Oceanic origins for wintertime Euro-Atlantic blocking

Although conventionally attributed to dry dynamics, increasing evidence points to a key role of moist dynamics in the formation and maintenance of blocking events. The source of moisture crucial for these processes, however, remains elusive. In this study, we identify the moisture sources responsible for latent heating associated with the wintertime EuroAtlantic blocking events detected over 31 years (1979-2010). To this end, we track atmospheric particles backward in time from the blocking centres for a period of 10 days, using an offline Lagrangian dispersion model applied to an atmospheric 5 reanalysis data. The analysis reveals that 36 55% of particles gain a massive amount of heat and moisture from the ocean over the course of 10 days. Via large-scale ascent, these moist particles transport low potential vorticity (PV) air of low-altitude, low-latitude origins to the upper troposphere where the amplitude of blocking is the most prominent, consistent with the previous studies. PV of these moist particles remains significantly lower compared to their dry counterparts throughout the course of 10 days, 10 preferentially constituting blocking cores. Further analysis reveals that approximately two-thirds of the moist particles source their moisture locally from the Atlantic, while the remaining one-third from the Pacific. The Gulf Stream and Kuroshio and their extensions, as well as the eastern Pacific northeast of Hawaii, not only provide heat and moisture to the particles but also act as “springboards" for their largescale, cross-isentropic ascent. While the particles of the Atlantic origin swiftly ascend just before their arrival at the blocking, 15 those of the Pacific origin ascend additional few days earlier, after which they carry low PV in the same manner as dry particles. Thus, our study reveals that what may appear to be a blocking maintenance mechanism governed by dry dynamics alone can, in fact, be of moist origin. 1 https://doi.org/10.5194/wcd-2020-39 Preprint. Discussion started: 19 August 2020 c © Author(s) 2020. CC BY 4.0 License.

1 Introduction location and time. These turbulent heat fluxes are useful in evaluating where the particles that eventually reach the Euro-Atlantic blocks are likely to be supplied with heat and moisture from the ocean.

Distinct characteristics of the moist and the dry particles
In order to elucidate the role of the ocean in providing moisture to the atmospheric particles en route to the Euro-Atlantic blocks, we first partition the individual particles into the moist and the dry particles. Here, the moist particles are defined as 130 those that are subject to evaporation from the ocean, evaluated with positive value of LHF, for at least one time step over the course of 10 days. Otherwise, the particles are regarded as dry particles. As our goal lies in identifying the oceanic sources of moisture for the blocking particles, we opt for this definition based on the moisture gain from the ocean over that used in Pfahl 5 https://doi.org/10.5194/wcd-2020-39 Preprint. Discussion started: 19 August 2020 c Author(s) 2020. CC BY 4.0 License. et al. (2015) and Steinfeld and Pfahl (2019), who defined the diabatic particles as those that are heated by more than 2 K within three days prior to their arrival in the blocking regions. 135 With our definition, we have identified 36.4% of the total particles as moist particles and 63.6% to be dry particles. This fraction of the moist particles in our study falls within the range of diabatic particles of 30 -45% reported by Pfahl et al. (2015), while it is underestimated compared to their fraction of 60 -70% when the 7-day period is considered. We speculate that this difference might arise from the difference in the definition of the moist particles and/or the fact that Pfahl et al. (2015) primarily used the anomaly-based blocking definition by Schwierz et al. (2004), which can also detect immature/onset stages 140 of blocking unlike the current study, when the direct diabatic effect tends to maximize (Pfahl et al., 2015;Steinfeld and Pfahl, 2019). In contrast, the fraction reported by the previous studies during the maintenance stage seems fairly consistent with the current study (see Figure 6 of Steinfeld and Pfahl, 2019). When we release the particles from the lower altitudes, the fraction of moist particles increases to 49.7% and 55.0% for the 5,000 m asl and 3,500 m asl release, respectively.
3.1 Spatial distribution of the moist and the dry particles 145 The spatial distributions of the moist and the dry particles 2, 5, and 9 days prior to their arrival at the blocking locations are shown in Figure 2, where the particle release locations (i.e. where blocking occurs, corresponding to day 0) are indicated by red contours. As shown in Figure 2, the particles in both categories are mostly advected eastward under the prevailing westerlies.
Compared to their moist counterpart, dry particles tend to travel slightly faster. This faster advection rate for the dry particles reflects their tendency to be situated at higher altitudes (see Section 3.3), where the background westerlies tend to be swifter. 150 Upon further inspection of Figure 2, one notices that moist particles 9 days prior (upper-right panel) tend to be situated within two distant regions separated by the Rockies, seemingly corresponding to the two major storm tracks in the wintertime Northern Hemisphere. Meanwhile, the maximum particle density 2 days prior is found over the midlatitude North Atlantic just south-west of the blocking region, which is an important ascent region for some of these moist particles, as will be shown later.
In contrast, the corresponding locations of the dry particles appear to be less concentrated. As such, it is speculated that these 155 two categories of particles tend to undergo different dynamical processes.

