Hemispheric Insolation Forcing of the Indian Ocean and Asian Monsoon: Local versus Remote Impacts*

Abstract

Insolation forcing related to the earth's orbital parameters is known to play an important role in regulating variations of the South Asian monsoon on geological time scales. The influence of insolation forcing on the Indian Ocean and Asian monsoon is studied in this paper by isolating the Northern and Southern Hemispheric insolation changes in several numerical experiments with a coupled ocean-atmosphere model. The focus is on the response of South Asian summer rainfall (monsoon strength) with emphasis on impacts of the local versus remote forcing and possible mechanisms. The model results show that both Northern Hemisphere (NH) and Southern Hemisphere (SH) summer insolation changes affect the Indian Ocean and Asian monsoon as a local forcing (in the same hemisphere), but only the SH changes result in remote (in the other hemisphere) forcing. The NH insolation change has a local and immediate impact on NH summer monsoons from North Africa to South and East Asia, while the SH insolation change has a remote and seasonal-scale delayed effect on the South Asian summer monsoon rainfall. When the SH insolation is increased from December to April, the sea surface temperature (SST) in the southern tropical Indian Ocean remains high from January to July. The increased SST produces more atmospheric precipitable water over the southern tropical Indian Ocean by promoting evaporation from the ocean. The enhanced precipitable water over the southern Indian Ocean is transported northward to the South Asian monsoon region by the lower-tropospheric mean cross-equatorial flows with the onset of the Asian monsoon increasing precipitable water over South Asia, eventually leading to the increase of Indian summer monsoon precipitation. Thus, these model experiments, while idealized and not fully representing actual orbitally forced insolation changes, confirm the broadscale response of northern monsoons to NH summer insolation increases and also illustrate how SH summer insolation increases can have a delayed influence on the Indian summer monsoon.

Southern Hemisphere forcing of Pliocene δ18O and the evolution of Indo‐Asian monsoons

“…This leads to maximum ocean to atmosphere latent heat flux and release of this latent heat during SM precipitation over Asia. Sensitivity tests using a coupled ocean-atmosphere model support this scenario; when SH insolation is increased from December to April, the sea surface temperature (SST) in the southern tropical Indian Ocean remains high from January to July resulting in increase evaporation, increased crossequatorial moisture transport, and increased SM precipitation [Liu et al, 2006]. If glacial boundary conditions were the only factors driving Indo-Asian monsoon variance, then the strongest summer monsoons would be in phase with ice minima and the strongest winter monsoons would be in phase with ice maxima in Figure 6.…”

The global monsoon across timescales: coherent variability of regional monsoons

“…Here, our goal is to note that the divergence in opinion is, at least partly, related to the different natures of the proxies used, with upwelling records based on wind being physically distinct from the speleothem records based on rain (e.g., Liu et al, 2006a;Clemens et al, 2010). Looking back at the evolution of paleo-monsoon research, it was initiated with both wind-and rain-based proxies, and the two kinds of sequences correlated fairly well at that stage.…”

Orbital Asian summer monsoon dynamics revealed using an isotope-enabled global climate model

“…8). Similarly, the monsoon winds could be affected by such ocean internal feedback not only from the Pacific 15 , but also from the southern Indian Ocean 6,7,[11][12][13][14]16,29 and lead to different timing. Our study therefore highlights a complex Asian monsoon dynamic with distinct response inside the monsoon system.…”