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

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

Obliquity forcing of low-latitude climate

Contains fulltext : 190091.pdf (publisher's version ) (Open Access

Futurestreams dataset

Repository of discharge and water temperature scenarios derived from 5 CMIP5 GCMs from the ISI-MIP project computed by the global water balance model PCRGLOBWB and the global water temperature model DynWat. Data are provided on a weekly temporal resolution and 10km spatial resolution. 4 Representative Concentration Pathways are presented, RCP2.6, RCP4.5, RCP6.0 and RCP8.5, up to 2099. The historical simulations are also provide for 1976 - 2005. A historical simulation derived from E2O reanalysis data is also provided. Data access is currently restricted, and will become directly available once the accompanying Data Descriptor (currently under review) is published. Contact persons are: - Joyce Bosmans ([email protected]) - Niko Wanders ([email protected]

Obliquity forcing of low-latitude climate

The influence of obliquity, the tilt of the Earth's rotational axis, on incoming solar radiation at low latitudes is small, yet many tropical and subtropical palaeoclimate records reveal a clear obliquity signal. Several mechanisms have been proposed to explain this signal, such as the remote influence of high-latitude glacials, the remote effect of insolation changes at mid- to high latitudes independent of glacial cyclicity, shifts in the latitudinal extent of the tropics, and changes in latitudinal insolation gradients. Using a sophisticated coupled ocean–atmosphere global climate model, EC-Earth, without dynamical ice sheets, we performed two idealized experiments of obliquity extremes. Our results show that obliquity-induced changes in tropical climate can occur without high-latitude ice sheet fluctuations. Furthermore, the tropical circulation changes are consistent with obliquity-induced changes in the cross-equatorial insolation gradient, suggesting that this gradient may be used to explain obliquity signals in low-latitude palaeoclimate records instead of the classical 65° N summer insolation curve

Response of the North African summer monsoon to precession and obliquity forcings in the EC-Earth GCM

We investigate, for the first time, the response of the North African summer monsoon to separate precession and obliquity forcings using a high-resolution state-of-the-art coupled general circulation model, EC-Earth. Our aim is to better understand the mechanisms underlying the astronomical forcing of this low-latitude climate system in detail. The North African monsoon is strengthened when northern hemisphere summer insolation is higher, as is the case in the minimum precession and maximum obliquity experiments. In these experiments, the low surface pressure areas over the Sahara are intensified and located farther north, and the meridional pressure gradient is further enhanced by a stronger South Atlantic high pressure area. As a result, the southwesterly monsoon winds are stronger and bring more moisture into the monsoon region from both the northern and southern tropical Atlantic. The monsoon winds, precipitation and convection also extend farther north into North Africa. The precession-induced changes are much larger than those induced by obliquity, but the latter are remarkable because obliquity-induced changes in summer insolation over the tropics are nearly zero. Our results provide a different explanation than previously proposed for mechanisms underlying the precession- and, especially, obliquity-related signals in paleoclimate proxy records of the North African monsoon. The EC-Earth experiments reveal that, instead of higher latitude mechanisms, increased moisture transport from both the northern and southern tropical Atlantic is responsible for the precession and obliquity signals in the North African monsoon. This increased moisture transport results from both increased insolation and an increased tropical insolation gradient

Monsoonal response to mid-holocene orbital forcing in a high resolution GCM

In this study, we use a sophisticated high-resolution atmosphere-ocean coupled climate model, EC-Earth, to investigate the effect of Mid-Holocene orbital forcing on summer monsoons on both hemispheres. During the Mid-Holocene (6 ka), there was more summer insolation on the Northern Hemisphere than today, which intensified the meridional temperature and pressure gradients. Over North Africa, monsoonal precipitation is intensified through increased landward monsoon winds and moisture advection as well as decreased moisture convergence over the oceans and more convergence over land compared to the pre-industrial simulation. Precipitation also extends further north as the ITCZ shifts northward in response to the stronger poleward gradient of insolation. This increase and poleward extent is stronger than in most previous ocean-atmosphere GCM simulations. In north-westernmost Africa, precipitation extends up to 35° N. Over tropical Africa, internal feedbacks completely overcome the direct warming effect of increased insolation. We also find a weakened African Easterly Jet. Over Asia, monsoonal precipitation during the Mid-Holocene is increased as well, but the response is different than over North-Africa. There is more convection over land at the expense of convection over the ocean, but precipitation does not extend further northward, monsoon winds over the ocean are weaker and the surrounding ocean does not provide more moisture. On the Southern Hemisphere, summer insolation and the poleward insolation gradient were weaker during the Mid-Holocene, resulting in a reduced South American monsoon through decreased monsoon winds and less convection, as well as an equatorward shift in the ITCZ. This study corroborates the findings of paleodata research as well as previous model studies, while giving a more detailed account of Mid-Holocene monsoons

