-
The model we use includes both the fully coupled and the slab ocean version of the Community Earth System Model version 1 (CESM1). The fully coupled model (CESM1-CPL) is comprised of the Community Atmosphere Model version 5, the Community Land Model version 4, the Community Ice CodE, and the Parallel Ocean Program version 2. For both atmosphere and land models, the horizontal resolution is 2.5° longitude × 1.9° latitude, with the atmospheric component discretized on 30 uneven vertical levels. For the sea ice model and ocean models, the horizontal resolution is at a nominal 1°, telescoped meridionally to ~0.3° at the equator. Vertically, the ocean model has 60 uneven levels with the thickness varying from 10 m near the surface to 250 m at the bottom. It should be noted that we run all the following experiments with perpetual equinox conditions, which reduces complexities in the results.
Starting from an equilibrium state that is available at NCAR, a control simulation (CTRL-CPL; Table 1) is integrated for 120 years under equinoctial solar radiation. We then perform two experiments with additional heating and cooling band forcing
$ {Q}^{*} $ added to the Southern Ocean, and they are denoted as HEAT-CPL and COOL-CPL. In such a way the total surface heat flux$ {Q}_{{\rm{t}}} $ into the Southern Ocean can be expressed as$ {Q}_{\mathrm{t}}={Q}_{\mathrm{a}\mathrm{o}}+{Q}^{*} $ , where$ {Q}_{\mathrm{a}\mathrm{o}} $ is the sum of radiative and turbulent heat fluxes.$ {Q}^{*} $ is designed to be a 24°-wide band with the maximum amplitude of +/−12$ \mathrm{W}\;{\mathrm{m}}^{-2} $ at 55°S (Fig. 1a), and the area mean heat flux perturbation over the forced ocean is +/− 4.8$ \mathrm{W}\;{\mathrm{m}}^{-2} $ . Each simulation is integrated for 70 years, and the data of the last 40 years are used for estimating the forced response by subtracting the CTRL-CPL data therefrom. To check how symmetric the climate response is to the heating and cooling thermal forcings in the Southern Ocean, we estimate the symmetric and asymmetric response as${{X}}_{\mathrm{l}}={(\mathrm{\delta }{X}}_{+}-{\mathrm{\delta }{X}}_{-})/2$ and${{X}}_{n}=({\mathrm{\delta }{X}}^{+}+{\mathrm{\delta }{X}}^{-})/2$ , respectively. Admittedly, the climate system in our work is still far from equilibrium because of the sluggish deep ocean (Long et al., 2014; Stouffer, 2004), and the precise depiction of the atmospheric and oceanic responses at their full equilibrium stage is beyond the scope of this study. A recent study by Lembo et al. (2020) examined the response of global ocean heat uptake to an abrupt doubling of CO2 and found that the anomaly of global ocean heat uptake displays two prominent time scales: the fast response in which the upper ocean is in quasi-equilibrium with the radiative forcing, and the subsequent slow response owing to the gradual adjustment of the deep ocean. The timescale of the fast ocean heat uptake response is on the order of a few decades, while the timescale of the slow response is thousands of years or longer, which is too computationally expensive with the fully coupled model. However, as shown later, most of the oceanic temperature, density and circulation changes in this study occur in the upper ocean (< 800 m), the timescales for these fast responses are relatively short and can be separated from the millennial timescale for deep ocean equilibration. In addition, we extend the HEAT-CPL run to 120 years, and the precipitation, TS, zonal mean temperature, and MOC anomalies averaged over years 70 to 120 are indeed very similar to those over years 30 to 70 (Fig. 2), giving confidence that the responses discussed in this study are steady and robust.NAME RUN (yrs) DESCRIPTION Fully coupled experiments CTRL-CPL 120 Fully coupled control run HEAT-CPL 120 Perturbed by an additional heating in the Southern Ocean COOL-CPL 70 Perturbed by an additional cooling in the Southern Ocean Wind stress overriding experiments HEAT-WS 70 Same as HEAT-CPL, but wind stress is specified to climatology COOL-WS 70 Same as COOL-CPL, but wind stress is specified to climatology Slab ocean experiments CTRL-SOM 100 Slab ocean control run HEAT-SOM 80 Perturbed by an additional heating in the Southern Ocean COOL-SOM 80 Perturbed by an additional cooling in the Southern Ocean Table 1. Experiments with fully coupled and slab ocean CESM1
Figure 1. (a) Geographical locations of the energy perturbation bands. The zonal mean climatology of (b) TS (K), (c) precipitation (mm d−1), and (d) AHT (PW) in CTRL-CPL (black) and CTRL-SOM (red). TS = surface temperature; AHT = atmospheric heat transport.
