Influence of the Madden-Julian Oscillation on Extreme Wet Bulb Temperature

Wet bulb temperature (T_w) has emerged as an important climate metric for measuring and predicting heat stress due to its direct physiological link to human health. When the wet bulb temperature of the air rises above skin temperature, humans' evaporative cooling mechanisms are rendered ineffective. Improving our understanding of what causes extreme wet bulb temperatures thus has the potential to reduce the human health impacts of extreme humid heat events. The Madden-Julian Oscillation (MJO) is a fundamental source of intraseasonal variability, bridging the scale from weather to climate. In order to help improve our understanding, and the predictability of extreme humid heat events on a subseasonal timescale, we characterize the relationship between the MJO and wet bulb temperature.

We find that in the Persian Gulf and South Asia wet bulb temperatures at thresholds sufficiently high to impact human health (28 °C to 33 °C) are more than twice as likely during certain MJO phases as in others. The MJO is found to have the greatest influence over the likelihood of extreme wet bulb temperatures in the transition seasons, particularly for the Persian Gulf in MJO phases 1 and 8 during months March-May and in South Asia in MJO phases 5 and 6 during months September-November. There is also a strong MJO signal observed in South Asia during months June-August, the season during which wet bulb temperatures in this region tend to be the highest. We link these identified relationships to potential dynamical drivers to provide physical insight into the mechanisms by which the MJO and the related Boreal Summer Intraseasonal Oscillation (BSISO) affect wet bulb temperatures in each region. As models of the MJO become more accurate, our understanding of wet bulb temperature variability will help to improve predictability of extreme humid heat in regions already experiencing dramatic threats to human health, the economy, and the electricity sector.