Advances in tropical cyclone prediction on subseasonal time scales

This review describes advances in understanding and forecasting tropical cyclone(TC)subseasonal variability during the past four years.A large effort by the scientific community has been in understanding the sources of predictability at subseasonal timescales beyond the well-known modulation of TC activity by the Madden-Julian Oscillation(MJO).In particular,the strong modulation of TC activity over the western North Pacific by the Boreal Summer Intra-Seasonal Oscillation(BSISO)has been documented.Progress has also been realized in understanding the role of tropical-extratropical interactions in improving subseasonal forecasts.In addition,several recent publications have shown that extratropical wave breaking may have a role in the genesis and development of TCs.Analyses of multi-model ensemble data sets such as the Subseasonal to Seasonal(S2S)and Subseasonal Experiment(SubX)have shown that the skill of S2S models in predicting the genesis of TCs varies strongly among models and regions but is often tied to their ability to simulate the MJO and its impacts.The skill in select models has led to an increase over the past four years in the number of forecasting centers issuing subseasonal TC forecasts using various techniques(statistical,statistical-dynamical and dynamical).More extensive verification studies have been published over the last four years,but often only for the North Atlantic and eastern North Pacific.

TROPICAL CYCLONE PREDICTION ON SUBSEASONAL TIME-SCALES

Here we discuss recent progress in understanding tropical cyclone(TC)subseasonal variability and its prediction.There has been a concerted effort to understand the sources of predictability at subseasonal time-scales,and this effort has continued to make progress in recent years.Besides the Madden-Julian Oscillation(MJO),other modes of variability affect TCs at these time-scales,in particular various equatorial waves.Additionally,TC activity is also modulated by extratropical processes via Rossby wave breaking.There has also been progress in the ability of models to simulate the MJO and its modulation of TC activity.Community efforts have created multi-model ensemble datasets,which have made it possible to evaluate the forecast skill of the MJO and TCs on subseasonal time-scales in multiple forecasting systems.While there is positive skill in some cases,there is strong dependence on the ensemble system considered,the basin examined,and whether the storms have extratropical influences or not.Furthermore,the definition of skill differs among studies.Forecasting centers are currently issuing subseasonal TC forecasts using various techniques(statistical,statistical-dynamical and dynamical).There is also a strong interest in the private sector for forecasts with 3-4 weeks lead time.