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Sparse Times in the Gulf of Tehuantepec – Evaluating the Effects of the 2015-2016 El Niño on the Wind-Mixed Productivity in the Eastern Tropical Pacific Ocean

El Niño winters reduce productivity in the Pacific Ocean off Central America

Sparse Times in the Gulf of Tehuantepec – Evaluating the Effects of the 2015-2016 El Niño on the Wind-Mixed Productivity in the Eastern Tropical Pacific Ocean

El Niño reduces the phytoplankton productivity of the coastal waters off Central America during winter.

Sparse Times in the Gulf of Tehuantepec – Evaluating the Effects of the 2015-2016 El Niño on the Wind-Mixed Productivity in the Eastern Tropical Pacific Ocean


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! article, The 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 chlorophyll-a concentration, indicative of lower phytoplanton concentrations and growth, was observed by remote sensing in all three of the eastern Pacific wind jet-mixing zones.

Because 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.

 

a Eastern Pacific SST, December 2014-January 2015  b  Eastern Pacific SST, December 2015-January 2016

 

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).   


a Eastern Pacific chlorophyll, December 2014-January 2015  Eastern Pacific chlorophyll, December 2015-January 2016


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.

 

a MERRA northward wind component, December 2014-January 2015   b  MERRA northward wind component, December 2015-January 2016

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.

 

a MERRA eastward wind component, December 2014-January 2015   b  MERRA eastward wind component, December 2015-January 2016

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 were attenuated.

 

a  Tehuano wind zone, SST, time series, December 2014 - January 2015


b  Tehuano wind zone, chl, time series, December 2014 - January 2015


c  Tehuano wind zone, northward wind component, time series, December 2014 - January 2015


d   Tehuano wind zone, eastward wind component, time series, December 2014 - January 2015


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 Niño

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 areas.

 

 

References

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D.B. Chelton, Michael H. Freilich, and S.K. Esbensen (2000) Satellite observations of the wind jets off the Pacific Coast of Central America. Part I: Case studies and statistical characteristics. Monthly Weather Review, 128, 1993-2018.

P.C. Fiedler (2002) The annual cycle and biological effects of the Costa Rica Dome. Deep-Sea Research, 49, 321-338.

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NASA Earth Observatory (2004) Tehuano Wind Colors the Ocean. http://earthobservatory.nasa.gov/IOTD/view.php?id=4145, accessed May 18, 2016.

NASA Ocean Color Science Focus (accessed May 18, 2016) The Papagayo Wind. http://oceancolor.gsfc.nasa.gov/cmsdocs/educational_material/ThePapagayoWind2.pdf

NASA Visible Earth (2014) Tehuano Winds. http://visibleearth.nasa.gov/view.php?id=83483, accessed May 18, 2016.

D.M. Schultz, W.E. Bracken, and L.F. Bosart (1998) Planetary- and synoptic-scale signals associated with Central American cold surges. Monthly Weather Review, 126, 5-27.

W.J. Steenburgh, D.M. Schultz, and B.A. Colle (1998) The structure and evolution of gap outflow over the Gulf of Tehuantepec, Mexico. Monthly Weather Review, 126, 2673-1691.

Y. Sasai, Y., H. Sasaki, K. Sasaoka, A. Ishida, and Y. Yamanaka (2007) Marine ecosystem simulation in the eastern tropical Pacific with a global eddy resolving coupled physical-biological model. Geophys. Res. Lett., 34, L23601, doi:10.1029/2007GL031507.

A. Trasviña, E.D. Barton, J. Brown, H.S. Velez, P.M. Kosro, and R.L. Smith (1995) Offshore wind forcing in the Gulf of Tehuantepec, Mexico: The asymmetric circulation. J. Geophys. Res., 100, 20,649-20,663.

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