Annual change of sea surface slope along the Equator of the Atlantic Ocean in 1983 and 1984

Dynamics of the equatorial undercurrent and its termination

“…1.3 Observations of the equatorial ZPG (or of the longitudinal structure of the dynamic topography) After an early discussion by Arthur (1960), measurements of the ZPG have been reported, for the Pacific by Knauss (1966), Lemasson and Piton (1968), Tsuchiya (1979), Meyers (1979), Halpern (1980), Leetmaa and Spain (1981), Mangum and Hayes (1984), Bryden and Brady (1985) and Lukas (1986), for the Atlantic by Katz et al (1977), Lass et al (1983), Arnault (1984), Merle and Arnault (1985), Weisberg and Weingartner (1986), Katz et al (1986), and Hisard and H~nin (1987), for the Indian ocean by Taft and Knauss (1967) and Eriksen (1979). The case of the Indian ocean will be considered separately in the following, because of the extraordinary seasonality of the wind system there.…”

“…However, Hisard and H~nin argue that they observe a lag of the order of two months between ZPG and~(x): ZPG force at 23·W and easterly wind stress at 40 D Ware both maximum in the fall and they both weaken in the winter, but the ZPG appears to reach its minimum and to start building back up before the winds reach their own minimum and reintensify. According to Katz et al (1986), the echo-sounder signal does not reproduce such an early reintensification of the zonal slope in January 1984, but there is no echo-sounder data to compare to the estimate from hydrography in January 1983. Hisard and H'nin's interpretation of their data differs from the interpretation published in Katz et al (1986): according to Hisard and H~nin, the maximum zonal slope and hence ZPG force estimated from the 1983 hydrographic casts occured in October (based on two data points only) while, in Katz et al, it is represented as occuring in July, and being followed by a weaker value in October in agreement with the echo-sounder signal.…”

“…According to Katz et al (1986), the echo-sounder signal does not reproduce such an early reintensification of the zonal slope in January 1984, but there is no echo-sounder data to compare to the estimate from hydrography in January 1983. Hisard and H'nin's interpretation of their data differs from the interpretation published in Katz et al (1986): according to Hisard and H~nin, the maximum zonal slope and hence ZPG force estimated from the 1983 hydrographic casts occured in October (based on two data points only) while, in Katz et al, it is represented as occuring in July, and being followed by a weaker value in October in agreement with the echo-sounder signal. However, Hisard and Henin's description of their data is not unlike Weisberg and Weingartner's "overshooting" argument, and their observation of a larger zonal slope at depth than at the surface in winter 1983 and 1984 agrees with Weisberg and Weingartner's suggestion that "subsurface gradients may not be in phase with those at the surface during a period of weak and variable winds".…”

“…East of about S·W in February-March and in September, when the westerly component of the south-west monsoon is most intense, the zonal slope of the equatorial dynamic height field at various depths relative to SOO db exhibits its largest upward slope towards the coast. In June, the slope in the Gulf (Katz et al, 1986). b) Thick and thin continuous lines link the point estimates of the zonal pressure gradient +Px at respectively O-dbar and 50-dbar relative to 500 dbar deduced from the hydrographic sections (compare with a).…”

“…Katz et al (1986) describe the variations in sea surface slope from combined measurements using hydrographic stations, pressure gauges and inverted echo-sounders (Figure 3a). On a seasonal cycle similar to the one observed before, an interannual variation is superimposed: during the boreal winter of 1983-1984, an almost complete relaxation of eastward wind stress occurred in the central basin, accompanied by a levelling off of the sea surface to the west and a strong rise against the African coast; through 1984, the zonal wind stress remained lower than in 1983, with however a larger surface ZPG west of the Gulf of Guinea.…”