The Climatology of Vertical Mixing in the Tropical Tropopause Layer

We present the climatology of parametrised vertical mixing in the ECMWF ERA-Interim reanalysis project, along with an estimate of vertical mixing using COSMIC GPS data. Vertical mixing has important implications for tracers and the diabatic heat budget in the TTL. Vertical mixing in ERA Interim is highly zonally asymmetric, leading to a zonally asymmetric diabatic forcing, with a localised heating of approximately 0.6 K/day and a zonal acceleration of approximately 1.5 m/s/day at 100°E at 125 hPa, and a corresponding signal of opposite sign at 100 hPa. The effect of this forcing on the mean flow is examined using a dry GCM forced with a steady forcing equal to the climatological diabatic heating and acceleration due to mixing. The order of magnitude of the parametrised mixing is validated by examining the decay of vertical wave momentum flux in the TTL, and comparing this with the dissipation rate due to parametrised mixing and radiative cooling on equatorial waves. Equatorial waves play a key role in triggering mixing, and the climatology of vertical mixing in ERA Interim is found to be similar to the climatology of Kelvin wave activity, which are also largest over the Indian Ocean in the TTL.