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# Ocean Response to a Climate Change Heat-Flux Perturbation in an Ocean Model and Its Corresponding Coupled Model

• State-of-the-art coupled general circulation models (CGCMs) are used to predict ocean heat uptake (OHU) and sea-level change under global warming. However, the projections of different models vary, resulting in high uncertainty. Much of the inter-model spread is driven by responses to surface heat perturbations. This study mainly focuses on the response of the ocean to a surface heat flux perturbation F, as prescribed by the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP). The results of ocean model were compared with those of a CGCM with the same ocean component. On the global scale, the changes in global mean temperature, ocean heat content (OHC), and steric sea level (SSL) simulated in the OGCM are generally consistent with CGCM simulations. Differences in changes in ocean temperature, OHC, and SSL between the two models primarily occur in the Arctic and Atlantic Oceans (AA) and the Southern Ocean (SO) basins. In addition to the differences in surface heat flux anomalies between the two models, differences in heat exchange between basins also play an important role in the inconsistencies in ocean climate changes in the AA and SO basins. These discrepancies are largely due to both the larger initial value and the greater weakening change of the Atlantic meridional overturning circulation (AMOC) in CGCM. The greater weakening of the AMOC in the CGCM is associated with the atmosphere–ocean feedback and the lack of a restoring salinity boundary condition. Furthermore, differences in surface salinity boundary conditions between the two models contribute to discrepancies in SSL changes.
摘要: 耦合的大气-海洋环流模式常被用来预估全球变暖情景下海洋热吸收和由于海水热力膨胀引起的海平面高度变化。然而，不同耦合模式对其的预估存在较大不确定性。模式间的不确定性很大一部分是由不同模式对全球变暖情景下的热通量扰动的响应不同所造成的。本文采用海洋模式及海气耦合模式对CMIP6中异常通量强迫比较试验（FAFMIP）给定的热通量扰动F的海洋响应进行了研究。结果表明海洋模式和耦合模式模拟的海水变暖均主要是由热通量扰动F所决定的。在全球尺度上，海洋模式模拟的全球变暖情景下的海洋变化（海温变化、海洋热含量和海表高度的变化）与耦合模式模拟的基本一致。两个模式模拟的海洋变化的差异主要发生在北冰洋、大西洋以及南大洋区域。这主要是由于环流变化导致的再分布的热通量的差异所决定的。相对于海洋模式来讲，耦合模式模拟的大西洋经向翻转环流的初始强度较大且减弱的幅度较强（约9%），这与在耦合模式中更好地考虑大气-海洋相互作用且并未采用恢复盐度边界条件有关。
• Figure 1.  The prescribed heat flux anomaly (F) from FAFMIP for (a) OExp1, (c) CExp1, and (e) their differences (OExp1 – CExp1). (b), (d), (f), describe the same results as (a), (c), (e), but for the heat flux anomaly due to the redistribution of the SST ($Q_{\rm{r}}'$) (units: W m–2), the positive value indicates downward.

Figure 2.  The Atlantic Meridional Overturning Circulation (AMOC) for the OGCM (a) control run (OCTRL), (b) perturbation run (OExp1), and (c) their difference (OExp1 – OCTRL). (d), (e), (f), show the same results as (a), (b), (c), but for CGCM (units: Sv).

Figure 3.  The total ocean heat content change due to (a) the ocean temperature anomaly (T), (b) added temperature change ($T_{\rm{a}}'$), and (c) the redistributive temperature anomaly ($T_{\rm{r}}'$) during the final decade of the experiments for OGCM. (d,) (e), (f), show the same results as (a), (b), (c), but for CGCM. (g), (h), (i), are the differences in T', $T_{\rm{a}}'$, and $T_{\rm{r}}'$ between the OGCM and CGCM, respectively (units: 109 J m−2).

