Impact of Marine Heatwaves on Ocean Carbon Sink: A Case Study of Coastal Areas in California, USA

An extreme event, Marine heatwave, has become a pressing concern in recent years. It is defined as a discrete event where the sea surface temperature remains above a specific threshold value of climatology for several consecutive days, and the intervals between two consecutive abnormal high-temperature events are less than two days. Due to climate change, there is an increasing trend in both the intensity and duration of marine heat waves. Marine heatwaves may not directly affect human society; however, they can pose significant threats to marine ecosystems, coastal communities, and the ocean carbon sink, thereby impacting human well-being. The ocean carbon sink is the most significant carbon sink among the world's three major carbon sinks. It absorbs around 25% of anthropogenic carbon dioxide emissions annually. Dissolved inorganic carbon within the ocean carbon sink relies on the carbon sequestration of biological pumps such as coral, seagrasses, and kelps to store it in the deep water. Influenced by the El Niño-Southern Oscillation and currents, the northeastern Pacific Ocean is a hotspot for marine heatwaves, typically beginning from the North Pacific offshore regions in the spring and impacting the U.S. West Coast in the fall. Consequently, the coastal area of California is selected as the study area and divided into three regions.

Previous studies have shown that the escalating severity of marine heatwaves may result in these biological pumps losing their functions or habitats. However, regarding ocean carbon sequestration, whether the incapacities of these biological pumps due to marine heatwaves will have a short-term impact on the carbon sequestration capacity in the ocean remains to be verified. This study aims to analyze the time series of marine heatwaves and ocean carbon sink capacity with the time series analysis and determine the impacts on ocean carbon sink. We categorize marine heatwave extreme events in California into three indicators and the ocean carbon sequestration capacity into physical and biological indicators. Improved Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (ICEEMDAN) is employed to extract the trends and interannual variations. Meanwhile, to identify the correlations between the marine heatwave and the ocean carbon sink at different time points and different time scales, we apply Time-Dependent Intrinsic Correlation (TDIC). Due to the longer temporal scales in changes in the ocean, the impact of marine heatwaves on the ocean carbon sink may have a potential delay. Therefore, we employ Time-Dependent Intrinsic Cross-Correlation (TDICC), a method based on TDIC that could be utilized to analyze the time-lag effects in the interaction between marine heatwaves and the ocean carbon sink.