region.
Furthermore, it has been previously reported that changes in surface chlorophyll are strongly dependent on changes in mixed layer depth (Doney et al 2006).However, we found that changes in surface chlorophyll in the SEIO are not strongly dependent on changes in mixed layer depth in a warming climate.We observed that the shallower mixed layer under a warming climate may not fully account for the increase in nutrients in the SEIO region that is critical for enhanced chlorophyll responses to the IOD.Thus, changes in the thermocline, rather than the mixed layer, appear to better explain variations in surface nutrients in the SEIO under global warming.
To further demonstrate the importance of nutrients, we examined the difference (SSP minus historical) in chlorophyll sensitivity, calculated as chlorophyll anomalies regressed on DMI, and the difference in mean nutrients in the mixed layer (surface to 50 m depth) in the SEIO region in individual models (figure 4).It is found that all the good models are consistent and show an increase in IOD-induced chlorophyll in the SEIO (figure 4(a)), and that is largely due to the increase in nutrient concentrations in the region (figures 4(b)-(e)).In other words, the mean concentrations of nitrate and phosphate are increased in the SEIO region, which favors enhanced chlorophyll blooming under global warming (figures 4(b) and (d)).In addition, due to the shallower thermocline, subsurface nutrients are enhanced in the SEIO region (figures 3 and S7), resulting in an increased vertical gradient of nutrients in the region (figures 4(c) and (e)).Since nutrients are enhanced below the mixed layer in the future climate, even the weaker IOD-induced upwelling can deliver more nutrients to the surface, maintaining favorable conditions for phytoplankton growth.We also calculated the nutrient supply to the surface layer by IODinduced upwelling as UI EK × ∂N / ∂Z , and found that MME mean is 0.030 ± 0.036 mmolm −2 s −1 • C −1 in the present climate, and it increases to 0.052 ± 0.033 mmolm −2 s −1 • C −1 under global warming.Thus, although the IOD-induced upwelling intensity gets weaker, the nutrient supply to the surface layer increases in the future climate.This suggests that the enhanced nutrients in the SEIO are likely to be an important factor in the noted increase in surface chlorophyll in the future climate.

Summary and discussion
In the IO, marine primary productivity plays a crucial role in both regional economic development and the global climate system, with its dynamics heavily influenced by tropical climate variations such as the IOD.Given its importance, previous studies have investigated the profound effect of IOD on IO chlorophyll and reported the robust changes in IOD-induced chlorophyll variability in the region in the present climate (Pathirana et al 2023).As IO phytoplankton biomass is likely to change significantly under a warming climate (Bopp et al 2013), it is important to find answers to the question of how the IOD-chlorophyll relationship will change in the future, and this is investigated here using CMIP6 model data.We found a significant increase in IOD-induced chlorophyll in the SEIO region under global warming.Interestingly, the IOD-induced coastal upwelling is found to be weakened, suggesting the importance of other factors that could modulate the IOD-chlorophyll relationship in this region.Therefore, we examined the mean state change, and it is noted that the mean thermocline is shallower, and nutrients are significantly enhanced in the upper layer of the SEIO region in the future climate.This suggests that intense nutrient concentrations in the subsurface could lead to an increase in nutrient supply to the surface, facilitating favorable conditions for phytoplankton growth, even if the IOD-induced upwelling intensity is weaker.Thus, the enhanced nutrients in the SEIO are likely to be an important factor in the observed increase in surface chlorophyll in the future climate.
Understanding the impact of global warming on IOD-induced chlorophyll variability depends heavily on the accuracy of the simulated IOD characteristics and biological properties in the IO by fully coupled climate models (Cai and Cowan 2013, Roxy et al 2016, Wang et al 2017, Kwiatkowski et al 2020, McKenna et al 2020).Although there are 13 models with ocean biogeochemistry, as shown in figure 1, the majority of climate models still cannot capture the climatology and interannual variability of chlorophyll in the tropical IO well (Pathirana et al 2023).Thus, the CMIP6 models have limitations in accurately simulating the biological properties in the IO and thus do not adequately capture the complex biological dynamics in the region.Such discrepancies in the simulation of biological properties highlight the need for improved models that can better represent the complex interactions within IO ecosystems.In addition, existing climate models have shortcomings in accurately reproducing the characteristics of the IOD (Cai and Cowan 2013, Weller and Cai 2013, McKenna et al 2020, Zheng et al 2024).Therefore, it is important to understand how well climate models simulate IOD characteristics in the current climate, as IOD simulations in the current climate can influence future IOD changes and associated environmental impacts.
Given the biases in the IOD and the associated biological responses, the deficiencies in model representation hinder a comprehensive understanding of the biological consequences of the IOD.There is therefore an urgent need for the development and use of advanced, fully coupled models that integrate biological and climatic components with greater precision (Kwiatkowski et al 2020, Wrightson and Tagliabue 2020, Tagliabue et al 2021).Therefore, we cannot exclude the possibility that the results are sensitive to model performance.However, with reference to previous studies (Roxy et al 2016, Pathirana et al 2023), we selected four models that can capture the biological variability relatively well to discuss the IOD-chlorophyll relationship in the IO (figure 1).In addition, we cannot exclude the risk of uncertainty in the results due to the limited number of models used.For example, among the 4 selected models, NorESM2-LM and NorESM2-MM are different versions of the same model and therefore are not independent samples.On the other hand, the ocean BGC models in both MPI-ESM1-2-LR and NorESM2 are based on the HAMOCC, but branched out (MPI-ESM1-2-LR-Mauritsen et al 2019, NorESM2-Tjiputra et al 2020, Séférian et al 2020).Therefore, it is important to further investigate the changes in surface chlorophyll in the IO induced by climate variability (e.g.IOD) using more independent samples.
Furthermore, we found that the IOD characteristics (e.g.JJASON amplitude) in the selected models are relatively close to the observed IOD characteristics in the present climate (table S2).However, the interannual variability of IOD amplitudes (from 1998-01 to 2019-12) shows that there are differences in IOD amplitude and phase locking between the observations and the four selected models (figure S11).In addition to the effects of interannual climate modes, there are other factors such as mesoscale eddies, SST fronts, waves, etc. that could influence surface chlorophyll dynamics (Liu et al 2020, Wang et al 2021).Therefore, to fully understand the changes in IO surface chlorophyll under global warming, it is necessary to investigate such factors.It is also worth noting that we have examined an extreme warming scenario (SSP5-8.5),and therefore it will be important to examine the IOD-chlorophyll relationship using low and moderate warming scenarios to have a broader understanding of future changes.
Nevertheless, the results of the current investigation provide valuable insights into projected changes in IOD-driven chlorophyll variability within the IO.The projected increase in surface chlorophyll within the SEIO has considerable implications for the socioeconomic conditions and marine ecosystems of the region.Given the nature of the SEIO's low productivity, the expected increase in phytoplankton blooms will modulate fisheries production and ecosystem functions (Iskandar et al 2010, Gaol et al 2015, Sari et al 2020, Horii et al 2023).However, it is essential to recognize the potential consequences of increased phytoplankton activity, including the formation of harmful algal blooms, which can pose significant environmental challenges (Griffith andGobler 2020, Dai et al 2023).The increased availability of key nutrients such as nitrate and phosphate could contribute to the exacerbation of such blooms (Heisler et al 2008).The subtle assessment of potential shifts in marine phytoplankton dynamics due to IOD is therefore of great importance, providing critical insights into the complex regional impacts and assisting in the formulation of strategic measures for ecological conservation and environmental management.
. Consistent with Pathirana et al (2023), it is clearly seen that the southeasterly winds are intensified and the chlorophyll anomalies are increased in the SEIO region in response to IOD in the present climate (figure 2(a)).However, there are robust changes in the biophysical properties of the SEIO under greenhouse warming (figure 2(b)).Figure 2(b) shows the differences in westerly wind in the SEIO region, suggesting that the IOD-induced southeasterly winds weaken significantly with global warming (Zhen et al 2010, Zhen et al 2013).Surprisingly, however, the IODinduced chlorophyll anomalies are significantly enhanced in the SEIO region (figure 2(b)