Spatial distribution of mean properties along the moist particles within and above the marine PBL
We further partition the moist particles into the time when they are located within the atmospheric PBL over the ocean (Figure 3) and the rest of the time (Figure 4), in order to unambiguously identify the role of the ocean in fuelling the moist particles. As such, Figure 3 illustrates where the particles are located and how the mean properties along them are distributed when the 160 moist particles can exchange heat and moisture with the underlying ocean, while Figure 4 shows when those particles are not in direct contact with the ocean. Note that the latter also includes the time when the particles are located within the PBL over land.  as the primary heat and moisture sources for the moist particles that are going to be advected into the Euro-Atlantic blocks.
One noticeable exception to this overall tendency is found off the west coast of North America, where the particles undergo topographical lifting ( Figure 3d) followed by dehydrating ( Figure 3c) and warming (Figure 3b) due to latent heating. As will be shown later in Section 4.3, this region turns out to be a common location for the organized large-scale ascent of the particles 170 that gain moisture from the Pacific.
In stark contrast, the dominant tendency for the moist particles when situated above the PBL (Figure 4) is such that an increase (decrease) in potential temperature is concurrent with a decrease (increase) in specific humidity, which is accompanied by their ascent (descent) (Figure 4d). This result means that latent heating and evaporative cooling are the dominant processes that control the property changes of the particles travelling above the PBL. The preferred locations for the particle ascent are 175 found just upstream of the blocking over the North Atlantic 2 days prior to their arrival at the blocking region, as well as off the west coast of the North American continent, found 5 and 9 days prior to the arrival at the blocking.
Meanwhile, the dry particles undergo barely any property change, except for a general cooling trend associated with radiative cooling (not shown).   consistent with the radiative cooling reported by Steinfeld and Pfahl (2019). Specific humidity of the dry particles is fairly low over the course of 10 days en route to the blocking regions, as these particles do not pick up any moisture from the ocean by definition. Despite its large standard deviation, PV of the dry particles is maintained around approximately 2 PVU (potential vorticity unit; 1 PVU ≡ 1.0 × 10 −6 m 2 s −1 K kg −1 ), a typical PV value at the dynamical tropopause.
Moist particles (red lines in Figure 5) display a fairly distinct picture from its dry counterparts: While also originating from the mid-latitudes and travelling northward, the moist particles tend to arise from the lower troposphere and then undergo a 190 substantial uplift over the course of 10 days. This uplift and the northward motions are particularly enhanced about 3 days prior to their arrival at the blocking system, coinciding with notable dehydration and warming due to intense latent heating during this period. The mean PV of the moist particles tends to retain fairly low values throughout without undergoing any substantial changes, reflecting their origins at the lower altitudes and latitudes. Until about 3 days before their arrival at the blocks, the moist particles tend to be steadily supplied with SHF and LHF, before undergoing strong uplift.

