Abstract
Decadal scale time series of the shelf seas are important for understanding both climate and process studies. Despite numerous investigations of long-term temperature variability in the shelf seas, studies of salinity variability are few. Salt is a more conservative tracer than temperature in shallow seas, and it can reveal changes in local hydrographic conditions as well as transmitted basin-scale changes. Here, new inter-annual salinity time series on the northwest European shelf are developed and a 13 year high resolution salinity record from a coastal mooring in western Scotland is presented and analysed. We find strong temporal variability in coastal salinity on timescales ranging from tidal to inter-annual, with the magnitude of variability greatest during winter months. There is little seasonality and no significant decadal trend in the coastal time series of salinity. We propose 4 hydrographic states to explain salinity variance in the shelf area west of Scotland based on the interaction between a baroclinic coastal current and wind-forced barotropic flow: while wind forcing is important, we find that changes in the buoyancy-driven flow are more likely to influence long-term salinity observations. We calculate that during prevailing westerly wind conditions, surface waters in the Sea of the Hebrides receive a mix of 62% Atlantic origin water to 38% coastal sources. This contrasts with easterly wind conditions, during which the mix is 6% Atlantic to 94% coastal sources on average. This ‘switching’ between hydrographic states is expected to impact nutrient transport and therefore modify the level of primary productivity on the shelf. This strong local variability in salinity is roughly an order of magnitude greater than changes in the adjacent ocean basin, and we infer from this that Scottish coastal waters are likely to be resilient to decadal changes in ocean climate.
Highlights
The exchange of water between the ocean and the shelf, and the behaviour of water on the shelf is important to numerous fields of study: while they account for only 0.5% of the oceans by volume, shallow seas are disproportionately important for the growth of phytoplankton and are thought to contribute around 16% of the primary production in the oceans (Chen et al, 2013; Jahnke, 2010; Simpson and Sharples, 2012)
Two new data products are constructed from historical observations in the region; a year time series of salinity in Tiree Passage and the same duration of surface salinity transects between Tiree Passage and the shelf edge
This work complements the analysis of temperature and current meter data obtained from the Tiree Passage Mooring (TPM) by Inall et al (2009) and adds to the small network of coastal time series on the European shelf capable of resolving climatic cycles and trends (Allen et al, 1998; Holt et al, 2012; Laane et al, 1996)
Summary
The exchange of water between the ocean and the shelf, and the behaviour of water on the shelf is important to numerous fields of study: while they account for only 0.5% of the oceans by volume, shallow seas are disproportionately important for the growth of phytoplankton and are thought to contribute around 16% of the primary production in the oceans (Chen et al, 2013; Jahnke, 2010; Simpson and Sharples, 2012). Studies of the freshwater plumes generated by riverine discharge onto the (saline) continental shelf are numerous, notable examples being Chesapeake Bay (Lentz et al, 2006), the continental shelf off Delaware Bay (Münchow and Garvine, 1993b; Wong and Munchow, 1995), the Louisiana coastal current (Wiseman et al, 1997) and the Rhine outflow into the North Sea (Simpson et al, 1993; Souza and Simpson, 1997) These regions all feature a coastal current driven by the differing density between the saline shelf water and the freshwater outflow. A somewhat different system emerges on shelves featuring high freshwater input, such as those bordering fjordic or high latitude coastlines where the sources of runoff merge into a large, coherent coastal current In these cases, the buoyancy-driven flow is not significantly impacted by wind events, most likely due to the short timescales typical of wind forcing (and the currents generated) not being long enough for the overall spatial extent of the flow to be impacted. This work complements the analyses of coastal current physics by investigating their implications over inter-annual time scales
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