Remote Insolation Forcing of Orbital‐Scale South Asian Summer Monsoon Variability

Whether the South Asian summer monsoon (SASM) is controlled by local or remote insolation at the orbital band remains uncertain. Here, we perform a transient simulation forced by Earth's orbital parameters between 400 and 350 ka BP, a period characterized by significant contrast between local and remote insolation, to identify the SASM's response to insolation forcing. Simulation results suggest that the primary driver of orbital‐scale SASM variability is the Northern Hemisphere high‐latitude June insolation, as opposed to local insolation. High June insolation in the Southern Hemisphere might reduce the SASM intensity. Remote insolation influences the SASM by altering the latitudinal thermal gradient and, consequently, the meridional position of the South Asian high (SAH). The SAH is associated with intense convection and hence drives the meridional shift of the intertropical convergence zone and the SASM rain belt. Thus, orbital‐scale SASM variability is strongly related to remote insolation forcing.

delay has led to the development of the "latent heat" hypothesis, highlighting the significance of Southern Hemisphere insolation (Clemens et al., 2008(Clemens et al., , 2021)).Other studies propose a relationship between the SASM and interhemispheric interactions, specifically the Hadley circulation.These interactions are governed by June insolation gradient between 30°N and 30°S (Beck et al., 2018;Cheng et al., 2021Cheng et al., , 2022)).
Several studies aim to isolate the relative contribution of local and remote insolation (Bosmans et al., 2015;Tuenter et al., 2003) to SASM variations through slice sensitivity experiments (Liu et al., 2006;Wen et al., 2022), in light of the aforementioned controversy.However, the controversy remains unresolved (Wen et al., 2022).
Transient simulations could represent an alternative approach to address this issue.Precession is the primary factor controlling insolation variations, resulting in local and remote insolation being nearly identical over the past 800 ka (Figure 1).However, there is a notable contrast between remote and local insolation between 400 and 350 ka BP (Figure 1c), which could help us detect whether the SASM follows remote or local insolation changes via direct phase matching.A transient simulation during this period is an effective approach for this detection.
The obvious contrast between remote and local insolation will resurface in the next 50 ka (Figure 1a).It is essential to identify the SASM's response to local and remote insolation to accurately project its future behavior.Here, we aim to determine the dominant insolation forcing (remote or local) that affects orbital-scale SASM variability.This will be achieved through a transient simulation spanning 400-350 ka BP forced by Earth's orbital parameters using the Community Earth System Model version 1.2 (CESM1.2).

Model and Simulation
The  (Hurrell et al., 2013).A low-resolution version (T31_g37) of the CESM1.2 is utilized in this study.10.1029/2023GL105003 3 of 9 The initial conditions for the transient simulation are obtained by simulating the 400 ka BP climate (Exp_400ka) using realistic orbital configurations (Berger & Loutre, 1991) and greenhouse gases (CO 2 : 280.15 ppmv, CH 4 : 607.0 ppbv, and N 2 O: 285.7 ppbv) for 400 ka BP (Bereiter et al., 2015;Loulergue et al., 2008;Schilt et al., 2010).The Exp_400ka is initialized from the pre-industrial (PI) simulation and run for 2000 model years.Ice sheets are set to the PI level.The transient simulation is then conducted using realistic Earth's orbital parameters (Berger & Loutre, 1991) between 400 and 350 ka BP initializing from the last year of the Exp_400ka, with a tenfold acceleration scheme (Lorenz & Lohmann, 2004).Since this study investigates the SASM's response to orbital forcing, the transient simulation maintains greenhouse gas concentrations and ice sheet levels the same as the Exp_400ka.

