Figure 1. Mean circulation and potential vorticity averaged over 50 years (195099) in the upper pycnocline of the Pacific Ocean. a, Geostrophic streamlines relative to 900 dbar (in m s) and b, absolute value of potential vorticity (in 10 m s) on the 25.0 kg m potential density surface. Velocity vectors are overplotted on a. Distribution of hydrocasts down to 900 dbar is overplotted on b, with the size of the dots representing total number of casts in regions of 5° latitude by 15° longitude (smallest, 50300; intermediate, 3011,000; largest, 1,001 or more). Winter season outcrop lines are drawn in both panels. The outcrop lines define locations where wintertime surface mixing penetrates deepest into the pycnocline, creating new water masses that are subsequently sequestered from the atmosphere as seasonal heating restratifies the upper ocean in spring and summer. Potential vorticity is defined as f(/z)/, where f is the Coriolis parameter, /z is the vertical density gradient, and is a constant reference density (10 kg m). Water parcels conserve their potential vorticity in an ideal fluid.
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 195665, 197077, 198089 and 199099. Values are integrated in the Northern Hemisphere from the eastern boundary to 145°E in density classes between 22 and 26 kg m, and in the Southern Hemisphere from the eastern boundary to 160°E in density classes between 22.5 and 26.2 kg m. Transports are in units of sverdrups (1 Sv = 10 m s) 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°N9°S, 90°W180°W) where equatorial upwelling is most intense (Wyrtki, 1981). The temperature time series is derived from monthly analyses (Smith et al., 1994) 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 195099 averages.
Figure 3. Decadal differences in the tropical Pacific between 199099 and 197077. a, Wind stress difference (in N m) for 199099 minus 197077, based on the Florida State University wind product (Goldenberg and O'Brien, 1981). Overplotted is the difference in sea surface temperature (in °C) the same time period (Smith et al., 1994). b, Depth difference (in m) of the 25.0 kg m potential density surface for 199099 minus 197077.