University of Southampton OES Undergraduate Falmouth Field Course 2016 - Group 3 databank and initial findings.
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The ADCP data we collected was mostly uniform and showed currents between 0.0m/s and 0.5m/s. However, between stations 33 and 34 a narrow band down to approximately 5.0m depth showed increased velocities. The maximum was 1.3m/s, at 50˚ 5.16’ N, and 4˚ 51.47’ W. This was probably due to an internal, breaking wave caused by density differences at the thermocline. This can be compared to the thermocline depth which was between 5.0m and 10m at these stations.
The instrument was unable to collect data from a large area below 35m due to a lack of particles. The ADCP works by sending sonar waves out and measuring how much is received back. When few particles are present at a depth relatively far away from the instrument, not enough backscatter is received to determine a current velocity.
Offshore Physical Data - CTD
Temperature
For each station temperature decreases with depth. Station 33 has the greatest difference between surface and maximum depth sampled, with a decline of 3.32°C and the thermocline present around 10m depth. On the other hand, Station 38 has the smallest difference (i.e. 0.69 °C) between surface and maximum depth temperatures. Stations 36 and 37 presented similar temperatures values, with a thermocline present at around 3m - 7m and comparable values from surface to depth. Similar values were also found at Stations 34 and 35, with thermocline around 3m – 5m.
Salinity
For each station salinity increases with depth. Station 33 has the greatest difference between surface and maximum depth sampled, with an increase by 0.38 psu, and halocline around 10m-15m. Station 38 has the smallest difference between surface and bottom layer measured (i.e. 0.02 psu), demonstrating constant salinity values through depth, and therefore, no presence of halocline. For all stations, the change in salinity is minimal, with the range of data lying between 34.6 and 35.6.
Offshore Physical Data - ADCP
The Stratification Index shows the stability of the water column and based on this parameter, the position of tidal fronts can be determined. S = log10(H/U3), where H = water depth and U = strength of tidal currents (Simpson and Sharples, 2012). Stations 33 and 37 presented strong stratification (Table 1), since strong stratification is related with values of S > 3.0, while low values (i.e. < 1.5) correspond to vertical mixing (Simpson and Sharples, 2012). Values in the transition between the regimes represents where the tidal mixing fronts occurs (Simpson and Sharples, 2012). Therefore, we can affirm that Stations 34, 35, 36 and 38 (Table 1) were located close to the tidal mixing front, providing optimum conditions for primary production.
The values obtained for Richardson Number (Ri) showed that in most of the stations, stratification could be observed. For Ri > 1, laminar flow is observed (Stations 33, 34, 36) (Table 1), implying higher influence of the buoyancy frequency (i.e. higher stratification) than the shear stress. This influence was observed especially at Station 33, what agrees with the calculation of S, indicating more stratification and less mixing for the station. For Ri < 0.25, mixing predominates, since there is a strong shear stress. Station 38 (Table 1) presented this behaviour, since the station is located in shallow waters, being affected by the mixing generated by friction of tidal currents with the seabed. Stations 37 and 35 (Table 1) presented transition between stability and mixing. The calculation of Ri for Station 37 could be inaccurate, since the ADCP data was available only for the 8m upper layer of the 62 m depth of the station.
Offshore Physical Data - Ri Number
The profiles made from the data collected at Station 33 show there to be strong stratification through the water column, supported by the Richardson number calculated giving a result of 2.17 (3s.f.). This is the greatest result of any of the stations sampled and shows that the flow of water is stable and laminar and no mixing occurs. The stratification index for this site (3.237) supports what is shown by the data and indicates a stratified water column [Simpson and Sharples, 2012]. A thermocline is present at approximately 15m depth and this station displays the greatest change in temperature from the surface to the maximum depth sampled
Station 34 was positioned on the seaward side of a front boundary within the coastal water body. At the front boundary two bodies of water (one cool and one warm) are meeting and the mixing shown to be present at Station 34 identifies it as being in close proximity to the front. The temperature and salinity profiles show these parameters to be uniform with depth, supporting the idea that mixing was present at this station. The Stratification Index for this Station (2.807) is between 1.5 and 3.0 and indicates that there is a tidal mixing front present [Simpson and Sharples, 2012].
Station 37 presented stratification of the water column (i.e. S = 3.16), with a thermocline around 15m to 20m depth. At the thermocline, there was an increase in chlorophyll that generated nutrient depletion around 20m to 30m depth.
On the other hand, salinity and temperature profile could be taken. Both these profiles presented a small variation in values of salinity and temperature from surface to depth, which represents a well-mixed water column. The calculation of Ri number and Stratification index also supported this mixing pattern (i.e. Ri = 0.00 and S = 2.46, respectively), since the station was close to the tidal front and produced strong shear stress. In addition, the shallower water contributes to the mixing character, due to increased friction at the seabed (Simpson and Sharples, 2012).
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Offshore Physical Data - Discussion
References
Simpson, J. H. and Sharples, J.: Introduction to the Physical and Biological Oceanography of Shelf Seas, Cambridge University Press, Cambridge, UK, 2012.