Displays

We start with the severe European winter of 1962/63, a winter when the North Atlantic Oscillation (NAO) index was strongly negative with persistent easterly wind anomalies across northern Europe and the British Isles. We then note that the NAO is a manifestation of synoptic Rossby wave breaking. The positive feedback with which synoptic eddies act to maintain the atmospheric jet stream against friction turns out to also be the mechanism by which the equatorial deep jets in the ocean are maintained against dissipation. We were fortunate to be able to demonstrate this in both a simple model set-up that supports deep jets and directly from mooring data on, and on either side of, the equator at 23 W in the Atlantic Ocean. The deep jets offer some potential for prediction over the neighbouring African continent on interannual time scales. This then leads to a brief discussion of the importance of the tropics for prediction on both seasonal and decadal time scales and longer, linking back to the winter of 1962/63.  The models we use for prediction not only contain surprisingly large biases but also require the parameterization of unresolved processes and some brief discussion will be given on the representation of mesoscale eddies in ocean models, such as are used in prediction systems and for making future climate projections.

How to cite: Greatbatch, R.: The North Atlantic Oscillation and related topics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4832, https://doi.org/10.5194/egusphere-egu2020-4832, 2020.

In this study we track and analyze eddy movement in the global ocean using 20 years of altimeter data and show that, in addition to the well-known westward propagation and slight polarity-based meridional deflections, mesoscale eddies also move randomly in all directions at all latitudes as a result of eddy-eddy interaction. The speed of this random eddy movement decreases with latitude and equals the baroclinic Rossby wave speed at about 25° of latitude. The tracked eddies are on average isotropic at mid and high latitudes, but become noticeably more elongated in the zonal direction at low latitudes. Our analyses suggest a critical latitude of approximately 25° that separates the global ocean into a low-latitude anisotropic wavelike regime and a high-latitude isotropic turbulence regime. One important consequence of random eddy movement is that it results in lateral diffusion of eddy energy. The associated eddy energy diffusivity, estimated using two different methods, is found to be a function of latitude. The zonal-mean eddy energy diffusivity varies from over 1500 m² s^-1 at low latitudes to around 500 m² s^-1 at high latitudes, but significantly larger values are found in the eddy energy hotspots at all latitudes, in excess of 5000 m² s^-1. Results from this study have important implications for recently-developed energetically-consistent mesoscale eddy parameterization schemes which require solving the eddy energy budget.  

How to cite: Zhai, X., Ni, Q., Wang, G., and Marshall, D.: Random movement of mesoscale eddies in the global ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-114, https://doi.org/10.5194/egusphere-egu2020-114, 2020.

We present a method to decompose the time mean vertically averaged transport, as simulated by an high-resolution ocean model, into its four dominant components. These components are driven by the gradient of potential energy per unit area (PE), the divergence of the flux of time mean momentum (MMF) and eddy momentum (EMF), and the wind stress. Since the local vorticity budget and the bathymetry are noisy and dominated by small spatial scales, a barotropic shallow water model is used as a filter to diagnose the respective transports instead of integrating along lines of constant f/H.
Applying this method to the output of a high-resolution model of the North Atlantic we find that PE is the most important driver, including the northwest corner. MMF is an important driver of transport around the Labrador Sea continental slope and, together with the EMF, it drives significant transport along the path of the Gulf Stream and North Atlantic current. Additionally, the circulation patterns driven by the EMF compares well with an estimate based on a satellite product. Hence, the presented method provides insights into the relative importance of the different dynamical processes that may drive barotropic transport in an ocean model. But it may also be used to isolate potential issues if a model misrepresents the barotropic transport.

How to cite: Claus, M., Wang, Y., Greatbatch, R., and Sheng, J.: Dissecting the Barotropic Transport in a High-resolution ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18490, https://doi.org/10.5194/egusphere-egu2020-18490, 2020.

The rapid propagation of boundary waves (or, equivalently, the strong influence of topography on vorticity balance) ensures that bottom pressure along the global continental slope reflects large scale ocean processes, making it possible to see through the fog of the mesoscale, which obscures many observable quantities. This fact is exploited in systems to monitor the Atlantic Meridional Overturning Circulation (AMOC). Here, we use diagnostics from an ocean model with realistic mesoscale variability to demonstrate two things. First: boundary pressures form an efficient method of monitoring AMOC variability. Second: pressures are remarkably constant along isobaths around the global continental slope, varying by less than 5 cm sea-level-equivalent over vast distances below the directly wind-driven circulation. In the latter context, the AMOC stands out as a clear exception, leading to a link between the AMOC and differences in the hydrography of entire ocean basins.

How to cite: Hughes, C. W., Williams, J., Blaker, A., and Coward, A. C.: The global dominance of the Atlantic circulation, seen through boundary pressures., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2211, https://doi.org/10.5194/egusphere-egu2020-2211, 2020.