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Despite the aforementioned significant differences in their origins and their evolution between the dry and the moist particles, these differences almost diminish as time nears the particle release time from the blocks (i.e. particle age = day 0). One notable exception is PV, which on average undergoes little change during the course of 10 days, retaining its initial values along the trajectories of both dry and moist particles even at day 0. This result is consistent with the previous study by Methven (2015), who showed that the average PV of an upper-tropospheric outflow of a strongly ascending air stream is approximately 200 equal to that of the lower-tropospheric inflow on average, as PV increases below the heating maximum but decreases above it. Figure 5 indicates that the mean PV values are significantly higher for the dry particles than for the moist particles at the 99% confidence level even at day 0. Given that blocks are high pressure systems associated with low PV values, feeding of lower PV air is expected to be crucial for their maintenance (Yamazaki and Itoh, 2013b). Our result thus suggests that these moist particles play an important role in transporting significantly lower PV into the block system, further corroborating the findings 205 of the previous studies (Pfahl et al., 2015;Steinfeld and Pfahl, 2019). This significant difference between the dry and moist particles in their mean PV values seen at day 0 in Figure 5 further suggests that there could be a difference in their spatial distribution at the time of their release from the blocks. Figure 6 illustrates the spatial distributions of the dry and moist particles separately, as well as their difference. Each row denotes different release altitudes: 7,500-m asl (∼350 hPa), 5,000-m asl (∼500 hPa), and 3,500-m asl (∼700 hPa). The figure reveals 210 that regardless of the release altitudes, the density of the moist particle (middle panels) at the centre of the blocks tends to be higher than that of the dry particles (left panels). The difference plots (right panels) highlight that this difference is more apparent for the particles released from the lower altitudes (5,000 m asl and 3,500 m asl). The moist particles released from 7,500 m asl tend to be situated slightly to the east of the dry particles, indicating the possibility of vertically-slanted injection of moist particles, which may be a reflection of the slantwise nature of the WCB (Oertel et al., 2020). Overall, our results suggest 215 that the moist particles act to transport the low PV across isentropic surfaces to the upper levels, such that a blocking core is preferentially constituted of moist particles with lower PV. and latent heat fluxes (magenta) supplied for the moist particles from the ocean when the particles reside within the marine PBL. (Second to last rows) mean potential temperature (second row), mean specific humidity (third row), mean potential vorticity (fourth row), mean pressure (fifth row), and mean latitude (last row) of the dry (cyan lines) and moist particles (red lines) along their respective trajectories. The envelopes around each line indicate ±0.5 standard deviation. The particle age on the x-axis denotes the days before the particles arrive at the blocking system (i.e., day 0 means the arrival time at the blocks). The closed (open) circles indicate where a given property distribution between the moist and dry particles is significantly different at the 99% (95%) confidence level.

Moisture sources for the moist particles
In the previous section we found that heat and moisture for the moist particles are not only locally sourced from the Atlantic, but the North Pacific also likely acts to supply necessary heat and moisture to some particles en route to the Euro-Atlantic 220 blocking ( Figure 3). In this section, we further separate the moist particles depending on their moisture sources, in order to uncover the characteristics associated with their different pathways and their roles in blocking maintenance.

Partitioning of the moist particles according to their moisture sources
We partition the moist particles into the two different moisture origins based on the ocean basin mask dataset with 1 when particles are tracked backward in time for the period of 10 days ( Figure 7a). Intriguingly, a minute fraction of the particles (1.3% of the total moist particles) receive heat and moisture from both the Pacific and Atlantic basins (denoted "twobasin pathways"). Note that we do not consider the moist particles fuelled over the Indian Ocean in the rest of the analyses, which account for only 0.9% of the total moist particles. These particles do not additionally source moisture from the other basins (not shown).

