FY 1989

Observations of long Rossby waves in the northern tropical Pacific

Kessler, W.S.

NOAA Tech. Memo. ERL PMEL-86, NTIS: PB89-196331, 169 pp (1989)

Long baroclinic Rossby waves are potentially important in the adjustment of the tropical Pacific pycnocline to both annual and interannual wind stress curl fluctuations. Evidence for such waves is found in variations of the depth of the 20°C isotherm in the northern tropical Pacific during 1970 through 1987. 190,000 bathythermograph profiles have been compiled from the archives of several countries; the data coverage is dense enough that westward-propagating events may be observed with a minimum of zonal interpolation. After extensive quality control, 20°C depths were gridded with a resolution of 2° latitude, 5° longitude and bimonths; statistical parameters of the data were estimated. A simple model of low-frequency pycnocline variability allows the physical processes of Ekman pumping, the radiation of long (non-dispersive) Rossby waves due to such pumping in mid-basin, and the radiation of long Rossby waves from the observed eastern boundary pycnocline depth fluctuations. Although the wind stress curl has very little zonal variability at the annual period in the northern tropical Pacific, an annual fluctuation of 20°C depth propagates westward as a long Rossby wave near 4°–6°N and 14°–18°N in agreement with the model hindcast. Near the thermocline ridge at 10°N, however, the annual cycle is dominated by Ekman pumping. The wave-dominated variability at 4°–6°N weakens the annual cycle of Countercurrent transport in the western Pacific. El Niño events are associated with westerly wind anomalies concentrated in the central equatorial Pacific; an upwelling wind stress curl pattern is generated in the extra-equatorial tropics by these westerlies. Long upwelling Rossby waves were observed to raise the western Pacific thermocline well outside the equatorial waveguide in the later stages of El Niños, consistent with the simple long-wave model. It has been suggested that El Niño events are initiated by downwelling long Rossby waves in the extra-equatorial region reflecting off the western boundary as equatorial Kelvin waves. The bathythermograph observations show that although such downwelling waves commonly arrive at the western boundary (the Philippines coast), there is a low correlation between these occurrences and the subsequent initiation of El Niño events.