Cross-slope flow in the Atlantic Inflow Current driven by the on shelf deflection of a slope current

Abstract We identify a newly described current, the Atlantic Inflow Current (AIC), a persistent pathway of Atlantic Water from the European Slope onto the Malin Shelf. Using drifters and gliders we examine the vertical and horizontal structure of the AIC and use Lagrangian statistics to quantify lateral mixing along its path. We estimate this current to have a transport of approximately 0.2 Sv, advecting 2.8 TW of heat onto the shelf (referenced to 7 °C) and 2.4 kT/day of the limiting nutrient, nitrate. This nutrient-rich AIC joins the Irish Coastal Current, continuing into the Minch and the outer Hebridean Shelf before ultimately entering the North Sea. The biological consequences of the influx of water masses and nutrients onto the shelf can range from altering primary production, and subsequent food web dynamics, to recruitment of fish larvae from oceanic water. A better understanding of the current dynamics described here is crucial for the assessment of shelf sea primary production and its impacts on carbon draw-down as well as recruitment for commercially important fisheries.

The Malin Shelf slope is a section of the European continental slope, which is approximately the current changes along its length, on average increasing poleward, fed by a zonal 26 geostrophic flow from the west (Marsh et al., 2017). 27 Along the slope regions of cross-slope exchange and varying slope current stability 28 (Huthnance et al., 2009)  In the region near to the canyon at 55.5 o N hydrodynamic models show a subsurface poleward 48 current on the outer shelf that turns eastward as the shelf deepens (for example Lynch et al.,49 2004; Xing and Davies, 2001). This modelled current then continues eastward, parallel to the 50 north coast of Ireland and adjacent to the Irish Coastal Current. However, these models do 2 Data, instrumentation and methods 88 During summer 2013, as part of the FASTNEt (Fluxes Across the Sloping Topography of the 89 North East Atlantic) project, we conducted a multi-platform campaign to observe the Slope 90 Current and the Atlantic Inflow Current, to assess their level of interaction as well as the 91 Inflow Current's magnitude, variability and structure. This work was primarily conducted 92 during cruise JC88 on the RRS James Cook in July 2013. As part of the study 30 satellite 93 tracked GPS drifters were released above the slope, upstream of the shallow canyon, while 94 simultaneously three buoyancy driven gliders were conducting cross slope transects. The timings of instrument deployments are summarised in Table 1, with the details provided in 100 the following section.

105
The data collected by the 75 kHz VM-ADCP during JC88 give the velocity of the water 106 down to depths of 800 m beneath the ship's track. During JC88 three cross-slope transects 107 were revisited multiple times, upstream of, downstream of and in the over the shallow slope 108 canyon at 55.5 o N (Jones, 2016). Time means of the velocity transects were computed in 109 conjunction with time mean CTD transects, allowing us to establish whether salinity acts as 110 an effective tracer of the slope current water, as has been seen previously (Porter et al.,111 2016b; Souza et al., 2001) .

112
Due to the complex nature of the tidal velocities on and around the slope, with a significant 113 internal component (Inall et al., 2001), the VM-ADCP data have not been detided. While it 114 has been shown that the along-slope velocity on the Malin Slope can be partly driven or    where ′ = − , with being the low pass filtered velocity component in the zonal 158 direction and the overbar indicates the mean of a local 0.25°x0.25° spatial box (Richardson,159 1983) ′ is defined similarly for the meridional component). The glider profiles were collected in a saw tooth pattern and then interpolated onto a 175 telescopic grid, using a Barnes optimal analysis scheme (Barnes, 1994) with a search radius 176 of 10 km, reflecting the local first baroclinic Rossby radius.

306
The glider transect (Figure 8 a-c), conducted in September, 30 days after the average day of 307 local drifter presence, shows that deep drifters between 55.75°N and 60°N tracked a warm 308 (10-10.5°C), high salinity (>35.6 g/kg) core of East North Atlantic Water (Figure 8).

309
Assuming that the speed of the drifters as they passed through this region (0.25 ms -1 (Figure 6    The impact of this current on nutrient fluxes and its continuation through the Hebrides Shelf 355 and onward are discussed further in the following section.

445
Satellite imagery shows bands of high chlorophyll on the shelf, associated with the AIC 446 (Figure 2), suggesting locally high productivity, either advected from the slope as 447 phytoplankton or as a response to increased nutrient levels. These features are often observed suggesting that this current and its impact on the shelf seas are persistent. On the shelf the recent past conditions and a potential future scenario. Biogeosciences 9, 97-117.