A strong influence of the precession and northern high latitudinal climate on the monsoon seasonality and productivity in the Andaman Sea

The Indian Monsoon (IM) is a vital component of the global climate system, driving seasonal changes in precipitation, productivity, and circulation in the Indian Ocean. However, despite its significance in affecting the socio-economic conditions of over a billion people living in South Asia, the factors influencing the spatio-temporal variability of IM remain uncertain. The Andaman Sea, a core monsoon region, is a unique and relatively under-sampled basin with limited connectivity to the global ocean. To better understand the stadial-interstadial variability in IM seasonality and associated productivity, we studied the relative abundance of planktic foraminifera from the International Ocean Discovery Program (IODP) Expedition 353 Site U1448 in the Andaman Sea. Our findings reveal a strong stadial-interstadial variability in the monsoon seasonality, with a stronger Indian Summer monsoon (ISM) during the interstadials and a stronger Indian Winter monsoon (IWM) during the stadials. Specifically, the strongest ISM was during the early Holocene, associated with higher insolation, while peak IWM intensity was during the Heinrich Event 1 (H1). Moreover, we observed a strong influence of the high latitudinal millennial scale North Atlantic cold and warm events, such as Younger Dryas (YD), Heinrich Event (H1−10), and Bolling-Allerod (B/A), on the Andaman Sea, with warm events typically leading to stronger ISM and cold events leading to stronger IWM. We also report a strong coupling between ISM and productivity, with stronger ISM associated with higher productivity. A distinct stadial-interstadial variability was observed in the magnetic susceptibility (MS) and Red/Blue (R/B) reflectance ratio which were used to understand the calcium carbonate (CaCO₃) and organic carbon (C_org) content variability. The higher CaCO₃ and C_org values were observed during the interstadials and lower values during the stadials. Both, ISM and IWM exhibited a significant periodicity of 24 and 21 kyr, respectively and the summer monsoon was in phase with maximum northern hemisphere solar insolation in the precession band with a minor lag of 2.4 kyr, suggesting the dominant role of precession in controlling the monsoon in the Andaman region. This study suggests a precession modulated insolation variability as the major factor controlling IM variability, along with which, internal feedbacks including the glacial boundary conditions, northern hemisphere climate and greenhouse gas concentration also influenced the IM during the last glacial-interglacial cycle. These findings provide crucial insights into the factors affecting IM variability and will aid in improving our understanding of the climate system's complex dynamics.