Large Scale Weather Patterns

The linkage between large scale weather patterns that are recurring and persisting is called ``teleconnection``. The teleconnections have a large impact in the nature, the humans and the society. Most of them can be understood by propagation of planetary waves, such as the Rossby or the Kelvin waves.

The major atmospheric jet streams can act as waveguides for Rossby waves as they can trap the energy from a vorticity source and transfer the information around the globe, alternating the weather patterns and creating teleconnections. We wonder how different aspects of the jet, such as the jet strength, width and location can constrain specific features of the trapped, zonal waves such as their wavenumber and amplitudes. The systematic analysis is done with the use of reanalysis data and a barotropic model.

The major source of atmospheric variability over the Northeast Atlantic, Europe and North America is the teleconnection North Atlantic Oscillation (NAO). We investigate the contribution of tropical and extratropical atmospheric forcing mechanisms to the formation of the NAO pattern, using a barotropic model and focusing on time scales of 10-60 days.

The tropical Indian Ocean (IO) exhibits notable atmospheric variability influencing the climate not only on regional, but also on global scale. The Seychelles Dome (SD) in the Southwest IO is a region characterized by a shallow thermocline that experiences large variability and strong seasonal upwelling. The variance of the SD is suggested to have an impact on the IO weather and on teleconnections. We investigate the local and remote response of the coupled ocean-atmosphere system to a shallow SD event, running ensemble experiments with the coupled EC-Earth model.

Samenvatting Περίληψη Acknowledgements Curriculum vitae viii Chapter 1.2.2 Rossby waves and their propagation characteristics Atmospheric Rossby waves are disturbances from the zonal symmetry in the atmospheric circulation that move westward under the inuence of the rotation of the Earth. In this thesis we study Rossby waves that are excited by tropical Rossby wave sources and travel into the extratropics, creating so-called teleconnection patterns. Rossby waves are named after Carl-Gustaf Arvid Rossby (Rossby (1939)) who identied them and explained their westward propagation in 1939. Rossby waves are dispersive, the longer the wavelength the faster the westward propagation. Their propagation is oset by the background winds such that in the region of the westerly winds, only the longest waves retrograde, move against the ow westward, while the smaller waves are carried eastward. The largest waves are called planetary waves. The planetary Rossby waves determine β * = β − ∂ 2Ū ∂y 2 (1.6) The jet stream is a fast westerly current owing around the globe in the upper troposphere with a sharp meridional prole, that is a consequence of the Earth's rotation and its dierential heating (section 1.2.1). Due to its sharp prole β * tends to have a larger relative maximum thanŪ (see Eq. 1.6). Therefore K s (Eq. 1.7) takes its maximum values along the jet core and reduces quickly toward the anks of the jet. This curvature and the attribute of the rays to bend towards larger K-values allows the changes the waveguiding properties and hence inuence the formation and propagation characteristics of Rossby waves. The pattern is most vigorous in the boreal winter when the circulation is strongest. The eect of NAO variability on the local weather conditions are distinct over Eastern North America, North Atlantic and Northern Europe and often across Southern Europe and the Middle East. Contrary to the extratropical mean ow, the NAO has a pronounced 1.4.2 The Indian Ocean Dipole Another important source of IO variability is the atmosphere-ocean coupled phenomenon widely known as the Indian Ocean Dipole (IOD) (Saji et al (1999)). On a regional scale the IOD has a signicant impact on the atmospheric circulation and precipitation anomalies and on a larger scale the phenomenon correlates with temperature and precipitation anomalies in remote regions, such as Europe, America, South Africa and Northeast Asia (Saji and Yamagata (2003a)). The IOD is characterized by an eastwest SST gradient along the central tropical IO that strongly impacts the atmospheric convection over the basin. When the Central-West IO is warmer than the Eastern IO the event is called a positive event and vice-Along the equator the strong east to southeasterly trade winds allow the cold deep currents, that ow from Antarctica to the west coast of South America, to upwell and keep the SSTs relatively cool along the equatorial cold tongue. In the Western Pacic the trade winds deepen the thermocline and the surface water is warmer. This area of warm ocean waters, the Pacic warm pool, is a major source for atmospheric convection program, which is nanced by the Netherlands Organization for Scientic Research (NWO). We also thank the two anonymous reviewers that provided valuable suggestions and remarks that improved the outcome of this work.