The Experts below are selected from a list of 4551 Experts worldwide ranked by ideXlab platform
Sandy P Harrison - One of the best experts on this subject based on the ideXlab platform.
-
Simulations of the impact of Orbital Forcing and ocean on the Asian summer monsoon during the Holocene
Global and Planetary Change, 2020Co-Authors: Yuefeng Li, Sandy P HarrisonAbstract:The importance of Orbital Forcing and ocean impact on the Asian summer monsoon in the Holocene is investigated by comparing simulations with a fully coupled ocean-atmosphere general circulation model (FOAM) and with the atmospheric component of this model (FSSTAM) forced with prescribed modem sea-surface temperatures (SSTs). The results show: (1) the ocean amplifies the Orbitally-induced increase in African monsoon precipitation, makes somewhat increase in southern India and damps the increase over the southeastern China. (2) The ocean could change the spatial distribution and local intensity of the Orbitally-induced latitudinal atmospheric oscillation over the southeastern China and the subtropical western Pacific Ocean. (3) The Orbital Forcing mostly enhances the Asian summer precipitation in the FOAM and FSSTAM simulations. However, the ocean reduces the Orbitally-induced summer precipitation and postpones the time of summer monsoon onset over the Asian monsoon region. (4) The Orbital Forcing considerably enhances the intensity of upper divergence, which is amplified by ocean further, over the eastern hemisphere. But the divergence is weaker in the FOAM simulations than in the FSSTAM simulations when the Orbital Forcing is fixed. (5) The Orbital Forcing can enhance the amplitude of precipitation variability over the subtropical Africa, the southeastern China and northwestern China, inversely, reduce it over central India and North China in the FOAM and FSSTAM simulations. The ocean obviously reduces the amplitude of precipitation variability over most of the Asian monsoon regions in the fixed Orbital Forcing simulations. (6) The areas characterized by increased summer precipitation in the long-term mean are mostly characterized by increased amplitude of short-term variability, whereas regions characterized by decreased precipitation are primarily characterized by decreased amplitude of short-term variability. However, the influences of Orbital Forcing or dynamical ocean on regional climate depend on the model. Crown Copyright (c) 2007 Published by Elsevier B.V. All rights reserved
-
mid holocene monsoons a multi model analysis of the inter hemispheric differences in the responses to Orbital Forcing and ocean feedbacks
Climate Dynamics, 2012Co-Authors: Sandy P Harrison, Y ZhaoAbstract:The response of monsoon circulation in the northern and southern hemisphere to 6 ka Orbital Forcing has been examined in 17 atmospheric general circulation models and 11 coupled ocean–atmosphere general circulation models. The atmospheric response to increased summer insolation at 6 ka in the northern subtropics strengthens the northern-hemisphere summer monsoons and leads to increased monsoonal precipitation in western North America, northern Africa and China; ocean feedbacks amplify this response and lead to further increase in monsoon precipitation in these three regions. The atmospheric response to reduced summer insolation at 6 ka in the southern subtropics weakens the southern-hemisphere summer monsoons and leads to decreased monsoonal precipitation in northern South America, southern Africa and northern Australia; ocean feedbacks weaken this response so that the decrease in rainfall is smaller than might otherwise be expected. The role of the ocean in monsoonal circulation in other regions is more complex. There is no discernable impact of Orbital Forcing in the monsoon region of North America in the atmosphere-only simulations but a strong increase in precipitation in the ocean–atmosphere simulations. In contrast, there is a strong atmospheric response to Orbital Forcing over northern India but ocean feedback reduces the strength of the change in the monsoon although it still remains stronger than today. Although there are differences in magnitude and exact location of regional precipitation changes from model to model, the same basic mechanisms are involved in the oceanic modulation of the response to Orbital Forcing and this gives rise to a robust ensemble response for each of the monsoon systems. Comparison of simulated and reconstructed changes in regional climate suggest that the coupled ocean–atmosphere simulations produce more realistic changes in the northern-hemisphere monsoons than atmosphere-only simulations, though they underestimate the observed changes in precipitation in all regions. Evaluation of the southern-hemisphere monsoons is limited by lack of quantitative reconstructions, but suggest that model skill in simulating these monsoons is limited.
