A model perspective on orbital forcing of monsoons and Mediterranean climate using EC-Earth

Abstract

This thesis focuses on orbitally forced changes of monsoons and Mediterranean climate. Changes in the shape of the Earths orbit around the Sun and its rotational axis govern the seasonal and latitudinal distribution of incoming solar radiation on time scales of thousands to millions of years. The three orbital parameters, eccentricity, precession and obliquity, are reflected in sedimentary records from all over the world. In this thesis a state-of-the-art coupled general circulation model, EC-Earth, is used to obtain a physical basis of climate response to orbital forcing. First, a Mid-Holocene experiment is discussed, performed within the framework of the Paleoclimate Modelling Intercomparison Project (PMIP3). Stronger northern hemisphere summer insolation results in intensified monsoons in North-Africa and Asia, while reduced southern hemisphere summer insolation results in a weaker South-American monsoon. Over North-Africa, the monsoon extends further poleward during the Mid-Holocene. These results corroborate the findings of paleoclimate proxy studies as well as previous model studies, while giving a more detailed account of Mid-Holocene summer monsoons. Secondly, the response of the North-African and Asian summer monsoons to separate precession and obliquity forcing is investigated. Strengthening of the North-African monsoon during minimum precession and maximum obliquity, when northern hemisphere summer insolation is increased, is mostly related to stronger monsoon winds carrying moisture from the tropical Atlantic Ocean. This is in contrast to previous studies suggesting high-latitude mechanisms. Furthermore, the monsoon winds, convection and precipitation extend farther north into the Sahara. The Asian monsoons are strengthened as well during minimum precession and maximum obliquity. Southerly monsoon winds over East-Asia are stronger due to an intensified west-east land-sea pressure gradient. The intensified North Pacific High is connected to anomalously high pressure over south-east Asia, and an Indian Ocean Dipole pattern emerges. The Indian monsoon is stronger, but the increased precipitation over the western Indian Ocean and decreased wind speed over the northern Indian Ocean damp the monsoon strengthening. The southern tropical Indian Ocean acts as a moisture source for the enhanced monsoon precipitation. Thirdly, the influence of obliquity on low-latitude climate is addressed. Obliquity-induced insolation changes at low latitudes are very small, therefore glacial cycles and other high latitude mechanisms are often invoked to explain low-latitude obliquity signals. However, the model results in this study show that obliquity-induced changes can occur through changes in the tropical cross-equatorial insolation gradient. This gradient is stronger during high obliquity, causing increased cross-equatorial wind and moisture transport and therefore a re-distribution of precipitation. Lastly, the effect of precession and obliquity on the freshwater budget of the Mediterranean is examined. Ample proxy studies suggest that the area was wetter and that the Mediterranean Sea was less saline at times of enhanced insolation seasonality, i.e. minimum precession and maximum obliquity. The EC-Earth results show that during these situations, both summer monsoonal runoff through the Nile as well as winter precipitation over the Mediterranean Sea are increased. The first is most important for precession, the latter for obliquity. The changes in winter precipitation are related to changes in the air-sea temperature difference.