ALBERT

Description: The benthic nitrogen (N) cycle is highly dynamic and diverse due to the strong redox gradients occurring in marine surface sediments and the variety of oxidation states accessible to nitrogen. Since N is a limiting nutrient for biological productivity, fluxes of nitrogenous species across the sediment-water interface may strongly affect the biogeochemistry of nitrogen, carbon and phosphorus in the oceanic water column. In particular, as a major sink for fixed N in the marine environment, benthic denitrification has a profound impact on the availability of bioavailable N in the oceans. Consequently, an understanding of N cycling in marine sediments is of major importance for constraining the global marine nitrogen budget and quantifying benthic-pelagic feedbacks. The thesis addresses benthic N turnover on local and global scales with a special focus on oxygen minimum zones where strong lateral redox gradients lead to unique and highly interesting interactions in N cycling.

Description: Cruise WND-V was the fifth and final expedition of the West Nile Delta project carried out at IFM-GEOMAR with support from RWE-Dea. On this concluding cruise of the project, the two mud volcanoes North Alex and Giza were once again visited, this time using the Greek R/V AEGAEO. The primary purpose of the WND-V cruise was the recovery of instruments and observatories which had been deployed in or near the active centers of North Alex and Giza MVs over the past 30 months. In addition, complementary new data were acquired that were needed to better quantify the methane flux from the centers of both MVs. From 15 to 25 June 2010, the following activities were carried out: · recovery of 12 long-term Ocean-Bottom-Seismometers (OBS) at North Alex MV, which had been deployed during POSEIDON cruise P388 in 2009. · recovery of 4 instruments for measuring long-term pore pressure variations (piezometers) on the slope near North Alex MV, deployed during POSEIDON cruise P388 in 2009. · recovery of 6 instruments for measuring long-term chemical flux (CATmeters) at Giza and North Alex MV. · recovery of 1 instrument (OBMets) for measuring methane flux from the seafloor at North Alex MV. · recovery of 2 tiltmeters for measuring seafloor deformation at North Alex MV. · recovery of 2 long-term temperature observatories at Giza and North Alex MV. · CTD casts in the central area of North Alex MV · Imaging of bubble streams from the active center of North Alex using an SIMRAD EK60 sonar system. Although hampered by poor weather during the final days of the cruise the recovery operations were finished two days earlier than expected. Bathymetric data acquisition however, which had been part of the original schedule had to be cancelled due to bad weather as well as technical problems with the deep water sonar systems.

Description: Cruise P453 & P458 with R/V POSEIDON aim to conduct 3D wide-angle ocean bottom recording (OBR) data during the acquisition of 3D multichannel seismic (MCS) reflection data (D. Sawyer, Rice University & T.J. Reston, Birmingham) and to analyze and interpret a highresolution densely sampled 2D OBR wide-angle profile. The primary goal of the wide-angle data will be to provide an accurate and detailed 3D P-wave velocity model for the 3D reflection data. Particularly the densely sampled 2D profile will help to determine the degree of thinning within the crust and the degree of serpentinisation of the uppermost mantle. The collection of 3D MCS data gives the unique opportunity to obtain densely sampled water column reflection data and to analyze spatial and temporal (4D) variations of the internal wave field which will yield new understanding and insights into water mass mixing processes offshore west Iberia. Contemporaneously hydrographic data will be collected to calibrate and analyze the oceanic thermohaline structures originating from the interaction between Atlantic waters and the Mediterranean Sea Outflow.

Description: R/V POSEIDON cruise P457 aimed further development of detailed marine tephrochronology of Iceland by sediment coring in order to improve our knowledge of the spatio-temporal evolution of Icelandic volcanism and related hazards. In addition, the marine record contains paleoclimatic information, which may relate recurrent glacier advances and land degradation periods to ocean and atmospheric circulation changes. A minor sub-project should contribute to a better understanding of Surtsey volcanism by investigations of marine Surtsey tephra. P457 conducted extensive sediment echosounding (11 surveys with a total length of c. 425 nm) in order to identify undisturbed sediment sequences for coring. At 20 appropriate sites, P457 deployed gravity and/or gaint box corers to recover ultra-high resolution sediment cores from 〈 100 m to ~1,600 m water depth at the south-western, southern and eastern sectors of the Icelandic shelf and slope. Of these deployments, 9 gravity corers yielded altogether 59.5 m core recovery and 11 box corers recovered surface sediment samples. Additionally 7 CTD/rosette water sampling stations have been performed at shallow sites close to Iceland and at deep sites further offshore in order to determine the REE distributions and the Nd and Hf isotope compositions of the sea water. No equipment was lost or significantly damaged. Cruise P457 was particularly successful in the working areas southwest and south of Iceland but failed to recover long sediment cores in the eastern working areas and at Surtsey. Sand, clayey silt, clayey sandy silt, sandy clayey silt, and volcanic ashes are the dominant lithologies in the P457 sediment cores. Preliminary studies of selected sedimentary records along the Iceland margin from ca. 24°W to 12°W on both sides of Reykjanes Ridge suggest that a correlation of these cores is possible, implying that sedimentary records are undisturbed and of high quality. Notably, distinct volcanic ash layers can apparently be traced in the P457 cores across the working area. Preliminary age models of selected cores show that the sedimentation rates around Iceland are low with only a thin Holocene. Sediment records cover approximately 120.000 years at most.

