Long-range prediction of monsoon onset over Kerala

ABSTRACT. Using the data of 33 years ( 1961-1993) the effect of the intensity of heat low over central India during the Month of April and Winter (December to February) Eurasian snow cover on interannual variation of monsoon date over Kerala were examined. Composite mean surface temperature over central India during the month of April was higher during early onset years by 3.5° C. April mean surface temperature index (MST) and Winter (December to February) Eurasian snow cover (WSC) are significantly correlated with Monsoon onset dates al 1% and 5% significant levels respectively. Lower surface temperature and excessive snow cover indicate a late onset. A regression equation was developed for long range prediction of onset date over Kerala using MST and WSC as independent variables. The root mean square error (RMSE) of the relationship was found to be 4.6 days. The model was tested using independent data of five years and was found performing well. Contingency tables were developed between the pairs MOD and WSC and MOD and MST. The tables can be used for probability forecasts of early and late onset years.

Momentum budget of low level westerly jet during monsoon onset

In this paper a diagnostic study is carried Out to the source and sink terms for the formation and acceleration/deceleration of low level westerly Jet during monsoon onset. For this purpose momentum budget technique is used. The budget equation is derived in the (x.y.p.t) system. The area is confined to a small box the boundaries over the central Arabian Sea the westerly flow prominent during the onset of monsoon. Each tem in the budget equation is calculated separately. FGG E III b. 1200 UTC data set is used for the analysis. The Coriolis force term is found to be sink term rather than a source tern Tram. Transient north-south term is prominent source terms when time averaged momentum budget is considered. When the time averaged momentum budget for each pressure slab is considered. it is concluded that, north-south force terms are prominent source terms of momentum for all slabs and large .cumulus type convection may contribute to frictional dissipation of momentum for the upper pressure slabs. Frictional force is the main sink term when one examines the momentum budget for each day and the source term are varying day-by-day. On the average, the jet is accelerated during the period. It is also found that the net momentum tendency is small and oscillatory in nature. it is also found that at Minicoy. rainfall is inversely related to momentum tendency and whenever westerly jet is strong (weak) the rainfall is less (more). Distribution of U-momentum is also found to be oscillatory ill nature.

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.

Wind steadiness up to 35 km and its variability before the southwest monsoon onset and the withdrawal

This paper brings out mainly on the quantitative approach to delineate wind direction variability through Wind Steadiness Factor (WSF) - a single parameter which depends on height, wind speed and wind direction. This can be used as a prognostic parameter for the onset and withdrawal of south west monsoon (SW Monsoon) over Kerala. A brief sketch on wind climatology up to 35 km over TERLS (8° 32' N / 76° 52' E) is also discussed to have a background knowledge. From the derived WSF climatology, it is seen that the region between 12.5 km to 18 km is of highest WSF during the SW Monsoon due to the Tropical Easterly Jet (TEJ). Vertical variation of annual WSF has shown well demarcated four layered structure and the wind rose constructed for each layer provides the contribution of WSF attributed by the dominant direction for that particular layer. A WSF value was estimated for the region between 12.5 km to 18 km over Thiruvananthapuram for each available rawin profile [0530 and 1730 hrs (IST)] and inferred that an early incidence and maintenance of WSF well above 80% prior the SW Monsoon supports an early SW Monsoon onset and reverse for a late onset. A late/early WSF decrease from a value of 80% followed by systematic further decrease is associated with late/early withdrawal of the SW Monsoon.

The leading mode and factors for coherent variations among the sub-systems of tropical Asian summer monsoon onset

Previous studies on the Asian summer monsoon (ASM) onset mainly focused on each monsoon sub-system. Mainly based on the monthly mean rainfall and low-level winds in May, this study investigated the dominant onset mode from the perspective of the entire tropical ASM region, which reveals the coherent features among the regional-scale onsets. The results of multivariate empirical orthogonal function (MV-EOF) analysis indicate that the MV-EOF1 presents reduced rainfall and anomalous low-level easterly winds at 850 hPa over the tropical ASM region in May during its positive phase. The corresponding principal component (PC1) is highly correlated with the local monsoon onset dates over Arabian Sea, Bay of Bengal, Indo-China Peninsula, and South China Sea, where the mean monsoon onsets occur in May. The only exception is India subcontinent, where the mean monsoon onsets occur in June. The results indicate that the leading mode captures the synchronized variation of monsoon onset over most of Asian monsoon sub-systems, which exhibits remarkably interannual and interdecadal changes. The factors that modulate the coherent variation of the tropical ASM onset are further examined. The simultaneously delayed ASM onset tends to occur during the easterly phase of the 30- to 80-day oscillation, the decaying phase of El Niño, and the positive phase of Pacific Decadal Oscillation (PDO). The 30- to 80-day oscillation serves as a background condition for the synchronized delayed or advanced ASM onset. El Niño-related sea surface temperature anomalies modulate the tropical ASM onset mode by modulating the tropical Walker Circulation and inducing an atmospheric Rossby wave response. The PDO affects the tropical ASM onset mode mainly via the equatorial Rossby wave response and the extratropical Rossby wave train.

Variability of scatterometer based surface vorticity over Arabian Sea and its use in monsoon onset forecasting

This paper presents the results of the study undertaken to investigate the changes in the surface wind and vorticity fields over south Arabian Sea and adjoining north Indian Ocean and their association with monsoon onset over Kerala. For this purpose QuikSCAT daily gridded data of recent 10 years (2000-2009) have been utilized. The results show that there is a rising trend in the surface wind speed and vorticity over south Arabian Sea about 4-7 days before the monsoon onset over Kerala. Thus, the rising trends in the surface wind speed and vorticity over south Arabian Sea seem to be a good precursor of monsoon onset over Kerala. Continuous monitoring of Oceansat-2 scatterometer surface wind and vorticity fields over south Arabian Sea and adjoining north Indian Ocean during 15th May to 15th June could provide predictive indications of monsoon onset over Kerala on medium range scale.

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.