Johannes Karstensen

Age determination of mixed water masses using CFC and oxygen data

We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section I5 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48 degrees-55 degrees E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55 degrees E after 15-20 years. It takes ICW some 25-30 years to reach 110 degrees E.

The seasonal pCO2 cycle at 49°N/16.5°W in the northeastern Atlantic Ocean and what it tells us about biological productivity

A 2-year record of mixed layer measurements of CO2 partial pressure (pCO2), nitrate, and other physical, chemical, and biological parameters at a time series site in the northeast Atlantic Ocean (49N/16.5W) is presented. The data show average undersaturation of surface waters with respect to atmospheric CO2 levels by about 40 ± 15 matm, which gives rise to a perennial CO2 sink of 3.2 ± 1.3 mol m2 a1. The seasonal pCO2 cycle is characterized by a summer minimum (winter maximum), which is due to the dominance of biological forcing over physical forcing. Our data document a rapid transition from deep mixing to shallow summer stratification. At the onset of shallow stratification, up to one third of the mixed layer net community production during the productive season had already been accomplished. The combination of high prestratification productivity and rapid onset of tratification appears to have caused the observed particle flux peak early in the season. Mixed layer deepening during fall and winter reventilated CO2 from subsurface respiration of newly exported organic matter, thereby negating more than one third of the carbon drawdown by net community production in the mixed layer. Chemical signatures of both net community production and respiration are indicative of carbon overconsumption, the effects of which may be restricted, though, to the upper ocean. A comparison of the estimated net community production with satellite-based estimates of net primary production shows fundamental discrepancies in the timing of ocean productivity.

Formation of Southern Hemisphere Thermocline Waters: Water Mass Conversion and Subduction*

Abstract The ventilation of the permanent thermocline of the Southern Hemisphere gyres is quantified using climatological and synoptic observational data. Ventilation is estimated with three independent methods: the kinematic method provides subduction rates from the vertical and horizontal fluxes through the base of the mixed layer, the water age uses in situ age distribution of thermocline waters, and the annual-mean water mass formation through air–sea interaction is calculated. All three independent estimates agree within their error bars, which are admittedly large. The subduction rates are mainly controlled through their vertical and lateral components with only minor transient eddy contributions. The vertical transfer, derived from Ekman pumping, ventilates over most of the areas of the subtropical gyres, while lateral transfer occurs mainly along the Subtropical and Subantarctic Fronts, where it injects mode and intermediate waters. For the permanent thermocline the overall ventilation of the Sout...

Water mass transformation in the Greenland Sea during the 1990s

Time series of hydrographic and transient tracer measurements were used to study the variability of Greenland Sea water mass transformation between 1991 and 2000. Increases in tracer inventories indicate active renewal of Greenland Sea Intermediate Water (GSIW) at a rate of 0.1 to 0.2 Sv (1 Sv = 1 × 106 m3 s−1) (10-year average). A temperature maximum (Tmax) was established at the base of the upper layer (500 m) as a consequence of anomalously strong freshwater input into the near-surface layer at the beginning of the 1990s. Tmax rapidly descended to 1500 m by 1995 followed by a much slower rate of descent. GSIW became warmer and less saline compared to the 1980s. During the deepening phase of Tmax, atmospheric data revealed above-average wind stress curl and oceanic heat loss. In addition, high Arctic Ocean sea-ice export and lack of local sea-ice formation have been documented for that period. A combination of all these factors may have evoked the renewal of GSIW with anomalously freshwater from the upper layers. The Tmax layer established a stability maximum that inhibits vertical exchange between intermediate and deeper waters. Temperature and salinity of deep waters continued to increase at rates of 0.01°C yr−1 and 0.001 yr−1, respectively. However, since 1993, decrease in and homogenization of deep water transient tracer concentrations indicate that renewal occurred predominantly by addition of Arctic Ocean waters. In 2000 the water column (500 m to 3400 m) required an additional 60 W m−2 (110 W m−2) over the annual mean heat loss to restore its heat content to 1989 (1971) values.

Water subducted into the Indian Ocean subtropical gyre

Subduction rates of the water into southern Indian Ocean permanent thermocline from 23.5 to Full-size image (<1 K) are calculated from a kinematic and a water-age approach and are compared with surface buoyancy flux calculations. The different estimates compare well within error margins, indicating that the effect of transient eddies on the subduction process is of second order. Considering only northward transport components, the overall transfer of water into the Indian Ocean thermocline is about Full-size image (<1 K) (Full-size image (<1 K)), with equal contributions from lateral and vertical (essentially Ekman pumping) components. Ekman pumping dominates the upper density range (Full-size image (<1 K)), while for denser water the largest contribution stems from lateral input of Mode Waters. High silicate concentrations in the Mode Waters indicate that Southern Ocean waters participate in the formation process. The source water properties in salinity, oxygen, and nutrients along the surface of the deepest winter mixed layer are given.

Atlantic Ocean Observing Networks: Cost and feasibility study

Results of a cost and feasibility study of the present and planned integrated Atlantic Ocean Observing System, including assessing the readiness and feasibility of implementation of different observing technologies