Role of eccentricity in early Holocene African and Asian summer monsoons

The effect of precession on paleoclimate changes depends on eccentricity. However, whether and to what degree eccentricity relates to millennial-scale monsoonal changes remain unclear. By investigating climate simulations with a fixed precession condition of 9 ka before the present, we explored the potential influence of eccentricity on early-Holocene changes in the Afro–Asian summer monsoons. Compared with the lower eccentricity of the present day, higher eccentricity in the early Holocene strengthened the continental summer monsoons, Pacific anticyclone, and Hadley circulation, particularly over the ocean. Over Africa, the eccentricity-induced “dry-gets-wetter” condition could be related to the Green Sahara, suggesting a superimposed effect of precession. Over the western Pacific, the tropical response to eccentricity may have been competitive in terms of what an extremely high obliquity may have caused. A downscaled modulation of eccentricity in relation to precession and obliquity cannot be ignored when paleomonsoon records are studied. Regarding early-Holocene monsoonal changes in South Asia, however, a high eccentricity may have had only a secondary effect on enhancing the monsoonal precipitation in the southern edge of the Tibetan Plateau, exhibiting the weak power of candle-like heating. This suggested that sizable monsoonal changes over the northern Indian Ocean and India–Pakistan region are unrelated to early-Holocene eccentricity.

Influence of the representation of convection on the mid-Holocene West African Monsoon

Global climate models experience difficulties in simulating the northward extension of the monsoonal precipitation over north Africa during the mid-Holocene as revealed by proxy data. A common feature of these models is that they usually operate on grids that are too coarse to explicitly resolve convection, but convection is the most essential mechanism leading to precipitation in the West African Monsoon region. Here, we investigate how the representation of tropical deep convection in the ICOsahedral Nonhydrostatic (ICON) climate model affects the meridional distribution of monsoonal precipitation during the mid-Holocene by comparing regional simulations of the summer monsoon season (July to September; JAS) with parameterized and explicitly resolved convection. In the explicitly resolved convection simulation, the more localized nature of precipitation and the absence of permanent light precipitation as compared to the parameterized convection simulation is closer to expectations. However, in the JAS mean, the parameterized convection simulation produces more precipitation and extends further north than the explicitly resolved convection simulation, especially between 12 and 17∘ N. The higher precipitation rates in the parameterized convection simulation are consistent with a stronger monsoonal circulation over land. Furthermore, the atmosphere in the parameterized convection simulation is less stably stratified and notably moister. The differences in atmospheric water vapor are the result of substantial differences in the probability distribution function of precipitation and its resulting interactions with the land surface. The parametrization of convection produces light and large-scale precipitation, keeping the soils moist and supporting the development of convection. In contrast, less frequent but locally intense precipitation events lead to high amounts of runoff in the explicitly resolved convection simulations. The stronger runoff inhibits the moistening of the soil during the monsoon season and limits the amount of water available to evaporation in the explicitly resolved convection simulation.

Impact of Climate Change and Surface Energy (Im) Balance on North-East India Monsoonal Rainfall

In recent decades, climate change and its impact on the ecosystem has remained a concern from global to regional to local scale. Many studies performed over India have highlighted the change in precipitation associated with the Indian summer monsoon (ISM) and its linkage with changed land surface processes. Over North-East India (NEI), changed surface and atmospheric energy imbalance due to increase in wasteland, deforestation and over cultivation have made the soil barren. In addition, soil moisture of barren land has decreased, latent (sensible) heat decreased (increased) with stimulating ground heat increment. This led to lower evapotranspiration and convection leading to precipitation decrement. To analyse this in detail, the present study shows a lower increase in the near surface temperature during 1956-1985 (period I), but a higher increasing trend has been seen during 1986-2015 (period II). In the case of precipitation trends, an increase during period I and a decrease at a 95% significant level during period II are seen. The average air temperature warming rate increase of 0.09 °C/year is observed. The monsoonal precipitation has decreased significantly in recent years (1986-2015) than that in the past (1956-1985). In addition, a decrease in monsoonal precipitation at 0.35 mm/year rate during period II is seen over NEI. A prominent increment of 0.12 W/m2 is observed in surface sensible heat flux over NEI. Land use land cover change (LULCC) is continuously altering the local rate of change of thermal radiation, evapotranspiration and convection, and has also played a critical role in defining monsoonal precipitation over NEI. However, the surface net solar and thermal radiation change are in equilibrium with the surface sensible and latent heat for sustaining the surface energy budget. Hence, a small change in surface net radiation causes an imbalance of surface energetics. It is one of the most prominent causes for the precipitation pattern changes over NEI. The LULCCs and earth’ surface energy imbalance reinforce climate variability and climate change over the study region.

