Influence of finite-time Lyapunov exponents on winter precipitation over the Iberian Peninsula

Garaboa-Paz, Daniel; Lorenzo, Nieves; Pérez-Muñuzuri, Vicente

doi:https://doi.org/10.5194/npg-24-227-2017

Articles | Volume 24, issue 2

https://doi.org/10.5194/npg-24-227-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

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Current perspectives in modelling, monitoring, and predicting...

https://doi.org/10.5194/npg-24-227-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 24, issue 2

Research article

|

24 May 2017

Research article |

| 24 May 2017

Influence of finite-time Lyapunov exponents on winter precipitation over the Iberian Peninsula

Daniel Garaboa-Paz, Nieves Lorenzo, and Vicente Pérez-Muñuzuri

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Final revised paper (published on 24 May 2017)
Preprint (discussion started on 20 Dec 2016)

Interactive discussion

Status: closed

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

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RC1: 'Review Garaboa-Paz et al', Anonymous Referee #1, 23 Jan 2017
- AC1: 'Answers to Referee 1', Vicente Perez-Munuzuri, 21 Feb 2017
RC2: 'An interesting approach calling for further research', Reik Donner, 27 Jan 2017
- AC2: 'Answers to Referee 2', Vicente Perez-Munuzuri, 21 Feb 2017

Peer-review completion

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

AR by Vicente Perez-Munuzuri on behalf of the Authors (08 Mar 2017)

ED: Referee Nomination & Report Request started (09 Mar 2017) by Emilio Hernández-García

RR by Anonymous Referee #1 (24 Mar 2017)

Suggestions for revision or reasons for rejection

The authors have partially addressed my concerns. I still believe the paper has interesting results that merit publication, but I do not think it is ready yet.

1) I still can not follow the reasoning behind the mechanism that links the summer FTLE and rainfall in winter. The authors say " In our opinion, large-scale tropospheric mixing drives summer SST anomalies that lead to changes in the next seasons storm tracks, and consequently changes in the location of action centers". However, it is not possible to deduce this from their results.
They show composites of different fields during summer for large + and - cases of FTLE anomalies. However, there is no causality implied there. SST lags atmospheric forcing for about 3 months, and thus the simultaneous maps shown can not be used to imply causality. Also, it is not obvious that midlatitude SST forces atmospheric circulation anomalies during winter that lead to rainfall.
An alternate possibility is that the FTLE in summer is a reflection of tropical SST anomalies, which may persist and then impact winter precipitation. Note that both winter rainfall and FTLE are correlated to SOI in JFM.

2) When describing Figure 3 (page 5) the authors propose a series of changes in synoptic activity without providing evidence for that. If they believe this is the right explanation they should at least say that this is an hypothesis, or speculation.

3) In the response the authors mention that
"the changes in the values of positive phase and negative phase of FTLEs are small, the
Wilcoxon rank sum test shows that these differences between positive and negative phase of
FTLEs are significant. The same is found between the differences of the positive and negative
anomalies of the other variables."
If this is certainly the case, they should explain in the manuscript the use of this test and mark in the figures the regions that are really significant. This is important given that the changes in FTLE are of the order of 3%.

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RR by Reik Donner (05 Apr 2017)

Suggestions for revision or reasons for rejection

The authors have addressed some of the recommendations from my original review to improve the presentation of their work. However, I feel that some further revision is necessary before this work can be considered ready for publication.

My main concern is still the definition of seasons, which is non-standard. Despite the statements in the response letter, there is a climatological standard referring to DJF (JJA) as (boreal) winter (summer). If a climatologist reads the term “(boreal) summer/winter”, they will most likely expect exactly these months to be considered. I accept that some authors may deviate from this general practice for probably good reason, but in this case, I strongly recommend clarifying throughout the manuscript that the three-month periods of JFM and JAS are used and NOT the common climatological winter and summer months. That is, I recommend replacing the terms “summer” and “winter” by JAS and JFM, respectively, throughout the manuscript to avoid any ambiguities.