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When we extend the duration of the particle tracking from 10 days to 20 days (Figure 7b), however, the fraction of the moist particles increases by 40%, especially escalating the fraction of the Pacific pathways and two-basin pathways, while the 20-day tracking accounts for almost the same fraction as the Atlantic origin. The twofold increase of the Pacific pathway particles indicates that the 10-day period is not long enough for some particles to be traced back to their moisture source in the Pacific. The over sixfold increase of the two-basin pathways is explained primarily by the detection of additional particles of 240 the Atlantic origin that previously stayed in the PBL over the Pacific between 10 and 20 days prior to blocking events, and to a lesser extent by some particles of the Pacific origin that are tracked back further to the Atlantic, across Eurasia. The fraction 13 https://doi.org/10.5194/wcd-2020-39 Preprint. Discussion started: 19 August 2020 c Author(s) 2020. CC BY 4.0 License.
of particles sourcing the moisture from the Indian Ocean remains lower than 1% even with the 20-day tracking, indicating that Indian Ocean does not act to provide much moisture for the wintertime Euro-Atlantic blocking events.
The Atlantic pathways tend to originate locally 10 days prior to the arrival at the block locations (the particle distribution at 245 -10 days indicated by the red contours in Figure 7a), with their maximum concentration found over the lee side of the Rockies over the North American continent. These particles then pick up a substantial amount of moisture from the Gulf Stream and its extension. A similar picture holds when they are tracked for 20 days (Figure 7b), although more particles tend to stem from the east of the block locations over Eurasia.
The Pacific pathways identified through the 20-day tracking tend to originate from farther upstream, primarily receiving 250 turbulent heat fluxes from the Kuroshio and its extension, with a clearer maximum concentrated along the Kuroshio Extension.
Still, the accumulated LHF maximum along the Kuroshio Extension is only about half of that along the Gulf Stream, despite no notable difference in the climatological turbulent heat fluxes between them. It remains to be investigated whether this suppressed LHF would be enhanced with particle tracking beyond 20 days.
The origin locations of the two-basin pathways appear to be a mixture between the Atlantic and Pacific pathways, though total LHF is affected by the following two factors: particles' 6-hourly concentrations and the actual LHF supplied to the particles. Further analysis indicates that these two factors both contribute, suggesting that the particles are preferentially situated over this location and that they are subject to a larger amount of LHF than its climatology (not shown). As discussed later in Section 4.3, this region coincides with starting locations of the large-scale ascent for those particles supplied with moisture 265 from the Pacific, thus acting as a "springboard".

Time evolution of variables along the trajectories of particles of the different moisture sources
In order to assess whether physical processes involved depend on the particles' moisture sources, we repeat the same analysis on the temporal evolution of variables along the particle trajectories, but this time after separated into different moisture sources ( Figure 8). Additionally, a three-dimensional view of the mean positions of these different pathways along with their mean PV 270 value are shown in Figure 9 in order to illustrate their typical trajectories relative to their geographical locations. Figure 8 reveals that the Atlantic pathways tend to originate from northern locations in the mid-troposphere on average (red lines). These particles gradually descend while travelling southward and gaining moisture until the last 3 -4 days, before being lifted up swiftly and undergoing latent heating over the Atlantic basin ( Figure 9).
In contrast, the Pacific pathways (orange lines in Figure 8) stem from the lower latitudes and altitudes. On the following days 275 (day 9 -day 4) these particles keep ascending and experiencing latent heating, especially off the west coast of North America  Figure 5, but for the moist particles categorized according to their moisture sources: the Atlantic pathways (red), Pacific pathways (orange), and two-basin pathways (magenta). Cyan denotes the properties for the dry particles.
( Figure 9), which tends to occur earlier than the Atlantic pathways by approximately 4 -5 days. After nearly all the moisture is lost by day -4, most of the properties of these particles become almost indiscernible from those of the dry particles (denoted by cyan lines), travelling along similar pressure and isentropic surfaces as well as at similar latitudes. In addition, they undergo radiative cooling at the same rate as the dry particles from then on. The Pacific pathway particles are thus transported in the 280 same manner as the dry particles once lifted up into the mid-troposphere.
The two-basin pathways evolve in a similar manner to the Pacific pathways at the beginning (magenta lines in Figure 8), as the particles originate from relatively low latitudes and altitudes with relatively high moisture content and then undergo ascent over the Pacific, especially off the west coast of North America. The ascent is, however, only by about 150 hPa till day -7, leading to slight latent heating. As will be shown in Section 4.4, this lesser extent of uplift off the west coast of North America 285 stems from the lower humidity content of those particles compared to the Pacific counterpart at the time of the ascent. These particles then start descending on the lee side of the Rockies ( Figure 9) and receive heat and moisture from the Atlantic, before ascending very rapidly into the blocking system in the last 2 days (Figure 8).
The PV values of the moist particles, regardless of their moisture sources, are significantly lower than those of the dry particles. In particular, although the Pacific pathways undergo a slight increase in PV after day -5, as they appear to be advected 290 indistinguishably from the dry particles as noted earlier, their PV values are still significantly lower than those of the dry particles. When compared to the winter-mean background PV, the PV values of these particles in the last 4 days just before arriving at the blocking system indeed tend to be significantly lower, indicating that they are associated with anticyclonic eddies (Figure 10). This result further corroborates the previous findings that once lifted up into the upper troposphere these particles constitute low PV anomalies in contrast to the background upper tropospheric PV at the same location (Pfahl et al.,295 2015; Steinfeld and Pfahl, 2019). In light of the blocking mechanism, therefore, these moist particles can be considered as those associated with anticyclonic eddies (low PV) that are selectively absorbed into the blocking system (Yamazaki and Itoh, 2013a), acting to prolong the blocking lifetime. While Yamazaki and Itoh (2013a) identified the physical process in the upper troposphere through an isentropic trajectory analysis by implicitly assuming dry processes (Pfahl et al., 2015), our result points towards a reconciled view such that at least a part of this low PV air can, in fact, be traced back to moist processes farther 300 upstream.
We have repeated the same analysis using particles tracked backward in time for 20 days. Extending the tracking time, however, does not change the qualitative picture shown in Figure 8, except that all categories of particles originate from the mid-latitudes. Despite that some particles are classified in different pathways under the extended tracking, the evolution of the properties during the last 10 days is indistinguishable from those obtained from the 10-day tracking (not shown), indicating the 305 significance of the processes that take place in the final 10 days for the downstream blocking. two-dimensional locations (green). The former is based on 10.1% of the Pacific pathways that reached above 400 hPa and whose potential temperature remains between 300 K and 310 K in the last 4 days before arriving at the Euro-Atlantic blocks, while the latter on the CFSR climatological PV values at 305 K level computed as the DJF-mean for each winter. The two distributions are significantly different at the 95% confidence level.

