ALBERT

Description: Seamounts, such as the Ewing Seamount in the south-east Atlantic, offer a habi- tat for Vulnerable Marine Ecosystems (VMEs) with increased biological activity compared to the surrounding deep sea. This is due to characteristic circulation features, such as a ’Taylor cap’ with a horizontal anticyclonic cell and vertical cir- culation cells that transport nutrient-rich water from the depths to the slopes and summit of the seamount. With the help of ocean model data and Lagrangian simu- lations, favourable conditions for marine species can be observed and quantified. One method for this is the description of the ’retention potential’, the likelihood of particles, for example nutrients or larvae, to remain in a certain area. Experiments with Parcels were carried out to investigate how the different model formulations and resolutions of INALT20 and ROMS-AGRIF affect the particle spreading at the Ewing Seamount. These model configurations differ both in the horizontal and ver- tical resolution, the formulation of the vertical axis and the atmospheric and tidal forcing. With coarser resolution and a vertical axis with levels of constant depth, there is no anticyclonic cell around the summit. Additionally, velocities near the bathymetry are generally low compared to when using terrain-following coordi- nates. As a result, particles are carried away from the seamount at a slower rate, which increases the retention potential. Tides, in turn, reduce this potential, and an atmospheric forcing with interannual variability contributes to greater variabil- ity in the particle distribution.

Description: The Agulhas Leakage (AL) transports warm and salty Indian Ocean waters into the Atlantic Ocean and as such is an important component of the global ocean circulation. These waters are part of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC) and AL variability has been linked to AMOC variability. The AL is expected to increase under a warming climate due to a shift in the Southern Hemisphere westerlies, which could further influence the AMOC dynamics. This study investigates the AL transport variability on long time scales in the pre-industrial and under a warming climate and its relation to the AMOC. It uses a high-resolution configuration of the Community Earth System Model (CESM) with a nominal horizontal resolution of 0.1° for the ocean and sea-ice and 0.25° for the atmosphere and land, which resolves the necessary spatial scales. The simulated AL transport of 19.7 ± 3 Sv lies well within the observed range of 21.3 ± 4.7 Sv. A positive correlation between the Agulhas Current and the AL is shown, meaning that an increase of the Agulhas Current transport leads to an increase in AL. Furthermore, the salt flux associated with the AL influences AMOC dynamics through the salt-advection feedback by reducing the AMOC’s freshwater transport at 34°S. In a warming climate, the AL transport was indeed found to increase due to strengthened and southward shifting winds while the Agulhas Current transport was found to decrease. Consequently, a larger fraction of the Agulhas Current will flow into the Atlantic Ocean rather than being recirculated into the Indian Ocean. The increase in AL is accompanied by a higher salt flux into the Atlantic Ocean, which destabilises the AMOC within the salt-advection-feedback. But whether and to what extent this additional salt advected to the North Atlantic could also dampen an AMOC weakening induced by increased meltwater input under climate change still needs further research.

Description: In the Labrador Sea, mesoscale eddies have been identified as a major exchange agent between the fast flowing boundary currents and the quiescent interior Labrador Sea. This way, the eddies contribute to heat, freshwater and property fluxes and impact deep convection and carbon uptake. It is therefore of interest to carefully analyse the occurrence, dynamics and water mass characteristics of mesoscale eddies in the Labrador Sea. Here, four years of moored instrument time series data are analyzed for eddy occurrences. A semi-automatic method for eddy detection in moored velocity data was developed and the eddie’s time series data were fit to a Rankine vortex model in order to estimate eddy characteristics. Over the four years, three cyclonic and seven anticyclonic eddies have been detected with this method. Surprisingly, most eddies differ in their characteristics and structures from the eddies reported in earlier studies, namely Irminger Rings, Convective Eddies, and Boundary Current Eddies. In particular, no Irminger Ring was found but a cyclonic, bottom-intensified warm core eddy, which has not been reported in this area before.

