The changing relationship between ENSO variability and moisture balance in the continental United States

The El Nino-Southern Oscillation (ENSO) system orchestrates a well-documented suite of climate anomalies worldwide. The details of ENSO's extratropical influence vary among events, but this variability has not been described or diagnosed beyond the past few decades, and previous descriptions have looked at inter-event differences rather than decadal patterns. We use a new tree-ring based drought reconstruction for the continental U.S. and instrumental ENSO indices to document systematic decadal changes in the U.S. drought-ENSO relationship since the late 19^th century. Significant ENSO-drought correlation occurs consistently in the southwest U.S., but the strength of penetration of moisture anomalies into the continent varies substantially. The most striking change over the past 130 years is the initiation of a `bipolar' ENSO-drought signature around 1920, producing opposite-sign moisture anomalies in the southwest and mid-Atlantic states. Shifts in teleconnection patterns coincide with variations in the strength of ENSO and in a North Pacific mode.