The changes in Earth's precession have an impact on the tropical precipitation. This has been attributed to the changes in seasonal solar radiation at the top of the atmosphere. The primary mechanism that has been proposed is the change in thermal gradient between the two hemispheres. This may be adequate to understand the zonal mean changes, but cannot explain the variations between land and oceans. We have used a simple model of the intertropical convergence zone (ITCZ) to unravel how precipitation changes with precession. Our model attributes the changes in precipitation to the changes in energy fluxes and vertical stability. We include the horizontal advection terms in this model, which were neglected in the earlier studies. The final response of the land and oceans is a result of complex feedbacks triggered by the initial changes in the insolation. We find that the changes in precipitation over the land are mainly driven by changes in insolation, but over the oceans, precipitation changes on account of changes in surface fluxes and vertical stability. Hence insolation can be a trigger for changes in precipitation on orbital timescales, but surface energy and vertical stability play an important role too. The African monsoon intensifies during a precession minimum (higher summer insolation). This intensification is mainly due to the changes in vertical stability. The precipitation over the Bay of Bengal decreases for minimum precession. This is on account of a remote response to the enhanced convective heating to the west of the Bay of Bengal. This weakens the surface winds and thus leads to a decrease in the surface latent heat fluxes and hence the precipitation.
The most dominant temporal mode in insolation and tropical precipitation is
the 23 000-year precession cycle of the Earth
It is attributed to the land–sea contrast theory in the previous studies
Some studies have used the changes in energy balance to understand the
response of precipitation to precession
Moisture and MSE equations can be used separately to understand the dynamics
of monsoon under different climate scenarios
The schematic diagram showing the orbital configuration of minimum
precession (
The paper is organized as follows. The next section describes the model and the experimental setup, and outlines the derivation of a simple model for the intertropical convergence zone (ITCZ). We have used this simple ITCZ model to understand the factors leading to the shift in precipitation between land and oceans, at the regional scale. The results are described in Sect. 3. It is followed by a discussion about the precipitation response to MH and obliquity forcing with the help of the simple ITCZ model.
EC-Earth is a fully coupled ocean–atmosphere GCM
The two precession extremes, precession minima
The difference in the incoming solar radiation at
the top of atmosphere between
The orbital configuration used for the extremes in precession,
precession minima
The regions used in this article and their coordinates.
The model is run separately for each of the orbital configurations. The
length of each simulation is 100 years, with the first 50 years being
considered spinup. We have used the climatology of the last 50 years, for all
our analysis. The orbital parameters remain constant throughout the
simulation. All other boundary conditions (e.g., the solar constant,
greenhouse gas concentrations, orography, ice sheets, vegetation) were kept
constant at the pre-industrial levels. Vernal equinox has been fixed at
21 March, and the present-day calendar is used. Since the length of the
season and the dates of equinoxes change along the precession cycle, the
autumn equinoxes do not coincide. This is known as the “calendar effect”.
It introduces some errors due to the phasing of insolation. We do not make
any corrections in order to be consistent with previous studies. Further
details about the experiments are provided in
The Hadley cell is a thermally direct overturning circulation in the tropics.
It takes energy away from the tropics and transports it towards the poles.
The Hadley cell has a rising branch in the deep tropics and a descending
branch in the extra-tropics. This leads to moisture convergence near the
rising branch. The ITCZ coincides with the rising branch of the Hadley cell
and is responsible for the zone of heaviest precipitation in the tropics. The
characteristics of the ITCZ can be described by using the conservation
equations for moist static energy (MSE) and moisture. Using this approach,
The quantities on the right-hand side are the sum of all sources and sinks.
Further details on the derivation of Eq. (
The dependence of
Figure
We have shown in Fig.
By taking the ratio of Eqs. (
To quantify the relative contribution of
This equation can further be modified as
In this section, we have explained the changes in
The difference in precipitation (
Figure
The difference in
In Fig.
During DJF,
The contribution of
In this section, we look at the various terms in Eq. (
In the southern tropics the dominant contribution is from changes in
The seasonal cycle of near-surface equivalent potential temperature
(
We have shown earlier that in
The JJA mean difference (
We take two regions: one over central India and the other over the Bay of
Bengal, to identify the flux which contributes most to the changes in
LHF is a function of surface wind speed, sea surface temperature (SST), and
near-surface relative humidity. LHF increases with an increase in SST and
wind speed. SST has increased over the Bay of Bengal and southern Arabian Sea
by about 2
The JJA mean wind speed (850 hPa) in shading for
The shift in LLJ leads to lesser moisture flux along the southern part of the
Western Ghats. Hence,
The difference between
To summarize, the decrease in
In this section, we have discussed the similarities between the sets of
idealized experiments (
Models with different levels of complexities: QTCM
Using a simple model for ITCZ, we have interpreted the response of a high
resolution fully coupled model EC-Earth to precession. The changes in
precipitation can be attributed to either the changes in total energy fluxes
going into the column (
Explanation for data not being publicly available: the data are being used for publication, and hence cannot be made public as yet. The data are available upon request.
The supplement related to this article is available online at:
CJ, JS and AC analysed and interpreted the GCM output. JHCB designed and ran the experiments. CJ wrote the manuscript with input from all authors. All authors reviewed the manuscript.
The authors declare that they have no conflict of interest.
We thank A. Nikumbh for useful comments. The authors acknowledge support from the Centre for Excellence in the Divecha Centre for Climate Change (DCCC). This work was partially funded by DST India.
This paper was edited by Qiuzhen Yin and reviewed by two anonymous referees.