Gregory C. Johnson

Expanding Oxygen-Minimum Zones in the Tropical Oceans

Oxygen-poor waters occupy large volumes of the intermediate-depth eastern tropical oceans. Oxygen-poor conditions have far-reaching impacts on ecosystems because important mobile macroorganisms avoid or cannot survive in hypoxic zones. Climate models predict declines in oceanic dissolved oxygen produced by global warming. We constructed 50-year time series of dissolved-oxygen concentration for select tropical oceanic regions by augmenting a historical database with recent measurements. These time series reveal vertical expansion of the intermediate-depth low-oxygen zones in the eastern tropical Atlantic and the equatorial Pacific during the past 50 years. The oxygen decrease in the 300- to 700-m layer is 0.09 to 0.34 micromoles per kilogram per year. Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies.

Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets*

AbyssalglobalanddeepSouthern Oceantemperature trendsarequantifiedbetweenthe1990sand2000sto assesstheroleofrecentwarmingoftheseregionsin globalheatandsealevelbudgets.Theauthors1)compute warming rates with uncertainties along 28 full-depth, high-quality hydrographic sections that have been occupied two or more times between 1980 and 2010; 2) divide the global ocean into 32 basins, defined by the topography and climatological ocean bottom temperatures; and then 3) estimate temperature trends in the 24 sampled basins. The three southernmost basins show a strong statistically significant abyssal warming trend, with that warming signal weakening to the north in the central Pacific, western Atlantic, and eastern Indian Oceans. Eastern Atlantic and western Indian Ocean basins show statistically insignificant abyssal cooling trends. Excepting the Arctic Ocean and Nordic seas, the rate of abyssal (below 4000 m) global ocean heat content change in the 1990s and 2000s is equivalent to a heat flux of 0.027 (60.009) W m 22 applied over the entire surface of the earth. Deep (1000‐4000 m) warming south of the Subantarctic Front of the Antarctic Circumpolar Current adds 0.068 (60.062) W m 22 . The abyssal warming produces a 0.053 (60.017) mm yr 21 increase in global average sea level and the deep warming south of the Subantarctic Front adds another 0.093 (60.081)mm yr 21 . Thus, warmingin theseregions,ventilatedprimarilyby AntarcticBottomWater,accounts for a statistically significant fraction of the present global energy and sea level budgets.

Direct measurements of upper ocean currents and water properties across the tropical Pacific during the 1990s

Abstract Meridional sections of upper ocean zonal currents, potential temperature, and salinity are estimated at ten longitudes from 143°E to 95°W using Conductivity–Temperature–Depth and Acoustic Doppler Current Profiler data from 172 synoptic sections taken in the tropical Pacific between 138°E and 86°W, mostly in the 1990s. Data reduction is carried out in a potential isopycnal and mixed layer framework to preserve a sharp pycnocline, a mixed layer, water property extrema, and velocity extrema. Mean zonal currents, potential temperatures, and salinities are produced at each longitude. The seasonal cycles of these fields are also estimated, as well as a simple El Nino Southern Oscillation (ENSO) cycle. Zonal sections along the equator are also presented. Properties of the near-equatorial zonal currents, including transports, temperatures, and salinities, are estimated separately from the synoptic sections. The seasonal cycles of these quantities and their correlations with the Southern Oscillation Index are investigated. The work is distinguished from most existing literature in that direct estimates of zonal velocity are combined with contemporaneous temperature and salinity data, allowing trans-Pacific estimates of near-equatorial current transports and properties, including those of the northern branch of the South Equatorial Current, the New Guinea Coastal Undercurrent, and the Equatorial Undercurrent.

Interior Pycnocline Flow from the Subtropical to the Equatorial Pacific Ocean

Abstract Interior circulation pathways from the subtropics to the equator are markedly different in the Northern and Southern Hemispheres of the Pacific Ocean. In the North Pacific the pycnocline shoals and strengthens dramatically under the intertropical convergence zone, separating the North Equatorial Current from the North Equatorial Countercurrent. While the high potential vorticity between these currents would intuitively seem to inhibit meridional water-property exchange between the subtopics and the equator, transient tracer analyses and some modeling studies have suggested an interior pathway from the subtropics to the equator in the pycnocline of the central North Pacific. This study delineates this pathway and estimates an upper bound for its magnitude at 5 (±1) × 109 kg s−1. In contrast, the southern branch of the South Equatorial Current clearly brings pycnocline water estimated at 15 (±1) × 109 kg s−1 from the southern subtropics directly to the equator in the South Pacific through an interi...

