Onset Of Southwest Monsoon Research Articles

Objective method of predicting monsoon onset over Kerala in medium and extended range time scale using Numerical Weather Prediction models

The southwest monsoon onset over Kerala (MOK) is very crucial for an Agricultural country like India hence its prediction in the extended range time scale (about 3 weeks in advance) is very useful for the Kharif season in India. As the declaration of MOK involves subjective interpretation of the forecasters, an objective method of prediction of MOK based on dynamical models could successfully avoid bogus onsets. Two objective prediction methods for MOK are developed based on the real-time extended range forecast (ERF) of rainfall measured over Kerala coast 080-120N, 740-780E and the strength and depth of the westerly wind over the Arabian Sea (050-120N, 550-750E) for the period from 2003 to 2022 are used in the first method. In addition to these 3 indices, meridional pressure gradient along the west coast of India is used in the second method with 4 indices. The MOK date is defined objectively based on these variables exceeding the thresholds in both the methods. The results indicate that the MOK forecast with four indices performed well compared to that with three indices during the whole period from 2003 to 2022 with the mean deviation days of MOK found to be 0.75 and 3.05 days respectively. Overall, the dynamically defined onset date over Kerala based on the real-time ERF indicated useful skill at least about 2–3 weeks in advance. It is also demonstrated that the district-level Multi-Model Ensemble forecast in the medium range (5 days in advance) based on 5 global models can add value to the categorical ERF of MOK for the exact forecast date of MOK.

South-West monsoon onset forecast by analysis of satellite images using objective dialectical classifier

South-West monsoon onset forecast by analysis of satellite images using objective dialectical classifier

EVALUATION OF BIO EFFICACY AND PHYTO TOXICITY EVALUATION OF INDAZIFLAM 500 SC IN ACID LIME

Field experiments were conducted to evaluate the bio efficacy and phyto toxicity of Indaziflam 500 SC in acid lime at Central Block, Horticultural College and Research Institute, Periyakulam during kharif and rabi seasons of the years 2017 and 2018. Experiments were carried out in Randomized Block Design with ten treatments and replicated thrice. Indaziflam 500 SC was sprayed as pre emergence as well as combination with post emergence herbicides in acid lime before the onset of South West Monsoon and North East Monsoon at kharif and rabi seasons respectively. Indaziflam 500 SC either alone or in combination with post emergence herbicide was completely dissolved while making stock solutions. There were no phyto toxicity symptoms on acid lime at higher doses of pre emergence application and combination with post emergence application of Indaziflam 500 SC. Pre emergence application of Indaziflam 500 SC @ 62.5 g a.i./ha recorded significantly lower no. of weed density on 30, 60, 90 and 120 DAA during both the seasons. Increased weed density was observed in untreated control at all stages of crop growth during both the seasons. Pre emergence application of Indaziflam 500 SC @ 62.5 g a.i./ha recorded significantly lower weed DMP at 90 DAA during both the seasons of the study followed by pre emergence application of Indaziflam 500 SC @ 50 g a.i./ha and resulted in higher weed control efficiency at all stages of observation. Significantly higher acid lime fruit yield of 12.45 and 13.85 t/ha was recorded with hand weeding during kharif and rabi respectively due to weed free condition maintained during entire growth stage of the crop and it was followed by pre emergence application of Indaziflam 500 SC @ 62.5 g a.i./ha.

Particulate organic matter dynamics and its isotopic signatures (δ13CPOC and δ15NPN) in relation to physical forcing in the central Arabian Sea during SW monsoon (2017–2018)

