1,721,007 research outputs found
Application of new parameterizations of gas transfer velocity and their impact on regional and global marine CO2 budgets
No full textOne of the dominant sources of uncertainty in the calculation of air–sea flux of carbon dioxide on a global scale originates from the various parameterizations of the gas transfer velocity, k, that are in use. Whilst it is undisputed that most of these parameterizations have shortcomings and neglect processes which influence air–sea gas exchange and do not scale with wind speed alone, there is no general agreement about their relative accuracy.The most widely used parameterizations are based on non-linear functions of wind speed and, to a lesser extent, on sea surface temperature and salinity. Processes such as surface film damping and whitecapping are known to have an effect on air–sea exchange. More recently published parameterizations use friction velocity, sea surface roughness, and significant wave height. These new parameters can account to some extent for processes such as film damping and whitecapping and could potentially explain the spread of wind-speed based transfer velocities published in the literature.We combine some of the principles of two recently published k parameterizations [Glover, D.M., Frew, N.M., McCue, S.J. and Bock, E.J., 2002. A multiyear time series of global gas transfer velocity from the TOPEX dual frequency, normalized radar backscatter algorithm. In: Donelan, M.A., Drennan, W.M., Saltzman, E.S., and Wanninkhof, R. (Eds.), Gas Transfer at Water Surfaces, Geophys. Monograph 127. AGU, Washington, DC, 325–331; Woolf, D.K., 2005. Parameterization of gas transfer velocities and sea-state dependent wave breaking. Tellus, 57B: 87–94] to calculate k as the sum of a linear function of total mean square slope of the sea surface and a wave breaking parameter. This separates contributions from direct and bubble-mediated gas transfer as suggested by Woolf [Woolf, D.K., 2005. Parameterization of gas transfer velocities and sea-state dependent wave breaking. Tellus, 57B: 87–94] and allows us to quantify contributions from these two processes independently.We then apply our parameterization to a monthly TOPEX altimeter gridded 1.5° × 1.5° data set and compare our results to transfer velocities calculated using the popular wind-based k parameterizations by Wanninkhof [Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. J. Geophys. Res., 97: 7373–7382.] and Wanninkhof and McGillis [Wanninkhof, R. and McGillis, W., 1999. A cubic relationship between air?sea CO2 exchange and wind speed. Geophys. Res. Lett., 26(13): 1889–1892]. We show that despite good agreement of the globally averaged transfer velocities, global and regional fluxes differ by up to 100%. These discrepancies are a result of different spatio-temporal distributions of the processes involved in the parameterizations of k, indicating the importance of wave field parameters and a need for further validation
A sea-state dependent parameterization of whitecapping and air-sea gas transfer velocities (abstract of paper presented at the EGU General Assembly 2005, Vienna, 24-29 April 2005)
No full textA parameterization of whitecapping that depends on sea state in addition to the friction velocity of the wind is justified in terms of the energetics of wind waves. This new parameterization has implications for processes wholly or partly dependent on whitecapping including sea-salt production and air-sea gas transfer. A parameterization of gas transfer velocities is derived from a previous model modified by the sea-state dependent parameterization of whitecapping. This new model is evaluated. The new model provides a rationale for the divergence of earlier gas transfer coefficient models, giving due consideration to the sea-state conditions prevalent in the underlying data sets. Contemporary gas transfer is evaluated over the globe at seasonal and regional resolutions (up to monthly and 1 degree) for both the new and traditional parameterisations using both reanalysis products (ECMWF ERA40) and earth observation (scatterometer and altimeter) products. The new model implies mean global transfer velocities, mean global exchange coefficients and a net global carbon dioxide sink broadly in line with previous estimates but with significant differences in detail. The sensitivity of the net carbon dioxide sink to the balance of non-whitecapping and whitecapping components of gas transfer is high. Regional differences in gas transfer between traditional formulations and the new model are substantial. Whitecapping in the North Atlantic is notable for strong inter-annual variability driven primarily by the high sensitivity of wave heights to the North Atlantic Oscillation
Parametrization of gas transfer velocities and sea-state-dependent wave breaking
No full textBoth experimental estimates and different parametrizations of the transfer velocity of poorly soluble gases exhibit a very broad range of values at a given wind speed. Transfer velocities also appear to depend non-linearly on wind speed, and for high wind speeds this non-linearity is widely attributed to the influence of wave breaking. Both theoretical and experimental studies suggest that wave breaking, and associated whitecapping, is not simply dependent on wind speed but depends also on sea state. New parametrizations of gas transfer velocity based on an existing model of the dependence of transfer velocity on wind stress and whitecapping, supplemented by two sea-state-dependent parametrizations of whitecapping, are developed. These new models predict a diversity of transfer velocities at a given wind speed comparable to the diversity of existing parametrizations. Further, the results suggest that some of the existing parametrizations of transfer velocity reflect in part the wind fetch and sea state typical of the experiments used as a basis of the parametrization. It is suggested that transfer velocities may be estimated much more accurately through satellite retrieval of both wind speed and significant wave height than by wind speed alone
