Impact of the Atlantic Multidecadal Oscillation and Mississippi River Discharge Anomalies on Gulf of Mexico Sea-Level Anomalies and Land Loss Rates in the Mississippi Delta

More than 90% of the Mississippi River Delta landscape in south Louisiana is <0.5 m elevation. Land loss was ongoing in the 1800s as a natural part of delta evolution but became more widespread in the 20^th century due to dams that reduced sediment load, levees that limited sediment dispersal to the delta plain, and acceleration of global sea-level rise. Mapping of land loss over time also shows land-loss acceleration in the late 1960s to mid 1990s and deceleration after 1995, changes that are commonly attributed to anthropogenic causes. Moreover, the Grand Isle, LA tide-gauge time series shows coeval accelerations and decelerations in relative sea-level rise, which have been correlated to changes in land loss rates and interpreted to reflect acceleration and deceleration of subsidence from local anthropogenic causes. However, the Galveston, TX and Pensacola, FL tide-gauge time series display similar timing, amplitude, and direction in detrended annual sea-level anomalies and 10-yr means over their shared 1947 to 2018 periods of record, which indicates a non-local cause.

We identify an alternative chain of causality for anomalaous land loss rates that is related to the Atlantic Multidecadal Oscillation (AMO) and associated anomalies in Mississippi River freshwater discharge and coastal sea level. Our analyses show that what has been called the AMO cool phase produces anomalously strong onshore winds that transport moisture deep into the midcontinent, which in turn produces anomalously high Mississippi River discharges and strong onshore wind stresses that pile up water to produce higher coastal sea level. By contrast, the AMO warm phase produces a negative anomaly in moisture flux into the US midcontinent that produces anomalously low Mississippi River discharge, as well as alongshore west-to-east directed wind anomalies along the northern Gulf Coast that favor offshore Ekman transport and lower coastal sea-level. Current research argues the AMO signal reflects climate system response to anthropogenic and natural forcing rather than intrinsic variability. Regardless of what causes the AMO signal, we interpret higher land-loss rates of the late 1960s to mid 1990s to reflect anomalously high sea level of the AMO cool phase, whereas lower land-loss rates since 1995 reflect anomalously low sea level of the AMO warm phase.