Machbarkeitsstudie zur Bestimmung turbulenter Flüsse auf FS Polarstern im Rahmen von OCEANET

Diese Diplomarbeit behandelt die Möglichkeit und Machbarkeit der Messung von Parametern, die zur Herleitung turbulenter Flüsse dienen. Die Messungen wurden auf den beiden Polarsternexpeditionen ANT XXIV-4 im April/Mai 2008 und ANT XXV-5 im April/Mai 2009 durchgeführt. Die Berechnung der turbulenten Flüsse folgte aus gemessenen mittleren Größen mit Hilfe verschiedener Bulkparametrisierungen und aus Turbulenzmessungen mittels der Dissipationsmethode. Auf der Expedition ANT XXV-5 wurden zwei voneinander unabhängige Messsysteme, bestehend aus je einem Ultraschallanemometer und einem Hygrometer, an der Reling auf dem Peildeck und an der Reling auf dem Krähennest aufgebaut. Auf der ersten Fahrt wurde nur ein Ultraschallanemometer kombiniert mit zwei Hygrometern an der Reling auf dem Krähennest aufgebaut. Die Messungen auf dem Krähennest zeigten sehr gute Ergebnisse, gerade im Vergleich zwischen Parametrisierungen und Dissipationsmethode, obwohl die Höhe des Krähennestes mit knapp 30 Metern an der oberen Grenze der Prandtl-Schicht, in der die turbulenten Flüsse als höhenkonstant angenommen werden, liegt. Für den Fluss fühlbarer Wärme ist eine sehr gute Übereinstimmung mit der Parametrisierung von Smith (1980) bzw. Large und Pond (1982) zu erkennen. Beim Fluss latenter Wärme stimmen die Dissipationsergebnisse am besten mit der Parametrisierung von Smith (1988) überein. Dagegen ergaben sich aus den Messungen auf dem Peildeck, welches mit ca. 21 Metern Höhe deutlich niedriger liegt, Ergebnisse, welche verdeutlichen, dass diese Position für Turbulenzmessungen ungeeignet ist. Die Dissipationsergebnisse weichen fast durchgängig stark von den Bulkparametrisierungen und den Turbulenzmessungen auf dem Krähennest ab und es kommt zu Überschätzungen von über 50 W/m2 für den Fluss fühlbarer Wärme und von über 300 W/m2 für den Fluss latenter Wärme. Auch beim Impulsfluss, in dieser Arbeit durch den Dragkoeffizient gezeigt, wurden ähnliche Ergebnisse erzielt. Bei den Messungen mit dem Sonic USA-1 auf dem Krähennest ergibt sich eine sehr gute Übereinstimmung mit der Parametrisierung von Large und Pond (1981). Die Messungen auf dem Peildeck dagegen zeigen eine große Streuung. Das ist bedeutend für den Einsatz eines Messcontainers auf dem Peildeck, welcher im Rahmen des OCEANET-Projektes entwickelt wird und auch mit einem Turbulenzmesssystem ausgestattet werden soll. Diese Position hat sich für Turbulenzmessungen als ungeeignet erwiesen. Die Messungen auf dem Container sind allerdings ca. 3 Meter höher als die Messungen auf dem Peildeck während der Fahrt ANT XXV-5, dies könnte eine weniger gestörte Anströmung begünstigen. Diese Messungen sollen auf der Polarsternfahrt ANT XXVI-1 durchgeführt werden, wobei auch dort wieder ein Vergleich mit dem alten Messsystem auf dem Krähennest durchgeführt werden soll. Auch bei den Messungen der CO2-Konzentration und der dadurch resultierenden Bestimmung des CO2-Flusses streuen die Ergebnisse der Fahrt ANT XXV-5 sehr stark. Es ist allerdings erwähnenswert, dass auch die alte Messposition des Licors, welches die CO2-Konzentration misst, auf dem Krähennest, keine, im Vergleich zum Peildeck, besseren Ergebnisse hervorgebracht hat