SASM Response to Insolation Forcing
Observed SASM circulation is characterized by a cross-equatorial clockwise gyre in the lower troposphere over the tropical Indian Ocean from the NCEP/NCAR reanalysis (Kalnay et al., 1996), which is well captured by the PI simulation (Figure S1a and S1b in Supporting Information S1).The SASM brings abundant rainfall to southern Asia in summer, contrasting to the dry condition in winter (Figure S2 in Supporting Information S1).Observed mean annual precipitation cycle in southern Asia (5º-30°N, 70º-110°E) from the Global Precipitation Climatology Project (GPCP) data (Adler et al., 2018) is perfectly modeled by the PI simulation.Observed SASM rainfall mainly concentrates over the eastern Bay of Bengal, southern Tibetan Plateau, and western India (Figure S1d in Supporting Information S1).The PI simulation overestimates the rainfall over western India and underestimates it over the southern Tibetan Plateau (Figure S1e in Supporting Information S1), which might be caused by the relatively smooth topography in the low-resolution model.There are no evident differences in the mean state of SASM circulation and precipitation pattern between the PI and transient simulations (Figures S1 and S2 in Supporting Information S1).In general, the CESM1.2reasonably simulates SASM circulation and precipitation.
The SASM strength is measured by various indices including the rainfall amount and circulation intensity (Wang & Fan, 1999).The paleo-SASM intensity is mainly reconstructed through rainfall-related moisture level (An et al., 2011;Chen et al., 2014).This study defines the SASM index (SASMI) as the mean summer (June-July-August-September) rainfall in southern Asia.Simulated SASM rainfall displays a notable increase from 400 to ∼390 ka BP, followed by a decline until ∼382 ka BP, with a significant increase until ∼372 ka BP.Subsequently, a continuous decline is simulated until 350 ka BP, with the exception of a period of no evident trend between ∼365 and 355 ka BP (Figure 1d).Simulated SASM history is generally consistent with the stalagmite δ 18 O record of Sanbao Cave in East Asia (Figure S3b in Supporting Information S1; Cheng et al., 2016), which is interpreted as a proxy of SASM rainfall according to the upstream depletion mechanism (Liu et al., 2015), and the siliciclastic mass accumulation rate (MAR) record from core U1456 in the eastern Arabian Sea (Figure S3c in Supporting Information S1; Chen et al., 2019).
The study period exhibits three wave peaks at 356 ka BP, 373 ka BP, and 390 ka BP, respectively, in local summer insolation (red line in Figure 1c).The lowest peak is linked to the SASM maximum at ∼373 ka BP, while the highest peak corresponds to a comparatively weak SASM at ∼356 ka BP (Figure 1d).Obviously, changes in SASM rainfall do not follow local insolation.Therefore, it appears that the SASM is not controlled by local insolation.The SASM corresponds closely with northern high-latitude insolation (Figure 1d) and inter-tropical insolation gradient (Figure 1b), with the exception of a small discrepancy at ∼357 ka BP.At this point, the SASM is relatively weak, despite a relatively high 65°N insolation and inter-tropical insolation gradient.Generally, orbital-scale SASM variability is driven by remote insolation forcing between 400 and 350 ka BP.

Mechanisms of Remote Insolation Forcing
SASM rainfall is mainly caused by intense convection resulting from the northward movement of the Intertropical Convergence Zone (ITCZ) (Fleitmann et al., 2007).Deep convection in southern Asia is associated with a powerful upper-level anticyclone (Figure S4a in Supporting Information S1), known as the South Asian high (SAH) (Nützel et al., 2016).The ITCZ's meridional movement coincides with the corresponding movement of the SAH, exerting a major effect on the SASM rainfall pattern.The SAH's ridge is characterized by no winds (Wei et al., 2015).The SAH's northward shift is associated with an increase in 200 hPa geopotential height to the north of its climatological ridge line (zero isoline of 200 hPa zonal wind).Conversely, the geopotential height decreases to the south of the ridge line.The latitudinal gradient of 200 hPa geopotential height thereby reflects the SAH's meridional shift.The SAH index (SAHI) is defined as the normalization of 200 hPa geopotential height gradient between the northern (27.5°-37.5°N,60°-110°E) and southern (15°-25°N, 60°-110°E) boxes in Figure S4a in Supporting Information S1, following the method from Wei et al. (2015).The elevated index indicates a northward shift of the SAH.
