10B.2 Diversity of Surface Easterly Wind, Upper Ocean Chlorophyll-a and Thermocline Depth Along the Equator in El Niño and La Niña Events During 1997 to 2016

Wednesday, 25 January 2017: 1:45 PM
609 (Washington State Convention Center )
David Halpern, JPL, Pasadena, CA; and B. Franz and P. Heimbach

An El Niño (La Niña) event has five consecutive 3-month average sea surface temperature anomalies (SSTAs) greater (less) than 0.5°C (-0.5°C) in the 5°S-5°N, 170°W-120°W region, and a super El Niño event occurs when a SSTA is greater than 2.0°C. The era of continuous satellite surface wind vector measurements (AMI-SCAT on ERS-2; SeaWinds on QuikSCAT; ASCAT-A on MetOp-A) captured two super El Niño events (May 1997 to May 1998 and March 2015 to May 2016), four typical El Niño events, and four La Niña events. Both super El Niño events had similar SSTA intensities of about 2.2°C. We chose the 1°S-1°N region along the equator to examine the behavior of the zonal wind component because this latitudinal width represented the narrow scale of Ekman upwelling, which is hypothesized to be an important mechanism of near-surface chlorophyll-a and thermocline depth variations. In the average typical El Niño event, the westward wind speed collapsed over 135°E-165°E with longitudinal-averaged eastward wind speed of about 1.0 m/s. In June-November 1997, a major collapse of the easterly wind occurred over 153°E-170°W, with a longitudinal-averaged eastward wind speed of about 2 m/s and maximum speed of 4.1 m/s at 168°E. A similar easterly wind collapse did not occur in the 2015-2016 super El Niño event. During the latter six months of the 1997-1998 event, the wind direction was westward throughout the Pacific with a uniform speed of about -3.0 m/s from the far west Pacific to 130°W, where the speed declined linearly, more or less, to approximately zero at 90°W. The average westward wind speeds at 150°W associated with the 1997-1998, 2015-2016 and typical El Niño events were 3.2, 4.6 and 6.0 m/s, respectively, which indicated a large diversity among super events and between super and typical El Niño events; east of 110°W, the westward wind speeds were comparable. Surface wind distributions in the equatorial Atlantic and Indian oceans will also be described, e.g., the surface zonal wind component in the equatorial Atlantic Ocean did not portray any impact of El Niño or La Niña. Correlations between inferences of Ekman vertical motions in the upper ocean at the equator and fluctuations in near-surface phytoplankton abundance, which were measured by satellites, and in the depth of the thermocline, which were determined by a model constrained with ocean observations, will be discussed.
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