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: Atmospheric blocking is a persistent pressure pattern, which blocks the westerly flow in the mid-latitudes and strongly impacts weather both in its immediate vicinity and in regions upstream and downstream of it. Extreme events cause droughts in summer ( e.g. Russian heat wave 2010) and cold spells in winter ( e.g. European winter 1962\63). Blockings are also connected to Sudden Stratospheric Warmings (SSWs). In winter 1962\ 63 a blocking occured both before and after an SSW. However, blocking dynamics are not well understood and the phenomenon is not well-represented in climate models. To detect blockings, different definitions were derived, but deduced blocking frequencies are dependent on the blocking index. In the first part of this study, climatologies are derived from reanalysis data, to detect differences between index definitions. The indices by Lejenas and 0kland (1983) (LO*), Tibaldi and Molteni (1990) (TM) Pelly and Hoskins (2003) (PH) detect patterns, which show a reversal of the climatological flow. Anomalous high pressure systems are detectable with the index of Sausen et al. (1995) (SA). All indices show frequency maxima over the Euro-Atlantic region and the Pacific. The highest frequencies occur in winter. The TM and LO* indices show further maxima in spring and the PH index shows an additional frequency maximum in summer. Short blockings are more evenly distributed in location and season than long-lasting blocking episodes, which are accumulated over the ocean basins in winter. The second aim of the study is the investigation of the insufficient representation of blocking in climate models. Hence, climatologies are derived for CESM(WACCM). The model underestimates blockings, especially over the Euro-Atlantic region in winter. The frequency changes over the Pacific are dependent on the index. Discrepancies between the sectors are probably caused by the misrepresentation of transient eddies in the Atlantic region, which are neccessary for the formation and persistence of blockings. It is suggested that the model resolution is not high enough to simulate transient eddies, but further investigation is needed. A realistic mean flow in the model is also required. Mean flow biases could cause the underestimation over the Pacific. To investigate influences of external forcings, different model experiments were used. Increasing greenhouse gases could lead to a decrease in blocking frequency; El Nifio Southern Oscillation events probably cause a decrease over the Pacific. But these results have to be handled with care as the misrepresentation of blocking frequencies predominates differences between model runs.

Description: Anomalies of the winter stratospheric polar vortex can propagate down to tropospheric levels and modulate variability patterns, such as the North Atlantic Oscillation (NAO), which is the leading mode of variability in the North Atlantic (NA) sector during boreal winter. Not only is the NAO important for European winter weather conditions, but the NAO related heat and freshwater fluxes, and the associated changes in westerly wind over the NA region, also influence the formation of deep water masses in the NA basin and can thereby influence the variability of the Atlantic meridional overturning circulation (AMOC). The northward transport of heat by the AMOC is very important for European climate and the variability of the AMOC is therefore of great interest. To investigate the role of the stratosphere for variability over the North Atlantic sector, two state-of-the-art ocean-atmosphere general circulation models are used: a high-top model (CESM1(WACCM)) and a low-top model (CCSM4). For each model, a Control simulation is analyzed and compared to a simulation under the Intergovernmental Panel on Climate Change (IPCC)’s RCP8.5 scenario, which represents the worst case scenario of greehouse gas (GHG) emissions. Strong and weak vortex events are defined using the Northern Annular Mode (NAM), which is also used to describe the downward propagation of these anomalies. In the low-top model the downward propagation of stratospheric NAM anomalies to the surface is not well captured, but it is very well represented in the high-top model. This simulated difference in stratosphere-troposphere coupling is also reflected in the simulated effects of the stratosphere on the surface atmosphere and ocean parameters. While stratospheric vortex events in the high-top model are connected to NAO-like anomalies at the surface (in sea level pressure, turbulent heat flux and surface wind stress), in the low-top model this connection is less pronounced. No significant changes in mixed layer depth (MLD), which is used as an indicator for deep water formation, are found in the low-top model. The high-top model, on the other hand, shows a strong connection between stratospheric polar vortex events and MLD anomalies (strong (weak) vortex events are connected to deeper (shallower) MLDs), especially in the Labrador Sea, which is an important area of deep water formation in the NA region. A cross-correlation analysis of the NAM/NAO and AMOC shows that the NAO leads the AMOC by about 4 years in both, the high and low-top model Control simulations. While the stratospheric NAM is also highly correlated with the AMOC in the high-top model (peaking when the NAM leads the AMOC by 2 years), there is no resonable correlation between NAM and AMOC in the low-top model. Under global warming the correlation between the AMOC and NAO decreases for both models. In the case of the high-top model, the NAM and AMOC are more strongly correlated than the NAO and AMOC under the GHG scenario.