Pacific atmospheric circulation from paleoclimate data assimilation via the ensemble Kalman filter

Climate models predict that anthropogenic warming should weaken the meridional tropical Pacific Walker Circulation as differential increases in precipitation and specific humidity reduce the vertical mass flux in the atmosphere. Such a weakening has not been detected in observations, however, potentially reflecting a combination of model biases and/or the influence of internal variability and sea surface temperature patterns. Instrumental observations of the Walker Circulation are also quite limited, preventing a long-term perspective on its range of variability and response to radiative forcing. Here, we use offline paleoclimate data assimilation and isotope-enabled climate model simulations to reconstruct the Walker Circulation during the last millennium. We evaluate a range of climate fields and circulation indices as reconstruction targets, and we test the sensitivity of the assimilation to the selection of paleoclimate proxies used in the analysis. These results can be used to place current Walker Circulation variability in a multi-centennial context, but highlight the challenges and sensitivities to the choice of methods and data employed in the assimilation process.