Delving into the analysis for each zone, we observed certain
characteristics in the particles’ spatial distribution associated with
the forcings and main currents as defined by cLCS (Table 1; Figure S2 in
Supplementary Information and 10):
- Zone 1: The largest particle density is found with 1% windage. These
particles follow the cLCS formed by LC to enter the GoM.
- Zone 2: The particle’s density increases by three orders of magnitude
with 1% windage compared to the particle density by only considering
currents. However, maximum particle confluence occurs with 2%
windage.
- Zone 3: The maximum particle density occurs with 2% windage as the
particles are advected to this area, crossing barriers imposed by the
cCLS from the currents.
- Zone 4: The maximum particle confluence occurs with 1% windage,
although the particle density is in the same order of magnitude
without windage. In comparison, with 2% windage, the particles
cluster west of the cLCS (zone 3) and follow their northward
trajectory over the continental platform (Sheng & Tang, 2004), as
currents follow the orientation of the isobaths over the continental
shelf (Brink, 2016; Lago et al., 2019)
- Zone 5: The maximum particle density occurs with 1% windage, as
particles are displaced south in the eastern CS, leading the particles
in this area and zone 4. With increased windage (2%), the particles
are advected further south, increasing the particle’s density in zone
3.
- Zone 6: The density of particles is larger with 1% windage, followed
by the case without windage. When considering 2% windage, the
particle density diminished considerably, as the particles coming from
the east are advected further south.
- Zones 7 and 8: The highest density of particles occurs in the absence
of windage, showing these areas as being located in the main transport
route by currents and without winds. This result denotes the wind’s
relevance for advecting the particles into the CS.
- Zone 9: For particles to arrive into this area, windage is essential
as it advects particles to the south, creating a transport route
towards the coast of Venezuela.
- Zone 10: The particles only reach this area when considering a 2%
windage, as a strong windage allows crossing into an isolated region
delimited by the cyclonic gyre located off the coast of Central
America (Andrade & Barton, 2000).
The most important result of these experiments is the particles’
geographical distribution according to the effect of wind. Zones 1, 4,
5, and 6 are dominated by windage (1% windage), while zones 2, 3, 10,
and 9 are dominated by windage and Stokes drift (2% of wind). The
distribution of particles can be explained by the persistent presence of
the cLCS identifying transport barriers that prevent particles from
reaching the coast (Figure 2 and Figure 9). For the particles to cross
these barriers, it is necessary for the winds to weaken the cLCS.
The dynamics of surface transport are partially controlled by the inflow
into the CS, which according to Johns et al. (2002), can be divided into
three main groups of passages: the Greater Antilles (zones 6 and 7), the
Leeward Islands (zone 8), and the Windward Islands passages (zone 9),
which coincide with the main transport routes identified by the cLCS.
The seasonal cycle of the mean currents inflow distribution in the
passages connecting the Atlantic Ocean with the CS has an annual and
semiannual variability with a maximum in late spring and summer and a
minimum in fall, continuing to the Yucatan Channel (Johns et al., 2002).
Nevertheless, the NBC is the single largest inflow source (40%) to the
Caribbean (Chérubin et al., 2005), which displays a strong mesoscale
variability in inter-island passage transports. Chérubin et al. (2005)
also found that the current’s maximum velocity position is in phase with
the transport variations and independently of its extension. The
Lagrangian experiments show that identifying transport pathways and
barriers can explain particle displacement distribution, where the wind
is a crucial parameter explaining particle intrusion into the Caribbean.
Once in the CS, the three westward jets described by Chérubin &
Richardson (2007) flow with their speeds decreasing with increasing
latitude. These jets and their relation to our results are described
below.
- The southern and fastest jet is located at ~11.5°N,
coinciding with the passage of particles at zone 9 with 2% windage.
This southern jet is characterized by southern cyclones, which move
northwestward as the anticyclonic CC circulation intensifies.
- The center and second fastest jet, at ~14°N, coincides
with the particle’s passage through zone 8 without wind or with 1%
windage. This center jet flows faster between August and December and
is seasonally intensified by the NBC; this coincides with the larger
particle density found in the zone during this period when considering
low wind influence (1% windage or less). It is important to note that
the intensification of the center jet is due to an increase in the
mean kinetic energy (negative potential vorticity anomaly) that
increases the number of cyclones during the fall. This is observed in
the particle’s trajectory once they enter the CS.
- The northern and slowest jet is found at ~16.8°N,
corresponding to zones 6 and 7. These zones show confluence only
without windage (zone 7) and 1% windage (zone 6). This area is
dominated by mesoscale anticyclones, sustaining westward currents
south of Puerto Rico and Hispañola (Baums et al., 2006), so particles
remain in this area with low wind conditions and are displaced further
south only with 2% windage. Therefore, the wind effect in this region
is decisive in the particle’s trajectory.
Besides the water inflow and current jets, the Caribbean basin is
influenced by atmospheric phenomena such as the easterly waves,
anticyclonic cold fronts (also known as Central American Cold Surges),
Caribbean Low-Level Jet (CLLJ), the trade winds, and the ITCZ. The
easterly waves are wave-type disturbances in the tropical easterly
current. These waves are associated with the hurricane season (summer),
characterized by a cyclonic circulation that deforms the pressure field,
causing the wind direction to change from northeast to the east
(Caviedes, 1991). As such, easterly waves could promote particle
displacement from the Equatorial Atlantic towards the CS by the windage
effect. The arrival of anticyclonic cold fronts typically occurs between
September and April and can extend as far south as 10oN latitude (DiMego et al., 1976). These cold fronts are associated with
a significant increase in wind intensity, cloud cover, atmospheric
pressure, wave height, and a decrease in temperature (Appendini et al.,
2014, 2018; Cao et al., 2020; Ortiz-Royero et al., 2013). Considering
the strong northerly wind component during cold fronts, they could
influence the particle transport by displacing them towards the south,
impending them to reach areas such as the Yucatan Peninsula. The
Caribbean Low-Level Jet (CLLJ) is a near-surface branch of the
easterlies that intensifies seasonally and has a nearly east-west
direction (García-Martínez & Bollasina, 2020; Hidalgo et al., 2015).
The Caribbean Counter Current (CCC) is controlled by the CLLJ, which
during relatively mild wind conditions, promotes the intensification of
the Panama-Colombia Gyre (Orfila et al., 2021). While the CLLJ can
promote windage displacing particles towards the west, during its
relaxation phase, the particle displacement into the CCC and the
Panama-Colombia Gyre could be expected. Finally, the trade winds’
seasonality and the ITCZ’s consequent latitudinal displacement (Aliaga
Nestares et al., 2022; Haffke et al., 2016; Henke et al., 2012; Skliris
et al., 2022) likely influence the currents and particularly the windage
effect on particle transport.