Stanley P. Hayes

On the variability of winds, sea surface temperature, and surface layer heat content in the western equatorial Pacific

In this study we examine the surface layer heat balance using wind, current, and temperature data from equatorial moorings along 165°E. The analysis focuses primarily on daily to monthly time scale variations during the 1986–1987 El Nino/Southern Oscillation event. The period is one of high mean sea surface temperatures (≳29°C) and frequent outbreaks of westerly winds. We infer that evaporative cooling related to wind speed variations accounts for a significant fraction of the observed sea surface temperature (SST) and upper ocean heat content variability. This evaporative heat flux converges nonlinearly in the surface layer, giving rise to larger temperature variations in the upper 10 m than below. Other processes examined (wind-forced vertical advection and entrainment, lateral advection) were negligible or of secondary importance relative to evaporative cooling. A large fraction of the SST and surface layer heat content variance could not be directly related to wind fluctuations; this unexplained variance is probably related to shortwave radiative fluxes at the air-sea interface.

Oceanographic Observations of the 1982 Warming of the Tropical Eastern Pacific

Moored current meter, sea level, hydrographic, and surface drifter measurements show the large changes that took place in the eastern tropical Pacific during the onset of the warm episode of 1982. In August the near-surface flow at 0�, 110�W reversed direction to eastward. By October the sea surface temperature in the equatorial zone increased by 5 degrees Celsius above the long-term monthly mean value, sea level rose by 22 centimeters at the Gal�pagos Islands, and the thermocline was displaced downward by 50 to 70 meters along the equator and the South American coast.

A Comparison of Tropical Pacific Surface Wind Analyses

Abstract Surface wind analyses from three data assimilation systems are compared with independent wind observations from six buoys located in the Pacific within 8 deg of the equator. The period of comparison is 6 months (February to July 1987), with daily sampling. The agreement between the assimilation systems and the independent buoy data is disappointing. The longterm mean differences between the buoy and the assimilated zonal and meridional winds are as large as 3.1 m s−1, which is comparable to the size of the means themselves. The zonal and meridional daily wind correlations range between 0.66 and 0.17. The wind field agreement was actually better among the different systems than between any system and the buoys. However, the agreement among the analysis products was usually better for the zonal winds than for the meridional winds. For the time period and locations presented, the comparisons with the independent data show that no assimilation system is clearly superior to any of the others.

Flow of Abyssal Water into the Samoa Passage

On the TEW Expedition a hydrographic section was made in June 1987 across the Samoa Passage; in addition, south of the Passage, a section was made through the Penrhyn and Samoa Basins and the Tonga Trench. In this paper the flow of the deep water with North Atlantic Deep Water and Antarctic characteristics below 3500 m is discussed. Temperature-salinity-oxygen relationships show a similarity of properties between the Tonga Trench and western boundary of the Samoa Basin. The salinity and silicate data show distinct signatures of North Atlantic Deep Water and Antarctic Bottom Water flowing into the Samoa Passage. Salinity and silicate data in the Samoa Basin and Passage are similar to those measured on the STYX Expedition (1968). Oxygen data show that the water along the Samoa Basins western boundary is oxygen-rich relative to water in the eastern Basin, but with no oxygen maximum in deep water, as was observed on STYX. Geostrophic flow, based on a reference surface determined from T-S. O2 curves, shows strong northward components (> 10 cm s−1) in the Samoa Passage. On the west side of the Passage the flow was southward (<1 cm s−1) below 4600 m. This flow reversal was associated with a reversal of the sign of the horizontal potential temperature gradient and also with lower values of oxygen, salinity and silicate next to the boundary. The northward transport of Lower Circumpolar Water between the Manihiki Plateau and the Tonga Trench was 12.3 ± 3.6 × 106 m3 s−1; the flow into the Samoa Passage was 6.0 ± 1.1 × 106 m3 s−1. Assuming that only flow above 4800 m can exit the northern Samoa Basin because of topographic restrictions, the total northward transport into the north Tokelau Basin was 9.6 ± 1.8 × 106 m3 s−1.

Upper ocean variability on the equator in the Pacific at 170°W

The monitoring of upper ocean velocity, temperature, and surface winds at 0°, 170°W was initiated in May 1988 as part of the Tropical Ocean-Global Atmosphere program. Located between regions of warm (cold) sea surface temperature in the western (eastern) equatorial Pacific, this west central region exhibits large interannual variations in surface wind stress associated with the El Nino-Southern Oscillation. Here we report on the first 3 years of data collected at 0°, 170°W, exclusive of an El Nino event. The west central Pacific is a region through which the Equatorial Undercurrent (EUC) accelerates downstream. The vertical position of the EUC core, the vertical penetration of the South Equatorial Current (SEC), and the vertically integrated zonal volume transport all show large annual cycles. Annual and higher-frequency transport variations are due primarily to fluctuations occurring above the thermocline, as opposed to within the EUC itself. Unlike the EUC position, the EUC core speed remains relatively steady annually, except during boreal fall and winter, when short duration, zonal momentum pulses generated as Kelvin waves to the west reduce the EUC to minimal values. Interannually, the EUC core speed was highest in 1988, and it has decreased, on average, since then. Also during 1988, tropical instability waves, generally observed farther east, were well developed at 170°W, while not in subsequent years. This suggests the same role interannually as annually for these waves: to smooth out heat and momentum gradients that form in response to the wind stress. (The winds in 1988, following the 1986–1987 El Nino, were stronger than in subsequent years.) At higher frequencies, evidence exists for an inertial-gravity wave mode previously identified in nearby Canton Island sea level records.