Simulated SAH experiences a significant northward shift from 400 to ∼390 ka BP, followed by a minor southward shift from ∼390 to ∼382 ka BP.After that, there is a notable northward shift until ∼372 ka BP.Subsequently, the SAH starts to move southward, with the exception between ∼363 and 355 ka BP, during which there is no significant trend (Figure 1e).The southward migration continues until 350 ka BP.The SAH's meridional shift generally corresponds to northern high-latitude insolation (Figure 1e) and inter-tropical insolation gradient (Figure 1b).However, there is a weak correlation at ∼357 ka BP when the SAH does not experience a significant northward displacement despite high 65°N insolation and inter-tropical insolation gradient.The SAH's northward shift corresponds well with an intensified SASM, suggesting the SASM responds to remote insolation by relocating the SAH.
Since the SAH's ridge line is characterized by zero zonal wind at 200 hPa, the SAH's northward migration is accompanied by a corresponding shift of the zero wind isoline.Compared to 400-398 ka BP (low local and remote insolation), orbital forcing dramatically weakens the Asian subtropical westerly jet (ASWJ) between 374−372 ka BP (high remote insolation and inter-tropical insolation gradient) (Figure 2a).The SAHI is negatively correlated with 200 hPa westerlies to the north of the SAH's ridge line (R = 0.75, p < 0.01) (Figure S5 in Supporting Information S1).As a result, both the zero wind isoline and the SAH shift northward between 374 and 372 ka BP.In addition, the reduction of the westerlies to the south of the westerly jet is larger than those to the north, causing a northward shift of the ASWJ between 374 and 372 ka BP (Figure S5c in Supporting Information S1).The ASWJ is accompanied by anticyclonic wind shear (i.e., the SAH) to the right of the jet axis (Zhang et al., 2022).The ASWJ's migration promotes the SAH's northward shift between 374 and 372 ka BP.During 357-355 ka BP (high local insolation), orbital forcing also leads to a weaker ASWJ compared to 400-398 ka BP (Figure 2b).The reduction is obviously smaller than that between 374 and 372 ka BP (Figure 2c).Particularly, the reduction between 357 and 355 ka BP is relatively minor in northern South Asia (Figure 2b).Accordingly, the SAH experiences a slight northward shift between 357 and 355 ka BP (Figure 1e).Thus, the SAH's meridional shift is primarily influenced by remote insolation.
Accompanied by the SAH's northward shift, the associated deep convection, that is, the ITCZ, would also migrate further northward due to insolation forcing.The deep convection is characterized by low outgoing longwave radiation (OLR) (Wang & Fan, 1999).The OLR over India, northern Southeast Asia, and South China is significantly weakened while it is enhanced over the southern Bay of Bengal and southern Southeast Asia due to orbital forcing with high remote insolation (Figure 2d).This indicates a northward shift of the ITCZ between 374 and 372 ka BP.The ITCZ also experiences a northward shift between 357 and 355 ka BP due to orbital forcing with high local insolation (Figure 2e).The intensification of the deep convection is considerably smaller compared to 374-372 ka BP (Figure 2f), likely due to the relatively minor northward shift of the SAH (Figure 1e) since the SAH is characterized by intense convection in the lower troposphere of southern Asia (Nützel et al., 2016).The ITCZ's northward shift exerts a direct impact on the SASM.Therefore, the SASM's response to remote insolation forcing is probably through the SAH's movement and linked ITCZ.It is notable that a weak signal of double ITCZ is observed in summer over the eastern tropical Indian Ocean, with the southern branch along ∼5°S (Figure S1d in Supporting Information S1).This double ITCZ is also simulated by the CESM1.2(Figure S1e and S1f in Supporting Information S1).The ITCZ's southern branch shows a weakening trend in conjunction with the northward shift of the ITCZ's northern branch in response to orbital forcing (Figures 2d-2f).
