The relationship between Indian summer monsoon rainfall and Atlantic multidecadal variability over the last 500 years

This study examines the role of increased Indian Ocean warming and positive Indian Ocean Dipole (IOD) events on the surface chlorophyll concentration in the Eastern Arabian Sea (EAS) during the period 1998 to 2014. Remotely sensed surface chlorophyll concentration – during the month of October when IOD strength is maximum – at three selected areas in the EAS, viz., the central eastern Arabian Sea (CEAS, 73° E–76° E, 13° N–18° N), south eastern Arabian Sea (SEAS, 74° E–77° E, 8° N–13° N) and the southern tip of India (TIP, 74° E–78° E, 5° N–8° N) shows a steady decreasing trend, though not statistically significant. The EAS also has a higher warming trend when compared to the western basin during the study period. Our analysis has shown that in the EAS, local surface winds, remote forcing by equatorial winds and the surface and sub-surface oceanic circulation features are less (respectively more) favorable for inducing coastal upwelling during positive (respectively negative) IOD years. The Dipole Mode Index (DMI) and surface chlorophyll concentration in the EAS is significantly and negatively correlated, pointing to the fact that in the event of occurrence of frequent positive IOD years under a global warming regime, the surface chlorophyll concentration is likely to decrease during fall.

A novel approach using Hybrid Bi-Gaussian fitting is developed and applied to ST Radar raw spectrum for the identification of clear air and rain echoes. The algorithm proved efficient in separating the echoes for all classes of rainfall which enables further computation of clear air vertical velocity and hydrometeor fall velocity from multi-peaked power spectra. The validity of this novel algorithm is established by comparing the profiles derived from adaptive fitting with that obtained using fall velocity directly obtained from Micro Rain Radar (MRR). In this study, the hydrometeor fall velocity derived from ST Radar rain echo after applying the adaptive hybrid fitting is validated against the fall velocity obtained from Micro Rain Radar(MRR) operating at 24.23 GHz. The vertical profiles of fall velocity derived from ST Radar spectrum compares well with that obtained from MRR during heavy and light rainfall intensities.