Seasonal Variation of Intra-Seasonal Eddy Kinetic Energy along the East Australian Current

Round 1

Reviewer 1 Report

Review on " Seasonal eddy kinetic energy variability along the East Australian Current" by Xu et al. submitted to Atmosphere.

The East Australian Current (EAC) is the weakest western boundary current in the ocean. Nevertheless, the mesoscale eddies in the EAC region are as active as those in the Gulf Stream and Kuroshio regions. This study tried to explore the modulation of the EAC on the eddy activity in the east of Australia, based on both satellite altimeter observations and high-resolution ocean model outputs. The results suggest that the seasonal variability of eddy kinetic energy along the EAC is associated with the mean flows. The paper is well organized. However, there are several weaknesses in the present manuscript. Some of gaps of this manuscript before publication are outlined below. These issues should be carefully addressed before further evaluation.

1. It should better note the AVISO SSH data used are adt or sla in section 2.1.

2. Were the observed surface geostrophic velocities calculated by the authors or from AVISO product?

3. Line 102, the “velocity perturbation” should be defined here.

4. Figure 1, I suggest to show AVISO SSH and AVISO surface currents as Fig.1a, and ECCO2 SSH and ECCO2 surface currents as Fig.1b.

5. Figure 2, showing AVISO EKE minus ECCO2 EKE as Fig.2c may better to understand the differences between them. In additional, I think the EKE is calculated from velocity, not SSH, please clarify in the caption of Figure 2.

6. Line 125-131, why the authors using a 170-day lowpass filter, rather than a 60-170-day bandpass filter?

6. Maybe the authors can explain why ECCO2 tend to simulate smaller seasonal amplitude as shown in Figure 3?

7. Please give the significance levels in Figure 4

8. In section 3, Line 125-131, and section 5, line 167-169, the authors use a 170-day highpass filter, whereas 150 day highpass filter in the caption of Fig. 6, please check.

9. Most of the figure captions are not clear, for example, Fig.8, maybe as ”Monthly mean climatological eddy kinetic energy (EKE, black line), barotropic conversion rate (BTR, red line) and baroclinic conversion rate (BCR, blue line) in the upper 500 m averaged over (150°E–160°E, 25°S–38°S) ”

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

In this manuscript, Xu et al., studied seasonal variability of EKE along the East Australian Current (EAC) using satellite observations and eddy-permitting model output. The key result is that the seasonal variation of the EKE is controlled by barotropic instability of the jet/current. The variation of the baroclinic instability is opposite to that of the barotropic instability and is of much smaller magnitude. I found these results rather interesting and worthy of publication in the journal. But there are a few key points that need to be clarified or explained better.

1. the paper focuses on the seasonal variation or change of the EKE that is induced by intra-seasonal variability (60-170 days, to be exact), it needs to clarify why this is the focus.

2. It also needs to be explained better how the barotropic/baroclinic instability is formulated and/or calculated and how the lateral rate of strain is related to the barotropic instability.

3. It is a drawback that the study used an eddy-permitting simulation, in which the EKE is significantly lower than observations (despite being data-assimilated). I do not expect the authors to change model (but such limitation should be mentioned or discussed).

Details

Line 13 & many places. model “outputs” instead of “output”

Line 18. “…barotropic instability of the background mean flows”, what do you mean by “background mean flows” do you mean the EAC itself? This sentence appears similar to the following “Further analysis … mesoscale signals in this region is related to the transport variation of the EAC”

Line 21. “transport variation” often the word “variation” can be changed to “variability”.

Line 70. “eddy-permitting” is more appropriate for ¼ degree. I am not sure if you can call ECCO as “OGCM simulation” as it is data-assimilated state estimate.

Section 2.2 “model data”, instead of model data you probably should call it “ECCO2 state estimate”

It is important to clarify that ECCO2 is data-assimilated, it is not pure model simulation.

Line 97, how do you define EKE as a function of time? i.e., u’ and v’ refer to velocity anomaly (reference to mean velocity of which time scales? long-term mean? Annual mean?).

Line 103. It needs to be pointed out that the ECCO2 assimilated AVISO data thus some good agreement in Figure 1 is automatic.

Line 117. Figures 2 and 3. Although the general patterns agree, the ECCO2 clearly show significantly lower variability, likely due to the eddy-permitting resolution. It would be a lot more useful to used simulations of high resolution (1/10 or higher).  

Line 122-127. How are the periods (60-170 days) chosen? It is useful to put the two panels into one and mark the period in the figure 4. It is particularly strange to choose 170 days, because 60 days is about 2 months, it seems to me that 180 days are easier to relate to (6 months) or 200 days where there is a drop in AVISO spectra?

Also, what happens to variability within 2 months/60 days? There is significant variability within 60 days, right?

To be clear, the study is about the EKE, but the EKE induced by intra-seasonal variability (or more specifically, 60-170 days). This needs to be very clear in the title (abstract, conclusion) if it is. And you should talk about why focus on intra-seasonal variability …

Line 129. It would be useful to have a vertical section of the EAC in comparison to the EKE as a function of depth, and an estimate of the volume transport in ECCO2 compared to observational estimate.  

Section 2.5

I think some good introduction (with references) is needed for equation 4, 5 (and equation 6 later); explaining how different term is connected or represents barotropic/baroclinic conversions.

Line 179, what do you mean “background horizontal currents (including time-mean flows)” do you mean they are the same as in “low-passed velocity”? as in equation 2 (line 124).

I think Figures 8 and 9 should be flipped.

Line 262. I do not think you can call “leads by one month”, the first 7 months seems in-phase to me. And Figure 11 maybe combined with Figure 8.

Section 3. the material from Line 245-274 seems to be a natural continuation of section 2.5.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors addressed all my previous comments and I do not have further suggestions.