The SAH's northward shift is related to a weakened ASWJ.The temperature gradient within the troposphere (500-200 hPa) is the determining factor for the ASWJ intensity (Chen et al., 2023;Zhang et al., 2022).The troposphere in the Northern Hemisphere generally experiences greater warming between 374−372 ka BP compared to 400-398 ka BP in response to insolation forcing (Figure 3a).The warming at mid-latitudes is more intense than at low latitudes, leading to a reduced tropospheric temperature gradient over Asia.Consequently, orbital forcing with higher remote insolation weakens the ASWJ.The troposphere experiences significant warming globally between 357−355 ka BP compared to 400-398 ka BP due to insolation forcing (Figure 3b).The impact of orbital forcing with higher local insolation forcing on the thermal gradient in the mid-latitude Northern Hemisphere is significantly weaker than that with higher remote insolation (Figure 3c).Therefore, orbital forcing with higher local insolation results in a relatively smaller reduction of the ASWJ intensity than that with higher remote insolation.
The tropospheric temperature fluctuations are mainly influenced by surface temperature and subsequently regulated by temperature advection (Chen et al., 2023;Zhang et al., 2022).Insolation forcing generally causes significant surface warming at mid-to high latitudes and slight cooling at low latitudes between 374−372 ka BP than 400-398 ka BP (Figure 3d).Surface warming directly impacts the tropospheric temperature at mid-to high latitudes in the Northern Hemisphere through longwave radiation.Tropospheric warming at low latitudes (Figure 3a) cannot be attributed to surface cooling (Figure 3d), which might be associated with temperature advection.The lower tropospheric temperature gradient over Asia is mainly related to surface warming at mid-to high latitudes in the Northern Hemisphere.This warming is directly driven by insolation forcing at high latitudes in the Northern Hemisphere.
Between 357−355 ka BP, there is moderate global surface warming caused by insolation forcing, compared to 400-398 ka BP (Figure 3e).The meridional surface temperature gradient changes are notably weaker than those between 374 and 372 ka BP (Figure 3f).Orbital forcing with higher local insolation results in a smaller reduction  (a, d), between the periods of 357-355 ka BP and 400-398 ka BP (b, e), and between the periods of 374-372 ka BP and 357-355 ka BP (c, f).Stippling and blue vectors denote that differences are significant at the 0.05 confidence level.

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Summary and Discussions
Prior studies aimed to differentiate the impact of remote and local insolation on orbital-scale SASM variability primarily through slice sensitivity experiments (Liu et al., 2006;Wen et al., 2022).These simulations yield divergent conclusions.Transient simulations have been conducted to investigate the SASM's response to insolation forcing, with a focus on the past 300 ka (Kutzbach et al., 2008;Liu et al., 2022).In the last 300 ka, there has been an analogue between local and northern high-latitude summer insolation (Figure 1).This has made it challenging for previous transient simulations to identify whether the SASM is affected by remote or local summer insolation.
Our transient simulation with orbital forcing for 400-350 ka BP is ideal for recognizing the impact of local and remote summer insolation forcing on orbital-scale SASM variability.This is due to the substantial disparity between local and remote insolation during this period (Figure 1c).Simulation results reveal that the primary driver of orbital-scale SASM variability is remote summer insolation in the Northern Hemisphere high latitudes,  (a, d), between the periods of 357-355 ka BP and 400-398 ka BP (b, e), and between the periods of 374-372 ka BP and 357-355 ka BP (c, f).Stippling denotes that differences are significant at the 0.05 confidence level. 10.1029/2023GL105003 7 of 9 rather than local summer insolation suggested by previous studies (Kutzbach, 1981;Wen et al., 2022).Northern high-latitude summer insolation influences surface temperature, which in turn affects tropospheric temperature in mid-to high latitudes.This alters the meridional tropospheric temperature gradient over Asia, which further affects the ASWJ intensity.The SAH's meridional position is closely related to the ASWJ strength.The ITCZ's meridional shift is correlated with the SAH.Therefore, orbital-scale SASM variability is driven mainly by remote summer insolation.Of course, there are probably some biases in simulated SASM rainfall, especially over the southern Tibetan Plateau, due to the relatively smooth topography described by the low-resolution CESM1.2.These biases might be amplified when the ITCZ shifts northward toward the Tibetan Plateau.
