GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L05605, doi:10.1029/2011GL050773, 2012
Why does the Loop Current tend to shed more eddies in summer
and winter?
Y.-L. Chang1 and L.-Y. Oey1
Received 27 December 2011; accepted 31 January 2012; published 7 March 2012.
[1] The observed seasonal preferences of Loop Current
eddy shedding, more in summer and winter and less in
fall and spring, are shown for the first time to be due to a
curious combination of forcing by the seasonal winds in the
Caribbean Sea and the Gulf of Mexico. The conditions are
favorable for the Loop to shed eddies in summer and winter
when strong trade winds in the Caribbean produce large
Yucatan transport and Loop’s intrusion, and concurrently
when weak easterlies in the Gulf offer little impediment to
eddy shedding. The conditions are less favorable in fall and
spring as the trade winds and Yucatan transport weaken, and
the strengthening of the Gulf’s easterlies impedes shedding.
Citation: Chang, Y.-L., and L.-Y. Oey (2012), Why does the
Loop Current tend to shed more eddies in summer and winter?,
Geophys. Res. Lett., 39, L05605, doi:10.1029/2011GL050773.
1. Introduction
[2] Early studies of the Loop Current in the Gulf of Mexico
in the 1960’s1980’s suggest that it may vary seasonally.
The northward penetration of the Loop Current was bimodal:
maximum penetrations occur, on average, in winter (DecJan)
and summer (JunJul) [Leipper, 1970; Behringer et al., 1977;
Molinari et al., 1978; Sturges and Evans, 1983]. Molinari
et al. [1978] concluded that the seasonal intrusion of the
Loop Current varied with the geostrophic transport through
the Yucatan Channel. Sturges and Evans [1983] suggested
that the Loop Current varied in response to wind. These
pioneering authors also recognized that there were substantial deviations from the seasonal cycle, and intrusions
and eddy-sheddings can occur in virtually any month of the
year. That the Loop Current can intrude into the Gulf and
eddies can separate from it without the need for a seasonal
forcing such as the inflow transport was first demonstrated
numerically by Hurlburt and Thompson [1980], and since
then confirmed by numerous studies using more elaborate
models.
[3] The idea of a seasonal Loop Current is nevertheless
very attractive; the system is more predictable, and understanding the underlying mechanisms can lead to improved
predictions of the strong currents and heat content associated
with the Loop, which have practical significance. In this
work, the old problem of a seasonal Loop Current is revisited
taking advantage of the order-of-magnitude increase in data
coverage from satellite, advent in models and forcing data,
1
Atmospheric and Oceanic Sciences Program, Princeton University,
Princeton, New Jersey, USA.
Copyright 2012 by the American Geophysical Union.
0094-8276/12/2011GL050773
and improved theoretical understanding of Loop Current
dynamics.
2. Observed Loop Current Shedding Events
[4] The dates of Loop Current eddy separation from 1974
to 1992 are from Vukovich [1988], Sturges [1994] and
Sturges and Leben [2000] using a combination of satelliteSST images as well as in situ and ship measurements to
identify eddy separations. From 1993 to 2010, satellite
altimetry data from AVISO [http://www.aviso.oceanobs.
com/] is used. For shedding period shorter than 2 months
(one in 1993, the other one in 2002), the two consecutive
events are taken as the same event, and the first shedding is
recorded. There are 47 eddy shedding events from 1974 to
2010. Figure 1a sorts the number of shedding events by
months (a seasonal histogram or SeH) and indicates that eddy
shedding has a bimodal (biannual) seasonal signal: maximum in summer (JulSep) and winter (Mar), and minimum
in late fall (NovDec) and late spring (MayJun). The
maximum difference in eddy count (Mde) is 7 between
the period of most and least eddies. Approximately 40%
of the eddies are shed during summer, but only one eddy is
shed in the late fall (NovDec). However, most of the seasonal signal is for the record after 1993 (bars in Figure 1a);
summer eddy sheddings then account for 45% of the total,
and no eddies were shed in NovDec. This difference suggests a shift in the Loop Current’s behaviors between the two
periods - a point we will comment on later. The seasonal
preference of eddy-shedding suggests that the system is, at
least in part, forced. Such a possibility was anticipated by
Chang and Oey [2010, hereinafter CO2010; see also Oey et al.,
2003, hereinafter OLS2003] whose process experiments show
the effects of wind on Loop Current eddy-shedding.
