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Decadal variations in equatorial Pacific ecosystems and ferrocline/pycnocline decoupling.

Abstract : The equatorial Pacific Ocean is known for its large interannual to decadal variability in circulation. In particular, the changes that occurred in 1976/1977 have received considerable attention in the climate dynamics literature, and recently there has been much attention focused on changes that may have occurred there in 1997/1998. Unfortunately, because of data sparsity, the impact of these changes or shifts on ocean biogeochemistry and ecosystems remains largely unknown. Here a three-dimensional ocean circulation model (the ORCA2 configuration of OPA) which has a food web/biogeochemistry model (PISCES) embedded in it, and which has been forced with both NCEP-1 and ERA-40 reanalysis fluxes over multiple decades, is used as a tool to investigate decadal changes and their associated mechanisms. Our main finding with the model is that a decrease in the amplitude of the surface zonal wind stress in the tropical Pacific in the mid-to-late-1970s leads to a decrease in Fe and Chl concentrations in the upwelling regions of the eastern equatorial Pacific after 1976/1977. These changes find expression predominantly during the upwelling season (the seasonal maximum for Fe and Chl concentrations), when surface Fe and Chl concentrations tend to be significantly higher pre-1976/1977 than post-1976/1977. The changes in Chl concentrations need to be understood as modulations of the amplitude of the seasonal cycle, rather than as a "biological regime shift" (an abrupt transition from one mean state to another). In contrast to what is found for Fe and Chl, for NO3 the decadal changes in surface concentrations in the upwelling region about 1976/1977 can be described as a shift in the mean state. It is shown that the response in surface Fe and Chl in the upwelling region about 1976/1977 is proportionally larger than the decadal changes in surface wind stress forcing, and it is also larger than the previously reported change in the strength of the meridional overturning strength of the subtropical cells (STCs). Importantly, this amplified response reflects a decoupling of the ferrocline and pycnocline within the equatorial Pacific. In this way, the presence of a time-invariant sediment source for Fe can substantially amplify the ecosystem response to decadal variability in ocean circulation.
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Contributor : Francoise Cudennec <>
Submitted on : Saturday, July 24, 2021 - 8:25:07 AM
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Keith B. Rodgers, Olivier Aumont, Christophe E. Menkès, Thomas Gorgues. Decadal variations in equatorial Pacific ecosystems and ferrocline/pycnocline decoupling.. Global Biogeochemical Cycles, American Geophysical Union, 2008, 22, pp.GB2019. ⟨10.1029/2006GB002919⟩. ⟨hal-00388077⟩

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