Rainfall over Oman and its teleconnection with El Niño Southern Oscillation

The Sultanate of Oman is located in the south-eastern part of the Arabian Peninsula and covers the larger part of the southern coasts of the Arabian Peninsula in both arid and semi-arid environments except for the southern part which is swept by the monsoon affecting the Arabian Sea during the period from June to September. The summer rainfall over Oman shows year-to-year variability, and this is caused by oceanic and atmospheric influences. In the present study, we tried to explore the influence of El Niño on the rainfall over Oman using different data sets. The empirical orthogonal function (EOF) technique employed to the zonal wind at 850 hPa for the 30-year period shows that the second and third modes of EOF are showing high variability over the Oman regions. The corresponding PCs were subjected to FFT analysis, and it showed a peak about 5–6 years. In addition to this, the zonal wind over the Oman regions is correlated with the global zonal wind and found a significant correlation (1 % significant level). It has already been proved that the wind and rainfall during summer monsoon is in phase. Moreover, the spectral analysis of rainfall at Masirah station and the Niño3.4 index show the similar mode of variability indicating a direct relationship. The correlation between rainfall and the Niño3.4 index is also showing a positive significant value, and therefore, it can be concluded that the El Niño in the Pacific favours rainfall over the Oman region.     

Dynamics of westward propagation and intensification of Lakshadweep low in the southern Arabian Sea

Ocean Dynamics - Westward propagation and the dynamics of intensification of Lakshadweep low (LL) have been investigated in detail. In this study, it is observed that LL is not propagated westwards...     

About the variability in thunderstorm and rainfall activity over India and its association with El Niño and La Niña

Thunderstorms are of much importance in tropics, as this region is considered to have central role in the convective overturn of the atmosphere and play an important role in rainfall activity. It is well known that El Niño and La Niña are well associated with significant climate anomalies at many places around the globe. Therefore, an attempt is made in this study to analyze variability in thunderstorm days and rainfall activity over Indian region and its association with El Niño and La Niña using data of thunderstorm day’s for 64 stations well distributed all over India for the period 1981–2005 (25 years). It is seen that thunderstorm activity is higher and much variable during pre-monsoon (MAM) and southwest monsoon (JJAS) than the rest of the year. Positive correlation coefficients (CCs) are seen between thunderstorms and rainfall except for the month of June during which the onset of the southwest monsoon sets over the country. CCs during winter months are highly correlated. Composite anomalies in thunderstorms during El Niño and La Niña years suggest that ENSO conditions altered the patterns of thunderstorm activity over the country. Positive anomalies are seen during pre-monsoon (MAM) and southwest monsoon months (JAS) during La Niña years. Opposite features are seen in southwest monsoon during El Niño periods, but El Niño favors thunderstorm activity during pre-monsoon months. There is a clear contrast between the role of ENSO during southwest monsoon and post-monsoon on thunderstorm activity over the country. Time series of thunderstorms and precipitation show strong association with similarities in their year-to-year variation over the country.     

Variations in vapor pressure and standardized precipitation evapotranspiration index since AD 1861 over the western Himalaya in India: inference from tree ring-width records

A regional tree ring-width index chronology prepared from various tree core samples of the western Himalaya has been analyzed in relation to climate fluctuations. The correlation analysis of tree ring chronology shows significant positive correlations with regional rainfall and standardized precipitation evapotranspiration index (SPEI) and negative correlations with temperature and vapor pressure (VP) during the spring season. The correlation coefficients (CCs) of tree ring-width index chronology with rainfall, temperature, SPEI, and VP during 1901–1990 are 0.50, −0.49, 0.65, and −0.51, respectively. All CCs are significant at 0.1% level. The highly significant CCs between tree ring-width index chronology and SPEI indicate that tree growth over the western Himalaya is more sensitive to soil moisture availability than rainfall, whereas the rising VP is found to have a significant moisture stress condition to tree growth by accelerating the evapotranspiration, which is not conducive for the development of tree growth in the region. So, based on the strong association between tree ring-width index chronology and SPEI; the reconstructions of SPEI and VP are developed back to AD 1861, that show the long period of dryness during 1936–1963.     

Indian Summer Monsoon Rainfall Characteristics During Contrasting Monsoon Years

The present paper presents a diagnostic study of two recent monsoon years, of which one is dry monsoon year (2009) and the other is wet monsoon year (2010). The study utilized the IMD gridded rainfall data set in addition to the Reynolds SST, NCEP-NCAR reanalysis wind and temperature products, and NOAA OLR. The study revealed that the months July and August are the most crucial months to decide whether the ISMR is wet or dry. However, during July 2009, most of the Indian subcontinent received more than 60 % in the central and western coastal regions. In a wet monsoon year, about 35–45 % of rainfall is contributed during June and July in most parts of India. During these years, the influence of features in the Pacific Ocean played vital role on the Indian summer monsoon rainfall. During 2009, Pacific SST was above normal in nino regions, characteristic of the El Nino structure; however, during 2010, the nino regions were clearly below normal temperature, indicating the La Nina pattern. The associated atmospheric general circulation through equatorial Walker and regional Hadley circulation modulates the tropospheric temperature, and hence the organized convective cloud bands. These cloud bands show different characteristics in northward propagation during dry and wet years of ISMR. During a dry year, the propagation speed and magnitudes are considerably higher than during a wet monsoon year.     

Microphysical characteristics of rainfall during different seasons over a coastal tropical station using disdrometer

Drop size distribution (DSD) over the tropical region exhibit pronounced variations during different monsoon seasons. Measurements from an impact type Joss–Waldovgel disdrometer is used for characterization of drop size distribution and its integral parameters over a tropical coastal station (Thiruvananthapuram, 8.31°N, 76.54°E, 20 m asl). Rain events were identified during the winter, premonsoon, summer monsoon and postmonsoon seasons from 8 years, computed rain duration (min) and accumulated rain water (mm). Rain intensity (mm h^?1), mean drop diameter (D_m, mm) and total number concentration of raindrops (N_T, m^?3) were calculated on each sampling interval and classified in to different bins. The different range bins of rain intensity and their relative contributions towards total rainfall are different for different seasons. Maximum events were reported on the R2 (heavy drizzle/light rain) type, but the contribution of rainfall (mm) is mainly registered on R4 (heavy rain) type. Similarly, the N_T and D_m are also showing different characteristics during different monsoon seasons. Frequency of occurrence of D_m is higher in D_m2 (1–2 mm) followed by D_m1 (D_m < 1 mm) and then D_m3 (2–3 mm) with difference in magnitudes for different seasons. On analysing relative rainfall contribution from different mean diameter bins, it can be observed that D_m2 and D_m3 (1–3 mm) are the major contributors to the total rainfall. In the case of N_T, both frequency and accumulated water are almost same or comparable for the different bins during all the seasons. The D_m and N_T are positively related with different intensity bins. The lower rainfall intensity bins show higher duration during the summer monsoon season and lower duration during the premonsoon season, the higher intensity range bins show lower duration for the premonsoon season and higher duration for the postmonsoon season.     

Significant changes in the ENSO-monsoon relationship and associated circulation features on multidecadal timescale

Climate Dynamics - We observe significant changes in the ENSO–Indian summer monsoon rainfall (ISMR) relationship in past three multidecadal epochs (early epoch: 1931–1960, middle epoch:...