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.