The Spring Transition of the North Pacific Jet and its Relation to Deep Stratosphere-to-Troposphere Mass Transport over Western North America

. Stratosphere-to-troposphere mass transport to the planetary boundary layer (STT-PBL) peaks over the western United States during boreal spring, when deep stratospheric intrusions are most frequent. The tropopause-level jet structure modulates the frequency and character of intrusions, although the precise relationship between STT-PBL and jet variability has not been extensively investigated. In this study, we demonstrate how the north Pacific jet transition from winter to summer leads to the observed peak in STT-PBL. We show that the transition enhances STT-PBL through an increase in storm track 15 activity which produces highly-amplified Rossby waves and more frequent deep stratospheric intrusions over western North America. This dynamic transition coincides with the gradually deepening planetary boundary layer, further facilitating STT-PBL in spring. We find that La Niña conditions in late winter are associated with an earlier jet transition and enhanced STT-PBL due to deeper and more frequent tropopause folds. An opposite response is found during El Niño conditions. ENSO conditions also influence STT-PBL in late spring/early summer, during which time La Niña conditions are associated with 20 larger and more frequent tropopause folds than both El Niño and ENSO neutral conditions. These results suggest that knowledge of ENSO state and the north Pacific jet structure in late winter could be leveraged for predicting the strength of STT-PBL in the following months.

For a measure of the depth of stratospheric intrusions that lead to STT-PBL over western North America, the tropopause fold identification scheme developed by Skerlak et al. (2015) was used at 6-hourly temporal resolution and 1 X 1° horizontal resolution. Building upon the fold identification scheme introduced by Sprenger et al. (2003), tropopause folds were identified 100 by first distinguishing air of stratospheric and tropospheric origin on a potential vorticity basis using a three-dimensional labelling scheme. Folds of stratospheric origin are then identified when there were multiple crossings of the tropopause (defined using surfaces of the +/-2-PVU surface and 380 K potential temperature, tropopause marked at whichever surface is lower), observed within a single vertical profile. At locations where folds were identified, the minimum and maximum pressure the fold reached was recorded (Fig. S1). To examine how the jet transition, and later ENSO phase, affects the depth of 105 tropopause folds, we tracked the maximum pressure a fold, if identified over the western US (box in Fig. S1), reached for each 6-hour timestep. The size of folds deeper than 400 hPa was also tracked by counting the number of grid points over the region that were characterized by a maximum pressure greater than 400 hPa. The number of grid points meeting this criterion was then divided by the number of grid points over the western US domain. The number of timesteps characterized by a fold with a maximum pressure greater than 400 hPa was also recorded, for a measure of the frequency of tropopause folds under various 110 large-scale conditions. Any mention of tropopause fold frequency therefore refers to only this subset of folds, as folds shallower than 400 hPa are unlikely to affect STT-PBL. Similar results are found when folds larger/deeper than 500 hPa are selected, although differences in the size of intrusions are more difficult to discern given the small-scale nature of intrusions this deep (e.g., Knowland et al. 2017). 115 We note that the 2-PVU surface, which is used to define the maximum pressure folds reached, does not always mark the terminus of the fold, and that ozone originating in the stratosphere more closely follows the 1-PVU surface which penetrates further downward, as shown in Albers et al. (2018;their Figure 2). Shapiro (1980) also discussed how ozone associated with a tropopause fold in March 1978 reached farther into the troposphere than the dynamic tropopause, indicative of crossisentropic mixing. Knowland et al. (2017) examined cross sections of two stratospheric intrusions that led to enhanced surface 120 ozone concentrations in Colorado in spring 2012, and showed that the dynamic tropopause only reached to the midtroposphere, with only small filaments of the intrusion reaching deeper than 500 hPa, while high stratospheric ozone values extended to the surface. For these reasons, we believe that tracking when the dynamic tropopause is deeper than 400 hPa captures the structures often associated with transport deep into the troposphere. This is confirmed by the strong relationship between STT-PBL and the fold characteristics discussed in Section 3.3. Changes in these measures of fold depth, size, and 125 frequency will be evaluated during the phases of the spring transition, as well as during El Niño, La Niña and ENSO neutral conditions. https://doi.org/10.5194/acp-2020-604 Preprint. Discussion started: 25 September 2020 c Author(s) 2020. CC BY 4.0 License.

