ALBERT

Description: Influences from the Tropics, the stratosphere and the specification of observed sea surface temperature and sea-ice (SSTSI) on Northern Hemisphere winter mean circulation anomalies during the period 1960/61 to 2001/02 are studied using a relaxation technique applied to the ECMWF model. On interannual time-scales, the Tropics strongly influence the Pacific sector but also the North Atlantic sector, although weakly. The stratosphere is found to be influential on the North Atlantic Oscillation (NAO) on interannual time-scales but is less important over the Pacific sector. Adding the observed SSTSI to the tropical relaxation runs generally improves the model performance on interannual time-scales but degrades/enhances the model’s ability to capture the 42-year trend over the Pacific/Atlantic sector. While relaxing the stratosphere to the reanalysis fails to capture the trend over the whole 42-year period, the stratosphere is shown to be influential on the upward trend of the NAO index from 1965 to 1995, but with reduced amplitude compared to previous studies. Influence from the Tropics is found to be important for the trend over both time periods and over both sectors although, across all experiments, we can account for only 30% of the amplitude of the hemispheric trend. Copyright c� 2012 Royal Meteorological Society

Description: Eden and Olbers have discussed the relationship between bottom pressure torque and bolus velocity in the western boundary current using the vertically truncated BARBI model approach. Here we revisit this issue using the much simpler residual mean framework. The central role played by a density equation that is linearised about a state of rest is discussed, as well as mechanisms required to maintain the baroclinicity of the western boundary current. We conclude that in the framework being considered by Eden and Olbers, frictional processes must play an important role in the western boundary current dynamics, otherwise the baroclinicity of the current is completely removed by the cross-front mixing effect of the eddies. We also derive the form of the Stommel equation obtained by Eden and Olbers in a manner which clarifies the approximations made by these authors. We argue that for their analysis to be valid, the flow must be concentrated in a shallow layer compared to the ocean depth, there must be no density structure at the sea floor, and any overturning circulation, whether directly wind-driven or as a part of the global thermohaline circulation, must be much smaller than the western boundary current transport.

Description: Highlights: • Global mean sea level simulated in interannual CORE simulations. • Regional sea level patterns simulated in interannual CORE simulations. • Theoretical foundation for analysis of global mean sea level and regional patterns. Abstract: We provide an assessment of sea level simulated in a suite of global ocean-sea ice models using the interannual CORE atmospheric state to determine surface ocean boundary buoyancy and momentum fluxes. These CORE-II simulations are compared amongst themselves as well as to observation-based estimates. We focus on the final 15 years of the simulations (1993–2007), as this is a period where the CORE-II atmospheric state is well sampled, and it allows us to compare sea level related fields to both satellite and in situ analyses. The ensemble mean of the CORE-II simulations broadly agree with various global and regional observation-based analyses during this period, though with the global mean thermosteric sea level rise biased low relative to observation-based analyses. The simulations reveal a positive trend in dynamic sea level in the west Pacific and negative trend in the east, with this trend arising from wind shifts and regional changes in upper 700 m ocean heat content. The models also exhibit a thermosteric sea level rise in the subpolar North Atlantic associated with a transition around 1995/1996 of the North Atlantic Oscillation to its negative phase, and the advection of warm subtropical waters into the subpolar gyre. Sea level trends are predominantly associated with steric trends, with thermosteric effects generally far larger than halosteric effects, except in the Arctic and North Atlantic. There is a general anti-correlation between thermosteric and halosteric effects for much of the World Ocean, associated with density compensated changes.

Description: A set of relaxation experiments using the ECMWF atmospheric model is used to analyse the severe European winter of 1962/63. We argue that the severe winter weather was associated with a wave train that originated in the tropical Pacific sector (where weak La Nina conditions were present) and was redirected towards Europe, a process we suggest was influenced by the combined effect of the strong easterly phase of the Quasi-Biennial Oscillation (QBO ) and unusually strong easterly winds in the upper equatorial troposphere that winter. A weak tendency towards negative North Atlantic Oscillation (NAO) conditions in December, associated with extratropical sea surface temperature and sea-ice anomalies, might have acted as a favourable preconditioning. The redirection of the wave train towards Europe culminated in the stratospheric sudden warming at the end of January 1963. We argue that in February, the sudden warming event helped maintain the negative NAO regime, allowing the severe weather to persist for a further month. A possible influence from the Madden-Julian Oscillation, as well as a role for internal atmospheric variability, is noted.

