Subseasonal Prediction of Significant Wave Heights over the Western Pacific and Indian Ocean Region

The bias and skill of multi-week predictions of significant wave height (SWH) in the western Pacific and Indian Ocean (WP-IO) region are investigated. The WaveWatch III (WW3) model is forced with daily 10-m winds from the National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2 (CFSv2), retrospective forecasts (CFSR). Reforecasts using January and May initial conditions for the period 1999-2009 are considered. The main features of the climatological mean 10-m winds in weeks 1-4 are well captured by CFSv2, although the magnitude of the bias increases with lead time over much of the region in both the January and May cases. The CFSv2 WW3 system similarly captures the magnitude and spatial structure of SWH in weeks 1-4 well in both cases; however, the magnitude of the positive biases increases with lead time over the Southern Ocean (SO), the South China Sea, and the northwestern Pacific region in the January cases, and over SO in the May cases. The magnitude of the SWH variability grows weaker with lead time over SO, which may be related to the weaker interannual variability of 10-m winds in weeks 1-4 over SO in CFSR. During the first two forecast weeks, the temporal anomaly correlation skill of SWH is significantly higher than it is during weeks 3 and 4 in the WP-IO region. Based on a categorical forecast verification, the CFSv2 WW3 can predict rare events at these lead times.