Effects of the “Almost El Niño” of 2014 on Phytoplankton Chlorophyll Concentrations in the Pacific Equatorial Upwelling
Changes in the Equatorial Pacific affect phytoplankton growth and concentration
MODIS-Aqua chlorophyll concentrations in the Pacific Equatorial Upwelling Zone, November 9-16, 2014. Arrow indicates a "Tehuantepec Wind" mixing event signature.
Using data from buoys floating in the warm Pacific Ocean, remotely sensed data from satellites, oceanographic models, and global climate models, scientists have been watching the Pacific Ocean during the year 2014 for unmistakable indicators of an El Niño event. The Pacific Ocean has been remarkably shy about its intentions this year, as many of the watched indicators have for months hovered close to (but not over) the edge of definitive El Niño conditions. Despite this uncertainty, the generally warmer-than-average waters of the Pacific have contributed to record high global surface temperatures in several recent months. These warm waters are a significant factor if 2014 ultimately becomes the warmest calendar year ever recorded (instrumentally).
Oceanographic scientists know that there are distinct physical and biological oceanographic indicators of an El Niño event taking place in the Pacific Ocean. The primary physical indicators are the height of the sea surface (measured by satellite altimetry) and the sea surface temperature (SST), which can be measured both in situ by buoys or ships and remotely by satellite instruments. When an El Niño takes place, the sea surface height rises by tens of centimeters, and the SST of equatorial waters increases significantly.
Both of these physical indicators of El Niño are due to warm Kelvin waves that originate in the “warm pool” of the tropical western Pacific and propagate eastward along the equatorial belt. The Kelvin waves have a surface and subsurface signature, with warm waters on the surface following the path of a warm mass of water below the surface, which eventually rises in the central and eastern Pacific Ocean.
The warm subsurface waters have an important effect on the Pacific Ocean’s Equatorial Upwelling Zone (EUZ). Under normal conditions, deeper waters rise to the surface (upwell) along the Equator, driven by large-scale ocean circulation patterns. The upwelled water carries increased concentrations of nutrients, primarily nitrate and phosphate, which are vital to the growth of phytoplankton, the floating plant cells that are the foundation of the oceanic food chain. Thus, the Pacific EUZ is observable in remotely sensed chlorophyll concentration data (also called “ocean color” data) as a broad swath of increased chlorophyll concentration along the Equator.
When an El Niño event takes place, the upwelling is suppressed, and the surface waters become depleted of their nutrients. The result is reduced growth and concentration of phytoplankton, and the classic biological signature of El Niño – a reduction in the equatorial chlorophyll concentrations. During strong El Niño events, the highly productive areas near the Galapagos Islands, along the coast of Peru, and near windy gaps in the Central American mountain cordillera are also suppressed by the overlying mass of warmer water. These effects propagate to seabirds, which may abandon their nesting areas, and mammals such as seals and sea lions, which starve due to the depletion of fish stocks that accompanies the decline in overall oceanic productivity.
Using the NASA Giovanni data visualization and analysis system, the effects of the “almost El Niño” on phytoplankton chlorophyll concentrations in the Pacific EUZ can be examined. Figure 1 shows the boundaries of the regions that are defined for analysis of SST and sea surface height in the Pacific for the purpose of diagnosing the ocean’s El Niño status.
Figure 1. Map of the regions defined for analysis of data to assess the status of the Pacific Ocean with respect to the occurrence of an El Niño event.
Using Giovanni, time-series plots of 8-day average Moderate Resolution Imaging Spectroradiometer-Aqua (MODIS-Aqua) chlorophyll concentration were generated for each of the El Niño regions shown in Figure 1. Figures 2 to 5 show the time-series from January 2013 to mid-November for, respectively, Niño 4, Niño 3,4, Niño 3, and Niño 1,2 regions. MODIS-Aqua chlorophyll concentration data are produced by the NASA Ocean Biology Processing Group (OBPG).
