Impact of cyclonic eddies and typhoons on biogeochemistry in the oligotrophic ocean based on biogeochemical/physical/meteorological time-series at station KEO

Abstract

In 2014, a sediment trap mooring was deployed adjacent to station Kuroshio Extension Observatory (KEO)'s National Ocean and Atmosphere Administration (NOAA) surface mooring. These data, from July 2014 to July 2016, are used here to investigate nutrient supply mechanisms that support ocean productivity in this oligotrophic region of the subtropical western North Pacific. Both years of sediment trap data show that biogenic material fluxes at~5000 m increased between late winter (March) and late spring (June). Based on sea surface temperature and upper ocean water temperature profiles, from the NOAA surface buoy, and satellite-based surface chlorophyll-a , this increase was likely due to an increase of ocean productivity in early spring (March) that was supported by nutrients supplied by winter mixing. On the other hand, biogenic material fluxes also increased in October 2014, and between late December 2014 and January 2015 when concentrations of nutrients near the surface typically are low. Sea surface height anomalies and vertical profiles of water temperature in the upper 500 m showed cyclonic eddies passing station KEO and causing upwelling in late July-early August 2014 and November 2014. It appears that these events supplied nutrients to the upper layer, which then caused ocean productivity in the subsurface layer to temporally increase, resulting in increased deep biogenic material fluxes in autumn and winter. This interpretation of the data is consistent with a simple 3D physical-biological model simulation that shows meso-scale cyclonic eddies can supply nutrient to support new production at KEO. During the 2-year-long sediment trap deployments, several typhoons also passed near station KEO and near-inertial internal waves were observed near the nitracline depth after the typhoons passed. Although turbulent mixing caused by near-inertial internal wave could have possibly supplied nutrient to upper oligotrophic euphotic layer, numerical simulations of the turbulent nutrient supply indicate that enhanced turbulent diffusion across the nutrient concentration gradient did not supply enough nitrate to support the increase in biogenic material flux in autumn.

Evidence of Eddy‐Enhanced Winter Chlorophyll‐a Blooms in Northern Arabian Sea: 2017 Cruise Expedition

“…Since the cyclonic eddy can perturb the thermohaline stratification via pumping effects, these stations can thus be grouped with the negative SLA (blue curves of Figures 2d), uplift thermocline depth, and a shallow MLD (Figures 2a and 2b). This agrees with the conclusion that cyclonic eddies (for the northern hemisphere) generally have a shallow thermocline at the centers, and the subsurface nutricline can thus temporarily rise into the euphotic zone (Martin & Pondaven, 2003; Honda et al, 2018; also see Figures S3 and S4).…”

“…When the winter phytoplankton bloom begins (usually in December), nutrients enhanced by convective mixing cause an increase in primary production and lower the phytoplankton growth limitation. In addition, vertical and horizontal advection caused by mesoscale eddies and filaments participate in the development of winter blooms and the continuously supply of nutrients to the upper layer (Honda et al, 2018; McGillicuddy et al, 1998; Resplandy et al, 2011). The amount of nutrients transported by eddies and filaments accounts for 40–50% of the total amount of nutrients supplied (Shenoi et al, 2004), and four kinds of mechanisms are involved: eddy stirring, eddy trapping, eddy pumping, and eddy‐Ekman pumping (Siegel et al, 2011; Gaube, 2012).…”

Time‐series observations of photosynthetic oxygen production in the subtropical western North Pacific by an underwater profiling buoy system

“…Winter mixing can supply enough nutrients to cause the diatom blooms in this region. In the oligotrophic regions in the western North Pacific, episodic events such as mesoscale eddies (Honda et al ), typhoons (Lin ) and eolian dust inputs (Taketani et al ) are considered as potential mechanisms for nutrient injection into the euphotic zone. The meteorological satellite data showed the propagations of cyclonic eddies in August 2012 (stratification period) and of anticyclonic eddies in December 2012 (mixing period) in the vicinity of Sta.…”

Deep Ocean Particle Flux in the Northern South China Sea: Variability on Intra-Seasonal to Seasonal Timescales

“…Cyclonic eddies could remarkably increase surface biomass (Chen et al, 2007;Xiu and Chai, 2011;Wang et al, 2016;Honda et al, 2018). Positive Chl-a anomalies from 1998 to 2011 were associated with cyclonic eddies in the SCS (He et al, 2016).…”