Large-scale ascent associated with moist particles with different moisture sources
Given the importance of large-scale cross-isentropic ascent, here we investigate the extent of uplift as well as the associated properties for the moist particles with different moisture sources. Here, we quantify large-scale ascent in accordance with the definition of WCB. Following Steinfeld and Pfahl (2019), we define those particles that undergo a prominent rapid ascent for 310 at least 600 hPa within 48 hours as the particles along the WCB. For simplicity, unlike Madonna et al. (2014), the particles are not required to be in the vicinity of cyclones to be identified as the WCB particles in our definition. Our WCB particles may thus include those particles that are not strictly classified as the conventional WCB. A detailed analysis of individual events, nonetheless, revealed that these events are almost always accompanied by cyclonic systems at the time of their ascent (not shown). Consistent with Madonna et al. (2014), the average increase in potential temperature associated with the WCB 315 particles is found to be about 20 K, while the average decrease in specific humidity about 7 g kg −1 . Figure 11 indicates that approximately 30% of the moist particles (i.e. about 10% of the total number of particles) is identified as the WCB particles at some point along their 10-day trajectories, regardless of their moisture sources. This number (∼10%) is fairly similar to 9.7% reported by Steinfeld and Pfahl (2019), who considered a 7-day period. Meanwhile, this fraction is much larger than the finding by Madonna et al. (2014), who reported only 0.36% of the 2-day trajectories released for the 320 period from 1979 to 2010 from the entire lower troposphere to be identified as the WCB trajectories. This difference in the fraction of the WCB trajectories from the climatology may thus indicate that the WCB trajectories are preferentially channeled towards the blocking systems. The fraction of the moist particles identified as WCBs exceeds above 60% (90%) of the total moist particles when the WCB definition is loosened to an ascent over 500 hPa (400 hPa) within 48 hours (Table 1).  accumulated LHF (Figure 7). These locations, including the region to the northeast of Hawaiian Islands agree with the reported climatological locations of the wintertime WCB occurrence (Madonna et al., 2014, their Figure 4d). This close collocation between the particles' acquisition of turbulent heat fluxes and their ascent is in line with the results shown by Boutle et al. (2010) in their idealized extratropical cyclone simulation. They showed that the augmented continuous evaporation is found 330 in the vicinity of the WCB as a consequence of the continual moisture export from these regions by the horizontal divergence forced by the boundary-layer drag. This moisture, in turn, converges at the base of the WCB forced by surface drag and largescale ageostrophic flow, thereby providing a substantial amount of moisture continuously to the WCB. Thus, the collocation of the LHF from the ocean and particle distributions is likely to indicate an active involvement of the moisture convergence associated with extratropical cyclones.

