A study on climate and biological variability in the Indo-Pacific and their future changes

Abstract

The Indo-Pacific region, comprising the interconnected Indian and Pacific Oceans, plays a central role in shaping global climate and biological variability. The mean state of the Indo-Pacific, characterized by its sea surface temperatures (SST), acts as a key link in the complex web of climate dynamics and is likely to modulate dominant climate variability such as the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation (ENSO), which have profound influences on regional and global climate patterns. These climate variations in turn have a significant impact on biological processes, including primary productivity. Therefore, understanding the detailed interactions between the Indo-Pacific mean state and climate and biological variability is critical to unravelling the complex mechanisms that govern marine ecosystems and climate regulation. Moreover, as the global climate undergoes unprecedented changes, future projections underscore the need to investigate how changes in the Indo-Pacific mean state may amplify or attenuate climate and biological variability. Therefore, the present work is an integrated scientific assessment to investigate different aspects of climate variability and biological responses in the Indo-Pacific region based on observations, Coupled Model Intercomparison Project phase 6 (CMIP6) models and a series of carbon dioxide (CO2) ramp-up and ramp-down ensemble experiments. Tropical convection plays a key role in regional and global climate variability and is predicted to change significantly in both magnitude and spatial patterns under a warming climate. As such changes can have significant climate impacts at global and regional scales, possible clarifications are sought which are crucial for projecting future changes and reducing climate model uncertainties in projected tropical precipitation. Therefore, given the importance of the Indo-Pacific warm pool in tropical climate dynamics and to address the associated uncertainties in Present Day (PD) climate simulations, Chapter II undertakes an in-depth analysis of the CMIP6 climate models. The investigation focuses on understanding how the warm pool intensity (WPI) during the PD climate influences future changes in tropical precipitation and the resulting consequences. It is found that the changes in projected tropical precipitation in the CMIP6 models vary between models, but are largely related to the model's WPI in the PD climate. In particular, models with stronger warm pools in the PD simulation tend to simulate an increase in precipitation in the Central Pacific (CP) and a decrease in precipitation in the Maritime Continent (MC) under greenhouse warming. Significant differences in precipitation between the CP and MC regions induce low-level westerly anomalies over the west-central Pacific, favouring SST warming in the CP region. This suggests that the associated air-sea interactions result in a particular tropical pattern in response to anthropogenic forcing. However, previous research has highlighted the shortcomings of existing climate models in accurately representing the characteristics of the IOD. Given that these model deficiencies extend to biases in the IOD and its associated biological responses, they hinder a comprehensive understanding of the consequential biological impacts of the IOD. On the other hand, as the IOD significantly affects the livelihoods of millions of people in Indian Ocean rim countries, the need for improved representation of the IOD in climate models becomes essential. In an effort to address this critical gap, Part 1 of Chapter III is focused on an investigation of climate model biases that contribute to substantial uncertainties in simulated IOD characteristics in current climate models. Through a detailed analysis of historical simulations from 39 CMIP6 models, the study reveals inter-model variations in IOD amplitude that are closely related to the simulation of the tropical Pacific mean state. In particular, models with warmer Pacific mean states show increased precipitation in the western-central Pacific alongside a decrease in the MC, inducing low-level easterly anomalies in the tropical Indian Ocean. These wind anomalies strengthen the local air-sea coupled feedback over the Indian Ocean during late spring and early summer. The results also highlight a significant correlation between IOD growth rates and the tropical Pacific mean state, suggesting that the local air-sea coupled feedback controlled by the tropical Pacific mean state plays a key role in controlling the pronounced intermodel diversity in IOD amplitude within the CMIP6 models. Changes in Indo-Pacific mean state exert a remarkable influence on tropical climate variability, thereby shaping the climate variability-induced impacts on the ecosystem. Therefore, part 2 of Chapter III focuses on how IOD modulates the biological variability in the Indian Ocean. As a prominent mode of climate variability in the Indian Ocean, the IOD has a significant impact on biological activities in this region. To elucidate the complex biological response to the IOD, previous research has introduced the Biological Dipole Mode Index (BDMI). However, the delineation of the region by the BDMI has limitations in capturing IOD-induced chlorophyll variations in the Indian Ocean. Through analysis of observational data and historical simulations from