Abstract
The history of the Polynesian civilization on Rapa Nui (Easter Island) over the Common Era has come to exemplify the fragile relationship humans have with their environment. Social dynamics, deforestation, land degradation, and climatic shifts have all been proposed as important parts of the settlement history and societal transformations on Rapa Nui. Furthermore, climate dynamics of the Southeast Pacific have major global implications. While the wetlands of Rapa Nui contain critical sedimentological archives for reconstructing past hydrological change on the island, connections between the island’s hydroclimate and fundamental aspects of regional climatology are poorly understood. Here we present a hydroclimatology of Rapa Nui showing that there is a clear seasonal cycle of precipitation, with wet months receiving almost twice as much precipitation as dry months. This seasonal cycle can be explained by the seasonal shifts in the location and strength of the climatological south Pacific subtropical anticyclone. For interannual precipitation variability, we find that the occurrence of infrequent, large rain events explains 92% of the variance of the observed annual mean precipitation time series. Approximately one third (33%) of these events are associated with atmospheric rivers, 21% are associated with classic cold-front synoptic systems, and the remainder are characterized by cut-off lows and other synoptic-scale storm systems. As a group, these large rain events are most strongly controlled by the longitudinal position of the south Pacific subtropical anticyclone. The longitudinal location of this anticyclone explains 21% of the variance in the frequency of large rain events, while the remaining variance is left unexplained by any other major atmosphere-ocean dynamics. We find that over the observational era there appears to be no linear relationship between the number of large rain events and any other major climate phenomena. With the south Pacific subtropical anticyclone projected to strengthen and expand westward under global warming, our results imply that Rapa Nui will experience an increase in the number of dry years in the future.
Highlights
The island of Rapa Nui (Easter Island; 27◦6′ S, 109◦21′ W) is located in the southeastern Pacific Ocean
To the west of this anticyclone can be found the eastern-most limb of the South Pacific Convergence Zone (SPCZ), a band of precipitation that stretches diagonally from Rapa Nui up towards the equator north of Papua New Guinea (Fig. 1c)
For Rapa Nui, we find that the strength of the south Pacific anticyclone and the distance of the anticyclone from the island best explains the seasonal cycle of precipitation
Summary
The island of Rapa Nui (Easter Island; 27◦6′ S, 109◦21′ W) is located in the southeastern Pacific Ocean. Dynamical explanations for these hydroclimate shifts are mostly speculative and have included the influence of ENSO (Mann et al 2008; Stenseth and Voje 2009), changes in the trade winds and Walker Circulation (Bridgman 1983; McCall 1993), changes to the south Pacific anticyclone, changes in the SPCZ, and changes in the strength and latitudinal position of the westerly storm track (Sáez et al 2009; Cañellas-Boltà et al 2013) These studies are important for their implications for regional and global-scale climatological changes and can provide the climate context for historical changes in Rapanui culture. We focus our precipitation analysis on the observational data, except when spatial patterns of precipitation are shown
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