Inter-annual variability of upwelling off the South-Vietnamese coast and its relation to nutrient dynamics

Abstract

It is the general consensus that the South China Sea (SCS) is influenced by the El Niño southern Oscillation (ENSO). Off southern Vietnam, coastal upwelling is one of the major features of the SCS, which is consequently influenced by the ENSO. However, the processes behind this influence are under discussion. It is the general thesis that the inter-annual changes of the monsoon wind cause the changes of the strength of the upwelling. This thesis has been tested and it is discarded. Instead it can be shown that the wind stress is not the main cause for the impact of the ENSO. Therefore, also nutrient dynamics must be completely re-evaluated.

This study investigates in the circulation and the hydrographical situation during the summer 2003 and 2004, an El Niño and a normal year, respectively. The influence of the circulation on the new biological production based on vertical nutrient fluxes is estimated. Numerical modelling is used to study the circulation inside the SCS. The results of the simulation discover two different states of the horizontal circulation: During the first state an anticyclonic circulation dominates the region. During the second state the cyclonic circulation in the north and the anticyclonic circulation in the south build the dipole. Two cruises during the summer 2003 and summer 2004 verify the two states of the simulated circulations. The ENSO influence on the summer circulation is by tending towards the first state in the summer after an El Niño, while during normal summers the second state is dominant.

Against earlier findings, this study features one new major key of the ENSO influence on the temperature distribution and primary production in the western SCS: This influence is not via the vertical velocities, but due to the different horizontal circulations. In the dipole circulation pattern, the decreased horizontal velocities cause longer residence times which come along with the increase in the efficiency of the upwelling. In this study, for the first time, the advection of the pronounced different water masses of the northern and southern SCS is considered. The different water masses support an important preconditioning of the nutrient concentrations in the water column. An experiment, which compares primary production with and without this preconditioning proves that the originating water masses moving into the upwelling region control the primary production.

Horizontal advection explains the substantial contributions of the strong ENSO signal in the region. Not the regional Ekman drift currents, but the basin wide horizontal circulation explains the inter-annual changes of the vertical nutrient uplift during the summer. It can be shown that the dipole circulation is an almost perfect nutrient pump for new production.