The authors’ responses to my previous comments and the revised version of the manuscript both keep ignoring my point that the proposed index CHOCO-D is erroneous and misleading to track the Choco jet. The Choco jet was discovered and introduced in the literature two decades ago (Poveda and Mesa, 1999; 2000), as a low-level jet acting on a well-defined geographical (3-D) setting, and having a very precise physical definition: a year-long westerly low-level jet, with a clear-cut annual cycle, which satisfies all the criteria put forward by Stensrud (1996) for low-level jets. As such, the Choco jet has been extensively studied by many authors since (see Yepes et al., 2019). The large latitude range that the authors chose to define the newly proposed index is under the action of the Choco jet and the Caribbean low-level jet, another very-well defined circulation mechanism acting over the Caribbean, Central America and the far eastern Pacific, also exhibiting a clear-cut annual cycle (Magaña et al., 1999: Poveda and Mesa, 1999; Wang, 2007; Muñoz et al., 2008; Cook and Vizy, 2010, etc.). The newly proposed CHOCO-D index does not exhibit a steady westerly flow, but a regime of seasonally varying westerly and easterly winds owing to the crashing dynamics (“tug of war”, as per my original review) of the Choco and Caribbean low-level jets over the far eastern north tropical Pacific. With all due respect, the proposed new index contaminates the dynamics of the Choco jet with that of the Caribbean jet, and does not make historical or physical sense. As it has been defined, the proposed index introduces unnecessary confusion in the study and understanding of the region’s weather and climate. I kindly urge the authors to refrain referring to the newly proposed index as an index of the Choco jet. At the most, it is an index for the tug of war between the Choco and the Caribbean jets. Therefore, I strongly recommend to change the title of the paper, and all the sections accordingly. What the authors built is not an index of the Choco jet but of the tug of war between the Choco and the Caribbean (CHOCAR, which by the way means to crash, in Spanish). As such, the authors need to justify the need and the relevance of such phenomenon to support the need for creating such an index. Therefore, the manuscript demands major revisions.
I also find fundamental problems in the authors responses to my original review, discussed next:
Response to comment No. 3: “When using an index only relying in wind direction (directional index) to characterise a jet, it is critical to define it in an area larger than that where the core of the jet is located. Otherwise it would be impossible to obtain a climatic signal related to the jet’s variability.” In this case, the authors must provide evidences to prove that the proposed index does comply with the requirements defined by Stensrud (1996). Moreover, as I previously mentioned it does not make physical sense to define a low-level-jet using a large latitudinal extent that is subject to the dynamics of two different low-level jets. Also, the second statement is incorrect. It is precisely because of the very well-defined setting of the historical Choco Jet that it is possible to obtain a climatic signal related to the jet variability, at diurnal, seasonal, intra-seasonal, annual and inter-annual timescales. Otherwise, the authors are mixing different phenomena and creating unnecessary confusion.
Response to comment No. 3: “if a directional index is defined for a region where the jet is permanently located as suggested (in this case the vicinity of 5ºN and 80ºW at 925 hPa), we would find no changes in the wind direction associated to the jet’s variability”. I do not understand why the authors keep arguing that there is the need to define the Choco jet in terms of a wind reversal. As historically defined, the all year-round westerly Choco jet does exhibit a clear-cut behaviour at diurnal, annual and inter-annual time scales. The so-called variability in the wind direction of the newly proposed index is solely due to the contamination with the dynamics of the Caribbean jet, and not a reversal of the Choco Jet.
Response to comment No. 3: “Instead, we want to build a series based solely on wind direction and longer than those currently available.” This objective is quite different than building an index of the Choco jet. Again, what the authors built is not an index of the Choco jet but of the tug of war between the Choco and Caribbean low-level jets.
Response to comment No. 4: “The Choco jet is not, by any way, a monsoonal circulation.” Yes, indeed. Although by the definition of the newly proposed index, it becomes a monsoonal (once-a-year) circulation, which contradicts the historical and physical definition of the Choco jet.
Response to comment 4: “This will also allow us to justify the necessity of using the large area [4N-15N; 120W-84W] plus the area [4N-9N;84W-77.5W] where the influence of both jets is mixed,…” Again, it is incorrect and confusing to introduce a new definition of the Choco jet by mixing winds from two jets. The original Choco jet does not get reversed during the annual cycle, unlike the newly proposed one. So, the authors completely change the physical interpretation, dynamics and effects.
Response to comment 4: “the CLLJ is clearly “winning” and easterlies dominate the selected area from 4N to 15N, while the weak westerlies associated with the Choco Jet are restricted to the 5N 80W vicinity.” It is misleading to trying to justify the proposed Choco jet index using a single month of a single year, in particular during the occurrence of extreme phases of ENSO (Yepes et al., 2019). Figure 1a of the responses document illustrates the winds during August 1997, just in the middle of one of one of the strongest El Niño events ever recorded. As such it is normal to witness an enhanced Caribbean jet and a weakened Choco jet, and therefore the Caribbean winning the tug of war over the Choco jet, moreover at 90W, as shown in Fig 1(c). A similar deficiency-limitation can be pointed out in the case of Figure 2, since it depicts the winds at 925 hPa during a strong La Niña event (August 2010), and therefore the predominance of the Choco jet over the Caribbean jet. This cherry-picking of months and ENSO events are inadequate and misleading to justify the climatological features of the newly proposed index for the Choco jet.
