Review of egusphere-2023-29452: “Life cycle dynamics of Greenland blocking from a potential vorticity perspective”.

The evolution of Greenland blocks are examined here from a PV perspective. The authors show, by tracking negative PV anomalies reaching the Greenland region, that there are two distinct pathways through which Greenland blocking typically develops. The focus then is on quantifying the contribution of various terms in the PV tendency equation to the onset, maintenance and decay of Greenland blocks. It is shown that moist processes (via quantification of the divergence of the divergent wind within the PV anomaly) are a leading contributor to the onset and maintenance of Greenland blocks (with difference between each pathway quantified and explained). These findings add to the growing body of work on the diabatic contributions to block dynamics.

The paper is well written, the analysis techniques appropriate for the aims of the paper and the figures included are high quality and support the conclusions reached by the authors. In particular, I thought the introduction was well written and contains a nice introduction to the causes and impacts of atmospheric blocking in general (with specific results relating to Greenland nicely highlighted) and the quasi-Lagrangian PV perspective nicely allows the interpretation on the evolution of the PV anomalies during their entire lifecycle. It is also an interesting result that there are clearly two distinct pathways for negative PV anomalies to develop over the Greenland region, which opens many avenues for further study on the two different pathways and e.g. their representation in models.

The article fits well within the scope of WCD and includes many results that will be of interest to its readers. For the reasons listed above I believe the manuscript is suitable for publication in the journal subject to the authors addressing the minor comments listed below. 

Minor comments:

1. The authors use a weather-regime classification of Greenland blocking rather than a typical (feature-based) blocking index. What do the days that get assigned the GB weather regime look like? E.g. how many days go into the composite in Figure 1 and what is the corresponding spread among these days? Did you check that there are not days that get assigned to GB that a synoptician would not describe as blocked? As previous studies have shown results can be sensitive to the choice of blocking index (e.g. Barriopedro et al. 2010), it would increase the robustness of your results if they could be replicated using another index. I understand if this is not feasible but some discussion should be added to reflect this at least.

L1: As currently written, the opening sentence reads like blocking over Greenland does not have impacts of surface weather over Greenland “Blocking over Greenland has substantial impacts on surface weather in particular over Europe and North America”. I assume this is not what is meant so I would consider rephrasing this sentence.

L65-80: Greenland blocks were also shown to be sensitive to upstream precursor cyclones and the upper-level wave pattern in Maddison et al. (2019). Consider including reference here.

L126: could you add (half) a sentence here explaining why coarser resolution data is needed for the piecewise PV inversion?

L131: Given that ERA5 extends much further back now than 1979, was there a reason for only including these dates?

L189: why are the threshold values in Fig. A1C positive?

L226-L243: is this section missing a sentence on the physical interpretation of “Bnd”?

L257 and elsewhere: many things are stated as being a “30-day running mean climatology”. This could have several different meanings so a more precise definition should be included somewhere.

Fig.3: do these patterns look different if separated into seasons? A sentence could be added to highlight any differences or state they are similar.

Fig.3: if the top row of Fig.3 is reproduced separately for the ‘retrogression’ and ‘upstream’ Greenland blocks, how different is it? Maybe the PV evolution in the days preceding the blocks are quite different for the different pathways leading to cancelation of features. Another sentence could be added on this if there are interesting differences.

L348:You state that LOW contributes to the amplification of onset PVA-s. Maybe I am confused, but an amplification of the PVA-s would be a negative tendency (as is seen in the total DIAG from day -2), whereas the tendency from LOW is positive throughout, I.e. acting against the block amplification? Also the text around L356. I assume I am interpreting this figure wrong as a positive PV tendency would not act to amplify the PVA-. Please clarify what you mean here.

L440-453: does the increased WCB outflow at day-5 for the retrogression indicate that moist processes were also key in establishing the European/Scandinavian ridge that eventually becomes the Greenland block?

Section 4.3: you show the net effect of the different terms on the evolution of the maximum PVA-s. Have you looked at the net effect of the terms for all the PVA-s contributing to the Greenland blocks? How different would that look? (I am still confused why the PV tendencies are positive here when the onset of a block is associated with negative PV tendencies.)

Technical corrections:

L95: adopting —> adapting?

L225: missing “)” after “Equation 5”

L291: is a word missing after “westward towards”? Do you mean “westward towards Greenland to build up a block”?

L340: does DIAG not underestimate the amplitude change? OBS ends at a more negative value and therefore the change is greater than in DIAG?

L466: the beginning decay phase —> the beginning of the decay phase?

L542: lost —> reduced

L550: remove “the” from “the sign”

L588: Sections 4.2 and 4.2 —> Sections 4.1 and 4.2?

L629: on —> of

References:

Barriopedro D, García-Herrera R, Trigo RM. Application of blocking diagnosis methods to general circulation models. Part I: a novel detection scheme. Clim Dyn. 2010;35(7–8):1373–91.

Maddison, J.W., Gray, S.L., Martínez-Alvarado, O. and Williams, K.D. (2019) Upstream cyclone influence on the predictability of block onsets over the Euro-Atlantic region. Monthly Weather Review, 147, 1277–1296.

