The dynamics of the Loop Current (LC) and the detached Loop Current eddies
(LCEs) dominate the surface circulation of the Gulf of Mexico (GoM) and
transport Caribbean Water (CW) into the gulf. In this work, 25 years
(1993–2017) of daily satellite data are used to investigate the variability
of these physical processes and their effect on chlorophyll a (Chl a)
concentrations from 1998 to 2017, including temporal changes, mean differences,
and regional concentration tendencies. The physical variables analyzed are
absolute dynamic topography (ADT) and oceanic currents. From the ADT and
oceanic current monthly climatologies, it is shown that there is an annual
intrusion of CW with an inward incursion that starts in spring, peaks in
the summer, reaches to 28∘ N and 90.45∘ W, and then
retreats in winter to approximately 26.5 ∘ N and 88.3 ∘ W. Minimum surface Chl a concentrations (< 0.08 mg m-3)
are found during the summer–autumn period inside the region of maximum
incursion of CW; the opposite is observed during the winter period when
Chl a concentrations were at a maximum, e.g., > 0.14 mg m-3. The
3-year running averages of the ADT 40 cm isoline qualitatively reproduce
the climatological pattern of 25 years showing that before 2002 CW was
less intrusive. This suggests that from 2003 onward, larger volumes of
oligotrophic waters from the Caribbean Sea have invaded the western GoM and
reduced mean surface Chl a concentrations. A direct comparison between the
1998–2002 and 2009–2014 periods indicates that in the latter time interval,
the Chl a concentration above waters deeper than 250 m has decreased
significantly.
Introduction
The effects of global warming on the circulation of the world's oceans and
its concomitant consequences on the oceans' biological productivity are some
of the most important scientific and economic issues of our times.
Forecasting the effects of global warming on ocean resources
depends on having a clear understanding of the manner in which physical
processes (e.g., solar radiation, winds, circulation, and vertical mixing)
affect primary production. This understanding is aided by the availability
of remote sensing observations, unparalleled in their spatial and temporal
coverage of the earth's surface. Since 1990, satellite data on absolute
dynamic height (ADT), chlorophyll a (Chl a) concentration, and derived
products (eddy kinetic energy (EKE), geostrophic and Ekman currents) have
been available to study the Gulf of Mexico (GoM), an important
socioeconomic region for fisheries, petroleum, natural gas, and tourism. We
have availed ourselves of a 25-year time series of satellite data to study
the relationship between the physical dynamics of the GoM and its effect on
primary production in the context of a global warming scenario. Unlike
previous studies, this work entails the analysis of the Loop Current (LC), the path footprint of the LC eddies (LCEs), and the dominant features of the
surface circulation that transport Caribbean Water (CW) into the GoM (Nowlin
and McLellan, 1967; Morrison et al., 1983). The LC in the eastern GoM is
part of the North Atlantic Ocean Subtropical Gyre, an essential contributor
to the interhemispheric Meridional Overturning Cell (Schmitz and McCartney,
1993; Candela et al., 2003; Schmitz et al., 2005). This current carries warm
waters from the gulf to the North Atlantic through the Straits of Florida via
the Gulf Stream (Hurlburt and Thompson, 1980), thereby also being an
important contributor to the upper ocean heat budget of the GoM (Liu et al.,
2012). Based on a detailed analysis in the central and western GoM by
Portela et al. (2018), within the gulf are seven water masses in order of
increasing mean density: the remnants of Caribbean Surface Water (CSWr: also
referred to as CW), North Atlantic Subtropical Underwater (NASUW), Gulf
Common Water (GCW), Tropical Atlantic Central Water (TACW), the nucleus of
the TACW (TACWn), Atlantic Intermediate Water (AAIW), and North Atlantic Depth
Water (NADW). Here, we are principally concerned with surface effects.
CW enters the GoM via the LC with specific biological (i.e., low Chl a)
and physical characteristics (warmer by ∼0.6 units and less
saline waters by ∼0.5 units). The current penetrates into the
gulf, reaching 28∘ N, near the Mississippi Delta. As it extends to
the north, it forms a loop (Austin, 1955) that turns southeast to ultimately
exit into the Atlantic Ocean.
Knowledge of how the thrust of the LC affects the intrusion of CW is based
on hydrographic data (Leipper, 1970; Niiler, 1976; Behringer et al., 1977;
Molinari et al., 1977; Huh et al., 1981; Paluszkiewicz et al., 1983), remote
sensing observations (Vukovich et al., 1979; Vukovich, 1988; Leben and Born,
1993; Leben, 2005), and, in the last 20 years, numerical modeling
(Hurlburt and Thompson, 1980; Candela et al., 2003; Oey et al., 2005; Sturges
and Lugo-Fernandez, 2005; Counillon and Bertino, 2009; Cardona and Bracco,
2016; Wei et al., 2016). More recently, novel developments based on
artificial neural networks and empirical orthogonal function analysis have
also been applied to predict LC variation (Zeng et al., 2015), effecting
reliable forecasts for up to 5 to 6 weeks. Knowledge of how the primary
forcing mechanism affects the Loop Current is important to the circulation
of the GoM as both a direct and indirect generator of surface-layer eddies
and as a source of lower-layer flows (Hamilton et al., 2016). Based on
satellite altimetry observations and the dynamic height gradient from 1993
to 2009, Lindo-Atichati et al. (2013) observed northward seasonal
penetration of the LC, peaking in summer. LC extension and anticyclonic eddy
separation are the result of the momentum imbalance (Pichevin and Nof, 1997)
and form the shape of future LCEs. Chang and Oey (2010), using a numerical
model, proposed that wind stress could be the primary forcing that
releases LCEs. In a second paper, supported by satellite observations, they
proposed that the LC intrusion and the shedding of the LCEs followed a
biannual cycle (Chang and Oey, 2013). A reanalysis of
archived data also detected statistically significant LCE separation
seasonality (Hall and Leben, 2016). Recently, Candela et al. (2019) analyzed
4 years of water current data and reported a seasonal cycle in the
transport through the Yucatán Channel, with the annual cycle as the main
harmonic peak in July.