Mixed layer heat and salinity variability in the equatorial Atlantic

The most striking sea surface temperature (SST) phenomenon in the tropical Atlantic is the seasonal appearance of the Atlantic Cold Tongue (ACT). The ACT, characterized by strongly reduced temperatures, develops in late boreal spring/early summer with the strengthening of the southeast trades along the equator and last until late fall/early winter. Onset, duration, spatial extent and strength of cooling vary significantly from year to year. The mixed layer (ML) heat budget terms in the ACT region are rather well estimated, but only locally or with model results. The role of near-surface salinity variations for ACT onset and development as well as the mixed layer salinity budget are less understood. In this thesis ML temperature and salinity (MLS) changes during ACT development were studied from May to July 2011 by a cold tongue experiment (CTE). The CTE was based on two successive research cruises, a glider swarm experiment, and moored observations. This in-situ dataset together with satellite data, atmospheric reanalysis data, and assimilation model output were used to evaluate the ML heat and salinity budget for two sub-regions: 1) the western ACT between 23°-10°W showing strong cooling during the CTE and 2) the region north of the ACT influenced by the northward migrating Intertropical Convergence Zone. The findings of the year 2011 were compared with mean seasonal cycles at three buoy sites (0°N, 23°W; 4°N, 23°W; 0°N, 10°W). The strong ML heat loss in the ACT region during the CTE was found to be the result of the balance between warming due to net surface heat flux and cooling due to diapycnal mixing and zonal advection. The dominant role of diapycnal mixing was observed as well as conjectured from the residual of the heat budget by using estimated diapycnal diffusivities and vertical temperature gradients at the base of the ML. In the region north of the ACT the weak cooling was achieved by similar contributions of net surface heat flux, zonal advection and entrainment. Only a small residual remained in the region north of the ACT. During the CTE, salinity in the ACT region slightly increased with a balance of overall freshening due to strongly varying zonal advection and salinity increase due to the net surface freshwater flux. A strong salinity increase in the ACT region occurred at the equator, 10°W before the CTE and was not captured by our measurements. The diapycnal mixing, which act to erode the high salinity core of the Equatorial Undercurrent, could only partly be related to the remaining residual. In the region north of the ACT, stronger precipitation resulted in a freshening effect due to a net surface freshwater flux. Zonal advection changed sign during the CTE contributing to a ML freshening at the beginning of the CTE and a salinity increase afterward. The salinity balance at the equator is characterized by weak seasonal changes. In the first half of the year the MLS changes are caused by the freshening due to precipitation, zonal advection and eddy salt advection, which are balanced by the salinity increasing contribution of evaporation, meridional salt advection and entrainment. In the western cold tongue region, a diapycnal salt flux during February/March and in June is a salt gain for the ML. Meridional salt advection increases MLS during the second half of the year in the cold tongue region. In particular in the central cold tongue region an imbalance during summer and fall remains. In summer this imbalance can be minimized by the implementation of the diapycnal salt flux. At the northern buoy at 4°N the seasonal cycle of MLS is caused by the semiannual cycle of precipitation and meridional advection and the annual cycle of entrainment and eddy salt advection

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

Sea surface temperatures (SSTs) in the eastern tropical Atlantic are crucial for climate variability within the tropical belt. Despite this importance, state-of-the-art climate models show a large SST warm bias in this region. Knowledge about the seasonal mixed layer (ML) heat budget is a prerequisite for understanding SST mean state and its variability. Within this study all contributions to the seasonal ML heat budget are estimated at four locations within the Atlantic cold tongue (ACT) that are representative for the western (0°N, 23°W), central (0°N, 10°W) and eastern (0°N, 0°E) equatorial as well as the southern (10°S, 10°W) ACT. To estimate the contribution of the diapycnal heat flux due to turbulence an extensive data set of microstructure observations collected during ten research cruises between 2005 and 2012 is analyzed. The results for the equatorial ACT indicate that with the inclusion of the diapycnal heat flux the seasonal ML heat budget is balanced. Within the equatorial region, the diapycnal heat flux is essential for the development of the ACT. It dominates over all other cooling terms in the central and eastern equatorial ACT, while it is of similar size as the zonal advection in the western equatorial ACT. In contrast, the SST evolution in the southern ACT region can be explained entirely by air-sea heat fluxes

