the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Warm conveyor belt activity over the Pacific: Modulation by the Madden-Julian Oscillation and impact on tropical-extratropical teleconnections
Christian Michael Grams
Edmund Kar-Man Chang
Stephan Pfahl
Heini Wernli
Abstract. Research in the last decades revealed that rapidly ascending airstreams in extratropical cyclones – so-called warm conveyor belts (WCBs) – play an important role in extratropical atmospheric dynamics. However on the subseasonal time scale, the modulation of their occurrence frequency, henceforth referred to as WCB activity, has so far received little attention. Also, it is not yet clear whether WCB activity may affect tropospheric teleconnection patterns, which constitute a source of predictability on this subseasonal time scale. Using reanalysis data, this study analyzes the modulation of WCB activity by the Madden-Julian Oscillation (MJO). A key-finding is that WCB activity increases significantly over the western North Pacific when the convection of the MJO is located over the Indian Ocean. This increased WCB activity, which is particularly pronounced during La Niña conditions, is related to enhanced poleward moisture fluxes driven by the circulation of subtropical Rossby gyres associated with the MJO. In contrast, when the convection of the MJO is located over the western North Pacific, WCB activity increases significantly over the eastern North Pacific. This increase stems from a southward shift and eastward extension of the North Pacific jet stream. However, while these mean increases are significant, individual MJO events exhibit substantial variability, with some events even exhibiting anomalously low WCB activity. Individual events of the same MJO phase with anomalously low WCB activity over the North Pacific tend to be followed by the known canonical teleconnection patterns in the Atlantic-European region, i.e., the occurrence frequency of the positive phase of the North Atlantic Oscillation (NAO) is enhanced when convection of the MJO is located over the Indian Ocean, and similarly for the negative phase of the NAO when MJO convection is over the western North Pacific. However, the canonical teleconnection patterns are modified when individual events of the same MJO phase are accompanied by anomalously high WCB activity over the North Pacific. In particular, the link between MJO and the negative phase of the NAO weakens considerably. Reanalysis data and experiments with an idealized general circulation model reveal that this is related to anomalous ridge building over western North America favoured by enhanced WCB activity. Overall, our study highlights the potential role of WCBs in shaping tropical-extratropical teleconnection patterns and underlines the importance of representing them adequately in numerical weather prediction models in order to fully exploit the sources of predictability emerging from the tropics.
Julian Quinting et al.
Status: open (until 10 Jun 2023)
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RC1: 'Comment on egusphere-2023-783', Anonymous Referee #1, 03 May 2023
reply
Review of the manuscript “Warm conveyor belt activity over the Pacific: Modulation by the Madden-Julian Oscillation and impact on tropical-extratropical teleconnections” by Quinting et al.
The authors investigate the sensitivity of the warm conveyor belts activity in the North Pacific to the different Madden-Julian Oscillation phases and El Niño Southern Oscillation, how the tropics-extratropics teleconnection associated withe MJO is modulated by the WCB activity, and if an idealised general circulation model can reproduce the teleconnection and its modulations. Overall, the manuscript is interesting and well written, the methods used appropriate, and the topic fits the scopes of the journal. My main remark concerns the idealised simulations, which would greatly benefit from a better description. I would recommend publication after minor revisions associated with my comments listed here after, following the order of the manuscript.
Line 42: Could the following more recent references also be appropriate here?
Jiang et al. (2016): The relationship between the Madden–Julian Oscillation and the North Atlantic Oscillation https://doi.org/10.1002/qj.2917
Fromang and Rivière (2020): The Effect of the Madden–Julian Oscillation on the North Atlantic Oscillation Using Idealized Numerical Experiments https://doi.org/10.1175/JAS-D-19-0178.1
Lines 60-61: “Warm conveyor belts (WCBs) are rapidly, mostly poleward ascending airstreams in the storm track regions…” As written in the next sentence, WCBs originate in the warm sector of the cyclones. Therefore, I suggest to replace “in the storm track regions” with something like “within mid-latitudes cyclones” or “within mid-latitudes cyclonic systems”.