Interacting seasonal and stochastic processes could trigger the separation
of the LCEs (Fratantoni et al., 1998; Zavala-Hidalgo et al., 2003, 2006) as well as forming Caribbean eddies and other
topographic features (Garcia-Jove et al., 2016). In this context, the LC
system has some similarities with the North Brazil Current retroflection
(Pichevin et al., 1999; Goni and Johns, 2001; Zharkov and Nof, 2010), the
Agulhas retroflection (de Ruijter et al., 1999; Baker-Yeboah et al., 2010), and
the Gulf Stream, wherein large meanders pinch off as warm rings (Brown et
al., 1983; Richardson, 1983; Savidge and Bane, 1999).
Despite extensive research, after more than a half-century we are still
struggling to completely understand LC variability, the processes
controlling the Loop Current extension, and the mechanism of the detachment of
anticyclones from the loop. Because positive time trends have been reported
in temperature, winds, sea level, and the greater number of detached eddies
separated from the LC, it can be expected that these phenomena would affect
primary productivity and, indirectly, surface Chl a concentration (Polovina,
et al., 2008; Laffoley and Baxter., 2016). In this work, 25 years
(1993–2017) of daily ADT data combined with monthly radiance data from
1998 to 2017 are used to investigate the variability of the transport of
Caribbean Surface Water into the gulf and its effect on Chl a concentration.
We examined temporal changes, mean differences, and regional concentration
tendencies.
Monthly means of absolute dynamic topography (ADT) and surface
currents averaged over a quarter of a century (1993–2017).
Data and methods
Three independent datasets were used to provide evidence of temporal
variability in the extension of CW into the GoM. We used ADT and surface
velocity fields (geostrophy and Ekman) from the GEKCO (Geostrophic Ekman
Current Observatory; Sudre et al., 2013) product from 1993 to 2017 with a
resolution of 0.25∘×0.25∘, in conjunction with Chl a ocean
color data derived from the reprocessing R2014.0 product suite from Aqua
MODIS (Moderate Resolution Imaging Spectroradiometer) and from SeaWIFS
(Sea-Viewing Wide Field-of-View Sensor), using the OCx algorithm with a
spatial resolution of 9×9 km (https://oceancolor.gsfc.nasa.gov/cgi/l3, last access: 19 November 2019). The 2003–2017 monthly Chl a ocean
color product was derived from Aqua MODIS, and the 1998–2002 monthly Chl a ocean
color product was derived from SeaWIFS.
Climatology was created from maps of ADT that result from the elevation of
the sea surface height referenced to the geoid using the product from DUACS
(Data Unification and Altimeter Combination System) available on the AVISO
(Archiving, Validation and Interpretation of Satellite Oceanographic data)
website at https://www.aviso.altimetry.fr/en/data (last access: 19 November 2019). The ADT
climatology was constructed using the 25 years of daily satellite maps from
1993 to 2017, averaging for each month in the different years. We considered LCEs in any stage of formation, detaching, and
reattaching to the LC as evidence of the incursion of CW. After the ADT
climatology was obtained, the predominant boundary contour of CW was
extracted from each climatological month. It was observed that the 40±2.2 cm ADT contour was well matched to the climatological maxima of
its respective EKE. For this reason, the ADT 40 cm contour is taken as the
main ADT reference that tracks the Caribbean Water Front (CWF).
Climatological monthly maps of eddy kinetic energy (EKE) in the GoM: red
contours correspond to the areas of maxima EKE. The heavy black line
corresponds to the 40 cm isoline of the CWF (the contour of the CWF is
significant at the 95 % level). The EKE was calculated using daily maps
of satellite-derived currents from AVISO (GEKCO) for a quarter of a century
(1993–2017).
Specifically, monthly CWF positions were obtained from short-term running
averages of daily satellite observations in 3-year periods. Each running
average was moved rearward by 1 year, e.g., 1993–1995, 1994–1996, 2014–2016, 2015–2017. For each 3-year period, a set of 12
monthly maps was obtained, resulting in a total of 23 sets of monthly CWF
maps: 10 sets from 1993 to 2002 and 13 sets from 2003 to 2017. We used the
40 cm contour of each set of 3-year averages because this was the
contour with the highest EKE observed in the 25-year dataset. To retrieve the CWF contours, we first determined the
initial latitudinal position of the CWF to be at 80.7∘ W with the
respective corresponding longitudinal positions between Cuba and Florida.
