Based on a simulation using a newly developed climate system model (Chinese Academy of Sciences-Earth System Model-Climate system component, CAS-ESM-C), the author investigated the Aleutian Low-Icelandic Low Seesaw (AIS) and its decadal variation. Results showed that the CAS-ESM-C can reasonably reproduce not only the spatial distribution of the climatology of sea level pressure (SLP) in winter, but also the AIS and its decadal variation. The period 496–535 of the integration by this model was divided into two sub-periods: 496–515 (P1) and 516–535 (P2) to further investigate the decadal weakening of the AIS. It was shown that this decadal variation of the AIS is mainly due to the phase transition of the Pacific Decadal Oscillation (PDO), from its positive phase to its negative phase. This transition of the PDO causes the sea surface temperature (SST) in the equatorial eastern (northern) Pacific to cool (warm), resulting in the decadal weakening of mid-latitude westerlies over the north Pacific and north Atlantic. This may be responsible for the weakening of the inverse relation between the Aleutian Low (AL) and the Icelandic Low (IL).