Post-Processing and Diagnostics
We use the leading empirical orthogonal function (EOF1) and principal component (PC1) time series of the daily mean 200-130 hPa zonal wind over the north Pacific basin (100-280°E, 10-70°N), smoothed with a 5-day running mean, to track the seasonal evolution of the jet. Zonal wind anomalies used for EOF analysis were calculated with respect to the February -June 1958-2017 average using JRA-55 reanalysis (black contours, Fig.1a), with the resultant anomalies intentionally including changes associated with the seasonal cycle (in contrast to the more canonical approach, in which the daily climatology is removed to eliminate the seasonal cycle). To be consistent with the STT-PBL and tropopause fold datasets, which are based upon ERA-135 Interim reanalysis (Sprenger et al. 2017), we recomputed the 200-hPa zonal wind EOFs in the same manner using ERA-Interim 200-hPa zonal wind (the correlation between PC1 using JRA55 and ERA-Interim for their common period, 1979-2017, is 0.99).
For a measure of the eddy characteristics and horizontal Rossby wave energy propagation during various phases of the spring 140 transition, the horizontal E-vector (Eq. 1; Hoskins et al. 1983) was calculated using daily zonal and meridional wind anomalies that have a) the 60-year daily climatology and b) the 11-day running mean removed. Regions where E points eastward (westward) are characterized by meridionally (zonally) elongated eddies (Fig. S2). Negatively tilted anomalies, indicative of cyclonic wave breaking, correspond to a northward-pointed E and energy propagation, while positively tilted anomalies indicate anticyclonic wave breaking and correspond to southward-pointed E and energy propagation. 145

Significance Testing
For a measure of confidence in the differences in fold characteristics and STT-PBL during different jet phases and ENSO 150 conditions, mean values were bootstrapped using a sample size Neff = N/tautocorr, where tautocorr is the number of timesteps at which the autocorrelation of the variables decreases to below 0.5, and N represents the number of samples in the smallest group being compared. For example, when comparing STT-PBL over North America during May El Niño, La Niña and ENSO neutral conditions, with corresponding samples sizes (N) equal to 20, 9 and 7 years, respectively, STT-PBL for the three groups is resampled using N=7 years* 124 timesteps/year = 868 timesteps, and tautocorr = 6 timesteps (36 hours), which equates to Neff 155 = 144 timesteps. To fairly compare the confidence intervals for each ENSO group, every group was resampled using the reduced sample size Neff, to calculate a new mean STT-PBL value. This process was repeated 10000 times to determine the 95 th and 5 th percentiles of the mean value for each group. A similar approach was applied to each variable considered.
The leading EOF pattern of 200-hPa zonal wind tracks the seasonal evolution of the north Pacific jet from February through June each year (Fig. 1). A positive PC1 value represents the stronger wintertime state (Fig. 1a), which gradually weakens on average from about March through June, as shown by the transition of PC1 from positive to negative each year (Fig. 1b). There is greater spread among the PCs of individual years during February, March and April than there is during May and June, 165 indicating the transition from winter to spring is more variable than the transition from spring to summer. The composite zonal wind on days when PC1 > 1s, hereafter referred to as the winter phase, most often occurring in February -March, is characterized by a strong jet extending well past the date line (Fig. 2a). During the winter phase, high-frequency eddy kinetic energy (EKE) is greatest in the jet exit region in the central Pacific, representing the wintertime Pacific storm track and tendency for eddies to amplify via deformation in the jet exit region (Rivière and Joly 2006;Breeden and Martin 2018). The 170 prevalence of equatorward-pointed E, signifying positively-tilted waves, over North America is consistent with the frequency of positively-tilted troughs and ridges identified during boreal winter by Schemm and Sprenger (2020).