-
simulations of the impact of Orbital Forcing and ocean on the asian summer monsoon during the holocene
Global and Planetary Change, 2008Co-Authors: Yuefeng Li, Sandy P HarrisonAbstract:The importance of Orbital Forcing and ocean impact on the Asian summer monsoon in the Holocene is investigated by comparing simulations with a fully coupled ocean–atmosphere general circulation model (FOAM) and with the atmospheric component of this model (FSSTAM) forced with prescribed modern sea-surface temperatures (SSTs). The results show: (1) the ocean amplifies the Orbitally-induced increase in African monsoon precipitation, makes somewhat increase in southern India and damps the increase over the southeastern China. (2) The ocean could change the spatial distribution and local intensity of the Orbitally-induced latitudinal atmospheric oscillation over the southeastern China and the subtropical western Pacific Ocean. (3) The Orbital Forcing mostly enhances the Asian summer precipitation in the FOAM and FSSTAM simulations. However, the ocean reduces the Orbitally-induced summer precipitation and postpones the time of summer monsoon onset over the Asian monsoon region. (4) The Orbital Forcing considerably enhances the intensity of upper divergence, which is amplified by ocean further, over the eastern hemisphere. But the divergence is weaker in the FOAM simulations than in the FSSTAM simulations when the Orbital Forcing is fixed. (5) The Orbital Forcing can enhance the amplitude of precipitation variability over the subtropical Africa, the southeastern China and northwestern China, inversely, reduce it over central India and North China in the FOAM and FSSTAM simulations. The ocean obviously reduces the amplitude of precipitation variability over most of the Asian monsoon regions in the fixed Orbital Forcing simulations. (6) The areas characterized by increased summer precipitation in the long-term mean are mostly characterized by increased amplitude of short-term variability, whereas regions characterized by decreased precipitation are primarily characterized by decreased amplitude of short-term variability. However, the influences of Orbital Forcing or dynamical ocean on regional climate depend on the model.
-
intercomparison of simulated global vegetation distributions in response to 6 kyr bp Orbital Forcing
Journal of Climate, 1998Co-Authors: Sandy P Harrison, John E Kutzbach, Ayako Abeouchi, D Jolly, F Laarif, Buwen Dong, K Herterich, C D Hewitt, S Joussaume, J F B MitchellAbstract:Abstract The response of ten atmospheric general circulation models to Orbital Forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated sh...
-
vegetation and soil feedbacks on the response of the african monsoon to Orbital Forcing in the early to middle holocene
Nature, 1996Co-Authors: John E Kutzbach, Gordon B Bonan, Jonathan A Foley, Sandy P HarrisonAbstract:FOSSIL pollen, ancient lake sediments and archaeological evidence from Africa indicate that the Sahel and Sahara regions were considerably wetter than today during the early to middle Holocene period, about 12,000 to 5,000 years ago1–4. Vegetation associated with the modern Sahara/Sahel boundary was about 5° farther north, and there were more and larger lakes between 15 and 30° N. Simulations with climate models have shown that these wetter conditions were probably caused by changes in Earth's Orbital parameters that increased the amplitude of the seasonal cycle of solar radiation in the Northern Hemisphere, enhanced the land-ocean temperature contrast, and thereby strengthened the African summer monsoon5–7. However, these simulations underestimated the consequent monsoon enhancement as inferred from palaeorecords4. Here we use a climate model to show that changes in vegetation and soil may have increased the climate response to Orbital Forcing. We find that replacing today's Orbital Forcing with that of the mid-Holocene increases summer precipitation by 12% between 15 and 22° N. Replacing desert with grassland, and desert soil with more loamy soil, further enhances the summer precipitation (by 6 and 10% respectively), giving a total precipitation increase of 28%. When the simulated climate changes are applied to a biome model, vegetation becomes established north of the current Sahara/Sahel boundary, thereby shrinking the area of the Sahara by 11% owing to Orbital Forcing alone, and by 20% owing to the combined influence of Orbital Forcing and the prescribed vegetation and soil changes. The inclusion of the vegetation and soil feedbacks thus brings the model simulations and palaeovegetation observations into closer agreement.
Yi Ming - One of the best experts on this subject based on the ideXlab platform.