The CWF contour lines that run from east to west and finish close to the tip
of the Yucatán Peninsula were separated by 0.2±0.1∘. However,
some ADT contour “islands” appeared next to the CWF with a typical distance
of > 0.3∘ from the CWF contour. Additionally, a spectral
analysis was done using a daily time series of 25 years of ADT data to build
a spatially averaged region influenced by the LC between 91.25∘ W and
23.125∘ N and between 83.5∘ W and 28.12∘ N.
When ADT island distances were > 0.3∘ from the front, we
used a MATLAB code procedure to eliminate them from the CWF contours. Once
the CWF's contours were retrieved, the next step was to visually corroborate
the quality and coherence of each CWF contour over the monthly field maps of
ADT, sea surface currents, and Chl a distribution. In this way, inconsistencies
were detected and corrected. The MATLAB code procedure satisfactorily
corrected 91.3 % of the contours. The remaining sets were corrected by
hand via visual analysis.
Geographical positions of the CWF tracked using the 40 cm ADT isoline
representing 1993–2017 monthly average values: (a) northward and (b) westward,
respectively; (c) ADT spectral analysis in a region influenced by the CWF
(91.25∘ W, 23.125∘ N and 83.5∘ W, 28.12∘ N).
The ADT quarter-century CWF (1993–2017) monthly climatology and its
standard deviation are shown in heavy and dotted lines, respectively. The
heavy line corresponds to the 40 cm isoline of the CWF. The dotted line
encloses values of the standard deviation > 15 cm.
The main mesoscale instabilities were obtained from calculations of the
climatological monthly EKE maps of geostrophic and Ekman currents obtained
from 25 years of daily satellite observations from GEKCO using the following
equation:
1u=u′+U;u′=u-U,2v=v′+V;v′=v-V,3EKE=1/2(u′2+v′2),
where u and v represent the total current (u=uE+ug and v=vE+vg),
uE and vE represent the Ekman current, ug and vg represent the geostrophic
current, U and V are the means of the oceanic currents, and u′ and v′ are the
anomalies of the current. To find the relationship between ADT and EKE
patterns, the 40 cm ADT isoline was overlaid on the monthly EKE maps. This
made the EKE means representative of the energy of the mesoscale eddy field
(Jouanno et al., 2012).
For consistency between the different satellite datasets, all monthly
climatological spatial fields were standardized at 0.25∘×0.25∘ spatial resolution by bilinear interpolation.
Results and discussionTracking the intrusion of Caribbean Water
The LC enters the gulf through the Yucatán Channel and exits through the
Straits of Florida, penetrating northward into the GoM until instabilities
form in the current and a ring-like LCE pinches off. There are two ways of
tracking the LC: (1) tracking the thermal signal (not possible in summer due
to weak thermal contrast in the GoM) and (2) tracking the sea surface height
through satellite altimetry. In 2005, Leben, using the 17 cm contour in
daily sea surface topography maps (this contour closely follows the edge of
the high-velocity core of the LCEs and LC), tracked the LC thermal fronts in
the sea surface temperature images during good thermal contrast. In a
different way, Lindo-Atichati et al. (2013) calculated the maximum
horizontal gradient of the sea surface height (SSH) to track only the
contours of the Loop Current Front (LCF). In this work, we used the ADT to track both the LC and
the LCEs formed by the influence of the CW. Monthly mean surface oceanic
currents from GEKCO overplotted on the ADT data are shown in Fig. 1. Maximum
satellite surface current velocities in the Caribbean Sea and the GoM, as
well as in the Yucatán current on the continental coast, were > 50 cm s-1, coinciding with in situ estimates of ∼60 cm s-1 (Badan
et al., 2005). The monthly GoM total current fields show the variability of
the primary forcing that coincides with the mean ADT edge; the vectors of
maximum velocity are tangent to the edge of the maximum slope change. To
locate the CW, the 40 cm mean ADT isoline was chosen. The ADT reference
corresponds to regions of maximum ADT gradients and maximum EKE. Figure 1
shows that (mostly) in autumn (October, November, and December) and winter
(January, February, and March), the CW retracts to its most southeasterly
location. In contrast, in spring (April, May, June) and summer (July,
August, September), CW penetration moves towards the northwest. In fact, the
extension begins in May and reaches maximum penetration in August, showing
an annual pattern. This movement is similar to that observed by Chang and
Oey (2013). They found that in summer, the maximum LC intrusion was forced
by the trade winds. Their and our observations are also consistent with the
work of Candela et al. (2019), who reported that water transport into the GoM
in July through the Yucatán Channel was at a maximum.
It is accepted that LCEs occur in a geographical control zone that is
based on momentum imbalance (Pichevin and Nof, 1997; Nof, 2005) rather than
instability. Also, we should not abandon the idea that the formation of
instabilities such as meanders and cyclonic eddies are due to high EKE
produced by upstream conditions that influence the circulation within the
GoM (Oey et al., 2003) and produce changes in the fluxes in the Yucatán
Channel (Candela et al., 2002), transport variations in the LC (Maul and
Vukovich, 1993), variations in the deep outflow (Bunge et al., 2002), and
cyclonic eddies in Campeche Bank and Tortugas (Fratantoni et al., 1998;
Zavala-Hidalgo et al., 2003). The areas of large EKE are related to the
intrusion and retreat of CW (Garcia-Jove et al., 2016) via baroclinic and
barotropic instabilities (e.g., Jouanno et al., 2009).
