Liu, Mian

English

Mauritanian upwelling region is locate d in subtropical Atlantic, off the northwest African shore. The parameters of seawater (e.g. temperature, salinity and oxygen) have certain impacts on the biological processes and the fisheries production Tracer gases (CFC 12 and SF6) are used to show the mean age and distribution of water masses and then to measure the ventilation from upwelling and currents near the Mauritanian coast. In this thesis, I report the measurements of tracer gases from three cruises (different years and seasons) along the latitude of 18°N. The analysis shows the following results:

Seasonal variation of sea surface temperature ( SST and sea surface salinity (SSS is directly influenced by solar radiation and wind stress. Salinity, whose distribution is similar with temperature, is highest in summer but lowest in spring instead of winter due to the precipitation. Oxygen concentration in seawater is a result of both physical and biological processes:

1) Solubility, which decreases with temperature, is highest in winter and lowest in summer.

2) Upwelling, which is strong in winter and early spring, can bring water from oxygen minimum zone (OMZ) to the surface.

3) Ventilation process depends on currents, which is strong in winter.

4) Apparent oxygen utilization (AOU under the euphotic layer is low in spring.

As a final result of all the above reasons, the oxygen concentration is highest in winter and lowest in summer. Transit time distribution (TTD) of tracer gases is used to show the distribution and movement of water masses. A value of (Δ/Γ) = 1.4 is chosen to show the mixing rate in the ocean. Since concentration and partial pressure of tracer gases have disadvantages to show the age of water masses, saturation of tracer gases in the seawater are used to calculate the mean age. According to the mean age and direction of water movement at the upper layer, this area can be divided into two parts by the longitude of 17°W 

1) The upwelling has impacts on the near coastal region, where the mean age is relative old at the upper layer (east of 17°W). During the upwelling season (spring and winter), water with old mean age is transported from deeper layer to the surface. The upwelling is strong in winter and early spring because the monsoon blows from land to sea and transports the surface water off the coast. As a result, temperature and salinity at surface is relative lower compared with the open ocean. Oxygen concentration is also influenced by the upwelling. Water from OMZ (200 to 500m depth) is transported upwards, so water samples with low oxygen in the upper layer can be found near the coast.

2) The currents (such as NECC and NEUC) influence the open ocean area, where the mean age is relative young at the upper layer (west of 17°W). Stratification is relative clear and the vertical water exchange in this region is not so obvious as the coastal region. Water from SEC and NBC is transported eastward by trade wind and forms the NECC and NEUC, which ventilate the subtropical North Atlantic. The currents are strongest in winter due to the trade wind. Temperature and salinity decrease with depth. Oxygen concentration is relative lower in the east because the NECC and NEUC with oxygen rich water come from the west

OceanRep

Evaluation of transiert tracer measurements from three cruises to the Mauritanian upwelling region: estimation of transit time distribution and ventilation

Evaluation of transiert tracer measurements from three cruises to the Mauritanian upwelling region: estimation of transit time distribution and ventilation

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