Subsurface thermal structure of the global ocean is a key factor that reflects the impact of global climate variability and change. Accurately determining and describing the global subsurface and deeper ocean thermal structure from satellite measurements are becoming even more important for understanding the ocean interior anomaly and dynamic processes during recent global warming and hiatus. The extent to which such surface remote sensing observations can be used to develop information about the global ocean interior is essential but challenging.
This work proposes a Support Vector Regression (SVR) method, a popular machine learning method for data regression used to estimate Subsurface Temperature Anomaly (STA) in the global ocean. The SVR model can well estimate the global STA upper 1000 m through a suite of satellite remote sensing observations of sea surface parameters [including Sea Surface Height Anomaly (SSHA), Sea Surface Temperature Anomaly (SSTA), Sea Surface Salinity Anomaly (SSSA), and Sea Surface Wind Anomaly (SSWA)] with in situ Argo data for training and testing at different depth levels. In this study, we employed the Mean Squared Error (MSE) and squared correlation coefficient (R2) to assess the performance of SVR on STA estimation.
Results from the SVR model were validated to test the accuracy and reliability using the worldwide Argo STA data (upper 1000 m depth). The average MSE and R2 of the 15 levels are 0.0090/0.0086/0.0087 and 0.443/0.457/0.485 for two attributes (SSHA, SSTA)/three attributes (SSHA, SSTA, SSSA)/four attributes (SSHA, SSTA, SSSA, SSWA) SVR, respectively. The estimation accuracy was improved by including SSSA and SSWA for SVR input (MSE decreased by 0.4%/0.3% and R2 increased by 1.4%/4.2% on average). The estimation accuracy gradually decreased with the increase in depth from 500 m. With the increase in depth, the absolute value of STA became smaller, i.e., it became more indistinctive in the spatial heterogeneity. The STA became less intensive in the deeper ocean due to the water stratification and stability.
Results showed that SSSA and SSWA, in addition to SSTA and SSHA, are useful parameters that can help estimate the subsurface thermal structure and improve the STA estimation accuracy. Moreover, an obvious advantage for SVR is the absence of limitation on the input of sea surface parameters. Therefore, we can figure out more potential and useful sea surface parameters from satellite remote sensing as input attributes to further improve the STA sensing accuracy from SVR machine learning. This study provides a helpful technique for studying thermal variability in the ocean interior, which has played an important role in recent global warming and hiatus from satellite observations over global scale.