Dynamics of ocean atmosphere exchange
The transfer rate of most gases between the atmosphere and ocean is controlled by processes just beneath the water surface.
When this region is highly turbulent, gases can be more rapidly transferred toward or away from 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 fluxes. This uncertainty will be particularly large at the high wind speeds typical of the Southern Ocean. This objective seeks to 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.
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 turbulent water velocity fluctuations, and through the effect on the seawater temperature structure. The relationship of the surface parameters and the resulting turbulent mixing is being established through process study experiments.
In parallel with this work, techniques are being developed to directly measure the transfer rates (or fluxes) of gas, heat and momentum on the atmospheric side of the sea surface. This is more difficult over the ocean than over land since at sea the observing platform is 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 important. The challenge is to develop a system which can be applied to a range of gases and which incorporates high accuracy gas analysers.
In 1987, Charlson, Lovelock, Andreae and Watson published a paper proposing what became known as the CLAW (from the initial letter of each author's name) hypothesis. The hypothesis was that phytoplankton in the oceans produce a gas dimethylsulfide (DMS) which escapes from the ocean and undergoes a series of transformations in the atmosphere to form small sulfate particles. These sulfate particles then act as cloud condensation nuclei (CCN) allowing water to condense on their surfaces creating clouds which reflect the suns radiation and cool the surface. So tiny marine organisms may be able to regulate climate through their emission of DMS.
New Zealand is an excellent place to study biogenic sulfate from the ocean because there is a much smaller industrial pollution background than there is in the northern hemisphere. NIWA's study of the CLAW hypothesis in the New Zealand region focuses on two fronts. First, we study the biological factors governing DMS production and, second, we study the atmospheric processes that connect DMS with clouds. Both require a combination of observational measurements and modelling work
Ocean measurements are being made from the RV Tangaroa in the highly productive marine areas around New Zealand and show that high levels of 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 sulfate 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 sulfate adding to existing particles and conversion that forms new particles. We are one of very few groups able to make this distinction by using sulfur isotopes (heavy and light versions of the sulfur atom). This technique relies on the fact that formation of new particles or accumulation on existing particles affects the ratio of heavy to light sulfur atoms differently.
Carbon dioxide (CO2) is a soluble gas which dissolves in the oceans and is taken up by marine plants (phytoplankton). A natural cycle results in which CO2 is absorbed from the atmosphere in some (generally cooler and more biologically active) parts of the ocean and released back to the atmosphere in other (generally warmer and less biologically active) parts.
This natural cycle has been modified through the addition of CO2 to the atmosphere by human activities. Increasing CO2 concentrations in the atmosphere tend to increase the amount dissolved in the surface ocean. Currently about 29 billion (thousand million) tonnes of CO2 are being added to the atmosphere each year due to fossil fuel burning and deforestation and the oceans are removing about 7 billion tonnes. A similar amount is removed due to increases in plant biomass and soil carbon. Predicting how the net ocean uptake will change in the future is critical to understanding how atmospheric CO2 concentrations will change in future and so to estimating long term climate change.
Our measurements have shown that large areas of the oceans around New Zealand remove CO2 from the atmosphere. Work is continuing to determine the patterns of this CO2 uptake, its magnitude, and the factors controlling spatial and temporal variability.
Large-scale spatial variability is being investigated through open ocean voyages to different water bodies such as subantarctic waters to the east and south of NZ, subtropical waters to the north, and the subtropical frontal area over the Chatham Rise. These studies are complemented by a regular series of measurements on voyages to the east of the South Island at approximately two-monthly intervals. This time series provides an understanding of the seasonal cycle and inter-annual variability of CO2 in subantarctic waters, and has shown a seasonal cycle in uptake which appears to be strongly influenced by the growth of phytoplankton.
Development of techniques capable of high precision measurements is an integral part of the programme. Underway measurements from a small boat in a highly variable frontal/coastal marine system requires specialised equipment. Equipment for more accurate underway measurement of pCO2 and pH is being developed, and compared with overseas groups making similar measurements.