March 25, 1998

Dear Colleagues,

We have been developing an analysis of tropical Pacific near-surface currents, based on Ekman and geostrophic dynamics, using high resolution wind and altimeter sea level data. This web page presents a series of maps tracing the evolution of surface circulation in the tropical Pacific during the NSCAT observation period (October 1996 - June 1997), which spans the transition from a cold phase in the fall-winter and the genesis of the 1997-8 warm event in the spring 1997. This work is a product of Topex/Poseidon primary and extended mission science team and NSCAT validation studies involving G. Mitchum (USF), A. Busalacchi (NASA/GSFC), R.Lukas(U.H.) and T. Liu (JPL).

The methodology for computing surface currents is described in Lagerloef,G. S. E., G. Mitchum, R. Lukas and P. Niiler, 1999: Tropical Pacific near-surface currents estimated from altimeter, wind and drifter data, J. Geophys. Res., 104, 23,313-23,326. The paper describes how near-surface Ekman velocity is estimated with two latitude-dependant linear coefficients, tuned to best represent the ageostrophic motion of the WOCE/TOGA 15 m drogue drifters relative to the surface wind stress. NSCAT scatterometer winds were used in this presentation. Geostrophic velocities were computed with sea level gradients derived from TOPEX/Poseidon sea surface height analyses. Near equatorial geostrophic currents were obtained with a weighted blend of the equatorial beta-plane and conventional f-plane geostrophic equations (also described in Lagerloef et al., 1999). For the images shown here, the surface height analysis was obtained from the University of Texas website. A 5-year mean surface height field was subtracted and replaced by the mean annual 0-1000 dbar dynamic height derived from the NOAA/NODC atlas (Levitus). We are presently computing a new surface height analysis for TOPEX/Poseidon data and the surface current estimates will be updated accordingly in the future.

Each image below shows the monthly mean from October 1997-June 1998. The top panels of each show the surface Ekman term and the middle panels show the geostrophic flow. The bottom panels superimpose the total surface velocity (Ekman plus geostrophic) on an image of the monthly SST anomaly.

A negative SST anomaly persisted throughout the eastern equatorial region during October-January. A zone of strong easterly trade winds, evident in the Ekman flow, moved westward into the central basin near the date line during December-February. Relaxation of trades in the eastern sector, particularly in February, coincided with the evolution of eastward geostrophic flow east of about 240 E, while westward currents prevailed along the equator in the central and western sectors. This flow reversal in the east, accompanied by locally weak trades and hence weak upwelling, coincided with the onset of warm SST anomaly which first appeared off South America in March and April. One can discern a geostrophic current anomaly, most likely a Kelvin wave signature, that propagated eastward from the date-line to South America between December and April, such that the arrival there also coincided with the SST anomaly onset. The surface geostrophic flow changed abruptly between March and April to a strong eastward jet spanning the width of the Pacific. This flow persisted through May and June as the El Niño SST anomaly intensified rapidly, even as the upwelling favorable trades shifted to the eastern sector. It is apparent that the evolution of the SST anomaly arose from a combination of factors including reduced Ekman upwelling, downwelling Kelvin waves and strong eastward currents that advected warm SST from the west.

Please address inquiries to:
Dr. Gary S.E. Lagerloef, Earth and Space Research, 1910 Fairview Ave E, Suite 102, Seattle WA 98102 USA, phone: 206-726-0501 xt 11, fax: 206-726-0524,
internet: lagerloef@esr.org, http://www.esr.org