Impacts of multi-year La Niña events on persistent drought conditions over the southern US

Abstract:

The interannual cooling of the equatorial Pacific associated with La Niña events has been known to cause drought conditions over the southern US by displacing the subtropical jet and storm track northward. Whereas strong El Niños usually terminate after the mature phase, strong La Niñas tend to persist for 2 years or longer, thereby potentially exacerbating the situation. To investigate the impacts of multi-year La Niña events on the extent and persistency of drought in the US, a suite of observational data is analyzed for the past century. Composite analyses based on 10 multi-year La Niña events reveal distinct seasonal evolution of oceanic and atmospheric anomalies during the course of events. During the second year, the peak cooling of the equatorial Pacific weakens while the meridional extent of the cooling is nearly unchanged. Despite the weaker equatorial cooling, precipitation anomalies over the US remain of similar magnitude and the region of reduced precipitation extends further to the north during boreal winter-spring of the second year. These features are robust across different datasets and composite periods. Analysis of atmospheric teleconnections suggests that the reduced meridional gradient of equatorial cooling results in a broader meridional structure of upper tropospheric circulation anomalies, shifting the subtropical jet further to the north. An atmospheric model forced with observed multi-year La Niña SST anomalies shows a similar change in the atmospheric teleconnection patterns from the first to the second year although the difference is much less pronounced compared to observations. Given the sustained impacts on drought, it is crucial to predict the duration of La Niña events in advance.