According to the Special Report on the Ocean and Cryosphere (SROCC) of the Intergovernmental Panel on Climate Change (IPCC), the global ocean will continue to warm during the 21st century (IPCC, 2019). By 2100, the ocean warming in the top 2,000 m is estimated to be 5-7 times higher under the business-as-usual scenario and 2-4 times higher under the low emission scenario, relative to the temperature reported since 1970 (very likely). Under this warming scenario, a rise in extreme temperature events in the ocean is also projected. Marine heatwaves (MHWs) are anomalous warm water events in response to the warming ocean-defined when the daily sea surface temperature (SST) exceeds the 90th percentile for 5 or more days (percentile threshold may vary and can be as high as the 99th percentile;
The Madden Julian Oscillation (MJO), the dominant subseasonal variability in the tropics, is widely represented using the Real-time Multivariate MJO (RMM) index. The index is limited to the satellite era (post-1974) as its calculation relies on satellite-based observations. Oliver and Thompson (J Clim 25:1996–2019, 2012) extended the RMM index for the twentieth century, employing a multilinear regression on the sea level pressure (SLP) from the NOAA twentieth century reanalysis. They obtained an 82.5% correspondence with the index in the satellite era. In this study, we show that the historical MJO index can be successfully reconstructed using machine learning techniques and improved upon. We obtain a significant improvement of up to 4%, using the support vector regressor (SVR) and convolutional neural network (CNN) methods on the same set of predictors used by Oliver and Thompson. Based on the improved RMM indices, we explore the long-term changes in the intensity, phase occurrences, and frequency of the winter MJO events during 1905–2015. We show an increasing trend in MJO intensity (22–27%) during this period. We also find a multidecadal change in MJO phase occurrence and periodicity corresponding to the Pacific Decadal Oscillation (PDO), while the role of anthropogenic warming cannot be ignored.
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