Regional variability and turbulent characteristics of the satellite-sensed submesoscale surface chlorophyll concentrations

Abstract

The regional variability and turbulent characteristics of submesoscale surface chlorophyll concentrations are examined with hourly maps of geostationary ocean color imagery-derived chlorophyll concentrations ata0.5-kmresolutionforaperiodoffiveyears(2011to2015)overthe East/JapanSeawithconcurrentmesoscaleandsubmesoscaleobservations. Two seasonal blooms occur in the spring and fall within 250 km off the coast that are associated with constructive combinations of light exposure, nutrients, and vertical stratification. Another bloom occurs in the summer and is closely related to regional wind-driven upwelling events. The spring and fall blooms are more significant near the coast (within 40 km from the coast) than offshore because of the more energetic submesoscale horizontal shear and vortical phenomena onshore as well as their propagation in the cross-shore direction. In addition, the regional spring bloom starts offshore and migrate onshore with a time delay of one month, which may result from the onshore propagation of geostrophic currents, the deepening of the mixed layer, and favorable nutrient fluxes from the subsurface. The wavenumber-domain energy spectra of chlorophyll concentrations exhibit anisotropy, which may be closely related to bathymetric effects andregionalcirculations. Thespectraldecayslopeschangefromk−5/3 to k−1 at the O(10) km scales and from k−1 to k≤−3 at the O(1) km scales and have weak seasonality. These results are consistent with the two-dimensional quasi-geostrophic turbulence theory and can be interpreted with the baroclinic instability energized from the moderate seasonal mixed layer under mesoscale regional circulations.