Koushik Dutta

Evidence for solar forcing on the Indian monsoon during the last millennium

Abstract Solar influence on the intensity of the Indian monsoon is demonstrated using a sediment core from the eastern Arabian Sea dating back to 1200 yr, through pattern matching as well as spectral analysis of proxy records of monsoon and solar activity. The intensity of the Indian monsoon is found to have decreased during periods of solar minima during the last millennium. Periodicities of 200±20, 105±15 and 60±10 yr are observed in the proxy records coinciding with those known for solar cycles. The ∼60-yr periodicity observed in the instrumental rainfall data appears to be of solar origin and supports the hypothesis of solar control on the Indian monsoon on a multi-decadal time scale. Evidence for the presence of a coupled atmospheric forcing for the Indian and East African monsoons on a centennial time scale is also seen.

ΔR correction values for the northern Indian Ocean

Apparent marine radiocarbon ages are reported for the northern Indian Ocean region for the pre-nuclear period, based on measurements made in seven mollusk shells collected between 1930 and 1954. The conventional (super 14) C ages of these shells range from 693+ or -44 to 434+ or -51 BP in the Arabian Sea and 511+ or -34 to 408+ or -51 BP in the Bay of Bengal. These ages correspond to mean Delta R correction values of 163+ or -30 yr for the northern Arabian Sea, 11+ or -35 yr for the eastern Bay of Bengal (Andaman Sea) and 32+ or -20 yr for the southern Bay of Bengal. Contrasting reservoir ages for these two basins are most likely due to differences in their thermocline ventilation rates.

Distribution of natural and man-made radionuclides during the reoccupation of GEOSECS stations 413 and 416 in the Arabian Sea: temporal changes

Measurement of natural and man-made 14 C and 3 H and man-made 90 Sr, 239,240 Pu and 241 Am at two GEOSECS stations, GX 413 and 416 reoccupied two decades later are reported. Deeper penetration of nutrient and oxygen-poor intermediate waters along with man-made radionuclides, especially at GX 413 (near Bab el Mandab), are noticeable. The one-dimensional advection-diffusion model using pre-nuclear 14 C data supports this observation as the vertical advection velocity at GX 413 is 11.8 m/y compared to the 3.6 m/y in the central north Arabian Sea (GX 416).

Decadal Changes of Radiocarbon in the Surface Bay of Bengal: Three Decades after GEOSECS and One Decade after WOCE

Radiocarbon was measured in the surface seawater dissolved inorganic carbon (DIC) of the Bay of Bengal during November 2006. A meridional transect of the ∆14C in DIC was obtained from measurements in closely spaced samples collected roughly along 88°E. The ∆14C of these samples ranged from 44‰ to 57.7‰ (mean 51.8 ± 1.1‰, n = 12), and 38‰ at one station in the northern Bay of Bengal. The overall pattern of 14C distribution in DIC of surface Bay of Bengal during 2006 was roughly similar to that during the WOCE expedition of 1995. These results indicate a ∆14C decline rate of ~4‰ per decade since WOCE in the surface Bay of Bengal, which is much smaller compared to a decline rate of ~25‰ per decade observed in the 2 decades between the GEOSECS and WOCE expeditions, due to the smaller atmosphere-ocean ∆14C gradient.

Estimates of upwelling rates in the Arabian Sea and the equatorial Indian Ocean based on bomb radiocarbon

Radiocarbon measurements were made in the water column of the Arabian Sea and the equatorial Indian Ocean during 1994, 1995 and 1997 to assess the temporal variations in bomb 14C distribution and its inventory in the region with respect to GEOSECS measurements made during 1977-1978. Four GEOSECS stations were reoccupied (three in the Arabian Sea and one in the equatorial Indian Ocean) during this study, with all of them showing increased penetration of bomb 14C along with decrease in its surface water activity. The upwelling rates derived by model simulation of bomb 14C depth profile using the calculated exchange rates ranged from 3 to 9 m a(-1). The western region of the Arabian Sea experiencing high wind-induced upwelling has higher estimated upwelling rates. However, lower upwelling rates obtained for the stations occupied during this study could be due to reduced 14C gradient compared to that during GEOSECS.