The influence of soil moisture deficits on Australian heatwaves

Abstract

Several regions of Australia are projected to experience an increase in the frequency, intensity and duration of heatwaves (HWs) under future climate change. The large-scale dynamics of HWs are well understood, however, the influence of soil moisture deficits-due for example to drought-remains largely unexplored in the region. Using the standardised precipitation evapotranspiration index, we show that the statistical responses of HW intensity and frequency to soil moisture deficits at the peak of the summer season are asymmetric and occur mostly in the lower and upper tails of the probability distribution, respectively. For aspects of HWs related to intensity, substantially greater increases are experienced at the 10th percentile when antecedent soil moisture is low (mild HWs get hotter). Conversely, HW aspects related to longevity increase much more strongly at the 90th percentile in response to low antecedent soil moisture (long HWs get longer). A corollary to this is that in the eastern and northern parts of the country where HW-soil moisture coupling is evident, high antecedent soil moisture effectively ensures few HW days and low HW temperatures, while low antecedent soil moisture ensures high HW temperatures but not necessarily more HW days.

Amplification of Australian Heatwaves via Local Land‐Atmosphere Coupling

“…(Kala, Evans & Pitman, ) focused on a single event and examined the sensitivity to soil moisture conditions 15 days prior to that event. In contrast, Herold et al () used a 3‐month timescale when examining the potential role of antecedent soil moisture conditions on heatwaves. Finally, Perkins et al () found that at longer timescales (~5 months) climate variability tends to have a larger impact on the background climate than soil moisture conditions.…”

“…Land surface conditions can amplify surface temperatures, particularly during heatwave events (Fischer, Seneviratne, Lüthi, & Schär, ; Fischer, Seneviratne, Vidale, et al, ; Gibson et al, ; Hauser et al, ; Herold et al, ; Lorenz et al, ; Miralles et al, ). Globally, this influence has been attributed to precipitation deficits enhancing negative soil moisture anomalies, facilitating further surface warming by partitioning more energy into sensible heating (Mueller & Seneviratne, ).…”

“…The land surface contribution to surface temperatures is not limited to single‐day extreme temperature events with several studies demonstrating the role of land surface drying both prior to and during a heatwave event (Fischer, Seneviratne, Lüthi, & Schär , ; Fischer, Seneviratne, Vidale, et al, ; Gibson et al, ; Hauser et al, ; Herold et al, ;Lorenz et al, ; Miralles et al, ). For example, Fischer, Seneviratne, Lüthi, and Schär () demonstrated that the presence of land‐atmosphere interactions typically increases the number of heatwave days per summer season by 50–80% in Europe.…”

“…The land surface drying that occurred prior to the drought increased the likelihood of exceptional temperature anomalies experienced during this event by sixfold (Hauser et al, ). Studies focusing on less extreme heatwaves over Europe (Lorenz et al, ; Miralles et al, ), North America (Cowan et al, ; Ford & Schoof, ; Teng et al, ), and Australia (Gibson et al, ; Herold et al, ; Perkins et al, ) all suggest that, in general, antecedent soil moisture conditions had a role in both the heatwave intensity and to a lesser extent also the duration. This is understandable given that these are regions where the atmosphere is sensitive to land surface variability (i.e., the land‐atmosphere coupling) (e.g., Hirsch et al, ; Lorenz et al, ; Seneviratne et al, ).…”

Comparing Australian heat waves in the CMIP5 models through cluster analysis

“…Other considerations include the land‐atmosphere coupling strength in climate models during simulated heat wave events [e.g., Miralles et al ., ]. The understanding of land atmosphere coupling on Australian heat waves remains limited compared to other regions, despite preliminary evidence for a significant soil moisture influence on heat waves for many parts of Australia [ Perkins‐Kirkpatrick et al ., ; Herold et al ., ]. As such, the ability for models to realistically capture these interactions will be important.…”