Description: Highlights: • North Atlantic sea surface temperature exhibits high decadal predictability potential. • Model bias hinders exploiting the decadal predictability potential. • An innovative method was developed to overcome some of the bias problem. • North Atlantic sea surface temperature will stay anomalously warm until about 2030. Abstract: The Atlantic Meridional Overturning Circulation (AMOC), a major current system in the Atlantic Ocean, is thought to be an important driver of climate variability, both regionally and globally and on a large range of time scales from decadal to centennial and even longer. Measurements to monitor the AMOC strength have only started in 2004, which is too short to investigate its link to long-term climate variability. Here the surface heat flux-driven part of the AMOC during 1900–2010 is reconstructed from the history of the North Atlantic Oscillation, the most energetic mode of internal atmospheric variability in the Atlantic sector. The decadal variations of the AMOC obtained in that way are shown to precede the observed decadal variations in basin-wide North Atlantic sea surface temperature (SST), known as the Atlantic Multidecadal Oscillation (AMO) which strongly impacts societally important quantities such as Atlantic hurricane activity and Sahel rainfall. The future evolution of the AMO is forecast using the AMOC reconstructed up to 2010. The present warm phase of the AMO is predicted to continue until the end of the next decade, but with a negative tendency.

Description: The seasonal and spatial variation of the suspended sediment concentration (SSC) in the Yellow Sea and the East China Sea (YSECS) is studied using turbidity, temperature and velocity field data from all seasons of 2011. The important hydrodynamic factors affecting the SSC distribution are discussed, based on the field data. The data provide a picture of the seasonal evolution of the SSC in the YSECS in a single year. The results show that the highest surface layer SSC was measured in spring and winter, while the summer had the lowest surface layer SSC. In the bottom layer, the SSC distribution patterns were similar in all seasons except that a much higher SSC water plume extended from the Subei Shoal to the East China Sea shelf in winter survey. The vertical SSC gradient was stronger in the shallow water area than the deep water area, and it was stronger in summer and autumn than in winter and spring. We argue that wind-induced vertical mixing dominated the SSC distribution in the surface layer while the tidal currents played an important role in the bottom layer and sub-tidal currents dominated the horizontal pattern of the SSC distribution pattern over the shelf area. In particular, the presence of the summer thermocline limited the vertical mixing in summer, decreasing the SSC in the surface layer. The front between the East China Sea Coastal Current and the Taiwan Warm Current appears to prevent the high SSC water from being transported to the shelf. The sediment discharged by the Changjiang River was found to be concentrated in the estuary area and influenced the shelf SSC distribution only through the action of the subtidal currents.

Description: The phase and the amplitude of the North Atlantic Oscillation (NAO) are influenced by numerous factors, which include Sea Surface Temperature (SST) anomalies in both the Tropics and extratropics and stratospheric extreme events like Stratospheric Sudden Warmings (SSWs). Analyzing seasonal forecast experiments, which cover the winters from 1979/80–2013/14, with the European Centre for Medium-Range Weather Forecast model, we investigate how these factors affect NAO variability and predictability. Building on the idea that the tropical influence might happen via the stratosphere, special emphasis is placed on the role of major SSWs. Relaxation experiments are performed, where different regions of the atmosphere are relaxed towards ERA-Interim to obtain perfect forecasts in those regions. By comparing experiments with relaxation in the tropical atmosphere, performed with an atmosphere-only model on the one hand and a coupled atmosphere–ocean model version on the other, the importance of extratropical atmosphere–ocean interaction is addressed. Interannual variability of the NAO is best reproduced when perfect knowledge about the NH stratosphere is available together with perfect knowledge of SSTs and sea ice, in which case 64% of the variance of the winter mean NAO is projected to be accounted for with a forecast ensemble of infinite size. The coupled experiment shows a strong bias in the stratospheric polar night jet (PNJ) which might be associated with a drift in the modelled SSTs resembling the North Atlantic cold bias and an underestimation of blockings in the North Atlantic/Europe sector. Consistent with the stronger PNJ, the lowest frequency of major SSWs is found in this experiment. However, after statistically removing the bias, a perfect forecast of the tropical atmosphere and allowing two-way atmosphere–ocean coupling in the extratropics seem to be key ingredients for successful SSW predictions. In combination with SSW occurrence, a clear shift of the predicted NAO towards lower values occurs.

