107 / 2023-07-30 23:47:35

The planktonic ecosystem's intraseasonal response to summertime Madden-Julian Oscillation in the South China Sea: A model study

Madden-Julian Oscillation,South China Sea,physical-ecosystem model,Pelagic ecosystem

In the summertime South China Sea (SCS), the intraseasonal variations of the structure and function of the planktonic ecosystem was attributed to various dynamic processes; however, these have never been investigated from the perspectives of Madden-Julian Oscillation (MJO) which dominates the intraseasonal climate variability. To bridge this gap, we first analyzed phytoplankton pigment data aggregated from >7000 in-situ data samples from 10-year cruises. Composite analysis showed a significant signature from MJO in the pelagic ecosystem, in particular, in the upper-layer chlorophyll (Chl) and phytoplankton size classes (PSCs). During the late phases of MJO events, the Chl significantly increased, with the PSCs shifting towards more nanophytoplankton and less picophytoplankton. To further elaborate on the ecosystem's responses in terms of primary and export production to MJO, we conducted experiments with a physical-ecosystem model (ROMS-CoSiNE) localized in prior studies. Forced by the idealized MJO atmospheric forcing, the ecosystem of the central SCS basin developed with more nanophytoplankton down to 100 m depth. The MJO-forced model showed increased surface Chl (~80%), primary production (~50%), and export production with a smaller magnitude (~35%), leading to an apparent reduction in the e-ratio. This reduction in e-ratio cannot be explained by PSCs changes, but can be explained by the lagged export production with respect to primary production. Such a lag was elongated by the physical processes of both upwelling and mixing, reducing the particle sinking speed. In contrast with prior studies, the use of coupled model highlights the role of basin-scale vertical motion in MJO events in regulating SCS ecosystem, and therefore complements the understanding of complex ecosystem dynamics in a typical subtropical marginal sea.