The influence of the North Atlantic Oscillation on sea-level variability in the North Atlantic region
No full textSatellite altimeter (Topex/Poseidon, 1992–2001) and tide-gauge measurements are used to explore the relationship of the sea level of the North Atlantic and neighbouring seas and coastlines to the North Atlantic Oscillation (NAO). Altimeter measurements suggest significant gyre-scale influence of the NAO in the North Atlantic, but also stronger influences on the continental shelf and inland seas of Europe. A north–south dipole in sea-level anomaly consistent with a hydrostatic response to the NAO sea-level pressure dipole is evident, but there are also large non-hydrostatic effects. The strongest response on the European Shelf is in the southeastern part of the North Sea where sea level is positively correlated to NAO Index. The sea level in two semi-enclosed seas, the Baltic Sea positively and the Mediterranean Sea negatively, is also strongly influenced by the NAO. A weak negative correlation is apparent around the northeastern coastline of North America. These features are confirmed by contemporary coastal tide-gauge data, but the tide-gauge data also show that the influence of the NAO was weaker early in the Twentieth Century (20C) on parts of the Northwest European coastline. Inter-annual sea-level variability associated with fluctuations in the NAO are generally much larger than those associated with secular trends. Inferred multi-decadal fluctuations associated with the NAO are very substantial compared to the 15(35) cm estimated for 20C global sea-level rise (Church, J.A., Gregory, J.M., Huybrechts, P., Kuhn, M., Lambeck, K., Nhuan, M.T., Qin, D. and Woodworth, P.L. (2001). Changes in sea level. Chapter 11 of the Intergovernmental Panel on Climate Change Third Assessment Report, pp. 639–694. Cambridge University Press, Cambridge.) and scenario forecasts for the 21C (350 cm). Therefore, the behaviour of the NAO in the next few decades will be a major regional factor in sea-level rise and coastal vulnerability in some European regions
Waves and climate change in the sea of the Hebrides
No full textThere is mounting evidence for the effects of climate change both globally and regionally. Global warming and sea level rise are now established but may appear insignificant locally, although the expected acceleration in rate may make this more noticeable. The most important issue for individuals and communities, who have to make decisions (based on existing evidence) about how to manage response to climate change, is the likely local impact. This may be in terms of secondary effects, e.g changes in rainfall, and may vary greatly from the global average. Wave height in the North Atlantic, as observed from in-situ and altimeter observations, has increased over the last quarter-century. The North Atlantic Oscillation (NAO) appears to be correlated with increasing wave height in the North Atlantic over recent decades. Prediction of future impacts requires understanding the role of such decadal oscillations and their likely future evolution as well as long-term trends in sea level and wave height due to global warming and possible rapid climate change scenarios. It is important to understand these effects on relatively small scales. Here we examine the impacts of changing wave climate on the rocky coast of NW Scotland, specifically the Sea of the Hebrides. The Sea of the Hebrides is the body of water that lies between the Outer and Inner Hebrides island groups in NW Scotland. The impact of any increase in wave height in the North Atlantic at the coastline will be most significant in this area Crofting, fishing, fish fanning and tourism are the most significant economic activities. Impacts of climate change in this area may include interference to ferries and fishing activity, changes in potential wave energy availability and changes in coastal erosion and habitats. Wave models provide a tool to study detailed impacts of various climate change scenarios. The model system used here comprises three nested models using both the PRO-WAM and SWAN models, from a V North Atlantic model, through a 7.5km Malin/Hebrides Shelf model to a 1.85km Sea of Hebrides model. This allows the effect of winds over the whole North Atlantic to be investigated while also studying the local coastal wave impact including refraction and shoaling around the Western Isles of Scotland and make the connection between the statistical results from altimeter data to the dynamics.</p
Waves and climate change in the north-east Atlantic
Full text linkWave height in the North Atlantic has been observed to increase over the last quarter-century, based on monthly-mean data derived from observations. Empirical models have linked a large part of this increase in wave height with the North Atlantic Oscillation. Wave models provide a tool to study impacts of various climate change scenarios and investigate physical explanations of statistical results. In this case we use a wave model of the NE Atlantic. Model tests were carried out, using synthetic wind fields, varying the strength of the prevailing westerly winds and the frequency and intensity of storms, the location of storm tracks and the storm propagation speed. The strength of the westerly winds is most effective at increasing mean and maximum monthly wave height. The frequency, intensity, track and speed of storms have little effect on the mean wave height but intensity, track and speed significantly affect maximum wave height
Sensitivity of ferry services to the Western Isles of Scotland to changes in wave climate
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