[5] Another way of displaying the eddy-shedding data is to
plot the eddy-shedding histogram (ESH; Figure 1b). The
ESH has peaks (e.g., 6, 9 months etc), but most importantly it
shows wide-ranging shedding periods P from 419 months:
the eddy-shedding process appears to be chaotic. However,
the “broad-spectrum” ESH can be a consequence of the
seasonal shedding preferences of eddy-shedding. The argument is straightforward as summarized in Figure 1c. For
example, suppose the forcing is such that the Loop sheds
eddies in August and September, the ESH then shows values
at 1, and 11–13 months. By including only 4 observed,
preferred shedding months: March, July, September and
October (3,7,9,10 in Figure 1c), a broad-spectrum ESH with
periods from 1–20 months can exist. The solution is not
unique, but this is not central to our argument. The point here
is that an orderly, seasonally forced Loop Current that sheds
eddies only in certain months is consistent with the existence
of a broad spectrum of shedding periods; in other words,
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CHANG AND OEY: SEASONAL LOOP CURRENT SHEDDING
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Figure 1. (a) Seasonal Histogram (SeH; eddies vs. Calendar months) using 1974–2010 data (solid line; dash is 3-mo
weighted (1/4-1/2-1/4) mean) and 1993–2010 data (bar). (b) Eddy-Shedding Histogram (eddies vs. periods P). (c) P’s (shaded
if shedding) vs. shed-months (8 = Aug etc). (d) Shed-month vs. P’s, shown for first shedding in Jan. For each P, summed
shades = peaks in SeH. The “∨” means “or.”
a chaotic system is not necessary for the existence of the
broad spectrum. In addition to possible contribution from
some natural shedding periods which depend on internal
physics [e.g., Hurlburt and Thompson, 1980; OLS2003],
peaks in the ESH may then be thought of as the result of some
interannual variations of the forcing that perturb the shedding
month from one year to the next, or even no shedding at all
until the following year. That the Loop Current and eddyshedding system may be non-chaotic was first suggested by
Lugo-Fernandez [2007].
[6] The contrary is not necessarily true. In other words,
a chaotic Loop Current with a broad-spectrum ESH which
may contain some prominent peaks (Figure 1b) does not
in general lead to seasonal preferences of eddy-shedding
(Figure 1a). With steady forcing a modeled Loop Current can
display a natural period (e.g., CO2010); on the other hand,
experiments can be designed to produce a chaotic system
with a broad-spectrum ESH (OLS2003). Assuming such a
system exists in the observed world, that the corresponding
ESH has a broad spectrum with prominent peaks around
some natural periods, what then can be deduced about its
SeH? Given P, the month Msh when shedding occurs is:
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Msh ¼ Msh0 þ 12:ðn 1Þ=FP ; n ¼ 1; 2; ::; FP ;
ð1Þ
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where FP = 12/gcd(12, P) is the number of peaks in the SeH
for that P, gcd = greatest common divisor, P = 1, 2, 3, …, 19,
20 months, and Msh0 = the month of the first shedding;
Figure 1d shows the case for Msh0 = 1. It is readily shown
that, despite the presence of biannual and/or annual peaks in
the shedding periods (that may therefore favor a seasonal
SeH), the existence in the observed ESH (Figure 1b) of a P =
PFull = 5, 7, or 11, etc for which gcd(12, PFull) = 1, can yield
a non-seasonal SeH (details in the auxiliary material).1
[7] The simple calculations above demonstrate the importance of order in the shedding events. It appears that nature
has selected an order that, in the case of the Loop Current, is
largely non-random. In other words, the shedding process is
largely controlled by some form of external forcing, such as
the winds. Model experiments support this assertion.
