The Arctic Oscillation at the mid-Holocene and the Last Glacial Maximum: A comparison between PMIP 2 models

Abstract

The changes in the Arctic Oscillation (AO) are studied for the mid-Holocene (MH) and the Last Glacial Maximum (LGM) compared to the pre-industrial (PI) simulation, using 5 coupled ocean-atmosphere models (i.e. CCSM, ECHAM5-MPIOM1, IPSL, MIROC3.2, and UBRIS-HadCM3M2) from the second phase of Paleoclimate Modeling Intercomparison Project (PMIP 2). In the MH, the amplitude of the simulated AO appears a little smaller than the PI with a small decrease in the standard deviation of the AO index except CCSM. The vertical structures of the zonal-mean zonal wind, geopotential height, and temperature in association with the AO are similar with that in the PI, while the center of the polar westerlies slightly decreases and displaces downward to the lower stratosphere accompanied with the weakening of the polar vortex and the warming of cold polar cap region. In the LGM, the amplitude of the AO largely decreases with an evident smaller standard deviation of the AO index than the PI. Accordingly, it appears to produce quite different vertical structure of the AO. The strength of the polar vortex and westerlies weakens further with the center of westerlies displaced into upper troposphere at mid-latitudes. Meantime, the polar cap region becomes slightly warm in the stratosphere, while it remains cold in the troposphere. The colder climate regimes in the MH and LGM winter are unfavorable for the intensity of the AO.