The 10-20 Day Tropical-Midlatitude Interactions during the Winter Monsoon Season

Abstract

The lower-frequency variations in the winter monsoon system have been examined using a l0-day low-pass filter. Results indicate that the lower tropospheric north-south thermal gradient over eastern Asia, the Siberian high pressure system, and the eastern Asia subtropical jet stream varied coherently with a time scale of 10-20 days. Composite study reveals that the 200-mb streamfunction variations on the 10-20 day time scale are longsynoptic-scale disturbances propagating slowly eastward. The cyclonic disturbances weaken as they move eastward south of the Tibetan Plateau and begin to strengthen when the lower tropospheric thermal troughs from Siberia move southward. The irrotational circulations on the 10-20 day time scale are closely related to the streamfunction variation with a quarter-wave phase shift. The tropical-midlatitude interactions in the winter monsoon system are seen to be the interactions between these long-synoptic-scale waves and the local monsoon circulations based on the phase relationships among various elements studied. The divergent circulations of the propagating waves trigger the cold air outbreak over eastern Asia when the wave trough is situated south of the Tibetan Plateau. The propagating waves are seen to intensify over the ocean suggesting a transfer of energy from the monsoon system to the waves.

The Extreme Cold Anomaly over Southeast Asia in February 2008: Roles of ISO and ENSO*

“…The variation of EAWM is also well linked to the ISO through the tropical-midlatitude interaction (Pan and Zhou 1985;Hsu et al1990;Compo et al 1999;Kiladis and Weickmann 1992;Meehl et al 1996;Jeong et al 2005;Pan and Li 2007). For example, the temporal variation of surface temperature anomalies at Guangzhou in southern China shows a clear 30-60-day oscillation (Li 1989).…”

Leading Modes of East Asian Winter Climate Variability and Their Predictability: An Assessment of the APCC Multi-Model Ensemble

“…Much effort has been made to understand the variability of the EA winter climate by meteorologists (Lau and Li 1984;Zhang et al 1997a;Gong et al 2001;Wu and Wang 2002;Jhun and Lee 2004;Takaya and Nakamura 2005). Fluctuations from the synoptic to intraseasonal scale have been identified (Murakami 1979;Pan and Zhou 1985;Takaya and Nakamura 2005). The interannual to interdecadal variability of the EA circulation was also examined in some observational studies (see, e.g., Zhang et al 1997a;Chen et al 2000;Zhu et al 2005).…”

Winter Time 10-20 Day Variations in the Upper Troposphere

“…Previous study by Pan and Zhou (1985) has demonstrated the importance of the upper tropospheric long-synoptic scale waves in the modulation of the cold air outbreak over the winter monsoon domain. Results of this study indicate that these longsynoptic scale waves are part of a hemispheric wavetrain that propagates slowly eastward.…”

“…Observational studies have demonstrated that the winter monsoon circulation undergoes strong variations on the medium-range time scale (Murakami and Unninayer, 1977;Murakami, 1981;Murakami and Sumi, 1981;Lau and Lau, 1984;Pan and Zhou, 1985). Composites of the winter monsoon circulation using the peaks in the low pass filtered surge index (Ramage,1971-;Pan and Zhou, 1985, hereafter referred to as PZ) as reference dates revealed that the lower tropospheric thermal anomalies (represented by the 500mb-1000mb thickness field) and the sea-level pressure anomalies propagate southeastward from northern Eurasia toward the subtropical western Pacific.…”

“…Composites of the winter monsoon circulation using the peaks in the low pass filtered surge index (Ramage,1971-;Pan and Zhou, 1985, hereafter referred to as PZ) as reference dates revealed that the lower tropospheric thermal anomalies (represented by the 500mb-1000mb thickness field) and the sea-level pressure anomalies propagate southeastward from northern Eurasia toward the subtropical western Pacific. This behavior is similar to the results from two similar studies (Joung and Hitchman,1982;Lau and Lau, 1984) despite the fact that the reference dates for these two studies are chosen from individual cold surge events rather than the low -pass filtered surge index.…”