Regional Impacts of the El Niño-Southern Oscillation on Hydrological Earth Rotation Excitation: A Cross-Correlation Approach

The El Niño-Southern Oscillation (ENSO) represents the most prominent inter-annual climate mode on Earth, characterized by quasi-periodic fluctuations in sea surface temperature and atmospheric pressure (Southern Oscillation) across the equatorial Pacific. Its warm (El Niño) and cold (La Niña) phases drive large-scale atmospheric circulation patterns, causing climate anomalies across all seasons. While ENSO primarily originates in the Pacific, its teleconnections extend globally, influencing the terrestrial water cycle worldwide. A growing body of research highlights significant linkages between ENSO and terrestrial water storage (TWS) in various regions, shedding light on its hydrological impacts. Given these connections, ENSO signals are expected to influence hydrological excitation functions of Earth rotation variations derived from terrestrial water mass distributions. On a wide range of time-scales, variations in Earth's rotation are caused by angular momentum exchanges of surface geophysical fluids with the solid Earth.

We investigate the influence of ENSO signals on the hydrological angular momentum using a two-step time domain cross-correlation approach. Lagged cross-correlation and regression analysis were performed between the MEI.v2 climate index and TWS anomalies using three datasets: GRACE/-FO time-variable gravity field solutions, the distributed hydrological rainfall-runoff model OS LISFLOOD, and the operational Land Surface Discharge Model (LSDM). The inter-annual time series were derived through decomposition using least squares fitting, followed by Butterworth low-pass filtering to capture ENSO periodicity. A global analysis of 100 hydrological basins enabled a spatial and temporal differentiation of ENSO impacts on regional TWS variability, forming the basis for computing regional hydrological angular momentum (HAM) functions. We will both discuss contributions from tropical latitudes that directly respond to modified atmospheric moisture flux pattern, but also extra-tropical regions that respond to ENSO conditions in the tropics only later in time. We thus aim to localize regional HAM contribution with a significant ENSO influence.