Ivana Herceg Bulić

Winter ENSO teleconnections in a warmer climate

Changes in the winter atmospheric response to sea surface temperature (SST) anomalies associated with the El Niño-Southern Oscillation (ENSO) in a warmer climate conditions are estimated from the two 20-member ensembles made by an atmospheric general circulation model of intermediate complexity. Warmer climate is simulated by a modification in the radiation parameterisation that corresponds to the doubled CO2 concentration, and SST forcing is represented by the same SST anomalies as in current climate (1855–2002) experiment superimposed on the climatological SST that was obtained from a complex atmosphere-ocean general circulation model forced with the doubled CO2. SST anomalies in the Niño3.4 region, categorised into five classes, enabled a composite analysis of changes in the Northern Hemisphere tropical/extratropical teleconnections. The main features of the tropical–extratropical teleconnections are maintained in both experiments; for example, irrespective of the sign of SST anomalies, the amplitude of the atmospheric response is positively correlated with the intensity of ENSO event and the El Niño impact is stronger than that of La Niña of the same intensity. The strongest extratropical signal in the warmer climate, particularly significant for strong warm events, is found over the Pacific/North American region; however, this extratropical teleconnections is reduced in a warmer climate relative to the current climate. Over the North Atlantic/European region, a detectable signal linked to ENSO is found; this model response is significantly strengthened in the experiment with the doubled CO2 concentration. Such an atmospheric response in a warmer climate is found to be associated with changes in the mean state followed as well as in the jet waveguiding effect and stationary wave activity.

Delayed ENSO impact on spring precipitation over North/Atlantic European region

The delayed impact of winter sea-surface temperature (SST) anomalies in tropical Pacific on spring precipitation over the North Atlantic/European (NAE) region is examined using both measured and modeled data for the period 1901–2002. In an AMIP-type Atmospheric General Circulation Model (AGCM) ensemble, the observed delayed spring precipitation response in Europe to winter ENSO-related SST anomalies is well reproduced. A series of targeted AGCM/coupled GCM experiments are performed to further investigate the mechanisms for this delayed influence. It is found that late winter ENSO SST anomalies lead to the well-documented Rossby wave train arching from the Pacific into the Atlantic region. A positive (negative) ENSO event leads to a quasi-barotropic trough (ridge) in the North Atlantic region. The resulting wind and cloud changes cause anomalies in the surface heat fluxes that result in negative (positive) SST anomalies in the central North Atlantic and anomalies of the opposite sign further to the south. The SST anomalies persist into spring and the atmospheric response to these anomalies is an extension of the ENSO-induced trough (ridge) into the European region, leading to enhanced (reduced) moisture flux and low-level convergence (divergence) and thus positive (negative) precipitation anomalies. Although the signal is overall relatively weak (correlation coefficients of European spring rainfall with winter ENSO SSTs of about 0.3), a proper representation of the outlined mechanism in seasonal forecasting systems may lead to improved seasonal predictions.

Thermodynamical and Airflow Conditions Within the Lower Troposphere During the Tropopause Fold Event Leading to Elevated Surface Ozone Concentrations

We investigated one episode of a winter stratospheric ozone intrusion over Zagreb, Croatia on February 6, 1990, when unusually high ozone concentrations were measured at two sites in the greater Zagreb area. Lisac et al. (1993) already studied the same episode. They argue that the air must be of stratospheric origin, since photochemistry cannot generate large amounts of ozone at this time of year. Their conclusion is also supported by the spiky character of the ozone level behavior, which was recorded on both stations, and which suggests rapid downward transport related to cross-tropopause exchange (Schuepbach et al., 1999). Lisac et al. confirmed the intrusion by means of analysis of available routine surface and upper air data. Finally, they roughly estimated a probable three-dimensional trajectory of an intruded air parcel. The trajectory starts on February 5 at 01 LST (Zagreb local time) at 300 hPa surface above the Baltic sea, and due to its anticyclonic curvature arrives in Zagreb on February 6 at 13 LST from the east. The above study was limited by the rough spatial and temporal resolution of the routine data, and, it did not offer a clear explanation of the time delay in the ozone peak, which was recorded at one of the measuring sites, which, we believe, is attained in the present study.