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Gregory C. Johnson

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Gregory C. Johnson

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Generation and evolution of an isopycnal potential temperature–salinity (θS), or spiciness, anomaly is studied around 20°–23°S, 110°W in the austral winter of 2004. Two profiling CTD floats deployed in the region in January 2004 provide the observations. The anomaly (defined as relative to water properties of the preceding summer) is very large (initially about 0.35 in S and about 0.9°C in θ). It is associated with the winter ventilation of a thick, low-potential-vorticity layer known as South Pacific Eastern Subtropical Mode Water. Regional lateral θ and S distributions at the surface predispose the ocean to formation of this water mass and allow significant anomalies to be generated there with relative ease. The water mass is potentially important for climate in that, after northwestward advection in the South Equatorial Current, it contributes to the Equatorial Undercurrent and eventually resurfaces in the cold tongue of the eastern equatorial Pacific Ocean. The anomaly studied is strong enough to predispose a portion of the water column to salt fingering, increasing vertical mixing. Although lateral processes are no doubt important in evolution of the anomaly, the vertical mixing appears to be sufficiently vigorous to reduce it significantly within 6 months after its formation by spreading it to denser horizons through diapycnal fluxes. By that time the anomaly is most likely sufficiently diffuse so that subsequent evolution from diapycnal fluxes is significantly reduced as it makes its way toward the equator.

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Gregory C. Johnson
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Gregory C. Johnson

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The southern tropical Indian Ocean contains a striking forced annual Rossby wave studied previously using satellite altimeter sea surface height data, surface wind fields, expendable bathythermograph ocean temperature data, and models. Here, the deep reach of this wave and its velocity are analyzed using density–depth profiles and 1000-dbar horizontal drift data from Argo. Significant annual cycles in isopycnal vertical displacements and zonal velocity persist to the deepest pressures to which Argo data can be mapped reliably in the region, 1600–1900 dbar. Phase propagation of the annual cycle of the directly measured zonal velocities at 1000 dbar suggests a zonal wavelength of about 6000 km—about the length of the deep basin in which the wave is found—and a westward phase speed of ~0.2 m s−1. Apparent upward phase propagation in isopycnal vertical displacements suggests energy propagation downward into the abyss. This pattern is clearer when accounting for both the potential and kinetic energy of the wave. The largest zonal current associated with this wave has a middepth maximum that decays rapidly up through the pycnocline and less rapidly with increasing depth, suggesting a first-vertical-mode structure. The anomalous zonal volume transport of this annually reversing current is ~27 × 106 m3 s−1 across 80°E in mid-November. The peak zonal velocity of 0.06 m s−1 implies a maximum zonal excursion of about 600 km associated with the wave over an annual cycle.

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Gregory C. Johnson and Dongxiao Zhang

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The equatorial deep jets in the Atlantic Ocean are described using vertical strain, ξ z, estimated from all available deep CTD stations in the region. Wavelet analysis reveals a distinct energy peak around 661-sdbar vertical wavelength, 1232-dbar pressure, and ±1.5° latitude from the equator. This high-vertical-wavenumber and off-equatorial maximum, coupled with previously published velocity data that show nodes in zonal velocity near ±1.5°, is grossly consistent with the structure of first-meridional-mode equatorial Rossby waves. However, the meridional scale obtained from the observations exceeds, by about 1.5, the theoretical meridional scale for these waves. The jets are strong, with zonal velocities similar in magnitude to the Kelvin wave phase speed for their vertical wavelength. Harmonics of ξ z at vertical wavelengths of 1/2, 1/4, and perhaps 1/8 that of the primary peak provide evidence of a large-amplitude structure. Although sparse, available phase data at the 661-sdbar vertical wavelength suggest downward and westward phase propagation. Assuming sinusoidal character in time and longitude gives estimates of a 5- (±1) yr period and a 70° (±60°) zonal wavelength. These vertical, temporal, and zonal scales are roughly consistent with first-meridional-mode equatorial Rossby wave dynamics. However, although vertical and zonal phase propagation are discernible, there is no obvious signature of upward energy propagation in the variance vertical maxima, which is problematic for a simple linear Rossby wave interpretation.

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Donata Giglio and Gregory C. Johnson

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Argo profiling floats initiated a revolution in observational physical oceanography by providing numerous, high-quality, global, year-round, in situ (0–2000 dbar) temperature and salinity observations. This study uses Argo’s unprecedented sampling of the Southern Ocean during 2006–13 to describe the position of the Antarctic Circumpolar Current’s Subantarctic and Polar Fronts, comparing and contrasting two different methods for locating fronts using the same dataset. The first method locates three fronts along dynamic height contours, each corresponding to a local maximum in vertically integrated shear. The second approach locates the fronts using specific features in the potential temperature field, following Orsi et al. Results from the analysis of Argo data are compared to those from Orsi et al. and other more recent studies. Argo spatial resolution is not adequate to resolve annual and interannual movements of the fronts on a circumpolar scale since they are on the order of 1° latitude (Kim and Orsi), which is smaller than the resolution of the gridded product analyzed. Argo’s four-dimensional coverage of the Southern Ocean equatorward of ~60°S is used to quantify variations in heat and freshwater content there with respect to the time-mean front locations. These variations are described during 2006–13, considering both pressure and potential density ranges (within different water masses) and relations to wind forcing (Ekman upwelling and downwelling).

