Search Results
1. Introduction The upper tropical oceans comprise the core of the global Warm Water Sphere ( Wüst 1949 ). The Tropics are strongly stably stratified in potential density ( Fig. 1a ), due primarily to the dominance of solar heating. A secondary influence on the stratification is the freshwater fluxes: evaporation exceeds precipitation away from the equator toward the Subtropics, but this is reversed by heavy precipitation nearer the equator. The strongest tropical currents are zonally oriented
1. Introduction The upper tropical oceans comprise the core of the global Warm Water Sphere ( Wüst 1949 ). The Tropics are strongly stably stratified in potential density ( Fig. 1a ), due primarily to the dominance of solar heating. A secondary influence on the stratification is the freshwater fluxes: evaporation exceeds precipitation away from the equator toward the Subtropics, but this is reversed by heavy precipitation nearer the equator. The strongest tropical currents are zonally oriented
1. Introduction Tropical instability waves (TIWs), a result of instability of the equatorial current systems of the Atlantic and Pacific Oceans, typically reach large amplitude in the Pacific during the boreal fall and winter months. The waves extract energy from the large-scale, wind-driven currents and have been extensively studied. TIW variability is one of the most prominent features of the surface dynamic topography of the tropics and the global ocean, with peak amplitudes in sea surface
1. Introduction Tropical instability waves (TIWs), a result of instability of the equatorial current systems of the Atlantic and Pacific Oceans, typically reach large amplitude in the Pacific during the boreal fall and winter months. The waves extract energy from the large-scale, wind-driven currents and have been extensively studied. TIW variability is one of the most prominent features of the surface dynamic topography of the tropics and the global ocean, with peak amplitudes in sea surface
advective anomalies within zonal jets do not show up for salinity but that staircases are present for the Tsuchiya jets and the LLICs ( Figs. 5 , 9b , and 11 ). Local homogenization processes, which gradually lead to the transformation of the off-equatorial water masses to the equatorial water mass, might then play an important role in the erosion of intermediate water masses in the tropics. Fig . 12. Background tracer fields of (a),(c) oxygen and (b),(d) salinity from CSIRO Atlas of Regional Seas
advective anomalies within zonal jets do not show up for salinity but that staircases are present for the Tsuchiya jets and the LLICs ( Figs. 5 , 9b , and 11 ). Local homogenization processes, which gradually lead to the transformation of the off-equatorial water masses to the equatorial water mass, might then play an important role in the erosion of intermediate water masses in the tropics. Fig . 12. Background tracer fields of (a),(c) oxygen and (b),(d) salinity from CSIRO Atlas of Regional Seas
Abstract
The shallow subtropical–tropical cells (STC) of the Atlantic Ocean have been studied from the output fields of a 50-yr run of the German partner of the Estimating the Circulation and Climate of the Ocean (GECCO) consortium assimilation model. Comparison of GECCO with time-mean observational estimates of density and meridional currents at 10°S and 10°N, which represent the boundaries between the tropics and subtropics in GECCO, shows good agreement in transports of major currents. The variability of the GECCO wind stress in the interior at 10°S and 10°N remains consistent with the NCEP forcing, although temporary changes can be large. On pentadal and longer time scales, an STC loop response is found between the poleward Ekman divergence and STC-layer convergence at 10°S and 10°N via the Equatorial Undercurrent (EUC) at 23°W, where the divergence leads the EUC and the convergence, suggesting a “pulling” mechanism via equatorial upwelling. The divergence is also associated with changes in the eastern equatorial upper-ocean heat content. Within the STC layer, partial compensation of the western boundary current (WBC) and the interior occurs at 10°S and 10°N. For the meridional overturning circulation (MOC) at 10°S it is found that more than one-half of the variability in the upper limb can be explained by the WBC. The explained MOC variance can be increased to 85% by including the geostrophic (Sverdrup) part of the wind-driven transports.
Abstract
The shallow subtropical–tropical cells (STC) of the Atlantic Ocean have been studied from the output fields of a 50-yr run of the German partner of the Estimating the Circulation and Climate of the Ocean (GECCO) consortium assimilation model. Comparison of GECCO with time-mean observational estimates of density and meridional currents at 10°S and 10°N, which represent the boundaries between the tropics and subtropics in GECCO, shows good agreement in transports of major currents. The variability of the GECCO wind stress in the interior at 10°S and 10°N remains consistent with the NCEP forcing, although temporary changes can be large. On pentadal and longer time scales, an STC loop response is found between the poleward Ekman divergence and STC-layer convergence at 10°S and 10°N via the Equatorial Undercurrent (EUC) at 23°W, where the divergence leads the EUC and the convergence, suggesting a “pulling” mechanism via equatorial upwelling. The divergence is also associated with changes in the eastern equatorial upper-ocean heat content. Within the STC layer, partial compensation of the western boundary current (WBC) and the interior occurs at 10°S and 10°N. For the meridional overturning circulation (MOC) at 10°S it is found that more than one-half of the variability in the upper limb can be explained by the WBC. The explained MOC variance can be increased to 85% by including the geostrophic (Sverdrup) part of the wind-driven transports.
