Competitive influences on droughts: present and future

The current California drought, induced by a combination of low precipitation and high temperature, is estimated to be the worst in at least 1200 years. According to media headlines, a “super El Niño” with above-normal precipitation over California may provide some drought relief. This particular case highlights an underlying perception that ENSO events (and other modes of climate variability) will always be a source of relief (or damage). We argue that this expectation is not realistic in regions where the mean change in terrestrial aridity/moistening in response to greenhouse warming becomes larger than the expected range of current variability.

With few exceptions, most projections studies focus either on the change in mean state or on the change in variability, and neglect to investigate the combined effects of mean and variability changes at regional scale. Here, we use a suite of state-of-the-art climate model simulations to identify the regions where a projected change in aridity exists, and consider whether this change is large enough to overwhelm the effect of local drying/moistening associated with ENSO variability.

By the end of the 21st century, warming is expected virtually everywhere, independent of the phase of ENSO. In contrast, expectations regarding the net anomalies in regional precipitation (compared to some historical reference) are much more complex. This is because changes in the mean state and variability are governed by a number of different, spatially-complex mechanisms (e.g., via the ‘rich-get-richer/poor-get-poorer' mechanism and through well-known teleconnections). Here, we provide a comprehensive assessment of the relative contributions to future drought from changes in moisture supply (precipitation) versus evaporative demand (potential evapotranspiration). We also investigate the competing effects of mean changes and ENSO variability in terms of ameliorating or exacerbating drought.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and is supported, among others, by the PCMDI SFA funding and C.B. Early Career Research Program award. This work is released as LLNL-ABS-675905.