The merged and superposed sub‐tropical jet and polar‐front jet in the southwest Pacific: A case study

In the southwest Pacific, a meandering jet‐stream in the upper troposphere is sometimes found at ~30° S during austral winters and is usually treated as a sub‐tropical jet (STJ) due to its low latitude. For two contrasting cases, we have conducted analyses from two perspectives to identify the STJ and PFJ: first, using previously published qualitative criteria to identify jet‐cores and second, investigating the jet‐stream axes of STJ and PFJ identified using 2‐PVU curves. The results showed that the chosen meandering jet‐stream case at ~30° S was a merged, and for a time, a superposed STJ and PFJ. Downstream of the jet‐streak, the PFJ split to the south and the STJ to the east. This is in significant contrast to the horizontally well‐separated jet‐stream case chosen in this study. Some processes likely contributing to the superposition of the STJ and PFJ were analyzed and discussed. The movement of PFJ that was closely associated with the movement of the low over the Tasman Sea and the convection in and near the tropical region may have played dominant roles.


| INTRODUCTION
In the upper troposphere, there are two different jet-streams, the sub-tropical jet (STJ) and the polar-front jet (PFJ).The thermally direct Hadley cell (Held & Hou, 1980) transports angular momentum poleward and the STJ is generated at $30 latitude in both hemispheres due to conservation of angular momentum.In contrast, the PFJ occurs as a result of momentum flux convergence by atmospheric waves or eddies that develop in the mid-latitude baroclinic region of both hemispheres (Panetta, 1993;Panetta & Held, 1988).Because the strong winds in the PFJ are driven by the meridional convergence of westerly momentum associated with the baroclinic eddies (Panetta, 1993;Panetta & Held, 1988), the PFJ is often called an eddy-driven jet.Even though different mechanisms are responsible for the generation of the two different jets, they cannot always be seen separately at the same time in a region.For example, in the southern Hemisphere, a single jet is often seen in warm seasons and two separated jets are often seen in cold seasons (Gallego et al., 2005;Harnik, 2014;Kim & Lee, 2004).Close inspection of fig. 4 of Gallego et al. (2005) containing the 1958-2002 velocity average shows that, in addition to the austral summer single jet, one may also find a major strong jet-stream located at $30 S in June and July.A similar feature can also be found in fig. 1 of Harnik (2014).This single strong jet-stream at $30 S in the southwest Pacific is a dominant feature in the 300 hPa average wind speed during austral winters (fig. 2, Harnik, 2014).Kim and Lee (2004) examined the wave-zonal mean flow interaction and showed that growing baroclinic waves between the STJ and PFJ pump westerly momentum into the inter-jet region and blend the two jets into a single jet.Using a two-layer modified quasi-geostrophic spherical model, Lachmy and Harnik (2014) investigated the maintenance and transition of two flow regimes: the subtropical jet regime and the merged jet regime.For the latter, in addition to the thermally driven mechanism by the Hadley cell, convergence of eddy momentum flux also plays a major role.These theoretical studies clearly explained the existence and maintenance of a single jet or a merged jet.Real case studies by Winters andMartin (2014, 2016) showed that the superposition of the STJ and PFJ in the northern Hemisphere can be seen as a single jet.
Based on these theoretical and real case studies, a scientific question is raised about whether the individual meandering jet-stream at $30 S sometimes observed during austral winters in the southwest Pacific is a STJ as it appears, or could be a merged or sometimes a superposed STJ and PFJ.In this study, a superposed jet refers to the situation where the STJ and PFJ are in the same horizontal position with STJ on top of PFJ, whereas a merged STJ and PFJ refers to a situation where the jetcores are very close horizontally but not superposed.In this study, a single meandering jet-stream case was analyzed in detail and contrasted with a horizontally well-separated STJ and PFJ case.The chosen single meandering jet-stream case occurred during an intensive observation period (IOP12) at the end of June during the Deep Propagating Gravity Wave Experiment over and around New Zealand (DEEPWAVE, June-July 2014; Fritts et al., 2016).While studying the weather background and wind profiles that affected the upward propagation of mountain waves, the strong flow over New Zealand that split from this jet-stream was called a STJ in Portele et al. (2018) and Gisinger et al. (2017), and a tropopause-level jet in Kruse et al. (2016).Yang et al. (2022) found non-orographic inertia-gravity waves over the Southern Alps on 29 June 2014 that may have originated from this meandering jet-stream, which was called an upper-tropospheric jet in their paper.The contrasting, well-separated jet-stream case occurred on 18 June 2015 in the southwest Pacific.For this case, heavy rainfall occurred on the west side of the Southern Alps while heavy snow was found over the high mountains and most of the east side of the Southern Alps.A separate study on the weather background of this case and the effect of mountain height on the heavy rainfall in the Hokitika region of New Zealand (Yang Yang et al., 2023), found the well-separated jet-streams.
Correctly identifying a jet-stream is vital to understanding its associated processes and their influence on weather and climate.In addition, jet superpositions were found to be associated with a class of high impact weather (e.g., Christenson, 2013;Winters & Martin, 2014, 2017).We hypothesized that the chosen meandering jetstream is a merged and even superposed STJ and PFJ.The objective of this study is to test this hypothesis from two perspectives by analyzing the meandering jet-stream case that occurred end of June 2014 and the wellseparated STJ and PFJ case that occurred on 18 June 2015.In addition, processes that may contribute to the jet superposition are also investigated and discussed.