Description: SO-234/1 was a training and capacity building cruise for students from southern Africa and Germany in the framework of the BMBF-funded SPACES program (Science for the Assessment of Complex Earth System Processes), a cooperative research project initiated by the relevant ministries in Namibia, South Africa, Angola and Germany. Scientifically, SO-234/1 continued geological studies regarding the temporal and geochemical evolution of the Walvis Ridge (Southeast Atlantic) conducted on the precursor SO-233 expedition, and was broadened by biological studies by University of Tübingen scientists, which aimed to get a better understanding of the adaptations of visual systems in mesopelagic animals to bioluminescence. The educational aspects on SO-234/1 comprised fully integration of the students into all scientific work on board and various lectures and courses given by the senior scientists during transit times. The working area of SO-234/1 included several seamounts, such as the Ewing Seamount, and a section of the southeastern margin of the Walvis Ridge. Due to the fair weather conditions and the excellent support from the master and crew, SO-234/1 completed 18 sampling stations within only four working days. Rock sampling has been conducted using chain bag dredges. The seven SO-234/1 dredge hauls recovered in situ rocks from up to 3,500 m water depth, among them fairly fresh lava fragments and volcaniclastic rocks suitable for volcanological, geochemical, and geochronological analyses. Additionally, a TV-grab station has been conducted at Ewing Seamount but the grab failed to return hard rocks. The group of biologists conducted ten trawls at depths between 400 and 1,000 m using a rectangular midwater Tucker Trawl with an opening of 16 square meters. All trawls were successful and brought numerous animals from the junction of the meso- and bathypelagic habitats including, apart from fishes, several cephalopods, crustaceans, deep-sea jellyfish, ctenophors and many salp colonies.

Description: R/V SONNE cruise SO-233 WALVIS II conducted geological, morphological, and biological studies in the area of the aseismic Walvis Ridge and the adjacent ocean floor (South Atlantic). The Walvis Ridge is a textbook example of a hotspot track connected to a continental flood basalt province and represents the Atlantic “type locality” for the enriched mantle one (EM-I) geochemical endmember in intraplate volcanic rocks. Despite its importance in the global hotspot reference frame, endmember geochemical composition, and uncertainties in its formation and evolution, basement sampling of the Walvis Ridge remained poor to date, in particular along its easternmost 1500 km. The geological studies carried out during SO-233 therefore aimed for extensive multi-beam mapping using a SIMRAD EM 120 echo-sounding system, sediment echo-sounding using a ATLAS PARASOUND sub-bottom profiling system, and hard rock sampling by dredging and TV-grab of the Walvis Ridge and associated features. The major targets of the WALVIS II project are (1) to test for age progressive volcanism along the ridge, (2) to differentiate between classical hotspot and plate fracturing models for its formation, and (3) to constrain the origin, temporal and spatial evolution of melting conditions and source compositions (in particular regarding the EM-I endmember and proposed zonation models of mantle plumes). The biology program conducted on SO-233 comprised sampling of benthic organisms and meiofauna using a TV-multi-corer, a TV-grab, sediment traps installed in the geological dredges, and by collecting marine invertebrates from the hard rocks yielded by dredging. The biological investigations of the WALVIS II project intend to describe the benthic diversity of deep-sea invertebrates of the Walvis Ridge and will help to identify proxies of species connectivity and dispersal between the Walvis Ridge and neighboring ridge like structures (e.g. Agulhas Ridge). Another objective is to test whether connectivity of benthic communities in the Angola and Cape Basins is interrupted by the Walvis Ridge. SO-233 multi-beam mapping revealed that the southern bifurcated section of the Walvis Ridge appears to have formed through the coalescence of former volcanic islands. The new bathymetric data also yielded several evidence for large-scale extensional tectonic movements which are most likely related to the separation of the Walvis Ridge and Rio Grande Rise that were rifted apart by the mid Atlantic Ridge. Seventy-one dredge hauls have been conducted during SO-233. Of these, 28 delivered massive lavas, 24 volcaniclastic rocks including breccias containing lava fragments, 22 sedimentary rocks, and 11 Mn-Fe-oxide crusts and nodules. The volcanic rocks comprise a broad variety of lavas as well as epiclastic, hydroclastic, and pyroclastic rocks. Carbonates dominate among the non-volcanic rocks, many of them represent relicts of fossil coral reefs. Despite technical problems with the EM 120 system and difficult weather and seafloor conditions occasionally constraining rock sampling, SO-233 achieved its major goals, i.e. bathymetric mapping and representative hard rock sampling of all major geomorphological units of the Walvis Ridge and of associated features. The set of rock samples recovered during SO-233 represents the by far most detailed sampling of the Walvis Ridge to date. Out of 91 collecting stations, 80 stations yielded the total amount of 80 kg of sediment from sediment traps in the geological dredges, TV-multi-corer tubes and TV-grab. At 44 stations we could collect macrofaunal organisms, partly in large quantities. Ninety specimens of living brachiopods representing 6 genera were found at all depths and will mainly be used for molecular diversity studies. The remaining living macrofauna was largely composed of sponges, octocorals, some deep water hexacorals, molluscs, polychaetes, bryozoans, cirriped crustaceans and a few isopods and amphipods, mainly occurring in small numbers and medium diversity. The most spectacular finding was a fossil cold water reef mound community, which shows similarities in species composition to North Atlantic cold water reefs and proofs the influence of Antarctic benthos communities on the Walvis Ridge fauna mediated by northbound cold water currents. The samples represent the most diverse collection of benthos organisms ever retrieved from the Walvis Ridge region.

