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- Author or Editor: John Gilson x
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Abstract
High-resolution XBT transects in the North Pacific Ocean, at an average latitude of 22°N, are analyzed together with TOPEX/Poseidon altimetric data to determine the structure and transport characteristics of the mesoscale eddy field. Based on anomalies in dynamic height, 410 eddies are identified in 30 transects from 1991 to 1999, including eddies seen in multiple transects over a year or longer. Their wavelength is typically 500 km, with peak-to-trough temperature difference of 2.2°C in the center of the thermocline. The features slant westward with decreasing depth, by 0.8° of longitude on average from 400 m up to the sea surface. This tilt produces a depth-varying velocity/temperature correlation and hence a vertical meridional overturning circulation. In the mean, 3.9 Sv (Sv ≡ 106 m3 s−1) of thermocline waters are carried southward by the eddy field over the width of the basin, balanced mainly by northward flow in the surface layer. Corresponding northward heat transport is 0.086 ± 0.012 pW. The eddy field has considerable variability on seasonal to interannual timescales. For the 8-yr period studied here, eddy variability was the dominant mechanism for interannual change in the equatorward transport of thermocline waters, suggesting a potentially important forcing mechanism in the coupled air–sea climate system.
Abstract
High-resolution XBT transects in the North Pacific Ocean, at an average latitude of 22°N, are analyzed together with TOPEX/Poseidon altimetric data to determine the structure and transport characteristics of the mesoscale eddy field. Based on anomalies in dynamic height, 410 eddies are identified in 30 transects from 1991 to 1999, including eddies seen in multiple transects over a year or longer. Their wavelength is typically 500 km, with peak-to-trough temperature difference of 2.2°C in the center of the thermocline. The features slant westward with decreasing depth, by 0.8° of longitude on average from 400 m up to the sea surface. This tilt produces a depth-varying velocity/temperature correlation and hence a vertical meridional overturning circulation. In the mean, 3.9 Sv (Sv ≡ 106 m3 s−1) of thermocline waters are carried southward by the eddy field over the width of the basin, balanced mainly by northward flow in the surface layer. Corresponding northward heat transport is 0.086 ± 0.012 pW. The eddy field has considerable variability on seasonal to interannual timescales. For the 8-yr period studied here, eddy variability was the dominant mechanism for interannual change in the equatorward transport of thermocline waters, suggesting a potentially important forcing mechanism in the coupled air–sea climate system.
Abstract
Multidecadal trends in ocean heat and freshwater content are well documented, but much less evidence exists of long-term changes in ocean circulation. Previously, a 12-yr increase, 1993 to 2004, in the circulation of the South Pacific Subtropical Gyre interior was described. That analysis was based on differences between early Argo and 1990s hydrographic data and changes in sea surface height. Here, it is shown that the trend of increasing circulation continues through 2014, with some differences within the Argo decade (2005 to 2014). Patterns that indicate or are consistent with increasing equatorward transport in the eastern portion of the South Pacific Gyre are seen in Argo temperature and steric height, Argo trajectory velocity, altimetric sea surface height, sea surface temperature, sea level pressure, and wind stress. Between 2005 and 2014 the geostrophic circulation across 35°S, from 160°W to South America, was enhanced by 5 Sv (1 Sv ≡ 106 m3 s−1) of added northward flow. This was countered by a southward transport anomaly between the date line and 160°W. Corresponding temperature trends span the full 2000-m depth range of Argo observations. The 22-yr trend, 1993 to 2014, in sea surface height at 35°S, 160°W is 8 cm decade−1. Trends in sea surface temperature over 34 yr, 1981 to 2014, show a similar spatial pattern to that of sea surface height, with an increase of 0.5°C decade−1 at 35°S, 160°W. These multidecadal trends support the interpretation of the 40°S maximum in global ocean heat gain as resulting from anomalous wind forcing and Ekman convergence.
Abstract
Multidecadal trends in ocean heat and freshwater content are well documented, but much less evidence exists of long-term changes in ocean circulation. Previously, a 12-yr increase, 1993 to 2004, in the circulation of the South Pacific Subtropical Gyre interior was described. That analysis was based on differences between early Argo and 1990s hydrographic data and changes in sea surface height. Here, it is shown that the trend of increasing circulation continues through 2014, with some differences within the Argo decade (2005 to 2014). Patterns that indicate or are consistent with increasing equatorward transport in the eastern portion of the South Pacific Gyre are seen in Argo temperature and steric height, Argo trajectory velocity, altimetric sea surface height, sea surface temperature, sea level pressure, and wind stress. Between 2005 and 2014 the geostrophic circulation across 35°S, from 160°W to South America, was enhanced by 5 Sv (1 Sv ≡ 106 m3 s−1) of added northward flow. This was countered by a southward transport anomaly between the date line and 160°W. Corresponding temperature trends span the full 2000-m depth range of Argo observations. The 22-yr trend, 1993 to 2014, in sea surface height at 35°S, 160°W is 8 cm decade−1. Trends in sea surface temperature over 34 yr, 1981 to 2014, show a similar spatial pattern to that of sea surface height, with an increase of 0.5°C decade−1 at 35°S, 160°W. These multidecadal trends support the interpretation of the 40°S maximum in global ocean heat gain as resulting from anomalous wind forcing and Ekman convergence.