Typical synoptic conditions for individual moist particle pathways
The question remains as to what kind of synoptic conditions possibly give rise to different moist particle pathways. To answer this question, we have analyzed the particles trajectories of different moisture sources with the corresponding uppertropospheric synoptic eddies. Great case-to-case spatiotemporal diversity is found in the associated synoptic fields, hampering their meaningful composite analyses (not shown). Instead, Figure 12 illustrates a more or less representative case that exhibits the common features found for each pathway. Many cases are found to exhibit roughly similar properties to these representative cases.
One common feature for the vast majority of the Atlantic pathways is that the moist particles experience an organized descent over the North American continent oriented in the northwest-southeast direction in the vicinity of a synoptic-scale low-pressure system, before they gain moisture from the North Atlantic. These features well correspond to those associated with the dry 345 intrusion, a deep descent of dry air from the upper atmosphere into the lower troposphere often associated with extratropical cyclones (Browning, 1997), as clearly presented in Figure 12 for January 22 1992 (top right-most panel). A climatological study conducted by Raveh-Rubin (2017) illustrates that the North American continent is one of the most prominent locations for dry intrusion air to start descending, before interacting with the warmer North Atlantic underneath approximately 48 hours later. Intrusion of a dry, cold atmospheric particle is likely to induce strong SHF and LHF from the North Atlantic, especially 350 the warm Gulf Stream, as effective moistening of the particle before lifted up into an upper-level blocking anticyclone. required, this aspect appears to be consistent with the finding by Yamazaki and Itoh (2013b), who suggested that upper-level westerlies upstream of a blocking high can act as a waveguide to effectively carry synoptic anticyclones towards the blocking high.
All cases of the two-basin pathways are fairly analogous to the Atlantic pathways but extended farther upstream into the Pacific 10 days prior to the arrival at the blocking events. Compared to the Pacific pathways, the particles do not ascend as high 360 off the west coast of North America, and as a result, they do not experience strong descent over the North American continent compared to the particles for the Atlantic pathways either. We found that this lack of strong ascent over the Pacific is likely attributable to the difference in moisture content of the particles. At the time of the ascent, the moisture content of the Pacific pathways is significantly higher than that of the two-basin pathways at the 95% confidence level, with the mean difference amounting to 1.4 g kg −1 (Figure 13a). This relationship between the ascent level and moisture content becomes more evident 365 when a different ascent criterion is applied (Figure 13b). The figure reveals a clear link between the moisture content and the ascent level of a particle, in such a way that the higher the particle's moisture content at the base of the ascent, the higher the particle tends to ascend subsequently. In contrast, no such clear relationship is discernible with the sea-level pressure (SLP) found at the base of the particle ascent, despite that stronger low-pressure systems are likely associated with stronger ascent (Figures 13c, d). Although more careful analyses, including an investigation of the lowest SLP found in the vicinity of the 370 ascent as included in the WCB detection (Madonna et al., 2014), are indispensable to draw a more solid conclusion, these The fraction of the moist particles for the Pacific blocking is 28.6%. These moist particles are predominantly located over the Pacific basin, in particular just south of the blocking regions 2 days before the blocking occurrence and over the Kuroshio Extension region 5 days prior to their arrival at the block (Figure 14a). The preferred locations of these moist particles when that the moist particles tend to source their moisture more locally compared to the Euro-Atlantic counterpart. This difference likely arises from the greater zonal extent of the Eurasian continent than that of the North American continent, impeding the influence from the Atlantic Ocean. We speculate that these differences associated with the moist processes may partly account for the difference seen between the Atlantic and Pacific blocking in their predictability and reproducibility in climate models Matsueda, 2009).

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Outstanding questions include whether the temporal variability of the western boundary currents on seasonal and longer time scales can affect blocking activity through modulations of moist processes. In this study we have focused on the climatological aspect of direct diabatic processes associated with blocking maintenance and shown that western boundary currents provide a substantial amount of heat and moisture to the atmospheric particles en route to the Northern Hemisphere blocking systems. Observational evidence indicates that both the Kuroshio (Qiu and Chen, 2005;Kwon et al., 2010)  variabilities can modulate the direct diabatic processes associated with the blocking events described in the current study, however, is yet to be known. Hence in a subsequent study we will investigate the effect of the oceanic temporal variability. Given a recent study that showed the projected strengthening of the Kuroshio and weakening of the Gulf Stream under the future 455 climate scenarios (Chen et al., 2019), understanding the sensitivity of the diabatic effect to the variabilities of these western boundary currents has important implications.