Description: The exchange of trace gases between the surface ocean and the lower atmosphere has received increasing attention during the last years especially in view of ongoing global environmental changes such as eutrophication, warming of the ocean, ocean acidification etc.. The ocean has been identified as a huge reservoir of various climate relevant trace gases. However, the distributions and the pathways of the trace gases such as isoprene, acetone and acetaldehyde, in the surface seawater is poorly understood. Even consensus regarding the intensively studied dimethylsulphide (DMS) pathways continues to elude researchers to date. This thesis compiles different studies which contribute new insight into the fates of a group of short-lived, climate-relevant traces gases including DMS, isoprene, acetaldehyde and acetone in the surface layers of the Pacific and Atlantic Oceans: • The distribution of and the interactions between DMS, dimethylsulphoniopropionate (DMSP) and dimethylsulphoxide (DMSO) in surface seawater were examined in conjunction with the phytoplankton composition and methane along a northsouth transit in the western Pacific Ocean on board the R/V Sonne (TransBrom) from Tomakomai (Japan) to Townsville (Australia) from 9th to 24th 2009. A close link between DMSP and DMSO was found based on correlations between the two compounds and similar phytoplankton pigments, which were identified as presumably sources of both DMSP and DMSO. The detected DMSPp:DMSOp seemed to be typical for an oligotrophic tropical ocean and might indicate stress conditions for phytoplankton due to intensive solar radiation and nutrient limitation which in turn may have led to an accumulation of DMSO in the surface water of the western Pacific Ocean. It seems that DMSP and DMSO were more closely related to each other than to DMS. It was evident that different factors influence the DMS distribution, as underlined by the failure to identify phytoplankton groups as sources for DMS. Moreover, DMSP and DMSO were identified as possible substrates for methane production along the entire north-south transit, emphasizing the potential role of both compounds as precursors of a climate relevant trace gas. • The atmospheric distribution of DMS was also examined along the same northsouth transit in the western Pacific Ocean. DMS emitted from the western Pacific Ocean appeared to be influenced by storms events which were encountered along the same north-south transit. The distribution of the computed DMS flux differed from the distribution of measured atmospheric DMS concentrations, indicating that atmospheric transport processes governed the pattern of atmospheric DMS concentrations. The Langrangian FLEXPART model was used to examine the influence of transport processes on the distribution of DMS in the troposphere. Although the concentration of DMS in the surface ocean was not elevated above background levels, there were certain instances of elevated flux relating to the high winds from storm events. In regions of the cruise track influenced by convective processes, the amount of DMS (and likely its oxidation products) transported to the tropical tropopause layer, as computed by the model, was regionally significant. The modeled DMS concentrations with altitude were in general agreement with aircraft measurements. However, because DMS has never been considered as a source of sulphur to the upper troposphere/lower stratosphere, there are hardly measurements above 12 km. The actual amount of DMS crossing the tropopause could not be determined, but it is possible that one regionally and seasonally important source of sulphur to the persistent stratospheric sulphur layer is marine derived DMS. • Because DMS and isoprene have both been identified as potential cloud condensation nuclei precursors over the remote oceans, the distribution and the relationship between the two gases were studied in surface seawater along a north-south transit in the eastern Atlantic Ocean on board of R/V Polarstern (ANTXXV-1) from Bremerhaven (Germany) to Cape Town (South Africa) in November 2008. Positive and negative correlations between DMS and isoprene were found in different regions which extended over two-thirds of the transit. Additionally, DMS and isoprene showed a similar distribution pattern together with phytoplankton groups like dinoflagellates, haptophytes and chrysophytes when clustered by N:P, indicating a biological source of both compounds. However, DMS is known to be produced by bacteria rather than by phytoplankton. Thus, isoprene and DMSP were most likely produced by phytoplankton, which was reflected in the correlation between the two compounds dependent on the N:P ratio. The relationship between DMS and isoprene observed in oligotrophic to eutrophic regions might be based instead on microbial activities. It might be possible that bacteria exist which can produce both DMS and isoprene concurrently. However, this is highly speculative and needs to be further investigated. In addition, DMS:isoprene corresponded to the hydrographic regimes in the eastern Atlantic Ocean. Enhanced DMS concentrations occurred in upwelling regions, while isoprene showed elevated concentrations in oligotrophic regions. However, both compounds showed the highest concentrations in a spring bloom near South Africa, likely pointing to biological activities as the main source. The possibility that transport could be an important control on the isoprene distribution in the surface ocean was also determined. • In order to identify marine sources and sinks of acetaldehyde and acetone, incubation experiments were conducted with water samples from the Baltic Sea taken at the GEOMAR institute pier (Kiel Fjord) and samples from the eastern equatorial Atlantic Ocean taken during the research cruise MSM 18/3 on board of the R/V Maria S. Merian from Mindelo (Cape Verde Islands) to Libreville (Gabon) from 21st of June to 19th of July 2011. The effect of biological and chemical processes on the production and consumption of the two compounds was investigated under light and dark conditions. Acetaldehyde and acetone were consumed rather than produced in both oceanic regions. The effect of solar radiation was of minor importance for the production and degradation of acetone and acetaldehyde. Although, the consumption of acetone in biology treatments was slightly higher compared to the chemical treatments, no overarching differences could be determined between biology and chemistry samples, in general. However, there were hints that the biota was important for the production and consumption of both acetaldehyde and acetone, which needs to be further examined in detail. The measured production and degradation rates were variable among the individual experiments, pointing to a complex interaction between different sources and sink processes under varying environmental controls. That will be elucidated in future work with a detailed examination of the individual experiments.