The second, yet minor point of criticism is a number of sentences where rephrasing is necessary for either scientific or grammatical reasons. I provide a list of these points below (page and line numbers refer to the final manuscript markup without tracked changes):
• Page 1, ll. 8-9: “other circulation and temperature patterns as the sea surface temperature (SST), the sea level pressure (SLP) or the geopotential” – you list here three climatic variables, not three circulation and temperature patterns
• Page 1, ll. 18-20: Please provide references for the statements that (i) teleconnection indices are used as predictors for seasonal forecasts and (ii) the NAO-ENSO relationship is non-stationary.
• Page 1, ll. 21-22: “in specific regions” or “in certain regions”
• Page 1, l. 22: “nonstationary” => “nonstationarity”
• Page 2, l. 2: do not capitalize “sea surface temperatures”
• Page 2, l. 2: “North Atlantic basin” or “northern Atlantic basin”(?)
• Page 2, l. 17: “the obtained time series” – of what?
• Page 2, ll. 24-25: “A gridded dataset (IB02) was used for daily precipitation (mm) over the Iberian Peninsula from 1979 to 2008. This dataset covers the continental…”
• Page 2, ll. 28: “have been used” – for what?
• Page 2, l. 28 and l. 30: “have been derived” sounds a bit weird in the present context
• Page 3, ll. 9-10: This sentence appears a bit misplaced at this point; better provide this information at the point where the correlations are discussed.
• Page 3, ll. 17-18: “the interference of most pf the turbulence effects” – unclear statement
• Page 3, l. 31: “an integration time of tau=…”
• Page 4, l. 6: “following the steps explained above”
• Page 4, ll. 10-14: Please detail the meaning of the terms “mean” and “total mean” (i.e., “mean” seems to denote the spatial mean at a given time step, and total mean the average FTLE over all time steps?). Also, the phrase “two time series (positive and negative phases)” is not clearly understandable as such.
• Page 4, l. 22: “is generally stronger”
• Page 4, l. 24: “their role for JFM rainfall”
• Page 4, l. 28: “for summer FTLE anomalies (Fig. 2(b)) shows large, spatially consistent and statistically significant negative correlations…”
• Page 4, l. 32: “in the considered area some months in advance”
• Page 5, l. 4: If you write “have been discussed”, this implies that this was done somewhere else or in the previous parts of this paper – in this case, please give a reference. Or do you mean “will be discussed”?
• Page 5, l. 21: Do you mean “extreme weather events”?
• Page 5, l. 23: “positive geopotential height anomalies… negative summer FTLE anomalies”
• Page 5, l. 24: “to a next winter trough, that is a region…”
• Page 5, l. 30: “travelling across the Atlantic”
• Page 5, l. 34: What do you mean here by “trajectory”?
• Page 5, l. 34: “a reduced transport of moisture”(?)
• Page 5, l. 35: “displaced northward”
• Page 6, l. 1: “is at is usual…”
• Page 6, l. 4: “weaker easterlies”
• Page 6, ll. 7-8: In this sentence, it is not clear what correlated with what. Please rephrase. I also think that it would improve the readability of some sentences if you explicitly write “positive/negative” instead of “(+)/(-)”, e.g., in l. 14 on the same page.
• Page 6, ll. 23-24: “The conclusions suggest…” – which conclusions?
• Page 6, l. 26: “over the western…”
• Table 1: Please clarify if the seasons (rows) correspond to the same year as “winter”/”summer” (should be “JFM”/”JAS”) or the preceding/following.
• Page 7, l. 4: “influence winter rainfall…”
• Acknowledgements: ERA-Interim data were rather provided than supported by ECMWF.
• Fig. 3, caption: What do you mean by “global summer FTLE mean” – surely not the summer FTLE mean around the globe. Moreover, when you select years with positive/negative FTLE anomalies – do you mean all years where the mean FTLE is above/below its global mean, or just such where this deviation is particularly strong (as one would commonly use when considering “climatological composites”)?

After these points have been clarified/corrected, the paper can be accepted for publication.

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RR by Anonymous Referee #3 (17 Apr 2017)

Suggestions for revision or reasons for rejection

The paper titled “Influence of finite-time Lyapunov exponents on winter precipitation over Iberian Peninsula” and authored by Daniel Garaboa-Paz, Nieves Lorenzo and Vicente Pérez-Muñuzuri, represents an interesting research work with important results that can improve the seasonal forecasting of an area like the Iberian Peninsula, where the meteorological and climatological forecast is hard to perform. Also, the implemented tool can be extrapolated to other regions. It is an interesting work focused on a nowadays topic where the authors used the FTLE technic to represent the winter anomalies over the Iberian Peninsula, and also they explain and hypothesize the dynamics below the statistical results obtained, connecting the precipitation events during winter with several variables as SST, geopotential height, wind, etc.

