ALBERT

Description: Mesoscale variability of velocities is an important part of the global ocean circulation, as it contains more kinetic energy than the mean flow over most of the ocean. Understanding its generation, dissipation and modulation processes therefore is crucial to better understand ocean circulation in general. In this thesis, a global 1/12◦ ocean model (ORCA12) is used to study the distribution of mean surface Eddy Kinetic Energy (EKE), its seasonal cycle and possible driving mechanisms, averaged over 26 years (1981-2007). For the calculation of EKE, the deviations from yearly mean horizontal velocities u, v are found to be best suitable. The model is then evaluated using EKE derived from satellite altimetry (AVISO). The total EKE from the model, including geostrophic parts, realistically reproduces the observed geostrophic mean EKE and its seasonal cycle. Seasonal cycles of surface EKE in the subtropical gyres, including most of the Western Boundary Currents (WBCs), peak in the summer months in both hemispheres. The mean EKE and amplitudes of the annual cycle are generally larger in the Pacific, compared to the Atlantic. The seasonal variations of EKE in the WBCs are driven by dissipation processes at the sea surface, namely the wind stress and thermal interactions with the atmosphere in winter. Only in the core regions of the currents other processes play a role as the surface EKE there peaks in winter/spring, not consistent with the dissipation hypothesis. The balance of dissipation and generation terms in the strong, chaotic WBCs, however, varies from year to year. In the subtropical gyres’ interior, dissipation is not solely responsible for the annual cycle. Instead, the vertical shear of near-surface horizontal velocities is found to peak in summer, in phase with the EKE. This seasonal cycle of the shear can be observed down to ∼ 150m depth, depending on the region. Inspections of profiles of horizontal velocity and EKE reveal the vertical shear to be associated with the velocity differences between the Mixed Layer and the interior ocean, possibly leading to instabilities which locally generate surface intensified EKE, largest in summer. Therefore, the seasonal cycle of near-surface vertical shear of horizontal velocities seems to be responsible for the seasonal variations of surface EKE, although the general source of EKE in the subtropical gyres remains unclear.

Description: We present a new framework for global ocean- sea-ice model simulations based on phase 2 of the Ocean Model Intercomparison Project (OMIP-2), making use of the surface dataset based on the Japanese 55-year atmospheric reanalysis for driving ocean-sea-ice models (JRA55-do).We motivate the use of OMIP-2 over the framework for the first phase of OMIP (OMIP-1), previously referred to as the Coordinated Ocean-ice Reference Experiments (COREs), via the evaluation of OMIP-1 and OMIP-2 simulations from 11 state-of-the-science global ocean-sea-ice models. In the present evaluation, multi-model ensemble means and spreads are calculated separately for the OMIP-1 and OMIP-2 simulations and overall performance is assessed considering metrics commonly used by ocean modelers. Both OMIP-1 and OMIP-2 multi-model ensemble ranges capture observations in more than 80% of the time and region for most metrics, with the multi-model ensemble spread greatly exceeding the difference between the means of the two datasets. Many features, including some climatologically relevant ocean circulation indices, are very similar between OMIP-1 and OMIP- 2 simulations, and yet we could also identify key qualitative improvements in transitioning from OMIP-1 to OMIP- 2. For example, the sea surface temperatures of the OMIP- 2 simulations reproduce the observed global warming during the 1980s and 1990s, as well as the warming slowdown in the 2000s and the more recent accelerated warming, which were absent in OMIP-1, noting that the last feature is part of the design of OMIP-2 because OMIP-1 forcing stopped in 2009. A negative bias in the sea-ice concentration in summer of both hemispheres in OMIP-1 is significantly reduced in OMIP-2. The overall reproducibility of both seasonal and interannual variations in sea surface temperature and sea surface height (dynamic sea level) is improved in OMIP-2. These improvements represent a new capability of the OMIP-2 framework for evaluating processlevel responses using simulation results. Regarding the sensitivity of individual models to the change in forcing, the models show well-ordered responses for the metrics that are directly forced, while they show less organized responses for those that require complex model adjustments. Many of the remaining common model biases may be attributed either to errors in representing important processes in ocean-sea-ice models, some of which are expected to be reduced by using finer horizontal and/or vertical resolutions, or to shared biases and limitations in the atmospheric forcing. In particular, further efforts are warranted to resolve remaining issues in OMIP-2 such as the warm bias in the upper layer, the mismatch between the observed and simulated variability of heat content and thermosteric sea level before 1990s, and the erroneous representation of deep and bottom water formations and circulations. We suggest that such problems can be resolved through collaboration between those developing models (including parameterizations) and forcing datasets. Overall, the present assessment justifies our recommendation that future model development and analysis studies use the OMIP-2 framework.