Recent Observed Changes in the Tropical and North Pacific Connection Examined in an Extended Context Provided by Proxy Reconstruction and Coupled Models

Tropical and north Pacific modes of climate variability separately and together are the two main determinants of climate in North America. The relationship between ENSO and the North Pacific Oscillation (NPO) has been observed to weaken in the recent past. We attempt to understand this weakening by considering it in a longer-term perspective extended into the past and future by proxy climate reconstruction and by simulation of anthropogenic climate change, respectively. Proxy-based reconstructions of Pacific climate typically use regional chronologies and directly target specific indices of ENSO and NPO. However, trees and other proxies integrate seasonal-annual climatic effects, and most likely represent a response to the full spectrum of climate variability rather than a single pattern. We use proxy data from around the Pacific Rim (tree ring records mostly), to reconstruct patterns of sea surface temperature (SST) in the entire Pacific basin. We first relate observed tropical and extratropical Pacific SST patterns to patterns in an extensive network of proxy chronologies using Canonical Correlation Analysis (CCA) and then use the CCA model to reconstruct the SST field back several centuries. ENSO and NPO indices are derived from this entire-Pacific reconstruction and compared with other available reconstructions. The ENSO - NPO relationship is scrutinized in the reconstruction, observations, and in coupled models coerced with comprehensive and realistically evolving anthropogenic forcing terms. An attempt is made to explain the apparent recent de-coupling of the tropical and north Pacific.