D.M. Anderson and W.L. Prell, "A 300 kyr record of upwelling off Oman during the late Quaternary: evidence of the Asian southwest monsoon" Paleoceanography, 8:193-208 (1993)

• land-sea pressure gradient extends from surface high near 30°S to surface low over Asia
• upwelling caused by combined influence of wind-driven coastal upwelling and open-ocean Ekman pumping

• changes in rel. abundance of bulloides indicate that upwelling was reduced during the LGM and stronger at the precessional max at 9 kyr (insolation 8% greater than modern)
• long time series so far contradict the assumption that precession drives upwelling variations; dominant cyclicity is 50 kyr at off-shore sites

• Leg 117 sites (723 etc) are on the Oman margin, closer to shore than earlier sites
• examined relative abundance of different species and changes in accumulation rates of foraminifer shells

• 800 m water depth; site 723 is at 18°N, 58°E
• examined upper 60 m of core in holes A and B; mag susceptibility to correlate
• ages assigned based on SPECMAP; used 17 tie-points; mostly to planktonic, but some to benthic record, when planktonic record lacked structure

• bulloides abundance ranged from <10% to >60% in last 300 kyr; measured relative to 33 other species
• average abundance in coretop samples is 36%
• more abundant than coretops in interglacials and less in glacials
• trend toward increasing abundance from 300 ka to present
• 20-30 kyr cycles are superimposed on the 100 kyr glacial cycles
• bulloides cycles are in phase with ice-volume changes

• number of whole shells in >150 µm fraction varies from <300 shells/gram to >5000 shells/gram
• concentrations are greatest during interglacials and lowest during glacials
• reduction can not be attributed to dilution by clay b/c sed rate is only 3-4x higher in glacial compared to interglacial
• shell accumulation variation is more regular than bulloides abundance
• 20 kyr cycles are larger during interglacial and smaller during glacial
• shell concentration max may precede bulloides abundance max at some transitions, but is usually coincident
• converted to shell accumulation rates; need sed rates and estimates of sed density
• shell accumulation rates were higher during interglacials even though sedimentation rates were several times lower
• dissolution appears to increase when shell accumulation rates are low (glacials) and vice versa

• variance density (sigma-squared/frequency)
• autocovariance function: correlations between observations as a function of the intervening time interval
• largest concentration of variance in bulloides abundance and shell accumulation is at 100 kyr
• second largest is at 23 kyr
• cross-spectral analysis to examine timing of effects relative to causes
• bulloides abundance/shell accumulation are in phase with insolation forcing at 100 kyr and with min ice volume
• bulloides abundance/shell accumulation lags radiative forcing by 4 to 6 kyr at 23 kyr cycle
• same result when cross-spectral analysis between upwelling proxies and the isotopic curve
• shells/gram index may lead the min ice volume by ~1 kyr
• is upwelling responding directly to summer insolation variations in precessional frequency or is it indirect, driven by glacial-interglacial changes at this frequency

• below 36% the two measures are proportional; above 36% percent abundance increases more slowly than shell/gram
• this transition is outside the range of modern values in Arabian Sea (3-36%)

• dominance of the 100 kyr cycle indicates that the land-sea pressure gradient is sensitive to glacial-interglacial cycles
• glacial-interglacial effects either inhibit SW monsoon response to precessional cycles in summer insolation or directly change tropical circulation in some other way that alters the land-sea pressure gradient

• bulloides abundance is more constant off-shore, so variance is less
• the dominant frequences are 1/50 kyr and smaller one at 1/23 kyr
• these lag the inshore cycles by 2 kyr
• difference is caused by positive wind-stress curl driven upwelling offshore
• jet-like structure of the SW monsoon; NW of the axis of the jet, wind-stress curl is positive (upwelling), while SE of the axis it is negative (downwelling)
• coastal upwelling is roughly an order of magnitude larger than wind-stress curl-driven upwelling
• variations in offshore upwelling largely driven by changes in the location of the low-level jet
• 1979-1982: strongest winds produce weaker offshore upwelling because wind stress curl is reduced; jet migrates to the NW when SW monsoon winds are stronger

• lower surface pressure over Asia are caused by increased summer insolation; result in increase in the land-sea gradient and stronger SW monsoon winds across Arabian Sea (model result)

• decrease in monsoon intensity during glacials caused by:
• warmer Indian Ocean SSTs, expanded NH ice volume, lowered SL and higher land albedo; all increase pressure over Asia and decrease pressure over the ocean
• monsoon more sensitive to albedo changes than SST (model result)
• snow cover increases albedo and melting of snow back absorbs heat and adds moisture to soil; both decrease land-sea temperature contrast