Figure 2 shows that the 40 cm isoline encloses the maximum EKE area of the
LC–LCEs during each climatological month, demonstrating that its
distribution is mainly centered in the LC region; consequently, the maximum
EKE borders the CW Front just where the abrupt horizontal gradients of ADT
exist and changes in current speed occur. It is clear that the 40 cm isoline of
ADT matches both the maximum EKE values and the maximum ADT
gradient very well and is a good tracker of the contours of LC–LCEs. Lindo-Atichati et
al. (2013) proposed a methodology using the SSH maximum horizontal gradient,
which is the addition of sea height anomaly and mean dynamic topography, to
obtain the contours of the LCF and LCEs. In our analysis, we chose the 40 cm
isoline as a general reference to track the LCF, LCEs, and CW transport.
The enhanced monthly EKE signals respond in the same way as the LCF,
repeating the mean monthly pattern as well as the total currents; the CW
intrusion starts in spring and peaks in summer to retract in autumn and
winter, and there are no relevant mesoscale EKE structures in the western
GoM. These results confirm an annual pattern of CW intrusion in summer
months and a retraction in winter.
Average monthly percentage surface areas of CW in the interior of
the Gulf of Mexico determined from climatology of the SD contour
> 15 cm; enclosed areas were calculated in relation to the GoM area
(1.56×106 km2).
West and northward Caribbean Water extension
The monthly intrusions of the CWF were tracked by taking as a reference the
northernmost latitudes and westernmost longitudes of the 40 cm ADT isoline
representing 1993–2017 monthly average values of the ADT (not spatially
averaged). The climatological position of the CWF for each month of the year
is shown in Fig. 3. These results confirm the annual intrusion of CW
as follows: (1) analysis of the maximum north and westward penetration of the
front over 25 years shows that from January to February, it is retracted
southeast to ∼26.55∘ N and ∼88.32∘ W (Fig. 3a and b, respectively) and intrudes to
28∘ N, 90.45∘ W in August; (2) an ADT spectral analysis
derived from 25 years of daily data from the CWF region shows a strong
annual signal that originates from the back and forth of the ADT signal
(Fig. 3c). In this work, the ADT signal also includes the seasonal steric
effect. Based on Hall and Leben (2016), a steric signal appears as an annual
sine wave with a 5.8 cm amplitude. When the estimated seasonal steric
influence is removed, the high energy peak diminishes by 74 %.
In winter, the “tongue” of the CWF moves slowly to the north without
westward advance; in spring it lengthens and travels slightly towards the
west. From January to March, the northward CWF position shifts slowly,
tracing a gently sloping line that starts at 26.5∘ N, reaches its
maximum northern position of 28∘ N in August, and then decreases
in December to 26.28∘ N (maximum travel of the CWF was
1.72∘ or 191 km). In summer, the CWF intrudes further into the
interior of the GoM in both the north and west: its maximum northern and
westward advance occurs in August to 28∘ N and 90.45∘ W,
but then the CWF retracts in the last month of summer. Regarding
CWF westerly movement (Fig. 3b), the CWF traveled little
from January to April; in May, however, it extended quickly and in July,
August, and September reached approximately 90.2∘ W. It peaked in
October at 90.76 ∘ W (maximum range was 2.56∘ or 253 km, calculated at 27.5∘ N latitude). In December, the CWF retracted
abruptly to 88.24∘ W.
Monthly means of absolute dynamic topography (ADT) from 1993 to 2002
(color) and the respective CWF computed with the 40 cm isoline (heavy black
line).
Monthly means of absolute dynamic topography (ADT) from 2003 to 2017
(color) and the respective CWF computed with the 40 cm isoline (heavy black line).
Another aspect of the CWF is the rate of intrusion and retraction. From
March to August, the CWF moves to the north with a penetration speed on the
order of ∼1.02 km d-1, covering a distance of 153 km or 1.37∘. On the other hand, the rate of retraction from August to November is
∼1.86 km d-1, equivalent to 168 km (1.51∘). The entire process of
northerly intrusion occurred in three stages: first, from January to April,
the front moved slowly northward, increasing its speed while maintaining its
westward position. Between May and July the front moved northwest and was then
quasi-stationary in July and August near 90.45∘ W; finally, in
September, it moved from 90.13 to 90.76∘ W,
equivalent to 63 km, at a rate of 2.1 km d-1. The retraction to the west
occurred relatively quickly as the front retracted 193 km towards the east in
a single month (October) at the rate of 6.3 km d-1, and in November
it traveled 41 km at a rate of 1.4 km d-1, also towards the east.