Response to comment 4: “… but this would not be effective for an index based on wind direction such as ours, as the wind direction scarcely varies at this location. What we have found is that by using an extended area, the changes in the relation of westward/eastward winds are a good “proxy” of the strength of the Choco jet at its core” Again, the historical definition of the Choco jet was justified by the presence of steady (all year long) westerly winds flowing from the Pacific Ocean towards inland Colombia, and showing a clear-cut annual cycle associated with the spatial gradient between air temperatures over western continental Colombia and SSTs over the Nino 1+2 region. The newly proposed index turns out to mix the dynamics of two different jets, and causes confusion in the interpretation and analysis of diverse land-ocean-atmospheric mechanisms operating at the region. Therefore, I strongly disagree that the enlarged area used to define this new index provides an adequate “proxy” of the Choco jet. It is precisely due to the reversal of the winds with the annual cycle, when taken over such a large area, that the newly proposed index does not make historical of physical sense, beyond the correlations between the Choco jet index and the CHOCO-D index. My concern is about the physics and dynamics and not about statistics.
Response to comment 10. “the winds typically reverses between January and July, which is one of the classical criteria to define monsoonal areas”. Again, the winds of the Choco jet do not get reversed. The problem is the selected region, where two different jets are acting at the same time, and by taking averages of the winds of both jets, the authors find an spurious reversal resulting from the mixing of both mechanisms, thus confusing the dynamics of two different phenomena/mechanisms as a single physical phenomenon.
Response to comment 13: “Our directional index is not capable of characterizing the Choco Jet in some months.” Indeed! This is precisely the main problem of the newly proposed index (inadequate region, mixing of dynamics), thus creating confusion in the scientific literature.
Response to comment 14: “As the CHOCO-D is defined as the % of westerly winds in the selected area, our index is essentially zero from December to April.” This points out the limitations and shortcomings of the newly proposed index. Owing to the enlarged region used to define the proposed index, and that there are no westerly winds from December to April within such region, the index is useless almost half of the year. Therefore, what would be the value of the proposed index to represent the Choco jet?
Response to comment 15. “So although it is true that June-October is not ideal to analyze the dynamic of the Choco- jet, it is the best that can be achieved with a directional index.” Indeed! The authors recognize one of the limitations of the proposed index. The problem is that Choco jet is defined as permanent (westerly) jet, and not as a directional (reversed once-a-year) circulation feature. Again, the confusion results from the proposed enlarged region under the dynamics of two distinct low-level jets.
Response to comment 15. “the benefit of our approach is that we can compute an index based on actual observations since the late 19th century.” This argument hardly applies, given the aforementioned shortcomings and limitations of the proposed new index in historical and physical terms. What would be the usefulness of such an index? On the other hand, it would valuable if the authors can produce a better historical record of the (properly defined) Chocó jet.
Response to comment 20. “the CHOCO-D does not produces reliable estimations of the jet strength between December and April (see our reply to question #13),…” Quite true! The newly proposed index does not provide reliable information about the Choco jet during almost half (5/12) of the year!
Response to comment 21. “it should be clear the necessity of considering an area where both the Choco Jet and the Caribbean LLJ are included in order to compute a directional index.” With all due respect, I disagree based upon my previous comments on the inadequacy to define the new index using an enlarged region, which is under the dynamics of two different low-level jets.
Response to comment 22. “Maybe we were a little bit enthusiastic. In the revised version we will substitute “excellent response” by “significant relationship” and we will explicitly indicate that this relation is restricted to the May-November period.” Again, I disagree. They differ almost half (5/12) of the year!
References
Cook, K.H. and E.K. Vizy, 2010: Hydrodynamics of the Caribbean low-level jet and its relationship to precipitation. J. Climate, 23, 1477–1494.
Magaña, V., J. Amador, and S. Medina, 1999: The midsummer drought over Mexico and Central America. J. Climate, 12, 1577–1588.
Muñoz, E., A. J. Busalacchi, S. Nigam, and A. Ruiz-Barradas, 2008: Winter and summer structure of the Caribbean low-level jet. J. Climate, 21, 1260–1276.
Poveda, G., and O.J. Mesa, 1999: La Corriente de Chorro Superficial del Oeste (“del CHOCÓ”) y otras dos corrientes de chorro atmosféricas sobre Colombia: Climatología y Variabilidad durante las fases del ENSO. Revista de la Academia Colombiana de Ciencias Exactas, Físicas y Naturales, Vol. 23, No. 89, 517-528.
Stensrud, D.J., Importance of low-level jets to climate: A review, J. Climate, 9, 1698-1711, 1996.
Wang, C., 2007: Variability of the Caribbean low-level jet and its relations to climate. Climate Dyn., 29, 411–422
Yepes, J., G. Poveda, J.F. Mejía, L. Moreno, and C. Rueda (2019): CHOCO-JEX: A Research Experiment Focused on the CHOCO Low-level Jet over the Far Eastern Pacific and Western Colombia. Bull. Amer. Meteor. Soc. In the press. https://doi.org/10.1175/BAMS-D-18-0045.1 |
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