-
simulated responses of the west african monsoon and zonal mean tropical precipitation to early holocene Orbital Forcing
Geophysical Research Letters, 2018Co-Authors: Jane E Smyth, Spencer A Hill, Yi MingAbstract:This study seeks to improve our mechanistic understanding of how the insolation changes associated with Orbital Forcing impact the West African monsoon and zonal‐mean tropical precipitation. We impose early Holocene Orbital parameters in simulations with the Geophysical Fluid Dynamics Laboratory AM2.1 atmospheric general circulation model, either with fixed sea surface temperatures, a 50‐m thermodynamic slab ocean, or coupled to a dynamic ocean (CM2.1). In all cases, West African Monsoon rainfall expands northward, but the summer zonal‐mean Intertropical Convergence Zone does not—there is drying near 10°N, and in the slab ocean experiment a southward shift of rainfall. This contradicts expectations from the conventional energetic framework for the Intertropical Convergence Zone location, given anomalous southward energy fluxes in the deep tropics. These anomalous energy fluxes are not accomplished by a stronger Hadley circulation; instead, they arise from an increase in total gross moist stability in the northern tropics.
-
Simulated Responses of the West African Monsoon and Zonal‐Mean Tropical Precipitation to Early Holocene Orbital Forcing
Geophysical Research Letters, 2018Co-Authors: Jane E Smyth, Spencer A Hill, Yi MingAbstract:This study seeks to improve our mechanistic understanding of how the insolation changes associated with Orbital Forcing impact the West African monsoon and zonal‐mean tropical precipitation. We impose early Holocene Orbital parameters in simulations with the Geophysical Fluid Dynamics Laboratory AM2.1 atmospheric general circulation model, either with fixed sea surface temperatures, a 50‐m thermodynamic slab ocean, or coupled to a dynamic ocean (CM2.1). In all cases, West African Monsoon rainfall expands northward, but the summer zonal‐mean Intertropical Convergence Zone does not—there is drying near 10°N, and in the slab ocean experiment a southward shift of rainfall. This contradicts expectations from the conventional energetic framework for the Intertropical Convergence Zone location, given anomalous southward energy fluxes in the deep tropics. These anomalous energy fluxes are not accomplished by a stronger Hadley circulation; instead, they arise from an increase in total gross moist stability in the northern tropics.
John E Kutzbach - One of the best experts on this subject based on the ideXlab platform.
-
calendar effect on phase study in paleoclimate transient simulation with Orbital Forcing
Climate Dynamics, 2011Co-Authors: Guangshan Chen, John E Kutzbach, R GallimoreAbstract:Several studies have shown that the use of different calendars in paleoclimate simulations can cause artificial phase shifts on insolation Forcing and climatic responses. However, these important calendar corrections are still often neglected. In this paper, the phase shifts at the precession band is quantitatively assessed by converting the model data of the transient GCM climate simulation of Kutzbach et al. (Clim Dyn 30:567–579, 2008) from the “fixed-day” calendar to the “fixed-angular” calendar with a new and efficient approach. We find that insolation has a big phase shift in September–October–November (SON) when the vernal equinox (VE) is fixed to March 21. At high latitude, the phase bias is up to 60° (about 3650 years). The insolation phase bias in SON in Southern Hemisphere (SH) is especially important because it can influence the timing of the SH summer monsoon response due to the large heat capacity of ocean. The calendar correction has minor effect (±2°) on the phase relationships between Forcing and precipitation responses of the six global summer monsoons studied in Kutzbach et al. (2008). After correcting the calendar effect, especial on SH ocean temperature, the new phase wheel results are more similar for both hemispheres. The results suggest that the calendar effect should be corrected before discussing the dynamics between Orbital Forcing and climatic responses in phase studies of transient simulations.
