Detection and characteristics analysis of the western subarctic front using the high-resolution SST product
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Abstract: Oceanic front plays a significant role in the ocean vertical mixing and the regulation of air-sea interaction, among others. The western branch of the subarctic front (WSAF) located in the Northwest Pacific has attained lots of attention given its strong intensity and widespread influence on this region. In this study, we take advantage of the merged sea surface temperature (SST) at a high spatial resolution of 0.05° to investigate the characteristics of WSAF. The front detection algorithm that combines the Sobel operator and histogram analysis is adopted. It is advantageous in both preserving the front intensity represented by the SST gradient as well as reducing the detection noise level. We systematically applied this algorithm to the daily SST products for front detection, based on which the WSAF characteristics including its intensity, occurrence of frequency, latitudinal position and coverage area are then extracted. WSAF is mostly located within a small latitude range between 40°N and 41°N with a clear seasonal trend in its intensity that peaks in the winter and troughs in the summer. The seasonal variation of WSAF intensity is almost consistent throughout the temporal period of interest from 2010 to 2018. Similar seasonality is observed for its occurrence of frequency with the winter-summer contrast reaching up to 5%. The findings presented here shall help better interpret the WSAF characteristics in the long-term run as well as their impact on the regional weather and climate patterns at high spatial resolution.
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Figure 1. The mean SST in the Northwest Pacific from March 2010 to February 2019 (a) and the seasonally averaged SST over the region of interest in winter (b), spring (c), summer (d), autumn (e). The isothermal lines at 5℃, 10℃, 15℃ and 20℃ are overlapped as black contours.
Figure 3. An example of the combined detection algorithm used in this study. The input SST data (a), the longitudinal and latitudinal SST gradient (b, c), magnitude of the SST gradient (d). The probability density function (PDF) of SST data over the red (black) rectangle (e, f) in a, SST front detected by the histogram analysis (g) and the combined algorithm (h). The black curves in g and h denote the central front lines obtained by the Canny algorithm.
Figure 4. Seasonal average of SST gradient and the occurrence of frequency over the detected ocean front by the combined algorithm in winter (DJF) (a, e), spring (MAM) (b, f), summer (JJA) (c, g) and autumn (SON) (d, h), respectively. The black contour in the bottom panel is the frequency of SST gradient larger than 0.01℃/km.
Figure 5. Occurrence of frequency of central front line obtained by the Canny detection algorithm in winter (DJF) (a), spring (MAM) (b), summer (JJA) (c) and autumn (SON) (d).
Figure 6. The monthly mean of different SST gradient percentile of 50th (blue), 90th (orange), 95th (green) and 100th (red), respectively (a); spectrum magnitude of the monthly percentiles (b). WSAF: western branch of the subarctic front.
Figure 7. Temporal variation of the daily western branch of the subarctic front (WSAF) intensity obtained by the combined algorithm (a), the seasonal intensity (b) and the annual intensity (c).
Figure 8. Time series of the western branch of the subarctic front (WSAF) location on a daily basis (blue line). The monthly mean and median are plotted in solid black and dashed red, respectively. The background color represents the averaged SST gradient between 148°E and 150°E.
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