Stratospheric dynamical signal propagating through TTL : a case study of SSW period using a global non-hydrostatic model

We have reported an enhancement of tropical convection and abrupt change of tropical circulation during the Stratospheric Sudden Warming (SSW) events by mainly the observational data [Eguchi and Kodera, GRL, 2007; Eguchi and Kodera, SOLA, 2010; Kodera et al., JMSJ, 2011]. These results suggest that the stratospheric meridional circulation impacts on the tropical convection. Investigation of the vertical velocity and the diabatic heating rate in the Tropical Tropopause Layer (TTL) region are crucial to understand the responsible process. However, these quantities cannot be measured directly. The present study used the data from numerical simulation using a global nonhydrostatic model, NICAM (Nonhydrostatic ICosahedral Atmospheric Model) [Satoh, et al 2008], which does not make use of the cumulus parameterization. The model integration was initiated from the observed field from 20 December 2009. The horizontal grid size of 14 km and vertically stretched 40-layers (z= 0 - 38 km) are used. There are five layers in the TTL (14-20 km). The daily mean gridded data (1 degree box) derived from three hourly (hourly) outputs for 3D (2D) variables are analyzed in the present study. In the NICAM simulation, a sudden warming event is reproduced around 15 January 2010, although the date of the onset was approximately one week earlier than that in the real atmosphere. The latitude of active convection moved to southward, and the temperature at the lower stratosphere and the TTL region became cooler after the onset of SSW, similar to the observation. The increase of the upward velocity occurred in the TTL at the southern tropics simultaneous to an increase in diabatic heating rate (DH) associated with cloud formation. From the time-pressure section at the equatorial southern hemisphere (SH), the anomalies of vertical wind, temperature and diabatic heating rate gradually penetrated from the lower stratosphere to the upper troposphere during the period 16-20 January, defined as the transition period. Upwelling due to an enhanced stratospheric meridional circulation produced a cooling in the TTL, which was compensated by a diabatic heating due to an increased convective activity. Intensification of upward velocity occurred in the whole troposphere after the transition period. Increase of the upwelling in the equatorial SH led to a suppression of convective activity in the tropical northern hemisphere (NH), which produces moisture convergence/divergence in the SH/NH at the lower troposphere. The moisture convergence in the SH further enhanced convective activity there and this process involving a positive feedback resulted in the meridional shift of the convective zone in the troposphere.