| DESCRIPTION OF MODEL AND METHODOLOGY
The model used in this study is a regional configuration of the UK Met Office Unified Model (UM) with the END-Game dynamical core (Walters et al., 2017;Wood et al., 2014).The regional model has a domain size of 324 by 324 horizontal grid points and a horizontal resolution of 0.11 ($12 km, Figure 1).It has 70 levels in the vertical with the top-level at 80 km above the mean sealevel.The initial and lateral boundary conditions are provided by the operational global UM at the UK MetOffice.For the two jet-stream cases chosen in this study, the model simulation started at 12:00 UTC 17 June 2015 and 00:00 UTC on 29 and 30 June 2014.The spin-up time of this regional model is usually $3 h (Yang et al., 2017) so for this study only the model outputs beyond 6 h were used for analyses.
Based on the observation analysis of Defant and Taba (1957), Winters andMartin (2014, 2016) summarized the qualitative attributes/criteria of the STJ and PFJ to identify the jet-core for each of them.The core of the STJ is located at the 200-250 hPa level and within the 340-355 K isentropic layer, whereas the PFJ core is located at $300 hPa and within the 315-330 K layer.The tropopause can be identified using the potential vorticity (PV) contour of 2 PVU (1 PVU = 10 À6 K m2 kg À1 s À1 ).The core of the STJ or PFJ lies below the tropopause and at the low PV edge of the strong horizontal PV gradient of 1-3 PVU in their respective layers.These qualitative attributes/criteria were used in this study as the first method to identify the jet-cores.Quantitative PV gradient criteria summarized by Winters and Martin (2014) and Christenson et al. (2017) based on different low-resolution datasets (1 and 2.5 ) were not used in this higher resolution study ($0.1 ) because calculation of PV gradient, and therefore gradient criteria, is affected by model resolution.As shown in the following, the qualitative attributes/criteria are sufficient to locate the jet-cores manually.
The jet-cores of STJ and PFJ are very close to the tropopause, which can be identified by the 2 PVU curves.Therefore, the 2 PVU curves in the jet-stream were good indicators of the positions of the PFJ core (or jet-stream axis) at 300 hPa and the STJ core at 200 hPa (Winters & Martin, 2016), and were used as the second method to identify the position of the jet-core in this study.