The idea is well explored, the hypotheses are well argued and the “science” is well understood. The scientific background is clear, being an interesting topic with clearly objectives and results. The statistical technics performed and the methodology implemented are appropriated, well implemented and with relevant results. Moving to the manuscript itself, I am not a native English speaker, but in my opinion, the English is good and it is understandable, and the manuscript is well written.

Also, this manuscript was previously evaluated by two referees, and the authors were very clear on the responses. I think that the present study is an interesting contribution and should be considered for publication at the Nonlinear Processes in Geophysics Journal. I have only several minor comments and suggestions that can help to improve the paper.

First of all, I think that include some references in several parts of the text can improve the quality of the manuscript. I suggest include references on the next points:
* Page 1: When the authors write: “However, the relationship between NAO and ENSO and the European variability is nonstationary; that is, the strength of the correlation between these two teleconnections and climate anomalies has changed over time.”
* Page 2: On the affirmation: “Changes in mid-latitude circulation can strongly affect the weather events”, I suggest to add a reference and also a region where this changes can be observed, referring the type of weather events that are affected.
* Page 2 and 3: Please include a reference or web for the IB02, ERSST, ICOADS, NCEP reanalysis and CPC (related with teleconnection indices) databases.

Other comments and suggestions are the next ones:

Page 2:
The authors include some works that link the Iberian precipitation with other variables. The paragraph is the next one:
“Previous works have shown a possible link between the Iberian precipitation and other variables like summer Sea Surface Temperature (SST) anomalies over the north Atlantic basin (Rodríguez-Fonseca and deCastro, 2002; Lorenzo et al., 2010; Hatzaki et al., 2015), other teleconnection patterns (deCastro et al., 2006; Casanueva et al., 2014) or the Euroasian snow cover in autumn (Brands et al., 2014). The storm track activity has been related to the occurrence of extreme events (Lehmann and Coumou, 2015). Changes in mid-latitude circulation can strongly affect the weather events.”
Please, can you clarify how the Iberian precipitation can be linked with the Euroasian snow cover in autumn, and also where the storm track activity was considered and where the occurrence of extreme events was analysed?

The authors said that:
“Among others, FTLE have been used to identify the presence of barriers to mixing in the atmosphere between the tropics and extratropics (Pierrehumbert and Yang, 1993), to study the zonal stratospheric jet (Beron-Vera et al., 2008), jet-streams (Tang et al., 2010), hurricanes (Rutherford et al., 2012), transient baroclinic eddies (von Hardenberg and Lunkeit, 2002), the spread of plankton blooms (Huhn et al., 2012), the polar vortex (Koh and Legras, 15 2002), or the atmospheric rivers (Garaboa-Paz et al., 2015).”
Can you considered to organize the use of FTLE and divide in atmospheric and oceanic uses? I suggest the next order: “FTLE have been used also in atmospheric and oceanic studies, to represent the zonal stratospheric jet, jet-streams, the polar vortex, atmospheric rivers, hurricanes, transient baroclinic eddies or even the spread of plankton blooms” including the respectively references.

I also suggest to include, in a short way, the main methods used to calculate the FTLE in the next paragraph: “Our goal in this study is to characterize the rainfall patterns in the Iberian Peninsula as a function of the large-scale tropospheric mixing over the Atlantic ocean. To that end, we have calculated a climatology of FTLE for the period 1979-2008. The obtained time series was then correlated with the precipitation over the Iberian Peninsula. Finally, we discuss the obtained results by considering their relationship to the main modes of circulation variability.”

Please, include the time period selected for the ERA-INTERIM database in the paragraph: “The atmospheric transport has been studied using wind field data retrieved from the European Center for Medium-Range Weather Forecast reanalysis, ERA-Interim (Dee et al., 2011), with a horizontal spatial resolution of 0.7º, a vertical resolution of 100 hPa and a temporal resolution of 6 hours.”

