3.2. Local Meteoric Water Line and hydroclimatic mechanisms
The resulting LMWLs (Fig. 7) follow the expected trends based on
depletion or enrichment of isotopic composition of
δ18O and δ2H from theoretical
considerations such as i) the orographic distillation that generates
depletion of δ18O and δ2H values due
to fractionation generated in the advance of moisture flow into the
continent (Dansgaard, 1964; Rozanski et al., 1993; Clark & Fritz,
1999; Mook, 2002; Aggarwal et al., 2005) ii) the proximity to
atmospheric moisture sources (Guan, Zhang, Skrzypek, Sun, & Xu, 2013);
iii) the thermodynamic conditions of sources; iv) the mixing ratio of
sources (Rindsberger, Magaritz, Carmi, & Gilad, 1983; Rindsberger,
Jaffe, Rahamim, & Gat, 1990), and v) the amount effect that generates
depletion in hydrogen and oxygen isotopes with the increase in monthly
and annual precipitation of different places and with the intensity of
the storms (Dansgaard, 1964).
[Insert Figure 7]
As in our Flexpart experiment, isotopic signals from the LMWLs also
highlight the seasonal behavior of moisture sources for the two regions.
More specifically, from December through February (Fig. 7a, 7b, 7c), the
Andean region exhibits a variation range of δ2H
(δ18O) that oscillates between -20 and -60 ( -5 and
-9) ‰ VSMOW, and the Caribbean also varies with the highest values of
δ2H oscillating between 0 and -20 ‰ VSMOW, and for
δ18O between 0 and -5 ‰ VSMOW. In the Andean region
these values represent the depletion of the heavy isotopes from coastal
regions towards the continental interior (orographic distillation),
while in the Caribbean, more depleted values represent the first
condensate from marine moisture, indicative of meteorological conditions
(such as relative humidity (72%-79%) and sea surface temperature
(25-28°С)) from warm sources (Yurtsevert,1981; Rozanski et al. 1993),
indicating the prevalence of oceanic sources (from the Atlantic ocean as
indicated in our modeling results in Fig. 5). More specifically, the
values of Oxygen and Hydrogen isotopes for both regions are similar to
those proposed by Bowen (2003) for the TNA zone, and the FLEXPART
results indicate that TNA was the most active source in this month for
the study area (>27%, Fig. 5).
For the MAM season (Fig. 7d, 7e, 7f) in the Andean region, March
represents the highest enrichment for this season, and April- May also
exhibit an oscillation range for δ2H between 0 and
-120 ‰ VSMOW, and for δ18O between 0 and -20 ‰ VSMOW.
In the Caribbean region, March exhibits similar variations to DJF, while
April-May observations vary along the LMWL with oscillations for
δ2H between 0 and -120 ‰ VSMOW, and for
δ18O between 0 and -20 ‰ VSMOW. The main regional
terrestrial contribution for MAM corresponds to ORIC and NOSA sources
(Fig. 5). For both regions, the footprint of these terrestrial
contributions can be seen in the local LMWL located above the GMWL
(Interpretation of Fig. 7, following guidelines on Fig. 3). Moisture
from ORIC is located in the lower part of the figure, indicating its
warm origin, and moisture from NOSA along the LMWL indicating sources
with different temperatures. The large variation for both regions in
April-May (Fig. 7d, Fig. 7e) responds to the mixing ratio effect. This
spread in the isotopic composition of rainwater is produced by the
mixing of different air masses that precipitate over Colombia. During
this season, the study area is characterized by a complex combination of
terrestrial and oceanic sources of moisture that contribute in different
relative amounts to regional precipitation, associated with the dynamics
of the ITCZ (Tables 2 and 3).
[Insert Table 2]
[Insert Table 3]
In JJA (Fig. 7g, 7h, 7i) the range of variations for
δ2H in both regions oscillates between -20 and -80 ‰
VSMOW, and δ18O between -3 and -12 ‰ VSMOW. Especially
for the months of June-July (Fig. 7g, 7h), observations are located in
the middle of the variation range, coinciding with a dry period, and the
contributions are a combination of terrestrial and oceanic moisture
sources. For the Caribbean region, although not predominant, Atlantic
sources are important (Fig. 6b), as indicated by enrichment observations
of the LMWL. Similarly, terrestrial sources make their contributions
from ORIC and NOSA (Fig. 5f), explaining the combination of terrestrial
and oceanic sources. This is similar for the Andean region, which
receives more contributions from terrestrial sources, followed by
contributions from the Pacific Ocean (Fig. 6a). Particularly, for the
Andean region the major contributions of terrestrial sources from NAMZ
occurred during June (>17%), and coincide with
observations localized above of the GMWL and the lower part of LMWL due
to the minor temperature of the NAMZ source.
In the SON season (Fig. 7j, 7k, 7l), the isotopic composition of
precipitation shows more depleted values compared to the rest of the
year, with a variation range for δ2H
(δ18O) between 0 and -120 (-2‰ and -15) ‰ VSMOW. In
particular, October and November exhibit more depleted values,
coinciding with an increment of the contribution from the cold Pacific
Ocean (Fig. 6a). Also, this season corresponds to one of the two rainy
seasons of the year, generated by the seasonal migration of the ITCZ,
producing an increase of monthly precipitation and intensity of the
storms, causing depletion in hydrogen and oxygen (amount effect).
Likewise, the Caribbean zone receives predominant moisture sources from
the Pacific Ocean, intensifying these contributions in October –
November due to the ITCZ staying in the northern hemisphere over the
Atlantic and eastern Pacific.