Figure 1 .
Figure 1.Pattern correlation coefficients (PCC) of JJASON surface chlorophyll climatology (CLM) and standard deviation (STD) between observations and 13 CMIP6 models.The correlations are calculated for the IO region (30 • S-20 • N, 45 • E-100 • E) and the last 40 years of historical simulation (from 1975 to 2014) were considered for the CMIP6 models.Four models were selected as 'Good models' based on PCC ≥ 0.5 for CLM and PCC ≥ 0.4 for STD.The 0.4 criterion for STD was chosen following Roxy et al (2016).

Figure 2 .
Figure 2. MME of regressed surface chlorophyll (color shading) and 850 hPa winds (vector) on DMI in (a) present climate (hist, 1975-2014) and (b) its difference (SSP-historical).Regressed thermocline on DMI in (c) the present climate and (d) its difference.Regressed nitrate (upper 50 m) on DMI in (e) present climate and (f) its difference.In (b) only significant winds are shown in black, and in (b), (d), (e) dots indicate significant regions at the 95% confidence level.In (b), the purple solid square indicates the SEIO region.The 31 year running correlation between DMI and chlorophyll, nitrate, and phosphate anomalies in the SEIO region is shown in (g).In (g), the mean correlation of the historical period is shown as a circle.The solid lines and shading indicate the MME mean and the 95% confidence level of the mean, respectively.The results are based on 4 selected CMIP6 models shown in figure 1.The chlorophyll, thermocline, and nutrient responses in the other nine models are shown in figure S5 and S6.

Figure 3 .
Figure 3. Mean state change in (a) SST (color shading), precipitation (contour), 850 hPa winds (vector), and (b) nitrate (N, color shading) and phosphate (P, contour) at 50 m.Mean state change in the vertical profiles of (c) potential temperature, (d) nitrate, and (e) phosphate.The solid and dashed lines in (c) indicate the thermocline depth in the present and future climate, respectively.In (d) and (e) the solid and dashed lines indicate the nutricline depth in the present and future climate, respectively.Meridionally averaged (10 • S-0 • ) values are given in (c)-(e).In (a) only the significant values are shown, in other figures significant regions at the 95% confidence level are marked with dots.Mean state changes in the other nine models are shown in figure S9.

Figure 4 .
Figure 4. Future changes (SSP minus historical) in (a) chlorophyll sensitivity to IOD, (b) mean upper layer (50 m) nitrate (N) concentration, (c) mean vertical gradient of nitrate ( ∂N / ∂Z ), (d) mean upper layer phosphate (P) concentration, and (e) mean vertical gradient of phosphate ( ∂P / ∂Z ) in the SEIO region (figure 2(b)).The chlorophyll sensitivity is calculated from the linear regression and the vertical gradients in (c), (e) are calculated as the difference between the mean nutrients in the surface layer (0-20 m) and below the mixed layer (50-60 m).The results shown are for the good models and the MME mean is shown in brown color.The error bar indicates the 0.5 standard deviation.Future changes in chlorophyll sensitivity and nutrients from other models are shown in figure S10.
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Table 1 .
Information on index regions, definitions, and acronyms used in the present study.
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