Abstract
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.
Highlights
In this study, we used the CESM version 1.0.4 with a resolution of 3.75° for both latitude and longitude for the atmosphere and a nominal resolution of 3° for the ocean57
Using the CESM model outputs under the transient orbital insolation forcing since 250 ka, we identified a precessional evolution of the summer middle-upper tropospheric thermal contrast between Northeast Asia and the Northwest Pacific, referred to as the paleo-APO
The variability in the paleo-APO is associated with the subtropical high pressure over the North Pacific and the southerly winds over East Asia at the precessional band
Summary
We used the CESM version 1.0.4 with a resolution of 3.75° for both latitude and longitude for the atmosphere and a nominal resolution of 3° for the ocean. During the last 3000 model years (CESM_transient experiment), the orbital parameters are advanced by 100 years at the end of each model year (that is, with an acceleration factor of 100). During the last 3000 model years (CESM_transient experiment), the orbital parameters are advanced by 100 years at the end of each model year (that is, with an acceleration factor of 100)4,10 This CESM_transient experiment was applied in a previous study on the Indian Ocean dipole, and its global annual averaged SAT exhibits a linear increasing trend of 4.04 × 10−4 K per model year (Fig. S3a). This indicates that the data processing methods have little effect on the result
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