Abstract
Small changes in Pacific temperature gradients connected with the El Niño Southern Oscillation (ENSO) influence the Walker Circulation and are related to global climate anomalies. Therefore, it is of paramount importance to develop robust indices of their past behavior. Here, we reconstruct the difference in sea surface temperature between the west and central Pacific during ENSO, coined the West Pacific Gradient (WPG), based on the Last Millennium Paleo Hydrodynamics Data Assimilation. We show that the WPG tracks ENSO variability and strongly co-varies with the zonal gradient in Pacific sea surface temperature. We demonstrate that the WPG strength is related to significant atmospheric circulation and precipitation anomalies during historical El Niño and La Niña events by magnifying or weakening droughts and pluvials across the Indo-Pacific. We show that an extreme negative WPG coupled to a strong zonal Pacific temperature gradient is associated with enhanced megadroughts in North America between 1400 CE and the late sixteenth century. The twentieth century stands out in showing the most extreme swings between positive and negative WPG conditions over the past Millennium. We conclude that the WPG is a robust index together with ENSO indices to reveal past changes in Pacific zonal sea surface temperature gradient variability.
Highlights
A lack of long instrumental climate records from the Indo-Pacific warm pool and the tropical Pacific, the heat engines of the global climate system and an essential player in global rainfall/drought cycles, is a central problem for reducing uncertainties in model-based climate change process studies and projections to plan for the future successfully
Our results reveal distinct periods of persistent El Niño or La Niña-like conditions during the past Millennium associated with shifts in the strength of the PWC and Maritime Continent temperatures affecting global climate and drought occurrences
The spatial pattern of the tropical Pacific sea surface temperatures (SST) field is characterized by a zonal SST gradient (Fig. 1a) which strongly resembles the spatial SST pattern related to the West Pacific Gradient (WPG) based on ERSST (Fig. 1b)[24]
Summary
A lack of long instrumental climate records from the Indo-Pacific warm pool and the tropical Pacific, the heat engines of the global climate system and an essential player in global rainfall/drought cycles, is a central problem for reducing uncertainties in model-based climate change process studies and projections to plan for the future successfully. Hoell and F unk[24] and Hoell et al.[26,27] demonstrated that during both El Niño and La Niña events, the global impacts in terms of atmospheric circulation and precipitation anomalies were larger when the SST anomalies in the western Pacific were strongly opposing those in the central Pacific than when the western Pacific SST anomalies were near neutral These studies show that the SST gradient between the central (Niño[4] region) and the western Pacific was an essential measure of interannual to multidecadal Pacific climate variability in addition to any previously derived metric of ENSO2 or combination of ENSO m etrics[24,28]. We make use of the Last Millennium reconstruction from PHYDA based on the Past Global Changes (PAGES2K) data archive for the Common E ra[33,34], which contains a multivariate proxy data array to assess the interannual and decadal variability in the WPG, Maritime Continent temperatures, the Niño[4] and Niño3.4 regions, and the zonal SST gradient at annual resolution since 1000 CE. Our results reveal distinct periods of persistent El Niño or La Niña-like conditions during the past Millennium associated with shifts in the strength of the PWC and Maritime Continent temperatures affecting global climate and drought occurrences
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