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Sunke Schmidtko and Gregory C. Johnson

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Antarctic Intermediate Water (AAIW) is a dominant Southern Hemisphere water mass that spreads from its formation regions just north of the Antarctic Circumpolar Current (ACC) to at least 20°S in all oceans. This study uses an isopycnal climatology constructed from Argo conductivity–temperature–depth (CTD) profile data to define the current state of the AAIW salinity minimum (its core) and thence compute anomalies of AAIW core pressure, potential temperature, salinity, and potential density since the mid-1970s from ship-based CTD profiles. The results are used to calculate maps of temporal property trends at the AAIW core, where statistically significant strong circumpolar shoaling (30–50 dbar decade−1), warming (0.05°–0.15°C decade−1), and density reductions [up to −0.03 (kg m−3) decade−1] are found. These trends are strongest just north of the ACC in the southeast Pacific and Atlantic Oceans and decrease equatorward. Salinity trends are generally small, with their sign varying regionally. Bottle data are used to extend the AAIW core potential temperature anomaly analysis back to 1925 in the Atlantic and to ~1960 elsewhere. The modern warm AAIW core conditions appear largely unprecedented in the historical record: biennially and zonally binned median AAIW core potential temperatures within each ocean basin are, with the notable exception of the subtropical South Atlantic in the 1950s–70s, 0.2–1°C colder than modern values. Zonally averaged sea surface temperature anomalies around the AAIW formation latitudes in each ocean and sectoral southern annular mode indices are used to put the AAIW core property trends and variations into context.

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Sarah G. Purkey and Gregory C. Johnson

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Freshening and warming of Antarctic Bottom Water (AABW) between the 1980s and 2000s are quantified, assessing the relative contributions of water-mass changes and isotherm heave. The analysis uses highly accurate, full-depth, ship-based, conductivity–temperature–depth measurements taken along repeated oceanographic sections around the Southern Ocean. Fresher varieties of AABW are present within the South Pacific and south Indian Oceans in the 2000s compared to the 1990s, with the strongest freshening in the newest waters adjacent to the Antarctic continental slope and rise indicating a recent shift in the salinity of AABW produced in this region. Bottom waters in the Weddell Sea exhibit significantly less water-mass freshening than those in the other two southern basins. However, a decrease in the volume of the coldest, deepest waters is observed throughout the entire Southern Ocean. This isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential temperature maximum. The water-mass freshening of AABW in the Indian and Pacific Ocean sectors is equivalent to a freshwater flux of 73 ± 26 Gt yr−1, roughly half of the estimated recent mass loss of the West Antarctic Ice Sheet. Isotherm heave integrated below 2000 m and south of 30°S equates to a net heat uptake of 34 ± 14 TW of excess energy entering the deep ocean from deep volume loss of AABW and 0.37 ± 0.15 mm yr−1 of sea level rise from associated thermal expansion.

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Annie P. S. Wong and Gregory C. Johnson

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The structure, formation, and destruction of South Pacific Eastern Subtropical Mode Water (SPESTMW) are analyzed. Geographic extent and water properties are discussed by using high-quality CTD sections collected between 1991 and 1996. Defined as having a planetary potential vorticity magnitude of less than 3 × 10−10 m−1 s−1, SPESMTW has a volume of about 1.1 × 1015 m3, estimated from CTD data. The ventilation of this mode water is described by using data from a high-resolution XBT section in concert with 30-month time series from profiling CTD floats, some of the first Argo deployments. Published subduction rates allow a mode-water formation rate estimate of 8.7 × 106 m3 s−1. Combining this estimate with the volume yields a residence time of about 4 years. The density-compensating covarying patterns of late winter surface temperature and salinity in the ventilation region of SPESTMW are shown to contribute to the strength of the mode water. However, while the destabilizing salinity gradient in SPESTMW contributes to its formation, it may also hasten its destruction by leaving it susceptible to double-diffusive convective mixing. SPESTMW spreads northwestward from its ventilation region within the subtropical gyre, eventually joining the South Equatorial Current. It is speculated that the proximity of the SPESTMW ventilation region to the Tropics, where winds and sea surface temperatures vary significantly, coupled with a direct interior circulation pathway to the equator, may allow SPESTMW to effect modulation of ENSO dynamics.

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Gregory C. Johnson and Alejandro H. Orsi

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Water-mass changes are estimated in the southwest Pacific Ocean by comparing a meridional hydrographic section along 170°W between 60°S and the equator occupied in 1968/69 during the Southern Cross cruise and again in 1990 during a NOAA Climate and Global Change cruise. Another comparison is made using a hydrographic section along 35°S between the date line and 169°W occupied in 1969 during USNS Eltanin cruise 40 and again in 1991 during a Mapkiwi cruise. The most robust change consists of cooling (and freshening) on isopycnals, with peak differences exceeding −1.0°C (−0.25 pss) at the base of the subtropical thermocline. The cooling and freshening starts above the stratification minimum of the Subantarctic Mode Water and persists to below the salinity minimum of the Antarctic Intermediate Water. Amplitudes are largest at 48°S, near where these water masses subduct, and decay toward 20°S, near the axis of the Subtropical Gyre. This change is likely the result of surface warming and/or freshening at high latitudes, where these water masses are formed before they ventilate the base of the subtropical thermocline. Isopycnals tend to deepen south of 35°S and to shoal more weakly from 35° to 20°S. These changes are consistent with a simple model response to high-latitude warming. Results from the section comparisons are put in a larger context by estimating interdecadal changes on isopycnals throughout the South Pacific Ocean. In addition, two changes consistent with a strengthened southern influence are found within the Lower Circumpolar Water from the Chatham Rise at 43°S to the Samoa Passage at 10°S. Cooling and freshening erode the top of the deep salinity maximum of the modified North Atlantic Deep Water. In the weakly stratified abyssal layer, the modified Antarctic Bottom water cools by about 0.025°C.

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