As PC1 decreases to values between +/-.5s, which we define as the transition phase, the jet core weakens substantially while the jet exit region shifts northward (Fig. 2b). The storm track is more energetic throughout the Pacific-North American region 175 compared to the winter phase, and shifts northward with the jet exit region. In the east Pacific, a distinct secondary jet maximum develops near Hawaii in the subtropics, creating a double-jet structure in the Pacific basin which differs substantially from the strong, merged wintertime jet. The formation of this secondary zonal wind maximum was also observed to develop in April by Newman and Sardeshmukh (1998). The magnitude of E increases during the transition phase, with meridionally-elongated, positively-tilted waves dominating the structure in the midlatitude Pacific. Such characteristics are related to frequent 180 anticyclonic Rossby wave breaking associated with the formation of the two jet maxima observed in the transition phase. A distinct region of nearly-zonal E is observed over the eastern Pacific/western US, indicating waves in this region are more amplified meridionally compared to when the jet occupies the winter phase. Zonal wind, EKE and eddy amplitude proceed to weaken by late spring/early summer, when PC1 < -1s, hereafter referred to as the summer phase ( Fig. 2c), with the subtropical jet in the eastern Pacific essentially disappearing altogether. Over the western US, eddies are still meridionally amplified but 185 less so than during the transition phase, characteristic of the weakened storm track.
To examine the variability in the spring transition, we tracked the date on which PC1 dropped below +.5s and remained below that value for the remainder of the season. We target this transition in particular given the high variability of PC1 early in the season, and the marked invigoration of the storm track associated with PC1 decreasing from strongly positive to neutral. The 190 mean transition date over the 60-year record is 4 April, with a standard deviation of +/-12 days. To test if there are dynamic differences in the transition if it occurs earlier or later than normal, we grouped each season into early, neutral and late transition years, requiring early (late) transition years to have a transition date at least 5 days earlier (later) than the 60-year average ( Table 1). The average timing of the transition for the three groups differs by about two weeks, with the early group https://doi.org/10.5194/acp-2020-604 Preprint. Discussion started: 25 September 2020 c Author(s) 2020. CC BY 4.0 License.
transitioning on average in mid-March, the late group in mid-April. Comparing the composite February -June evolution of 195 PC1 for the early and late groups (the neutral transition years fall in-between), PC1 in the early group begins to decrease near the beginning of March, although these differences are not significant until later in the month (Fig. 3). During April, the late group PC1 value is roughly .5s higher than the early group, an average zonal wind difference of 10 m s -1 within the jet core, while by May the two groups are indistinguishable from one another. An early winter-to-spring transition is not associated with an early spring-to-summer transition, with PC1 decreasing below -.5s in mid-May for all transition groups. To test 200 whether early transitions are more abrupt (and therefore more dynamically disruptive) than later transitions, we compared the composite evolution of PC1 with respect to each year's transition date, and did not find any significant differences in the vigor of the transition (Fig. S3).

Relationship between the Spring Transition and STT-PBL 205
This section will show how the spring transition modulates STT-PBL over western North America. Earlier transitions enhance the amount of the time the jet occupies its transitional phase, corresponding to a more invigorated storm track, more folds and therefore more STT-PBL than later transitions. Early in the season, deeper folds enhance STT-PBL, while later in the season more expansive folds enhance STT-PBL. 210 STT-PBL is modulated by the phase of the jet and corresponding invigoration of the storm track (Fig. 4). Transport increases by roughly threefold when the jet is in its transition phase, compared to the composite STT-PBL during both the winter and summer phases. STT-PBL was averaged over western North America (box in Fig. 4b) for each day in the record and subsequently binned by PC1, confirming STT-PBL is strongest when the jet is closer to its transitional phase than at either 215 extremity ( Fig. 5a; Fig. S4). Both the highest STT-PBL days in the record and the highest median STT-PBL values occur during the transition phase, while the distributions during the winter and summer phases are indistinguishable from one another ( Fig. 5a). Consistent with the STT-PBL changes, tropopause folds reach farthest into the troposphere during the transition phase, on average to 450 hPa, in contrast to median values near 400 hPa during the winter phase and 300 hPa during the summer phase (Fig. 5b). During the winter phase, shallow (<300 hPa) and deep (>400 hPa) folds are equally likely, while deep 220 folds are more frequent than shallow folds during the transition phase. Shallow folds are overwhelmingly more likely during the summer phase, which might be related to the weaker jet and associated ageostrophic circulation during summer. Thus, while the STT-PBL distributions during the winter and summer phases are indistinguishable from one another, the fold depth distributions differ substantially.

