The Influence Of Mesoscale Eddies And Oceanic Dynamic Factors On The Distribution Of Chlorophyll

The chlorophyll in the ocean can be used not only to be a good estimator of algal biomass in the ocean,but also to estimate the ocean primary productivity.Ocean primary productivity indicates the capacity of producing organic compunds by phytoplankton.It plays an important role in the global carbon cycle,and has a profound impact on the discussion of global climate warming,sea level rising and other climate changes.In this article,we focus on two subjects:one is to quantify the influence of mesoscale eddies on the distribution of chlorophyll;the other is to study the impact of oceanic dynamic factors（sea surface temperature,wind stress and mixed-layer depth）on chlorophyll changes in open ocean.Firstly,we carried out the research on the influence of mesoscale eddies on chlorophyll based on the observational data and the model of the vertical distribution of chlorophyll.Oceanic mesoscale eddies with typical scales of O（100 km）in the world’s oceans are ubiquitous occupying about 20-30%of the ocean surface.They contain enormous kinetic energy and play a very important role in the transport of material,heat,salt and energy.Previous studies have quantified the transport of water mass,heat and salt by mesoscale eddies.The water transport induced by mesoscale eddies is comparable with that of the large-scale circulation in magnitude.Both the heat and salt transport induced by eddies take a sizeable fraction of total transports respectively,but the chlorophyll trapped by global mesoscale eddies and its influence on chlorophyll transport are still not clear.Based on satellite altimetry observations and Argo observation by the principle of potential vorticity conservation,the water mass transport by mesoscale eddies was quantified.At this time,the fluid inside the eddy moves together with the eddy as a whole,because the fluid trapped by the eddy tends to be trapped within the closed PV contours.When the mesoscale eddies move with the inside water column,the mesoscale eddies move together with the phytoplankton in the water column.The chlorophyll concentration is commonly used as a proxy for phytoplankton biomass.Inspired by the estimation of eddy-induced water mass transport,we believe that the chlorophyll trapped by mesoscale eddies can be quantified.The mesoscale eddies penetrate approximately 100 m to more than 2000 m underwater.The satellites can only observe chlorophyll at the sea surface,but can not do anthing with the chlorophyll below the euphotic zone depth.Here,we use the empirical model of the vertical distribution of chlorophyll,combined with the sea surface chlorophyll concentration to reconcstruct the chlorophyll concentration below the sea surface inside the eddy.Therefore,when mesoscale eddies trap and transport the chlorophyll within the water columns,we can estimate the transport of eddy-induced chlorophyll based on satellite observations,Argo float measurements,and an empirical model.The combination of satellite altimeter data and Argo measurements is used to detect eddy boundaries by tracking the outermost closed contours of potential vorticity at depth;then,sea surface chlorophyll from satellite observations,together with their vertical distributions estimated from models are used to derive the chlorophyll within eddy boundaries.The chlorophyll trapped by eddies can reach 3.2×10¹²g,which is about half of the total CHL in the ocean;the global time-mean chlorophyll eddy-induced zonal transport adds up to 5.7×10³ g/s westward.The results show that oceanic mesoscale eddies play an important role in the elevated chlorophyll in the interior of the ocean.Secondly,we explored the impact of oceanic dynamic factors（sea surface temperature,wind stress and mixed-layer depth）on chlorophyll based on satellite observation and Argo observation data.Many previous studies of the impact of oceanic oceanic dynamic factors on chlorophyll in a specific region focused on sea surface temperature,mixed-layer depth,or wind stress alone.Now remote sensing and Argo buoys have become common methods in the marine observation and monitoring because they can provide long-term data with a wide range of coverage.In this study,relationship between chlorophyll and all those three oceanic dynamic factors in the open ocean was quantified for five regions within the subtropical gyres and the variation trend of 13-year（2003-2015）was analyzed using satellite observations and Argo measurements.The correlation analysis results showed that mixed-layer depth was correlated positively with chlorophyll,sea surface temperature was correlated negatively with chlorophyll,and the correlation between chlorophyll and wind stress was either positive or negative.Based on the significance of the correlations,we established models to represent their relationships using the multiple linear regression method and analyzed which dynamic factors were the major determinants of chlorophyll change.The regression coefficients showed that both sea surface temperature and mixed-layer depth have remarkable effect on chlorophyll.The derived models could be used to diagnose the past changes,understand present variability,and predict the future state of chlorophyll changes based on oceanic dynamic factors.The relationships between chlorophyll and oceanic dynamic factors help us understand the dynamic mechanism of chlorophyll variation in the open ocean.Photosynthetic assimilation of carbon dixide by phytoplankton constitutes a necessary prerequisite for sustaining marine life,this process is closely related to the concentration of carbon dixide in the ocean.With the accumulation of more observed carbon dixide data,if the absorbed carbon dixide is taken into account when constructing the mathematical model,the accuracy of the model might be improved.