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Abstract
Abstract. The recently proposed global monsoon hypothesis interprets monsoon systems as part of one global-scale atmospheric overturning circulation, implying a connection between the regional monsoon systems and an in-phase behaviour of all northern hemispheric monsoons on annual timescales (Trenberth et al., 2000). Whether this concept can be applied to past climates and variability on longer timescales is still under debate, because the monsoon systems exhibit different regional characteristics such as different seasonality (i.e. onset, peak and withdrawal). To investigate the interconnection of different monsoon systems during the pre-industrial Holocene, five transient global climate model simulations have been analysed with respect to the rainfall trend and variability in different sub-domains of the Afro-Asian monsoon region. Our analysis suggests that on millennial timescales with varying orbital forcing, the monsoons do not behave as a tightly connected global system. According to the models, the Indian and North African monsoons are coupled, showing similar rainfall trend and moderate correlation in centennial rainfall variability in all models. The East Asian monsoon changes independently during the Holocene. The dissimilarities in the seasonality of the monsoon sub-systems lead to a stronger response of the North African and Indian monsoon systems to the Holocene insolation forcing than of the East Asian monsoon and affect the seasonal distribution of Holocene rainfall variations. Within the Indian and North African monsoon domain, precipitation solely changes during the summer months, showing a decreasing Holocene precipitation trend. In the East Asian monsoon region, the precipitation signal is determined by an increasing precipitation trend during spring and a decreasing precipitation change during summer, partly balancing each other. A synthesis of reconstructions and the model results do not reveal an impact of the different seasonality on the timing of the Holocene rainfall optimum in the different sub-monsoon systems. Rather they indicate locally inhomogeneous rainfall changes and show that single palaeo-records should not be used to characterise the rainfall change and monsoon evolution for entire monsoon sub-systems.
Highlights
Monsoon systems are coupled atmosphere–ocean–land phenomena that are primarily driven by seasonal and latitudinal differences in incoming solar radiation
As the pre-industrial simulations have been performed with a CO2 concentration of 280 ppm, considerably lower than those observed for the present day, we expect small biases in the strength of the monsoon system and precipitation amount resulting from the different CO2 levels
The strong connection of the North African, Indian and northern East Asian monsoon region may be related to the El Niño–Southern Oscillation variability, which is very prominent in the COSMOS model
Summary
Monsoon systems are coupled atmosphere–ocean–land phenomena that are primarily driven by seasonal and latitudinal differences in incoming solar radiation. The in-phase movement of the global monsoon systems partly visible in the seasonal migration of the Intertropical Convergence Zone (ITCZ) led to the development of a new monsoon concept, i.e. the global monsoon hypothesis (Trenberth et al, 2000). This concept suggests that all monsoon systems are part of one seasonal varying global-scale atmospheric overturning circulation in the tropics and subtropics forced by the annual cycle of insolation and the resultant global divergent motion. Whether the global monsoon concept can be applied to past climates and variability on longer timescales is not yet clear (Trenberth et al, 2000), palaeomonsoon studies reveal similarities to the modern global monsoon concept regarding palaeomonsoon evolution, e.g. similar longterm monsoon intensity oscillations in different monsoonsubdomains (Cheng et al, 2012)
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