Dear Reviewer 1,
Thank you very much for your very constructive comments and suggestions. We deeply appreciate your spending your valuable time for the review. We revised the manuscript based on your comments and suggestions. Please find following replies to your comments. Your comments and suggestions are indicated in italic font, and modifications in the revised manuscript are indicated with red color.
1)Motivation and the methods used in the study
In the response to my last review, the authors clarify that the motivation of the study is to investigate "to what extent the forcings by diabatic heating and intra-seasonal disturbances driving the variabilities of EAJS", and approach the topic by using a linear model to compare the jet response to forcings of the diabatic heating and the nonlinear terms associated with the intra-seasonal disturbances.
First, I don't think diagnosing the anomalous diabatic heating/intra-seasonal disturbances, or investigating the jet response to those anomalous terms using a linear model will help identify the dominant driver of the jet variabilities.This is because, in the extra-tropics, both the intra-seasonal disturbance and the diabatic heating are strongly driven by the westerly jet. For example, the diabatic heating in the midlatitude, which the authors argued as the dominant driver, is mostly driven by the atmospheric circulations. Thus, the diagnosed anomalies of those fields in the extra-tropics are more likely a consequence of the anomalous jet, instead of a driver.
Thank you for your comments. We agree that the diabatic heating/intra-seasonal disturbances are affected by the westerly jet as described:
L. 409-410:However, in the real atmosphere, the accelerated (or decelerated) zonal wind of the EAJS influences storm activities, thus the diabatic heating and the nonlinear terms.
And we agree that our method of a linear model cannot discuss such influence from the jet to the diabatic heating/intra-seasonal disturbances:
L. 406-408: As previously mentioned, in the model used in this study, the forcings of the diabatic heating and the nonlinear terms are prescribed and are not influenced by simulated linear dynamics.
However, we thinkthe diabatic heating/intra-seasonal disturbances are also affecting the jet variabilities. We are trying to quantify the effects from the diabatic heating/intraseasonal diturbances to the jet variabilities:
L. 100-105: As will be described in Section 4, a linear model used in this study isolates and quantifies only the impacts of the forcings on the EAJS variabilities. A linear model is a very effective tool for this purpose as shown in many previous studies [Hoskins and Karoly 1981; Branstator 1990; Valdes and Hoskins 1989; Held et al. 1989; Hoskins and Valdes 1990; Watanabe and Kimoto 2000; Watanabe and Jin 2003; Hirota et al. 2005; Mori and Watanabe 2008; Hirota et al. 2012].
Second, using a linear model will not help quantify the contributions of the diabatic heating and the intra-seasonal disturbances either. This is not only because that the diagnosed "forcing" actually in a large portion represents a response, but also that a linear model can not well simulate the nonlinear eddy feedback of high and low frequency eddies.
We are sorry for the confusion. We are not trying to simulate the nonlinear processes. The effects of the nonlinear processes are diagnosed from the reanalysis data and prescribed in the model:
L. 259-261:This model does not include the nonlinear processes and the moist processes. Instead, we prescribed (not neglected) the corresponding external forcings diagnosed from the reanalysis data.
Please note that quantifying the eddy feedback by using a linear model is also discussed in many previous studies [e.g. Branstator 1990; Valdes and Hoskins 1989; Held et al. 1989; Hoskins and Valdes 1990; Hirota et al. 2005; Mori and Watanabe 2008; Hirota et al. 2012]
2)Robustness of the results
The authors added significance test in the revised manuscript. From the new figures, I found that, for the anomalous intra-seasonal disturbances, almost no values in the extra-tropics passed the significance test. Even for the midlatitude diabatic heating, which the authors argued as the dominant driver, there are very few values in the extra-tropics passed the significance test. This also questions the numerical results, in which those anomalous fields are input as anomalous forcings. The results in the manuscript can not well support the authors' main conclusions.
Thank you for your comments. The relatively small significance may be because of the small number of the sample. We added a discussion about this issue as follows:
L. 228-232:The significance of the anomalous heating over the East China Sea, which will be shown to be important for the EAJS variabilities in the next section, is relatively small. This is possibly becauseof the small number (7 years) of the sample for the strong and weak monsoon composites. When we examine a regression map of the diabatic heating with respect to the EAXMI using the 32-year data, the anomalous heating is largely enhanced (not shown).
The omitted Figure in the manuscript is shown below:
Fig. R1: A regression map of the diabatic heating (K day-1) with respect to theEAWMI at 500 hPa. The hatchings indicate the areas exceeding the 90% significance level.
As for the intraseasonal disturbances, we think the important anomalies supporting our main conclusion is significant:
L. 199-201: In strong monsoon years, a significant divergence anomaly of ∇•W appears around northern Japan and over the eastern North Pacific, which is generally consistent with the westerly wind intensification of the EAJS (Fig. 2a).
L. 214-216: In the strong monsoon years, a weak divergence anomaly of ∇•W appears near Japan (30°N, 140°E), and a significant divergence anomaly is identified over the Central Pafic (20°N, 170°W) (Fig. 6a).
L.216-217: Similarly, in weak monsoon years, a significant convergence anomaly of ∇•Wexists near Japan (Fig. 6b).
3) On the numerical results
I'm not surprised that in the authors' numerical results, the diabatic heating plays a dominant role. Because such linear model is capable to capture the stationary waves induced by diabatic heating, but most time it cannot capture the eddy feedback, as a linear model cannot well simulate such nonlinear process. Thus, I don't think the numerical results can help quantify the relative contribution of diabatic heating and intra-seasonal disturbance to the EAJS variabilities.
Please see the latter part of the response to your comment 1).
Second, I don't quite trust the numerical results in the manuscript either. The authors only integrate the numerical model for 30 days and the results shown in the manuscript are merely the time average of the last 10 days. I understand that, for such a simplified model, not many processes can be simulated. However, given that the time scale of the diabatic heating in free troposphere is set to be 30 days in the model, a common sense would be to run the model much longer than this time scale to reach an equilibrium state and obtain stable statistics. Thus, I doubt that a 10 day averaged model result is not reliable to represent the seasonal mean atmospheric circulation as well as the influence of the intra-seasonal disturbance.
Thank you for your comment. We examined the time evolutions of the model output. Figure R2 shows the linear responses to the total forcings (diabatic heating + nonlinear terms associated with the high and low frequency disturbances) at day 10, day 20, and day 30 for the strong monsoon years. We describe the results as follows:
L. 271-274: The model response reached a near-steady state on the 20th day (not shown). This fast response of the atmosphere is consistent with previous studies [Rodwell and Hoskins 1996; Enomoto et al. 2003; Watanabe and Jin 2003].
Please note the damping time scale and integration time are based on previous studies examining the seasonal mean atmospheric circulations [Rodwell and Hoskins 1996; Enomoto et al. 2003; Watanabe and Jin 2003].
Fig. R2. The steady-state response of the zonal wind (m s-1) at 300 hPa to the total forcings for strong monsoon winters at (a) day 10, (b) day 20, and (c) day 30. (d) The time series of the zonal wind averaged over (25--45°N,80--180°E) for strong and weak monsoon winters.
Thank you very much again.
Sincerely yours,
Nagio Hirota
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