Northern Hemisphere summer Sea Level Pressure reconstructed from proxy-climate data

This study employs tree-ring data from the International Tree Ring Database and data from the GRIP/GISP2 and Agassiz ice cap cores to reconstruct July sea level pressure patterns over the Northern Hemisphere back to 500 BP. Pressure was selected for reconstruction because it provides an overall view of the large-scale atmospheric circulation. The spatial extent of the data employed in the study is hemispheric and terrestrial. Sites are relatively evenly distributed over the boreal and temperate forest zones, with ice-core data providing data from north of the tree line.

The method of principal components was used to reduce dimensionality in both the SLP and proxy data sets. Reconstructions retaining 3-4 SLP components, explaining approximately 40-45% of the observed variance, and 9-11 proxy components, explaining approximately 38-45% of the observed variance, provided the most reasonable patterns. Variance in the reconstructed patterns coincided with observed variance. The zone of greatest residual standard deviation, 5 mb, was in the Canada Basin, in the Arctic Ocean, with a small secondary maximum just west of the British Isles, where residual standard deviation approached 4 mb. Beyond these specific regions, overall predictive capacity was generally good for the training period, with RMSE values for individual years ranging between 0.9 and 2.5 mb and averaging around 1.3 mb over the 1977 element grid. These were considered reasonable considering the size and diversity of the region of study.