An index of wind-wave coupling and its global climatology

Using the fully developed wind-wave relation and simultaneous data of surface wind speed and wave height, an index of wind-wave coupling is presented. The validity of this index is demonstrated by comparing it to other wind-wave indices using 45-year European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40). The comparison shows that the angle between wind and wave directions need not be considered in practice when mapping climatology of wind-wave interaction, and the rationality behind it is explained from a statistical point of view. The results also indicate that probability-based indices are not robust enough to represent wave climate. A global climatology of this index is developed using both ERA-40 and jointly-calibrated altimeter data. Its spatial distributions show that the wind-wave coupling is strong in the mid-latitudes while weak in the low latitudes except for the trade-wind regions. Mixing of wind-sea and swell is found to be common in the ocean, especially in the trade-wind region, where two-way momentum transfer may exist because the directions of wind-sea and swell are different. The weakly coupled regions usually have a clear preference for the eastern sides and low latitudes of ocean basins, because the eastward swells there are mainly from the storms in westerlies. However, in the Pacific Ocean, trade-wind-generated swells may propagate westward, causing western side to be also weakly coupled.