Comparative analysis of four types of mesoscale eddies in the North Pacific Subtropical Countercurrent region - part II seasonal variation

Abstract

The North Pacific Subtropical Countercurrent area (STCC) is high in mesoscale eddy activities. According to the rotation direction of the eddy flow field and the sign of temperature anomaly within the eddy, they can be divided into four categories: cyclonic cold-core eddy (CCE), anticyclonic warm-core eddy (AWE), cyclonic warm-core eddy (CWE) and anticyclonic cold-core eddy (ACE). CCE and AWE are called normal eddies, and CWE and ACE are named abnormal eddies. Based on the OFES data and vector geometry automatic detection method, we find that at the sea surface, the maximum monthly number of the CCE, AWE, CWE, and ACE occurs in December (765.70 ± 52.05), January (688.20 ± 82.53), August (373.40 ± 43.09) and August (533.00 ± 56.92), respectively. The number of normal eddies is more in winter and spring, and less in summer and autumn, while abnormal eddies have the opposite distribution. The maximum rotation velocity of the four types of eddies appears in June (11.71 ± 0.75 cm/s), June (12.24 ± 0.86 cm/s), May (10.63 ± 0.99 cm/s) and June (9.97 ± 0.91 cm/s), which is fast in winter and spring. The moving speed of the four types of eddies is almost similar (about 10 ~ 11 cm/s). The amplitude of normal and abnormal eddies is both high in summer and autumn, and low in winter and spring, with larger amplitudes in normal than abnormal eddies. The eccentricity (defined as the eccentricity of the ellipse obtained by fitting the eddy boundary) of the four types of eddies is also close to each other, and their variation ranges from 0.7 to 0.8, with no apparent seasonal variation. The vertical penetration depth, which has no significant seasonal difference, is 675.13 ± 67.50 m in cyclonic eddies (CCE and CWE), which is deeper than that 622.32 ± 81.85 m in anticyclonic eddies (ACE and AWE). In addition, increasing the defined temperature threshold for abnormal eddies can significantly reduce their numbers but does not change their seasonal variation trend.

Statistical Analysis of Multi-Year South China Sea Eddies and Exploration of Eddy Classification

“…An eddy, as a physical derivative rather than a real ocean observation, can indeed yield different results using different eddy detection methods. The Euler algorithm has been validated in multiple regions, including the SCS, the Bay of Bengal, the subtropical countercurrent zone, the Gulf Current, North Pacific Ocean, Southern Ocean, and the Tasman Sea [6,12,23,24,[28][29][30][31][32][33][34][35][36][37][38][39]. Compared with the Okubo-Weiss (O-W) function method and angle winding method, the Euler vector geometry eddy detection method can achieve a higher success of detection rate and a lower excess of detection rate [28].…”

Composite vertical structures and spatiotemporal characteristics of abnormal eddies in the Japan/East Sea: a synergistic investigation using satellite altimetry and Argo profiles

“…Sun et al (2019) 2021) utilized a band-pass Gaussian filter to process microwave SST anomalies with a 2r-6r's half-power cutoff wavelengths before distinguishing abnormal eddies according to whether or not SLA and SST anomalies are of the opposite sign. An et al (2022) and Sun et al (2023) adopted a relatively simple classification criteria that averaged temperature anomalies inside eddies and definite cyclonic/anticyclonic eddies with positive/negative. Temperature anomalies are cyclonic warm-core/anticyclonic cold-core eddies, and the lifespan and spatial scales of eddies were considered to enhance the method's robustness.…”

“…The former recognizes the entire life cycle of an eddy as one case, whereas the latter treats eddy snapshots detected at every moment as data points. Here, we adopt the Euler method to statistically analyze normal and abnormal eddies in the Japan/East Sea because it has been revealed in a previous study that some eddies tend to change their temperature sign and then transition between abnormal and normal states during their entire lifespan, especially in the generation and corruption stages (Dilmahamod et al, 2022;Sun et al, 2023). The matching algorithms between eddies and Argo profiles inevitably discard a considerable number of eddy snapshots because only a limited number of eddies capture Argo profiles inside their boundaries.…”

“…From a conventional perspective, AEs/CEs are often associated with a warmer/colder cores in comparison to that of the background water mass. Nevertheless, recent studies have identified numerous abnormal eddies with opposite-temperature structures, including AEs with cold cores and CEs with warm cores (Itoh and Yasuda, 2010;Liu et al, 2021;Ni et al, 2021;An et al, 2022;Qi et al, 2022;Sun et al, 2023). In this study, these eddies are respectively named cold-core anticyclonic eddies (CAEs) and warm-core cyclonic eddies (WCEs).…”

“…An abnormal anticyclonic eddy surface, characterized by a lensshaped structure and a cold core, was investigated by 3-day insitu observations in the northern South China Sea (SCS), the conductivity-temperature-depth(CTD) and acoustic doppler current profiler(ADCP) measurements produced comprehensive information about the eddy's characteristics (Qi et al, 2022). Using eddy-resolving model data (OFES) from 2008 to 2017, a considerable amount of abnormal eddies were found to exist in the Kuroshio-Oyashio extension region and North Pacific Subtropical Countercurrent (STCC) region, and their spatial and seasonal variations have been exhaustively investigated (An et al, 2022;Sun et al, 2023). Although their definitions and detection methods of abnormal eddies are different, it can be confirmed that abnormal mesoscale eddies are ubiquitous in the global ocean.…”