ALBERT

Description: Understanding the dynamical influence of the 11-year solar cycle on Earth’s climate andits tropospheric effects is important in order to improve decadal climate prediction. Moststudies which investigate the downward transfer of the solar cycle signal focus on theNorthern Hemisphere (NH). In contrast, the dynamical influence of the 11-year solarcycle on the Southern Hemisphere (SH) is analyzed in this thesis with an exceptionallylong (999 years) high-top model simulation of CESM1(WACCM)under pre-industrialconditions but time-varying solar cycle forcing. The constructed solar forcing only includessolar variabilities on timescales up to the 11-year solar cycle and hence neglects long-termsecular changes of the solar irradiance such as the Gleissberg cycle.The strong relative increase in UV radiation during solar maximum (SolMax) induces awarming at the tropical upper stratosphere which leads to a strengthening of the merid-ional temperature gradient in the upper stratosphere from July to September in the SH.As a result of the thermal wind balance, a westerly wind anomaly appears in the mid-latitudinal upper stratosphere during SolMax which propagates downward and polewardfrom July to October. The interactions of planetary waves with the zonal mean flow leadto a dynamical solar response indicated by the onset of the Eliassen–Palm flux (E–Pflux) divergence difference in July which coincides with the beginning of the downwardpropagation.The zonal mean westerly wind anomaly is expected to strengthen the polar vortex whichwould result in a positive annular mode pattern as seen in theNH. However, the horizontalstructure of geopotential height anomalies exhibits a polar vortex shift towards the Pacificfrom August to October in the SH during SolMax instead of a strengthening. Althoughthese geopotential height differences are found by other studies, these studies only examinethe downward transfer in zonal mean analysis. However, in this study the regional sectoranalyses reveal that the strongest westerly wind anomaly inthe Indian ocean occurs inJuly whereas in the Atlantic sector the strongest anomaly is found in August. Both regionsshow the typical poleward propagation of the westerly wind anomaly which is missing inthe Pacific region. The Pacific ocean sector is the only sectorwhich reveals statisticallysignificant tropospheric impacts on the zonal wind from August to October which areconnected to the stratosphere by a downward transfer. This highlights the importance toinvestigate regional differences and suggests that the zonal mean approach is insufficientto examine the top-down mechanism in the SH.

Description: The inflow of fresh or brackish groundwater into the sea is referred to as Submarine Groundwater Discharge (SGD). The SGD is enforced by a terrestrial component which mainly depends on freshwater extraction and recharge by meteoric water and on aquifer permeability. And a marine component that is mainly controlled by the spatial distribution of outflows and water depth (hydraulic gradients between land and sea). This studyis motivated by the importance of freshwater in arid regions and, in particular, by the continuous challenges posed by the exploration and exploitation of fresh water sources inthe Sultanate of Oman. Moreover, there is a lack of studies on SGD phenomena along the 1000 km coastline in the South of Oman. The objective of this study is to develop a method to detect SGD spots in the offshore region, autonomously, and understanding the hydrodynamics of the discharge seepage for future backtracking, quantification and coastaland groundwater management. The study area Salalah, Dhofar Governorate, South of Oman is known to have a high natural groundwater recharge during the monsoon seasonand a karstic coastal seafloor, which results in a high potential of submarine groundwater discharge spots. A geochemical tracer (radon-222) was selected to detect SGD using anRTM1688-2 radon sensor instrument. This sensor underwent experimental design performance tests to adopt to mobile offshore platform monitoring i.e Wave Glider (WG). Groundwater characteristics (i.e.Rn222, salinity, temperature) aquifers were first deter-mined by measuring Salalah’s coastal onshore groundwater wells. The preliminary radonactivity results offshore Salalah demonstrated a distinctive radon concentration gradient between groundwater and seawater with an enrichment factor of up to 4000, which is idealfor signal preservation in a freshwater plume until reaching the sea surface. Accordingto the collected morphological data and the hydrological data from literature, Salalahis found to be the best potential location for SGD. A numerical integral plume model(TAMOC - Texas A&M Oil spill Calculator) was used to investigate the detectability of single-phase freshwater plumes. The model considers the local freshwater characteristics, geomorphological and oceanographic constraints at different discharge rates anddischarge buoyancy. After validating the plume model with literature data derived from laboratory experiment, detection limit guidance for autonomous monitoring of SGDs was established from each parameter: ambient cross-flow velocity, initial discharge salinity/temperature, initial discharge velocity, initial diameter discharge and initial dischargeradon activity concentration. Moreover, groundwater flow rates of a recently investigated SGD (Dhalkut SGD) in the area could be estimated using the plume model. It is shown that measured physical and chemical oceanographic parameters’ combined with ground-water well data provide a well constrained data set to simulate SGD off Salalah quite well and could give realistic values for SGD plumes in the area. The outcome of the model was utilized to find best practices for SGD detection by using autonomous surface vehicles (i.e. Wave Glider).

