Yuko M. Okumura

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.

Pronounced centennial-scale Atlantic Ocean climate variability correlated with Western Hemisphere hydroclimate

Surface-ocean circulation in the northern Atlantic Ocean influences Northern Hemisphere climate. Century-scale circulation variability in the Atlantic Ocean, however, is poorly constrained due to insufficiently-resolved paleoceanographic records. Here we present a replicated reconstruction of sea-surface temperature and salinity from a site sensitive to North Atlantic circulation in the Gulf of Mexico which reveals pronounced centennial-scale variability over the late Holocene. We find significant correlations on these timescales between salinity changes in the Atlantic, a diagnostic parameter of circulation, and widespread precipitation anomalies using three approaches: multiproxy synthesis, observational datasets, and a transient simulation. Our results demonstrate links between centennial changes in northern Atlantic surface-circulation and hydroclimate changes in the adjacent continents over the late Holocene. Notably, our findings reveal that weakened surface-circulation in the Atlantic Ocean was concomitant with well-documented rainfall anomalies in the Western Hemisphere during the Little Ice Age.Knowledge of surface-ocean circulation in the Atlantic over the late Holocene is incomplete. Here, the authors show that Atlantic Ocean surface-circulation varied in concert with Western Hemisphere rainfall anomalies on centennial timescales and that this link played an essential role during the Little Ice Age.

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.

Oyster reef die-offs in stratigraphic record of Corpus Christi Bay, Texas, possibly caused by drought-driven extreme salinity changes

An ultra-high resolution acoustic reflection survey within Corpus Christi Bay, Texas, reveals numerous oyster reefs that have died off and been buried by subsequent sedimentation. The die-offs occurred in four temporal clusters, as evidenced by nearly coterminous capping horizons across multiple reef heads. These horizons can be correlated to published, calibrated radiocarbon dates (2-σ uncertainty: ± ~900 years) derived from cored sediments, and placed into the stratigraphic context. The reefs began growing with the initial flooding of the bay at ~9600 ybp. The first die-offs are coincident with the transition from upper bay to open bay environment at ~8140 ybp, with the greatest concentration of die-offs seaward. Subsequent die-offs occurred ~7600, 6870, and 5800 ybp, with a seaward-to-landward progression. These ages appear to either post-date or be concurrent with published periods of accelerated sea level rise of ~1–4 m. However, because bay oysters (Crassostrea virginica) are robust with respect to salinity and depth changes, we cannot directly link die-offs with punctuated sea level rise. We hypothesize instead that reef die-offs are associated with extreme salinity changes caused by droughts, based on observations during the 1950s Texas drought. During that event, bay reefs were colonized by open-ocean species (Crassostrea equestris), which gradually replaced bay oysters progressing seaward to landward in concentration. Subsequent flooding and rapid freshening of the bay caused massive mortality of the colonizing species. These floods also brought abundant fine-grained sedimentation that could bury reefs before they could be recolonized. Such a sequence of events could explain widespread early Holocene, seaward-to-landward oyster reef die-offs during times of more frequent and severe drought conditions. A plausible climatological link exists between periods of low rainfall in Texas and periods of accelerated melting of the polar ice cap, which could explain the evident correlation between reef die-off and sea level rise.