2:15 PM - 2:30 PM
[AAS02-03] Comparisons of the intra-seasonal fluctuation of the monsoon trough simulated in a coupled and uncoupled model.
Keywords:monsoon trough, air-sea interaction, high resolved numerical model, atmosphere-ocean coupled model
The monsoon trough (MT) over the Western North Pacific (WNP) influences various extreme events, such as tropical cyclones and heavy rainfall, in east/south-east Asia during boreal summer. Expanding our knowledge of the MT activity is crucial for understanding and predicting weather phenomena in this region. During boreal summer over the WNP, some intra-seasonal variations, including the boreal summer intra-seasonal oscillation, dominate and have impacts on the intra-seasonal fluctuation of the MT. Additionally, sea surface temperature (SST) tends to respond passively to the atmosphere in this region. It remains unclear how the SST influenced by the atmosphere feeds back to the atmosphere, especially to convection which in turn may affect the fluctuation of the MT.
The present study investigated how the intra-seasonal variability of the MT is modulated by the air-sea coupled process by comparing atmosphere-only and atmosphere-ocean coupled model simulations. We conducted 10-member ensemble numerical experiments with a non-hydrostatic global atmospheric model “NICAM” and its ocean coupled version “NICOCO”. Both models simulated the structure of the MT under a La Niña condition. The meridional fluctuation of the MT and associated westerlies and convective activities was successfully simulated in NICOCO experiments, whereas the fluctuation was underestimated in NICAM experiments. In both experiments, the cyclonic circulation around active convections acted to accelerate (decelerate) the background monsoonal wind on the southern (northern) portion of convections, and latent heat flux was larger and upward anomaly of total surface heat flux was more dominant on the southern portion than the northern portion during strong MT activities. In NICOCO experiments, SST tendency was largely negative, and convective activities were suppressed on the southern portion. Consequently, convections moved toward the northern SST positive anomaly region, which favors convections. In NICAM experiments, the northward movement of convections was underestimated or too slow, because such SST fluctuation does not exist. Our results suggest that coupling the atmosphere with the ocean influences the northward propagation speed of convections over the WNP and the intra-seasonal fluctuations of the MT.
The present study investigated how the intra-seasonal variability of the MT is modulated by the air-sea coupled process by comparing atmosphere-only and atmosphere-ocean coupled model simulations. We conducted 10-member ensemble numerical experiments with a non-hydrostatic global atmospheric model “NICAM” and its ocean coupled version “NICOCO”. Both models simulated the structure of the MT under a La Niña condition. The meridional fluctuation of the MT and associated westerlies and convective activities was successfully simulated in NICOCO experiments, whereas the fluctuation was underestimated in NICAM experiments. In both experiments, the cyclonic circulation around active convections acted to accelerate (decelerate) the background monsoonal wind on the southern (northern) portion of convections, and latent heat flux was larger and upward anomaly of total surface heat flux was more dominant on the southern portion than the northern portion during strong MT activities. In NICOCO experiments, SST tendency was largely negative, and convective activities were suppressed on the southern portion. Consequently, convections moved toward the northern SST positive anomaly region, which favors convections. In NICAM experiments, the northward movement of convections was underestimated or too slow, because such SST fluctuation does not exist. Our results suggest that coupling the atmosphere with the ocean influences the northward propagation speed of convections over the WNP and the intra-seasonal fluctuations of the MT.