Description: Large tropical volcanic eruptions have been observed to have a significant influence on the large-scale circulation patterns of the Northern Hemisphere, through mechanisms related to the radiative effects of the sulfate aerosols resulting from the volcanic injection of sulfur dioxide into the stratosphere. While no such volcanically induced anomalies in Southern Hemisphere circulation have yet been observed, we find that in general circulation model simulations, eruptions with sulfur dioxide injections larger than that of the 1991 Mt. Pinatubo eruption do result in significant circulation changes in the SH, specifically an enhanced positive phase of the Southern Annular Mode (SAM). We explore the mechanisms for such a SAM response, as well as the corresponding changes in SH temperature, sea ice and precipitation. We also explore how the anomalously strong zonal winds characteristic of the positive SAM regime affect the rate of sulfate deposition to the Antarctic ice-sheet. We suggest that the use of ice-core sulfate records as a proxy for past volcanic activity may benefit from including knowledge of, or better assumptions regarding the changes in large scale atmospheric circulation after large tropical eruptions.

Description: The effects of clouds and sea-ice on the surface radiative balance in the Arctic are studied. Clouds during arctic summer are characterized by a low optical thickness and high cloud amounts. To determine the effect of these clouds in combination with sea-ice, measurements onboard the research vessel FS Polarstern were obtained in August and September 2009 during the expedition ARK24-3 in the Greenland Sea. Data evaluation was accompanied by model calculations with the radiative transfer model GRIMALDI. In particular, the effect of multiple reflexions between sea-ice surface and clouds on the shortwave radiative balance was determined. The GRIMALDI model from R. Scheirer is based on the Monte-Carlo method and was adjusted within this work to radiative transfer calculations in the broadband solar spectrum. The average downward shortwave and longwave irradiance of 80W/m2 and 296W/m2 measured during ARK24-3, as well as the average sea-ice albedo of 0.45 at this time of the year are within the scope of expectations from previous studies (Intrieri et al., 2002b; Shupe and Intrieri, 2004; Wang and Key, 2005; Persson et al., 2002). This applies also for the shortwave and longwave cloud radiative forcing (-70W/m2 and 46W/m2) which depends not only on sun zenith angle but also from surface albedo and temperature. For zenith angles less than 82°, surface cloud radiative forcing had a cooling effect during ARK24-3 and for zenith angles greater than 82° a warming effect. Sea-ice increases the downward shortwave irradiance at the surface by multiple reflexions between sea-ice and clouds. In case of a broken cloud cover the effect is amplified in places where the irradiance is already increased due to cloud distribution. In the local mean, the effect depends on the cloud cover and the optical thickness of clouds. The absolute magnitude of the effect increases with cloud fraction. With an increasing optical thickness, the relative magnitude of the effect increases, while the absolute irradiance decreases. This results in an optical thickness tau with a maximal increase of the absolute sea-ice effect. This optical thickness is near tau = 5 which equals the average optical thickness of clouds during ARK24-3. With a zenith angle of theta = 60°, sea-ice albedo of 0.5 and 100% cloud cover the total increase is 45W/m2, representing a relative increase of the downward irradiance of 15%. With the effect of increased downward shortwave irradiance over sea-ice compared to water, a difference in the cloud radiative forcing is to be expected. This difference was not observed during ARK24-3. Because of the small dataset it remains uncertain if this is due to additional low-level clouds over sea-ice or to increased cloudiness in relation with frontal systems.

