Variability of the Mozambique Channel Throughflow

Abstract

The Mozambique Channel is an important link in the thermohaline circulation, as the variability in its throughflow affects the exchange between the Indian and South Atlantic Ocean south of Africa. In this dissertation, we study the variability of the Mozambique Channel throughflow in the range from eddy to interannual time scales and investigate the origin of this variability. A 4.2-year time series obtained by a mooring array in the Mozambique Channel at 17S was the main object of study, together with output from (Ocean General Circulation) models and satellite observations. Over the length of the observational time series, the mean volume transport 16.7 Sv southward, with daily values ranging between 45 Sv northward and 65 Sv southward. The throughflow is highly variable on a large range of time scales. Interannual variability was observed both in the volume transport and subsurface salinity in the Mozambique Channel. The amplitude of these variations was large, in the order of 9 Sv for the transport time series and about 0.2 PSU for a salinity anomaly. The interannual variability of the transport time series has a dominant period of two years. This signal is related to the Indian Ocean Dipole index and is transported via the South Equatorial Current. A negative phase of the dipole induces an increase of the southward transport in the Mozambique Channel with a lag of roughly one year. The salinity anomaly in the years 2000 – 2001 is related to the weakening of the northern part of the South Equatorial Current. This results in a reduced inflow of tropical, relatively fresh waters. The seasonal cycle of the transport has an amplitude of about 5 Sv and originates from upstream variability in the wind forcing west of the Mascarene Ridge. In the observations, this signal is overshadowed by variability at other frequencies. In Ocean General Circulation Models on the other hand, this frequency dominates the throughflow, as these models underrepresent variability at other, especially higher, frequencies. Variability at the eddy time scale is very strong in the observed volume transport time series. This is due to the formation of Mozambique Channel eddies around the location of the mooring section. The alternation between a strong southward current and eddies formed out of this current is causing the strong variability. In eddy resolving Ocean General Circulation Models, eddies are formed further upstream and therefore no alternation takes place at the mooring section. Nevertheless, the velocity structure of eddies in these models is quite well simulated. Using relatively simple models, it is shown that the estimated transport through the channel is very sensitive to the model setup and the simulation of barotropic and / or baroclinic variability. Rectification of the mean flow by this variability was strongest along the north and south boundary of the island. Also, the amount of rectification was sensitive to the basin setup.