Every winter, a
unique seasonal pattern of primary productivity occurs off of the Pacific Coast
of southern Mexico and Central America. Strong
winds (which are caused by differences in atmospheric pressure on the eastern
and western sides of the Central American cordillera) funnel through mountain passes
and blow for miles over the Pacific Ocean, stirring deeper waters to the
surface. This wind-mixing process brings
colder water with increased nutrient concentrations to the sunlit zone,
fostering episodic blooms of phytoplankton.
(A Science Focus!
Papagayo Wind (PDF), discusses the phenomenon, with illustrations.)
During a large El Niño
event, the elevated sea surface temperature (SST) of the surface ocean, and the
concomitant increased thermocline depth, reduces the effectiveness of wind
mixing as a nutrient transport mechanism.
During the 1997-1998 El Niño, a significant negative anomaly of
indicative of lower phytoplanton concentrations and growth, was observed by
remote sensing in all three of the eastern Pacific wind jet-mixing zones.
the mechanisms that enhance winter productivity in this region are recurring, the
strong El Niño event occurring in 2015-2016 would be expected to result in a
similar pattern of reduced phytoplankton activity. Using an increased suite of data variables
than could be employed by prior examinations of the El Niño effects in this
region, it is possible to compare conditions between the essentially average
winter of 2014-2015 and the El Niño winter of 2015-2016.
Satellite remote sensing can indicate the occurrence of a
wind event in several ways. Frequently,
the leading edge of a Tehuano wind event (where the winds blow through the
Chivela Pass over the Gulf of Tehuantepec) is indicated by a thin arc cloud
over the Pacific Ocean. These brutal
winds can also carry dust through the passes.
Wind-induced mixing of the water column is detected by reduced sea
surface temperature (SST), as colder subsurface waters are mixed to the
surface. As described earlier, because these
colder waters have higher nutrient concentrations than surface waters, the wind
events usually initiate phytoplankton blooms when the increased nutrient availability
stimulates phytoplankton growth. Such
blooms can be observed by satellite-borne ocean color sensors, due to increased
concentrations of phytoplankton chlorophyll.
The occurrence of a strong El Niño event will affect the thermal
and biological oceanographic signatures of the Pacific wind events. An El Niño results in warmer surface waters,
and below the surface, the mass of warmer water increases the depth of the
thermocline (the transition zone between warm surface water and cold deep water). As a result, the wind events will be less
effective at bringing the colder subsurface waters with their higher nutrient
concentrations to the surface. The
result of this suppression is twofold: the
reduction in SST caused by the winds will be less, and there will be a correspondingly
reduced phytoplankton response because significantly less nutrients are brought
to the surface.
Comparing a ‘Normal’
winter to an El Niño winter with remote sensing
Data from the Moderate Resolution Imaging Spectroradiometer
(MODIS) on NASA’s Aqua satellite was used for the generation of SST and
chlorophyll concentration images, using the NASA Giovanni analysis and
visualization system. Images were
created for the large area encompassing the outflow region of all three wind
areas (Tehuano, Papagayo, and Panama).
These images averaged the monthly data products for December
2014-January 2015 and December 2015-January 2016. Time-series plots of chlorophyll-a concentration and SST from September
2014 to January 2016 were also created for the Tehuano wind offshore zone,
which showed the most striking difference between the two time periods.
To evaluate the wind regime, northward and eastward component
wind speeds from the Modern Era Retrospective-analysis for Research and
Applications (MERRA) were also plotted for the same period, and time-series plots
for each component were generated for the Tehuano wind zone. These data were acquired from the reprocessed
data set known as “MERRA-2”, also available in Giovanni.
Figure 1 shows a comparison of SST for December 2014-January
2015 (Figure 1a) and December 2015-January 2016 (Figure 1b). Elevated SST attributable to El Niño can be
clearly seen in Figure 1b. The distinct
difference in SST in the wind mixing zones is also evident, with SST in the
wind mixing zone for December 2015-January 2016 about two degrees C higher than
in December 2014-January 2015.
Figure 1. (a) MODIS SST averaged over December
2014-January 2015. (b) MODIS SST
averaged over December 2015-January 2016.
The same color scale is used for both images. In this color scale, light blue indicates the
lowest temperatures in the selected temperature range. Click on each image to view the full-size version.