Description: Stratospheric sulfate aerosols produced by major volcanic eruptions modify the radiative and dynamical properties of the troposphere and stratosphere through their reflection of solar radiation and absorption of infrared radiation. At the Earth's surface, the primary consequence of a large eruption is cooling, however, it has long been known that major tropical eruptions tend to be followed by warmer than usual winters over the Northern Hemisphere (NH) continents. This volcanic "winter-warming" effect in the NH is understood to be the result of changes in atmospheric circulation patterns resulting from heating in the stratosphere, and is often described as positive anomalies of the Northern Annular Mode (NAM) that propagate downward from the stratosphere to the troposphere. In the southern hemisphere, climate models tend to also predict a positive Southern Annular Mode (SAM) response to volcanic eruptions, but this is generally inconsistent with post-eruption observations during the 20th century. We review present understanding of the influence of volcanic eruptions on the large scale modes of atmospheric variability in both the Northern and Southern Hemispheres. Using models of varying complexity, including an aerosol-climate model, an Earth system model, and CMIP5 simulations, we assess the ability of climate models to reproduce the observed post-eruption climatic and dynamical anomalies. We will also address the parametrization of volcanic eruptions in simulations of the past climate, and identify possibilities for improvement

Description: To evaluate the importance of anti-herbivore resistance for algal invasion success we compared resistance traits among specimens of the red macroalga Gracilaria vermiculophylla from six native populations in Korea and China and eight invasive populations in Europe and Mexico that were maintained under identical conditions in the laboratory. Herbivorous snails both from the native range (Littorina brevicula) and from the invaded range (Littorina littorea) consumed significantly less of seaweed specimens originating from non-native populations. Metabolome profiling revealed that this preference was correlated with an increased woundactivated production of deterring prostaglandins and hydroxyeicosatetraenoic acids. Thus, invasive populations of G. vermiculophylla are more strongly defended against challenge by herbivores and other biological enemies that cause local tissue or cell disruption and activate oxylipin production. Anthropogenic distribution of genotypes adapted to resist elevated feeding pressure probably contributed to the invasion success of this species.

Description: Large explosive volcanic eruptions can inject massive amounts of sulfuric gases into the Earth's atmosphere and, in so doing, affect global climate. The January 1835 eruption of Cosigüina volcano, Nicaragua, ranks among the Americas’ largest and most explosive historical eruptions, but whether it had effects on global climate remains ambiguous. New petrologic analyses of the Cosigüina deposits reveal that the eruption released enough sulfur to explain a prominent ca. AD 1835 sulfate anomaly in ice cores from both the Arctic and Antarctic. A compilation of temperature-sensitive tree-ring chronologies indicates appreciable cooling of the Earth's surface in response to the eruption, consistent with instrumental temperature records. We conclude that this eruption represents one of the most important sulfur-producing events of the last few centuries and had a sizable climate impact rivaling that of the 1991 eruption of Mount Pinatubo.

Description: In this study, results from the Baltic Sea Tracer Release Experiment (BATRE) are described, in which deep water mixing rates and mixing processes in the central Baltic Sea were investigated. In September 2007, an inert tracer gas (CF3SF5) was injected at approximately 200 m depth in the Gotland Basin, and the subsequent spreading of the tracer was observed during six surveys until February 2009. These data describe the diapycnal and lateral mixing during a stagnation period without any significant deep water renewal due to inflow events. As one of the main results, vertical mixing rates were found to dramatically increase after the tracer had reached the lateral boundaries of the basin, suggesting boundary mixing as the key process for basin-scale vertical mixing. Basin-scale vertical diffusivities were of the order of 10−5 m2 s−1 (about 1 order of magnitude larger than interior diffusivities) with evidence for a seasonal and vertical variability. In contrast to tracer experiments in the open ocean, the basin geometry (hypsography) was found to have a crucial impact on the vertical tracer spreading. The e-folding time scale for deep water renewal due to mixing was slightly less than 2 years, the time scale for the lateral homogenization of the tracer patch was of the order of a few months. Key Points: Mixing rates in the Gotland Basin are dominated by boundary mixing processes; The time scale for Gotland Basin deep water renewal is approximately 2 years; Mixing rates determined from the tracer CF3SF5

Description: We investigate the contribution of oceanic methyl iodide (CH3I) to the stratospheric iodine budget. Based on CH3I measurements from three tropical ship campaigns and the Lagrangian transport model FLEXPART, we provide a detailed analysis of CH3I transport from the ocean surface to the cold point in the upper tropical tropopause layer (TTL). While average oceanic emissions differ by less than 50% from campaign to campaign, the measurements show much stronger variations within each campaign. A positive correlation between the oceanic CH3I emissions and the efficiency of CH3I troposphere–stratosphere transport has been identified for some cruise sections. The mechanism of strong horizontal surface winds triggering large emissions on the one hand and being associated with tropical convective systems, such as developing typhoons, on the other hand, could explain the identified correlations. As a result of the simultaneous occurrence of large CH3I emissions and strong vertical uplift, localized maximum mixing ratios of 0.6 ppt CH3I at the cold point have been determined for observed peak emissions during the SHIVA (Stratospheric Ozone: Halogen Impacts in a Varying Atmosphere)-Sonne research vessel campaign in the coastal western Pacific. The other two campaigns give considerably smaller maxima of 0.1 ppt CH3I in the open western Pacific and 0.03 ppt in the coastal eastern Atlantic. In order to assess the representativeness of the large local mixing ratios, we use climatological emission scenarios to derive global upper air estimates of CH3I abundances. The model results are compared with available upper air measurements, including data from the recent ATTREX and HIPPO2 aircraft campaigns. In the eastern Pacific region, the location of the available measurement campaigns in the upper TTL, the comparisons give a good agreement, indicating that around 0.01 to 0.02 ppt of CH3I enter the stratosphere. However, other tropical regions that are subject to stronger convective activity show larger CH3I entrainment, e.g., 0.08 ppt in the western Pacific. Overall our model results give a tropical contribution of 0.04 ppt CH3I to the stratospheric iodine budget. The strong variations in the geographical distribution of CH3I entrainment suggest that currently available upper air measurements are not representative of global estimates and further campaigns will be necessary in order to better understand the CH3I contribution to stratospheric iodine.