Page 2 and 3:
The authors said that they considered “Yearly anomalies of the SST, geopotential at 500 hPa, sea level pressure (SLP) and wind speed at 200 hPa and 850 hPa for the same period have been used. The SST anomalies have been derived from The Extended Reconstructed Sea Surface Temperature (ERSST) dataset which is a global monthly sea surface temperature dataset derived from the International Comprehensive Ocean-Atmosphere Dataset (ICOADS). It has been derived on a 2 x 2 grid with spatial completeness enhanced using statistical methods. This monthly analysis begins in January 1854 continuing nowadays. The newest version of ERSST, version 4, is based on optimally tuned parameters using the latest datasets and improved analysis methods. The geopotential, SLP and wind speed anomalies have been obtained from the National Center of Environmental Prediction (NCEP) reanalysis with a spatial resolution of 2.5 x 2.5. The climatology used for the anomaly plots is for the 1981-2010 period.” (page 2 and 3). Nevertheless, in the page 4 they said that “Seasonal composites (averages) of the anomalies (mean - total mean) of the SST, geopotential height and wind speed were obtained from NCEP for the same period.” Please, can you clarify which kind of data is used and with which temporality? Also the authors said that the performed a climatology for the anomaly plots using the 1981-2010 period. Why this period was selected and not the 1979-2008 period that is the one chosen for the entirely work?

Page 3
Please, include the meaning of the JFM, AMJ, JAS and OND acronyms on the text.

Please, considered to place the sentence: “The monthly indices were averaged for the seasons JFM, AMJ, JAS and OND from 1979 to 2008. The most representative atmospheric patterns for the Northern Hemisphere were considered in order to analyse their influence on precipitation for the Iberian Peninsula and with the summer FTLEs.” at the beginning of its paragraph on this way: “The monthly indices were averaged for the seasons JFM, AMJ, JAS and OND from 1979 to 2008. The most representative atmospheric patterns for the Northern Hemisphere were considered in order to analyze their influence on precipitation for the Iberian Peninsula and with the summer FTLEs. The teleconnection indices NAO, SCA (Scandinavia pattern), EA (East Atlantic pattern), EA/WR (East Atlantic/ West Russia pattern), POL (The Polar/ Eurasia pattern), SOI (Southern Oscillation Index), PNA (Pacific-North American Pattern) and the atmospheric mode AO (Artic Oscillation) were obtained from the Climate Prediction Center (CPC) at NCEP at monthly time scales.”

Why the authors considered an initial separation of 0.35º in the distribution of the particles?

Page 4:
It will be interesting to comment that the FTLE were initially calculated globally, and then an area of the Eastern Atlantic was extracted to perform the study. Please, considered to include a square in the Figure 1 that represent the Eastern Atlantic area used to extract the FTLE results (between 30ºW and 0ºW and between 25ºN and 65ºN).

Please, considered to write “the precipitation of the western half of the Iberian Peninsula and the FTLE” or “the precipitation of the western half of the study area and the FTLE.” instead “the precipitation of the western half of the peninsula and the FTLE.”

Page 5:
Please considered to write “The size of this area was varied considering the entirely North Atlantic Ocean without modifying significantly the results shown here.” instead ““The size of this area was varied within the North Atlantic Ocean without modifying significantly the results shown here.”
Please, modify the paragraph: “Previous studies have shown that SST anomalies in certain areas of the subtropical North Atlantic are statistically related to winter precipitation on the IPNA (Iberian Peninsula North Atlantic) region with a five-month lag (Rodríguez-Fonseca and deCastro, 2002).” There is only one previous study cited in this paragraph.

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ED: Publish subject to minor revisions (further review by Editor) (18 Apr 2017) by Emilio Hernández-García

AR by Vicente Perez-Munuzuri on behalf of the Authors (27 Apr 2017) Author's response Manuscript

ED: Publish as is (28 Apr 2017) by Emilio Hernández-García

AR by Vicente Perez-Munuzuri on behalf of the Authors (28 Apr 2017)

Short summary

This paper evaluates the connection between winter precipitation over the Iberian Peninsula and the large-scale tropospheric mixing over the eastern Atlantic Ocean. Finite-time Lyapunov exponents (FTLEs) have been calculated from 1979 to 2008 to evaluate this mixing. Our study suggests that significant negative correlations exist between summer FTLE anomalies and winter precipitation over Portugal and Spain.