NZ SOLAS (Ocean–Atmosphere Interactions)

 

This research aims to provide better predictions of changes in the ocean and climate system, particularly the way in which the ocean around New Zealand regulates greenhouse gases and clouds.

The problem

The world’s oceans absorb and release different gases which can have a major influence on the climate. Not enough is know about these processes to predict how the ocean will respond to and alter the path of climate change in the future. We know that the oceans around New Zealand

  • take up excess CO2 emitted to the atmosphere by the burning of fossil fuels and clearing of forests. We do not know the amount of CO2 uptake accurately and more importantly we are not sure whether this uptake will increase or decrease in future.
  • release sulphur gases which are transformed in the atmosphere to small particles known as cloud condensation nuclei (CCN) which are required for cloud formation. The rates at which the ocean produces sulphur gases and the processes which lead to the formation of clouds are not well enough understood for us to be sure whether the amount or character of clouds will change as the planet warms up.
  • have biomass which is limited by iron availability in sub-Antarctic waters and nitrogen availability in sub-tropical waters. We need to know how these limiting nutrients are supplied to these systems, and how variation in their delivery as a result of climate change will influence regional productivity.
  • are dynamic and represent important regions for trace gas exchange. We need to understand and model the physical and dynamic processes that determine this exchange.

Research in these areas is needed to reduce uncertainty in predicting future states of the atmosphere, ocean and climate system, and to provide an improved scientific basis for managing human impacts on climate.

The solution

Study the dynamic of ocean atmosphere exchange.

The transfer rate of most gases between the ocean and the atmosphere is controlled by processes just beneath the surface of the water. When this region is highly turbulent, gases can be more rapidly transferred across the surface. The turbulence is in turn controlled by dynamical factors such as wind speed, sea-state, and wave breaking. In addition, other effects such as bubbles, surfactants and rain can have a significant influence. Of these factors, wind speed is the most easily measured and has often been used to parameterise the transfer process. Not surprisingly, the use of this single parameter results in a good deal of variability of the estimated transfer rates (fluxes). This uncertainty will be particularly large at the high wind speeds typical of the Southern Ocean. This project will improve estimates of gas transfer rates through a better understanding of the underlying physical processes. These results will then be used to improve gas flux estimates and modelling capabilities, helping us to understand the climate and ocean.

Wave breaking is one of the key factors producing near-surface turbulence but also the most difficult to measure. It also disrupts the sea surface and injects air bubbles into the water column. NIWA has developed remote sensing techniques using radar and video to measure the coverage and scale of wave breaking at sea, as well as sea state. These instruments are being used on NIWA's research vessel, RV Tangaroa, and at shore-based sites. The turbulence is measured in two ways: through the velocity fluctuations of the water, and through the effect on the seawater temperature structure. The relationship of the surface parameters and the turbulent mixing that results is being established through experimental and theoretical investigation.

In parallel with this work, techniques are being developed to directly measure the transfer rates of gas, heat and momentum across the surface of the sea. This is more difficult over the ocean than over land since the area you are observing will be in motion and the fluxes are very small. However, the vast expanse of the oceans and their large storage capacity makes their contribution to the global climate system enormous. The challenge is to develop a system which can be applied to a range of gases and that incorporates high accuracy gas analysers.

In order to study the exchange of sulphur between ocean and atmosphere, ocean measurements are being made from the RV Tangaroa in the highly productive marine areas around New Zealand. These show that high levels of dimethyl sulfide (DMS) are associated with large plankton blooms. We have also measured changes in DMS in the remote Southern Ocean which were stimulated by addition of iron as a micronutrient to the ocean during an international project run by NIWA.

Atmospheric measurements are carried out at the Baring Head clean air station near Wellington to determine variations in sulphate aerosol and relate these to atmospheric chemistry. We have developed a computer model of the large number of chemical reactions involved and used this to assess the role of different oxidants.

To quantify the potential climatic impact of DMS, it is important to be able to distinguish between DMS conversion to sulphate adding to existing particles and conversion that forms new particles. We are one of very few groups able to make this distinction by using sulphur isotopes (heavy and light versions of the sulphur atom). This technique relies on the fact that formation of new particles or accumulation on existing particles affects the ratio of heavy to light sulphur atoms differently.

Find out more in 'Ocean-Atmosphere Interactions'

The result

International SOLAS group 

Page last updated: 
22 February 2014
Many gases are transmitted across the ocean surface.