Congratulations to Srishti Dasarathy for her first first-authored publication! Srishti’s paper “Multi-year Seasonal Trends in Sea Ice, Chlorophyll Concentration, and Marine Aerosol Optical Depth in the Bellingshausen Sea” is out in advance of print in JGR Atmospheres. This paper was a really long time in coming. For this study, Srishti made use of several different satellite products including measurements of marine aerosol optical depth (MOAD) derived from the CALIPSO satellite. We are not a remote sensing lab and Srishti doesn’t come from a remote sensing or physics background, so the learning curve was pretty steep. It took a couple of years, a lot of Matlab tutorials, and an internship with the CALIPSO team at NASA’s Langley Research Center just to crack the CALIPSO data and start testing hypotheses. Srishti’s main hypothesis was that MOAD would be positively correlated with ocean color and negatively correlated with sea ice, since phytoplankton are known to be a source of volatile organic compounds that can form aerosol particles. Confounding this is that sea spray – which like phytoplankton is associated with open water periods – is also a source of aerosols.
One challenge that we faced was that CALIPSO represents data with high spatial resolution along a 2D path or “curtain”, as shown above. The orbital geometry is such that not every point on the globe gets covered; the same curtains get sampled every 16 days. Thus, while spatial resolution is high along the curtain, it is poor orthogonal to the curtain, and temporal resolution is limited to 16 days. This makes it a bit challenging to capture signals associated with relatively ephemeral events (such as phytoplankton blooms).
To work around these limitations Srishti took a basin-scale view of the CALIPSO data and looked for large scale trends that would link CMOD with chlorophyll a or ice cover. This approach isn’t idea and glosses over a lot of interesting details, but it is nonetheless sufficient to reveal some interesting relationships. Most notably that MOAD and chlorophyll a are weakly but significantly correlated in a time-lagged fashion, with a delay of approximately 1 month yielded the strongest correlation. This makes sense, as the volatile organics compounds that link phytoplankton (and ice algal) communities to MAOD are thought to be maximally produced near the end of the phytoplankton bloom as the biomass starts to decay. In the near future new satellite missions like PACE and improved land/sea observing campaigns will allows us to get into the details a bit more, including direct observations of specific blooms and the time- and space-lagged MOAD response!
Congratulations Srishti! Amazing work.