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Offshore Water Column

Offshore water masses experience low levels of wind and currents and higher levels of solar radiation during summer months. The increased solar input along with the variations in wind lead to stratification in the water column between the surface waters and deeper water (Rippeth et al., 2002). During the daily time series, the water column encountered increasing solar input leading to a greater buoyancy of the warmer water as the day progressed. This led to greater stratification in the water column. The temperature salinity depth profiles exhibit the general warming of the water over the time period recorded taken at a station of coordinates 50°15.938’ N, 004°13.068 W near the L4 coastal buoy where the western channel observatory is located.

A two-step rapid change in temperature can be seen from 10m to 20m and from around 22m to 35m confirming the stratification of the column. As the day progressed the water column became partially mixed shown by the flattening of the rapid changes in temperature exhibited (Figure 1-6). This would be caused by the increase in solar input over the day being distributed throughout the 35m of water. The temperature changes over the different depths during the day (Figure 7). There is a clear increase in temperature in the top 20m of the water column from around 12:00 UTC(+1) to 13:00 UTC(+1). During the day the surface waters cool from 16.3°C to 15.5°C from 10:03 UTC(+1) to 11:18 UTC(+1) then the temperature slowly increases during the day back to 16.3°C at 14:58 UTC(+1). The continuous heating during the day and lower levels of wind produce a very small mixed layer only across the top 2m of the water column. This would be due to the increased buoyancy instilled by the warm waters leading to a smaller chance of a mixed layer developing as more force would be required to overcome the buoyancy of the warmer water. The absence of any meaningful mixed layer leads to a rapid drop in temperature just below the surface from 2m to 13m shown, this is the thermocline (Figure 1) (Van Haren et al., 1999). The thermocline spans from 16.3°C to around 14.5°C at 13m. During the day the thermocline deepens from between 2m to 13m to 20m. As the day progresses due to the increased levels of solar radiation warming of the water column occurs. This can be seen as there is a 0.3°C rise at 20m depth (Figures 2-6). A rapid change in temperature occurs at 38m until 42m (Figures 3-5). This can be attributed to the area at which the limit of the warming of the water column comes into contact with colder bottom water.

Salinity remains relatively constant during the day at around 35.3 this is to be expected due L4 being far from any freshwater input and the lack of rainfall on previous days that could influence the salinity.

References

Rippeth, T.P., Palmer, M.R., Simpson, J.H., Fisher, N.R. and Sharples, J., 2005. Thermocline mixing in summer stratified continental shelf seas. Geophysical Research Letters, 32(5).

van Haren, H., Maas, L., Zimmerman, J.T.F., Ridderinkhof, H. and Malschaert, H., 1999. Strong inertial currents and marginal internal wave stability in the central North Sea. Geophysical Research Letters, 26(19), pp.2993-2996.

ADCP Analysis

Throughout the day sampling at the L4 buoy, the ADCP recorded flow magnitude and direction.  From the ADCP data, flow was found to be in a southerly direction with slight variations between SSE and SSW. The flow had an average velocity of approximately 400mm/s.

The ADCP failed to superimpose the seabed depth on top of the recorded flow velocity, causing readings below the maximum depth of the water column.

On the 8th of July 2019 high tide was at 10:55 UTC(+1) and low tide was at 17:01 UTC(+1).  With the initial ADCP samples recorded from 10:03 UTC(+1) and the final samples taken at 14:58 (UTC+1), the tide was falling throughout the course of the experiment, bar the first 50 minutes when the incoming flow was slowing down before the tide changed.

There was minimal wind on the 8th of July 2019 meaning there was very little transfer of momentum from the atmosphere to the water, reducing the amount of wind input on the surface current.

The ADCP recordings at every hour showed a homogeneous flow in both magnitude and direction throughout the water column.