Equatorial long‐wave characteristics determined from satellite sea surface temperature and in situ data

Abstract

Sea surface temperature (SST) maps and imagery derived from the NOAA 6 satellite Advanced Very High Resolution Radiometer (AVHRR) for June and July 1981 in the eastern tropical Pacific portray the wavelike structure of the cool water along the equator from 93°W to 125°W. Cusped waves of approximately 1000‐km zonal wavelength and 25‐day period propagated westward with a phase speed of 40 km/day. The observed meridional extent between the crest and trough of the waves is about 300 km. Details in the imagery show cooler water at the cusps advected north and then east with the north equatorial countercurrent (NECC), consistent with the suggestion of a series of anticyclonic eddies occupying the shear zone between the NECC and the westward flowing south equatorial current. Absolute SST estimates from the AVHRR data agree to within 0.6°C with shipboard data taken along 110° W between 5°N and 5°S. The wavelike structures in the SST maps are also in agreement at the surface with a vertical expendable bathythermograph temperature section made along the equator between 93°W and 125°W, which shows the phase of the waves tilting westward with increasing depth over the upper 75 m. Such a phase shift, if it extended 100–200 km meridionally in either direction from the equator, would be associated with an equatorward flux of heat. Similar phase shifts appear in temperature time series at depths of 20 and 50 m, from a mooring at 0°33′N, 110°30′W. Near‐surface currents measured at this and a second mooring on the equator at 109°40′W indicate a regular pattern of northward advection when wave cusps pass them, followed by southwest flow during the passage of wave troughs, again consistent with an equatorward flux of heat, as well as with earlier theoretical and drift buoy findings.

Monthly period oscillations in the Pacific North Equatorial Countercurrent

“…Analysis of the upper ocean temperature balance at 7øN, 140øW reveals that meridional advection is important in the surface layer and the thermocline. For near-surface temperature variations, this result is consistent with that of others who have examined the relationship between SST and meridional currents associated with instability waves [e.g., Pullen et al, 1987;lmawaki et al, 1988]. However, from the meridional mooring array along 140 øW we have also been able to examine the temperature balance in the thermocline.…”

“…Specifically, Halpern et al [1988] identified a spectral peak in meridional velocity at periods centered near 20 days and due to instability waves, based on multiyear moored time series measurements in the eastern equatorial Pacific. Conversely, oscillations in the SST front north of the equator, as determined from satellite imagery, appear to have a slightly longer 25-day period [e.g., Legeckis, 1977;Legeckis et al, 1983;Pullen et al, 1987]. The difference between these two period estimates is on the order of 15-20%, comparable to that expected from the simple schematic model described above.…”

Wave-Mean Flow Interactions in the Equatorial Ocean

“…Philander et al 1985. These waves were first observed in satcllitc sea-surface temperature maps like that shown in Figure 1 (Legeckis 1977, Legeckis et al 1983 in which the sharp equatorial thermal front north of the equator is distorted by wave induced velocity perturbations (Pullen et al 1987). The waves are energized primarily by instability of the meridional shear in the large-scale zonal current system.…”

Distinct 17- and 33-Day Tropical Instability Waves in Subsurface Observations*

“…These meanders, as observed in early satellite SST images, were estimated to have 25-day periods and 1000-km wavelengths (Legeckis 1977). The description of TIWs has since been broadened to include additional analyses of sea surface height (SSH: Miller et al 1985;Malardé et al 1987;Musman 1989;Périgaud 1990), velocity (Halpern et al 1988;Bryden and Brady 1989;Qiao and Weisberg 1995;McPhaden 1996;Kennan and Flament 2000), wind stress (Xie et al 1998;Chelton et al 2001;Hashizume et al 2001), ocean color (Strutton et al 2001), subsurface temperature (McPhaden 1996;Flament et al 1996;Kennan and Flament 2000), and SST (Legeckis et al 1983;Legeckis 1986;Pullen et al 1987;Contreras 2002). These investigations have expanded the definition of TIWs to include variability occurring north and south of the equator with a large range of periods (15-40 days) and wavelengths (700-1600 km).…”