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Slowdown of the meridional overturning circulation in the upper Pacific Ocean

M. J. McPhaden1 and D. Zhang2

1Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington, 98115
2Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, 98115

Nature, 415(7), 603–608 (2002).
Copyright ©2002 by Macmillan Publishers Ltd. Further electronic distribution is not allowed.

Gallery of Figures and Tables

Figure 1

Figure 1. Mean circulation and potential vorticity averaged over 50 years (1950–99) in the upper pycnocline of the Pacific Ocean. a, Geostrophic streamlines relative to 900 dbar (in mto the minus 2 sto the minus 2) and b, absolute value of potential vorticity (in 10to the minus 10 mto the minus 1 sto the minus 1) on the 25.0 kg mto the minus 3 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, 50–300; intermediate, 301–1,000; 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(partialrho/partialz)/rhosub 0, where f is the Coriolis parameter, partialrho/partialz is the vertical density gradient, and rhosub 0 is a constant reference density (10to the 3 kg mto the minus 3). Water parcels conserve their potential vorticity in an ideal fluid.

Figure 2

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 mto the minus 3, and in the Southern Hemisphere from the eastern boundary to 160°E in density classes between 22.5 and 26.2 kg mto the minus 3. Transports are in units of sverdrups (1 Sv = 10to the 6 mto the 3 sto the minus 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 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 1950–99 averages.

Figure 3

Figure 3. Decadal differences in the tropical Pacific between 1990–99 and 1970–77. a, Wind stress difference (in N mto the minus 2) for 1990–99 minus 1970–77, 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 mto the minus 3 potential density surface for 1990–99 minus 1970–77.

Table 1


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