Figure 4 shows the calculated climatological areas of the standard deviation (SD)
of the CWF contours > 15 cm (dotted line) and CWF contours
> 40 cm (heavy black line). From these areas we calculated ratios
between the two (15 cm / 40 cm). The SD contour of 15 cm was selected because
this value was 3 times greater than the annual steric signal reported by
Hall and Leben (2016). Ratio values were 1.37 in January, increased to 1.45
in February and to 1.60 in March, then peaked in April (1.63) to decrease
in May (1.47) and June (1.46), reaching a low value in August (1.27). The
monthly ratio then increased in September to 1.55, decreased slightly in
October (153), reached a maximum value in November of 1.65, and settled to
1.55 in December. A plot of these monthly ratios clearly shows a strong
biannual signal peaking in April and November (not pictured). Chang and Oey (2012, 2013) proposed that the LC intrusion and the shedding of the LCE
followed a biannual cycle. This biannual cycle can also be related to the
annual lowest and highest values of the ratio.
Average bold numbers for Chl a concentrations (mg m-3) and differences
(mg m-3; %) between early and contemporary averages at two
geographical areas: 95.5∘ W, 22.12∘ N and
91.5∘ W, 25.87∘ N (Western GoM) as well as 86∘ W,
22.12∘ N and 84.75∘ W, 23.37∘ N (LC–LCEs)
during “early” (1998–2002), “middle” (2003–2008), and
“contemporary” (2009–2014) epochs. The table shows the compared
averages in bold. The standard deviations and the number of pixels considered
are shown in parentheses.
Monthly climatologies of Chl a (SeaWIFS 1998–2002 and MODIS data source
2003–2017). The heavy black line represents the contour of the 40 cm ADT data
that represents the CWF (1998–2017). Chl a values larger than 1 mg m-3 are
plotted in red.
Monthly spatial variability of the Caribbean Water
Front
It was found that where penetration–retraction of the CWF occurs, SD
variability varies from 15 to 35 cm, extending west to 90.8 in
winter and 93.5∘ W in summer (Fig. 4). West of the CWF, in the
deep zone of the GoM, the observed variability was close to 10 cm distributed
in the band of latitude between 23 and 28.5∘ N. The
regions of maximum variability (SD > 15 cm) occur in the CWF zone
and extend outside the irregular area of reference (isoline of the 40 cm ADT).
The effect of CWF penetration and regions of anticyclonic circulation was
determined from the area of the variability of ADT, with maximum values
close to ∼35 cm in the central region of the CWF at
86.67∘ W and 25.6∘ N. The percentage of the area of
influence of SD > 15 cm in relation to the area of the gulf
(1.56×106 km2) is presented in Fig. 5, where a gradual monthly
increase is observed from January to October, followed by a decrease in
November and December. In January, the direct influence of the CWF on the
gulf by area was 12.4 %, rising to 21.5 % for October and subsequently
decreasing in December to 15.4 %. We suppose that the greater percentage
area of the SD may be attributed to a greater influence of Caribbean Sea
water.
From top left to bottom right, average Chl a values according to period:
column 1 is SeaWIFS 1998–2002, column 2 is MODIS 2003–2008, and column 3 is MODIS
2009–2014. From top to bottom the panels correspond to the mean seasons.
The average Chl a concentration is computed inside the white and red squares (white
corresponds to the western GoM and red corresponds to the LC area). Average
values for each time period and season are in Table 1.
Changes in Caribbean Water incursion from 2003 to
the present
Using the 40 cm reference, a 3-year running average of the ADT data was
calculated to extract the minimum number of years that would produce a
similar pattern over a quarter century of the CWF. The results indicate a
difference in CWF path and westward penetration before and after 2002. It is
observed that before 2002 the CWF was less intrusive in the west (Fig. 6);
after 2002 it extended towards the west in both summer and autumn (Fig. 7).
It is important to note that the intrusion of the CWF is due to the
influence of LCEs that have a strong presence in the western GoM. This fact
is supported by a statistical analysis of the lifetimes of LCEs during
two time periods (1993–2002 and 2003–2015) (http://www.horizonmarine.com/loop-current-eddies.html, last access: 19 November 2019). The data show
that the LCEs in the 1993–2002 period had a mean life of 6.8 months, while
the average life in 2003–2015 was 11.7 months. To prove that there is
a significant difference between these periods, a Student's t test was applied
with the result that the difference between them is significant (t=-3.098, p=0.005). The LCE mean life difference is clear evidence that
the incoming volume of water from the Caribbean Sea (with oligotrophic features;
Aguirre-Gómez and Salmerón-García, 2015) reached farther
in the western GoM after 2002. These observations also agree with the
results of Lindo-Atichati et al. (2013), confirming that, on average, the LC
northward intrusion starts to increase in 2002. These authors also report an
increase in the number per year of LC rings over the same period that also coincided
with a significant increase in sea height residuals (2.78±0.26 cm per decade from 1993 to 2009). This supports the finding that from 2003 onward,
larger volumes of oligotrophic waters from the Caribbean Sea have invaded the
western GoM.
Differences in Chl a concentration (mg m-3) for 2009–2014 average
values of MODIS data minus 1998–2002 average SeaWIFS values. The broken line
represents the 250 m isobath. White contoured areas indicate no significant
differences.
Chl a concentrations (mg m-3) at four stations (a to d) in the GoM;
daily time series derived from SeaWIFS from 1998 to 2002 (green) and MODIS
from 2003 to 2017 (blue). Least squares regressions for SeaWIFS (red line),
MODIS (cyan line), and the overall linear regressions for each station
(dashed black line).