-
simulation of the evolutionary response of global summer monsoons to Orbital Forcing over the past 280 000 years
Climate Dynamics, 2008Co-Authors: John E Kutzbach, Guangshan ChenAbstract:We describe the evolutionary response of northern and southern hemisphere summer monsoons to Orbital Forcing over the past 280,000 years using a fully coupled general circulation ocean-atmosphere model in which the Orbital Forcing is accelerated by a factor of 100. We find a strong and positive response of northern (southern) summer monsoon precipitation to northern (southern) summer insolation Forcing. On average, July (January) precipitation maxima and JJA (DJF) precipitation maxima have high coherence and are approximately in phase with June (December) insolation maxima, implying an average lag between Forcing and response of about 30° of phase at the precession period. The average lag increases to over 40° for 4-month precipitation averages, JJAS (DJFM). The phase varies from region to region. The average JJA (DJF) land temperature maxima also lag the June Orbital Forcing maxima by about 30° of phase, whereas ocean temperature maxima exhibit a lag of about 60° of phase at the precession period. Using generalized measures of the thermal and hydrologic processes that produce monsoons, we find that the summer monsoon precipitation indices for the six regions all fall within the phase limits of the process indices for the respective hemispheres. Selected observational studies from four of the six monsoon regions report approximate in-phase relations of summer monsoon proxies to summer insolation. However other observational studies report substantial phase lags of monsoon proxies and a strong component of Forcing associated with glacial-age boundary conditions or other factors. An important next step will be to include glacial-age boundary condition Forcing in long, transient paleoclimate simulations, along with Orbital Forcing.
-
intercomparison of simulated global vegetation distributions in response to 6 kyr bp Orbital Forcing
Journal of Climate, 1998Co-Authors: Sandy P Harrison, John E Kutzbach, Ayako Abeouchi, D Jolly, F Laarif, Buwen Dong, K Herterich, C D Hewitt, S Joussaume, J F B MitchellAbstract:Abstract The response of ten atmospheric general circulation models to Orbital Forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated sh...
-
vegetation and soil feedbacks on the response of the african monsoon to Orbital Forcing in the early to middle holocene
Nature, 1996Co-Authors: John E Kutzbach, Gordon B Bonan, Jonathan A Foley, Sandy P HarrisonAbstract:FOSSIL pollen, ancient lake sediments and archaeological evidence from Africa indicate that the Sahel and Sahara regions were considerably wetter than today during the early to middle Holocene period, about 12,000 to 5,000 years ago1–4. Vegetation associated with the modern Sahara/Sahel boundary was about 5° farther north, and there were more and larger lakes between 15 and 30° N. Simulations with climate models have shown that these wetter conditions were probably caused by changes in Earth's Orbital parameters that increased the amplitude of the seasonal cycle of solar radiation in the Northern Hemisphere, enhanced the land-ocean temperature contrast, and thereby strengthened the African summer monsoon5–7. However, these simulations underestimated the consequent monsoon enhancement as inferred from palaeorecords4. Here we use a climate model to show that changes in vegetation and soil may have increased the climate response to Orbital Forcing. We find that replacing today's Orbital Forcing with that of the mid-Holocene increases summer precipitation by 12% between 15 and 22° N. Replacing desert with grassland, and desert soil with more loamy soil, further enhances the summer precipitation (by 6 and 10% respectively), giving a total precipitation increase of 28%. When the simulated climate changes are applied to a biome model, vegetation becomes established north of the current Sahara/Sahel boundary, thereby shrinking the area of the Sahara by 11% owing to Orbital Forcing alone, and by 20% owing to the combined influence of Orbital Forcing and the prescribed vegetation and soil changes. The inclusion of the vegetation and soil feedbacks thus brings the model simulations and palaeovegetation observations into closer agreement.
Caroline L. Prescott - One of the best experts on this subject based on the ideXlab platform.
