Monsoon Onset Dates Research Articles

Overview of the South China sea monsoon experiment

The present paper gives an overview of the key project “ South China Sea Monsoon Experiment (SCSMEX)” operated by the Ministry of Science and Technology of China during the period of 1996–2001. The SCSMEX is a joint atmospheric and oceanic field experiment which aims to better understand the onset, maintenance, and variability of the summer monsoon over the South China Sea (SCS). It is a large-scale international effort with many participating countries and regions cooperatively involved in this experiment. With the field observation in May–August 1998, a large amount of meteorological and oceanic data was acquired, which provides excellent datasets for the study of the SCS monsoon and the East Asian monsoon and their interaction with the ocean. The preliminary research achievements are as follows. (1) The earliest onset of the Asian monsoon over the SCS and Indo-China Peninsula has been well documented. From the viewpoint of the synoptic process, its onset is closely related to the early rapid development of a twin cyclone to the east of Sri Lanka. The conceptual model of the SCS monsoon onset in 1998 was put forward. The 50-year time series of the SCS monsoon onset date was also made. (2) Two major modes, namely the 30–60-day and 10–20-day oscillations were ascertained. The influences of the abnormal SCS monsoon on the precipitation over eastern China and its modes were identified. A strong (weak) monsoon over the SCS usually leads to less (more) precipitation over the middle and lower reaches of the Yangtze River basin, and more (less) precipitation in North China. (3) During the monsoon onset over the SCS, a wide variety of organized mesoscale convective systems (MCSs) were observed by a Doppler radar array deployed over the northern SCS. The relationship between large-scale circulations and MCSs during the monsoon onset process in 1998 was clearly revealed. It was suggested that there is a kind of positive feedback mechanism between large-scale circulations and MCSs. (4) The SST over the SCS during the early period influences the timing of the monsoon onset date and the monsoon’s intensity. During the monsoon onset, the ocean undergoes a process of energy release through air-sea interaction. During the break phase of the SCS monsoon, the ocean demonstrates the process of energy re-accumulation. Obvious differences in the air-sea turbulent flux exchange between the southern and northern parts of the SCS due to different characteristic features of the atmosphere and sea structure were observed in those regions. (5) The verification of impact of intensive observations on the predictive performance is made by the use of regional models. The air-sea coupled regional climate model (CRCM) was also developed under the SCSMEX Project. The simulation of the oceanic circulation in 1998 produced with the model was well compared with the observations.

Globally Unified Monsoon Onset and Retreat Indexes

Abstract Different criteria have been used in the past to define the monsoon onset and retreat over different monsoon regions and even over different parts of the same monsoon region. Here an objective criterion is proposed to define, for the first time, globally unified summer monsoon onset (or retreat) dates using a single meteorological variable (i.e., the global daily 1° × 1° normalized precipitable water data) with the threshold value being the Golden Ratio (0.618). Results are found to be consistent with those determined using long-term rainfall data over most monsoon regions. The precipitable water data have also been used to refine the definition of monsoon regions on a grid-cell-by-cell basis. The objective definitions of these basic monsoon characteristics would provide one of the necessary foundations for global monsoon research. They, along with the onset/retreat data over a 10-yr period (1988–97), would also facilitate the diagnostics and validation of global monsoon simulations.

Hydrological and meteorological features associated with onset of southwest monsoon over Tamil Nadu

A simple statistical concept has been advanced to determine the onset dates of southwest monsoon over Tamil Nadu. The normal onset date has been found to be 4th June with a standard deviation of 6 days based on 22 years of data. It has been shown that the feeble increase in rainfall over Tamil Nadu in association with the onset is not sustained subsequently and the rainfall decreases in the second half of June. The daily upper air data over Chennai and Karaikal for 10 years have been studied and the change in the temporal profiles of these parameters in response to the monsoon onset over Tamil Nadu has been clearly brought out. Some of the unique features of monthly rainfall distribution of Tamil Nadu vis-à-vis those of other meteorological sub-divisions of India in relation to southwest monsoon onset have been discussed.

Study on south-west monsoon rainfall characteristics in arid zone of Haryana

The daily rainfall data of arid zone for a period of 50 years (1946-95) at three locations. Hisar, Bhiwani and Sirsa were analyzed to find out characteristics of south-west monsoon. Mean onset date was observed 4 July at Hisar and Bhiwani with standard deviation of 7 to 9 days respectively, whereas 7 July at Sirsa with standard deviation of 9 days. The onset date of monsoon was inversely related to amount of rainfall at all stations under study. The mean withdrawal dates of south-west monsoon were 15 September at Hisar and Bhiwani and 11 September for Sirsa with standard deviation of 9 days at all three locations. The withdrawal dates also showed inverse and significant correlation with rainfall amount. The mean number of rainy days at Hisar, Bhiwani and Sirsa were 18, 17 and 13 with standard deviation of 5, 6 and 6 days, respectively.

