Atmospheric response to wintertime Tibetan Plateau cold bias in climate models

By Alice Portal et al.

Based on the composites of CMIP6 models, this paper shows a cold bias over the Tibetan Plateau (TP), and finds that the negative temperature anomalies over TP intensify the East Asia winter monsoon by enhancing the low-level baroclinicity in the region of the East China Sea. Then, the southern flank of the Pacific jet is reinforced. The responses of AGCM experiments support the results of CMIP6 composite. Results are interesting and a cause for concern. The manuscript is generally well written and the methods appear sound. Since there are still some points need to be revised, I would like to recommend a moderate revision before this paper can be accepted for publication.

General comments:

1. Why exclude December from the analysis? The period of December-January-February is usually considered as the deep winter in East Asia, and December should be included in order to assess the full climatology of TP temperature and Pacific jet. In addition, for the climate conditions in Asian region, the January-February sometimes denotes the late-winter. The temperature variability and atmospheric climatology associated with the East Asian winter monsoon have obviously subseasonal variations (e.g., Zhong & Wu, 2022, https://doi.org/10.1007/s00382-022-06610-9; Park & Kim, 2021, https://doi.org/10.1007/s00382-020-05544-4; Tian & Fan, 2020, https://doi.org/10.1007/s00382-019-05068-6) from the early- to late-winter. So, if the cold TP bias and the related dynamic processes proposed in the study are also applicable in the early-winter (i.e., November-December)?

1. Some model evaluations for the SPEEDY are needed, and at least one to be considered is the climatology of East Asian jet in AGCM and in CMIP6. Although the authors use the same surface temperature forcing as those in TP composite to drive the AGCM, the strength and position of cold advection and eddy growth rate are different (Figures 4 & 5). Compared to the CMIP6 composite, the temperature advection and eddy growth rate in AGCM are distributed farther east and closer to the Pacific and may contribute to less climate effects over the East Asian continent. Is the climatology of Asian jet in SPEEDY different from that in CMIP6 MME?

1. How does the cold TP bias construct in CMIP6 climate modes, snow cover over TP or other processes related to the surface heat fluxes? More discussions should be provided in the manuscript. Additionally, for the temperature advection, the authors only present the cold advection by the mean flow (i.e., Figure 4a). However, the anomalous low-level winds are also important to advect the surface temperature. How about the temperature advection by the anomalous winds?

1. In fact, the cold bias of winter temperature is not limited to TP, but the whole East Asia which is similar to those in CMIP5 models (e.g., Gong et al., 2014, https://doi.org/10.1175/JCLI-D-13-00039.1; Wei et al., 2014, https://doi.org/10.1007/s00382-013-1929-z). I suggest that the authors should give a brief discussion about the cold bias between CMIP5 and CMIP6 models.

Specific comments:

Suggest to change the title to reflect the East Asian winter monsoon. How about “Atmospheric responses in East Asia to wintertime Tibetan Plateau cold bias in CMIP6 models”?

The physical processes associated with the atmospheric responses to the cold TP bias are in line with expectations and previous analyses as illustrated in Introduction (i.e., L42-L47). So the novelty of the study needs to be better explained given these works.

Throughout the paper, words like the “bias” and “spread” are cross-used. They should have different meanings, and it’s better to define them more clearly in the paper.

“Cold bias” in your title should mean the temperature difference between the model simulation and observation. However, no observation data are used and the definition of “cold TP composite” in the paper does not meet the meaning of “bias”. I suggest a more appropriate word.

In Table 1, it would be more helpful to provide the latitude & longitude resolution in degrees or grid cells for each model.

L187: Figure 2(b, d) may be Figure 3(b, d).

This paper investigated the atmospheric response to wintertime cold Tibetan Plateau (TP) bias with CMIP6 multi-model mean (MMM) simulations and idealized SPEEDY experiments. The authors found that the cold bias over Asian orography intensifies the East Asia winter monsoon (EAWM) through enhancing the low-level baroclinity and reinforcing the southern Pacific jet. The EAWM is a three-dimensional climate system and more details should be examined to measure its strength. Thus, I recommend a major and mandatory revision before the paper could be accepted. The details of the comments are listed below.

Major comments:

This study investigated the impacts of the cold bias over Asian orography on East Asia winter monsoon (EAWM). The EAWM is a three-dimensional climate system (e.g., Jhun et al. 2004) and its strength could not be simply measured by the wind at 850hPa. Thus, the authors should carefully check the atmospheric anomaly (e.g., Z500, U300, SLP) to measure the strength of EAWM (Jhun et al. 2004, Wang et al. 2010).

Other comments:

1)    Line 101: ‘the January and February months are referred to as winter’. Why December is not considered? In general, the the boreal winter is refered as “December-January- February” (DJF). 

2)    Line 37: 40°N could be better.

3)    Line 128: As mentioned in line 125, the LST is prescribed in SPEEDY model. However, it is also proposed that the model includes a freely evolving LST scheme. I wonder how the LST is treated in the SPEEDY simulations? Could the LST be affected by upper-level circulation, or it is just prescribed as a model input? Please clarify.

4)    Line 150: ‘1979-2008’ could be better. 

5)    Figure 2: Please check the unit of the heat flux. It could be W m-2.

6)    Figure 2: Positive value means upward or downward heat flux? Please provide the information in figure captions.  

7)    Line 170: The statement could be misleading. The heat flux change is negative over TP regions. 

8)    Line 171: If the heat flux change is not significant over TP and CP, why the authors show the heat flux change here? It could confuse the readers. 

9)    Line 184: The jet stream distributes around 300hPa during winter (Jhun et al. 2004). The statement here could be misleading.

10)    Line 187: Please check the figure captions. 

11)    Line 190: Increased instability favors acceleration of upper-level zonal winds (e.g., Nie et al., 2016). Please show the zonal wind change of upper troposphere. 

12)    Line 214: Please check the figure captions.

13)    Figure 5: Please show the significant information of the changes as in Figure 4.

14)    Line 199: Please show the surface wind anomaly with vectors. Otherwise, one may not understand the heat flux anomaly.   

15)    Line 200: More upward heat flux? Please clarify. 

Reference:

Jhun, J., & Lee, E. (2004). A New East Asian Winter Monsoon Index and Associated Characteristics of the Winter Monsoon, Journal of Climate, 17(4), 711-726.

Nie, Y., Zhang, Y., Chen, G., & Yang, X. (2016). Delineating the Barotropic and Baroclinic Mechanisms in the Midlatitude Eddy-Driven Jet Response to Lower-Tropospheric Thermal Forcing, Journal of the Atmospheric Sciences, 73(1), 429-448. 

Wang, B., Wu, Z., Chang, C., Liu, J., Li, J., & Zhou, T. (2010). Another Look at Interannual-to-Interdecadal Variations of the East Asian Winter Monsoon: The Northern and Southern Temperature Modes, Journal of Climate, 23(6), 1495-1512.