3. Processes That Control the Seasonal Shedding
of the Loop Current Eddies
[8] The importance of wind forcing on eddy-shedding has
previously been noted (OLS2003; CO2010). We now demonstrate that the existence of a bimodal SeH (Figure 1a) is
caused by a curious complementary effect (on the Loop
Current) of the zonal component of the seasonal winds in the
Caribbean Sea and the Gulf of Mexico.
3.1. Seasonal Winds
[9] Winds in the Caribbean Sea vary depending on the
movement and intensity of the North Atlantic Subtropical
High and, in winter, on the North American High also
(Figures S2–S3). In the Gulf of Mexico, winds are additionally modified by the North American monsoon in summer,
the high pressure over the northeastern US in fall, and the low
pressure over the western US in spring. The combined effect
is that the seasonal winds are 180 out of phase in the two
regions: the Caribbean easterly is strong in winter and summer and weak in spring and fall while the Gulf’s easterly
wind is stronger in fall and spring and weak in summer and
winter (Figure 2a).
3.2. Numerical Experiments
[10] This out-of-phase relation between the seasonal winds
in the Caribbean Sea and the Gulf of Mexico is central to the
understanding of why the Loop Current tends to shed more
eddies in some months than others. Within the Gulf, easterly
wind forces an eastward return flow across the middle of the
basin which counters the westward-growing Loop Current by
Yucatan inflow and Rossby-wave dynamics and delays
eddy-shedding [Chang and Oey, 2010]. We may expect then
that the easterly peaks in the Gulf of Mexico in OctNov
and, to a lesser degree, in AprMay, would delay eddyshedding, which would be consistent with the observed
SeH (Figure 1a) that less eddies are shed in those months.
However, explanations based on the Gulf’s forcing alone
are incomplete; the dynamics of the Caribbean Sea are
necessary.
[11] The NW Atlantic Ocean model (5 –50 N and 98 W–
55 W; see Figure S4 in the auxiliary material which also
contains model descriptions) that we have previously tested
(e.g., OLS2003; CO2010) for studying Loop Current dynamics
1
Auxiliary materials are available in the HTML. doi:10.1029/
2011GL050773.
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is set up to run various experiments to isolate processes. The
“Basic” experiment is forced by the CCMP wind stresses
(0.25 0.25 , 6-hourly satellite+NCEP blended dataset )
from 1988–2009. The “NoWind” experiment has no
wind. In the “Atl” experiment, the wind is applied to the
east of 82 W only, and the experiment “GOM+NWCar,”
has wind applied to the west of 82 W only. Finally,
the “GOM+NWCarNoCurl” also has winds applied west of
82 W but they are zonal only and are spatially constant
averaged over the Gulf of Mexico and the NW Caribbean
Sea (Figure 2a). This last experiment has the essentials of the
out-of-phase relation between the seasonal winds in the
Caribbean Sea and the Gulf of Mexico. Each experiment was
conducted for 22 years (1988–2009). To ensure robustness of
our results, the Exp.Basic, Atl and GOM+NWCarNoCurl
were repeated for additional 22 years with different initial
fields and with a reduced Smagorinsky’s constant (0.05
instead of 0.1) for the horizontal viscosity.
[12] The Exp.NoWind yields P ≈ 710 months around
a peak ≈ 8 months (e.g., OLS2003; CO2010). Its SeH is
basically full (no seasonal preference with small standard
deviation (sd) = 0.5 and an Mde of only 1; not shown) as may
be anticipated from the discussions (Figures 1c and 1d) of the
previous section. Exp.Atl also gives a full SeH, also with
small sd = 0.4 and Mde =1 (Figure 3a, grey). Remote winds in
the eastern Caribbean Sea and the North Atlantic Ocean
are therefore unlikely to force a seasonal shedding. The
Exp.Basic (Figure 3a, solid) has sd = 1.8 and Mde = 6; it
shows eddy-shedding preferences in winter (FebMar) and
summer (JulAug), with less shedding in late spring (May,
4 less) and early fall (OctNov, 6 less), in general agreements with observations. This suggests that the seasonal
eddy-shedding is wind-forced. This deduction is confirmed
by the SeH from Exp. GOM+NWCar (Figure 3b; sd = 1,
Mde=4), which shows similar winter (Mar) and summer
(Aug) shedding preferences. Experiments GOM+NWCar
and Exp.Atl show that it is the regional wind in the Cayman
Sea (i.e., NW Caribbean Sea) and the Gulf of Mexico
that influences the seasonal eddy-shedding of the Loop
Current. Finally, when the wind stress curl is removed,
Exp. GOM+NWCarNoCurl (Figure 3c; sd = 1.3, Mde = 5)
shows that the zonal component of the wind alone can
explain the seasonal preferences with more sheddings in
winter (Mar) and summer (JulSep). While there are some
differences in the preferred months of shedding amongst the
three experiments, we do not consider them to be significant.
