3.1 Experimental design
The main simulations presented here were the five members from the
Atmospheric Model Inter-comparison Project (AMIP) protocol simulation
(for the period 1979–2014), three groups of simulations named CTL, LC,
and LGC (corresponding to the use of the FTF, FTF and HWR, FTF, HWR and
GLF modules), and an additional single member simulation of the
land-atmosphere coupled simulation named NP (Table 1). The AMIP
simulations were performed using the time-varying external forcing
recommended by CMIP6
(https://esgf-node.llnl.gov/search/input4mips/).
The atmosphere and land resolution was 2 in both latitude and
longitude in these simulations, and the monthly observations of the sea
surface temperatures (SSTs) and sea ice concentration from the HadISST
data set (Rayner et al. 2003) were used for prescribing the ocean
surface conditions. The forcing for the AMIP runs included the monthly
mean total solar irradiance (TSI) (Matthes 2017), greenhouse gas (GHG)
concentrations, including latitudinal changes and seasonality
(Meinshausen et al. 2017), ozone concentrations, anthropogenic aerosol
optical properties and the associated Twomey effect (Stevens 2017), land
use changes (Land Use Harmonization v2, http://luh.umd.edu/), and
historical stratospheric aerosols. In addition, the anthropogenic
groundwater exploitation forcing data from 1965–2014 were added (Zeng
et al. 2017).
For the AMIP runs, there were five members that shared the same setup,
but which had different initial conditions. The simulation time for the
AMIP runs was 1979–2014. For the CTL, LC and LGC simulations, they
generally followed a similar setup to the AMIP runs, except that the
simulation time for these runs was for a 41-year duration from 1976 to
2010 (using the period 1970–75 for spin-up). To reduce the internal
noise, and enhance the forced signal caused by the HWR and GLF, ensemble
averages were used here, where each group of experiments contained three
ensemble simulations using different initial conditions, the results of
which were then averaged over the ensemble for evaluation (Koster et al.
2002, 2006). In the current study, three individual simulations,
typically differing only in their initial atmospheric and land surface
conditions, comprised each ensemble (CTL, GC, or LGC). Simulations with
corresponding ensemble members in the three sets of simulations
typically shared the same initial conditions (for instance, CTL1, LC1,
and LGC1 shared the same initial conditions). For the NP transport
simulation, it generally followed a similar setup to the AMIP runs,
except that the bio-geochemical module and nitrogen transport module
were opened to obtain the nitrogen transport-related variables. The
simulation time for this simulation was 1960–2014.