Accuracy of wind observations from open-ocean buoys: Correction for flow distortion

The comparison of equivalent neutral winds obtained from (a) four WHOI buoys in the subtropics and (b) scatterometer estimates at those locations reveals a root-mean-square (RMS) difference of 0.56-0.76 m/s. To investigate this RMS difference, different buoy wind error sources were examined. These buoys are particularly well suited to examine two important sources of buoy wind errors because: (1) redundant anemometers and a comparison with numerical flow simulations allow us to quantitatively assess flow distortion errors, and (2) one-minute sampling at the buoys allows us to examine the sensitivity of buoy temporal sampling/averaging in the buoy-scatterometer comparisons. The inter-anemometer difference varies as a function of wind direction relative to the buoy wind vane and is consistent with the effects of flow distortion expected based on numerical flow simulations. Comparison between the anemometers and scatterometer winds supports the interpretation that the inter-anemometer disagreement, which can be up to 5% of the wind speed, is due to flow distortion. These insights motivate an empirical correction to the individual anemometer records and subsequent comparison with scatterometer estimates show good agreement

Physical and Biogeochemical Studies in the Subtropical and Tropical Atlantic

Maria S. Merian Cruise Report MSM18/L2 Cruise No. 18, Leg 2 May 11 – June 19, 2011 Mindelo (Cape Verde Islands) – Mindelo (Cape Verde Islands

Detailed investigation of the role of buoy wind errors in buoyscatterometer disagreement

The comparison of equivalent neutral winds obtained from (a) four WHOI buoys in the subtropics and (b) scatterometer estimates at those locations reveals a very low root-mean-square difference (RMS) on the order of 0.5-0.7 m/s and a seasonal cycle in the RMS. To investigate this RMS, different buoy wind error sources were examined. Our buoys are particularly well suited to examine two important sources of buoy error: (1) redundant anemometers and a comparison with numerical flow simulations allow us to quantitatively assess flow distortion errors, and (2) one-minute sampling at the buoys allows us to examine the sensitivity of buoy temporal sampling/averaging in the buoy-scatterometer comparisons. The flow distortion can be estimated to up to 5% of the relative difference of the anemometers. Application of this error to the individual anemometer and subsequent comparison with scatterometer estimates show a good agreement. Application of a reasonable time averaging, subtraction of a mean bias, and application of a viscosity correction generally reduces the RMS but cannot explain the seasonal cycle of it

Detailed investigation of the role of buoy wind errors in buoyscatterometer disagreement

The comparison of equivalent neutral winds obtained from (a) four WHOI buoys in the subtropics and (b) scatterometer estimates at those locations reveals a very low root-mean-square difference (RMS) on the order of 0.5-0.7 m/s and a seasonal cycle in the RMS. To investigate this RMS, different buoy wind error sources were examined. Our buoys are particularly well suited to examine two important sources of buoy error: (1) redundant anemometers and a comparison with numerical flow simulations allow us to quantitatively assess flow distortion errors, and (2) one-minute sampling at the buoys allows us to examine the sensitivity of buoy temporal sampling/averaging in the buoy-scatterometer comparisons. The flow distortion can be estimated to up to 5% of the relative difference of the anemometers. Application of this error to the individual anemometer and subsequent comparison with scatterometer estimates show a good agreement. Application of a reasonable time averaging, subtraction of a mean bias, and application of a viscosity correction generally reduces the RMS but cannot explain the seasonal cycle of it