Monsoonal changes in particulate organic matter (POM) and its isotopic signatures (δ13CPOC and δ15NPN) were studied (Aug 2017 and 2018) in the central Arabian Sea (21–11°N 64°E) . A low-level atmospheric jet (Findlater Jet) forms during the south-west (SW) monsoon and results in “open ocean upwelling” to the north of the jet's axis. Higher wind speed and jet-induced wind stress curl coupled with low sea surface temperature (SST) and higher nutrients were noticed in 2018 than in 2017. The region to the north of the jet was characterized by substantially shallower Mixed Layers Depths (MLDs) and higher POM contents relative to the jet's axis and southern stations. The molar ratio between particulate organic carbon (POC) and nitrogen (PN) (6.2 ± 1.9in 2017; 6.4 ± 0.9 in 2018) was very close to the canonical Redfield ratio (6.63). The δ13CPOC values (−26.3 ± 1.4‰ in 2017; 25.5 ± 1.4‰ in 2018) exhibited typical marine signature reported from this area and revealed a noticeable inter-annual difference. Relatively higher δ15NPN values noticed in the north (7.68 ± 2.6‰ in 2017; 9.24 ± 3‰ in 2018) indicated regenerated dissolved inorganic nitrogen coming from the oxygen minimum zone (OMZ) into the well-lit zone. The lower δ15NPN values along the jet axis and south of it were attributed to lateral advection of nitrogen from the Somali upwelling. Despite the higher nutrient availability, POC contents in 2018 did not exceed the values in 2017. Considering the total consumption of nitrogen (according to C:N: P = 106:16:1), the potential POC development in 2018 could be double the value in 2017. The interannual differences in SW monsoon onset and wind speed seem to directly control the nutrient supply, affecting plankton community structure and POM variability. Thus, any future change in the physical forcing may directly influence the POC pool and consequent export flux to the mesopelagic.

Effect of south-west monsoon onset on rice area in different agro-climatic zones of Chhattisgarh

Our current knowledge of the climatic impacts on cropping area is limited. This study investigates the impact of monsoon onset on kharif rice area. There are frequent changes in the Indian summer monsoon. The Indian agriculture is highly vulnerable to any change in south-west monsoon, crop production and crop acreage to be influences by variability of monsoon onset. Fall in the yield leads to shortage of food grains and price rise which affects food security of poor most. This study was carried out to identify impact of SW monsoon onset on acreage of kharif rice at different agro-climatic zones of Chhattisgarh during 1989 to 2017. On the basis of correlation analysis we found that there is positive relationship between onset dates of monsoon and kharif rice acreage. The R-values were 0.136 (In CPZ), 0.06 (In BPZ) and 0.122 (In NHZ) for the correlation between monsoon onset dates and rice area. The highest value of correlation co-efficient was found for Chhattisgarh plain zone (r = 0.136) which is significant at the 5% level of significance. In comparison of average rice acreage data under different categories of monsoon onset, we found that late onset conditions are favorable for kharif rice acreage.

The effect of seasonally and spatially varying chlorophyll on Bay of Bengal surface ocean properties and the South Asian monsoon

Abstract. Chlorophyll absorbs solar radiation in the upper ocean, increasing the mixed layer radiative heating and sea surface temperatures (SST). Although the influence of chlorophyll distributions in the Arabian Sea on the southwest monsoon has been demonstrated, there is a current knowledge gap regarding how chlorophyll distributions in the Bay of Bengal influence the southwest monsoon. The solar absorption caused by chlorophyll can be parameterized as an optical parameter, h2, which expresses the scale depth of the absorption of blue light. Seasonally and spatially varying h2 fields in the Bay of Bengal were imposed in a 30-year simulation using an atmospheric general circulation model coupled to a mixed layer thermodynamic ocean model in order to investigate the effect of chlorophyll distributions on regional SST, the southwest monsoon circulation, and precipitation. There are both direct local upper-ocean effects, through changes in solar radiation absorption, and indirect remote atmospheric responses. The depth of the mixed layer relative to the perturbed solar penetration depths modulates the response of the SST to chlorophyll. The largest SST response of 0.5 ∘C to chlorophyll forcing occurs in coastal regions, where chlorophyll concentrations are high (> 1 mg m−3), and when climatological mixed layer depths shoal during the inter-monsoon periods. Precipitation increases significantly (by up to 3 mm d−1) across coastal Myanmar during the southwest monsoon onset and over northeast India and Bangladesh during the Autumn inter-monsoon period, decreasing model biases.

The Impact of the Madden–Julian Oscillation on Cyclone Amphan (2020) and Southwest Monsoon Onset

Cyclone Amphan was an exceptionally strong tropical cyclone in the Bay of Bengal that achieved a minimum central pressure of 907 mb during its active period in May 2020. In this study, we analyzed the oceanic and surface atmospheric conditions leading up to cyclogenesis, the impact of this storm on the Bay of Bengal, and how the processes that led to cyclogenesis, such as the Madden–Julian Oscillation (MJO) and Amphan itself, in turn impacted southwest monsoon preconditioning and onset. To accomplish this, we took a multiparameter approach using a combination of near real time satellite observations, ocean model forecasts, and reanalysis to better understand the processes involved. We found that the arrival of a second downwelling Kelvin wave in the equatorial Bay of Bengal, coupled with elevated upper ocean heat content and the positioning of the convective phase of the MJO, helped to create the conditions necessary for cyclogenesis, where the northward-propagating branch of the MJO acted as a trigger for cyclogenesis. This same MJO event, in conjunction with Amphan, heavily contributed atmospheric moisture to the southeastern Arabian Sea and established low-level westerlies that allowed for the southwest monsoon to climatologically onset on June 1.

