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Sarah Ineson and Michael K. Davey

Abstract

A Pacific Ocean–global atmosphere general circulation model is used to simulate the climatic mean state and variability in the Tropics, up to interannual timescales. For this model no long-term trend in climate occurs, but there are systematic differences between the model mean state and observations: in particular, the east equatorial Pacific sea surface temperature is too high by several degrees. Along the equator the seasonal variability in sea surface temperature is good although some features of the seasonal cycle are unrealistic: for example, the east Pacific convergence zone crosses the equator twice a year, residing in the summer hemisphere.

Despite some deficiencies in the simulation of the mean state, there is substantial interannual variability, with irregular oscillations dominated by a 2-yr cycle. A principal oscillation pattern analysis shows that the interannual anomalies are typically generated in the west Pacific and move eastward along the equator, with closely connected oceanic and atmospheric components. The patterns are similar to those associated with observed El Niño events. Rainfall anomalies associated with the model El Niño events also have several realistic features.

Idealized seasonal prediction experiments were made by slightly perturbing the atmospheric component: three 6-month hindcasts were thus made for each of several start times spread through an El Niño cycle. Predictability of central Pacific sea surface temperature anomalies was best for hindcasts starting near a warm El Niño peak. Generally, hindcasts starting in September and December were more accurate, with less spread, than those starting in March and June. The behavior and predictability of seasonal rainfall in several regions was also analyzed. For example, a warm model El Niño produces enhanced rainfall in the central equatorial Pacific and reduced rainfall in the Indian region, which is reproduced consistently in the hindcasts.

The model also shows variability on shorter timescales, and an example is presented of a spontaneous westerly wind burst in the west Pacific and its oceanic impact.

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Chris K. Folland, Jeff Knight, Hans W. Linderholm, David Fereday, Sarah Ineson, and James W. Hurrell

Abstract

Summer climate in the North Atlantic–European sector possesses a principal pattern of year-to-year variability that is the parallel to the well-known North Atlantic Oscillation in winter. This summer North Atlantic Oscillation (SNAO) is defined here as the first empirical orthogonal function (EOF) of observed summertime extratropical North Atlantic pressure at mean sea level. It is shown to be characterized by a more northerly location and smaller spatial scale than its winter counterpart. The SNAO is also detected by cluster analysis and has a near-equivalent barotropic structure on daily and monthly time scales. Although of lesser amplitude than its wintertime counterpart, the SNAO exerts a strong influence on northern European rainfall, temperature, and cloudiness through changes in the position of the North Atlantic storm track. It is, therefore, of key importance in generating summer climate extremes, including flooding, drought, and heat stress in northwestern Europe. The El Niño–Southern Oscillation (ENSO) phenomenon is known to influence summertime European climate; however, interannual variations of the SNAO are only weakly influenced by ENSO. On interdecadal time scales, both modeling and observational results indicate that SNAO variations are partly related to the Atlantic multidecadal oscillation. It is shown that SNAO variations extend far back in time, as evidenced by reconstructions of SNAO variations back to 1706 using tree-ring records. Very long instrumental records, such as central England temperature, are used to validate the reconstruction. Finally, two climate models are shown to simulate the present-day SNAO and predict a trend toward a more positive index phase in the future under increasing greenhouse gas concentrations. This implies the long-term likelihood of increased summer drought for northwestern Europe.

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Blanca Ayarzagüena, Sarah Ineson, Nick J. Dunstone, Mark P. Baldwin, and Adam A. Scaife

Abstract

It is well established that El Niño–Southern Oscillation (ENSO) impacts the North Atlantic–European (NAE) climate, with the strongest influence in winter. In late winter, the ENSO signal travels via both tropospheric and stratospheric pathways to the NAE sector and often projects onto the North Atlantic Oscillation. However, this signal does not strengthen gradually during winter, and some studies have suggested that the ENSO signal is different between early and late winter and that the teleconnections involved in the early winter subperiod are not well understood. In this study, we investigate the ENSO teleconnection to NAE in early winter (November–December) and characterize the possible mechanisms involved in that teleconnection. To do so, observations, reanalysis data and the output of different types of model simulations have been used. We show that the intraseasonal winter shift of the NAE response to ENSO is detected for both El Niño and La Niña and is significant in both observations and initialized predictions, but it is not reproduced by free-running Coupled Model Intercomparison Project phase 5 (CMIP5) models. The teleconnection is established through the troposphere in early winter and is related to ENSO effects over the Gulf of Mexico and Caribbean Sea that appear in rainfall and reach the NAE region. CMIP5 model biases in equatorial Pacific ENSO sea surface temperature patterns and strength appear to explain the lack of signal in the Gulf of Mexico and Caribbean Sea and, hence, their inability to reproduce the intraseasonal shift of the ENSO signal over Europe.

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