Influence of ENSO on extreme precipitation and peak river flow in the US

Flooding in the US results in billions of dollars of losses every year. This is projected to increase further in many regions as the climate warms, due to a combination of more frequent and severe extreme rainfall events, with resulting impacts on flooding, and increased exposure as the population increases and development in flood-prone areas continues. Superimposed on this warming signal are the impacts of different internal cycles operating within the climate system on various timescales, such as El Niño Southern Oscillation (ENSO). These cycles may act to either exacerbate or reduce the severity of extreme precipitation and flooding, and on interannual timescales, ENSO is a dominant mode of variability. A better understanding of the influence of ENSO and other modes of variability on extreme precipitation and flooding, including under climate change, is important for a number of applications. These include climate change impact assessments, policy and decision-making, early warning systems for flooding and disaster response planning, and climate-related risk planning in the (re)insurance sector.

Here, we investigate the influence of ENSO on extreme precipitation and peak river flow in the US, under both historical and future climate conditions. For the historical period, we calculate annual maximum (AMAX) daily precipitation and flow, from the Multi-Source Weighted-Ensemble Precipitation (MSWEP) precipitation and USGS river gauge datasets, respectively. To assess whether positive, neutral, or negative phases of ENSO have a significant impact on extreme precipitation and flood magnitude, we calculate the correlation between AMAX and different ENSO phases. We use a number of different ENSO indices, including the Oceanic Niño Index (ONI) used operationally by NOAA, in order to test the sensitivity of these relationships to the method used to characterise ENSO.

We also assess the impacts of ENSO on projected future changes in AMAX precipitation, using climate model data from the Community Earth System Model Large Ensemble Project Phase 2 (CESM2-LENS). For this, we calculate the relative change in AMAX daily precipitation for positive, neutral, and negative phases of ENSO, to determine how projected extreme precipitation changes differ between the phases, and how this varies spatially across the US.