Figure 4.  The global and basin mean changes in ocean temperature anomaly (T), added temperature change ($T_{\rm{a}}'$), and redistributive temperature anomaly ($T_{\rm{r}}'$) simulated by the OGCM and CGCM for (a) global, (b) the Indo-Pacific (IP), (c) the Arctic-Atlantic (AA), and (d) Southern (SO) Oceans (units: °C).

Figure 5.  The global and the basin means time series of the heat flux anomaly (F) from FAFMIP, the heat flux anomaly ($Q_{\rm{r}}'$) resulting from the redistribution of SST and the total surface heat flux anomaly (F + $Q_{\rm{r}}'$) simulated by the OGCM and CGCM for the (a) global, (b) the Indo-Pacific (IP), (c) the Arctic-Atlantic (AA), and (d) the Southern (SO) Oceans (units: W m–2).

Figure 6.  The spatial distribution of changes in SSL [steric sea level,${\int }_{0}^{-{H}}\rho \left(T,S\right)-\rho ({T}_{0},{S}_{0})/\rho \left({T}_{0},{S}_{0}\right)\mathrm{d}z$] for (a) OGCM, (d) CGCM, and (g) their differences (OGCM – CGCM); (b), (e), (h), and (c), (f), (i), show the same results as (a), (d), (g), but for TSSL [thermosteric sea level, ${\int }_{0}^{-{H}}\rho \left(T,{S}_{0}\right)-\rho ({T}_{0},{S}_{0})/\rho \left({T}_{0},{S}_{0}\right)\mathrm{d}z$)] and HSSL [halosteric sea level, ${\int }_{0}^{-{H}}\rho \left({T}_{0},S\right)-\rho ({T}_{0},{S}_{0})/\rho \left({T}_{0},{S}_{0}\right)\mathrm{d}z$](units: m).

Figure 7.  Diagram describing the difference in T' (corresponding to OHU), $T_{\rm{a}}'$ (corresponding to OHUa), and $T_{\rm{r}}'$ (corresponding to OHUr) between the OGCM and CGCM. The figures show whether they exhibit warming (red) or cooling (blue) in the OGCM compared to CGCM. MHTa is the northward meridional heat transport for $T_{\rm{a}}'$, and MHTr is the northward meridional heat transport for $T_{\rm{r}}'$.

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## Manuscript History

Manuscript revised: 11 August 2021
Manuscript accepted: 17 August 2021
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Ocean Response to a Climate Change Heat-Flux Perturbation in an Ocean Model and Its Corresponding Coupled Model

###### Corresponding author: Hailong LIU, lhl@lasg.iap.ac.cn;
• 1. International Center for climate and Environment Sciences (ICCES), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100049, China
• 2. State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China
• 3. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100049, China
• 4. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
• 5. School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York, NY 11790, USA

Abstract: State-of-the-art coupled general circulation models (CGCMs) are used to predict ocean heat uptake (OHU) and sea-level change under global warming. However, the projections of different models vary, resulting in high uncertainty. Much of the inter-model spread is driven by responses to surface heat perturbations. This study mainly focuses on the response of the ocean to a surface heat flux perturbation F, as prescribed by the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP). The results of ocean model were compared with those of a CGCM with the same ocean component. On the global scale, the changes in global mean temperature, ocean heat content (OHC), and steric sea level (SSL) simulated in the OGCM are generally consistent with CGCM simulations. Differences in changes in ocean temperature, OHC, and SSL between the two models primarily occur in the Arctic and Atlantic Oceans (AA) and the Southern Ocean (SO) basins. In addition to the differences in surface heat flux anomalies between the two models, differences in heat exchange between basins also play an important role in the inconsistencies in ocean climate changes in the AA and SO basins. These discrepancies are largely due to both the larger initial value and the greater weakening change of the Atlantic meridional overturning circulation (AMOC) in CGCM. The greater weakening of the AMOC in the CGCM is associated with the atmosphere–ocean feedback and the lack of a restoring salinity boundary condition. Furthermore, differences in surface salinity boundary conditions between the two models contribute to discrepancies in SSL changes.

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