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New paper

The Southern Ocean leaving the footprints on the chlorophyll

Phytoplankton in the ocean contains chlorophyll in their body which absorbs CO2 and emits O2 during photosynthesis. Thus the amount of chlorophyll plays an important role in air-sea CO2 flux. The growth of phytoplankton depends on mainly two factors, light, and nutrient (iron in the Southern Ocean) which phytoplankton takes up as its food. The light provided to phytoplankton is abundant in summer, deficient in winter. Besides that, the light for phytoplankton also depends on the vertical mixing of the ocean. Think of the cinnamon on the top of a cappuccino. If there's stirring motion by a straw, cinnamon spreads vertically and the concentration of cinnamon on the top of the cappuccino decreases. Similarly, vertical mixing of the ocean affects the vertical distribution of phytoplankton and the amount of light they experience. Satellites show that there are full of coherent patterns of anomalously high and low sea level in the ocean with a scale of tens or hundreds of kilometers. They are called mesoscale eddies. The features with higher sea level are called anticyclones while those with lower sea level called cyclones. The observations reveal that they are different in the surface vertical mixing: anticyclones are characterized by deeper vertical mixing but cyclones by shallower vertical mixing. If the mixed layer is deeper, it can enrich nutrient (iron) concentration near the surface. Phytoplankton lives near the surface to secure the sunlight and consumes iron. This leads to the concentration of iron lower near the surface but higher in deeper water. Thus stronger vertical mix can enrich the iron concentration near the surface. At the same time it may reduce the amount of light for phytoplankton as they are brought down to deeper depth where the light level approaches to zero. Phytoplankton needs both light and iron. In summer, light is abundant and the background vertical mixing is not very intense. So even though there's strong vertical mixing, it doesn't have a great impact on the amount of light near the surface. Iron concentration near surface plays the important role instead. Stronger mixing will provide more iron to the surface, hence the concentration of chlorophyll can go up along with the intensity of vertical mixing. In winter, however, due to stronger vertical mixing, the concentration of iron near the surface is quite high, attributing relatively small impact to the concentration of chlorophyll, while the light is almost always scarce. Thus the amount of light near the surface is more important to phytoplankton. Stronger vertical mixing will reduce the light exposure for phytoplankton resulting to the reduction of chlorophyll concentration. These relationship is expressed as a positive correlation between the anomalies of sea level and chlorophyll in summer but a negative correlation in winter in the Southern Ocean. The influence of vertical mixing associated with mesoscale eddies may have direct impact on other biogeochemical fields such as carbon and the exchange of CO2 between air and sea.

Original paper: Seasonal Variation in the Correlation Between Anomalies of Sea Level and Chlorophyll in the Antarctic Circumpolar Current (Hajoon Song)

Seasonal Variation in the Correlation Between Anomalies of Sea Level and Chlorophyll in the Antarctic Circumpolar Current is published on Geophysical Research Letters