Isentropic Analysis of Polar Cold Air Mass Streams in the Northern Hemispheric Winter Monsoon

Isentropic Analysis of Polar Cold Air Mass Streams in the Northern Hemispheric Winter Monsoon

Toshiki IWASAKI, Yuki KANNO and Muhammad Rais ABDILLAH Graduate School of Science, Toshoku University, Sendai, Japan

1. Mass-weighted isentropic zonal mean (MIM) cold air streams

A diagnostic method is presented of polar cold air mass streams defined below a potential temperature. In NH winter, MIM's mass stream functions have a distinct extratropical direct (ETD) meridional circulation with a maxima around (280K, 45N), indicating that ć=280K is appropriate to the threshold of polar cold air mass. The high-latitude downward motion indicates the diabatic generation of cold air mass, whereas the mid-latitude equatorward flow does the cold air outbreak. The strength of low-level equatorward flow is under control of wave-mean flow interactions, where the equatorward flow consists of the extratropical pumping relation (Iwasaki and Mochizuki, 2012).

2. Geographical distribution of the cold air mass stream in the NH winter

In the NH winter, the polar cold air mass flux below the threshold potential temperature of 280K has two distinct mainstreams, i.e., East Asian (EA) stream and North American (NA) stream (Iwasaki et al., 2014). The former grows over the northern part of the Eurasian continent, turns over the Siberia toward East Asia, and disappears over the western North Pacific Ocean. The latter grows over the Arctic Ocean, flows toward the East Coast of North America, and disappears over the western North Atlantic Ocean. These match with main routes of intermittently occurred cold surges which bring severe cold waves over East Asia and eastern North America. In East Asia, the equatorward cold air mass flux, which is induced by the eastward pressure gradient caused by the Siberian high and Aleutian low, is a good indicator to evaluate cold surge events quantitatively (Shoji et al, 2014).

3. Extratropical-tropical interactions associated with cold air outbreaks in East Asia

The East Asian cold air stream is decomposed into the western and eastern CAOs at 45N by EOF analysis. These are significantly exerted by remote forcing from the tropics, such as the ENSO and the MJO. The western and eastern CAO's are dominant in La-Nina and El-Nino, respectively. The western CAOs become active when the MJO enhances the precipitation over the Maritime Continents, whereas the eastern CAOs become active when the MJO enhances the precipitation over the central Pacific Ocean. Particular attentions are paid to the effectiveness of these precipitations as a precursor of cold surges over East Asia from the viewpoint of medium- and long-range forecasts.

4. A two box model

A companion paper will be presented in this session by Kanno. A two box model composed of residence time in the genesis box, and life time in the loss box is proposed (Kanno et al., 2015). It will compare the cold air mass amounts between the two hemispheric winters, and examine the validity of the conceptual charge-discharge model for the winter monsoon in the NH winter, based on the two box model.

Key words: isentropic analysis, winter monsoon, cold air outbreaks, extratropical-tropical interactions

References Iwasaki, T., and Y. Mochizuki, 2012: SOLA, 8, 115-118. Iwasaki, T., T. Shoji, Y. Kanno, M. Sawada, M. Ujiie, and K. Takaya, 2014: J. Atmos. Sci., 71, 2230-2243. Shoji, T., Y. Kanno, T. Iwasaki, and K. Takaya, 2014: J. Climate, 27, 9337-9348. Kanno, Y., T. Shoji, and T. Iwasaki, 2015: Atmos. Sci. Lett., 16, 70-76.