Picture: Zapotitlan salinas Botanical Garden, Mexico. Credit: Andrés Sanz (Unsplash).

Annual precipitation over Central America and large areas of Mexico shows an interesting and potentially predictable variation over the course of a year.  This region sees high precipitation rates from May to July and then again from August to October but between these two peaks, from July to August, rainfall is at its lowest for the entire year.

Many theories have been proposed to explain what causes this sharp drop in precipitation between these peaks – which has been named the mid-summer drought (MSD) – but most fail to address how this summertime decrease in rainfall varies over different regions of Central America and Mexico.

Now, a study by CLEX researchers has used a new method to detect and quantify these MSD events, with a particular focus on Central America and southern Mexico.

The results suggest that how this potentially predictable period of lower rainfall manifests in different regions can be explained by a combination of two primary influences:

  • The reversal of onshore and offshore winds during the development of the MSD, and
  • How the wind is forced by the steep mountainous terrain.

The researchers used a range of computer algorithms in combination – known as an artificial neural network – to tease out the regional influences that produced different MSD characteristics across the region.

In doing this, they found that different factors proposed in prior studies change the timing and characteristics of each mid-summer drought event. This suggests that that while the MSD development for the region is potentially predictable, it will be very difficult to summarise this development in a single theory.

  • Paper: Zhao Z, NJ Holbrook, ECJ Oliver, D Ballestero and JM Vargas-Hernandez, 2020: Characteristic atmospheric states during mid-summer droughts over Central America and Mexico. Climate Dynamics, doi:10.1007/s00382-020-05283-6.