Characterizing convective inhibition during the sub–tropical mid–summer droughts

The Mid-Summer Droughts (MSD) of the Caribbean/Gulf of Mexico and Northern Philippine Sea regions manifest as a minimum in rainfall in June/July, splitting the rainy season in two. During this period the Bermuda and Bonin High pressure features halt their retreat eastward which results in a temporary local pressure increase. This extension pushes the southerly moisture flow around the monsoon low region further west and advects in dryer tropospheric air and more suppressed climatological conditions.

In this talk the climate signal of the MSD is characterized using a comprehensive radiosonde sounding dataset for key sites within the MSD regions. From the same sounding dataset the convective potential is determining using a technique of successively entraining plumes or generations - this technique will be described. The aim of this approach is to characterize the reduced convective potential during the MSD, with particular emphasis on sensitivity to the lower tropospheric moisture which arrives with the returning high pressure ridge.

We find that strong entrainment scenarios can usefully differentiate between the reduced convection of the MSD and the enhanced convection of the rest of the rainy season. Undilute CAPE based characterization of convection strongly biases surface forcing, which is at its strongest during the MSD, and erroneously predicts the peak convective response during that time. The reasons for observed differences between the Pacific and Atlantic MSD will also be discussed.