Lines 106-107: Could the authors provide an explanation about the “matching” between WCBs (or the trajectories) and the cyclones mask? What is performed at this stage is no clear. Does it select all ascending trajectories starting within the cyclone mask? Is there also a warm sector mask? Is it assumed that all ascending trajectories overlapping a cyclone mask will start in the warm sector?
Section 2.4: It is not clear how the MJO days/events are treated. Are the authors detecting events with the first day of the event being day 1 of pentad 0 (or lag 0)? Are the authors using all days with RMM index greater than 1 as first days of pentads (or lag0)? Please clarify.
In addition, no indication is given on the number of events or days considered in each composite, especially Fig. 1. Please provide these numbers.
Section 2.8: I find that the description of the idealised simulations could be improved:
- It seems that the imposed heating in the tropics or extratropics covers all longitudes and not only the specific range of longitudes associated with the MJO phases. Is it true? If yes, the authors should explain why they did that and how this is a “realistic heating”.
- The latitudinal range of the heating anomalies should be provided.
- Line 195: In several places, I found the use of “diabatic heating” confusing as the authors use a dry atmospheric model. Here, I would replace “The diabatic heating is derived from ERA-Interim precipitation anomalies” with “The imposed heating mimics the diabatic heating derived from ERA-Interim precipitation anomalies”.
- Line 201: same comment as above for “anomalous diabatic heating imposed”. The authors impose a heating, not a diabatic heating.
- How long are the simulations? 10 days to get two pentads?
Line 223: “into upper-tropospheric ridges”: The ridge does not seem well pronounced especially for phase 3 and there is no positive anomaly of Z300 (contrary to what is seen for phase 7 in Fig. 1l). Please check your sentence.
Lines 223-224: It is not clear what the authors mean with this sentence, especially as there is no notion of velocity in Fig. 1, and how to relate what it says to Fig. 1.
Line 225: Figs. 1g-k -> Fig. 1g,h,j,k
Line 263: Here, I would refer to Fig. 2 instead of Fig. 1 as the authors are now using the CNN-based data.
Line 275: I would add that in addition to being weaker, the anomalies are also less persistent for phases 2 and 3 especially.
Longitude/latitude values (lines 321, 333, 336): these locations are a bit difficult to identify on the figures as there are no lon/lat labels on the figures. Please consider adding some or add in the captions what the grid lines refer to.
Line 330: “enhance the ascent of WCBs” -> “enhance the ascent frequency of WCBs”.
Line 354: “anomalous ridge” I do not see a ridge. Do the authors mean a positive anomaly?
Line 361: “of the Azores high,” Please add a reference to Fig. 7a,c and Fig. 7d for the high WCB activity.
Line 362: Add reference to Fig. 7b.
Line 399: “diabatic” In line with a previous comment above, I find the use of “diabatic heating” here confusing.
Line 400: “The diabatic heating”. It is not clear here to which data the authors refer to. I suspect they write about the reanalyses (and not the idealised simulations). If so, change the start of the sentence to “In ERA-Interim, the diabatic heating ….”. Otherwise, clarify.
Lines 407-408: It is not clear to me what the authors mean here. Do they add heating in all areas with anomalous precipitation in ERA-Interim or at all longitudes regardless of the precipitations anomalies? In the first case, what threshold in precipitation anomalies is used (>0)? Please clarify here and in the methods section.
Lines 428-429 and 436-438: the authors attribute the differences between ERA-Interim and the idealised simulations to the different state of the atmosphere at the initial time between the simulation and the reanalysis. Could not the different Z300 anomalies be also linked to the atmospheric internal variability, ERA-Interim being just one possible realisation? Have you found an ensemble member giving the same result as ERA-Interim (here Fig. 7f but applies to all composites shown in Fig. 7)?
Line 494: “diabatic heating”: again the model is forced by an anomalous heating somewhere and not by diabatic heating because diabatic heating does not exist in a dry model, does it? Please rephrase.
Figures 3 and 4: I suggest to merge Figs. 3 and 4 to make the comparison between them easier. If they are not displayed on the same page, one cannot look at them at the same time and the comparison is difficult.
Citation: https://doi.org/10.5194/egusphere-2023-783-RC1
Julian Quinting et al.
Julian Quinting et al.
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