Unravelling the roles of orbital forcing and oceanic conditions on the mid-Holocene boreal summer monsoons

Abstract

Northern Hemispheric summer monsoons were more intense during the mid-Holocene (MH ~ 6000 years ago) and coincided with a northward shift of the Intertropical Convergence Zone (ITCZ) compared to the pre-industrial (PI) climate. Ancient civilizations in the Indus valley, Mesopotamia, and Egypt appear to have flourished during this period, thanks to abundant water availability. This study exploits a high-resolution variable grid global atmosphere model to understand the role of orbital forcing and ocean surface conditions in strengthening the monsoons and shifting the ITCZ northward over Africa, India, and East Asia during the MH. The combined impact of orbital forcing and sea surface temperature (SST) boundary conditions led to a change in monsoon rainfall of around 42, 30, 21, and 41% over Africa, East Asia, India, and northwest India (NWI) relative to the PI conditions. Changes in orbital parameters alone account for more than 36 and 26% of total rainfall increases in Africa and East Asia. Over the Indian subcontinent, the strengthening of monsoon was primarily a combined effect of SST and orbital forcing. In contrast, the SST boundary condition alone could explain the 39% of rainfall increase over NWI, where the Indus valley civilization once existed. Through moisture budget analysis, the study further illustrates the role of dynamic and thermodynamic factors responsible for the changes in monsoon precipitation. The enhanced monsoon resulted in a northward shift of ITCZ by around 3°N, 1.9°N, and 2.5°N over Africa, East Asia, and India, respectively, compared to its PI position. Analogous to the precipitation changes, orbital forcing mostly mediated ITCZ changes across Africa and East Asia, but the combined impact of orbital forcing and SST was responsible for the changes over India.

Appendices

Appendix-1: Moisture budget analysis

The analysis of the moisture budget is done as follows. We used a linearized formulation following Seager et al. (2010) and Seager and Naik (2012).

$$\delta P=\delta TH+\delta DY+\delta E+\delta TE+Res$$

(1)

$$\delta TH= - \frac{1}{{\rho }_{w}g}{\int }_{0}^{\overline{{P}_{s}}\left(pi\right)}\nabla . ({\overline{u}}_{Pi} \left[\delta \overline{q}\right]) dp$$

(2)

$$\delta DY = - \frac{1}{{\rho }_{w}g}{\int }_{0}^{\overline{{P}_{s}}\left(pi\right)}\nabla . (\left[\delta \overline{u}\right] {\overline{q}}_{Pi}) dp$$

(3)

$$\delta TE = - \frac{1}{{\rho }_{w}g} {\int }_{0}^{{\overline{{P}_{s}}}_{\left(Pi\right)}}\nabla . \delta \left(\overline{{u}^{^{\prime}}{q}^{^{\prime}}}\right) dp$$

(4)

$$\delta \left(.\right)={\left(.\right)}_{EXP}-{\left(.\right)}_{PI}$$

(5)

In Eq. 1, variables P and E are precipitation and evaporation from the surface. Changes in specific humidity and no changes in mean circulation is mentioned as thermodynamic (δTH) term, while changes in mean circulation and no changes in specific humidity is denoted as dynamic (δDY) terms. The change in transient eddy flux is δTE, and Res is the residual term. In Eqs. 1–5, u is horizontal wind, q is specific humidity, p is pressure and P_s is surface pressure, ρ_w is the density of water, and the subscript s denote the surface values. Climatological monthly means are denoted by over bars and deviation from monthly means by primes. Subscripts EXP and PI represent the MH or the two sensitivity experiments, and the PI control experiment respectively. The terms δP, δE, δTH, and δDY are calculated by using monthly outputs, and δTE is calculated by using daily outputs. The vertical integration is conducted from the surface to the top of the atmosphere (1000–200 hPa). We have shown only δTH and δDY components from this analysis and tried to relate with δP instead of δP-E since the changes are mostly identical.

Appendix-2: Identification of ITCZ location

We followed the method used by Braconnot et al. (2007) to locate ITCZ positions. The method identifies the northern limit of latitudinal location considering the centre of gravity of precipitation, which is located to the north of the maximum precipitation for each longitude, and the mean location is defined as follows:

$$lo{c}_{ITCZ\left(lon\right)}=\frac{\sum_{y=lat\left(pr max\right)}^{3{0}^{0}N} pr\left(y\right)lat\left(y\right)}{{\sum }_{y=lat\left(pr max\right)}^{3{0}^{o}N} pr\left(y\right)}$$

(6)

Where lon() stands for longitude, lat() for the latitude at which precipitation is calculated, pr is precipitation, and pr max for maximum precipitation.

Appendix-3: Cross-equatorial atmospheric heat transport

We calculated the cross-equatorial Atmospheric Heat Transport (AHT_EQ) following Donohoe and Battisti (2013), who determined the hemispheric contrast of net energy into the atmosphere from the energy budget formulation as given below.

$${\text{AHT}}_{{{\text{EQ}}}} { = }\left\langle {{\text{SW}}_{{{\text{ABS}}}} } \right\rangle - \left\langle {{\text{OLR}}} \right\rangle { + }\left\langle {{\text{SHF}}} \right\rangle - \left\langle {{\text{STOR}}_{{{\text{atmos}}}} } \right\rangle$$

(7)

$${\text{SW}}_{{{\text{ABS}}}} = {\text{SW}} \downarrow_{{{\text{TOA}}}} - {\text{SW}} \uparrow_{{{\text{TOA}}}} + {\text{SW}} \uparrow_{{{\text{SURF}}}} - {\text{SW}} \downarrow_{{{\text{SURF}}}}$$

(8)

$${\text{SHF}} = {\text{SENS}} \uparrow_{{{\text{SURF}}}} + {\text{LH}} \uparrow_{{{\text{SURF}}}} + {\text{LW}} \uparrow_{{{\text{SURF}}}} - {\text{LW}} \downarrow_{{{\text{SURF}}}}$$

(9)

$${\mathrm{STOR}}_{\mathrm{atmos}}=\frac{d}{dt}\left[\frac{1}{g}{\int }_{0}^{{P}_{s}}\left(Cp T+Lq\right) dP \right]$$

(10)

where SW_ABS is the shortwave radiation absorbed in the atmosphere by the direct heating of radiation from the sun, which is the sum of the upwelling and downwelling shortwave radiations at the top of the atmosphere (TOA) and the surface (SURF). The brackets indicate the spatial integral of the anomaly from the global average over the Southern Hemisphere or negative of the spatial integral over the NH. OLR is Outgoing Longwave Radiation at the TOA. SHF is the sum of the total energy flux from the surface to the atmosphere in the form of sensible heat fluxes (SENS), latent heat fluxes (LH), and long-wave fluxes (LW). STOR_atmos is the energy stored in the atmospheric column, where T is temperature, Cp is the specific heat capacity at constant pressure, L is the latent heat of vaporisation of water, q is specific humidity, and P_s is the surface pressure.