Figure 2. Time-series of 8-day MODIS-Aqua chlorophyll concentrations, January 2013- mid-November 2014, for the Niño 4 region.
Figure 3. Time-series of 8-day MODIS-Aqua chlorophyll concentrations, January 2013- mid-November 2014, for the Niño 3,4 region.
Figure 4. Time-series of 8-day MODIS-Aqua chlorophyll concentrations, January 2013- mid-November 2014, for the Niño 3 region.
Figure 5. Time-series of 8-day MODIS-Aqua chlorophyll concentrations, January 2013- mid-November 2014, for the Niño 1,2 region.
Figures 2, 3, and 4 all show a declining trend in chlorophyll concentration, which actually commenced in October 2013. Chlorophyll concentrations reached their lowest values in April and May 2014, when the oceanographic and climate communities were expecting the arrival of an El Niño event due to the mass of warm water that had been detected moving eastward along the Equator. Subsequently, however, more normal conditions returned, making the confident El Niño predictions less certain. A rebound in chlorophyll concentration can be seen in the figures, particularly Figure 4 for the Niño 3 region.
Figure 5 shows that there was no apparent influence from the warming equatorial waters in the Niño 1,2 region along the coast of South America, where the cold waters of the coastal Peru Upwelling Zone are located. This observation is consistent with the weak influence of the “almost El Niño” conditions.
The changes in the Pacific EUZ can be observed in maps of chlorophyll concentration over the course of the event. Figure 6 shows four maps of 8-day MODIS-Aqua chlorophyll concentration (created with Giovanni), covering the Pacific EUZ. The color palette of chlorophyll concentrations was modified to emphasize the lower concentrations (compared to highly productive coastal waters) that are present in the Pacific EUZ.
Figure 6. Maps of 8-day MODIS-Aqua chlorophyll concentration in the Pacific Equatorial Upwelling Zone. (a) December 3-10, 2013; (b) May 17-24, 2013; (c) August 5-12, 2014; (d) November 9-16, 2014. The white arrow indicates the signature of a Tehuantepec wind mixing event. Click on an image to view it full-size.
In Figure 6a, the medium-blue colors show the higher chlorophyll concentrations of the Pacific EUZ. In Figure 6b, the low concentrations occurring in May 2014 correspond to a narrower, attenuated upwelling zone. Figure 6c depicts the remarkable rebound to higher chlorophyll concentrations that occurred in August, notably adjacent to the Galapagos Islands and also slightly west of the islands.
In Figure 6d, reduction in equatorial chlorophyll concentrations is again observed, compared to the concentrations in August. However, the effects did not reach to the coast of Central and South America. To the south, the higher chlorophyll concentrations of the Peru Upwelling Zone can be seen. To the north, a plume of chlorophyll extending southward from the Mexican coast indicates the occurrence of a Tehuantepec wind mixing event. During these events, high winds rushing through the Tehuantepec Gap in the Mexican Cordillera stir nutrients from below the surface of the Pacific, feeding a short-lived phytoplankton bloom. (These events occur episodically during the winter months.) The occurrence of this event indicates that the underlying nutrient-rich waters in this region have not been forced deeper by warmer El Niño waters, so that they can still be mixed to the surface by strong winds.
In summary, phytoplankton chlorophyll concentrations in the Pacific EUZ during 2014 have clearly responded to the rise and fall of ocean surface height and temperature characteristic of the “almost El Niño” of 2014. It is still not clear (as of early December 2014) if the Pacific Ocean will progress to actual El Niño conditions; the aggregation of models leads the National Oceanic and Atmospheric Administration (NOAA) to maintain its prediction of a 2-in-3 chance of the emergence of El Niño. With Giovanni data, it will be possible to keep watching as it happens (or see if it doesn’t).
Feldman, Gene., Clark, Dennis., and Halpern, David (1984) Satellite color observations of the Eastern Equatorial Pacific during the 1982-1983 El Niño. Science, 226(4678), 1069-71.
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