SYNOPTIC CLIMATOLOGY FOR OCTOBER 2013

SYNOPTIC CLIMATOLOGY FOR OCTOBER 2013The averaged monthly circulation at the lower troposphere for October 2013 is portrayed inFigure 1. The northern subtropical ridge lies within its October long-term mean position ofbetween 30 and 35°N latitude. It extends from Eastern China (110°E) until the Western PacificOcean (160°E). Two anticyclonic circulations border it at the above mentioned locations. Itsoverall shape strongly resembles that of its long-term averaged form. There exists a very slightsouthward shift in the ridge position by around 5°S relative to September 2013. Meanwhile, thenorthern near equatorial monsoon trough is positioned between 10 and 20°N extending from theBay of Bengal to the Western Pacific Ocean. It has shifted slightly northwards compared to itsaverage position, but remained quasi-stationary in relation to last month. Two prominentcyclonic vortices were embedded within, that is one near coastal Odisha, India, and the other atthe Western Pacific Ocean, east of Luzon Island. The cyclonic circulation over Odisha, Indiamay be related to the point where very severe cyclonic storm Phailin made landfall on October10, 2013 in coastal Odisha. The southern hemisphere near equatorial trough remained close to itslong-term average position for the month of October which is in between 0 and 5°S latitude,stretching from the southern Indian Ocean until the Western Pacific Ocean (slightly to the east of150°E). Finally, light southwesterlies at speeds of 5-10 knots occurred over our region thismonth. These westerlies have slightly weakened compared to last month by around 5 knots.The monthly averaged upper tropospheric wind circulation is presented in Figure 2. Thenorthern subtropical ridge at 200hPa stretches from the Northern Indian subcontinent to theWestern Pacific close to the 20°N latitude line. It continued its southward shift by moving 5°south. Nonetheless, it maintained its long-term position for this month that is around 20°Nlatitude. At the same time, the southern hemisphere ridge stretches from the southern IndianOcean to the Western Pacific Ocean between 10°S and 15°S. It remained quasi-stationarycompared to the last month. The tropical upper tropospheric trough (TUTT) also remained quasistationaryrelative to the previous month, lying within 10 and 25°N latitude and east of 150°E inthe Western Pacific Ocean. Prevailing north-easterlies has diminished significantly this month,dropping from 30 knots in September to 20 knots in October. The weakening of prevailing lower

and upper level monthly averaged wind velocity signals the gradual withdrawal of the summermonsoon from Asia.The decadal mean circulation patterns at 850hPa are shown in Figures 3A, Figures 3B, and 3C,respectively. The most prominent synoptic change throughout the decades is the consistentsouthward shift of the near equatorial northern monsoon trough. During the first decade, thetrough lies between 15 to 25°N latitude, stretching zonally from northern India to the WesternPacific Ocean, in the second decade, it drops south to 15-20°N, before finally settling over 5°Nto 15°N latitude. The trough bisects Malaysia during the final decade of October 2013, leading tomore rainfall during this period. Apart from that, the southward shift of the near equatorialmonsoon trough is a sign that the northeast monsoon approaches. In contrast, the northernsubtropical ridge at 850hPa remains consistently between 30 and 35°N throughout the decades.Meanwhile, the lower level winds over our region has weakened drastically between the firstdecade and the subsequent decades. The first decade saw 10-20 knot westerlies over our region,before slackening to light and variable winds (5 knots) in the middle and final decades ofOctober 2013. This dwindling of decadal westerlies is a precursor to the northeast monsoon andalso contributes to enhanced convection during the final part of October 2013.The 10-day averaged 200hPa circulation patterns are displayed in Figures 4A, 4B and 4C. Thenorthern ridge gradually descends southwards throughout the decades. By the third decade itsmean position has shifted by around 5°S. At the meantime the TUTT has also moved southwardand is seemingly less prominent by the final decade. Only the southern ridge remained steadfastat 10-15°S from the Indian Ocean till the Western Pacific Ocean progressing through thedecades. The 200hPa wind direction above our region has shifted from northeasterly during thefirst decade to easterly during the second and third decades. Wind speeds have shown a slightdecrease in the following decades relative to the first decade that is from 20-25 knots in the firstto 15-25 knots in the final decade.Notes :First decade : 01 – 10 October 2013Second decade : 11 – 20 October 2013Third decade : 21 – 31 October 2013