|Multi-decadal variability of phytoplankton and related physical forcing in the North Atlantic Ocean |
Auteur(s): Martinez E., Antoine David, Raitsos Dionysios
Conference: Earth Observation for Ocean-Atmosphere Interactions Science (Frascati, IT, 2011-11-29)
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The spring, bloom is a renowned feature of many seasonal seas in the global ocean. Perhaps most famous of all is the spring bloom that occurs at middle and high latitudes of the North Atlantic. Since the 1950’s the Sverdurp’s theory prevails to explain the spring bloom initiation in the North Atlantic subpolar region. Photosynthesis is light limited during winter, so the bloom occurs in spring when the mixed layer shoals sufficiently to allow phytoplankton to remain within the sunlit region and enable net growth. Therefore an increase of stratification, due to global warming for instance, would lead to a strengthened bloom through phytoplankton spending more time in the euphotic zone. In this context, a stepwise increase in biomass has been reported in the mid-1980s correlated with the sea surface temperature (SST), an indicator of stratification, and the Atlantic Multidecadal Oscillation in the North Sea and Northeastern Atlantic. Chlorophyll-a (Chl), a measure of phytoplankton biomass, derived from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) from 1997–2002 was combined with in situ measurements of the Phytoplankton Color Index (PCI) collected by the Continuous Plankton Recorder survey since 1946. Recently, an increase of Chl has also been observed in satellite ocean color observations of the northeastern Atlantic (30°-50°N and 40°-0°W), from the 1980s to the 2000s (Coastal Zone Color Scanner ― CZCS, and SeaWiFS missions respectively) in parallel to an increase of SST. However, this increase of SST appeared to be related with a deepening of the Mixed Layer Depth (MLD) in this region rather than an increase of stratification. This result gives substance to the “dilution-recoupling hypothesis”. The Dilution–Recoupling hypothesis suggests, under climate warming conditions, that weaken of winter mixing may lead to decreased net phytoplankton growth rates and vernal biomass (i.e., an opposite conclusion from one based on the Sverdrup’s theory). However, because these contradictory results are restricted to PCI and SST data in an remote northeastern region in one case and to a discontinue radiometric time series of 10 years of data over a 20 year time period in the other case, it is of primordial importance to go further in the assessment of the Chl, and related physical parameters, decadal variability over a longer time period in the north Atlantic. The objective of the present study is to provide new elements to understand the response of phytoplankton to climate multi decadal changes through its physical forcing to better understand and forecast phytoplankton evolution in future. Here, we present the multi decadal variability of Chl in the North Atlantic (40°-60°N; 40°-0°W) in parallel to different physical forcing as the wind, SST and mixed layer depth. We combined extended in situ time series with modern high coverage satellite observations. The PCI, an index of Chl since 1946, is combined with a global merged satellite product of Chl (GlobColour, ESA) since 1997. MLD is derived from in situ vertical temperature profiles since 1941. The wind stress data (one important dynamical driver of MLD) and SST are issued from an extensive collection of surface marine data since 1960 (ICOADS). These in situ observations are respectively combined with ERS1-2/QuikSCAT and AVHRR satellite data since 1991 and 1981. The overlap between the various kinds of observations shows a good agreement for the different data sets. Since 1950-1960, while SST oscillates following the Atlantic Multidecadal Oscillation, the other parameters rather show a trend to increase. Beginning of the 1980s, there is a shift toward deeper MLD which is likely related to stronger wind stress. It is followed by a shift toward stronger Chl values. These patterns are emphasised in the north over 50°-60°N compared to 40°-50°N. The Chl increase in parallel to the MLD deepening might support the Dilution-Recoupling hypothesis rather than the Critical Depth hypothesis.