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The total hemispheric polar cold air mass (PCAM) amount below a threshold potential temperature is conserved under an adiabatic condition and is a good indicator of polar warming. While the long-term PCAM trends have been investigated, the cause of its interannual variability remains uncertain. Here we find that the El Nino-Southern Oscillation has a strong impact on the total northern hemispheric PCAM amount below a potential temperature of 280K. The correlation coefficient between the Nino-3.4 and the total PCAM reaches -0.66 over 36 recent winters. During El Nino, the PCAM decreases significantly because of the shorter PCAM residence time at high latitudes. The Aleutian Low intensifies and effectively transfers the PCAM from North America to the North Pacific, causing a significant decrease in the PCAM amount over the northern part of North America and a strengthening of cold air outflows over the central North Pacific. The PCAM coming to the warm ocean disappears rapidly and results in the imbalance of the hemispheric PCAM. This study reveals a notable influence of the tropical forcing on the variability of polar climate.

Plain Language Summary The polar cold air mass (PCAM) is an important component of Earth's lower troposphere. This study finds a strong influence of the tropical signal of the El Nino-Southern Oscillation on the PCAM budget in the Northern Hemisphere using a quantitative estimation of the PCAM. During El Nino, the total hemispheric PCAM amount decreases significantly because of large dissipation of the PCAM over the North Pacific Ocean. This result improves our understanding of how El Nino-Southern Oscillation affects the polar climate.