Association between mid-latitude circulation and onset of Indian summer monsoon over Kerala

Teleconnections between mid latitude circulation over different sectors and onset of Indian summer monsoon over Kerala for the period of 1971-89 have been examined. Significant negative relationships are observed between the meridional circulation indices and general perturbation of zonal flow and the monsoon onset over Kerala. Strong and significant positive relationships are also observed between frequency of zonal index over sector 160° E to 45° W and onset dates of monsoon over Kerala.

Onset, withdrawal and intra-seasonal variation of northeast monsoon over coastal Tamil Nadu, 1901-2000

The onset and withdrawal dates of northeast monsoon over coastal Tamil Nadu have been objectively determined for the ten year period 1991-2000. The statistics of onset and withdrawal dates for the 100 year 20th century period 1901-2000 have been presented and discussed. It has been shown by employing techniques such as harmonic/spectral analysis and cross lag correlation over 100 year rainfall data, that the daily rainfall of coastal Tamil Nadu during September-February displays a prominent 40-days periodicity and that rainfall anomalies move from south to north taking nearly 2 days to traverse between Pamban and Chennai separated latitudinally by nearly 400 km. In 70% of the years the onset and withdrawal process of northeast monsoon gets excited by the 40-day periodicity.

Influence of sea surface temperature anomalies in the Gulf of California on North American monsoon rainfall

Linkages between sea surface temperatures (SSTs) in the Gulf of California and the North American monsoon rainfall were examined using observational data and regional model experiments. SSTs in the Gulf start to increase in late June and reach a maximum in August. Monsoon onset dates in the Southwest were determined from the gridded rainfall analysis. Monsoon onsets can occur before SSTs in the Gulf reach a maximum. There is no relationship between monsoon rainfall onset dates or seasonal total rainfall over the Southwest and SSTs in the Gulf. Regional model experiments were then performed to study the impact of local SSTs on monsoon rainfall using the NCEP regional spectral model (RSM). RSM experiments were performed with the observed SSTs for four summers (July–September 1997–2000). The experiments were repeated with the same initial and boundary conditions but with the climatological SSTs in the Gulf of California and its vicinity. The model is able to capture the general features of monsoon rainfall and the diurnal cycle. Warm (cold) SSTs in the Gulf are responsible for more (less) rainfall along the western slopes of the Sierra Madre Occidental. Over the Southwest, the impact of local SSTs is small. Large‐scale flow has more influence on monsoon rainfall than SSTs in the Gulf of California because the SST forcing does not produce significant changes in the low level flow needed to influence rainfall over the Southwest.

A Hydrological Definition of Indian Monsoon Onset and Withdrawal

A diagnostic criterion that retrospectively assesses the onset and withdrawal dates of the Indian monsoon is derived from variability in the large-scale hydrologic cycle. The method is proposed as an improved means with which to understand interannual variability in the monsoon transitions as compared to criteria that rely heavily on rainfall variability over limited spatial domains (e.g., individual Indian districts). The hydrologic cycle is chosen as a key physical basis for monitoring the monsoon due to the essential roles played by zonal and meridional gradients in water vapor, clouds, and rainfall in driving the large-scale monsoon circulation. Moreover, as rainfall is greater than evaporation in wet monsoonal areas, lateral transports of water vapor are required for the existence of monsoonal rains. To diagnose onset and withdrawal, vertically integrated moisture transport (VIMT) is therefore used instead of rainfall, which over the large scale is often poorly measured and modeled. In contrast to rainfall, VIMT is generally well modeled and observed, and its variability, particularly over the Arabian Sea, is substantial during both monsoon onset and withdrawal. An index, named the hydrologic onset and withdrawal index (HOWI), is thus formed from those regions where VIMT variability is pronounced at the beginning and end of the monsoon season. The HOWI offers several advantages as the index is based on fields that are better modeled and measured than rainfall, and the index is indicative of the transition in the largescale monsoon circulation rather than being highly sensitive to synoptic variability and the spatial complexity of the monsoon transitions. The HOWI is shown to be both robust to bogus monsoon onsets and reflective of the timing, rather than the spatial character, of the transitions. Analysis of interannual variability in monsoon onset and withdrawal dates based on the HOWI reveals robust associations that are weak and insignificant when assessed using other onset criteria. For example, the associations between total June‐July‐August‐September (JJAS) rainfall and both monsoon onset and withdrawal are weak (correlations are weaker than 20.11) when onset dates from the Indian Meteorological Department (IMD) or other objective methods are considered. However, the HOWI criterion shows strong correlations between total JJAS rainfall and both onset (0.30) and withdrawal (20.49). Thus, the length of the monsoon season is shown to be strongly related to its overall strength. In addition, while the correlation between IMD onset date and

