Mesoscale variations of biogeochemical properties in the Sargasso Sea

Abstract

Abstract. A mesoscale resolution biogeochemical survey was carried out in the vicinity of the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time-series Study (BATS) site during the summer of 1996. Real-time nowcasting and forecasting of the flow field facilitated adaptive sampling of several eddy features in the area. Variations in upper ocean nutrient and pigment distributions were largely controlled by vertical isopycnal displacements associated with the mesoscale field. Shoaling density surfaces tended to introduce cold, nutrient-rich water into the euphotic zone, while deepening isopycnals displaced nutrient-depleted water downward. Chlorophyll concentration was generally enhanced in the former case and reduced in the latter. Eddy-induced upwelling at the base of the euphotic zone was affected by features of two different types captured in this survey: (1) a typical mid-ocean cyclone in which doming of the main thermocline raised the near-surface stratification upward and (2) a mode water eddy composed of a thick lens of 18øC water, which pushed up the seasonal thermocline and depressed the main thermocline. Model hindcasts using all available data provide a four-dimensional context in which to interpret temporal trends at the BATS site and two other locations during the 2 weeks subsequent to the survey. Observed changes in near-surface structure at the BATS site included shoaling isopycnals, increased nutrient availability at the base of the euphotic zone, and enhanced chlorophyll concentration within the euphotic zone. These trends are explicable in terms of a newly formed cyclone that impinged upon the site during this time period. These observations reveal that eddy upwelling has a demonstrable impact on the way in which the nitrate-density relationship changes with depth from the aphotic zone into the euphotic zone. A similar transition is present in the BATS record, suggesting that eddy-driven upwelling events are present in the time series of upper ocean biogeochemical properties. The variability in main thermocline temperature and nitrate in this synoptic spatial survey spans the range observed in these quantities in the 10-year time series available at BATS to date (1988-1998).

Monitoring the mesoscale eddies of the Lofoten Basin: importance, progress, and challenges

“…They also found a clear difference in the vertical profiles of Argo floats co-located within the cyclonic and anticyclonic eddies. Since the dynamics associated with the centres of anticyclonic and cyclonic eddies are completely different (McGillicuddy et al 1999), the difference in the vertical profiles further confirms the capability of the eddy detection method to even differentiate the anticyclonic and cyclonic eddies in the Lofoten Basin. The details of this method are presented next (Section 3.1).…”

Mesoscale physical–biological–biogeochemical linkages in the open ocean: An introduction to the results of the E-Flux and EDDIES programs

“…It is likely due to a combination of variations in the magnitude, timing, and duration of nutrient input caused by differences in eddy formation, intensity, age, and movement as shown in this volume and previously (Bibby et al, 2008;Brown et al, 2008;McGillicuddy et al, 1999;Nencioli et al, 2008;Olaizola et al, 1993;Sweeney et al, 2003). For example, Hawaiian lee cyclones (and anticyclones) appear to be wind-driven, formed by a combination of strong northeasterly winds and island topography (Chavanne et al, 2002;Lumpkin, 1998;Patzert, 1969), whereas midocean eddies in the Sargasso Sea are formed by instability processes that feed on the larger scale flow (Robinson, 1983).…”

Winter phytoplankton bloom induced by subsurface upwelling and mixed layer entrainment southwest of Luzon Strait

“…The intrusion of the Kuroshio results in mesoscale eddies in the vicinity of Luzon Strait (Wu and Chiang, 2007). The dynamics of mesoscale eddies influences biogeochemical budgets in the upper ocean (McGillicuddy et al, 1999). Vertical nutrient flux can be induced by mesoscale eddies, leading to enhanced new production (McGillicuddy and Robinson, 1997;McGillicuddy et al, 1998).…”