Annual cycle and trends of wave driving of the tropical lower Stratosphere

Diagnostics from ECMWF analyses indicate that a significant factor in controlling the mean state and annual cycle of the tropical tropopause is breaking by synoptic-scale waves on the equatorial side of the subtropical jets. This wave breaking extends further into the tropics during northern hemisphere winter than northern hemisphere summer where is displaced northward by the Asian monsoon. This annual cycle in wave breaking can explain the large annual cycle temperatures just above the tropical tropopause and hence it is unnecessary to invoke a "stratospheric pump". In contrast, the tropical temperatures at 10 hPa have a semiannual variation. This is partly due to the timing of strong wave breaking in the northern and southern hemisphere winter stratosphere. However there is also a significant solar heating effect which means the coldest tropical temperatures at this altitude are in southern hemisphere winter.

Radiosonde temperature measurements at 100 hPa show cooling of the tropical tropopause. It will be shown that this is consistent with increased Eliassen and Palm (EP) flux divergence in the subtropics near the tropical tropopause. Analysis of the vertical EP-flux through the extratropical tropopause shows that there has been an increase in the contribution from synoptic-scale waves. Changes in storm track activity, and other processes in the troposphere, are investigated to explain the changes in vertical wave flux through the extratropical tropopause.