225
In addition to changes in tropopause fold depth during the spring transition, the daytime BLH in the interior West increases dramatically, meaning shallower folds can reach the top of the boundary layer. Consistent with the STT data, 6-hour forecasts of PBL height valid at 1800 UTC were averaged over western North America and grouped by jet phase (Fig. 5c), confirming https://doi.org/10.5194/acp-2020-604 Preprint. Discussion started: 25 September 2020 c Author(s) 2020. CC BY 4.0 License. the PBL deepens as the jet transitions. Thus, while folds deeper than 500 hPa still occur somewhat frequently during the winter phase, the PBL is far lower, meaning a smaller subset of folds is deep enough to penetrate the boundary layer compared to the 230 transition phase. Conversely, when the jet occupies the summer phase, despite a very deep boundary layer, there is limited transport due to a relative lack of intrusions deeper than 350 hPa. The transition phase is associated with higher STT-PBL through the fortuitous coincidence of both more frequent deep tropopause folds and a deepening PBL. We note that STT-PBL can be displaced spatially from the position of the tropopause-level folds measured here, and can be aided by lower- Reconsidering the eddy characteristics associated with the three jet phases (Fig. 2), it appears that tropopause folds deep enough to produce STT-PBL occur most often when waves are highly amplified and the storm track is most energetic. Highly amplified Rossby waves are associated with strong curvature, particularly on the western edge of troughs, producing the subsidence that 240 forms deep tropopause folds and STT-PBL (e.g., Sprenger et al. 2007). Amplified waves propagating over western North America, which occur most often during the transition phase, bring more folds over the high terrain of the Rocky Mountains as the PBL deepens, leading to the STT-PBL maximum observed in boreal spring.
Given the longer period of time the jet is within its transition phase (when PC1 +/-0.5s), hereafter referred to as the residence 245 time, (Fig. 3), we hypothesize that early transition years are characterized by more STT-PBL than late transition years. To that end, we compared monthly mean STT-PBL for the early and late transition groups, revealing there is indeed more STT-PBL during early transition years in March, April and May, coinciding with more frequent folds deeper than 400 hPa ( Fig. 6a-b).
In February and March, folds are deeper and larger in early transition years as well, while in later months folds are larger but not deeper ( Fig. 6c-d). The residence time of the jet is much greater during early years by definition, and monthly mean EKE 250 over the North Pacific (180-250°E, 40-60°N; box in Fig. S6), is greater in April during early years ( Fig. 6e-f). Compositing each variable with respect to each year's transition date reveals an upward shift in STT-PBL in the two weeks following the transition in both groups, coincident with a marked increase in tropopause folds, residence time and EKE (Fig. 6g,h,k,l). Fold depth and area, conversely, are not systematically affected by the transition (Fig. 6i,j).

255
The relationship between STT-PBL and each related but distinct fold characteristic -maximum depth, fold area and frequency -evolves over the course of the spring transition. This is evident from timeseries of the correlation between monthly mean STT-PBL, February -June, with each fold characteristic, residence time of the jet, and median PBL height ( Fig. 7; Fig. S4).
During February and March, STT-PBL has the strongest correlation with fold depth and frequency, consistent with intuition.
