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°o f 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.

[1] The Milankovitch paradigm links the timing (phase) of ice volume minima to summer insolation maxima in the hemisphere where ice volume dominates; consistent application of this paradigm dictates that Pliocene ice volume minima should lag Southern Hemisphere summer insolation maxima. We infer the magnitude of this lag on the basis of the phase relationship between equatorial sea surface temperature and benthic d18 O. We infer that Pliocene d18 O minima should lag obliquity maxima by 19°(2.2 ka), broadly consistent with the current global marine d18 O chronology, and precession maxima by 32°(2 ka), a difference of 160°(10.2 ka) relative to the current global marine d18 O chronology. Only in the context of this revision are Pliocene summer and winter monsoon phase relationships consistent with direct orbital forcing across the entire Indo-Asian region, including marine and terrestrial proxies from the Chinese Loess Plateau, the South China Sea, and the Arabian Sea. Strong Pliocene summer and winter monsoons were in phase with one another, strengthened at obliquity minima and precession minima; the summer monsoon was also strengthened at precession maxima, yielding a semiprecession spectral signal. Strong Pliocene monsoons at orbital extremes indicate a direct response to fast physics processes including sensible heating and cooling of the Asian landmass and, for the summer monsoon, the export of latent heat from the southern Indian Ocean. As Northern Hemisphere ice volume grew into the Pleistocene, the timing of strong winter and summer monsoons drifted apart becoming influenced by the combined effects of fast physics and slow physics (ice volume) variables. The phase of strong winter monsoons shifted toward ice maxima, and the phase of strong summer monsoons shifted toward ice minima.

Observations have shown that the Tibetan Plateau (TP) has experienced elevation-dependent warming (EDW) during recent decades, that is, greater warming at higher elevations than at lower elevations. However, the factors and their mechanisms driving these changes remain unclear, due to scarce radiation-related observations. In the present study, four CCSM3 experiments using the 1990 control and quadrupled (4×) CO 2 levels, with fine and coarse resolutions, were examined to shed light on the mechanisms driving EDW. The differences in annual and seasonal surface temperatures (TS) between the 4× CO 2 and 1990 control runs, using T85 resolution, feature clear changes with elevation. In addition, EDW 500 m above the ground surface is much weaker and almost disappears at the surface elevations higher than 2000 m. This implies that the greater warming mainly occurs at the near surface with higher elevations and should be attributed to changes in the surface energy budget. In the 4× CO 2 , there are greater increases (compared to the 1990 control run) in the net solar, net longwave and sensible heat fluxes at the surface at higher elevations, but lower levels of the parameters are simulated at lower elevations. These differences lead to increases in the heat storage at the surface and finally result in greater warming at higher elevations. Compared with the net longwave flux, the relative net shortwave flux at the surface increases more evidently at higher elevations, implying that the increase in net shortwave flux at the surface plays a dominant role in producing greater warming at higher elevations. The elevation range between 2000 and 3000 m appears to be a turning point; below 2000 m, the total cloud increases and subsequently constrains the surface net solar radiation. Above 3000 m, the total cloud decreases but shows little elevation dependency, favoring the increases in the surface net solar radiation, and decreases in the snow depth, with more differences with increasing elevation, lead to the reduced surface albedo. This further facilitates Climatic Change (2016) 135:509-519 the absorption of solar radiation at higher elevations. Therefore, the combined effects of changes in the snow depth and cloud cover in response to 4× CO 2 levels result in greater heat storage at the surface at higher elevations than at lower elevations, leading to EDW over and around the TP.