Increase in storm track activity during the second half of the twentieth century, and its potential impacts on atmosphere-ocean CO2 exchange

Recent studies have suggested that the storm track activity, both in terms of cyclone activity and eddy variance and covariance quantities, have increased significantly over the second half of the twentieth century. Both NCEP/NCAR and ERA40 reanalysis data suggest that surface wind speed has increased over the Northern Hemisphere mid and high latitude oceans.

It is well known that carbon dioxide in the atmosphere has been increasing with time due to anthropogenic production. Not all CO2 produced by human activity stays in the atmosphere -- a significant portion is taken up by the ocean via air-sea gas exchange. One of the largest sinks occurs over the mid to high latitude oceans during winter. The rate of gas exchange depends on the difference in pCO2 between the atmosphere and ocean, as well as on the wind speed. Hence changes in surface wind speed could potentially affect the rate of oceanic uptake of CO2. The purpose of this study is to assess whether the effects of changes in wind speed are negligible.

In this study, the potential impact of the trend in surface wind speed on the rate of air-sea exchange is assessed by calculating the potential change in air-sea CO2 exchange due solely to changes in the surface wind speed, by fixing the air-sea difference in pCO2 at 1995 levels. Our results suggest that increase in storm track activity in Northern Hemisphere winter alone could lead to about 10% increase in the rate of air-sea CO2 exchange. Potentials exist for even larger impacts if the storm track activity in the Southern Hemisphere higher latitudes has also been increasing, as indicated by trends in the ERA40 data. Our results suggest that secular trends in surface wind speed should probably be taken into account in quantitative modeling of ocean CO2 uptake, at least in terms of uncertainty estimations.

In this presentation, we will also address the possibility of the existence of biases in the trend in storm track activity in the reanalysis data due to inhomogenous data quantity and quality over the years. The trend in storm track activity seen in the reanalysis will be assessed by comparisons with similar quantities computed from observations.