Pedro N. DiNezio

On the Fragile Relationship Between El Niño and California Rainfall

The failed influence of the 2015–2016 El Niño on California rainfall has renewed interest in the relationship between El Niño and U.S. rainfall variability. Here we perform statistical data analyses and simple model experiments to show that sufficiently warm and persistent sea surface temperature anomalies (SSTAs) in the far eastern equatorial Pacific are required to excite an anomalous cyclone in the North Pacific that extends to the east across the U.S. West Coast and thus increases rainfall over California. Among the four most frequently recurring El Niño patterns considered in this study, only the persistent El Niño, which is often characterized by the warm SSTAs in the far eastern equatorial Pacific persisting throughout the winter and spring, is linked to such extratropical teleconnection patterns and significantly increased rainfall over the entire state of California. During the last 69 years, only three of the 25 El Niño events (i.e., 1957–1958, 1982–1983, and 1997–1998) are clearly identified as the persistent El Niño. In addition, the monthly rainfall variance explained by El Niño is less than half that caused by internal variability during the 25 El Niño. Therefore, the rarity of persistent El Niño events combined with the large influence of internal variability effectively explains the fragile relationship between El Niño and California rainfall.

A 2 Year Forecast for a 60–80% Chance of La Niña in 2017–2018

Historical observations show that one in two La Nina events last for two consecutive years. Despite their outsized impacts on drought, these 2-year La Nina are not predicted on a routine basis. Here, we assess the predictability of 2-year La Nina using retrospective forecasts performed with a climate model that simulates realistic multi-year events, as well as with an empirical model based on observed predictors. The skill of the retrospective forecasts allows us to make predictions for the upcoming 2017-2018 boreal winter starting from conditions in November 2015. These two-year forecasts indicate that the return of La Nina is more likely than not, with a 60% probability based on the climate model and an 80% probability based on the empirical model; the likelihood of El Nino is less than 8% in both cases. These results demonstrate the feasibility of predictions of the duration of La Nina.

Influence of Westerly Wind Events stochasticity on El Niño amplitude: the case of 2014 vs. 2015

The weak El Niño of 2014 was preceded by anomalously high equatorial Pacific Warm Water Volume (WWV) and strong Westerly Wind Events (WWEs), which typically lead to record breaking El Nino, like in 1997 and 2015. Here, we use the CNRM–CM5 coupled model to investigate the causes for the stalled El Niño in 2014 and the necessary conditions for extreme El Niños. This model is ideally suited to study this problem because it simulates all the processes thought to be critical for the onset and development of El Niño. It captures El Niño preconditioning by WWV, the WWEs characteristics and their deterministic behaviour in response to warm pool displacements. Our main finding is, that despite their deterministic control, WWEs display a sufficiently strong stochastic component to explain the distinct evolutions of El Niño in 2014 and 2015. A 100-member ensemble simulation initialized with early-spring equatorial conditions analogous to those observed in 2014 and 2015 demonstrates that early-year elevated WWV and strong WWEs preclude the occurrence of a La Niña but lead to El Niños that span the weak (with few WWEs) to extreme (with many WWEs) range. Sensitivity experiments confirm that numerous/strong WWEs shift the El Niño distribution toward larger amplitudes, with a particular emphasis on summer/fall WWEs occurrence which result in a five-fold increase of the odds for an extreme El Niño. A long simulation further demonstrates that sustained WWEs throughout the year and anomalously high WWV are necessary conditions for extreme El Niño to develop. In contrast, we find no systematic influence of easterly wind events (EWEs) on the El Niño amplitude in our model. Our results demonstrate that the weak amplitude of El Niño in 2014 can be explained by WWEs stochastic variations without invoking EWEs or remote influences from outside the tropical Pacific and therefore its peak amplitude was inherently unpredictable at long lead-time.

Glacial reduction of the North American Monsoon via surface cooling and atmospheric ventilation

NSF [AGS-1204011, OCN-1304910, OCE-1651034]; David and Lucile Packard Foundation Fellowship in Science and Engineering

Evolving Impacts of Multiyear La Niña Events on Atmospheric Circulation and U.S. Drought

Wintertime precipitation over the southern US is known to decrease with interannual cooling of the equatorial Pacific associated with La Nina, which often persists two years or longer. Composite analysis based on a suite of observational and reanalysis datasets covering the period 1901-2012 reveals distinct evolution of atmospheric teleconnections and US precipitation anomalies during multi-year La Nina events. In particular, atmospheric circulation anomalies strengthen and become more zonally-elongated over the North Pacific in the second winter compared to the first winter. US precipitation deficits also remain large while the region of reduced precipitation shifts northeastward in the second winter. This occurs despite a significant weakening of the equatorial Pacific cooling in the second winter, and suggests that the large-scale atmospheric circulation is more sensitive to tropical SST anomalies of broader meridional extent. Given the extended climatic impacts, accurate prediction of La Nina duration is crucial.