Description: The Aleutian Islands of Alaska are home to a number of major volcanoes which periodically present a significant hazard to aviation. During summer of 2008, the Okmok and Kasatochi volcanoes experienced moderate eruptive events. These were followed a dramatic, major eruption of Mount Redoubt in late March 2009. The Redoubt case is extensively covered in this paper. Volcanic ash and SO2 from each of these eruptions dispersed throughout the atmosphere. This created the potential for major problems for air traffic near the ash dispersions and at significant distances downwind. The NASA Applied Sciences Weather Program implements a wide variety of research projects to develop volcanic ash detection, characterization and tracking applications for NASA Earth Observing System and NOAA GOES and POES satellites. Chemistry applications using NASA AURA satellite Ozone Monitoring System (OMI) retrievals produced SO2 measurements to trace the dispersion of volcanic aerosol. This work was complimented by advanced multi-channel imager applications for the discrimination and height assignment of volcanic ash using NASA MODIS and NOAA GOES and POES imager data. Instruments similar to MODIS and OMI are scheduled for operational deployment on NPOESS. In addition, the NASA Calipso satellite provided highly accurate measurements of aerosol height and dispersion for the calibration and validation of these algorithms and for corroborative research studies. All of this work shortens the lead time for transition to operations and ensures that research satellite data and applications are operationally relevant and utilized quickly after the deployment of operational satellite systems. Introduction

Description: Observations and simple theoretical arguments suggest that the Northern Hemisphere (NH) stratospheric polar vortex is stronger in winters following major volcanic eruptions. However, recent studies show that climate models forced by prescribed volcanic aerosol fields fail to reproduce this effect. We investigate the impact of volcanic aerosol forcing on stratospheric dynamics, including the strength of the NH polar vortex, in ensemble simulations with the Max Planck Institute Earth System Model. The model is forced by four different prescribed forcing sets representing the radiative properties of stratospheric aerosol following the 1991 eruption of Mt. Pinatubo: two forcing sets are based on observations, and are commonly used in climate model simulations, and two forcing sets are constructed based on coupled aerosol–climate model simulations. For all forcings, we find that simulated temperature and zonal wind anomalies in the NH high latitudes are not directly impacted by anomalous volcanic aerosol heating. Instead, high-latitude effects result from enhancements in stratospheric residual circulation, which in turn result, at least in part, from enhanced stratospheric wave activity. High-latitude effects are therefore much less robust than would be expected if they were the direct result of aerosol heating. Both observation-based forcing sets result in insignificant changes in vortex strength. For the model-based forcing sets, the vortex response is found to be sensitive to the structure of the forcing, with one forcing set leading to significant strengthening of the polar vortex in rough agreement with observation-based expectations. Differences in the dynamical response to the forcing sets imply that reproducing the polar vortex responses to past eruptions, or predicting the response to future eruptions, depends on accurate representation of the space–time structure of the volcanic aerosol forcing.

Description: Simulations of tropical volcanic eruptions using a general circulation model with coupled aerosol microphysics are used to assess the influence of season of eruption on the aerosol evolution and radiative impacts at the Earth's surface. This analysis is presented for eruptions with SO2 injection magnitudes of 17 and 700 Tg, the former consistent with estimates of the 1991 Mt. Pinatubo eruption, the later a near-"super eruption". For each eruption magnitude, simulations are performed with eruptions at 15° N, at four equally spaced times of year. Sensitivity to eruption season of aerosol optical depth (AOD), clear-sky and all-sky shortwave (SW) radiative flux is quantified by first integrating each field for four years after the eruption, then calculating for each cumulative field the absolute or percent difference between the maximum and minimum response from the four eruption seasons. Eruption season has a significant influence on AOD and clear-sky SW radiative flux anomalies for both eruption magnitudes. The sensitivity to eruption season for both fields is generally weak in the tropics, but increases in the mid- and high latitudes, reaching maximum values of ~75 %. Global mean AOD and clear-sky SW anomalies show sensitivity to eruption season on the order of 15–20 %, which results from differences in aerosol effective radius for the different eruption seasons. Smallest aerosol size and largest cumulative impact result from a January eruption for Pinatubo-magnitude eruption, and from a July eruption for the near-super eruption. In contrast to AOD and clear-sky SW anomalies, all-sky SW anomalies are found to be insensitive to season of eruption for the Pinatubo-magnitude eruption experiment, due to the reflection of solar radiation by clouds in the mid- to high latitudes. However, differences in all-sky SW anomalies between eruptions in different seasons are significant for the larger eruption magnitude, and the ~15 % sensitivity to eruption season of the global mean all-sky SW anomalies is comparable to the sensitivity of global mean AOD and clear-sky SW anomalies. Our estimates of sensitivity to eruption season are larger than previously reported estimates: implications regarding volcanic AOD timeseries reconstructions and their use in climate models are discussed.

