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: 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: 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: 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.

Description: Recent studies using reanalysis data and complex models suggest that the Tropics influence midlatitude blocking. Here, the influence of tropical precipitation anomalies is investigated further using a dry dynamical model driven by specified diabatic heating anomalies. The model uses a quasi‐realistic setup based on idealized orography and an idealized representation of the land‐ocean thermal contrast. Results concerning the El Niño Southern Oscillation and the Madden‐Julian Oscillation are mostly consistent with previous studies and emphasize the importance of tropical dynamics for driving the variability of blocking at midlatitudes. It is also shown that a common bias in models of the Coupled Model Intercomparison Project Phase 5 (CMIP5), namely, excessive tropical precipitation, leads to an underestimation of midlatitude blocking in our model, also a common bias in the CMIP5 models. The strongest blocking anomalies associated with the tropical precipitation bias are found over Europe, where the underestimation of blocking in CMIP5 models is also particularly strong.

Description: Northern Europe and the UK experienced an exceptionally warm and wet winter in 2019/20, driven by an anomalously positive North Atlantic Oscillation (NAO). This positive NAO was well forecast by several seasonal forecast systems, suggesting that this winter the NAO was highly predictable at seasonal lead times. A very strong positive Indian Ocean dipole (IOD) event was also observed at the start of winter. Here we use composite analysis and model experiments, to show that the IOD was a key driver of the observed positive NAO. Using model experiments that perturb the Indian Ocean initial conditions, two teleconnection pathways of the IOD to the north Atlantic emerge: a tropospheric teleconnection pathway via a Rossby wave train travelling from the Indian Ocean over the Pacific and Atlantic, and a stratospheric teleconnection pathway via the Aleutian region and the stratospheric polar vortex. These pathways are similar to those for the El Niño Southern Oscillation link to the north Atlantic which are already well documented. The anomalies in the north Atlantic jet stream location and strength, and the associated precipitation anomalies over the UK and northern Europe, as simulated by the model IOD experiments, show remarkable agreement with those forecast and observed.

Description: Climate variability in the tropical Atlantic Ocean is determined by large-scale ocean–atmosphere interactions, which particularly affect deep atmospheric convection over the ocean and surrounding continents1. Apart from influences from the Pacific El Niño/Southern Oscillation2 and the North Atlantic Oscillation3, the tropical Atlantic variability is thought to be dominated by two distinct ocean–atmosphere coupled modes of variability that are characterized by meridional4, 5 and zonal6, 7 sea-surface-temperature gradients and are mainly active on decadal and interannual timescales, respectively8, 9. Here we report evidence that the intrinsic ocean dynamics of the deep equatorial Atlantic can also affect sea surface temperature, wind and rainfall in the tropical Atlantic region and constitutes a 4.5-yr climate cycle. Specifically, vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5 yr and amplitudes of more than 10 cm s−1 are observed, in the deep Atlantic, to propagate their energy upwards, towards the surface10, 11. They are linked, at the sea surface, to equatorial zonal current anomalies and eastern Atlantic temperature anomalies that have amplitudes of about 6 cm s−1 and 0.4 °C, respectively, and are associated with distinct wind and rainfall patterns. Although deep jets are also observed in the Pacific12 and Indian13 oceans, only the Atlantic deep jets seem to oscillate on interannual timescales. Our knowledge of the persistence and regularity of these jets is limited by the availability of high-quality data. Despite this caveat, the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical Atlantic. Deep-jet generation and upward energy transmission through the Equatorial Undercurrent warrant further theoretical study.

Description: An ensemble of idealized experiments with the simplified general circulation model PUMA is used to analyze the response to reduced surface friction, that is a strengthening of the eddy-driven jet, a weakening of the Eulerian mean overturning, and a suppression of baroclinic instability. The suppression of baroclinic instability is caused by an effect called the barotropic governor by which increased horizontal shear restricts the ability of baroclinic disturbances to convert available potential energy into kinetic energy. This governor effect ensures that the residual circulation and Eliassen–Palm flux (EP flux) divergence are largely invariant to the surface friction parameter despite the connection between surface friction, the Eulerian mean overturning, and the eddy-momentum flux. The suppression of instability leads to an increase in persistence measured by the period of peak variance on synoptic time-scales and a strengthened signal-to-noise ratio on seasonal time-scales. These findings suggest that the signal-to-noise paradox seen in the context of seasonal prediction can be caused by excess mechanical damping in atmospheric prediction systems inhibiting the barotropic governor effect.