Cs. Law et al., A Lagrangian SF6 tracer study of an anticyclonic eddy in the North Atlantic: patch evolution, vertical mixing and nutrient supply to the mixed layer, DEEP-SEA II, 48(4-5), 2001, pp. 705-724
Citations number
36
Categorie Soggetti
Aquatic Sciences","Earth Sciences
Journal title
DEEP-SEA RESEARCH PART II-TOPICAL STUDIES IN OCEANOGRAPHY
Biological and biogeochemical change in the surface mixed layer of an antic
yclonic eddy at 60 degreesN in the North Atlantic were monitored within a L
agrangian time-series study using the tracer sulphur hexafluoride (SF6). Fo
ur ARGOS buoys initially released at the patch centre remained closely asso
ciated with the SF6 patch over a 10-day period, with the near-circular eddy
streamlines contributing to the stability and coherence of the patch. Prog
ressive deepening of the surface mixed layer was temporarily interrupted by
a storm, which increased mixed-layer nitrate and accelerated the transfer
of SF6 to the atmosphere. Diapycnal exchange of SF6 was relatively rapid du
e to the shallow pycnocline gradient, and a vertical eddy diffusivity (K-z)
of 1.95 cm(2) s(-1) at the base of the mixed layer was estimated from vert
ical SF6 profiles at the patch centre. Application of K-z to the nutrient g
radients inferred vertical nitrate and phosphate fluxes of 1.8 and 1.25 mmo
l m(-2) d(-1), respectively, for the pre-storm period, which accounted for
33% and 20% of the reported in vivo uptake rates. Integration of the vertic
al nitrate flux and decline in surface layer nitrate suggest a total loss o
f 0.54 mmol N m(-3) d(-1) during the 5-day pre-storm period, of which in vi
vo nitrate consumption only accounted for 49%. Vertical transport of ammoni
um regenerated in the pycnocline accounted for up to 25% of in vivo phytopl
ankton uptake. The results suggest that the contribution of vertical turbul
ence to the mixed-layer nutrient pool was less important than that recorded
in other regions of the open ocean, inferring that advective processes are
more significant in an eddy. This study also emphasises the potential of S
F6 for oceanic Lagrangian time series studies, particularly in dynamic regi
ons, and in constraining estimates of new production. (C) 2001 Elsevier Sci
ence Ltd. All rights reserved.