a CMIP6 model, I show that chlorophyll anomalies in the Indian Ocean exhibit a dipole pattern in response to IOD. During the developing and mature phases of the positive IOD, there is a substantial decrease in chlorophyll in the south-southwest of India, contrasting with a pronounced increase in the southeastern Indian Ocean (SEIO). This response is attributed to anomalous southeasterly winds induced by the IOD, which enhance nutrient upwelling in the SEIO and suppress it in the south-southwest of India, resulting in corresponding changes in surface chlorophyll blooms. Based on these results, a new biological dipole index is proposed that provides a more robust explanation for the surface chlorophyll response to IOD in the tropical Indian Ocean. Consequently, these results highlight the profound influence of IOD on oceanic chlorophyll and underscore the importance of a more comprehensive understanding of the associated biophysical interactions in the region. Furthermore, given the limited investigations of future changes in the biological impacts of the IOD, I extend my investigation to examine the CMIP6 models and to disentangle the complex interaction between IOD and chlorophyll concentrations in Part 3 of Chapter III. The results reveal an enhanced IOD-chlorophyll coupling, leading to increased primary productivity during the June to November period under the influence of global warming in the SEIO. Despite a notable reduction in IOD-induced upwelling in the SEIO, there is a significant increase in chlorophyll anomalies attributed to the shallowing of the mean thermocline in the region. This shallower thermocline is shown to be a key factor that favors an increase in the mean nutrient concentration in the upper layer, thus facilitating enhanced nutrient supply to the surface layer, even in the presence of weakened upwelling. Consequently, these results highlight the consistent effects of IOD on chlorophyll dynamics in the Indian Ocean under a warming climate. However, it is noted that most climate models struggle to accurately capture the climatology and interannual variability of chlorophyll in the tropical Indian Ocean. Thus, the limitations of CMIP6 models in reproducing the biological properties of the Indian Ocean underscore the complexity of biophysical interactions in the region. Given the identified biases in IOD and Indian Ocean chlorophyll simulations within current climate models, there is an urgent need for the development and use of advanced, fully coupled models to improve our understanding of the complex interactions between biological and climatic variability in the Indian Ocean. In addition to the variability observed in the Indian Ocean, shifts in the mean state of the Indo-Pacific will have a significant impact on the dynamics of the tropical Pacific, with consequent environmental and socio-economic impacts. In particular, Convective Extreme El Niño (CEE) events, which are characterized by intense convective events in the eastern Pacific and are known to be directly linked to anomalous global climate conditions, are expected to become more frequent as global warming continues. Investigating future changes in the characteristics of CEE, including frequency and intensity, in response to different climate change scenarios is emerging as a key scientific question. Given the changes in global mean precipitation patterns in response to CO2 forcing, a key investigation in Chapter IV focuses on understanding how CEE events respond not only to CO2 forcing, but also to CO2 removal scenarios. Through a comprehensive analysis of a series of idealized CO2 ramp-up and ramp-down ensemble experiments, I show a persistent increase in the frequency and maximum intensity of CEE events during the ramp-down period compared to the ramp-up phase. These changes in CEE dynamics are closely related to the southward shift of the Intertropical Convergence Zone and the enhanced nonlinear response of precipitation to SST changes during the ramp-down period. Also, the results highlight that the increased frequency of CEE events has a significant impact on regional extreme events, suggesting that potentially devastating weather events are likely to occur more frequently in the future, even in the context of reduced or negative CO2 emissions, and the impacts could be much worse than expected due to hysteresis/irreversible behavior of the climate system. Therefore, climate change mitigation policies need to consider not only the mitigation of immediate climate change damages, but also the prevention of expected irreversible changes. Thus, this thesis highlights the complex interactions between the mean state of the Indo-Pacific and its significant impact on climate and biological variability. The results emphasize the linkages between atmospheric and oceanic processes in the Indo-Pacific, highlighting the need for a thorough understanding in order to accurately project future changes. In the context of environmental crossroads, this research provides valuable insights that can inform adaptive strategies and policies that promote sustainable coexistence with the dynamic forces operating in the Indo-Pacific region. Furthermore, my detailed analysis reveals the urgent need for improved climate models to effectively address the environmental changes taking place in the Indo-Pacific, underscoring the urgency for advances in climate science to address the challenges of a changing climate in this important region.