the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Sep 2024
Extreme weather anomalies and surface signatures associated with merged Atlantic-African jets during winter
Abstract. The winter-long merging of the African and Atlantic jets is associated with extreme winter weather across the Northern hemisphere. Past studies have shown that merging of the Atlantic and African jets is linked to weaker Atlantic eddy activity and stronger tropical heating, and is strongly correlated with a negative NAO state. In this study we examine the relation between jet merging and extreme weather, taking care to separate out the effects of the NAO and El Nino, in order to be left with the added influence of Atlantic-African jet merging. Our analysis, considering percentile exceedance and anomaly composites of surface temperature, surface wind and precipitation, identifies distinct weather signatures of merged jet winters, notably affecting the Iberian Peninsula, North Africa, southern Mediterranean, southwest Greenland, and Northern Europe. Additionally, we analyse the relationship between merged jets and shifts in cyclone track orientation contributing to the observed extreme weather patterns over these regions. Furthermore, once we remove the NAO effect from the merged-jet surface temperature anomaly signal, we find that winter-long jet merging coincides with anomalous warm Arctic, cold Eurasia, and strong El Niño conditions. This weakens the high latitude temperature gradient which affects the midlatitude baroclinicity resulting in the weakening of eddy activity and ultimately leading to persistent jet merging. Thus the African and Atlantic jet merging during winter appears to further align with the theoretical concept that includes a dynamical regime transition of the jet from eddy driven to mixed eddy–thermally driven, consistent with the corresponding weakened mid-latitude eddies and intensified tropical heating.
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RC1: 'Comment on egusphere-2024-2745', Anonymous Referee #1, 08 Dec 2024
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Major comments.
This paper examines 2m temperature, surface winds, and precipitation extremes associated with merged Atlantic-African jet during northern winter. It is well written with an easy-to-follow structure. The introduction is particularly well-written. However, I do have one major concern about how the statistical significance is performed.
My worry is that the merged-jet regime is treated as an independent dynamic regime from the NAO and ENSO. It could well be that those winters involve both extratropical and tropical forcing, with the latter further involving latent heat realise in additional to tropical SSTs. More rigorous statistical significance tests are thus required to be sure that the signals are associated with the merged jets rather than the NAO, EI Nino or their combined effect.
- Quite a lot of the samples of the merged jets are associated with the negative NAO phases (-NAO, 9 out of 14) and with EI Nino (7 out of 14). With only 5 or 7 independent samples, the statistics of composite or linear regression analysis become limited and may lead to unreliable estimates. Furthermore, the merged-jet subgroup contains individual months for a given winter, e.g. 12/2009, 01/2010, 02/2010 and the signal has a strong imprint of tropical SST anomalies, which may persist over a winter. Thus, whether those months can be viewed as independent samples remains a question. To make sure that the results are truly different from -NAO and EI Nino, the authors need to explain exactly how the Monte Carlo simulation is performed. They may consider using other statistical techniques that are more robust with small sample sizes, such as Partial Least Squares Regression (PLS) or Ridge Regression that adds a penalty to the regression coefficients. Have the authors cross-validation the stability and reliability of the results using bootstrapping?
- Statistical significance tests are required for the composite differences shown in Figures 4, 6, 8, and 9.
- Have you examined the composite anomalies based on the eight winter months during which -NAO and EI Nino occurred at the same time and compare with the anomalies associated with the -ZJI regime?
- The discussion about the difference of daily average surface temperatures (figures 2 and 3), surface winds (figure 5), and total precipitation using box plot for the selected regions is somehow confusing. Because no confidence intervals are shown for the median nor for the mean, these box plots do not prove those measures are either larger or smaller than the climatology statistically.
Specific comments:
- Title: during winter -> during northern winter
- Line 62-64. This sentence has little to do with the 2009/2010 winter or add anyting to the paragraph. Consider removing.
- There are quite a few “Northern Hemisphere” in the paper. Consider abbreviating it to “NH”.
- Line 67: make it clearer that the warm extremes over North Africa and Arab are linked to positive NAO and during summer, or delete this part of the sentence because your focus is on the winter season.
- Line 73: insert “subtropical” before “eastern US coast”.
- Line 125: do you mean that the eddy kinetic energy shown in figure 9 is estimated based on 10-day highpass filter?
- Line 126-127: I am not should that mean flow corresponds to the data filtered with a 10-day low pass filter. Quasi-stationary waves will stay in the low-passed part but they should not be regarded as the mean state of the flow, also in which figure that the low-pass covariances between the high-pass fields are assessed?
- Lines 142-150. This is a very long sentence. Try to break it into two so that the readers can understand exactly what is involved.
- Line 150: how “significantly”. Consider reserve the word “significant” to statistically tested quantity.
- Line 174: NAO -> -NAO and other similar expression.
- Lines 190-215: reads like discussion rather than results. Move to last section and make the section title to “conclusions and discussion”?
- Lines 202 and 204: I do not think that you have studied the warming trend. These are anomalies associated with either the -NAO, EI Nino or the -ZJI.
- Line 239: can you be a little specific about “the sources of momentum”? Is the merged jet eddy driven or thermal driven? If it is too elaborated to go into the details, I would suggest removing these texts. Mixing lengthy discussion of previous work with new results can be confusing sometimes.
- Line 249: This explains -> This is consistent with.
- Lines 251-254: This may indicate an anomalous …”. This is too speculative especially figure 6b does not involve any statistical test, the rather small sample size, and the small magnitude of the wind anomalies (< 1 m s^-1).
- Line 272: “Additionally, there are notable extreme daily precipitation from the climatology that fall within the ZJI winter days”. I wonder how can you make a statement like this just based on the box plot. Each box plot is associated with different subgroup of winter days with no information about the climatology. For a given day that is shared by climatology, -NAO or -MJI group, it can be shown outside of the whiskers in one boxplot, e.g. climatology, but included in other subgroups.
- Line 314: “A significant proportion”. This is not supported by the results because no statistical significance is shown in figure 9.
- Lines 247-350: again speculative sentence without any concrete results backing.
- Line 318: Conclusions -> Conclusions and Discussion. Also, I would think that the conclusion can be much more concise than what has been written.
- Line 335: no trend has been studied here.
- Line 337: A connection between -ZJI and Arctic Amplification may be related to the paper https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL082714.
- Line 362: remove due to merged jet formation.
Citation: https://doi.org/10.5194/egusphere-2024-2745-RC1
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