Tropical Intraseasonal Weather Analysis in December 2016

This study aims to analyze the large-scale weather systems and their interaction in December 2016. The climatological background in December 2016 is under La Niña condition. Area over western Pacific to eastern Indian Ocean (IO) has 0.5 to 2^○C warmer SST and is more convective than climatology while region over eastern Pacific and central IO has colder SST and is unfavorable for convection developement.

For intraseasonal to synoptic scale weather systems, over equator, there are three distinct deep convective system accompanying westerly wind moving eastward from eastern Indian Ocean to western Pacific. They are identified as convectively coupled Kelvin wave (KW). In off-equatorial area, four prominent deep convective system propagates westward. They are tropical cyclones, Borneo vortex (BV), and most of them except BV are associated with equatorial Rossby (ER) wave and mix Rossby-gravity/TD-type (MRG/TD) wave.

Multiscale interaction between weather systems and topography is obvious. In early December, climatological westerly wind over eastern Indian Ocean was enhanced by arriving ER wave, therefore deep convection appeared to the west of Sumatra due to wind-terrain interaction. KW then amplified and coupled with the deep convection. Borneo vortex formed through interaction between cold surge and shape of South China Sea (SCS) and strengthened with the help from cyclonic circulation provided by MRG/TD. Around 18^th, the initial circulation of tropical depression 06U, tropical cyclone Yvette and Nock-ten formed almost simultaneously in the cyclonic circulation produced by interaction between westerly brought by KW and easterly trade wind over western north and south Pacific. Deep convection initiated over east coast of Vietnam and Philippine is noticeable due to wind-terrain interaction and its activity is modulated by MRG/TD. Based on Australian summer monsoon (ASM) index defined by Hung and Yanai (2004) and Kajikawa et al. (2010), the ASM onset took place on 11^th and 15^th and it coincides with passage of KW over maritime continent.

We investigate diurnal cycle over western maritime continent and separate this month into convective and suppressed phase. The rain rate and anomalous low-level flow in each phase is affected by Borneo vortex and KW. In convective phase, diurnal amplitude is largest over north western oceanic region of Borneo due to Borneo vortex while it is confined over eastern part of the continents in suppressed phase. The diurnal peak time is broadly similar in both phase but a bit different over southern and north western oceanic region of Borneo. Borneo vortex and KW is critical in modulating diurnal cycle over this region.

Keywords: La Niña, convectively coupled tropical waves, Australian summer monsoon, South China Sea, diurnal cycle, topography