The atmosphere-ocean interaction and development of the 2015 Asian monsoon in coupled NAVGEM/HYCOM Simulations

The development of the Indian monsoon in June-September 2015 was strongly influenced by the climate oscillatory modes. The anomalously strong June MJO episode contributed to the intense monsoon rainfall at the beginning of the season. Later in the season, SST anomalies associated with intensifying El Nino and Indian Ocean dipole, provided competing forcing for the monsoon circulation with increased SST in the eastern IO contributing to the monsoon strengthening and positive anomalies in the Central and Eastern Pacific weakening the monsoon circulation. The monsoon break in August 2015 and the decreased monsoon rainfall (about 15 % below normal) for the entire season suggest the significant influence of El Nino. The 2015 Asian monsoon development was simulated using the experimental Navy Earth System Model (NESM) lagged ensemble of 3 month forecasts. The Navy system consists of Navy Global Environmental Model (NAVGEM), the Hybrid Coordinate Ocean Model (HYCOM), and the Los Alamos sea ice model (CICE). The initial examination of the coupled ensembles indicates that the forecasts correctly captured the weak Indian monsoon, especially in July, as well as the increased precipitation in the western and central Pacific, consistent with intensifying El Nino. The development of SST anomalies in the coupled system, and the impact of these anomalies on MJO and monsoon circulations, are investigated. The role of model biases and coupled feedbacks between the atmosphere and ocean circulation anomalies in development and maintenance of SST anomalies is examined.