The effects on the biological oceanography of the region are
shown in Figure 2. All three wind zones
show higher chlorophyll-a
concentrations in the ‘normal’ 2014-2015 winter (Figure 2a) as compared to the lower concentrations during the 2015-2016 El Niño winter (Figure 2b).
Figure 2. (a) MODIS-Aqua chlorophyll-a concentrations averaged over December
2014-January 2015. (b) MODIS-Aqua chlorophyll-a concentrations averaged over December 2015-January 2016. The same
color scale is used for both images, where yellow, orange, red, and magenta indicate
higher chlorophyll-a concentrations
than shades of blue. Click on each image to view the full-size version.
The wind speed component comparison is shown in Figures 3
and 4. The northward wind component is
most significant for the Tehuano wind zone, where the winds are directed nearly
due south, resulting in negative northward wind component values. The Papagayo winds blow more in the western
direction, so the eastward wind component is negative in the Papagayo wind
zone. Figures 3 and 4 indicate that the
wind regime during the 2014-2015 ‘normal’ winter was slightly stronger than
during the 2015-2016 El Niño winter.
Figure 3. (a) The MERRA-2 northward wind component
averaged over December 2014-January 2015.
(b) The MERRA-2 northward wind component averaged over December 2015-January
2016. The same color scale is used for
both images. The black box in (a) is the
area used for time-series analysis. Click on each image to view the full-size version.
Figure 4. (a) The MERRA-2 eastward wind component
averaged over December 2014-January 2015.
(b) The MERRA-2 eastward wind component averaged over December
2015-January 2016. The same color scale
is used for both images. Click on each image to view the full-size version.
The time-series plots for SST, chlorophyll-a concentration, and the northward and
eastward wind components are shown in Figure 5. These plots are for the Tehuano wind zone
region, shown in Figure 3a. The SST
time-series (Figure 5a) indicates, in accord with Figure 1, that the SST in the
wind outflow region was definitely higher during El Niño winter as compared to
the previous winter. Chlorophyll-a concentration (Figure 5b) also shows
the expected difference between the two winters, with much higher chlorophyll
concentrations corresponding to the episodic bloom events in December
2014-January 2015 than the subsequent El Niño winter, during which these events
Figure 5. Time-series plots for the Tehuano wind zone,
for the period September 2014 – January 2016. (a) MODIS-Aqua SST. (b) MODIS-Aqua chlorophyll-a concentration.
(c) MERRA-2 northward wind component.
(d) MERRA-2 eastward wind component. Negative values for these wind components
indicate winds blowing to the south and west, respectively. Click on each image to view the full-size version.
The wind component time-series suggest, as is also indicated
in Figures 3 and 4, that the wind speeds are slightly reduced during the El
Niño winter. The average northward wind
speed (Figure 5c) is less negative in January 2016 than January 2015, but is
approximately equal to the wind speeds in the other winter months. The eastward wind component (Figure 7d) is
also slightly less negative from November 2015 – January 2016 compared to the
same period a year earlier.
Intriguingly, May 2015 shows a distinct increase in the positive
eastward wind speeds, which corresponds roughly to the initiation of definitive
El Niño conditions in the Pacific Ocean, when the atmosphere began to respond
to the elevated SST in the central Pacific.
The Effects of El
Comparing SST, chlorophyll-a concentration, and wind speed data between the normal winter
conditions of 2014-2015 and El Niño winter conditions of 2015-2016 for the wind
jet-mixed zone of the eastern tropical Pacific Ocean (off of the Pacific Coast
of Central America and Mexico) indicated patterns consistent with the expected
influence of El Niño oceanographic conditions.
The El Niño water mass with elevated SST influenced this oceanic region
by reducing the efficacy of wind mixing during the high wind speeds of wind jet
events. Higher SSTs in the wind jet-mixed
zones during El Niño winter are attributed to the increased depth of the
thermocline, which makes it more difficult for wind mixing to bring subsurface
water to the surface. This results in suppressed phytoplankton productivity,
due to reduced nutrient availability from subsurface waters.
The northward and eastward wind components exhibited only a
small decrease in wind speed during the El Niño winter as compared to normal
winter conditions. Thus, even though
Tehuano, Papagayo, and Panama wind events continue to take place during an El
Niño winter, their effect on the physical and biological oceanographic
environment of the adjacent Pacific Ocean is significantly reduced due to the overriding
influence of El Niño, as seen in the markedly higher SST values in the wind jet-influenced
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