On the onset of the south china sea summer monsoon in 1998

Through analyzing the NCEP/NCAR reanalysis data, the satellite observational data and the ATLAS-2 mooring buoy observational data, it is shown that May 21 is the onset date of the South China Sea summer monsoon in 1998. There were abrupt variations in the general circulation pattern at the lower troposphere and the upper troposphere, in upper jet stream location and in the convection and rainfall over the South China Sea region corresponding to the outbreak of the South China Sea summer monsoon. It is also indicated that there was rainfall in the southern China coastal region before onset of summer monsoon, but it resulted from the (cold) front activity and cannot be regarded as the sign of summer monsoon outbreak in the South China Sea.

Simulated changes in Southeast Asian monsoon precipitation resulting from anthropogenic emissions

Examined here is the effect on simulated monsoon precipitation of the Intergovernmental Panel on Climate Change (IPCC) IS92a non-intervention anthropogenic emissions scenario (Leggett et al., 1992) over Southeast Asia. By the middle of the next century, increases in greenhouse gases enhance monsoon precipitation by 1% (a net reduction of 7% with aerosol added), though the changes are far from uniform across the region. The monsoon onset date is delayed by about 10 days in the west (15 days when sulphate effects are included). The range of precipitation intensity over the region broadens, with decreases prevalent in the west and increases more widespread in the east. Although the details of the changes are likely to be model dependent, the simulations indicate that changes in mean monsoon precipitation are likely to be accompanied by substantial changes in other characteristics of the monsoon including the date of onset and precipitation intensity. © 1998 Royal Meteorological Society

Forecasting the onset date of southwest monsoon over Delhi

Using latest 32 years (1964-95) data, upper air temperatures and zonal and meridional components of winds of several selected stations for various standard isobaric levels (850 to 10 hPa) are screened for the pre-monsoon months of April & May in order to study their association with onset date of southwest monsoon at Delhi. Data for temperature and wind components for May for several stations exhibit significant correlations with onset-date. Some well known parameters presently in use in long range forecast models of monsoon seasonal rainfall have also been screened similarly. With a multiple regression technique, equations have been developed using suitable parameters from those which showed significant linear correlations.

Variability of the Australian Summer Monsoon at Darwin: 1957–1992

The variability of the Australian summer monsoon is reexamined using data covering 35 monsoon seasons. A new, easily applied objective definition of active and break phases of the monsoon, based solely on the zonal wind at Darwin, is proposed. Attempts to define “wet westerly” onsets are shown to be misleading, since no clear relationship is found between westerly winds and rainfall, on the timescales associated with the transition between active and break phases. The resulting dates of monsoon onset at Darwin differ from those reported in some recent studies, resulting in significantly different relationships with the Southern Oscillation. In particular, the date of monsoon onset is shown to be significantly related to, and hence predicted from, prior values of the Southern Oscillation index. Also in contrast to a number of recent studies that have highlighted the so-called 40–50-day oscillation in the Australian summer monsoon, no dominant timescales are found in the length of the active periods or in the recurrence time between active phases.

Climatic response of the Indian subcontinent to doubled CO2 concentrations

AbstractResults from the United Kingdom Meteorological Office (UKMO) coupled climate model have been analysed over the Indian subcontinent in order to validate the model's performance and to assess the changes in climate and its variability in a simulation with a 1 per cent increase per year in CO2 (compound).The model produces a reasonable simulation of present‐day climate over the Indian subcontinent. At the time of CO2 doubling, the model simulates temperature increases of the order of 1 K to 4 K over the Indian subcontinent during winter and monsoon seasons. The model‐simulated monsoon circulation shifts by 10° latitude towards the north and intensifies by approximately 10 per cent in the warmer atmosphere. The interannual variability of monsoon onset dates and intraseasonal variability of monsoon rainfall are not significantly different when the CO2 concentration doubles. However, the model simulates increased interannual variability of the monsoon rainfall and a greater number of heavy rainfall days during the monsoon.

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.