Description: The preconditioning of major sudden stratospheric warmings (SSWs) is investigated with two long time series using reanalysis (ERA-40) and model (MAECHAM5/MPI-OM) data. Applying planetary wave analysis, we distinguish between wavenumber-1 and wavenumber-2 major SSWs based on the wave activity of zonal wavenumbers 1 and 2 during the prewarming phase. For this analysis an objective criterion to identify and classify the preconditioning of major SSWs is developed. Major SSWs are found to occur with a frequency of six and seven events per decade in the reanalysis and in the model, respectively, thus highlighting the ability of MAECHAM5/MPI-OM to simulate the frequency of major SSWs realistically. However, from these events only one quarter are wavenumber-2 major warmings, representing a low (similar to 0.25) wavenumber-2 to wavenumber-1 major SSW ratio. Composite analyses for both data sets reveal that the two warming types have different dynamics; while wavenumber-1 major warmings are preceded only by an enhanced activity of the zonal wavenumber-1, wavenumber-2 events are either characterized by only the amplification of zonal wavenumber-2 or by both zonal wavenumber-1 and zonal wavenumber-2, albeit at different time intervals. The role of tropospheric blocking events influencing these two categories of major SSWs is evaluated in the next step. Here, the composite analyses of both reanalysis and model data reveal that blocking events in the Euro-Atlantic sector mostly lead to the development of wavenumber-1 major warmings. The blocking-wavenumber-2 major warming connection can only be statistical reliable analyzed with the model time series, demonstrating that blocking events in the Pacific region mostly precede wavenumber-2 major SSWs.

Description: A comprehensive quality assessment of the ozone products from 18 limb-viewing satellite instruments is provided by means of a detailed inter-comparison. The ozone climatologies in the form of monthly zonal mean time series covering the upper troposphere to lower mesosphere are obtained from LIMS, SAGE I, SAGE II, UARS-MLS, HALOE, POAM II, POAM III, SMR, OSIRIS, SAGE III, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, and SMILES within 1978-2010. The inter-comparisons focus on mean biases based on monthly and annual zonal mean fields, on inter-annual variability and on seasonal cycles. Additionally, the physical consistency of the data sets is tested through diagnostics of the quasi-biennial oscillation and the Antarctic ozone hole. The comprehensive evaluations reveal that the uncertainty in our knowledge of the atmospheric ozone mean state is smallest in the tropical middle stratosphere and in the midlatitude lower/middle stratosphere, where we find a 1σ multi-instrument spread of less than ±5%. While the overall agreement among the climatological data sets is very good for large parts of the stratosphere, individual discrepancies have been identified including unrealistic month-to-month fluctuations, large biases in particular atmospheric regions, or inconsistencies in the seasonal cycle. Notable differences between the data sets exist in the tropical lower stratosphere and at high latitudes, with a multi-instrument spread of ±30% at the tropical tropopause and ±15% at polar latitudes. In particular, large relative differences are identified in the Antarctic polar cap during the time of the ozone hole, with a spread between the monthly zonal mean fields of ±50%. Differences between the climatological data sets are suggested to be partially related to inter-instrumental differences in vertical resolution and geographical sampling. The evaluations as a whole provide guidance on what data sets are the most reliable for applications such as studies of ozone variability, model-measurement comparisons and detection of long-term trends. A detailed comparison versus SAGE II data is presented, which can help identify suitable candidates for long-term data merging studies.