Chlorophyll <inline-formula><mml:math id="M169" display="inline"><mml:mi>a</mml:mi></mml:math></inline-formula> satellite
imagery, climatology, and changes in the last decade
Another product that tracks the effect of CW inside the western GoM is
Chl a satellite imagery, being an index of primary productivity (Boyer et al.,
2009). Physical processes that affect the distribution and abundance of
Chl a include estuarine influxes, the depth of the nutricline, wind stress, thermal
stratification, and eddy advection. However, over deep waters of the GoM, it
is wind stress and thermal stratification that principally affect
the Chl a concentration (Martínez-López and Zavala-Hidalgo, 2009;
Müller-Karger et al., 2015, Damien et al., 2018). It was found that the
oligotrophic CW contrasts seasonally with the gulf waters and allows for the
observation of two levels of Chl a (high and low, Müller-Karger et al., 1989).
Here, the temporal relationship between the CWF and Chl a concentration was
constructed from SeaWIFS and MODIS monthly climatological images (Fig. 8).
The highest concentrations of Chl a in the interior of the GoM are observed
during autumn and winter months when high concentrations are triggered by
vertical mixing (Pasqueron de Fommervault et al., 2017; Damien et al., 2018)
and values were > 0.14 mg m-3, in agreement with Dandonneau et al. (2004), whereas in spring–summer they decreased to 0.08–0.09 mg m-3.
During spring–summer, when the maximum CW penetration occurs, our data
confirm that the “footprint” of the CWF water (delineated by the 40 cm
isoline of ADT) is in general oligotrophic, indicating that Caribbean Water
has indeed entered the GoM. During this period, the Chl a surface concentration
remains low as the increase in surface temperature strengthens
stratification. Additionally, the winds from the southeast are weak, thereby
reducing the mixing of nutrients to the surface. In contrast, during the
autumn–winter months, the northerly winds are stronger, increasing vertical
mixing, deepening the mixed layer, and carrying cold, nutrient-rich
subsurface water into the euphotic layer (Müller-Karger et al., 1991, 2015; Pasqueron de Fommervault et al., 2017).
In seeking relationships between the spatial–seasonal distribution of the
Chl a concentration and the incursion signaled by the ADT-generated data, three
spatial–temporal periods were selected; each was averaged pixel by pixel,
and the three were labeled: “early” (1998–2002), “middle” (2003–2008), and
“contemporary” (2009–2014) epochs. The 5-year averages of the early and
contemporary periods of two separate areas were compared: (1) an area
located in the western GoM at 95.5∘ W, 22.12∘ N and
91.5∘ W, 25.87∘ N, as well as (2) a smaller area located in the
center of the LC at 86∘ W, 22.12∘ N and 84.75∘ W, 23.37∘ N (Fig. 9). The differences in the means were
tested for significance with a two-tailed z test at the 95 % confidence
level (Fowler et al., 2013). The results are shown in Table 1 and may be
summarized as follows.
Temporal differences. (1) Western GoM differences between early
and contemporary Chl a concentrations are significantly different in all seasons;
(2) Loop Current differences between early and contemporary Chl a concentrations
are significantly different during winter, spring, and autumn but not in
summer.
Spatial differences. (1) In winter, the western GoM is
significantly higher in Chl a than the LC during both the early and contemporary
periods; (2) in the spring, the western GoM is significantly higher than the
LC during the early period but not in contemporary period; (3) in summer,
the LC is significantly higher than the western GoM during both the early and
contemporary periods; and (4) in autumn, the western GoM is significantly higher
than LC during the early period but not significantly different from the
LC in the contemporary period.
Seasonal differences. In the western GoM and the LC in both the
early and contemporary periods, Chl a decreases from winter to spring and from
spring to summer, and it increases from autumn to winter, but autumn
concentrations do not exceed winter (see also Fig. 9). All differences are
significant.
Examination of Table 1 indicates that at both areas, the winter season is
the most productive, followed by autumn, with the lowest Chl a concentrations
occurring in summer (see also Fig. 9). There is also a time-dependent trend,
with contemporary values that are, in general, lower than the values in the
early and middle epochs. Both areas exhibit identical climatic trends over
time and during each season, indicating that these effects are applicable
outside the continental shelf. The early spring epoch is more eutrophic
than the middle and contemporary epochs, indicating a decline in nutrient
concentrations over time. This effect is also evident in the LC core, where
Chl a concentrations decreased with time and signals the entrance to the gulf of
more oligotrophic water during the middle and contemporary epochs. Perhaps
the most notable seasonal scenario occurs in the summer to early October
period, when the CWF “tongue” extends into the interior of the GoM. Although
the concentration of Chl a in the western GoM declines gradually with time to
from ∼0.09 to ∼0.08 mg m-3, the interesting
fact is that the area of oligotrophic water expands and becomes larger in the
contemporary period. On the other hand, in the LC core, the Chl a concentrations
in the three epochs do not significantly differ, suggesting that the water
entering the GoM is from a single source, namely the Caribbean Sea. In
general, the extensive penetration of the LC within the GoM, as well as the
increase in the life periods and sizes of LCEs, coincides with the intrusion
of nutrient-poor CW.
