10
Summary &
Outlook
temperature and salinity, reproduced and observed, respectively.
In the fifth chapter, the six typical states of the horizontal circulation are presented for several depths. The states highlight the change of the circulation due to the changing monsoon wind, as well as some features which occur independent from the wind forcing.
The main characteristic is the appearance of a southward and a northward boundary current . The pathways of the boundary currents are strongly steered by the bathymetry. This yields to a complex mesoscale current system, with different meandering of the currents in different depths. Both boundary currents can exist at the same time during late summer.
Together they build a confluence zone and form an offshore jet.
The vertical velocities in the VUA, according to the states of the horizontal circulation, are described. Upwelling was found at pronounced regions also during northeast monsoon and inter-monsoon. Significant positive vertical velocities can be found not only near the coast, but at two regions they spread cross the shelf. The magnitude of the vertical velocities of O(10 m day-1) appears to be strong compared with other regions in the SCS, but the velocities cover the same range as off Yucatan.
Chapter 6 shows the hydrographic results of the ship experiments. The observations are set in relation to the vertical velocities of the model experiment. A complex structure in the distribution of temperature and salinity had been found. The two summer monsoon cruises give the feasibility to compare the observed hydrography due to different states of the circulation. Indications of upwelling can be found during the inter-monsoon cruises as well as in winter. The effectiveness of the upwelling on the primary production is documented by the observed data of the fluorescence. They identifies that the upwelling during summer as well as during spring and winter influence the primary production in a positive way.
In chapter 7, the processes, which explain the vertical velocities in two dimensions, hence neglecting the alongshore direction, are deducted. A spin-up experiment discovers that the wind driven upwelling, due to offshore movement of the surface layer, which will be compensated through the geostrophic interior, vanishes after three days. After this period the offshore flow is balanced by an onshore flow of the BBL. With the onset of the basin-wide circulation the upwelling due to the BBL is the most important process. On the shallow Sunda Shelf, where both boundary layers overlap, the equilibrium is achieved on the first day. The relation between the two dimensional processes and the results of the reconstruction of the vertical velocities of the model illustrates, that the two dimensional processes can explain the upwelling in the north of the VUA, where the bathymetry do not vary in the alongshore direction. However, in the center of the upwelling, where the shelf changes its depth, there is no correlation.
Chapter 8 deals with the three-dimensional processes. Due to the high Froude number of the southward boundary current, the inertial forces can become more important than the gravitational forces. Thus, if the shelf changes its depth a flow across the isobathes is possible. This causes a squashing of the water column, which results due to the conservation of the potential vorticity in an anticyclonic rotation. Both, the inertial forces and the anticyclonic rotation dispose the current into shallower water. With the assumption that the flow is aligned with the sloping bottom, upwelling is established. This process can occur independent of the local wind forcing.
A model experiment without the Mekong river illustrates, that during summer the stronger stratification due to the river discharge can decrease the vertical velocities.
The inter-annual variability and the correlation with the ENSO is pointed out in chapter 9, by using satellite based chlorophyll a concentrations. Increased concentrations can be found during winter and summer. Moreover, the magnitude of the concentration both during summer and winter is correlated with the ENSO status during winter. This means that an El Nino during winter, lowers the upwelling during winter and summer.
According to the research goals, defined in the introduction, the seasonal variability, and the influence of the monsoon on the variability of the coastal processes are studied.
Summarizing, upwelling took place independent from the direction of the monsoon and can be found during summer, spring and winter.
In this work, the main governing processes are identified: The classical two dimensional processes can not explain the upwelling at all. Upwelling is forced mainly by the western boundary currents. Hence, the interaction of the boundary currents with the bathymetry is the main physical process, that is that 40 % - 100 % of the upwelling is forced by the basin-wide circulation and not by the direct influence of the local wind.
The stratification of the water column is the second important factor, which influences the strength of the upwelling. The discharge of the Mekong increases the stratification and with that the discharge decreases the vertical velocities.
The third goal was the inter-annual variability of the coastal activity. Therefore the relation between the ENSO and the upwelling was studied. A correlation of the ENSO with the upwelling, represented by the chlorophyll concentration was found. Both, the concentration during winter and the concentration during summer are correlated with the ENSO during winter.