| 03 Jun 2026
A synoptic perspective on the role of the upstream diabatic cooling in modulating the North Pacific storm track
Abstract. Cold air outbreaks (CAOs) are an important driver of near-surface baroclinicity at the entrance of the northern hemispheric storm tracks during boreal winter. They originate over the upstream continent in regions of surface radiative cooling, suggesting that this diabatic process is not only important for CAO formation, but potentially also a relevant contributor to downstream storm track variability. Here, we aim to provide mechanistic insights into the underexplored link between upstream diabatic cooling and storm track variability in the context of CAOs. For this purpose, we analyze and compare distinct subsets of CAOs, characterized by stronger and weaker diabatic cooling over Siberia, occurring over the Japan Sea in the 1979–2023 period. To investigate the potential role of upstream diabatic cooling on storm track activity after CAOs, we quantify modulations of Eady growth rate (EGR) in the western North Pacific. In addition, we quantify the impact of diabatic cooling on the Siberian High, a semi-permanent weather system that has previously been associated with the occurrence of strong CAOs at the entrance of the North Pacific storm track.
With regards to low-level baroclinicity, we find that CAOs featuring enhanced upstream cooling are characterized by anomalously high EGR along the eastern coast of Russia and China. Further, peaks in EGR at the entrance of the storm tracks during winters between 1979–2023 are preceded by efficient radiative cooling over the continent, therefore contributing to a large temperature contrast between land and ocean. The contribution of diabatic processes to the intensification of the Siberian High is evaluated using the-sea level pressure (SLP) tendency equation. The diabatically driven SLP tendency at the time of maximum intensification amounts on average to 3.4 hPa 6h-1, and is of the same order of magnitude as the adiabatic contribution by horizontal and vertical motion. We also provide evidence that the land-based diabatic cooling indirectly supports, through baroclinic interaction, the amplification of an upper-level ridge-trough couplet that propagates into the storm tracks. The increased low-level baroclinicity and the deep upper-level trough at the entrance of the storm tracks, both enhanced by the upstream diabatic cooling, facilitate cyclogenesis at the entrance of the storm track. The resulting cyclones are more likely to be particularly deep, "bomb" cyclones and are associated with cyclonic wave breaking at upper-levels over the western North Pacific. On the other hand, we find no significant change in the number of cyclones developing in the 1–4 days following these CAO events: those subsequent cyclones remain small in size and do not exhibit strong intensification rates compared to climatology. In conclusion, our findings suggest that diabatic cooling over land takes an active role in shaping storm track variability, allowing us to mechanistically link upstream land–atmosphere thermodynamic processes to downstream extratropical cyclone activity.
This is an interesting and relevant study which focusses on the role of upstream diabatic cooling associated with Cold Air Outbreaks over the Japan Sea on the North-Pacific storm tracks. The authors select 98 CAO events over the Japan sea region to investigate the role of diabatic cooling on the weak and strong cooling events. The paper is in general well written, however some further clarification is needed, especially in the motivation of the selection of the CAO events and the motivation behind the definition of weak and strong CBAR events, see detailed comments below.
Selection of CAO events (section 2.1): To me it is not completely clear how the CAO events used in this study are selected. For example, the authors write that “CAO events are selected based on particularly high values in the cold air outbreak index…. In the Japan sea.” (Lines 85-86). What threshold is used for the selection of the CAO events and over which region exactly? What fraction of total CAO in the region is selected?
The motivation of the definition of C_BAR (Equation 1): I struggled a bit to understand the motivation behind the definition of C_BAR. The equation has subscripts (a) and (b) which are not defined. The authors write that θ_BCAR is defined as “the mean of the lowest θ values along each BCAR trajectory”. Is this the mean over the lowest value per trajectory? Why is then this value subtracted from the coldest value over all trajectories, and divided by the difference between the maximum and minimum θ among all CAO events? I understand that the motivation behind this definition might be explained in Schyder and Riboldi (2026) already, but it would be helpful for the reader if part of this is repeated , especially since this is vital for the interpretation of the results within this manuscript.
Oscillate behaviour of the baroclinicity (Line 200-201 and further): The authors argue that “the composite analysis centered on CAO events suggests that baroclinicity at the entrance of the storm tracks oscillates as a function of upstream radiative cooling and the cyclones within the storm tracks”. I find that hard to see from the composite results (Figure 3-4). Can the authors argue why they see this in the composites? I mainly see a slight weakening of the EGR/baroclinicity, but I find it hard to see when the subsequent replenishment of baroclinicity occurs. Complementary to this, isn’t it expected that this oscillation occurs at longer time scales (e.g. around 5 days, as suggested by e.g. Marchegianni and Spengler, 2023). Have the authors looked at longer time scales to see if the baroclinicity is replenished?
Minor comments:
General: The refer to large and small CBAR events, this could imply to me CBAR events with a large and small area. However if I interpret the text well they want to distinguish between strong and weak events, so I would suggest to explicitly name them so throughout the manuscript.
Line 39: Isn’t the source term not more generic related to diabatic processes, instead of just diabatic cooling?
Line 86: At which resolution (vertically and horizontally) are those parcels released, and in which region?
Line 95: I think this should be maximum in the subscript instead of minimum?
Line 132: Don’t you want per definition a single SLP maximum per anticyclone mask?
Equation 2/Line 146: The pressure surface P2 is not defined.
Line 160: What is the motivation for using the 500 km radius instead of the anticyclone mask?
Line 167: The first box is never mentioned in the main text, so maybe just refer to as a box instead of ‘second box’?
Line 172: The (beginning of the) sentence could be phrased better, I think you maybe want to say “We additionally count the number….” instead of “The additionally count the number….”?
Figure 3: At which interval are the pressure contours plotted?
Figure 7: What are the intervals for the geopotential height anomalies? Moreover, the regions which are statistically significant are slightly hard to see
Line 335: I would remove/rephrase ‘impressive’, since it is rather subjective.
Line 345: I would remove/rephrase ‘unspectacular’, since it is rather subjective.
Figure 9: Units are missing at the axis labels, I assume it is lon/lat degrees from the cyclone centre?
References:
Marcheggiani, A. and Spengler, T.: Diabatic effects on the evolution of storm tracks, Weather Clim. Dynam., 4, 927–942, https://doi.org/10.5194/wcd-4-927-2023, 2023.