-
Indian monsoon variability in response to Orbital Forcing during the late Pliocene
Global and Planetary Change, 2019Co-Authors: Caroline L. Prescott, Alan M. Haywood, Aisling M. Dolan, Stephen J. Hunter, Julia C. TindallAbstract:Abstract The Asian monsoon is a major component of the global climate system and can be divided into two subsystems, the Indian monsoon and the East Asian monsoon. Insights into monsoon behaviour and dynamics can be gained through studying past warm intervals in Earth's history. One such interval is the Pliocene epoch, specifically the mid-Piacenzian Warm Period (mPWP; 3.264–3.025 Ma). This time is characterised as a period of sustained warmth, with annual mean temperatures 2–3 °C higher than the pre-industrial era. Studies have examined the East Asian monsoon during the mPWP from both a geological data and climate modelling perspective. However, there has been little investigation into the behaviour of the Indian monsoon. Using a coupled atmosphere-ocean global climate model (HadCM3), the Indian summer monsoon response to Orbital Forcing during the mPWP is studied. Of the simulated interglacial events (Marine Isotope Stages KM5c, KM3, K1 and G17), MIS KM5c is the only one with a near-modern Orbital Forcing. This experiment is compared to a pre-industrial simulation to determine the nature of the mPWP Indian summer monsoon in the absence of a different pattern if insolation Forcing. The monsoon at MIS KM5c, is simulated to be stronger than pre-industrial, with higher surface air temperatures and precipitation over land due to higher levels of CO2. MIS G17, K1, and KM3 represent interglacial events of similar magnitude with different insolation Forcing than MIS KM5c. The Indian summer monsoon is simulated to be significantly stronger for the interglacials K1 and KM3, compared to KM5c. This is due to stronger precession Forcing causing an increase in summer surface air temperature and precipitation. When combined with Pliocene geological boundary conditions, these results highlight the significant effect of Orbital Forcing on the strength of the Indian summer monsoon. The sensitivity of the Indian monsoon to Orbital Forcing has important implications for any parallels drawn between Pliocene and future monsoon behaviour.
-
regional climate and vegetation response to Orbital Forcing within the mid pliocene warm period a study using hadcm3
Global and Planetary Change, 2018Co-Authors: Caroline L. Prescott, Alan M. Haywood, Aisling M. Dolan, S J Hunter, Julia C. TindallAbstract:Abstract Regional climate and environmental variability in response to Orbital Forcing during interglacial events within the mid-Piacenzian (Pliocene) Warm Period (mPWP; 3.264–3.025 Ma) has been rarely studied using climate and vegetation models. Here we use climate and vegetation model simulations to predict changes in regional vegetation patterns in response to Orbital Forcing for four different interglacial events within the mPWP (Marine Isotope Stages (MIS) G17, K1, KM3 and KM5c). The efficacy of model-predicted changes in regional vegetation is assessed by reference to selected high temporal resolution palaeobotanical studies that are theoretically capable of discerning vegetation patterns for the selected interglacial stages. Annual mean surface air temperatures for the studied interglacials are between 0.4 °C to 0.7 °C higher than a comparable Pliocene experiment using modern Orbital parameters. Increased spring/summer and reduced autumn/winter insolation in the Northern Hemisphere during MIS G17, K1 and KM3 enhances seasonality in surface air temperature. The two most robust and notable regional responses to this in vegetation cover occur in North America and continental Eurasia, where forests are replaced by more open-types of vegetation (grasslands and shrubland). In these regions our model results appear to be inconsistent with local palaeobotanical data. The Orbitally driven changes in seasonal temperature and precipitation lead to a ~ 30% annual reduction in available deep soil moisture (2.0 m from surface), a critical parameter for forest growth, and subsequent reduction in the geographical coverage of forest-type vegetation; a phenomenon not seen in comparable simulations of Pliocene climate and vegetation run with a modern Orbital configuration. Our results demonstrate the importance of examining model performance under a range of realistic Orbital Forcing scenarios within any defined time interval (e.g. mPWP). Additional Orbitally resolved records of regional vegetation are needed to further examine the validity of model-predicted regional climate and vegetation responses in greater detail.
-
Orbital Forcing and its importance in understanding the warm Pliocene
2017Co-Authors: Caroline L. PrescottAbstract:The Pliocene is traditionally viewed as an epoch with a warm and stable climate. Data-Model comparisons for the mid-Pliocene (~3.3 – 3 Ma) have identified regions where models do not agree with geological proxies. Palaeoenvironmental syntheses used in these comparisons are time-averaged. It has been hypothesised that Orbital cyclicity within the mid-Pliocene, not accounted for in previous model simulations or data syntheses, could contribute to data-model discord. Study of the Pleistocene (~11.7ka – 2.6 Ma) has established the importance of understanding climate variability and distinguishing the specific character of separate interglacial or glacial events. This thesis confirms such variability should be expected in the Pliocene. Using a climate model, two interglacials (MIS KM5c and K1) in the Pliocene are compared, and results demonstrate changes in the surface air temperatures (SATs) due to changes in Orbital Forcing can be substantial and differ between interglacials. A further two interglacials (G17 and KM3) are investigated, and changes in regional vegetation patterns and the summer Indian monsoon in response to Orbital Forcing over the four interglacial events are analysed. A notable vegetation response is seen in the continental interiors of North America and Eurasia, where forests are replaced by grassland and shrubland, and is most widespread for interglacials with the strongest Orbital Forcing (most different from present day). The Indian monsoon is slightly stronger than pre-industrial in KM5c (an interglacial with near-modern orbit), driven by higher CO2, and is significantly more intense in G17, K1 and KM3 than KM5c, due to Orbital-driven increased seasonal SATs. Orbital Forcing throughout Pliocene interglacials is found to have a significant effect on the simulation of regional climate, vegetation and the Indian monsoon system within the modelling framework used here. Time-averaged palaeoenvironmental synthesis therefore, cannot be expected to concur with climate model outputs using time specific Orbital Forcing.
