Improved boreal summer intraseasonal oscillation simulations over the Indian Ocean by modifying moist parameterizations in climate models

Northward propagation of the boreal summer intraseasonal oscillation (BSISO) system over the Indian Ocean significantly affects Asia summer monsoon and extreme weather events including typhoons but many climate models poorly simulate the norward movement of BSISO. Here, we suggest that the modified parameterizations can improve BSISO northward propagation and three-dimensional dynamic and thermodynamic structures by enhancing the vorticity anomalies under the mean vertical wind shear and meridional gradient of mean moisture in the climate models. We hypothesize that poor BSISO simulation in a climate model may result from too frequent deep convections without abundant moisture in the boundary layer (BL). To represent them, the modified parameterizations include (a) a BL depth-dependent convective trigger, (b) a bottom-heavy diffusivity in the shallow convection scheme, (c) Relative humidity-dependent convective entrainment rate, and (d) reduced conversion rate of convective cloud water to rainwater. The modified parameterizations increase vertical shear over the northern Indian Ocean by strengthened easterly anomalies in the upper troposphere, inducing positive barotropic vorticity anomalies. The enhanced positive vorticities generate boundary layer moisture convergence and positive convective instability in the north of the BSISO convection center, inducing next convection and thereby improving northward propagation of the BSISO. The modified parameterizations also improve the meridional gradient of mean-moisture advection in the lower troposphere with increasing upward transport of moisture in the boundary layer. The increased air-sea interaction by modified parameterization tends to intensify the signal of the northward movement of the BSISO. It is shown that the modified parameterizations work properly in different types of convective parameterizations. The possibility that our hypothesis may be applied to other climate models for improvement of the BSISO northward propagation is discussed.