Work Package 1

Upwelling variability

Involved Scientists

  • Dr. Volker Mohrholz (IOW)
  • Dr. Mohammed Hadi Bordbar (IOW)
  • Dr. Anja van der Plas (NatMIRC, Namibia)
  • Dr. Deon Louw (NatMIRC, Namibia)

Description

At present, it is not clear whether upwelling systems act as net sources or sinks for CO2. The Benguela upwelling system (BUS) can act as both a source and a sink depending on the season and upwelling intensity (Schmidt et al. 2016). In the northern BUS, upwelling strength displays a moderate seasonal cycle overlaid by intense short term upwelling pulses. Consequently, primary production (Louw et al, 2016) and oxygen supply have a pulse-like character. On time scales of days to months, the upwelling process itself is driven by the local wind forcing. The remote forcing controls the water mass distribution and sets the preconditions for nutrient supply and oxygen concentrations. The drivers of remote forcing are still a topic of ongoing research. During recent years, a number of interdisciplinary projects (GENUS, SACUS, PREFACE) investigated the upwelling dynamics in the Northern Benguela, focusing mainly on seasonal to interannual variability. However, the short term variability of upwelling plays an important role for the retention time and stabilization of upwelled nutrient-rich water masses in the euphotic layer, and thus for the efficiency of primary production and atmosphere-ocean gas exchange.

 

General Questions and Research topics:

  • What impact has the short term upwelling variability on oxygen and nutrient supply to the shelf and how does primary production respond to intense short term peaks in upwelling?

To answer this question, high resolution time series measurements are required. It is planned to extend the existing time series measurements at the long term mooring off Walvis Bay (23°S) and to equip the mooring with additional sensors for near surface chlorophyll-a concentration. A second mooring will be deployed near 25°S at the southern edge of the oxygen minimum zone where oxic and anoxic conditions alternate. The time series will supply concurrent collected information about the temporal variability of currents, water mass properties, oxygen supply and chlorophyll-a concentration to better link the occurrence of phytoplankton blooms to physical forcing events and changes in water mass properties. The time series data will be related to the cross shelf distribution of these parameters gathered during the planned field experiments along two transects at 23°S and 25°S. Both transects are also sampled up to 5 times per year within the framework of the Namibian environmental monitoring program.