-
the transient response of ice volume to Orbital Forcing during the warm late pliocene
Geophysical Research Letters, 2017Co-Authors: Aisling M. Dolan, Alan M. Haywood, S J Hunter, Bas De Boer, Caroline L. PrescottAbstract:Examining the nature of ice sheet and sea level response to past episodes of enhanced greenhouse gas Forcing may help constrain future sea level change. Here, for the first time, we present the transient nature of ice sheets and sea level during the late Pliocene. The transient ice sheet predictions are forced by multiple climate snapshots derived from a climate model set up with late Pliocene boundary conditions, forced with different Orbital Forcing scenarios appropriate to two Marine Isotope Stages (MISs), MIS KM5c, and K1. Our results indicate that during MIS KM5c both the Antarctic and Greenland ice sheets contributed to sea level rise relative to present and were relatively stable. Insolation Forcing between the hemispheres was out of phase during MIS K1 and led to an asynchronous response of ice volume globally. Therefore, when variations of precession were high, inferring the behavior of ice sheets from benthic isotope or sea level records is complex.
Spencer A Hill - One of the best experts on this subject based on the ideXlab platform.
-
simulated responses of the west african monsoon and zonal mean tropical precipitation to early holocene Orbital Forcing
Geophysical Research Letters, 2018Co-Authors: Jane E Smyth, Spencer A Hill, Yi MingAbstract:This study seeks to improve our mechanistic understanding of how the insolation changes associated with Orbital Forcing impact the West African monsoon and zonal‐mean tropical precipitation. We impose early Holocene Orbital parameters in simulations with the Geophysical Fluid Dynamics Laboratory AM2.1 atmospheric general circulation model, either with fixed sea surface temperatures, a 50‐m thermodynamic slab ocean, or coupled to a dynamic ocean (CM2.1). In all cases, West African Monsoon rainfall expands northward, but the summer zonal‐mean Intertropical Convergence Zone does not—there is drying near 10°N, and in the slab ocean experiment a southward shift of rainfall. This contradicts expectations from the conventional energetic framework for the Intertropical Convergence Zone location, given anomalous southward energy fluxes in the deep tropics. These anomalous energy fluxes are not accomplished by a stronger Hadley circulation; instead, they arise from an increase in total gross moist stability in the northern tropics.
-
Simulated Responses of the West African Monsoon and Zonal‐Mean Tropical Precipitation to Early Holocene Orbital Forcing
Geophysical Research Letters, 2018Co-Authors: Jane E Smyth, Spencer A Hill, Yi MingAbstract:This study seeks to improve our mechanistic understanding of how the insolation changes associated with Orbital Forcing impact the West African monsoon and zonal‐mean tropical precipitation. We impose early Holocene Orbital parameters in simulations with the Geophysical Fluid Dynamics Laboratory AM2.1 atmospheric general circulation model, either with fixed sea surface temperatures, a 50‐m thermodynamic slab ocean, or coupled to a dynamic ocean (CM2.1). In all cases, West African Monsoon rainfall expands northward, but the summer zonal‐mean Intertropical Convergence Zone does not—there is drying near 10°N, and in the slab ocean experiment a southward shift of rainfall. This contradicts expectations from the conventional energetic framework for the Intertropical Convergence Zone location, given anomalous southward energy fluxes in the deep tropics. These anomalous energy fluxes are not accomplished by a stronger Hadley circulation; instead, they arise from an increase in total gross moist stability in the northern tropics.