3.3. Why Can Wind Force a Seasonal Preference
in the Shedding of Loop Current Eddies?
[13] Yucatan transport (TrYuc) also varies biannually:
stronger in summer and winter and weaker in spring and fall
[Molinari et al., 1978; Rousset and Beal, 2010]. Simulated
TrYuc and Caribbean wind stress (t o, and wind stress curl
r t o) are significantly correlated with wind leading by
03 months. Correlation maps show that winds in the
Cayman Sea are effective in driving transport fluctuations
(Figures 2d and 2e): westward wind stress (t ox < 0) and
negative r t o drive stronger TrYuc. The TrYuc is positively
correlated with t ox in the eastern Gulf: TrYuc decreases as
westward wind in the Gulf becomes stronger (CO2010).
[14] The seasonal preferences of eddy-shedding can now
be explained. It is well-known that the Loop Current tends to
shed eddies more readily when it extends northward into the
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Figure 2. Seasonal cycles (1988–2009) of (a) zonal wind stresses averaged over Gulf of Mexico and NW Caribbean Sea
(negative westward), and (b) Yucatan transport anomaly from Exp.Basic with mean = 25.6 Sv shown. (c) Regression of
Loop’s northern boundary vs. z/f from Exp.Basic. Maps: correlations (wind leading 1 month; above the 95% significance,
otherwise white) between Yucatan transport and (d) zonal wind stress and (e) wind stress curl; contours are 0.2 and 0.4, black
positive and white negative.
Gulf, and that once the Loop is in the extended state and
ready to shed, the process is relatively fast (a few weeks [e.g.,
OLS2003]). The fundamental variable for the Loop’s intrusion is TrYuc. In summer and winter, TrYuc increases as the
negative wind stress and wind stress curl in the Caribbean
Sea increase (see wind plots in Figure S3 in the auxiliary
material); the easterly peaks (Jul and Jan) in the Caribbean
correspond well to the peaks in TrYuc especially for summer
(Figures 2a and 2b). The larger TrYuc leads to stronger inflow
velocity vo and cyclonic vorticity z o on the western (50 km)
portion of the Yucatan Channel, and a more extended Loop
Current [Oey, 2004; OLS2003]. The z o /f (f = Coriolis
parameter) is an excellent predictor of the Loop Current’s
northern boundary with high R2 = 0.83 for their linear
regression (Figure 2c). While this linear relation agrees with
the Reid’s formula [Reid, 1972; OLS2003], we treat it to be
merely an empirical one. The Loop Current therefore tends to
be extended in summer and winter. As TrYuc decreases (Sep
and Mar) when the Caribbean (westward) windstress weakens (JulSep, and JanMar), the Loop retracts as z o also
weakens. The mass influx (Qi) feeding the Loop also
decreases, providing a favorable condition for the westward
Rossby wave speed of the extended Loop (Ci b R2d,
where Rd = Rossby radius based on the depth of the matured
Loop) to overcome Qi, hence also a favorable condition for
eddies to separate [Nof, 2005]. The weakening of the wind
(and transport) are abrupt especially in summer (Figures 2a
and 2b). Moreover, because the Gulf of Mexico’s easterlies
are weak during those periods (Figure 2a), the eastward
momentum flux that impedes eddy-shedding (CO2010) is
also weak. This combination of strong Caribbean easterly,
abrupt weakening, and weak easterly in the Gulf of Mexico
favors a larger proportion of eddies being shed from
JulAug and FebMar (Figure 3). In fall and spring, TrYuc
and the Caribbean easterly remain weak but at the same time
westward wind in the Gulf of Mexico intensifies (Oct and
May; Figure 2a). The Loop Current’s expansion and eddyshedding are now impeded by the eastward momentum flux
that intensifies along the mid-latitudes within the Gulf. These
factors lead to a reduced number of eddies being shed in fall
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Figure 3. Seasonal histograms (eddies vs. Calendar months, 3-month weighted (1/4-1/2-1/4) mean, and plotted over two
cycles) for model experiments forced by CCMP wind: (a) Basic (circle symbols connected with solid line) and Atl (grey) both
44 years, (b) GOM+NWCar (22 years) and (c) GOM+NWCarNoCurl (44 years).