Description: The decadal variability of El Niño / Southern Oscillation (ENSO) is investigated in the preindustrial control run of the GFDL-ESM2M fully coupled climate model. Overall, the climate model has quite a realistic representation of relevant ENSO properties: the probability distribution of Niño3.4 sea surface temperature (SST) anomalies is positively skewed, the highest equatorial Pacific SST variability is observed in boreal winter with the corresponding decrease in variability during spring, and the decadal climate variability shows a shift of the ENSO spatial pattern. Nevertheless, compared to the ERA-20C reanalysis product, the model shows problems most climate models have: the anomalous cold equatorial Pacific SST with the largest bias located on the eastern side, strong easterly winds over the western equatorial region, the rising branch of the Walker Circulation located too far west and the too strong subsidence regime east of the date line. Two main periods of about 60 years with high and low ENSO amplitudes are observed, ranging between l.5° C and 0.7°C. Here it is shown, that the High and Low epochs have remarkably different mean states, which can explain the differences in simulated ENSO amplitudes. The High epoch is characterized by a weaker zonal equatorial SST gradient and a warmer Niño3 SST. The less intense Walker Circulation reduces the subsidence branch, and the negative shortwave (SW) feedback during El Niño events is extended over the Niño3 domain. The stronger convective response over the eastern equatorial Pacific enhances the SST variability, increasing considerably during boreal winter, and the strong non-linearities in atmospheric feedbacks are kicked forming strong East Pacific-like (EP) El Niño events. Hence, the ENSO asymmetry is remarkably incremented. During the Low epoch, the zonal equatorial SST gradient is increased with cooler Niño3 SST. The Walker Circulation is intensified and the subsidence branch over the Niño3 region is strengthened. The Niño3 domain also coincides with the reduction of the negative SW feedback during El Niño events, as well as the incapability of the atmospheric regime to turn into a convective state, when SST anomalies are turned positive. In addition, the Niño3.4 SST variability and the wind feedback are considerably decreased during boreal winter. There are indications that the reduced SST variability of the Low epoch is caused by the too strong subsidence branch over the Niño3 region, which restricts the seasonal southward migration of the Intertropical Convergence Zone (ITCZ), and hampers the evolution of strong EP El Niño events. However, the convective response is maintained over the western equatorial Pacific, outside of the strongest mean subsidence region, as shown by the highest negative SW feedback. Therefore, during this time period the frequency of Central Pacific-like (CP) El Niño events is increased, shifting the ENSO spatial pattern, and reducing SST variability in lack of strong EP El Niños. Correspondingly, the non-linearities between the positive and negative phases of ENSO are reduced, diminishing the ENSO asymmetry. In summary, these results show how important the mean state is for the ENSO amplitude and asymmetry.

Description: The biogeochemistry of an observed cold filament off Peru and the representation of filaments in submesoscale-permitting (1/45°) coupled biogeochemical model simulations are the focus of this thesis. Furthermore, we use the simulations to investigate the effect of submesoscale frontal processes on the distribution of biogeochemical tracers in the shallow oxygen minimum zone off Peru. The observed filament contains relatively cold, fresh and nutrient-rich waters originating in the coastal upwelling. Enhanced nitrate concentrations and offshore velocities of up to 0.5 m/s within the filament suggest an offshore transport of nutrients. Despite low chlorophyll α concentrations in the core of the filament, depth integrated primary production is 40% higher than at the upwelling front and 25% higher than offshore. The highly variable relationship of surface chlorophyll α and depth-integrated primary production highlights the inherent uncertainty of primary production estimates based on ocean-color measured by satellites. The observations are used to assess the results of two different biogeochemical model simulations (PISCES / BioEBUS). Both simulations exhibit filaments that are similar in lateral scale, horizontal and vertical structure and offshore extent to those observed, but differences exist in the biogeochemistry: While the PISCES simulation exhibits nitrate concentrations within filaments comparable to observations, filaments are largely depleted of nitrate in the BioEBUS simulation. This difference can be related to a higher pyhtoplankton growth rate and faster nitrate uptake in BioEBUS. The importance of sufficiently slow phytoplankton growth for maintaining realistic concentrations of upwelled nutrients offshore is therefore stressed. Furthermore, the simulations suggest that submesoscale frontal processes increase subduction and offshore export of nitrate which leads to reduced primary production. An increase in oxygen that resembles the pattern of the decrease in nitrate suggests a ventilation of the shallow oxygen minimum zone off Peru by vertical and horizontal eddy-fluxes.

Description: The study investigates salinity, depth and thickness of the isopycnals long term changes in the north northern tropical Atlantic through the density range 1026.8:1027.3 kg m-3, where oxygen minimum zone is situated. The salinity front is situated at around l8° N in the chosen density range and hydrographic property evolution is found being different to the north and south domains of the salinity front. Higher salinities during 70s than 2000-2009 period are estimated in the northern domain for all isoclines, but lower in the southern domain. Strong increase in density layer depths is observed in the southern domain intermediate water masses, i.e. Antarctic Intermediate water masses. The strongest correlation between depth and salinity anomalies is found to be -0.5 for the Antarctic Intermediate water masses, being about twice !arger than the estimates for any other water mass. In the end, the salinity front parameters such as width, gradient and variability are described over entire north tropical Atlantic.