Two points summarize the result of the seasonal analysis of the three
epochs: first, the extent of the CW intrusion confirms the northwest
migration of eddies during each epoch. Second, the Chl a concentration declines
over time.
The second point was confirmed by calculating the average Chl a concentrations
outside the continental shelf over two time periods, considering only the
concentrations above waters deeper than 250 m. Using data from 1998 to 2002
(SeaWIFS) and from 2009 to 2014 (MODIS), we conducted a Student's t test for
differences in the means (Fig. 10). The latter period was significantly lower
with t=4.75 and p < 0.001 (n1=1.825; n2=2.190).
This analysis confirms that the Chl a concentration of the GoM decreases over
time and appears to disagree with the results of Müller-Karger et al. (2015), who did not indicate a time trend in Chl a concentration in the GoM. As
the data were taken with different sensors and to eliminate the uncertainty
that this difference is not caused by a systematic difference between the
SeaWIFS and MODIS datasets used in our analysis, we calculated least squares
regressions to the SeaWIFS and MODIS time series at four stations
corresponding to the northwest, northeast, southwest, and southeast regions
of Müller-Karger et al. (2015) (Fig. 11). For each dataset, inner
slopes and overall slopes were calculated. For all four stations, the
SeaWIFS (1998–2002) and the MODIS (2003–2017) data series merged exactly and
all stations show negative trends; equivalently, the combined time series
(1998–2017) also show a negative tendency, supporting the conclusion that
the Chl a concentration over the deep GoM has decreased over time.
The difference between our results and those obtained by Müller-Karger
et al. (2015) may be attributed to the different way in which the two groups
treated the data. Müller-Karger et al. (2015) divided the GoM into four
quadrants with depths of over 1000 m: Region 1 northeast (RO1), Region 2
(RO2 northwest), Region 3 (RO3 southeast), and Region 4 (RO4 southwest). They
calculated the spatial average in each quadrant to build four time series
from 1993 to 2012. In their words, “Time series of anomalies of wind speed,
SST, SSHA and Chl a concentration were obtained by subtracting the monthly mean
(climatology) from the monthly field for that variable”. Time series of wind
speed, sea surface temperature (SST), sea surface height (SSH), and Chl a data
obtained at these stations from satellite products were analyzed
statistically and plotted. Other variables plotted by Müller-Karger et al. (2015) were mixed layer depth (MLD) as calculated from a hydrodynamic
model and net primary production (NPP) calculated from MODIS data using the
vertically generalized productivity model (VGPM) of Behrenfeld and Falkowski (1997).
On the other hand, we calculated the average of the Chl a concentration pixel by
pixel in waters over 250 m of depth for two time periods (1998–2002 and
2009–2014) and subtracted the respective monthly (climatological) means to
find the difference (Fig. 10). From 2009 onward, the difference indicated a
small reduction of Chl a in the first optical depth (1–20 or 40 m of depth)
that is increasing with time. A Student's t test was used to conclude that the
reduction was significant. We also treated the data exactly as
Müller-Karger et al. (2015) did and obtained slightly negative slopes
over the entire 1998 to 2013 period.
We suggest that Müller-Karger et al. (2015) did not detect the small
negative trend in their Chl a plots because their calculated slopes indicated no
time-dependent change. We surmise that they were also influenced by the lack
of slope in the modeled MLD plot despite clear positive trends for SST,
SSHA, and wind force. Actually, although close to zero, the slopes, as
indicated in Müller-Karger et al. (2015), were not zero but -0.03 for
RO1, -0.01 for RO2, and simply given in as -0.0 for RO3 and 0.0 for RO4 (see
their Table 1). Müller-Karger et al. (2015) also ignored the fact that
the time–Chl a correlation coefficients (R) for all four regions were negative.
To confirm our findings, we chose four stations, each one centrally located in
each quadrant (Müller-Karger et al., 2015), and conducted regression
analyses of the logarithmic transform of the SeaWIFS and MODIS Chl a
concentrations. All four regions showed a negative slope and a negative R, and
the negative slopes in the southern gulf (RO3 and RO4) were significantly
different from 0 (p≪0.05). This is shown in Fig. 11.
The observed small but persistent decline in Chl a from 1993 to 2017 may be
attributed to the AMOC's overall effect of warming the surface water and
thereby promoting stratification. However, we wish to make clear that our
conclusion about the recent time-dependent lowering of Chl a pertains only to
the near surface and may not indicate a decrease in the primary
productivity integrated over the entire water column. In the GoM, the
chlorophyll maximum as measured by fluorescence occurs at about 75 m, e.g.,
below one optical depth, and is greater in summer than in winter (Pasqueron
de Fommervault et al., 2017), indicating that the relationship between water
column productivity and near-surface Chl a concentration in the GoM requires
further study. Our own results and conclusions are based on SeaWIFS and
Aqua MODIS chlorophyll data, which for Type One water
correlate very well with chlorophyll measured with standard laboratory
methods (Mati Kahru, personal communication). In our work we can only say
that according to these satellite products, we find a time-dependent
diminution of the Chl a signal. This diminution has been widely observed by
others although in other waters (Behrenfeld et al., 2006; Polovina et al.,
2008; Irwin and Oliver, 2009; Laffoley and Baxter., 2016).