Influence of the representation of convection on the mid-Holocene West African Monsoon

<p>Global climate models have difficulties to simulate the northward extension of the monsoonal precipitation over north Africa during the mid-Holocene as revealed by proxy data. A common feature of these models is that they usually operate on too coarse grids to explicitly resolve convection, but convection is the most essential mechanism leading to precipitation in the west African monsoon region. Here, we investigate how the representation of tropical deep convection in the ICON climate model affects the meridional distribution of monsoonal precipitation during the mid-Holocene, by comparing regional simulations of the summer monsoon season (July to September, JAS) with parameterized (40km-P) and explicitly resolved convection (5km-E). <br>The spatial distribution and intensity of precipitation, are more realistic in the explicitly resolved convection simulations than in the simulations with parameterized convection.<br>However, in the JAS-mean the 40km-P simulation produces more precipitation and extents further north than the 5km-E simulation, especially between 12° N and 17° N. The higher precipitation rates in the 40km-P simulation are consistent with a stronger monsoonal circulation over land. <br>Furthermore, the atmosphere in the 40km-P simulation is less stably stratified and notably moister. The differences in atmospheric water vapor are the result of substantial differences in the probability distribution function of precipitation and its resulting interactions with the land surface. The parametrization of convection produces light and large-scale precipitation, keeping the soils moist and supporting the development of convection. <br>In contrast, less frequent but locally intense precipitation events lead to high amounts of runoff in explicitly resolved convection simulations. The stronger runoff inhibits the moistening of the soil during the monsoon season and limits the amount of water available to evaporation. </p>

Influence of the representation of convection on the mid-Holocene West African Monsoon

Global climate models have difficulties to simulate the northward extension of the monsoonal precipitation over north Africa during the mid-Holocene as revealed by proxy data. A common feature of these models is that they usually operate on too coarse grids to explicitly resolve convection, but convection is the most essential mechanism leading to precipitation in the west African monsoon region. Here, we investigate how the representation of tropical deep convection in the ICON climate model affects the meridional distribution of monsoonal precipitation during the mid-Holocene, by comparing regional simulations of the summer monsoon season (July to September, JAS) with parameterized (40 km-P) and explicitly resolved convection (5 km-E). The spatial distribution and intensity of precipitation, are more realistic in the explicitly resolved convection simulations than in the simulations with parameterized convection. However, in the JAS-mean the 40 km-P simulation produces more precipitation and extents further north than the 5 km-E simulation, especially between 12° N and 17° N. The higher precipitation rates in the 40 km-P simulation are consistent with a stronger monsoonal circulation over land. Furthermore, the atmosphere in the 40 km-P simulation is less stably stratified and notably moister. The differences in atmospheric water vapor are the result of substantial differences in the probability distribution function of precipitation and its resulting interactions with the land surface. The parametrization of convection produces light and large-scale precipitation, keeping the soils moist and supporting the development of convection. In contrast, less frequent but locally intense precipitation events lead to high amounts of runoff in explicitly resolved convection simulations. The stronger runoff inhibits the moistening of the soil during the monsoon season and limits the amount of water available to evaporation.

Contrasting Southern Hemisphere Monsoon Response: MidHolocene Orbital Forcing versus Future Greenhouse Gas–Induced Global Warming

Past changes of Southern Hemisphere (SH) monsoons are less investigated than their northern counterpart because of relatively scarce paleodata. In addition, projections of SH monsoons are less robust than in the Northern Hemisphere. Here, we use an energetic framework to shed lights on the mechanisms determining SH monsoonal response to external forcing: precession change at the mid-Holocene versus future greenhouse gas increase (RCP8.5). Mechanisms explaining the monsoon response are investigated by decomposing the moisture budget in thermodynamic and dynamic components. SH monsoons weaken and contract in the multimodel mean of midHolocene simulations as a result of decreased net energy input and weakening of the dynamic component. In contrast, SH monsoons strengthen and expand in the RCP8.5 multimodel mean, as a result of increased net energy input and strengthening of the thermodynamic component. However, important regional differences on monsoonal precipitation emerge from the local response of Hadley and Walker circulations. In the midHolocene, the combined effect of Walker–Hadley changes explains the land–ocean precipitation contrast. Conversely, the increased local gross moist stability explains the increased local precipitation and net energy input under circulation weakening in RCP8.5.

Mechanisms for Spatially Inhomogeneous Changes in East Asian Summer Monsoon Precipitation during the Mid-Holocene

The East Asian summer monsoon (EASM) intensified during the early to mid-Holocene relative to the present primarily due to orbital forcing. However, on the regional scale, changes in the monsoonal precipitation exhibit considerable spatial disparity, and the underlying mechanisms remain unresolved. In this study, the dynamic processes responsible for the difference of the EASM precipitation between the mid-Holocene and preindustrial period are systematically examined using the CMIP5 multimodel simulations. The moisture budget diagnostic identifies vertical motion as the key factor determining the cross-like precipitation pattern in East Asia. Relative to the preindustrial period, the mid-Holocene anomalous ascending motion corresponds well with the excessive precipitation over northern and southern China, and vice versa for west-central China, the Korean peninsula, Japan, and its marginal seas. In the framework of the moist static energy budget, the increased insolation and the attendant intensification of land–sea thermal contrast give rise to anomalous ascending motions, while descending motions are fundamentally forced by the decreased latitudinal insolation gradient. In particular, thermodynamic changes, namely, the reduced pole–equator temperature and humidity gradients, account for the downward motions over the northwestern Pacific. Dynamic changes, namely, the weakened westerlies, play a leading role in suppressing updrafts in west-central China. This study highlights that the orbital-scale monsoonal precipitation changes are not solely determined by local radiative forcing as repeatedly emphasized before. The latitudinal uneven distribution of insolation is crucial to explain the spatial inhomogeneity in the EASM precipitation changes during the Holocene.