The interannual cooling of the equatorial Pacific associated with La Niña events has been known to cause drought conditions over the southern tier of the US by displacing the subtropical jet and storm track northward. Whereas El Niño usually terminates after one year, La Niña often persists for 2 years or longer, exacerbating its impacts. It is therefore critical to predict the occurrence of such events with sufficient lead time. Our ongoing work funded by the NOAA CPC MAPP Program laid the groundwork for the prediction of La Niña-driven drought. Through “perfect model” experiments using the Community Earth System Model version 1 (CESM1), we showed that the duration of La Niña is predictable with lead times of 18-24 months. Furthermore, our observational analyses indicate that multi-year La Niña events have robust, sustained impacts on cold season precipitation deficits over the southern half of the US. To further advance our research toward operational predictions of persistent drought driven by multi-year La Niña, we propose the following three sets of research activities:
Task 1 (La Niña duration): We will assess the predictability of La Niña duration in the real world using two suites of long-range CESM1 forecasts initialized with observed oceanic conditions every July and November during 1954-2014. July and November are chosen because they correspond to the peak of El Niño and subsequent discharge of heat from the equatorial Pacific, respectively, after which La Niña typically develops. We will analyze the predictability of La Niña duration and the dynamical processes underlying that predictability for all events occurring during 1954-2014.
Task 2 (La Niña teleconnections): We will assess the predictability of atmospheric teleconnections forced by La Niña by quantifying the influence of various factors including internal atmospheric variability using the long-range CESM1 forecasts and two other suites of atmospheric model simulations in which tropical Pacific SSTs are constrained to observed values during 1880-2015. We will also analyze the mechanisms responsible for the evolution of the teleconnections over the course of multi-year La Niña events using a simple atmospheric model, as well as the atmospheric component of CESM1.
Task 3 (La Niña impacts on drought): We will investigate how the cold season precipitation deficits induced by La Niña teleconnections affect drought over the southern US during the course of multi-year La Niña events through analyses of observational data. We will use soil moisture and accumulated precipitation anomalies as indicators of drought. In particular, we will examine how precipitation deficits during the second cold season of multi-year La Niña events increase the severity and duration of drought compared to single-year La Niña events. We will also explore the predictability of La Niña-driven drought with the same suites of atmospheric model simulations used in Task 2.
Persistent drought driven by multi-year La Niña exerts tremendous socioeconomic impacts on the southern tier of the US. The proposed research activities will fill important gaps in our current understanding of La Niña-driven drought by exploring the predictability of three underlying processes: the duration of La Niña (Task 1); atmospheric teleconnections driven by La Niña (Task 2); and impacts of La Niña teleconnections on drought (Task 3). The project will thus represent a major contribution toward the Drought Task Force activities and a stated objective of the MAPP Competition: “advancing drought understanding, monitoring and prediction”, as well as NOAA’s long-term climate goal: “an informed society anticipating and responding to climate and its impacts”.
Climate Risk Area: Water Resources