Spectrum analysis and prediction possibilities of the onset dates of southwest monsoon over Kerala (India)

Maximum Entropy Spectral Analysis of the time series for the onset dates of the southwest monsoon over Kerala (India) revealed several periodicities significant at a 2a a priori level. some at a 3 C a priori level However these contributed only 40-50% to the total variance thus indicating 50-60% as purely random component. Also many of the significant periodicities observed were in the QBO region (T = 2-3 years) which. due to their variable periodicities and amplitudes, are almost equivalent to a random component. Hence predictions were possible only with a limit exceeding 5 days which are probably not very useful for any planning purposes agricultural or otherwise. No relationship was found between onset dates of established monsoon rainfall and the 50 hPa mean monthly equatorial zonal wind for the months of March, April, May or June. However there is a possibility that a relationship may exist between westerly (easterly) winds in May and early (late) onset of the first monsoon (or pre-monsoon ?) rainfall in Kerala. Meager or otherwise.
 

The interannual variability of mid-latitude meridional circulation and its teleconnection with Indian monsoon activity

The statistical relationship between the summer monsoon rainfall over all India, northwest India and peninsular India, onset dates of monsoon and the index of mid latitude, (35° to 70°N) meridional circulation at 500 hPa level over different sectors and hemisphere based on 19 years (1971–1989) data, have been examined. The results indicate that (i) the summer monsoon rainfalls over all India, northwest India and peninsular India show a significant inverse relationship with the strength of meridional index during previous January over sector 45°W to 90°E. (ii) The summer monsoon rainfalls over all India and peninsular India show a significant inverse relationship with the strength of meridional index during previous December over sector 90°E to 160°E, (iii) The summer monsoon rainfall over northwest India shows a significant direct relationship with the meridional index during previous May over sector 160°E to 45°W.

インドネシア・オーストラリア・ニューギニアにおける夏のモンスーンの開始・終了日の解析

インドネシア・オーストラリア・ニューギニアにおける夏のモンスーンの開始・終了日の解析

Influence of southern oscillation index on the variability and predictability of Indian monsoon. A reappraisal

The relationship between the monsoon rainfall throughout all India, northwest India and peninsular India as well as the onset dates of the monsoon and two indices of southern oscillation (SOI), namely Isla de Pascua minus Darwin (I-D) and Tahiti minus Darwin (T-D) pressure anomaly have been studied for different periods. The study indicates that the monsoon rainfall shows a strong and significant direct relationship with SOI for the concurrent, succeeding autumn and succeeding winter seasons. The magnitude of the direct correlation coefficient for the SOI using (I-D) is enhanced over all India and peninsular India if the above seasons happen to be associated with an easterly phase of the QBO (Quasi-Biennial Oscillation) at 50 mb. The result indicates that the strength of the monsoon plays an important role in the following southern oscillation events in the Pacific Ocean. The premonsoon tendency of the SOI anomaly spring minus winter SOI shows a significant positive correlation with monsoon rainfall over all India, northwest India and peninsular India. The absolute value of the positive correlation coefficient becomes highly enhanced over all India, northwest India as well as peninsular India if the 6-month period from December to March is associated with the westerly phase of the QBO. Hence, the premonsoon SOI tendency parameter can be a useful predictor of Indian monsoon rainfall especially if it happens to be associated with the westerly QBO. Significant negative association is also found between the anomaly of monsoon onset dates and SOI of the previous spring season, the absolute value being higher for SOI (T-D) than for SOI (I-D). The negative correlation coefficient becomes enhanced if the previous springs are associated with a westerly phase of the QBO. It shows that the previous spring SOI has some predictive value for the onset date of Indian monsoon, a positive SOI followed by an early onset of monsoon, andvice versa, especially if it is associated with a westerly phase of the QBO.

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.

Objective determination of northeast monsoon onset dates over coastal Tamil Nadu for the period 1901-90

The onset dates of northeast monsoon over coastal Tamil Nadu have been determined by adopting an objective method for the years 1901-90. The various statistical parameters associated with onset dates have been computed and interpreted. Relation between onset dates of easterlies and northeast monsoon over Tamil Nadu has been examined. The normal date obtained has been shown to be by and large consistent with the characteristics of normal daily rainfall of coastal Tamil Nadu. The superposed epoch method has been, used to compute the mean rainfall with reference to onset date and the abrupt increase in rainfall at onset has been clearly brought out The spatial distribution and intensity of rainfall at the time of onset have also been discussed. Daily rainfall has been graphically presented for some years with spectacular onset as well as for some years with subdued onset.