Description: The parameterization of sub-grid scale processes is one of the key challenges towards improved numerical simulations of the atmospheric and oceanic circulation. Numerical weather prediction models as well as climate models would benefit from more sophisticated turbulence closures that allow for less spurious dissipation at the grid-scale and consequently higher and more realistic levels of eddy kinetic energy (EKE). Recent studies propose to use a hyperviscous closure in combination with an additional deterministic forcing term as a negative viscosity to represent backscatter of energy from unresolved scales. The sub-grid EKE is introduced as an additional prognostic variable that is fed by dissipation at the grid scale, and enables recycling of EKE via the backscatter term at larger scales. This parameterization was previously shown to work well in zonally re-entrant channel configurations. Here, a generalization in the form of a Rossby number-dependent scaling for the strength of the backscatter is introduced to represent the emergence of a forward energy-cascade in unbalanced flows near the boundaries. We apply the parameterization to a shallow water model of a double gyre basin and provide evidence for its general applicability. In terms of mean state and variability, a low resolution model is considerably improved towards a high resolution control run at low additional computational cost.

Description: This paper develops an analysis framework to identify how physical processes, as represented in ocean climate models, impact the evolution of global mean sea level. The formulation utilizes the coarse grained equations appropriate for an ocean model, and starts from the vertically integrated mass conservation equation in its Lagrangian form. Global integration of this kinematic equation results in an evolution equation for global mean sea level that depends on two physical processes: boundary fluxes of mass and the non-Boussinesq steric effect. The non-Boussinesq steric effect itself contains contributions from boundary fluxes of buoyancy; interior buoyancy changes associated with parameterized subgrid scale processes; and motion across pressure surfaces. The non-Boussinesq steric effect can be diagnosed in either volume conserving Boussinesq or mass conserving non-Boussinesq ocean circulation models, with differences found to be negligible. We find that surface heating is the dominant term affecting sea level arising from buoyancy fluxes, contributing to a net positive tendency to global mean sea level, largely due to low latitude heating and because the thermal expansion coefficient is much larger in the tropics than high latitudes. Subgrid scale effects from parameterized quasi-Stokes transport, vertical diffusion, cabbeling, and thermobaricity are also found to be significant, each resulting in a reduction of global mean sea level. Sea level rise through low latitude heating is largely compensated by a sea level drop from poleward eddy heat transport and ocean mixing. Spatial variations in the thermal expansion coefficient provide an essential modulation of how physical effects from mixing and eddy induced advective transport impact global mean sea level. Highlights: ► Theoretical framework for how physical processes impact global mean sea level in ocean models. ► Mathematical and physical specification of the non-Boussinesq steric effect. ► How boundary buoyancy fluxes and interior processes impact global mean sea level. ► Global model examples of the non-Boussinesq steric effect with associated budget for global mean sea level.

Description: We quantify seasonal prediction skill of tropical winter rainfall in 14 climate forecast systems. High levels of seasonal prediction skill exist for year‐to‐year rainfall variability in all tropical ocean basins. The tropical East Pacific is the most skilful region, with very high correlation scores, and the tropical West Pacific is also highly skilful. Predictions of tropical Atlantic and Indian Ocean rainfall show lower but statistically significant scores. We compare prediction skill (measured against observed variability) with model predictability (using single forecasts as surrogate observations). Model predictability matches prediction skill in some regions but it is generally greater, especially over the Indian Ocean. We also find significant inter‐basin connections in both observed and predicted rainfall. Teleconnections between basins due to El Niño–Southern Oscillation (ENSO) appear to be reproduced in multi‐model predictions and are responsible for much of the prediction skill. They also explain the relative magnitude of inter‐annual variability, the relative magnitude of predictable rainfall signals and the ranking of prediction skill across different basins. These seasonal tropical rainfall predictions exhibit a severe wet bias, often in excess of 20% of mean rainfall. However, we find little direct relationship between bias and prediction skill. Our results suggest that future prediction systems would be best improved through better model representation of inter‐basin rainfall connections as these are strongly related to prediction skill, particularly in the Indian and West Pacific regions. Finally, we show that predictions of tropical rainfall alone can generate highly skilful forecasts of the main modes of extratropical circulation via linear relationships that might provide a useful tool to interpret real‐time forecasts.