Effect of simulated freefall lifeboat training on launch skill acquisition

Freefall lifeboats (FFLB) are used worldwide as a means for evacuation and escape. Currently, FFLB launch training is normally restricted to benign weather conditions due to the inherent risk to personnel safety and asset integrity. Under such circumstances, the coxswain cannot develop the heuristic techniques necessary for launching under more likely dangerous and unpredictable evacuation and environmental conditions. Simulators can provide enhanced training opportunities for these conditions, so long as the simulation technologies and training paradigms address the contextual, mathematical and behavior demands of the physical training. A high level of fidelity should invoke a level of participant presence suitable for performance-based learning and training objectives. The purpose of this research was to determine the effect of post-launch feedback on the rate of skill acquisition of novice participants performing simulated FFLB launches. Participants in two independent groups each went through 24 consecutive simulated launches under varying sea-states and visual conditions. One group was given pictorial feedback about the quality of each launch. The rate of skill acquisition and time to launch of this group was compared to a group that had no feedback. Results show that: pictorial feedback did not affect launch success or time to launch of our FFLB launching trials, wave height had the greatest affect on launch success, visual clarity only had a significant affect on launch time in the no feedback group, and sense of presence was not affected by the inclusion of feedback or correlated to performance measures

Precession‐ and obliquity‐induced changes in moisture sources for enhanced precipitation over the Mediterranean Sea

Enhanced winter precipitation over the Mediterranean Sea at times of minimum precession and maximum obliquity, that is, times of enhanced insolation seasonality, could provide freshwater required to form orbitally paced sedimentary cycles across the Mediterranean, offering a possible alternative to monsoonal runoff. We investigate the sources of the enhanced winter precipitation, by applying a moisture tracking model on the results of idealized orbital extreme experiments with a state‐of‐the‐art climate model. Precession and obliquity enhance precipitation in fall and winter. Our study shows that the source of enhanced precipitation over the Mediterranean Sea differs during the winter half‐year. In fall, the majority of the precession‐induced precipitation increase originates from the Mediterranean itself. However, in late winter, the increase can be attributed to enhanced moisture advection from the Atlantic. This agrees with changes in evaporation and air‐sea temperature differences over the Mediterranean. The obliquity‐induced precipitation increase shows much less differences, with an equal contribution of local and Atlantic sources. The mechanism behind the Atlantic source of moisture, particularly important in late winter for precession‐induced precipitation changes, is related to a weakened Azores High and slightly higher surface pressure over North Africa. The resulting anomalous circulation patterns generate enhanced Atlantic moisture transport toward the Mediterranean. These mechanisms coincide with weaker storm track activity over the North Atlantic, opposite to previous studies that often attribute enhanced Mediterranean winter precipitation to a southward shift and intensification of the Atlantic storm track. We thus provide an alternative mechanism for Atlantic sources of orbitally paced Mediterranean precipitation changes

Precession and obliquity forcing of the freshwater budget over the Mediterranean

There is strong proxy and model evidence of precession- and obliquity-induced changes in the freshwater budget over the Mediterranean Sea and its borderlands, yet explanations for these changes vary greatly. We investigate the separate precession and obliquity forcing of the freshwater budget over the Mediterranean using a high-resolution coupled climate model, EC-Earth. At times of enhanced insolation seasonality, i.e. minimum precession and maximum obliquity, the area was wetter and the Mediterranean Sea surface was less saline. The latter has been attributed to increased runoff from the south as a consequence of a strengthened North African monsoon, as well as to increased precipitation over the Mediterranean Sea itself. Our results show that both mechanisms play a role in changing the freshwater budget. Increased monsoon runoff occurs in summer during times of enhanced insolation seasonality, especially minimum precession, while increased precipitation is important in winter for both precession and obliquity. We relate changes in winter precipitation to changes in the air-sea temperature difference and subsequently, convective precipitation. The freshening in the minimum precession and maximum obliquity experiments has a strong effect on Mediterranean sea surface salinity and mixed layer depth, thereby likely influencing deep sea circulation and sedimentation at the ocean bottom