Séminaire : Keerthi Madhavan Girijakumari (LOCEAN): Untangling the timescales in surface chlorophyll fluctuations to better constrain the driving processes
Tuesday 02 March 2021, 11:00am - 12:00pm
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Abstract :

In the context of our changing climate, an ongoing challenge is to be able to reliably detect and attribute trends in ocean color records of surface chlorophyll‐a (SChl), which is the only observable variable representative of the state of marine ecosystem available at global scale. However, the magnitude of the natural variability of SChl is much stronger than that of the SChl long‐term trend, and occurs over a large range of unrelated time scales. Thus understanding long‐term variations of ocean ecosystems requires untangling the time scales involved in their natural fluctuations. Here we applied a temporal decomposition procedure to two decades of satellite ocean color observations and characterized the variability of surface chlorophyll‐a (SChl) at different timescales, with a special focus on the intraseasonal timescale that have been often overlooked. In order to assess the reliability of the satellite data at capturing intraseasonal, seasonal, and interannual variability, we first focused over a regional ocean (Mediterranean Sea) which have a complete data coverage and have a long term insitu timeseries (BOUSSOLE time series). Comparison with the high frequency insitu data shows that the satellite SChl products are reliable at all timescales. Our results reveal similar amplitude for the seasonal and intraseasonal SChl variations in the northwestern bloom region, together explaining about 90% of the SChl variance.  Seasonal SChl variations occur at the scale of the bloom region (~400 km) and are tightly connected with seasonal mixing-layer depth (MxLD) changes while intraseasonal SChl fluctuations occur at smaller spatial scales (~100 km) and are weakly correlated with MxLD. Weekly SChl fluctuations are in phase with weekly changes in wind stress and net heat flux during the intial state of the bloom in winter and early spring, suggesting that intermittency in vertical stability due to short-term episodes of calm weather in winter or to stormy conditions in early spring leads to short-term variations in light exposure or to events of vertical dilution, and to strong intermittency in phytoplankton bloom. Over the oligotrophic part of the Mediterranean Sea, about 80% of the variability in both SChl are explained by basin‐scale (~1,000 km) seasonal variations. We further extended our analysis over the global ocean and  characterised the variability of SChl at different timescales, and its spatial scales. We partition the world’s ocean into regions dominated by intraseasonal variations, seasonal variations & interannual variations.

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