Increasing heat low size and frequency in major monsoon regions.

Heat lows are key components of monsoon systems, forming as areas of low pressure in response to strong surface heating. Heat lows can affect the intensity, timing and location of monsoon rainfall by altering horizontal pressure gradients, encouraging low-level convergence and generating mid-level dry air outflow. It may be expected that heat lows will strengthen in response to surface warming, particularly as they form in arid regions which are heating faster than the global average. Despite this, trends in heat lows globally have neither been fully investigated nor compared, and the role of heat lows in monsoon change remains uncertain.

Here we analyse trends across the planet’s five strongest heat lows in reanalysis data spanning the last 45 years. We demonstrate that heat lows have increased in average size (50,000–120,000 km² per decade) and frequency of occurrence (3.2–12.7 heat low days per decade) in North America, the Sahara, the Arabian Peninsula and southern Africa. Between regions, however, we note diversity in the spatial and seasonal characteristics of heat low trends. For example, trends in the Southern African heat low are uniquely concentrated in the pre-monsoon period, consistent with delayed regional rainfall onset. Moreover, we point to regionally variable mechanisms of heat low change, whereby trends are either driven by increased downward longwave radiation associated with increased atmospheric moisture (the Sahara, West Asia, Australia), or by increased downward shortwave radiation caused by reductions in cloud cover (North America, southern Africa).

Results point to rapid changes to heat lows which are likely to have significant impacts on adjacent monsoon systems, particularly during the pre-onset period. Critically, we show that heat low trends and their respective driving mechanisms are not globally uniform, hence their impact on monsoons is likely to be regionally dependent, motivating further research into heat-low–monsoon interactions at the regional scale.