Figure 2. Annual climatological Lagrangian Coherent Structures. The colors indicate the strength of attraction, where red is the strongest, and white indicates low persistent stirring and isolated regions behind a transport barrier.
While cLCS allow us to characterize transport routes and barriers, SOMs enable identifying main patterns of surface velocity and their temporal occurrence. Using SOMs, we identified eight dominating patterns in a 3x3 array (Figure 3). Patterns 8 and 9 are slightly more frequent than the others. They occur from August-December when the North Equatorial Recirculation Region (NERR) and the NBC are well-defined and reach their maximum intensity. The cLCS showed the most extensive spatial coverage during these months in the NERR area. From January to April, patterns 7 and 4 show a significant decrease in the intensity of the NERR and the NBC, while there is an increase in the CC and LC magnitude. These four surface current patterns (8, 9, 7, and 4) occur 54.79% of the time, showing that the most intense surface currents in the study area are present about half the time. Of the four remaining patterns, pattern 3 stands out, as it shows an intensification of the currents from the Equatorial Atlantic to Florida (NERR, NBC, GC, CC, LC, FC), while pattern 2 shows the most well-defined LC, followed by pattern 4. Also, pattern 6 is relevant, as it thickens the NERR before reaching its maximum intensity. BMUs give the temporal evolution of the patterns: the NBC intensifies in the summer months to give rise to the formation and separation of the NERR, reaching its maximum latitudinal extension in September, between 10oN and -5oS. In this same period, the currents within the CS intensify, mainly in July and August, while the LC weakens in late August. Between January and March, the NERR weakens, the CS current intensifies, and the LC is re-established. Each pattern’s occurrence percentage was larger than 10% in all cases. Thus the SOM analysis provides a clear overview of the current variability observed in the HYCOM climatology and its evolution over a year.