Relative roles of land- and ocean-atmosphere interactions in Asian-Pacific thermal contrast variability at the precessional band.

In a 250-kyr transient simulation of the Community Earth System Model (CESM), we identified a precessional forced seesaw of the summer middle-upper tropospheric eddy temperature between Asia and the North Pacific as the paleo-APO (Asian-Pacific oscillation). The paleo-APO variability is out of phase with the precession parameter. Corresponding to a positive paleo-APO phase, both the subtropical anticyclonic circulation over the North Pacific and the East Asian summer monsoon (EASM) strengthen. Summer anomalous sea surface temperature shows a western cold-eastern warm pattern over the extratropical North Pacific and a zonal positive-negative-positive pattern over the tropical Pacific. The variations in the simulated paleo-APO and East Asian southerly wind at the precessional band agree well with the geological proxies at the Dongge, Sanbao, Linzhu, and Hulu caves in China, which also implies that these proxies may well reflect the variability in the southerly wind over East Asia. Sensitivity experiments further reveal that the reduced precession parameter may enhance the positive paleo-APO phase and the associated EASM because of the response of the land-atmosphere interactions to the precessional insolation changes. The effect of the ocean-atmosphere interactions on the paleo-APO is secondary.