Synoptic-scale variability of surface winds and ocean response to atmospheric forcing in the eastern austral Pacific Ocean

Pérez-Santos, Iván; Seguel, Romanet; Schneider, Wolfgang; Linford, Pamela; Donoso, David; Navarro, Eduardo; Amaya-Cárcamo, Constanza; Pinilla, Elías; Daneri, Giovanni

doi:https://doi.org/10.5194/os-15-1247-2019

Articles | Volume 15, issue 5

https://doi.org/10.5194/os-15-1247-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-15-1247-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 5

Research article

|

16 Sep 2019

Research article |

| 16 Sep 2019

Synoptic-scale variability of surface winds and ocean response to atmospheric forcing in the eastern austral Pacific Ocean

Iván Pérez-Santos, Romanet Seguel, Wolfgang Schneider, Pamela Linford, David Donoso, Eduardo Navarro, Constanza Amaya-Cárcamo, Elías Pinilla, and Giovanni Daneri

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Final revised paper (published on 16 Sep 2019)
Supplement to the final revised paper
Preprint (discussion started on 16 Nov 2018)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review', Anonymous Referee #1, 09 Jan 2019
- SC1: 'RC1 comment', Iván Pérez-Santos, 15 Jan 2019
- AC2: 'Respond to RC1', Iván Pérez-Santos, 29 Mar 2019
RC2: 'Review of "Synoptic scale variability of surface winds and expected changes inthe ocean-atmosphere dynamics of the eastern Austral Pacific Ocean" by Pérez-Santos et al.', Anonymous Referee #2, 30 Jan 2019
- AC1: 'Respond to RC2', Iván Pérez-Santos, 06 Feb 2019
- AC3: 'Respond to RC2', Iván Pérez-Santos, 29 Mar 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Iván Pérez-Santos on behalf of the Authors (29 Mar 2019) Manuscript

ED: Referee Nomination & Report Request started (20 Apr 2019) by Markus Meier

RR by Anonymous Referee #1 (13 May 2019)

RR by Anonymous Referee #2 (27 May 2019)

Suggestions for revision or reasons for rejection

I appreciate the efforts the authors have made to answer my comments, in particular the addition of ERA5 data and the time series of Chl-a and SST. The new analysis of ERA5 allows to clarify that the different behavior between QUICKScat and ASCAT is not due to instrumental issues but to variability (Figure 8). Nevertheless there are a couple of important issues that should be addressed. In general I think the results do not support robust conclusions. I understand that it could be difficult and that maybe other statistical approaches should be followed. Therefore, in general I would suggest to moderate the conclusions. For instance, the results indicate that LAP/HAP system may affect the nightime heatwaves, but it is by no means demonstrated.

In the analysis of the EOFs is stated (L240) that " The southerly and northerly winds were associated with the passage of intense HAP (Fig. 7a) and LAP (Fig. 7b) systems, throughout the study region." and in (L396) " LAP and HAP systems dominated mode 1 of the EOF, contributing 30 % of the total variance (Fig. 3-6). In this mode, southerlies related to the passage of HAP systems, and northerlies produced by LAP systems (Fig. 7)" . But this has not been demonstrated at all. The EOF patterns do not show anything similar to a HAP/LAP structure. This should be rewritten throughout the paper.

The comparison of time series of Chl-a and SST with respect to Ekman Transport (Figure 11) shows relatively low correlation (section 3.2), with values of around 0.3. Therefore, it can't be concluded that EP/ET are the responsible of Chl-a and SST variations. This should be acknowledged in the abstract and discussion.

In Section 3.3 the figure 12 is analysed. I repeat my comment, I don't think the figure demonstrates solar radiation drives the diurnal cycle of SAT. The maximum is shifted: maximum solar radiation is at 14h while maximum SAT is at 16h. In any case, the interest of this section is not the diurnal maximum but the night heat waves, so that part could be removed.

Regarding night heat waves, the mechanism driving them is not clear, as was pointed out in my previous review. Correlation between atmospheric pressure and air temperature (Figure 13), seems too high for what is shown in the figure. It is difficult to compare both time series as it is presented. Maybe plotting both time series in the same plot (i.e. normalized or using different y axis) would help to clearly show that both time series are correlated. As it is shown now it is hard to see that both time series are correlated at the 0.96 level as is stated in the figure title.

Moreover, the relating SAT and atmospheric pressure is not probably the right way to confirm that winds are the responsible of night heatwaves. Pressure is only related to winds through the gradients, not the absolute value. In the previous review I suggested to use some statistics as composite images associated with nighttime heat wave periods or to show histograms of wind components in the periods when night heatwaves occured. In any case , to show only two hand-picked cases is not robust enough to conclude that LAP systems are the responsible for night heatwaves, as is stated in the abstract.

Detailed comments:
Abstract. It is stated that between 41-43ºS Ekman Pumping dominates in spring-summer, but from Figure 10 it looks the opposite. Total Transport is clearly driven by Ekman Transport, but in spring-summer it changes its sign with respect to what happens in the other locations/seasons. In any case, Ekman Pumping is similar in the three locations and secondary.

The computation of the wind curl is only done over the sea, while in coastal areas this means losing data points. I think the authors could compute it with ERA5 to get something more reliable in the coastal area.

Figure 11, the figure caption should mention from which point are the time series extracted.

In figure 12 (before figure 9), I don't understand yet what panels g and h represent. The authors haven't done any modification on that, in spite of what they answer in the reply. By elimination I deduce they refer to maximum SAT , but it is not clear what maximum refers to. Is it the maximum measured in the whole observational period? The daily maximum?.

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ED: Reconsider after major revisions (03 Jun 2019) by Markus Meier

AR by Iván Pérez-Santos on behalf of the Authors (22 Jul 2019) Manuscript

ED: Publish as is (09 Aug 2019) by Markus Meier

AR by Iván Pérez-Santos on behalf of the Authors (11 Aug 2019) Manuscript

Short summary

Satellite wind data were used to understand surface wind variability in the eastern austral Pacific Ocean, a region dominated generally by strong westerlies, but the empirical orthogonal function demonstrated that wind variability was dominated by a synoptic scale. Nighttime heatwave events were detected to produce air temperature maxima which exceeded the normal midday maxima caused by solar radiation. Downwelling conditions prevailed in the study region due to onshore Ekman transport.