|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:
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?
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?
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.”
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.