Pre-monsoon convective events and thermodynamic features of southwest monsoon onset over Kerala, India – A case study

The thermodynamic characteristics of the atmosphere during the pre-monsoon thunderstorm period and the transition phase from pre-monsoon to monsoon period over the gateway of Indian summer monsoon is investigated in this study. The study utilizes Stratosphere-Troposphere (ST) wind profiling Radar, Microwave Radiometer, and GPS radiosonde observations. A sharp increase in the boundary-layer and middle-layer moisture content, abrupt changes in the mixing height, lifting condensation level (LCL), convective available potential energy (CAPE), and other thermodynamic variables are noted. The pre-monsoon period is characterized by intermittent deep convection that moistens the middle layers, while stratiform clouds are observed in the monsoon period and deep moist layer is observed. The dry middle-layers observed before the monsoon onset got moistened consistently as the low-level jet strengthened. The LCL height was lower than the mixing height after the onset of monsoon. The wet-bulb zero level (WBZ) height was reduced to 2 km during the pre-monsoon and shifted gradually to higher altitudes as the onset of monsoon occurs. Variations in WBZ height may help in the predictability of monsoon onset. The study has also investigated the characteristics of a thunderstorm event before the onset and suitable indices for early indication of the storm.

Atmospheric turbulence and land - atmosphere energy transfer characteristics in the surface layer of the Northern slope of Mt. Qomolangma area : abstract

Based on the turbulent data collected at Quzong site, on the northern slope of Mt. Qomolangma, from April 2005 to March 2006, macro-scale turbulent statistical characteristics and land-atmosphere energy transfer before and after the onset of southwest monsoon were acquired by the eddy correlation method. It was found that Monin-Obukhov similarity theory is applicable for Mt. Qomolangma area. The relationship between normalized wind speed standard deviation and atmospheric stability, variances of normalized temperature and humidity standard deviation with atmospheric stability were simulated in the study. It was also found that energy balance components (net radiation flux, sensible heat flux, latent heat flux and soil heat flux) and surface heating filed had evident diurnal and seasonal changes. Especially under the influence of southwest monsoon, the sensible heat flux and latent heat flux in Quzong area have evident opposite changing trends. The variation characteristics of other surface parameters (surface reflectance and Bowen ratio) is very clear before and after the breakout of southwest monsoon.

Trends of south-west monsoon onset and withdrawal in different agro-climatic zones of Chhattisgarh

Trends of south-west monsoon onset and withdrawal in different agro-climatic zones of Chhattisgarh

Climatological seasonal changes of wind and rainfall in the Philippines

AbstractClimatological seasonal changes of rainfall and lower tropospheric circulation in and around the Philippines are analysed by utilizing the TRMM 3B42 to obtain the 5‐day mean rainfall and ERA‐Interim wind data for the period 1998–2013. In particular, climatological onset and withdrawal processes of the southwest (SW) and northeast (NE) monsoon are investigated. It is found that the onset of the SW monsoon occurs abruptly in mid‐May almost simultaneously over the Philippines except in the southern region. The start of the SW monsoon in the Philippines occurs earlier in the north than in the south. In addition, a climatological increase in rainfall on the west coast precedes the SW monsoon arrival by approximately two pentads, due to tropical cyclone influences in some years. After the SW monsoon onset, the monsoon trough is located over the southern Philippines. In mid‐June, the monsoon trough begins to deepen, and migrates northward to the central Philippines, and easterly winds intrude shortly to the northern Philippines. Then, the entire Philippines is covered by the summer monsoon westerly in early July, which is followed by the rainfall peak over the west coast region occurring in early August. The SW monsoon begins to retreat from the north in mid‐September, and fully retreats from the Philippines rather suddenly in late September. In general, the rainfall amount in the west coastal region remains high for approximately 2 months after this wind reversal, showing relatively larger post monsoon rainfall due mainly to tropical cyclone effects. During the NE monsoon season, the rainfall centre in the east coast is located in the northern and central region until mid‐December. Afterwards it is anchored in the southern region from late December to mid‐March, indicating the fully established NE monsoon season.

