Seasonal to interannual variability of water mass characteristics and currents on the Namibian shelf

- Long-term records of temperature, salinity and currents from the Namibian shelf.
- Seasonal to interannual variability discussed on the base of climatological mean values.
- Alongshore advection demonstrated to be an important driver for the seasonal fluctuation of water masses on the shelf.
- Alongshore transport is closely related to the sea level elevation.
- Extreme warm and cold events in the Benguela system described by means of in situ temperature data.

We present long-term current meter records from the Benguela system together with salinity and temperature observations gathered by a mooring on the Namibian shelf across 13 years (2002-2015). From this unique data set a climatological mean state is estimated enabling us to investigate seasonal to interannual variations of these variables on the Namibian shelf. The present study highlights the importance of the alongshore advection for the water mass characteristics in the Benguela system on a seasonal time scale. The annual cycle of the alongshore transport is characterized by a biannual flow reversal. Poleward directed currents dominate from October to April, and from May to September equatorward currents prevail. In addition, we present observational evidence for a biannual intrusion of tropical waters into the Benguela system with maxima in October and February.Based on the in situ temperature data, several anomalous events are described that affect the whole water column. During the outstanding warm event in austral fall 2011 the monthly temperature anomaly exceeds one Kelvin for five consecutive months peaking in March (2.4 K) in the upper layer of the water column. Our study suggests, that the occurrence of such extreme temperature events in the Benguela upwelling system is closely related to the strength of the alongshore advection in austral summer.