Muhammad Rais Abdillah

Charge and discharge of polar cold air mass in northern hemispheric winter

This study shows the variability of polar cold air mass amount below potential temperature of 280 K, and north of 45°N can be understood with a concept of charge and discharge, where anomalously large daily discharge indicates an intermittent occurrence of cold air outbreak. The polar cold air mass amount north of 45°N gradually charges up due to diabatic cooling but dramatically discharges due to cold air outbreak with a pulse width of about 5 days. Cold air outbreaks tend to bring colder winter in East Asia and the east coast of North America, while warmer winter prevails on the northern side of these regions. The cold air mass amount south of 45°N increases just after a cold air outbreak but returns to the normal level soon because of its life time of about 3 days. Therefore, monthly mean of total cold air mass amount in the Northern Hemisphere is negatively correlated with the monthly mean discharge.

Tropical–Extratropical Interactions Associated with East Asian Cold Air Outbreaks. Part I: Interannual Variability

AbstractInterannual variability of winter-mean East Asian cold air outbreaks (CAOs) and its relationship with the tropical climate system during 56 boreal winters (DJF) are investigated. The magnitude of CAO is quantified as winter-mean equatorward cold airmass (CAM) flux below 280-K potential temperature across the 45°N latitude. EOF analysis shows that the interannual variation of East Asian CAOs is attributed mainly to the contributions from western and eastern CAOs. In particular, the western and eastern CAOs tend to be remotely forced by La Nina and El Nino events, respectively.The western and eastern CAOs have distinct climate variability. The western CAO, which is enhanced under the climatic anomalies of high pressure over northern Eurasia and low pressure over the western North Pacific, causes negative CAM anomalies over northern Eurasia and positive ones over midlatitude East Asia. In the tropical region, the western CAO negatively correlates with the eastern Pacific and Indian Ocean SST, both of...

Tropical–Extratropical Interactions Associated with East Asian Cold Air Outbreaks. Part II: Intraseasonal Variation

AbstractIntraseasonal variations of East Asian cold air outbreaks (CAOs) in relation to the tropical atmosphere during 34 winters (DJF) are investigated. This study is a continuation of Part I, which discussed the interannual variability of East Asian CAOs. Two types of quantitative East Asian CAOs, western and eastern CAOs, are examined. Their variations are identified by the zonal integration of equatorward flux of cold air mass (CAM) below 280 K at 45°N over 90°–135°E and 135°E–180°. A day-lagged regression analysis reveals that peaks of intraseasonal western and eastern CAO events are preconditioned by large-scale tropical convection anomalies resembling particular phases of the Madden–Julian oscillation (MJO). Western CAO events tend to occur when the convective phase of the MJO crosses over the Maritime Continent. In contrast, eastern CAO events are triggered by the MJO over the western Pacific. Observations of MJO-related atmospheric anomalies indicate the important roles of poleward Rossby wave tr...

Long‐term trend of cold air mass amount below a designated potential temperature in Northern and Southern Hemispheric winters using reanalysis data sets

This study shows that the total hemispheric cold air mass (CAM) amount below a designated potential temperature of 280 K serves as a good indicator of long-term climate change trends in the polar region. We conduct quantitative analyses of the warming trends of Northern Hemisphere (NH) and Southern Hemisphere (SH) winters using five reanalysis datasets (CFSR, ERA-interim, JRA-55, JRA-25, and NCEP-NCAR) and JRA-55-related datasets (JRA-55C and JRA-55AMIP). Total hemispheric CAM amount for the NH winter shows a significant decreasing trend for most of the reanalyses at a rate of approximately -1.37 to -0.77% decade-1 from 1959 to 2012 and at a rate of approximately -1.57 to -0.84% decade-1 from 1980 to 2012. The CAM amount trends show very different geographic patterns during the two periods, which suggests that geographical patterns of the trends estimated for these periods are largely controlled by internal low-frequency dynamics. The CAM flux across 45°N, which serves as an indicator of hemispheric scale cold air outbreaks, has so large interannual variability that its trend becomes insignificant. In the SH winter, trends of total hemispheric CAM amount and its equatorward flux across 50°S are not consistent among the new reanalyses for 1980-2012. Although total hemispheric CAM amount decreases for 1959-2012, sparse observations and changes in systematic bias, due to the implementation of assimilation of satellite observation data in 1979, compromise the reliability of the trends. Improvements to the reanalyses are desirable as a means of monitoring historical changes in CAM for the SH winter.