The large-bodied copepods off Masirah Island, Oman: An investigation of Southwest Monsoon onset and die-off

Abstract Masirah Island, situated off the southern coast of Oman in the Arabian Sea, is surrounded by upwelled water during the Southwest Monsoon (SWM) and was used as a base for time-series studies of the copepod community for a decade beginning in 2006, covering the Spring Intermonsoon (SIM), April–May, and including June, and the SWM period, August–October. Subeucalanus pileatus and Calanoides natalis were the two most abundant copepod species (33.9% and 32.7% respectively of the total abundances). These two species shifted their dominance during the two main seasonal periods. During the SWM, C. natalis was the dominant species occurring mainly at sea surface temperatures (SST) between 18 and 27 °C while S. pileatus dominated during the SIM occurring at warmer SSTs, frequently between 25 and 28 °C. Results from generalized linear models indicated that the season and the SST were the best predicted variables affecting the abundances of C. natalis and S. pileatus. Calanoides carinatus was also affected by wind speed and wind direction, meaning that strong SWM winds and low temperatures (seasonal upwelling) favor the occurrence of this species. Upwelling favorable winds lasted longer in 2015/2016 than in 2009/2010; peak winds shifted from June in 2009–2011 to July in 2012–2016. The SST was higher in 2016 than in the previous five years. Time-series of C. natalis life stages showed three large peaks of early copepodites from June through September 2016 during the SWM season, in contrast to only one peak observed at the beginning of the SWM season from previous years. This result suggests that C. natalis was able to capitalize on the longer SWM in 2016 by continuing to reproduce throughout September. Diapausing C. natalis reached the surface in mid-to-late June as stage CV, ruling out any reproduction by this species at its overwintering depth. Unfavorable SST at the end of the SWM corresponded to a downward migration by C. natalis in 2015. Overall, the physical forcing of this upwelling region appears to be changing and C. natalis seems to be adapting to these changing conditions by continuing to reproduce at the surface as long as SST is favorable.

Climate change and public health: a study of vector-borne diseases in Odisha, India

The construction of paleo-climatic calendar based on various evidences as well as instrumental records of climatic elements tell that climate has changed to a great extent over time. Scientific evidences establish the fact that the impacts of climate change are having wide immediate as well as long-term indirect effects on public health. Especially the focus is on climate change impacts in terms of increased severity, frequency and spread of vector-borne diseases. Climate changes in terms of increased average temperatures, more intense rainfall, extended summer season and less intense winters can impact the range and incidence of infectious and vector-borne diseases. Odisha, an eastern coastal state of India, is not an exception in experiencing the effects of climate change in spreading of vector-borne diseases. Malaria is one of the most important and widely distributed vector-borne disease observed in Odisha. Transmission of malaria is a dynamic process influenced by the changes in ecological and meteorological conditions. The other vector-borne diseases observed in Odisha since last decade include dengue, Japanese encephalitis and chikungunya. The mid-June period is usually characterized by onset of South West monsoon in Odisha in past. However, the analysis indicates an increase in number of heat wave days in the month of June in recent years, which may be an indicator of gradual shift of monsoon season in Odisha. The results also indicate that vector-borne disease conducive season extends from July till November, nearly half of the year. Further, the dengue and chikungunya are more prevalent in coastal districts, whereas malaria and JE have been experienced in interior districts.

Teleconnection between Cold ENSO Phases and Seasonal Rainfall Over Sri Lanka: Case Study in Kotmale Catchment

Cold ENSO phase or La-Nina is the least considered atmospheric oscillation in the hydro-meteorological sector in Sri Lanka. Therefore, this study focused on understanding teleconnection between cold ENSO phases and the seasonal rainfall over Kotmale catchment. Daily rainfall data of 9meteorological stations were used to investigate behaviour of annual rainfall climatologies of the catchment. Daily mean inflow data of Kotmale reservoir for the duration of 1984 - 2012 was analysed to understand the trends of the reservoir inflow. Sea surface temperature values were extracted from the Japan Meteorological Agency to determine cold ENSO phase. Rainfall trend for each station and inflow trend of the reservoir both declined throughout the selected time duration. However the rate of decreasing trend changes with each station. Seasonal rainfall of all the stations showed negative correlation with mean sea surface temperature over Nino 3 region for December to September. Cold ENSO phases encounter prominent above-average seasonal rainfall during and the onset of South-West monsoon. Further, onset and during of North-East monsoon rainfall over the catchment was weakened due to the impact of cold ENSO phase over Nino 3 region. However, further studies are suggested to identify hidden opposite phenomenon observed during cold ENSO phases on 1964 and 1967 in Kotmale catchment, where all the stations experienced worst below-average seasonal rainfall.