Prior studies suggest that local summer insolation forcing primarily occurs through direct heating, which alters the surface land-ocean thermal contrast (Kutzbach, 1981;Wen et al., 2022).Our simulation results indicate that the impact of heating over the land due to local summer insolation is less significant compared to that caused by northern high-latitude summer insolation (Figure 3f).In contrast, local summer insolation results in comparatively greater heating effect over the tropical Indian Ocean (Figure 3f).Thus, local summer insolation has a relatively minor impact on orbital-scale SASM variability.Several studies focus on the role of inter-tropical summer insolation gradient on orbital-scale SASM variability (Beck et al., 2018;Cheng et al., 2021Cheng et al., , 2022)).This gradient is nearly identical in phase and amplitude to northern high-latitude summer insolation (Figure 1b).Our simulation results indicate that the stronger SASM is associated with a northward shift of the clockwise gyre over the tropical Indian Ocean, which enhances the monsoonal moisture transport toward southern Asia (Figure S6 in Supporting Information S1).This northward shift is probably promoted by anomalous cyclone from North India to South China and anomalous anticyclone over the southern tropical Indian Ocean, which are linked with high 30°N insolation and low 30°S insolation (i.e., enhanced inter-tropical summer insolation gradient), respectively, due to the direct thermal effect (Figures 3d-3f).However, high 30°N insolation is not always in conjunction with low 30°S insolation (Figure 1c), and changes in inter-tropical summer temperature contrast between 30°N and 30°S averaged over 40º-120°E do not follow inter-tropical insolation gradient (Figure S7a in Supporting Information S1).Hence, the role of inter-tropical summer insolation gradient may be important for the stronger SASM between 374 and 372 ka BP (Figure S7 in Supporting Information S1), when there is obvious contrast between 30°N and 30°S insolation (Figure 1c).With respect to Southern Hemisphere forcing, SASM rainfall variations (Figure 1d) do not conform to June insolation at 30°S (Figure 1c).Our simulation does not support the dominant role of Southern Hemisphere insolation in SASM variability (Clemens et al., 2008(Clemens et al., , 2021)).This is in line with simulation results from Wen et al. (2022).However, the low SASM intensity during 357-355 ka BP (Figure 1d), despite the relatively higher northern high-latitude insolation, may be due to increased southern June insolation (Figure 1c).Increased southern insolation raises temperature over the tropical Indian Ocean (Figure 3e), leading to a weakening of the meridional thermal gradient (Figure 3b) and a resulting small SAH's northward shift (Figure 1e).Therefore, the SASM intensity deviates from the 65°N summer insolation at ∼357 ka BP (Figure 1d).
This study emphasizes the role of the direct insolation forcing in orbital-scale SASM variability.The oceanic feedback has also been suggested to be crucial for SASM changes (Liu et al., 2004;Zhang et al., 2016).SASM variability is closely related to sea surface temperature (SST) changes in the tropical Pacific (Hrudya et al., 2021) and North Atlantic (Goswami et al., 2006).The model simulates La Niña-like SST anomalies over the tropical Pacific between 374−372 ka BP than 400-398 ka BP (Figure S8a in Supporting Information S1).This SST pattern gives rise to an enhanced SASM by shifting the Walker circulation and hence inducing anomalous upwards in southern Asia (Hrudya et al., 2021;Wang & Fan, 1999).As a result, the SASM is further enhanced by La Niña-like SST anomalies between 374 and 372 ka BP.However, La Niña-like SST anomalies are very weak between 357−355 ka BP (Figure S8b in Supporting Information S1), contributing a corresponding lower SASM enhancement compared with 374-372 ka BP.In the North Atlantic, there is a more positive Atlantic Multidecadal Oscillation (AMO) between 374−372 ka BP than 357-355 ka BP (Figure S8 in Supporting Information S1).
