Role of the Moist and Dry Components of Moist Isentropic Mass Circulation in Changing the Extratropical Surface Temperature in Winter

• Published in: Geophysical research letters 2021-02, Vol.48 (3), p.n/a
• Main Authors: Liang, Ruxue, Yu, Yueyue, Shi, Chunhua, Guan, Zhaoyong
• Format: Article
• Language: English
• Subjects: Arctic surface warming; extratropical surface temperature; mid‐latitude surface cooling; moist isentropic mass circulation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL091587

This study separates the dry and moist components of the moist isentropic mass circulation (MIMC) on a daily timescale and investigates their relationships with extratropical surface temperature changes in winter (November to February). Results from ERA5 reanalysis data set (1979–2018) show that the MIMC is composed of a poleward warm branch in the upper layers and an equatorward cold branch below. The warm branch (WB) is dominated by the moist component in the mid‐latitude troposphere, but by the dry component in other regions. The stronger moist component of WB at 50°N‐70°N (WB_M) is a better precursory indicator than the dry component (WB_D) for the Arctic surface warming as a result of its dominant role in modifying the downward longwave radiation via water‐vapor‐related processes. The stronger WB_D is coupled with a negative Arctic Oscillation and a stronger, longer‐lasting equatorward transport of colder air, therefore a better precursory indicator of mid‐latitude cold events. Plain Language Summary It is widely known that the stronger poleward transport of warm air into the polar region is always accompanied by a stronger equatorward transport of cold air into mid‐latitudes, leading to Arctic warming and cold events in the mid‐latitudes in winter. However, the abundant water vapor in the mid‐latitude troposphere may complicate this relationship. We separated the meridional transport of the dry and moist components of the cold and warm air masses and put forward two useful precursory circulation indicators for the surface temperature changes in the Arctic region and mid‐latitudes, respectively. In particular, the poleward transport of moist, warm air across the polar circle is a better precursory indicator of Arctic surface warming with a lead time of one week. This is because the water‐vapor transported by the moist component of circulation and its subsequent radiative processes, rather than the warm air mass together with its static energy transported into the polar region, dominates the anomalous warming of the Arctic surface. On the other hand, the stronger poleward transport of dry, warm air is more closely related to a stronger and longer‐lasting equatorward transport of colder air and is therefore a better predictor of cold events at mid‐latitudes. Key Points We designed a transformed method of separating the daily dry and moist components of the moist isentropic mass circulation The poleward transport of warm, moist air in the subpolar region