In order to optimize frontal detection in sea surface temperature fields at 4km resolution, a combined statistical and expert-based approach is applied to test different spatial smoothing of the data prior to the detection process. Fronts are usually detected at 1km resolution using the histogram-based, single image edge detection (SIED) algorithm developed by Cayula and Cornillon in 1992, with a standard preliminary smoothing using a median filter and a 3×3pixel kernel. Here, detections are performed in three study regions (off Morocco, the Mozambique Channel and north-western Australia) and across the Indian Ocean basin using the combination of multiple windows (CMW) method developed by Nieto, Demarcq and McClatchie in 2012 which improves on the original Cayula and Cornillon algorithm. Detections at 4km and 1km resolution are compared.Fronts are divided into two intensity classes (“weak” and “strong”) according to their thermal gradient. A preliminary smoothing is applied prior to the detection using different convolutions: three type of filters (median, average and Gaussian) combined with four kernel sizes (3×3, 5×5, 7×7, and 9×9pixels) and three detection window sizes (16×16, 24×24 and 32×32pixels) to test the effect of these smoothing combinations on reducing the background noise of the data and therefore on improving the frontal detection. The performance of the combinations on 4km data are evaluated using two criteria: detection efficiency and front length. We find that the optimal combination of preliminary smoothing parameters in enhancing detection efficiency and preserving front length includes a median filter, a 16×16pixel window size, and a 5×5pixel kernel for strong fronts and a 7×7pixel kernel for weak fronts. Results show an improvement in detection performance (from largest to smallest window size) of 71% for strong fronts and 120% for weak fronts. Despite the small window used (16×16pixels), the length of the fronts has been preserved relative to that found with 1km data.This optimal preliminary smoothing and the CMW detection algorithm on 4km sea surface temperature data are then used to describe the spatial distribution of the monthly frequencies of occurrence for both strong and weak fronts across the Indian Ocean basin. In general, strong fronts are observed in coastal areas; whereas weak fronts, with some seasonal exceptions, are mainly located in the open ocean.This study shows that adequate noise reduction achieved by a preliminary smoothing of the data considerably improves the frontal detection efficiency as well as the global quality of the results. Consequently, the use of 4km data enables frontal detections similar to 1km data (using a standard median 3×3 convolution) in terms of detectability, length and location. This method is easily applicable to large regions or at the global scale, with far less constraints of data manipulation and processing time using 4 km data relative to 1 km data.
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