The Mississippi River is an economic artery of the United States, and federal efforts to understand, predict, and manage flooding along its course have been underway since the 19th century1. On the lower Mississippi (below the Mississippi’s confluence with the Ohio River), flood protection is provided by a system of earthen levees and spillway structures designed to contain discharges exceeding those associated with the largest floods observed during the early 20th century2. Floods remain costly despite the protection offered by modern river engineering, with economic damages from flooding in 2011 estimated to be $3.2 billion3. Failure of key elements of the current flood control system, which nearly occurred during a major flood in 1973, would be an economic and humanitarian disaster of unprecedented severity4. Forecasting flood occurrence over seasonal to decadal time-scales, and thus affirming the viability of these flood protection measures, remains a major challenge – especially in light of the brevity of the instrumental record and the confounding effects of flood control infrastructure on the behavior of fluvial systems5, 6, both of which limit our ability to characterize hydrological systems’ sensitivity to climate variability and change7, 8.

Improving flood forecasting for the lower Mississippi depends on understanding the links between flood occurrence and the slowly varying, more predictable modes of climate variability that influence hydrological processes over central North America, including the Pacific-North American Pattern (PNA), the Atlantic Multi-Decadal Oscillation (AMO), the North Atlantic Oscillation (NAO), and the El Niño-Southern Oscillation (ENSO)9,10,11,12,13. Analyses of historical datasets have identified relationships of varying strength and direction between these modes of climate variability, precipitation, and streamflow over portions of the Mississippi River basin, particularly the Missouri/upper Mississippi10, 14, 15 and Ohio River basins11, 16, 17. Establishing the dynamical controls on increased flood risk on the lower Mississippi River has proven more challenging due to the multiple interacting controls that govern flood occurrence at the outlet of a continental drainage system18, 19, and the limited number of extreme floods available for study during the period of instrumental record.

On major river systems, extreme floods arise from the interaction of atmospheric processes that transport large amounts of oceanic moisture inland18 with the properties of the land surface that dictate the rate of surface runoff delivered to the main channel19. The relatively slow movement of water through soils relative to atmospheric moisture tranport processes creates lags between river discharge and the state of the climate system20, 21. Efforts to understand the causes of floods on the lower Mississippi River have typically focused on the extreme precipitation event(s) in the weeks prior to peak discharge. These rainstorms occur when moist air from the subtropical North Atlantic is concentrated along a frontal zone positioned across the basin, and are linked to the strength and position of the North Atlantic Subtropical High (NASH)22,23,24, a correlate to the phases of the PNA, AMO, and NAO11, 15, 25. Considerably less attention has been paid to the climatic controls on antecedent soil moisture – a key element in the development of a flood that evolves gradually but preconditions a basin to be vulnerable to flooding by reducing the infiltration capacity of the land surface19, 21 – and its role in generating discharge extremes of the lower Mississippi River.

Soil moisture over the lower Mississippi basin is strongly influenced by ENSO10, 26, 27 – a dominant mode of climatic variability associated with sea surface temperature anomalies in the eastern equatorial Pacific28 – and the two largest historical floods of the lower Mississippi River in the springs of 1927 and 2011 were preceded by El Niño events in the winters of 1925/1926 and 2009/2010, respectively (Fig. 1). More moderate floods, including those in springs of 1973 and 1983, were preceded by El Niño events in the winters of 1972/1973 and 1982/1983, respectively. Of all 14 major Mississippi River floods observed at Vicksburg, Mississippi (defined as peak annual stage >15.24 m)29 from 1858–2015, 64% have occurred within a year of an El Niño event (Supplemental Table 1). Through ENSO’s influence on the position and strength of the subtropical and polar jet streams28, El Niño events are associated with increased surface water storage over the lower Mississippi River basin10, 26, 27 that can persist for months due to the slow release of water stored in soils20. Based on these observations, we hypothesized that ENSO modulates lower Mississippi River discharge – and thus flood occurrence – within a year of an El Niño event through its influence on surface water storage.

Figure 1 Left panel: The Mississippi River basin and its soil moisture in relation to ENSO, expressed as a Pearson correlation between monthly soil moisture anomalies46 and the Niño 3.4 index35 from 1870–2014. The locations of the river gauging stations at Memphis, Tennessee (black square) and Vicksburg, Mississippi (grey square) are shown in relation to the lower Mississippi River (box). Right panels: Monthly Niño 3.4 index in relation to daily river stages for the Mississippi River at Memphis and Vicksburg for floods in (b) 1927, (c) 2011, (d) 1973, and (e) 1983. River stages are expressed as a height above the flood stage as defined for each gauge23. Map in left panel generated in ArcMap v.10.2.2 (http://arcgis.com). Full size image

To investigate the relationship between ENSO and lower Mississippi River floods, we used the Last Millennium Ensemble (LME) of the Community Earth System Model (CESM1)30. We evaluated all ‘full-forcing’ ensemble members in the CESM–LME, comprised of 10 realizations for the period A.D. 850–2005 (i.e., 1,155 years for each realization). The CESM–LME includes a coupled river transport module30, and simulates a greater number of discharge extremes than are available in the short (i.e., last 100–150 year) instrumental record. From the CESM–LME simulations, we extracted peak annual discharge for the lower Mississippi River basin and sea surface temperatures in the Niño 3.4 region (see Methods for details). We then compared the magnitude and return intervals of peak annual discharges that occurred within 12 months of an El Niño episode with those that did not. We also analyzed the trends in mean monthly soil moisture and precipitation anomalies over the lower Mississippi basin, as well as surface temperature and sea level pressure anomalies across the western hemisphere in relation to extreme floods (defined here as peak annual discharges with an annual exceedance probability ≤1%; i.e., ‘a 100-year flood’).

Prior work validating CESM–LME output has demonstrated that the full-forcing realizations reproduce major modes of observed internal climate variability, including ENSO and its teleconnections30,31,32,33; we performed additional validation to demonstrate that the mean, variance and seasonality of simulated and observed lower Mississippi River discharge is similar (Supplemental Fig. 1) and that CESM’s soil moisture field in relation to ENSO is comparable to that observed historically (Supplemental Fig. 2). The CESM–LME does not simulate the effects of engineering infrastructure (e.g., artificial levees, dams, and spillways), irrigation, or groundwater extraction on discharge, allowing us to evaluate the climate controls on discharge independently of the effects of most human alterations to the basin that confound analyses of instrumental datasets2, 5, 6. Land use is a transient forcing in the CESM–LME that could influence simulated discharge5,6,7, but we found no significant difference in peak annual discharge when we compared the pre- and post-agricultural periods (i.e., AD 850–1800 and AD 1800–2005) in the simulations (unpaired t-test, t = 0.2356, df = 2605.3, p = 0.8138).