The Asian–Pacific oscillation over the past millennium in PMIP3 and PMIP4

Atmospheric oscillations in the upper troposphere can exert considerable impacts on regional and global climate change. The Asian–Pacific Oscillation (APO), a recently identified upper-tropospheric teleconnection is not yet well documented during the last millennium. Here we use 13 models from Paleoclimate Modeling Intercomparison Project phase 3 and 4 (PMIP3 and PMIP4, respectively) to first assess their ability in simulating the climatology and interannual variability of the APO in recent decades, and then examine the changes in APO over the last millennium and attribute the associated sources. The evaluation indicates that the models show high fidelity to the observed climatological horizontal and vertical distributions of the boreal summer APO, with the PMIP4 models outperforming the PMIP3 ones. And the majority of the models can reasonably reproduce the out-of-phase teleconnection pattern over the Asia–North Pacific sector and the interannual variability of the APO index (APOI). During the last millennium, the APOI exhibits a remarkable decreasing trend, and the indices averaged in the MCA (LIA) are significantly larger (smaller) referring to the whole millennium. In warm/cold periods, APO changes over East Asia can be attributed to the Tibetan Plateau heating-induced condensation latent heat and the northern part of the East Asian summer monsoon-associated latent heat, whereas the changes over the North Pacific originate from dry adiabatic heat and the superposed weak offsetting latent heat. Additionally, the APOI reveals a notable interannual scale cycle at around five years.