Contrasting trends in southwest monsoon rainfall over the Western Ghats region of India

This study presents the contrasting trends of rainfall in the northern and southern Western Ghats (WG) and examines possible reasons for the phenomenon. The WG is one of the important mountain ranges that run parallel to the west coast of India. The mountain chain lies almost perpendicular to the low level jet stream (LLJ) and hence, receives about three times the average rainfall in India. The onset of southwest monsoon also occurs at this region, and thus WG plays a key role in regulating Indian climate through regional climate modulations. Therefore, detecting changes in the rainfall in WG is necessary to identify changes in regional climate. Here, we examine the features of rainfall received and reasons for the observed rainfall patterns in the northern and southern WG. In general, the rainfall peaks are observed in low elevated areas with high inter-annual variability. We find an increase (decrease) southwest monsoon rainfall of about 1.6 mm day−1 decade−1 in certain pockets of the northern (southern) WG. However, an average trend of + 0.3 (− 0.39) mm day−1 decade−1 is estimated in the northern (southern) WG for the 1931–2015 period. Our analyses reveal that this contrasting trend in rainfall (i.e. positive in the north and negative in the south WG) is due to the northward movement of LLJ; from 10°N to 15°N. This shift in LLJ is triggered by an abnormal increase in the surface temperature of the northern Arabian Sea and tropospheric temperature of the north India in the recent decades. The warming helped the LLJ core to move northwards and that weakened (strengthened) the westerly winds over the southern (northern) WG to significantly change the pattern of southwest monsoon rainfall. Henceforth, this study cautions the changes in the rainfall pattern over WG, which can have significant long-term implications for regional climate change.

Southwest monsoon onset dates over Malaysia and associated climatological characteristics

In Malaysia precipitation prevails throughout the year. However, the southwest monsoon (late May to September) is characterised with low precipitation, less cloud, high outgoing long-wave radiation (OLR) and often featured by dry epochs. Therefore, onset of the monsoon here is best determined by considering multiple onset parameters such as wind, OLR, rainfall and relative humidity. We used modified Malaysian Meteorological Department wind shear index based on major convection centres during the monsoon onset. The 850 hPa winds were chosen to investigate the onsets of the monsoon in view of the marked orographic and mesoscale processes. The next criterion was the presence of sustained westerlies averaged between 850 hPa and 600 hPa from all the available radiosonde stations data over Malaysia for at least 5 days. As the strongest convective activity in the tropics is represented by OLR of less than 220 W m-2, the third criterion was to check whether the value of OLR was greater than 220 W m-2 over the region. The mean date of the summer monsoon onset over Malaysia is found to be 19 May, with a standard deviation of 8 days. Further, climatological composites show that there is a gradual change from easterlies to westerlies from the surface up to 500 hPa in Malaysian stations both in Peninsular Malaysia and East Malaysia during May. OLR and rainfall analysis reveal that, the southwest monsoon daily rainfall over Malaysia is less than 10 mm and OLR is greater than 220 W m-2. Additionally, monsoon onset tends to be late during the El Niño years and earlier during the La Niña years.