| WEATHER BACKGROUND
For the upper-tropospheric jet-stream in the southwest Pacific on 18 June 2015, a high was found to the northeast of New Zealand and another high to the southwest (Figure 1a).Between the two highs was a band of low pressure with northwest-southeast orientation across the South Island of New Zealand.At 300 hPa (Figure 1d), a straight upper-tropospheric jet was found at $25 S with the maximum wind speed about 60 m/s.Another jet was found to the south of the South Island of New Zealand around 47-55 S with maximum wind speed of $75 m/s.As will be shown later, these were the STJ and PFJ, respectively.For this case, heavy rainfall occurred on the west side of the Southern Alps while heavy snow was found over the high mountains and most of the east side of the Southern Alps.
For the second jet-stream case on 29 June, a surface high was found to the east of New Zealand and a low over the Tasman Sea (Figure 1b).On 30 June, after an eastward movement, the surface low and high were found over New Zealand and to the east of the country, respectively (Figure 1c).At 300 hPa on 29 June (Figure 1e), a meandering jet-stream was found at $30 S. Inside the jet-stream, a curved jet-streak (the maximum wind speed area inside the jet-stream) with a maximum wind speed of $90 m/s was found over eastern Australia and the North Tasman Sea.On 30 June (Figure 1f), with the eastward movement of the jetstream, the jet-streak was wholly over the northern part of the Tasman Sea (Figure 1f).Very heavy rainfall was found in many areas of New Zealand on 30 June.For example, at Waitara, a town in the North Island, the daily precipitation valid at 0900 NZST on 1 July was $70 mm, while the climate mean monthly precipitation in June and July is $170 mm.

| Horizontally well-separated STJ and PFJ
Figure 2a shows the vertical structure of the jet-stream to the south of the South Island (Figure 1d, T1).The jet-core was found between 250 hPa and 300 hPa within the 315-330 K layer below the tropopause (contour of 2 PVU) and above the 1 PVU contour.These features indicated that this jet-stream was a PFJ.In contrast, for the jet-stream at low latitude (Figure 2b along T2 in Figure 1d), the jetcore was found at a height between 250 hPa and 200 hPa within the 340-355 K layer below the tropopause, indicating that the jet-stream at low latitude was a STJ.About 800 km to the south of this STJ was another jet-core with the maximum wind speed of $60 m/s (Figure 2b), which shared almost the same features of the PFJ jet-core described earlier (Figure 2a).In fact, it was part of the PFJ that curved from south of the South Island to lower latitudes over the Tasman Sea with weaker wind speed at 300 hPa (Figure 1b).Thus, Figure 2b shows the vertical structures of the horizontally well-separated STJ and PFJ.A pronounced 'three step tropopause structure' (Defant & Taba, 1957;Winters & Martin, 2014) was found, as shown from the 1 and 2 PVU contours.

| Based on qualitative criteria/ attributes of STJ and PFJ
Across the jet-streak on 29 June 2014 (Figure 2c along T3 in Figure 1e), the core of the STJ was located at $200 hPa and the core of the PFJ was located at $300 hPa.The meridional locations of both jet-stream cores were separated by only $100 km.The 'three-step tropopause structure' for horizontally well-separated STJ and PFJ described earlier was not so pronounced here.The second step between the STJ and PFJ almost disappeared.These facts indicated a merged STJ and PFJ.
At 12:00 UTC on 30 June (Figure 2e along T5 in Figure 1f), the meridional locations of both jet-stream cores were the same, but the STJ core was at a higher altitude than the PFJ core.The 'three-step tropopause structure' changed to two-step structure, consistent with fig.4d of Winters and Martin (2014), indicating superposition of the STJ and PFJ.
Across the exit of the jet-streak on 29 June (Figure 2d along T4 in Figure 1e), the cores of the STJ and PFJ were found at $200 hPa and $290 hPa, respectively.The meridional locations of both cores were separated by about 400 km at the exit, which was much larger than inside the jet-streak (Figures 2c,d).The tropopause structure was not the two-step structure found in the jet-streak on 30 June.Instead, another step for the tropopause was found between both jet-stream cores and made the tropopause structure like the three-step structure (Figure 2d).The maximum wind speed downstream of the jet-streak was at least $5 m/s weaker than the jet-streak itself.Similar features were also found downstream of the jet-streak on 30 June (Figure 2f along T6 in Figure 1f).
For the superposed jets the jet-core was vertically elongated (Figures 2e), that is, the strong wind speed of the jet-core extended downward from the level of the STJ core.This is in contrast to the jet-core of the STJ (Figures 2b,d,f).The maximum wind speed in the superposed jet on 30 June was stronger ($5 m/s) than that of the merged jets.Similar features can also be found in previous publications (Christenson et al., 2017;Handlos & Martin, 2016;Winters & Martin, 2016).

