Anticipatory assessment: Future US droughts under the influence of evolving modes of variability and long-term change

Introduction to the problem and rationale: The US is currently in respite from two decades of
persisting and recurring severe droughts. This might well be connected to the El Niño-Southern
Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation
(AMO) all being unusually close to climatological values. However long-term trends in
temperature and precipitation are steadily influencing US climate. While we know the trends will
continue and ENSO will oscillate, it is not known if the PDO and AMO will move towards positive
or negative states or remain near neutral. Consequently, while we cannot tell exactly what
conditions in the climate system will drive future US droughts, sometime in the near future an
extreme drought will occur in the US and it will be asked: What is the cause, why did it start and
when will it end, is it natural climate variability or made more likely or severe due to human-driven
long-term trends? Our rationale is to enable real-time assessment of causes and mechanisms of
extreme hydroclimate events over the US.

Brief summary of proposed work: We propose to develop a library of atmosphere model
simulations for 2020 to 2040 forced by imposed sea surface temperature (SST) anomalies that span
the range of possible future PDO, AMO and ENSO conditions together with long-term trends. The
future SSTs will be derived from statistical models trained on observations to ensure realistic
spatial and temporal characteristics. We will generate 20-member ensembles for all the
combinations of neutral, future most positive and future most negative states of the PDO and
AMO. Each ensemble member will have a different sequence of ENSO behavior. The ensembles
will have long-term trends of SST imposed from (i) extrapolation of observed trends and (ii) the
CMIP6 multimodel mean trend, as well as projected changing trace gas concentrations. With the
ensembles we will determine, 1) how precipitation, temperature and soil moisture within droughts
change as a function of driver (ENSO, PDO, AMO) and as a function of time in response to the
background climate state, 2) how continued warming influences soil moisture during droughts and,
consequently, extreme high temperatures and heat waves, 3) how teleconnections from the tropics
to North America evolve as climate modes evolve and warming continues, 4) how trends in storm
tracks and jet streams will impact droughts driven by climate modes, 5) how future droughts are
influenced by uncertainty in trends in equatorial Pacific cold tongue SSTs. The physical
mechanisms of anticipated future droughts will be contrasted with past droughts (e.g. California
2011-16, Texas 2011, Midwest 2012 etc.) to identify similarities and differences and pinpoint how
US droughts are evolving. As future droughts occur, with known combination of long-term change
and climate modes, the ensembles will be available to be mined to rapidly assess the contributions
to the extreme event from atmosphere variability, ENSO, decadal modes and long-term change
together with the dynamical mechanisms involved.

Relevance to competition and broader impacts: The work aims to “increase … process-based
understanding of the climate mechanisms that influence” droughts as extreme events and the
“interaction between … interannual to multidecadal variability … and long-term trends”. The
project will focus on historical case studies and quantify how droughts evolve in the future. There
are broader impacts to society by improving intelligence on (i) causes of droughts leading to
improved predictability and (ii) how droughts of the future will differ from those of the past
guiding adaptation measures.

Climate Risk Area: Water Resources