• central question: to what extent does low-latitude climate respond to glacial-interglacial changes at higher latitudes?
• no single pattern of variability has been identified from the low-latitude proxy records
• two separate patterns:
• precipitation variation: lake levels, Nile outflow, NW Africa eolian dust, equatorial Atlantic upwelling (McIntyre and Molfino), Asian monsoon intensity all vary with precessional forcing
• other marine records of eolian dust, pollen, grass cuticle abund. and lake level and terrestrial vegetation records vary with obliquity and eccentricity, suggesting a high-latitude origin

• dust entrained in Arabian and NE Africa after early summer drying of immature soils and subsequent deflation by NW winds (Arabian sources) and SW winds (NE African sources (contrast with SCS where dust was a winter monsoon phenomenon)
• sensible and latent heating over northern Africa during summer causes inflow of moisture-laden air from eastern equatorial Africa
• Latent heat The heat released or absorbed per unit mass by a system in a reversible isobaric-isothermal change of phase. In meteorology, the latent heats of evaporation (or condensation), fusion (melting), and sublimation of water substance are of importance.
• se trades cross equator to deliver rain sporadically
• ne trades blow across North Africa in winter
• two dust plumes:
• summer plume centered @ ~10-25°N; Saharan source; midtroposphere path
• winter plume between 10û and 5°N; s. Sahara and Sahel sources; lowest troposphere

• primary objective: use GCM to understand how seasonal components of Asian and African monsoon climate respond to specific boundary condition changes
• model prescribes changes in:
• season distribution of solar insolation
• high-latitude glacial ice cover
• North Atlantic sea surface temperature
• Tibetan plateau orography
• climate responses are looked for in:
• surface wind
• pressure
• precipitation
• temperature field responses

• differences between reality and model world
• winter SLP between Asian high and Indian Ocean low 30% too strong
• ne trades are too week and southward of their present position
• too much winter precipitation in s. Asia and excess precip in Bay of Bengal
• summer monsoon rains don't penetrate Himalayas sufficiently

• 11K.FULL: 11 kyr B.P. max insolation, glacial ice, sea level boundary conditions
• 11K.NOICE: same as above, but with modern glacial ice distribution
• 18K.FULL: 18 kyr B.P. boundary conditions including lower sea level and different orbital configurations, but with modern CO₂ levels
• 18K.NOICE: as above except with modern ice distribution
• CONT.SST: modern conditions except for 18 kyr SST for North Atlantic n. of 25 degrees N
• CONT.NM: modern conditions except Himalayan/Tibet reduced to <500 m

• winter climate: 11K.NOICE vs. CONT shows dramatic high lat changes and only moderate winter monsoon changes; no change in NE trades, but lower SLP over Asian continent
• expected: lower winter insolation causes cooler NH temps, high SLPs and stronger anticyclonic circ. over Asia
• actual result: North America cooler, but Asia and Europe have higher temps and lower SLPs
• summer climate: 11K.NOICE run-summer monsoon enhanced; stronger SW winds over Arabian Sea and e. Asia; SLP lower over India and central Asia; precip over s. Asia increased; greater penetration of Asian interior
• African monsoon less responsive but there is significant increase in precipitation (contra Prell and Kutzbach)
• Prell and Kutzbach found the Asian summer monsoon to be linearly related to summer insolation

• winter climate: enhanced tradewinds, decreased precip over Arabia, ne Africa and s. Asia ... nw Africa unaffected
• 11K.FULL-11K.NOICE: ice causes stronger tradewinds over n. Indian Ocean, s. Asia drier ... no significant changes in North Africa
• 18K.FULL-18K.NOICE: anticyclonic circ strengthened, esp. zonal component b/c of intensified high over Himalayas; e. Med, Arabia and ne Africa all drier; assoc. w/ advection of cooler and drier air from high-lat ice sheets
• summer climate: weakened summer monsoon
• 11K.FULL-11K.NOICE: no consistent changes in cyclonic circ; minor reduction in precip over s. Asia
• 18K.FULL-18K.NOICE: reduced cyclonic circ. over s. Asia; reduced northward penetration of summer heat low over s. Asia; significant precip. reductions over India and Arabia attributable to albedo changes
• the Clausius-Clapeyron Equation: relates the vapor pressure at certain temperatures to the heat of vaporization,
• ln(P2/P1) = -DH_vap/R * (1/T2- 1/T1)
• DH_vap/R= heat of vaporization in kilojoules/kj/molK

• winter climate: cooler SSTs cause dramatic cooling over N. Atlantic, Europe and central Asia, NW Africa and Arabia; strengthens NE trades along NW Africa, but only minor precip reductions
• summer climate: cooler SST cause significant reductions in the northern limb of the African monsoon, while the Asian monsoon is rel. unaffected; reduced inflow to North Africa w/ corresponding decreases in air temp and precip; increased outflow of surface air from Africa and Asia

• winter climate: most striking increase in winter trade wind intensity
• enhances anticyclonic circ over Asia w/ incr. zonal flow
• enhanced Hadley circ and corresponding higher SLP over S. Asia
• decreased precip. and surface warming of s. Asia
• no obvious changes over North Africa
• cold surges are caused by coalescence of orographically enhanced surges of cold Siberian air and downward sector of Hadley circ
• summer climate: weakens, but doesn't eliminate Asian cyclonic flow
• reduction in land-sea SLP gradient
• attritbutable to severe reductions in release of latent heat in the midtroposphere
• severe reductions in precipiation across broad areas of s. Asia
• westward dislocation of precip maximum
• mountains are important for focussing moist convection and latent heat release

See Table 2 on pg. 7283 for summary of relative sensitivities of both regions to various forcing factors.

• South Asia: attain max values with precession max at 10.4 kyr
• Arabian Sea upwelling
• SW Monsoon wind speed
• Arabian and western Indian lake levels
• West Africa: precessional forcing seen in
• west African lake levels
• Niger River outflow
• Nile River outflow
• marine records of fresh-water diatoms from lake beds
• east Atlantic upwelling
• tropical lake levels are highest at 7-9 kyr, apparently lagging precession max at 11 kyr
• C¹⁴ dates from lakes were too young (checked against U/Th dates from Barbados corals)

• glacial climate much drier
• increased eolian quartz and dust in e. Atlantic
• eolian sands and evaporites in w. Africa
• marine pollen
• phytoliths increased in marine record
• increased eolian grain size; stronger winds
• increased organic carbon values off nw Africa; more upwelling

• glacial climate much drier
• increased eolian dust in Arabian Sea
• pollen evidence of vegetation changes
• macrofaunal changes in east Africa
• stronger NE trade winds
• stable isotope (C-13) analyses of organic carbon in Bay of Bengal suggests stronger wind stress curl-driven upwelling
• model results suggesting reduced Asian monsoon with greater ice cover are not borne out by paleoclimate records