Storm-driven across-shelf oceanic flows into coastal waters

Abstract

Abstract. The North Atlantic Ocean and northwest European shelf experience intense low-pressure systems during the winter months. The effect of strong winds on shelf circulation and water properties is poorly understood as observations during these episodes are rare, and key flow pathways have been poorly resolved by models up to now. We compare the behaviour of a cross-shelf current in a quiescent period in late summer, with the same current sampled during a stormy period in midwinter, using drogued drifters. Concurrently, high-resolution time series of current speed and salinity from a coastal mooring are analysed. A Lagrangian analysis of modelled particle tracks is used to supplement the observations. Current speeds at 70 m during the summer transit are 10–20 cm s−1, whereas on-shelf flow reaches 60 cm s−1 during the winter storm. The onset of high across-shelf flow is identified in the coastal mooring time series, both as an increase in coastal current speed and as an abrupt increase in salinity from 34.50 to 34.85, which lags the current by 8 d. We interpret this as the wind-driven advection of outer-shelf (near-oceanic) water towards the coastline, which represents a significant change from the coastal water pathways which typically feed the inner shelf. The modelled particle analysis supports this interpretation: particles which terminate in coastal waters are recruited locally during the late summer, but recruitment switches to the outer shelf during the winter storm. We estimate that during intense storm periods, on-shelf transport may be up to 0.48 Sv, but this is near the upper limit of transport based on the multi-year time series of coastal current and salinity. The likelihood of storms capable of producing these effects is much higher during positive North Atlantic Oscillation (NAO) winters.

Observation-based estimates of volume, heat, and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents, and the atmosphere

“…Along the European continent there is outflow south of Ireland and then inflow to the north, perhaps a suggestion of cyclonic circulation over the Porcupine Bank at shallower depths. North of Ireland some outflow is evident, suggesting transport onto the Malin and Hebridean shelf (Jones et al, 2018(Jones et al, , 2020Porter et al, 2018). Between Scotland and Iceland, −10 to −12 Sv of outflow marks the exit of the NAC towards the Iceland-Scotland Ridge.…”

The Impact of Ocean‐Wave Coupling on the Upper Ocean Circulation During Storm Events

“…The term u′(x(t), t) = α R with R ∈ [−1, 1] represents random fluctuations in the velocity field to simulate sub-grid turbulent diffusion. We use α = 0.04 m s −1 which corresponds to a horizontal eddy diffusivity K h of ≈1 m 2 s −1 when modeling sub-grid turbulence as a random walk diffusive process (e.g., De Dominicis et al, 2013), in agreement with S. Jones et al (2020).…”

Observation-based estimates of volume, heat and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents and the atmosphere

“…Along the European continent there is outflow south of Ireland and then inflow to the north, perhaps a suggestion of cyclonic circulation over Goban Spur at shallower depths. North of Ireland some outflow is evident, suggesting transport onto the Malin and Hebridean shelf (Jones et al, 2018;Jones et al, 2020;Porter et al, 2018). Between Scotland and Iceland, -10 to -12 Sv of outflow marks the exit of the NAC over the Wyville Thompson Ridge.…”