Description: This PhD thesis consists of three scientific articles. In the first one the tropical sea level pressure (SLP) response under global warming is investigated in a multi-model ensemble (MMEns) of climate models from the 3rd phase of the Coupled Model Intercomparison Project (CMIP3) and in ERA Interim reanalysis data. In this article we follow the idea to split up the tropical warming into a spatial homogeneous part and a spatial inhomogeneous part. Even though the inhomogeneous warming is roughly 10 times smaller than the homogeneous warming, the tropical SLP changes are strongly related to the inhomogeneous tropospheric temperature (Ttropos) changes under global warming. According to the Bjerknes Circulation Theorem, this strong relationship can be explained with a nearly direct circulation between a warmer and a colder air column, due to weak Coriolis force in the tropics, with rising air and low SLP in the warmer air column and sinking air and high SLP in the colder air column. The inhomogeneous Ttropos warming is dominated by the land-sea warming contrast. Also SLP trends show a land-sea contrast due to the strong link with Ttropos. It is charactericsed by a SLP decrease over South America and Africa and a SLP increase over the Indo-Pacific. This result is confirmed in an idealised sensitivity experiment. The trends of SLP and Ttropos in ERA Interim over the period from 1989 until 2010 are also strongly linked to each other, but here the land-sea warming contrast is not the dominant driver of Ttropos warming, indicating that the trends over the last decades are mostly due to internal variability. The second article presents a method for comparing the spatial patterns of variability on the basis of Empirical Orthogonal Function (EOF) modes. This method is called Distinct Empirical Orthogonal Function (DEOF) analysis and can be used to find differences in the patterns of variability e.g. between past and future climate or between models and observations. It compares the whole multivariate structure of two datasets and finds the patterns with the largest difference in explained variance between those. We illustrate in well-defined artificial examples how changes in the modes of variability (an intensification, a shift or a multivariate change) can be revealed with this method. Further we compare our approach with examples from the literature. This includes changes in the modes of variability under global warming in SLP over the North Atlantic and Europe, SLP of the Southern Hemisphere, surface temperature of the Northern Hemisphere, sea surface temperature of the North Pacific and precipitation in the tropical Indo-Pacific. In the third article the changes in the equatorial zonal circulation cells, with its most prominent part the Walker Circulation, are investigated in a MMEns of CMIP3 and CMIP5 models under global warming. In the mean state we found a weakening of the zonal circulation cells as well as an eastward shift of the Walker cell. The global warming trend has a similar structure to the variability in zonal stream function that is associated with El Nino Southern Oscillation (ENSO). Indeed, two third of the Walker Circulation changes be explained by a trend towards more El-Nino-like conditions. Under global warming the modes of variability show an eastward shift of the dominant mode, which is consistent with the El-Nino-like trend and a spatial non-linearity found in ENSO associated variability. The CMIP3 and CMIP5 MMEns show in nearly all aspects very similar results, which underlines the robustness of these results. The trends in ERA Interim reanalysis data over the period from 1979 to 2012 show a westward shift and a strengthening of the Walker Circulation, hence in contrast to the CMIP MMEns a trend towards more La-Nina-like conditions. The trend in dominant mode of variability can here explain three quarter of the Walker Circulation changes. Thus long-term trends of the Walker Circulation seem to follow, to a large part, the pre-existing dominant mode of internal variability.

Description: Reconstructions of the atmospheric sulfate aerosol burdens resulting from past volcanic eruptions are based on ice core-derived estimates of volcanic sulfate deposition and the assumption that the two quantities are directly proportional. We test this assumption within simulations of tropical volcanic stratospheric sulfur injections with the MAECHAM5-HAM aerosol-climate model. An ensemble of 70 simulations is analyzed, with SO2 injections ranging from 8.5 to 700 Tg, with eruptions in January and July. Modeled sulfate deposition flux to Antarctica shows excellent spatial correlation with ice core-derived estimates for Pinatubo and Tambora, although the comparison suggests the modeled flux to the ice sheets is 4–5 times too large. We find that Greenland and Antarctic deposition efficiencies (the ratio of sulfate flux to each ice sheet to the maximum hemispheric stratospheric sulfate aerosol burden) vary as a function of the magnitude and season of stratospheric sulfur injection. Changes in simulated sulfate deposition for large SO2 injections are connected to increases in aerosol particle size, which impact aerosol sedimentation velocity and radiative properties, the latter leading to strong dynamical changes including strengthening of the winter polar vortices, which inhibits the transport of stratospheric aerosols to high latitudes. The resulting relationship between Antarctic and Greenland volcanic sulfate deposition is nonlinear for very large eruptions, with significantly less sulfate deposition to Antarctica than to Greenland. These model results suggest that variability of deposition efficiency may be an important consideration in the interpretation of ice core sulfate signals for eruptions of Tambora-magnitude and larger.

Description: The atmospheric deposition of both macronutrients and micronutrients plays an important role in driving primary productivity, particularly in the low-latitude ocean. We report aerosol major ion measurements for five ship-based sampling campaigns in the western Pacific from similar to 25 degrees N to 20 degrees S and compare the results with those from Atlantic meridional transects (similar to 50 degrees N to 50 degrees S) with aerosols collected and analyzed in the same laboratory, allowing full incomparability. We discuss sources of the main nutrient species (nitrogen (N), phosphorus (P), and iron (Fe)) in the aerosols and their stoichiometry. Striking north-south gradients are evident over both basins with the Northern Hemisphere more impacted by terrestrial dust sources and anthropogenic emissions and the North Atlantic apparently more impacted than the North Pacific. We estimate the atmospheric supply rates of these nutrients and the potential impact of the atmospheric deposition on the tropical western Pacific. Our results suggest that the atmospheric deposition is P deficient relative to the needs of the resident phytoplankton. These findings suggest that atmospheric supply of N, Fe, and P increases primary productivity utilizing some of the residual excess phosphorus (P*) in the surface waters to compensate for aerosol P deficiency. Regional primary productivity is further enhanced via the stimulation of nitrogen fixation fuelled by the residual atmospheric iron and P*. Our stoichiometric calculations reveal that a P* of 0.1 mu mol L-1 can offset the P deficiency in atmospheric supply for many months. This study suggests that atmospheric deposition may sustain similar to 10% of primary production in both the western tropical Pacific.