Review of doi:10.5194/egusphere-2023-2945: “Life cycle dynamics of Greenland blocking from a potential vorticity perspective”, Hauser, S., Teubler, F., Riemer, M., Knippertz, P., and Grams, C. M.

This study identifies consistent life cycles of Greenland blocking with objective onset, maximum and decay stages in ERA5 reanalysis data from 1979–2021. The periods of blocking are identified from the perspective of 7 weather regimes in the North Atlantic-European region, and a weather regime index computed to quantify the similarity of an instantaneous geopotential height field to one of these regimes. These indices are used to define onset and decay times and thus regime life cycles, of at least 5 days. One of these weather regimes is an anticyclonic type, Greenland Blocking (GL). This method identifies 177 GL episodes in the data set.

The study then employs a quasi-Lagrangian potential vorticity (PV) perspective to track negative, upper-tropospheric PV anomalies (PVAs^-) associated with the block, to quantify the contribution to the evolution of the amplitude of these anomalies from different physical processes, including dry and moist dynamics.

The authors find that the PVAs^- linked to GL do not form locally, but follow two distinct pathways: upstream and retrogression; these pathways are identified based on two areas east and west of the centre of mass longitude of the year-round composite vertically averaged negative PV anomaly. Upstream PVAs^- propagate north-eastward towards Greenland from the northern United States, whereas retrogression PVAs^- are characterised by a north-westward propagation against the mean flow from norther Europe. There are more differences in the development of PVAs^- between these two pathways than there is between seasons.

An Eulerian method based on convolutional neural networks is used to identify warm conveyor belts (WCBs) and identify inflow in the lower troposphere, ascent in the mid-troposphere, and outflow in the upper troposphere.

Using divergent PV tendencies, the study finds that moist processes play a key role in the amplification of PVAs^- regardless of pathway. The authors further link  the increase in PVAs^- amplitude to WCB activity over the North Atlantic. Upper-level wave dynamics and baroclinic interaction also play a role.

The article is very well written with high quality figures, and fits well within the scope of WCD. The analysis techniques are appropriate, and make use of recent novel methods to further the understanding of the formation of GL. The conclusions reached are supported by the analysis, and there is the potential for further valuable study based on these results, e.g. the representation of the identified mechanisms in models. The results will be of broad interest, and I recommend the publication of this manuscript in the journal subject to some minor comments.

Minor comments:

1. How do the frequency and seasonality of Greenland blocking events found using the weather regime perspective, compare over a similar time period with events found using other blocking indices? For example, in Parker et al (2018), 26 events were found in a 10-year period from 2006-2015.; this study finds 177 events in a 43-year period. A short discussion of this would frame the results in the context of other studies.
2. The GL life cycle varies in length from 5 days to more than a month. Are there any major differences in the evolution of the PVAs^- between shorter- and longer-lived GLs?
3. Lines 334-342: questionable time steps when OBS and DIAG exhibit very different values have been filtered out. Can you motivate the choice of the condition for this filtering, and quantify the “large fraction” of values that are still included in the composite?
4. You state that the differences in the development of PVAs^- is greater between the two pathways than between seasons. Do you have any figures readily available to show this? – this is purely out of curiosity, but perhaps some supplementary material would be of interest to readers. The seasonality in the contribution of WCBs/moist processes would be interesting to see.

Edits:

1. Line 95 – “adopting” should be “adapting”.
2. Line 225 – missing ) after Equation 5.
3. 3 caption – for the bottom row, what is the contour interval for the orange and green contours? – is it the same as the grey shading?
4. Line 291 – “propagated westward towards” – towards where?
5. Lines 318-319 – “the chosen period of three days before onset is sufficient, as this is the time period with the largest differences in the propagation of onset PVAs^- “: perhaps note here that this is shown by the white crosses in Fig 4 which mark the point of three days before onset (-96 hrs). This motivates the choice of the centre of mass location to determine the pathway classification, and makes it clearer for the reader.
6. Line 337 – Section B should be Appendix B.
7. Line 340 – “DIAG somewhat over-estimate” – should this be “DIAG somewhat under-estimates”?
8. Line 466 – “the beginning decay phase” – perhaps rephrase as “the beginning of the decay phase”.
9. Line 521 - “indicating a starting decrease in max PVA^- amplitude” - perhaps rephrase to “indicating the beginning of a decrease in maximum PVA^- amplitude” .
10. Line 542 – “the ridge further weakened and lost in northward extent” – perhaps rephrase to “the ridge further weakened and contracted to the south”.
11. Line 543 – “an even retrograde behaviour of the ridge” – perhaps rephrase to “a possible retrogression of the ridge” or the like.
12. Line 550-551 – “which switches the sign” should be “which switches sign”.
13. Line 581 – after the first sentence of this paragraph, reference Figure 12, right side.
14. Line 588 – “Sections 4.2 and 4.2”??
15. Line 629 – “are on minor” should be “are of minor”.
16. Line 654 – “Taken the two perspectives…” should be “Taking the two perspectives..”