The positive AMO enhances the SASM by setting up anomalous tropospheric warming over Eurasia and hence prolonging the monsoon duration (Goswami et al., 2006;Zhang et al., 2016).This also contributes a stronger SASM between 374−372 ka BP than 357-355 ka BP.SST changes over the North Atlantic and tropical Pacific are also results of orbital forcing.Therefore, these oceanic feedbacks further enhance the SASM's response to orbital forcing, especially northern high-latitude insolation forcing.
In addition, tropospheric warming is the most obvious over Northeast Asia (Figure 3a), which is crucial for anomalous stronger SASM between 374 and 372 ka BP.This extraordinary warming cannot be directly explained by changes in surface temperature (Figure 3d), but might be related to anomalous positive SST over the North Pacific (Figure S8a in Supporting Information S1).This SST warming would induce upward anomalies over the North Pacific and hence compensating adiabatic descent over Northeast Asia (Figure 2a).The adiabatic descent leads to significant adiabatic heating, especially in the mid-troposphere, over Northeast Asia (He et al., 2023).Hence, the tropospheric warming is especially obvious over Northeast Asia in response to orbital forcing.
The earth is characterized by quasi-periodic (∼100 ka) changes in glaciers and greenhouse gases during the past 800 ka (Lisiecki & Raymo, 2005;Lüthi et al., 2008).Our focused interval is a transition from an interglacial period to a glacial age with a drop of CO 2 concentration by ∼100 ppmv (Lisiecki & Raymo, 2005;Lüthi et al., 2008).Changes in ice volume and CO 2 concentration dramatically affect surface temperature in northern mid-to high-latitudes, which influence the ASWJ (Dai et al., 2021;Jin et al., 2023) and might further modulate the SASM at orbital timescales.However, these effects are neglected in this study.This study just provides a test of the SASM's response to orbital forcing, which might provide a deeper insight into orbital-scale SASM variability during interglacial periods.
Similar to the period of 400-350 ka BP, there will be obvious difference between remote and local summer insolation in the future 50 ka (Figure 1a).The identification of the SASM's response to remote and local summer insolation is of particular significance.Our findings are thus important for the future projection of the SASM.

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Local insolation is not the direct driver for orbital-scale variability of the South Asian summer monsoon (SASM) • The meridional shift of the South Asian high (SAH) affects orbital-scale SASM variability • Remote insolation influences the meridional shift in the SAH through altering the latitudinal thermal contrast Supporting Information: Supporting Information may be found in the online version of this article.
CESM1.2 is a fully coupled model of the Earth system that couples the spectral atmospheric model Community Atmospheric Model version 4 (CAM4), the land model Community Land Model version 4.0 (CLM4.0), the ocean model Parallel Ocean Program version 2 (POP2), and the sea-ice model Community Ice Code version4.0(CICE4.0)through the flux coupler CPL7

Figure 2 .
Figure 2. Composite differences of 200 hPa summer mean winds (m/s, left panel) and OLR (W/m 2 , right panel) between the periods of 374-372 ka BP and 400-398 kaBP (a, d), between the periods of 357-355 ka BP and 400-398 ka BP (b, e), and between the periods of 374-372 ka BP and 357-355 ka BP (c, f).Stippling and blue vectors denote that differences are significant at the 0.05 confidence level.

Figure 3 .
Figure 3. Composite differences of summer mean troposphere (500-200 hPa) temperature (°C, left panel) and surface temperature (°C, right panel) between the periods of 374-372 ka BP and 400-398 kaBP (a, d), between the periods of 357-355 ka e), and between the periods of 374-372 ka BP and 357-355 ka BP (c, f).Stippling denotes that differences are significant at the 0.05 confidence level.