Offshore

Figure 1. Track from RV Callsta’s onboard computer showing sampling stations.

Physical Findings…

Figure 2 shows that all stations temperature varies over 1°C near the surface and this variability decreases with depth, with the only exception being C42 and C44. At these stations there is a markedly warmer sublayer. Station C42 was nearer the coast and sat above the continental shelf where the water depth was shallower. Station C44 was also further shoreward in the shelter of Bigbury Bay where the depth shoaled somewhat onto the continental shelf. This shallower depth at these two locations may explain why there was overall warmer temperatures below the thermocline at this depth as surface heating is being distributed over a shallower depth. A thermocline is present that lies at a depth of around 22m. Additionally, figure 2 shows how the general surface temperature increases at each station. Figure 3 shows the data from the time series stations C45-C47 of temperature and clearly shows that the observed water temperature increased with time coinciding with increasing light intensity throughout the day and wind conditions that became fairer. It is also possible to see a deepening of the thermocline with respect to time at this location. There was also some warming of the sublayer below the thermocline between 12:30 and 13:29 UTC which wasn’t so prominent as between 13:29 and 14:30 UTC.

 TEMPERATURE

Salinity remains mostly constant with depth with only minor fluctuations enhanced in figure ? given that the largest range in salinity was around 0.2. There seems to be a large spike in salinity at C43 around the thermocline but this may be an issue with the salinity read out due to temperature effects around the thermocline.

 SALINITY

The density profiles very closely match the inverse of temperature and therefore suggests that temperature is dominating density at the offshore sites.

 DENSITY

Time of transect 1428 – 1445 (for location see map in background section of ship track heading northwards towards Plymouth)

The following ADCP shows the return journey from the L4 station where right is north and left is south on the ADCP transect. As such, time starts from the left and proceeds to the right, as during the time the transect was recorded the boat was navigating back towards Plymouth from L4. This profile shows how the west to east tidal flow is changing with respect to depth. The ADCP plot clearly shows that faster tidal currents are noticeable at and above the thermocline located around 22m and the currents typically weakest at depth. Additionally, the strong tidal currents near the surface varies with depth along the transect.

 TIDAL FLOW VELOCITIES

Figure 10. Plot to show temperature with depth for the offshore stations, sampled by RV Callista on 10/07/18.

Figure 11.Time series of temperature at the L4 station. Measurements taken through the water column by CTD on the RV Callista- 10/07/18.

Figure 12. Averaged Salinity (PSU) against Depth (m), for offshore stations C42-C47, as sampled by RV Callista on 10/07/18.

Figure 14.CTD density profiles adjusted using a moving average and plotted against depth for offshore stations C42-C47 sampled by RV Callista on 10/07/18.

Figure 13.  Ship track of RV Callista on 10.07.18

Figure 15. ADCP from Callista on 10.07.18