), and a similar T – S curve for AAIW is observed here. Lower oxygen intermediate waters that have mixed in the tropics, such as the North Pacific Tropical Intermediate Water (NPTIW), are found east of 130°E ( Bingham and Lukas 1995 ). The poleward transport of AAIW has been used to verify some connectivity between the MUC and southern North Equatorial Undercurrent ( Schönau et al. 2015 ; Wang et al. 2015 ). Fig . 4. Mean structure of the MC/MUC from an objective map of glider profiles within 80
), and a similar T – S curve for AAIW is observed here. Lower oxygen intermediate waters that have mixed in the tropics, such as the North Pacific Tropical Intermediate Water (NPTIW), are found east of 130°E ( Bingham and Lukas 1995 ). The poleward transport of AAIW has been used to verify some connectivity between the MUC and southern North Equatorial Undercurrent ( Schönau et al. 2015 ; Wang et al. 2015 ). Fig . 4. Mean structure of the MC/MUC from an objective map of glider profiles within 80
although clearly the atmospheric and ocean conditions are different. The ETNP has the lowest SSS in the tropics corresponding to the eastern Pacific fresh pool, which extends westward north of the equator from the west of Costa Rica ( Fiedler and Talley 2006 ). The eastern Pacific fresh pool is due to heavy precipitation associated with the seasonal march of the intertropical convergence zone (ITCZ; Alory et al. 2012 ) and the intra-American monsoon system ( Amador et al. 2006 ). In the ETNP, BLs are
although clearly the atmospheric and ocean conditions are different. The ETNP has the lowest SSS in the tropics corresponding to the eastern Pacific fresh pool, which extends westward north of the equator from the west of Costa Rica ( Fiedler and Talley 2006 ). The eastern Pacific fresh pool is due to heavy precipitation associated with the seasonal march of the intertropical convergence zone (ITCZ; Alory et al. 2012 ) and the intra-American monsoon system ( Amador et al. 2006 ). In the ETNP, BLs are
., CLIVAR Publication Series 133, 42 pp . Gouriou , Y. , and J. Toole , 1993 : Mean circulation of the upper layers of the western equatorial Pacific Ocean . J. Geophys. Res. , 98 ( C12 ), 22 495 – 22 520 . Gu , D. F. , and S. G. H. Philander , 1997 : Interdecadal climate fluctuations that depend on exchanges between the tropics and extratropics . Science , 275 , 805 – 807 . Guilderson , T. P. , M. Kashgarian , and J. Southon , 1998 : Radiocarbon variability in the western
., CLIVAR Publication Series 133, 42 pp . Gouriou , Y. , and J. Toole , 1993 : Mean circulation of the upper layers of the western equatorial Pacific Ocean . J. Geophys. Res. , 98 ( C12 ), 22 495 – 22 520 . Gu , D. F. , and S. G. H. Philander , 1997 : Interdecadal climate fluctuations that depend on exchanges between the tropics and extratropics . Science , 275 , 805 – 807 . Guilderson , T. P. , M. Kashgarian , and J. Southon , 1998 : Radiocarbon variability in the western
, 99 pp . Long , B. , and P. Chang , 1990 : Propagation of an equatorial Kelvin wave in a varying thermocline. J. Phys. Oceanogr. , 20 , 1826 – 1841 . Madden , R. A. , and P. R. Julian , 1972 : Description of global-scale circulation cells in the tropics with a 40–50-day period. J. Atmos. Sci. , 29 , 1109 – 1123 . Madden , R. A. , and P. R. Julian , 1994 : Observations of the 40–50-day tropical oscillation: A review. Mon. Wea. Rev. , 122 , 814 – 837 . Matsuno , T
, 99 pp . Long , B. , and P. Chang , 1990 : Propagation of an equatorial Kelvin wave in a varying thermocline. J. Phys. Oceanogr. , 20 , 1826 – 1841 . Madden , R. A. , and P. R. Julian , 1972 : Description of global-scale circulation cells in the tropics with a 40–50-day period. J. Atmos. Sci. , 29 , 1109 – 1123 . Madden , R. A. , and P. R. Julian , 1994 : Observations of the 40–50-day tropical oscillation: A review. Mon. Wea. Rev. , 122 , 814 – 837 . Matsuno , T
, not downwelling, because the stratification increases with height off the bottom, resulting in a convergent buoyancy flux. Integrated over the tropics and zonally over a typical wavelength of an ETW, we estimate that the mixing could yield of order 10 Sv of diapycnal transport in the few hundred meters above the seafloor and hence could contribute significantly to the diapycnal upwelling of the abyssal equatorial circulation inferred by inverse models. Our study is, however, potentially limited by
, not downwelling, because the stratification increases with height off the bottom, resulting in a convergent buoyancy flux. Integrated over the tropics and zonally over a typical wavelength of an ETW, we estimate that the mixing could yield of order 10 Sv of diapycnal transport in the few hundred meters above the seafloor and hence could contribute significantly to the diapycnal upwelling of the abyssal equatorial circulation inferred by inverse models. Our study is, however, potentially limited by
tropics . J. Geophys. Res. , 97 , 7305 – 7316 , https://doi.org/10.1029/92JC00407 . 10.1029/92JC00407 Thadathil , P. , and Coauthors , 2008 : Seasonal variability of the observed barrier layer in the Arabian Sea . J. Phys. Oceanogr. , 38 , 624 – 638 , https://doi.org/10.1175/2007JPO3798.1 . 10.1175/2007JPO3798.1 Veronis , G. , 1972 : On properties of seawater defined by temperature, salinity, and pressure . J. Mar. Res. , 30 , 227 – 255 . Vialard , J. , and P. Delecluse , 1998
tropics . J. Geophys. Res. , 97 , 7305 – 7316 , https://doi.org/10.1029/92JC00407 . 10.1029/92JC00407 Thadathil , P. , and Coauthors , 2008 : Seasonal variability of the observed barrier layer in the Arabian Sea . J. Phys. Oceanogr. , 38 , 624 – 638 , https://doi.org/10.1175/2007JPO3798.1 . 10.1175/2007JPO3798.1 Veronis , G. , 1972 : On properties of seawater defined by temperature, salinity, and pressure . J. Mar. Res. , 30 , 227 – 255 . Vialard , J. , and P. Delecluse , 1998
