Atmospheric Low Frequency Variability: The Examples of the North Atlantic and the Indian Monsoon

Abstract

Great efforts, sometimes taking the form of a race, are exerted by climate scientists to provide medium and long-term future climate predictions on large and regional, or even local scales. This exercise has proved to be a really challenging one. There is a wide variety of climate characteristics between different regions on the globe. For example, tropical and subtropical regions tend to be more influenced by what happens in the equatorial Indian and Pacific oceans such as the El Nino Southern Oscillation (ENSO). Midlatitude regions, on the other hand, are more affected by the north-south migration of the polar front or synonymously the midlatitude jet stream. It is important to notice that even within the midlatitude band climate variation differs from region to region. For example, climate variability over the North Atlantic European region is different from that of the Pacific North America (PNA) region and is particularly more difficult to predict. The jet stream is a belt of strong westerly wind that goes around the globe in the subtropics (subtropical jet) or the midlatiudes (eddy-driven jet). The subtropical jet results from the westerly acceleration of poleward moving air associated with the upper branch of the Hadley cell. The midlatitude jet stream, on the other hand, results from the momentum and heat forcing by midlatitude eddies, i.e. weather systems. Weather and climate variations in the extratropics are associated to a large extent with meridional shifts of the midlatitude westerly jet stream. For instance, major extratropical teleconnections, including the North Atlantic Oscillation (Fig. 1) and the PNA pattern, describe changes in the jet stream (Wittman et al. 2005; Monahan and Fyfe 2006). Over the North Atlantic region, the North Atlantic Oscillation (NAO) is the dominant large scale mode of variability with its north-south dipole anomaly centres (Hurrell et al. 2002). It is a seesaw in atmospheric mass between the subtropical high and and the polar low and affects much of the weather and climate in the North Atlantic, east of North America, Europe and parts of Russia. The positive phase of the NAO (Fig. 1b) is generally associated with a stronger subtropical high pressure and a deeper than normal Atmospheric Low Frequency Variability: The Examples of the North Atlantic and the Indian Monsoon 1