In April, however, the relationship between STT-PBL and fold depth diminishes, while fold frequency maintains a strong 260 relationship with transport through June. In contrast to fold depth, the relationship between fold size and STT-PBL is strongest in May, when it has the second-strongest relationship after frequency. A longer residence time of the jet within the transition https://doi.org/10.5194/acp-2020-604 Preprint. Discussion started: 25 September 2020 c Author(s) 2020. CC BY 4.0 License. phase enhances STT-PBL in March and February, a relationship which disappears later in spring, in part because PC1 continues decreasing towards its summertime state. Daytime PBL height and STT-PBL are modestly correlated in February and March, with no relationship in later months when the daytime PBL has deepened to several kilometres and appears to no longer be the 265 limiting factor for deep transport ( Fig. 5c; Seidel et al. 2012;Langford et al. 2017). Since fold frequency maintains a strong relationship with STT-PBL throughout the transition, we correlated fold frequency to EKE, confirming a more energetic storm track produces more folds and supporting the relationship between storm track variability, folds and STT-PBL. The correlation drops off by May, however, for reasons which are not immediately clear but might reflect the more convective nature of transport during this time of year, which can be important for transport to the surface (Langford et al. 2017;Skerlak et al. 270 2019). Overall, fold frequency maintains the strongest relationship with STT-PBL throughout the transition, while fold depth and area also affect STT-PBL early and late in the transition, respectively.

Impact of ENSO on the Spring Transition and STT-PBL
What drives the substantial variability in the timing of the spring transition? While prior research has alluded to a connection 275 between ENSO and STT-PBL, the precise nature of the ENSO-fold-STT relationship during boreal spring is not fully understood. Here we demonstrate that ENSO conditions do influence the jet, tropopause fold characteristics and STT-PBL, and that this influence evolves throughout the spring transition.
ENSO markedly affects the jet from February -April, with La Niña conditions corresponding to a much lower PC1 value than 280 neutral or El Niño conditions, while in May and June the differences are weaker (Fig. 8). There is some asymmetry in the PC1 response, with La Niña weakening the jet more substantially than El Niño strengthens it. Given the positive relationship between STT-PBL and residence time of the jet, we hypothesize that La Niña conditions are associated with enhanced STT-PBL, which is broadly confirmed in Figures 9-10 and is consistent with the conclusions of Lin et al. (2015). In February and March, El Niño conditions are associated with a zonally extended jet that connects to the jet over North America, while the jet 285 is zonally confined to the central Pacific during La Niña (Fig. 9a-f). STT-PBL is overall weak in February, but is strongest during La Niña years, consistent with the most neutral PC1 value. STT increases in all three ENSO groups during March, as PC1 values decrease. Note that the jet has already transitioned during many of the La Niña and some of the ENSO neutral years (Table 1). In April, the jet transition is either underway or has already occurred, and correspondingly STT-PBL peaks for the El Niño and ENSO neutral groups, and remains elevated for La Niña (Fig. 9g-i; Fig. 10a). During May and June, STT-290 PBL remains elevated during La Niña years, although the difference compared to ENSO-neutral is somewhat uncertain with the number of samples available (Fig. 9j-o). The (presumably eddy-driven) jet core in the western Pacific is stronger during La Niña years, reflecting an increase in storm track activity compared to neutral and El Niño conditions coincident with a more negative PC1 value ( Fig. 10f; Fig. S6).
Which of the various tropopause fold characteristics explored in the prior section do ENSO conditions affect? Just as the influence of ENSO on PC1 evolves over the course of the transition, so too does the influence of ENSO on tropopause folds.
During February and March, La Niña conditions are characterized by significantly deeper and more frequent folds, driving an increase in STT-PBL (Fig. 10a-c). Folds are also larger, particularly in May when the relationship between fold area and STT-PBL is the strongest (Fig. 10d). While STT-PBL during April is similar in all three groups, folds are still more frequent and 300 potentially deeper during La Nina (Fig. 10b-c). STT-PBL is elevated during La Niña in May, when folds are more common and larger in areal extent. Mean fold depth, in contrast to fold size, is insensitive to ENSO phase in May and June. Finally, the residence time of the jet within its transition phase is significantly enhanced in February and March (as suggested in Fig. 8), while it is reduced in May when PC1 is more negative (Fig. 10e; Fig. 9l). EKE during La Niña is most enhanced in April, similar to the early transition years ( Fig. 10f; Fig. 6f), due to the notable increase in EKE following the transition ( Fig. 10l; 305 Fig. 6l). Note that during May, EKE is enhanced during La Niña in a smaller region over the eastern Pacific (Fig. S6d), accompanied by a more zonal E-vector, consistent with elevated STT-PBL observed in Fig. 10a but not reflected in Fig. 10f.
