Observations of Equatorial Pacific Planetary Boundary Layer Wind Shear During February 2000–November 2009

The Jet Propulsion Laboratory (JPL) Multi-angle Imaging SpectroRadiometer (MISR) instrument on the National Aeronautics and Space Administration (NASA) Terra satellite provides global observations of both cloud height and velocity for the first time. We analyzed MISR cloud motion vectors with heights from 500-900 m (ftp://l4ftl01.larc.nasa.gov/MISR/MI3MCMVN.002), which were designated 700-m wind vectors. From June 1999 to November 2009 the JPL SeaWinds instrument on the NASA Quick Scatterometer (QuikSCAT) satellite recorded 10-m wind vectors (http://podaac.jpl.nasa.gov/dataset/QSCAT_LEVEL_2B_V2?ids=Measurement&values=Ocean%20Winds). Terra and QuikSCAT equator crossing times were in the morning at 10:30 and 06:00, respectively. MISR and SeaWinds wind vector footprint sizes were 17.6 and 25 km, respectively. Individual MISR east-west and north-south wind components were averaged in 2°-latitude by 2°-longitude regions; similarly for SeaWinds data. Wind direction is oceanographically defined as the direction towards which the wind is blowing.

We analyzed the planetary boundary layer wind speed component difference, or wind shear, between 10 and 700 m at 2°S-2°N, 130°E-80°W, initially emphasizing time series along the equator at 147°E, 165°E, 170°W, 140°W and 110°W, which are sites of long-term in-situ moored buoy wind recorder measurements. During 2007, which the National Oceanic and Atmospheric Administration (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml) indicated as a moderate La Niña year, the average number of days for which MISR and SeaWinds observations were collocated at the five sites was 30. Time series of zonal wind components were strongly correlated, with the average correlation coefficient equal to 0.78; the minimum correlation coefficient (0.50) occurred at 110°W. The MISR and SeaWinds average annual-mean zonal wind speeds at 147°E, 165°E, 170°W, 140°W and 110°W were -6.2 and -4.9 m/s, respectively. Higher easterly wind speed occurred at 700 m compared to 10 m, as expected, although the longitudinal distribution was not uniform. In the western equatorial Pacific at 147°E and 165°E, the magnitudes of the zonal wind speed difference between 10 and 700 m were both approximately 0.5 m/s. However, at 165°E the easterly wind component was stronger at 700 m than at 10 m, which was opposite that at 147°E. In the central and eastern equatorial Pacific at 170°W, 140°W and 110°W the easterly wind speed at 700 m was nearly 2.3 m/s higher than at the surface, with the shear increasing eastward from 170°W to 110°W. At the five sites, the north-south wind component was generally northward with MISR meridional wind speed larger than SeaWinds data. Longitudinal variations of the collocated annual-mean zonal wind speed difference at 10- and 700-m altitudes showed that the MISR and SeaWinds westward wind speeds were largest in the 170°E-170°W region and that between 170°E and 100°W the wind difference was approximately -2 m/s, producing wind shear magnitudes greater than 0.003/s; maximum shear (0.004.3/s) occurred at 140°W. The 10- to 700-m wind shear along the equator was considerably larger in the central Pacific compared to the Atlantic, Indian and western Pacific. Seasonal-to-interannual variations in wind shear along the Pacific equator will be described during February 2000 to November 2009, including El Niño and La Niña events. Comparisons made with numerical weather prediction wind vector data products will be discussed.