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Figure 4. A schematic plot of seasonal eddy shedding according to the dynamics explained in text. (top) From left to right:
extended Loop when Caribbean wind and Yucatan transport are strongest (Jul and Jan), wind and transport weaken (Sep and
Mar; squiggly arrow represents Rossby wave), and wind in the Gulf is strongest (Oct and May; blue arrows indicate windforced near-surface circulation). (bottom) Base line represents the zero wind when the Loop Current sheds eddies at or near
its natural period. The solid up arrow “↑” indicates increased shedding and dashed down arrow “↓” decreased shedding. The
easterly wind is stronger away (up or down) from the base line: solid for Caribbean wind and dotted line for the Gulf. The time
lag is approximate indicating a range rather than a fixed value.
and spring (Figure 3). These processes are summarized
schematically in Figure 4. In the auxiliary material, the
dynamics are further examined using a simple reducedgravity model (Exp.RG). The Exp.RG confirms that easterly
wind in the NW Caribbean Sea drives a seasonal shedding.
The Gulf’s easterly wind accentuates the seasonality by
delaying eddy-shedding in fall and spring: it increases the
summer-fall (or winter-spring) difference in the number of
eddies shed. We also compared the RG experiments with the
3D Exp.Basic (and Exp.GOMCarNocurl) using the ensemble
averaging idea of the Loop Current Cycle described by
Chang and Oey [2011]. In the 3D experiments, we found that
on average eddy-shedding follows shortly (1 month) after
the maximum Yucatan transport, but that in Exp.RGCarib
there is an additional time-lag of 12 months. The RG
response is similar to the EOF modes 1 + 2 of the 3D
experiments while interestingly the EOF mode 3 accelerates
the shedding in the 3D experiments and closely resembles the
Campeche Bank instability mode [Oey, 2008]. Therefore,
dynamical instability takes part in the eddy-shedding process, but it does not control the seasonal timing.
seasonal winds in the Caribbean Sea and the Gulf of Mexico.
The Loop sheds more eddies in summer and winter in
response to intensified Yucatan transports driven by the
stronger trade winds in the Caribbean, and concurrently when
weak easterlies in the Gulf offer little impediment to eddy
shedding. The conditions are reversed in fall and spring when
the Caribbean’s (Gulf’s) easterlies weaken (strengthen).
Since wind plays a central role, our results suggest the
existence of an interannual variation of the eddy-shedding
process. Indeed, Figure 1a indicates that the biannual seasonal preferences are much less distinct for the first half of
the data period from 1974–1992. The second half (1993–
2010) has more shorter (biannual) periods, and why that is so
may be due to a basic change in the wind. This and other
consequences will be examined in a future study.
4. Summary and Conclusions
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[15] The Loop Current is observed to shed more eddies
in summer and winter. Numerical experiments also yield
seasonal preferences with more sheddings in winter and
summer, and less in fall and spring in agreement with
observations. The seasonal preferences are forced by the
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[16] Acknowledgments. We gratefully acknowledge the supports by
the Bureau of Offshore Energy Management contract M08PC20007 and
the Portland State U. contract 200MOO206.
[17] The Editor thanks the anonymous reviewers for their assistance in
evaluating this paper.
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Y.-L. Chang and L.-Y. Oey, Atmospheric and Oceanic Sciences
Program, Princeton University, 300 Forrestal Rd., Sayre Hall, Princeton,
NJ 08544, USA. ([email protected])
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