Land-sea Thermal Contrast in Relation With Summer Monsoon Onset Over the Chao Phraya River Basin

An earlier onset of the Southeast Asian summer monsoon (SAM) was observed over the Chao Phraya River basin in Thailand using Thai Meteorological Department (TMD)-derived high-resolution merged rainfall from 1981 to 2016. As the SAM is precipitous, its variability depends on many local and global factors, such as thermal conditions over the Bay of Bengal (BoB) and Tibetan Plateau (TbT). Despite tremendous studies in the past, the role of thermal heat contrast over SAM is still not fully understood. Using the observation and reanalysis datasets, it was found that the absolute value of total heat over the BoB was higher. However, the interannual variability in total heat is higher over the TbT. Significant changes in surface temperature (±1.5°C), air thickness (±20 meters) and geopotential height found over the TbT were associated with early (late) SAM onset. The results also suggested that the significant changes in air thickness were influenced by the surface temperature difference over the TbT, and the changes in the integrated apparent heat source and integrated apparent moisture sink were up to ± 100 Wm−2, which resulted in stronger (weaker) convective activities over the BoB and mainland of the Indochina Peninsula during early (late) SAM onset. At the intraseasonal timescale, the instance MJO found over the Indian Ocean and Western Hemisphere at 4 to 10 days span during early SAM onset. An opposite scenario is found for a late SAM onset years with MJO location over Western Pacific and Maritime continent.

EMPIRICAL ORTHOGONAL FUNCTIONS ASSOCIATED WITH PARAMETERS USED IN LONG RANGE FORECASTING OF INDIAN SUMMER MONSOON

EEmpirical Orthogonal Functions (EOF),. associated with the; parameters for long range forecasting of Indian summer monsoon onset and seasonal. rainfall have been discussed. It was found that the percentage of variance explained was 77 and 67 respectively through the first four EOF. The highest correlation coefficient with the onset date was found for the first function which showed the maximum influence of Cobar (Australia) and Darwin (Australia) zonal winds on the onset date. It was interesting to note that for rainfall prediction predominant effect on the first EOF was noticed of 50 hPa ridge over northern hemisphere, Eurasian snow cover, Argentina pressure (negatively correlated) and 500 hpa ridge, 10 hPa Balboa wind, north, central India and east coast minimum temperatures, and northern hemisphere temperature. However, the Influence of EI-Nino, equatorial pressure and Darwin pressure (Including Tahiti minus Darwin) and Himalayan snow cover was almost negligible. The eigen index for the onset date suggests a complementary method for its application In long range prediction of summer monsoon onset date.

Re-determination of normal dates of onset of southwest monsoon over India

The onset of southwest monsoon over various parts of the country can be early, timely or late as compared to a set of normal dates. Advance of monsoon could be either rapid or slow or sometimes there could be prolonged stagnation in the advance of monsoon. The timing of the monsoon onset is of vital importance to the agricultural sector and water replenishment. The climatological normal dates of onset which are being used at present are based on a much older data set (1901-1940) obtained from 149 stations. In this study, the climatology of the summer monsoon onset over different parts of India is derived using the mean pentad precipitation data of 569 stations spread all over the country, from 1971-2000. It has been observed that the revised dates of onset of monsoon over the Andaman Sea is 20 May, over Kerala is 1 June, over the northeastern parts is 5 June and the date of monsoon covering the entire country is 15 July (same as the existing dates). Considerable differences between the existing and the re-determined dates of onset are noticed over parts of south peninsula and western parts of central and adjoining northwest India where the re-determined dates are advanced by 1 to 3 days and delayed by 10 to 15 days respectively with respect to the existing normal dates of arrival of monsoon. The Standard Deviation of the re-determined normal dates ranges between 7 to 14 days with larger values over the northwest and west central parts and interior peninsular India.

Progress of Indian summer monsoon onset and convective episodes over Indo-Pacific region observed during 2009-2014

Summer monsoon onset progress from the oceanic region of Southeast Bay of Bengal / Andaman Sea (Oceanr) up to extreme southwestern part of India (Kerala) for the years 2009 to 2014 is investigated. Synoptic weather information, INSAT/KALPANA-1 as well as cloud imageries archived from Dundee Satellite Receiving Station for May and early June for these years are used in the analysis. Upper-air reanalyzed winds from NCEP/NCAR and OLR data archived through NOAA satellites are also used. During the study period, the dates of monsoon onset as well as the time required for the advancement of onset from Oceanr to Kerala have shown a large variation. An attempt is made to investigate the causes for such variations. The results indicate that intense disturbances which formed over north Indian Ocean in 2009, 2010, 2013 and 2014 and over west-north Pacific Oceanic region in 2011 and 2012 have contributed for the same. Analysis is carried out, limiting its focus to bring out the role of these convective events in the observed variation of onset timing and its progress by taking case to case review of these events and bringing out their influence through synoptic analysis. Utility of this information in prediction of the progress of Indian summer monsoon onset is also brought out.