Influence of abiotic factors on behaviour and adult emergence pattern of coconut white grub, Leucopholis coneophora Burmeister (Coleoptera: Scarabaeidae)

Leucopholis coneophora Burmeister is a subterranean pest associated with coconut based cropping systems in south India. Feeding damage causes yellowing of fronds and yield reduction. To develop appropriate IPM strategy a basic knowledge on insect behaviour is essential. Four years studies indicated that, adult emergence of L. coneophora was commenced with summer shower in April in Kerala. Delay in summer shower delayed the emergence. After a pause in May, the emergence resumed with the setting of south west monsoon in June. The beetles did not emerge during dry spells in between the rainy days, when the soil temperature (at 10 cm depth) was ≥34.5 °C. Emergence of the beetles started at an illuminance of 124.37 ± 75.5 l in evening and remained active till 2 ± 0.4 l with a maximum swarming at 32.6 ± 15.1 l. Female emergence and mating occurred at 12.04 ± 8.1 l. Female based sex pheromone mediated communication is evident. Strong competition among the males for mating with emerging female, which was evident by a wider operational sex ratio in the initial period (1:10.11) that narrowed down to 1:4.33 in later days. The beetles neither congregate on any host plant nor exhibit phototaxis. Number of beetles entrapped in light traps varied from 1.5–16.5% and hand picking is highly significant over light trapping. Hence hand picking of beetles daily in the evening for 2 weeks commencing from the onset of south west monsoon in Kerala, in Indian subcontinent is suggested as a tool in IPM.

Spatial variability of copepod species distribution in the eastern Arabian Sea in pre-monsoon conditions

This work examines the spatial variability among mesozooplankton abundance and copepod species diversity along the continental margin prior to the onset of southwest monsoon in the eastern Arabian Sea during the year 2011. The eastern Arabian Sea experiences seasonal hypoxia during the receding phase of monsoon that corresponds to low copepod abundance in the slope region. Collectively, fifty copepod species were identified over the continental margin where the diversity index (H′) ranged between 2.59 and 3.73. Bray–Curtis cluster analysis clustered the stations into three groups that corresponded to the continental shelf, slope and break reaches of the eastern Arabian Sea. This spatial variability among the groups is mainly ascribed to changes in the relative contributions of the prevailing species. The distinct genera of the continental shelf were Microsetella spp. and Temora turbinata; at the slope region, Oncaea spp. and at shelf-break, Pleuromamma indica. Overall, herbivorous copepods were numerically abundant at the continental shelf and carnivores at the slope. The discrepancy between the distributions of herbivorous versus carnivorous copepod assemblages may be attributed to environmental factors such as ambient water dissolved oxygen and phytoplankton biomass.

Upwelling induced changes in the abundance and community structure of archaea and bacteria in a recurring mud bank along the southwest coast of India

The present study reports the influence of upwelling on the abundance and community structure of archaea and bacteria in a mud bank formed along the coastal waters of Alappuzha, the southwest coast of India. The signatures of upwelling were observed with the onset of southwest monsoon, characterized by a decrease in sea surface temperature and dissolved oxygen in bottom waters and increase in salinity. Nitrite, nitrate, and phosphate showed a significant increase during upwelling compared to pre-upwelling conditions. There were no significant differences in the abundance of archaea across seasons (pre- and during upwelling conditions) and regions (Station S1, inside and Station S2, outside the mud bank), while the bacterial abundance showed a six-fold increase for S1 and four-fold increase for S2 during upwelling. The PCR–DGGE results show the existence of a transient population of archaea and bacteria in the mud bank region during upwelling. The step up multiple regression models could explain 66.5% and 56.3% of the temporal variations in archaeal and 96.5% and 53.63% in bacterial distribution inside and outside of mud bank respectively, at pre-upwelling conditions. About 67% of the temporal variations in the distribution of archaea and 41.4% in bacteria inside and 49.9% in bacteria outside of mud bank during upwelling conditions could be explained by the same model. The temporal variation in the distribution of archaea outside mud bank during upwelling conditions could not be explained by the present regression model. Our studies indicate that the upwelling induces changes in the abundance and community structure of bacteria in the mudbank, which are modulated by the interactive effects of hydrographic variables. Further studies to unravel the interactive effect of hydrographic variables on the biogeochemical activities of microorganisms would be appreciated.

Southwest Monsoon Change in Thailand Using the Regional Climate Model-WRF

Monsoon onset is one of important characteristics of southwest monsoon which directly influence to Thailand’s water management and agricultural planning. Most of total rainfall amounts in Thailand are provided by the monsoon. Weather Research Forecasting model (WRF) was used as a tool to simulate dialy total rainfall, upper and lower horizontal winds over current period (1990-1999) and future period (2030-2039). These variables are analyzed to study southwest monsoon onset change over Thailand. The simulations show that average monsoon onsets during 1990s and 2030s are in 13th and 20th May, respectively. Standard deviations for the onset were 3 days and 4 days for current and future period, respectively.