In summary, in late winter/early spring, the teleconnection to the extratropics during La Niña projects onto the seasonal transition of the jet represented by PC1, often expediting the transition. This large-scale modulation, in turn, enhances the 310 depth of tropopause folds and fold frequency over western North America, enhancing STT-PBL. In May, La Niña conditions continue to increase the frequency and size of folds and therefore STT-PBL, also through invigoration of the storm track as in February and March (Fig. S6). An opposite response is observed during El Niño conditions.

Discussion and Conclusions 315
The present study seeks to further clarify the relationship between the north Pacific jet, tropopause folds, and deep mass transport, and how these connections evolve from February -June over the western United States using JRA-55 and ERA-Interim reanalysis. The leading EOF and corresponding PC of 200-hPa zonal wind are demonstrated to track the winter-tosummer jet evolution. The nature of this transition is consistent with previous studies of the annual cycle of the jet (Newman and Sardeshmukh 1998), and the associated changes in the storm track (Nakamura 1992; Hoskins and Hodges 2019). We find 320 that the spring jet transition modulates folds and STT-PBL, and that the timing of the transition varies from mid-March to late April. In February and March, early transitions lead to enhanced STT-PBL through an increase in the depth and frequency of tropopause folds over western North America. Conversely, late transitions are characterized by shallower, less frequent folds and weaker STT-PBL. Early transitions preferentially occur during La Niña conditions, while there is a weaker but still notable link between El Niño conditions and late transitions. In February and March, La Niña conditions enhance STT-PBL through 325 an increase in fold depth and frequency, while in May, STT-PBL is greater through an increase in fold size and frequency.
The peak in STT-PBL during boreal spring over western North America found by previous studies occurs through the simultaneous occurrence of the dynamic north Pacific jet transition and seasonal deepening of the PBL. The highly amplified flow observed during the spring transition increases the frequency of deep stratospheric intrusions, as the PBL deepens due to 330 enhanced solar heating, strengthening STT-PBL. The association between more STT-PBL and highly amplified flow found here is consistent with case studies of notable stratospheric ozone intrusion events over the western US (Langford et al. 2009;Lin et al. 2015;Knowland et al. 2017) and the established role of Rossby wave breaking in facilitating STT (Homeyer and Bowman 2013). The zonal wind anomalies associated with the transitional phase also resemble the April-May zonal wind anomalies found during years with the greatest mixing ratios of ozone observed in stratospheric intrusions (Albers et al. 2018). 335 The present analysis offers a simple metric to track such jet variability and situates it within the context of the seasonal transition.
Our results are consistent with the differences in STT-PBL of ozone observed between ENSO phase during April-May over western North America by Lin et al. (2015). This is notable because we only consider mass transport without measuring how 340 the ozone concentrations within folds varies between ENSO, which can be quite substantial This study took advantage of recently-developed products specifically targeted at understanding STT-PBL using ERA-Interim reanalysis fields (Sprenger et al. 2007;Dee et al. 2011;Skerlak et al 2014;Sprenger et al. 2017). We note that, as a 350 consequence, our results concerning STT-PBL are limited to the ERA-Interim record and the frequency of ENSO events within the 1980-2016 period (excluding section 3.1 which used the 60-year JRA-55 reanalysis record). To minimize the possibility of overstating subsequent conclusions regarding folds and STT-PBL, we have applied rather strict significance testing to account for sampling and autocorrelation, confirming the differences we have highlighted are frequently statistically significant. Future work could employ model simulations using many ensembles to increase the sample size of early/late 355 transition years and ENSO events, to revisit the connections found in this study.

Code and Data Availability
The code used to perform this analysis can be accessed by personal communication with the corresponding author. The    show each year's PC1 evolution, during 18 early and 19 late transition years, respectively. b) shows confidence intervals for the composite PC1 values for the early (green) and late (purple) groups.