Summary and conclusions
The availability of a large spatial extension of satellite observations of
ADT, sea surface currents, wind stress over a quarter century, and Chl a
over 20 years has enabled us to confirm the LC and CW dynamics observed in
the 1960s and 1970s with more recent in situ observations. The verification of the
CWF climatologies developed in this work is important as a reference
baseline for further numerical modeling, and it impacts assessments of the
gulf's biogeochemistry, energy, heat transport, and Chl a concentration. A
recent committee of the National Academic of Sciences (2018) suggested
three main study topics to advance the knowledge of the processes that
characterize the GoM: (1) the LC system active area, (2) the variation of the
inflows of the LC system, and (3) the dynamic interactions of the LC system
in the west. Following these suggestions, we have confirmed that the maximum
influence of the CW into the GoM (e,g., its maximum extension into the gulf
or intrusion) has a temporal variability, being stronger in summer and
weaker in the late fall and winter. This is supported by the fact that the
generated monthly EKE maps have the maximum gradient at the periphery of the
CWF and have a similar monthly pattern of extension and retraction as the
CWF.
We noted that in the summer months the wind stress from the southeast is
weak, thereby minimizing the flow of nutrients to the surface and causing
Chl a to be low, specifically for three reasons: (1) the increase in the
surface temperature of the water column strengthens stratification, (2) the
intrusion of the CW to the western gulf's surface thickens
the surface layer, and (3) the eddy-driven anticyclonic circulation deepens
the nutricline. This contrasts with the cold seasons when the surface
temperature of the water is lower and the northerly winds are stronger,
favoring the flow of nutrients to the surface.
The 3-year running averages of the ADT 40 cm isoline qualitatively reproduce
the climatological pattern of a quarter of a century, showing that before
2002 the CWF was less intrusive and the LCE sizes were smaller. In the
1993–2002 period, we calculated that the mean life cycle of the eddies was
6.8 months and that in the 2003–2015 period the mean life cycle was 11.7 months. This difference suggests that after 2003, larger volumes of
oligotrophic waters from the Caribbean Sea invaded the western GoM and
reduced mean surface Chl a concentrations. This work shows that the intrusion of CW by LC–LCEs extends further into the western GoM than
was previously known.
Chl a concentrations respond to the dynamics inside the GoM and are influenced by
the CWF and the LC anticyclonic and cyclonic eddies.
Since 2002, near-surface Chl a concentrations over bathymetry deeper than 250 m
have decreased, and GoM surface waters may be turning more oligotrophic than
in the previous decade.
This work, based on 25 years of remotely sensed data, emphasizes the role of
climatology in determining GoM circulation and its productivity and suggests
that further climatologically induced changes are probably imminent.
Data availability
Sea surface water and absolute dynamic topography were processed by SSALTO/DUACS and distributed by AVISO+ (https://www.aviso.altimetry.fr/en/data, last access: 19 November 2019) with support from CNES.
The GEKCO (Geostrophic Ekman Current Observatory; Sudre et al., 2013;
http://www.legos.obs-mip.fr/members/sudre/gekco_form with support, last access: 19 November 2019) product used in this study was developed by Joël Sudre at
LEGOS, France.
MODIS and SeaWIFS Chl a maps were derived from two NASA products: Aqua MODIS (Moderate Resolution Imaging Spectroradiometer; https://oceancolor.gsfc.nasa.gov/l3/, last access: 19 November 2019) and SeaWIFS (Sea-Viewing Wide Field of View Sensor) using the OCx algorithm
with a spatial resolution of 9×9 (https://oceancolor.gsfc.nasa.gov/l3/, last access: 19 November 2019).
Finally, the general features of the Gulf of Mexico Loop Current eddies were taken from the Woods Hole Group: https://www.horizonmarine.com/loop-current-eddies (last access: 19 November 2019).
Author contributions
JH-A and JAD initiated the collaboration and designed the study. AZ, JH-A, and JAD contributed to the result interpretation. The conceptualization and arrangement of the original paper were under the charge of AZ, JH-A, JS, IM, ST-R, and JAD. Preprocessing of all data was carried out by JS and JAD. JAD enhanced the figure quality. All authors contributed equally to editing and review. All authors worked on the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thanks the reviewers for their positive criticisms and comments that helped
us to improve the paper. This study was carried out as part of the PhD
thesis research conducted by the lead author at the Faculty of Marine
Science and the Oceanographic Research Institute (FCM-IIO/UABC),
Postgraduate Coastal Oceanography Program, and it was supported by the
Graduate Professional Development Mexican Program grants (PRODEP:
DSA/103.5/16/5801), the National Institute of Technology of Mexico (TecNM),
and the Mexican Energy Bureau and Hydrocarbons Mexican Trust, project
201441. This is a contribution of the Gulf of Mexico Research Consortium
(CIGoM).
Financial support
This research has been supported by the National Council of Science and Technology of Mexico – Secretariat of Energy – Hydrocarbons Trust (grant no. 201441).
Review statement
This paper was edited by Piers Chapman and reviewed by two anonymous referees.
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