Ocean currents evident in satellite wind data

Satellite-mounted radar scatterometers designed to quantify surface winds over the ocean actually measure the relative motion between the air and the ocean surface. Estimates of the wind stress from conventional surface wind measurements are usually derived neglecting ocean currents. However, when the relative motion is used, the differences in the estimated stress can be as large as 50% near the equator and may even reverse sign during an El Nino. This assertion is supported by the strong relationship between the surface currents measured by the Tropical Atmosphere-Ocean (TAO) array in the Pacific Ocean and the differences between the winds estimated from scatterometer data and those measured by TAO anemometers. The fact that the scatterometer measures relative motion, and not wind alone, makes scatterometer-derived stress a more accurate representation of the boundary condition needed for both atmospheric and oceanic models than stress fields derived neglecting ocean currents.

Delayed-Mode Calibration of Autonomous CTD Profiling Float Salinity Data by θ–S Climatology*

Autonomous CTD profiling floats are free-moving floats that report vertical profiles of salinity, temperature, and pressure at regular time intervals. The Argo program plans to deploy 3000 such floats to observe the upper 2000 m of the global ocean. These floats give good measurements of temperature and pressure, but salinity measurements may experience significant sensor drifts with time. The moving nature of these floats means that it is too expensive to retrieve them regularly for physical calibrations. Thus a system has been set up to correct the drift in these profiling float salinity data by using historical hydrographic data. An objective mapping technique is used to estimate the background climatological salinity field onu surfaces from nearby historical data. Temporal variations in water mass properties are accounted for in the objective estimate. The float salinity data are fitted to the background climatology in potential conductivity space by weighted least squares with a time-varying slope. The error associated with estimating the background climatology is carried through in the weighted least squares calculations. The result is a set of calibrated salinity data with error estimates. Because of the need to accumulate a time series for calculating a stable slope correction term, this system is a delayed-mode quality control system, with reliable calibrations available a few months after float data are obtained. However, contemporary ship-based measurements are essential in determining whether a measured trend is due to sensor drift or due to natural variability.

Mediterranean outflow mixing and dynamics

The Mediterranean Sea produces a salty, dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern Gulf of Cadiz. The current accelerates to 1.3 meters per second, which raises the internal Froude number above 1, and is intensely turbulent through its full thickness. The outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline North Atlantic Central water. Within 100 kilometers downstream, the current is turned by the Coriolis force until it flows nearly parallel to topography in a damped geostrophic balance. The mixed Mediterranean outflow continues westward, slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass.

Global Contraction of Antarctic Bottom Water between the 1980s and 2000s

AbstractA statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world’s deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below θ = 0°C at a rate of −8.2 (±2.6) × 106 m3 s−1. This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are see...

Equatorial Pacific Ocean Horizontal Velocity, Divergence, and Upwelling*

Abstract Upper-ocean horizontal velocity and divergence were estimated from shipboard observations taken from 1991 to 1999 in the equatorial Pacific between 170°W and 95°W. Mean transports were estimated for the zonal currents at the mean longitude of the sections, 136°W. Mean meridional currents for the entire longitude range included poleward surface flows reaching −0.09 m s−1 in the south and 0.13 m s−1 in the north as well as equatorward flow within the thermocline reaching 0.05 m s−1 in the south and −0.04 m s−1 in the north near 23°C (85 m). Vertical velocity was diagnosed by integrating horizontal divergence estimated for the entire region down from the surface. Equatorial upwelling velocities peaked at 1.9 (±0.9) × 10−5 m s−1 at 50 m. The upwelling transport in the area bounded by 3.6°S–5.2°N, 170°W–95°W was 62 (±18) × 106 m3 s−1 at 50 m. Strong downwelling was apparent within the North Equatorial Countercurrent. An asymmetry in the meridional flows suggested that on the order of 10 × 106 m3 s−1 o...

Recent cooling of the upper ocean

We observe a net loss of 3.2 (± 1.1) × 10 22 J of heat from the upper ocean between 2003 and 2005. Using a broad array of in situ ocean measurements, we present annual estimates of global upper-ocean heat content anomaly from 1993 through 2005. Including the recent downturn, the average warming rate for the entire 13-year period is 0.33 ± 0.23 W/m 2 (of the Earths total surface area). A new estimate of sampling error in the heat content record suggests that both the recent and previous global cooling events are significant and unlikely to be artifacts of inadequate ocean sampling.