FIGURE 2. Meridional transports in the pycnocline and smoothed sea surface temperatures over the past 50 years.

a, Mean zonally integrated meridional transports in the pycnocline relative to 900 dbar along 9° N and 9° S, computed for 1956–65, 1970–77, 1980–89 and 1990–99. Values are integrated in the Northern Hemisphere from the eastern boundary to 145° E in density classes between 22 and 26 kg m^-3, and in the Southern Hemisphere from the eastern boundary to 160° E in density classes between 22.5 and 26.2 kg m^-3. Transports are in units of sverdrups (1 Sv = 10⁶ m³ s^-1) which is the volumetric equivalent of mass for a constant reference density. Error bars are for one standard error. b, Mean meridional transport convergence (in Sv) in the pycnocline across 9° N and 9° S. Convergence is calculated as the difference between Southern Hemisphere minus Northern Hemisphere transports in a. Also plotted in b are areally averaged sea surface temperature anomalies in the eastern and central equatorial Pacific (9° N–9° S, 90° W–180° W) where equatorial upwelling is most intense³¹. The temperature time series is derived from monthly analyses⁵⁰ smoothed twice with a 5-year running mean to filter out the seasonal cycle and year-to-year oscillations associated with ENSO. Anomalies are relative to 1950–99 averages.