Physical Drivers and Biogeochemical Effects of the Projected Decline of
the Shelfbreak Jet in the Northwest North Atlantic Ocean
Abstract
A solid understanding of the mechanisms behind the presently observed,
rapid warming of the northwest North Atlantic Continental Shelf is
lacking. We hypothesize that a weakening of the Labrador Current System
(LCS), especially the shelfbreak jet along the Scotian Shelf, is
contributing to these changes and that the future evolution of the LCS
will be key to accurate projections. Here we analyze the response of a
transient simulation of the high-resolution GFDL Climate Model 2.6
(CM2.6) which realistically simulates the regional circulation but
includes only a highly simplified representation of ocean
biogeochemistry. Then, we dynamically downscale CM2.6 using a
medium-complexity regional biogeochemical ocean model to obtain
projections of several ecosystem-relevant variables. In the simulation,
the shelfbreak jet weakens throughout the century because of a reduction
of the along-shelf pressure gradient caused by a buoyancy gain of the
upper water column along the shelf edge. This buoyancy gain is the
result of an increased presence of subtropical waters in the continental
slope. Importantly, we find that the weakening of the shelfbreak jet is
not in response to a northward shift of the Gulf Steam, as has been
hypothesized by others, and that previous reports of a northward shift
of the Gulf Stream North Wall (GSNW) are an artifact of the
temperature-based GSNW criterion in common use. The projected weakening
of the shelfbreak jet is likely to lead to a reduction in nutrient
availability and a subsequent decline in productivity on the Scotian
Shelf, Gulf of St. Lawrence, and Grand Banks.