Figure 2. (a) Zonal mean precipitation (blue lines; left axis) and TS (red lines; right axis) responses in CPL-HEAT averaged over years 30 to 70 (solid lines) and years 70 to 120 (dashed lines). (b−c) The total MOC response (Sv) in CPL-HEAT averaged over years 30 to 70 and years 70 to 120.
To disable the wind-driven oceanic processes, we further perform a pair of partially coupled heating and cooling experiments (HEAT-WS and COOL-WS; Table 1). The partial coupling is realized through overriding the wind stress at the air-sea interface to a daily mean climatology (also see Liu et al., 2017a for details), which is derived from the 120-year CTRL-CPL run. Therefore, the OHT, as well as ocean circulation changes, are a result of air-sea thermal interaction, and the wind stress contribution to the oceanic changes can be estimated by subtracting the results of CPL-WS from those in CPL. This technique has been successfully used to examine the formation processes of SST in response to global warming in many previous studies (Lu and Zhao, 2012; Luo et al., 2015; Liu et al., 2017a, b).
-
To test the impact of the active ocean dynamics, we also use a slab ocean version of CESM1 (CESM1-SOM). In this model, the ocean and atmosphere are only thermodynamically coupled, and SST is computed from surface heat flux and q-flux that accounts for the missing ocean dynamics. We integrate a slab ocean control run (CTRL-SOM) for 100 years, both the mixed layer depth and the q-flux are derived from the climatology of CTRL-CPL. The q-flux is allowed to vary in space and has a repeating seasonal cycle, while the mixed layer depth is only allowed to vary in space. Branching out from the 21st year of the control run, a pair of heating and cooling experiments are integrated for 80 years with the same band forcing
$ {Q}^{*} $ added to or subtracted from the Southern Ocean (HEAT-SOM and COOL-SOM; Table 1). Again, only the last 40 years of the model integration are used for analysis.The zonal mean climatology of surface temperature (TS), precipitation, and AHT in the coupled and the slab models are compared in Figs. 1b–d. Although some minor differences can be found between them, the large-scale distributions are remarkably similar, legitimizing the following comparison of their responses to the same external forcing.
-
Following Yang et al. (2015), the zonally integrated full-depth OHT (
$ {\mathrm{O}\mathrm{H}\mathrm{T}}_{\mathrm{T}\mathrm{o}\mathrm{t}} $ ) can be calculated as the residual of three components:where
$ {\rho }_{0} $ is the density of seawater,$ {c}_{p} $ is the specific heat of seawater,$ \theta $ is potential temperature,$ \overline {v} $ and$ {v}^{*} $ are Eulerian-mean and eddy-induced meridional velocity, respectively,$ D $ denotes diffusion and other subgrid processes. Therefore, the total OHT is decomposed into components induced by Eulerian-mean flow, eddies, and diffusion. Among the three components, the Eulerian-mean component can be easily calculated with model output potential temperature and Eulerian-mean velocity. The eddy component, which results from both mesoscale and sub-mesoscale processes, is not resolved in our ocean model. Instead, the mesoscale eddies are parameterized by the Gent–McWilliams scheme (Gent and McWilliams, 1990), in which a variable coefficient enables an appropriate ocean response to surface momentum forcing. The sub-mesoscale eddies are parameterized following Fox-Kemper et al. (2008).The response of the Eulerian-mean OHT can be further decomposed into the advection of mean temperature by the circulation anomaly, the advection of temperature anomaly by mean circulation, and a nonlinear component, i.e.,
Following Yu and Pritchard (2019), we term them as dynamic, thermodynamic, and nonlinear components, respectively.
On the other hand, the total OHT can be separated into contributions from individual basins. Since the mass flow in the Indian Ocean or the Pacific Ocean alone is not closed due to the Indonesian Throughflow, the heat transport of the two basins is summed together.
-
The MOC is calculated by integrating meridional velocity zonally and vertically. Since the meridional velocity can be decomposed into Eulerian-mean and eddy-induced components
$ v=\overline {v}+{v}^{*} $ , the MOC can also be decomposed into these two components:
NAME | RUN (yrs) | DESCRIPTION |
Fully coupled experiments | ||
CTRL-CPL | 120 | Fully coupled control run |
HEAT-CPL | 120 | Perturbed by an additional heating in the Southern Ocean |
COOL-CPL | 70 | Perturbed by an additional cooling in the Southern Ocean |
Wind stress overriding experiments | ||
HEAT-WS | 70 | Same as HEAT-CPL, but wind stress is specified to climatology |
COOL-WS | 70 | Same as COOL-CPL, but wind stress is specified to climatology |
Slab ocean experiments | ||
CTRL-SOM | 100 | Slab ocean control run |
HEAT-SOM | 80 | Perturbed by an additional heating in the Southern Ocean |
COOL-SOM | 80 | Perturbed by an additional cooling in the Southern Ocean |