A Double-Peak Intraseasonal pattern in the Chlorophyll Concentration Associated With Summer Upwelling and Mesoscale Eddies in the Western South China Sea

The southwest monsoon between June and October induces upwelling and offshore cold-water transport in the western South China Sea (SCS). A seasonal collocated chlorophyll bloom also occurs, resulting in a productivity increase in the oligotrophic SCS basin. To accurately quantify productivity transported toward the open ocean, it is necessary to determine whether the transport is continuous or intermittent (intraseasonal oscillation). However, intraseasonal chlorophyll oscillations during summer have not been investigated. The influences of upwelling and an eddy dipole on intraseasonal chlorophyll oscillation also remain unclear. Merged daily chlorophyll data obtained from a multisatellite ocean color product are used to investigate intraseasonal chlorophyll oscillations. Our results indicate that summer chlorophyll development is characterized by an intraseasonal double-bloom pattern in July-August and September-October in 11 out of 20 years (1998-2017). The time interval between the two blooms varies between 23 and 45 days. The timing of the first bloom is correlated with upwelling (R-2 = 0.70), but the secondary bloom is not (R-2 = 0.26). During the second chlorophyll bloom period, high chlorophyll is observed in the interaction region of an eddy dipole, which is locally generated rather than transported from nearshore areas. A new index is proposed based on finite-size Lyapunov exponents and the orientations of the associated eigenvectors to represent frontogenetic processes. The index coincides not only spatially with the chlorophyll bloom but also temporally with the chlorophyll variation during the second bloom period (R-2 = 0.73). Plain Language Summary Wind-driven coastal upwelling occurs in the western South China Sea in summer during the southwest monsoon. Cold water near the coast extends offshore and develops into a cold eastward jet. The upwelling can transport deep nutrient-rich water to the surface and support phytoplankton growth, generating chlorophyll blooms on a seasonal time scale. We find that due to the synergistic influences of upwelling and an eddy dipole, this seasonal chlorophyll bloom fluctuates on intraseasonal time scales. There are two chlorophyll blooms during summer in 11 out of 20 years (1998-2017). Upwelling contributes to the first chlorophyll bloom, and submesoscale frontogenetic processes associated with an eddy dipole are responsible for the second chlorophyll bloom.