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

Abstract

Abstract.In this study, we use a sophisticated highresolution atmosphere-ocean coupled climate model, ECEarth, 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.

Mechanisms rectifying the annual mean response of tropical Atlantic rainfall to precessional forcing

“…Numerous studies have shown that the WES feedback is an important amplifier of meridional ITCZ shifts over the Atlantic Ocean (e.g. Braconnot et al 2000;Biasutti et al 2005;Zhao et al 2005;Bosmans et al 2012).…”

“…In addition, there are numerous studies focusing on time slice experiments of Pleistocene climates that also involve precessional change, such as the mid-Holocene and the last interglacial (e.g. Kutzbach and Otto-Bliesner 1982;Kutzbach and Liu 1997;Zhao et al 2005;Braconnot et al 2008;Khon et al 2010Khon et al , 2012Bosmans et al 2012). These studies emphasize the importance of differential heating of Fig.…”

Obliquity forcing of low-latitude climate

“…EC-Earth is used for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Hazeleger et al, 2011;Flato et al, 2013;Collins et al, 2013) and was also used to perform the pre-industrial and mid-Holocene experiments of the Paleoclimate Modelling Intercomparison Project (Bosmans et al, 2012). We performed two idealized obliquity experiments, using the same experimental setup as Tuenter et al (2003): one with a low obliquity (22.04 • , T min ) and one with a high obliquity (24.45 • , T max ).…”

The South American monsoon variability over the last millennium in climate models

“…It is well established that monsoon systems respond to orbital forcing (Kutzbach and Liu, 1997;Kutzbach et al, 2007;Bosmans et al, 2012). At orbital timescales (especially related to the precessional cycle of ca.…”

“…19 and 21 kyr), changes in the latitudinal insolation gradient, and hence temperatures, force the monsoon circulation globally (e.g. Bosmans et al, 2012). In the precession frequency band the summer insolation is in anti-phase between hemispheres (for example, when Northern Hemisphere (NH) summer insolation is at its maximum, summertime insolation in the Southern Hemisphere (SH) is at its minimum).…”