This blog comprises two scientific cruises with the German research vessel SONNE in the Indian Ocean.
The tropical Indian Ocean is a diverse area that exerts important influence on atmospheric and oceanic processes. It is unlike the other ocean basins, as it has a northern land boundary. In addition there are only tropical zones in the Indian Ocean, which prevents the typical “conveyor belt” water transport found in the other ocean basins (e.g ventilation, deep water formation). Some specific features that make the Indian Ocean unique are the Indonesian Throughflow (exchange with Pacific Ocean), the South West Monsoon, the North East Monsoon, Wyrtki Jets, the Indian Ocean dipole, and the Madden-Julian Oscillation. The Indian Ocean is also more rapidly warming than the other ocean basins and is impacted more acutely by the rising populations of the surrounding countries, resulting in more damage to important fish species and higher rates of eutrophication. The dust and aerosol transport to the region is high and the Indian Ocean contains one of only three open ocean oxygen minimum zones (OMZ).
The study program onboard the SPACES cruise SO234-2 (08 July – 20 July 2014) is designed to teach the students about “Air-Sea Interactions in the western Indian Ocean”. The western tropical Indian Ocean experiences the Somali current, which reverses with the different monsoon systems, the Agulhas current, open ocean denitrification and high rates of nitrogen fixation in the Arabian Sea, and a large area in the western equatorial region of high carbon dioxide drawdown. All of these phenomena make the tropical Indian Ocean an excellent laboratory and classroom, allowing special focus on anthropogenic effects and global climate change on ocean biogeochemical and physical processes. The study onboard will be carried out by pairs of students, one from an African country and one from Germany. The pairs will rotate through 5 different stations: 1) meteorology, involving radiosonde launches, 2) atmospheric physics, including spectrophotometric determination of reactive trace gases in the atmosphere 3) physical oceanography, involving deep ocean profiles (CTD casts), 4) biogeochemical cycling, involving gas chromatographic and mass spectrometric analysis of seawater samples; 5) atmospheric chemistry, involving air sampling of more than 50 trace gases, direct trace gas air-sea flux measurements.
SO234-2 is directly followed by the cruise SO235 OASIS (Organic very short lived substances and their Air sea Exchange from the Indian Ocean to the Stratosphere, 23 July – 7 August 2014). It will focus on surface ocean trace gas cycling, flux to the atmosphere, and transport to upper troposphere and lower stratosphere. The gases that will be measured are mainly brominated and iodinated halocarbon compounds and dimethylsulphide, both in the ocean and in the atmosphere. CO2, N2O, and CH4 will also be measured. The Indian Ocean, with high influx of nutrients from land and special current and eddy systems, supports phytoplankton blooms that generate trace gases in the surface ocean. In addition, the western Indian Ocean experiences areas of high wind speed, related to the monsoon systems there. Both the high ocean production and wind speed events enhance air-sea exchange. The convective area above India/ Bay of Bengal is instrumental for transporting ocean-derived trace gases to the upper troposphere/lower stratosphere.
Chiefscientist on both cruises is Prof. Dr. Kirstin Krüger (Department of Geosciences, University of Oslo).
Further information about the expedition you find on the expedition website of GEOMAR.