Zonal wind variations in the stratosphere and mesosphere over the equatorial region of Asia and possible association with Indian summer monsoon rainfall

Behaviour of stratospheric and mesospheric zonal winds in the equatorial region of Asia has been studied using rocket wind data from 921 flights for the period from 1970 to 1993 over Thumba (India) and upper wind data in the lower stratosphere over Singapore (S.E. Asia) from 1970 to 1997. Monthly means of these zonal winds are subjected to spectrum analysis. It is inferred that the quasi-biennial oscillation (QBO) with periodicity of 22 to 30 months and annual cycle with periodicity of 11 to 12 months dominate in the lower stratosphere over Thumba, while the semi-annual cycle is predominant in the upper stratosphere and mesosphere. QBO in the lower stratosphere over Singapore has been observed to be significant with periodicity of 22 to 28 months, whereas the annual cycle has no significance. There exists a prognostic value between easterly & westerly phases of QBO over Thumba during the antecedent months of January to May and the performance of succeeding summer monsoon rainfall over India with strong easterlies favouring decreased rainfall and westerlies or light easterlies favouring increased rainfall over the country a s a whole. Correlation coefficients are statistically significant between the seasonal rainfall over India and zonal winds at 10 hPa (30 km) during winter months and also zonal winds at 30 hPa (24 km) during pre-monsoon months. However, there is an improvement in correlation if combined circulation of zonal winds at 10 hPa during winter and at 30 hPa during pre-monsoon is considered. Zonal winds at 50 hPa (20 km) and 30 hPa (24 km) over Singapore during monsoon months show diagnostic value with seasonal rainfall over India having significant concurrent correlation coefficients; however winds during winter and pre-monsoon do not show significant correlation with Indian monsoon. Thus, this could be used to monitor the progress of south-west monsoon performance but not in predicting it. Over Thumba, the mean zonal winds at 30 hPa during ENSO and anti- ENSO years differ significantly in the annual feature.

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Response of a Global Spectral Model for Simulation of Indian Summer Monsoon Rainfall

Journal of Climate Change ◽

10.3233/jcc200010 ◽

2020 ◽

Vol 6 (2) ◽

pp. 33-46

Author(s):

Srabanti Ballav ◽

Sandipan Mukherjee ◽

Ashok P. Dimri

Keyword(s):

Summer Monsoon ◽

Indian Summer Monsoon ◽

Monsoon Rainfall ◽

Indian Summer Monsoon Rainfall ◽

Model Performance ◽

Summer Monsoon Rainfall ◽

Seasonal Rainfall ◽

Spectral Model ◽

Global Spectral Model ◽

Rainfall Forecast

The present work highlights response of a global spectral model T80L18 with respect to Indian summer monsoon rainfall (ISMR) during 8 years period of 1996-2003. The model performance is evaluated for day-1, day-3 and day-4 retrospective 24-hour accumulated rainfall forecasts from 0300 UTC to the next day 0300 UTC using in-situ rainfall observations of 4491 stations. The model performance is evaluated by assessing: (i) percentage departure and root mean square error (RMSE) of seasonal rainfall forecast, (ii) coefficient of variation (CoV) of seasonal rainfall forecast and observation, along with percentage departure of monthly rainfall forecast and (iii) model performance during a drought and a normal year of 2002 and 2003, respectively. Generally, it is noted that the T80L18 model underestimated high rainfall and overestimated low rainfall, however, with increasing forecast duration prediction over low rainfall areas improved. The model RMSE over central and western India is found to increase with increasing forecast duration; however, the same was found to decrease over Jammu and Kashmir. The CoV of day-1 rainfall forecast is found to be low over all India in comparison to the observed data. In the case of model performance evaluation during a drought and a normal year of 2002 and 2003, it is noted that the model produced higher rainfall over the rainfall deficit regions of observed distribution; whereas the heaviest observed rainfall region (>250 cm) is not well resolved by the model. In general, the T80L18 model performance is noted to be better over central India for mean seasonal rainfall prediction.

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Activity of Madden Julian Oscillation during 2002 and 2006 – A comparative analysis

MAUSAM ◽

10.54302/mausam.v59i3.1263 ◽

2021 ◽

Vol 59 (3) ◽

pp. 321-326

Author(s):

MEDHA KHOLE

Keyword(s):

Summer Monsoon ◽

Indian Summer Monsoon ◽

Seasonal Variability ◽

Monsoon Rainfall ◽

Indian Summer Monsoon Rainfall ◽

Monsoon Season ◽

Rainfall Variability ◽

Summer Monsoon Rainfall ◽

Equatorial Region ◽

Madden Julian Oscillation

The Indian summer monsoon is characterized by very significant intra-seasonal variability. Madden-Julian Oscillation (MJO) is one of the dominant modes of the intra-seasonal variability of the Indian summer monsoon rainfall. The activity of Madden Julian Oscillation during the monsoon seasons of the two years of contrasting intra-seasonal rainfall variability has been examined in terms of rainfall activity over India and eastward propagation of convection in the near-equatorial region. The study shows the contrasting nature, viz., in the monsoon season of 2002, eastward mode dominated whereas in 2006, it remained suppressed.

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