Monsoonal mixed layer heat budget of the Indian Ocean: Understanding the biases in coupled forecast models.

The Indian Ocean plays an important role in modulating the global weather and climate. However, many state-of-the-art climate models can't predict the dynamically complex mechanisms of the Indian Ocean accurately. Studies show that the biases in the earlier versions of the Met Office Climate Model developed during the initial days of model simulation and persisted up to climate time scales. To investigate biases in the revised GC5 model, we analyzed 208 monthly forecasts initialized every five days from June to November (2018–2023). The spatial evolution of the SST biases over the Indian Ocean from these forecasts showed specific regions of warm and cold biases with up to a magnitude of ~ -1°C to 1°C. This regional bias formation is examined using the mixed layer heat budget analysis during the Indian summer and winter monsoons to understand the relative contribution of the various parameters in driving this variability. We have selected three warm SST bias regions, on the east coast of Africa, near the Indian Peninsula and on the west coast of Sumatra. The cold bias regions are in the northern Arabian Sea and on the west coast of Java. The primary analysis from the mixed layer heat budget shows that the warm and cold SST biases in the model are modulated mainly by some common parameters such as the net heat flux and total advection. However, further analysis showed that the total advection is more important in the warm bias regions. The vertical mixing term is also significant in generating cold SST biases and this can be a consequence of the positive wind speed biases in the model. Our study also concludes that even though the biases have comparable spatial and temporal magnitude and evolution, the parameters which modulate the SST variability have regional variations. Additionally, anomalously positive precipitation in the equatorial Indian Ocean and the west coast of India and a negative precipitation bias along the east coast of India were also identified. Hence removing these discrepancies in the SST might be crucial for accurately simulating the Indian monsoon.