Appendix A: A hybrid blocking index
As is extensively discussed in Barriopedro et al. (2010) and Woollings et al. (2018), there is no one definite measure to define blocking and thus numerous definitions have been used in the literature. Each index aims to highlight different aspects of 460 blocking systems, entailing its pros and cons. The two mainstream blocking indices include ones that detect the meridional gradient of geopotential height, focusing on the fact that blocks obstruct the prevailing westerlies (e.g., Tibaldi and Molteni, 1990;Scherrer et al., 2006) and others based on anomalies of variables such as geopotential height or potential vorticity from its climatological mean, highlighting that blocking highs are associated with strong anticyclonic anomalies (e.g., Dole and Gordon, 1983;Schwierz et al., 2004). Previous studies have reported that the former tends to erroneously detect the subtropical highs 465 as blocks (Scherrer et al., 2006), while also having a deficiency in detecting omega or immature blocks (Doblas-Reyes et al., 2002;Pelly and Hoskins, 2003). The latter, in comparison, might suffer from cases when the geopotential height anomalies do not necessarily accompany a reversal of the westerlies (Woollings et al., 2018), while also having disadvantages in requiring arbitrary blocking anomaly thresholds and a robust climatology (Dunn-Sigouin et al., 2013).
In the current study, we identify blocks by using a hybrid index introduced by Dunn-Sigouin et al. (2013), which combines 470 the basic ideas of both of these two types of blocking indices. Namely, blocks are detected by identifying a local maximum of 500-hPa geopotential height anomaly as was done in Dole and Gordon (1983), and then by ensuring that there exists a meridional height reversal around the identified maximum as in Tibaldi and Molteni (1990). More specifically, the following steps are followed: 1. 500-hPa geopotential height anomaly (Z ) is computed by removing the climatological seasonal cycle and long-term 475 variability as in Sausen et al. (1994) and then by imposing a weight proportional to the inverse of the sine of latitude. As such, Z represents a quasi-geostrophic stream function anomaly.
2. Closed positive contours of Z larger than a minimum amplitude threshold (A) with spatial scale (S) are then identified and tracked in time, ensuring that they meet the overlap criterion in blocking areas (O) for two consecutive days. maximum difference between the two grid points separated by a meridional range (∆φ), is examined on the equatorial side of the anomaly maximum. The reversal criterion is met if the height gradient is negative at any longitude within a longitudinal range (∆λ).
4. The anomaly is identified as a blocking event if the above three criteria are met during a given period (D).
We have set the arbitrary thresholds to be identical to those used by Dunn-Sigouin et al. (2013)

Appendix B: Atmospheric dispersion model FLEXPART
FLEXPART is a comprehensive offline atmospheric Lagrangian dispersion model, widely used for diverse atmospheric transport applications. FLEXPART computes the particle trajectories under the assumption of zero acceleration as X(t + ∆t) = X(t) + v(X, t)∆t,

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where t denotes time, ∆t the time increment, and X the particle's position vector. v = v +v t +v m , is the three-dimensional wind vector consisting of grid-scale winds (v), turbulent wind fluctuations (v t ), and mesoscale wind fluctuations (v m ). Turbulent wind fluctuations are evaluated by solving Langevin equations (Thomson, 1987) in assuming Gaussian turbulence.
Mesoscale wind fluctuations, whose spectral interval falls between the resolved winds and the turbulent wind fluctuations, are approximated by solving an independent Langevin equation for the mesoscale wind velocity fluctuations in a similar manner 500 to Maryon et al. (1998).
In addition, in FLEXPART, one can optionally evaluate the effect of moist convective transport that takes place within convective clouds by turning on the moist convection scheme by Emanuel and Živković-Rothman (1999), at the expense of the substantial increase in the computational time (Stohl et al., 2005). This transport is grid-scale in the vertical but subgrid-scale in the horizontal. A recent study by Oertel et al. (2020) has highlighted the importance of convection embedded in WCBs by 505 using a high-resolution model with a 0.02 • horizontal resolution. Switching on the moist convection scheme in FLEXPART, however, does not qualitatively or quantitatively alter our results, suggesting that the importance of the subgrid-scale horizontal convection is only secondary to the resolved grid-scale convection in the current study with a much coarser resolution (not shown). For this reason, we turn off the moist convection scheme in our analyses.