| Based on jet-stream-axis
The 2 PVU curves in the jet-stream were good indicators of the positions of the PFJ core (or jet-stream axis) at 300 hPa and the STJ core at 200 hPa (Winters & Martin, 2016).This was true for the PFJ and STJ cases on 29 and 30 June 2014 and 18 June 2015.As shown in Figure 3a, the PFJ (black and blue dotted lines) and STJ F I G U R E 2 Cross sections at 00:00 UTC on 18 June 2015 (a) along T1 across the PFJ at higher latitudes and (b) along T2 across the STJ at lower latitudes in Figure 1d, at 12:00 UTC 29 June 2014 (c) along T3 across the jet-streak, and (d) along T4 at the exit region of the jetstreak in Figure 1e, and at 12:00 UTC 30 June 2014 (e) along T5 across the jet-streak and (f) along T6 at the exit region of the jet-streak in Figure 1f.Contours of the 1, 2 and 3 PVU (black contours); potential temperature every 5 K (dashed green); and isotachs every 10 m/s beginning at 30 m/s (red contours) are shown.The 315-330 K and 340-355 K isentropic layers, used to identify the locations of the jets, are shaded grey.The blue (yellow) dot and solid straight lines denote the PFJ (STJ) cores and the meridional positions of the jet-cores.
(black and blue solid lines) separated downstream of the jet-streak, converged upstream of the jet-streak, and gradually transitioned from a merged jet on 29 June to vertical superposition on 30 June in the jet streak (Figure 3a).However, this feature was not found for the wellseparated PFJ and STJ on 18 June 2015 (Figure 3b, green lines).This is consistent with the results described earlier based on qualitative attributes/criteria of STJ and PFJ.Thus, the complete coincidence of the 2 PVU curves at 300 and 200 hPa in the jet-streak on 30 June (Figure 3a) is further evidence to show the superposition of the STJ and PFJ.

| DISCUSSION
Figure 4 shows the ageostrophic winds at 300 hPa and 200 hPa.At 300 hPa, northeasterly ageostrophic winds were found at the entrance of the jet streak, and westerly and southwesterly ageostrophic winds were found at exit of the jet streak (Figures 4a,b).Similar features were also found at 200 hPa (Figures 4c,d).These ageostrophic winds are the upper branch of the jet-streak induced circulation cells upstream and downstream of the jet-streak (Bjerkness, 1951;Shapiro, 1981;Uccellini, 1986;Uccellini & Johnson, 1979;Uccellini & Kocin, 1987).Within the jetstreak, the ageostrophic winds were southeasterly at 300 hPa (Figures 4a,b) but weak and erratic in direction at 200 hPa (Figures 4c,d).
For the superposed jet-stream case during the 18-20 December 2009 Mid-Atlantic Blizzard, Winters and Martin (2016) showed that ageostrophic transverse circulations within and between the STJ and PFJ played a primary role in the production of a superposition.For the jet superposition case studied here, from 0600 UTC to 1800 UTC on 29 June the jet-stream axis inside the jetstreak for the PFJ at 300 hPa moved northward about 150 km (Figure 4a).On 30 June, the western part (upstream of the jet-streak) of the PFJ axis moved southward $300 km while the remaining part of PFJ axis within the jet-streak and downstream moved northward $300 km.Similar features of the PFJ axis movement from 0600 UTC to 1800 UTC were also found for the STJ axis at 200 hPa (Figures 4c,d).
As described earlier the low over the Tasman Sea moved eastward from 29 to 30 June.From 0600 UTC to 1800 UTC on 29 June, the low centre moved eastward $4.0 and southward 0.5 .Almost the same movement was found for the low from 0600 UTC to 1800 UTC on 30 June.Assuming the PFJ at 300 hPa moved with the low and had the same movement during the 12 h, we plotted the new position of the PFJ axis at 1800 UTC (Figure 4a,b, blue dashed lines) after 12 h movement from 0600 UTC (black dotted lines).In the jet-streak the black solid line (the actual PFJ axis) and the blue dashed line (the expected PFJ axis if moving with the low) were almost coincident, indicating that the PFJ movement from 29 to 30 June was closely associated with the movement of the low.However, the movement of the STJ at 200 hPa was different from that of the PFJ on 29 June.The blue dashed line was $100 km to the north of the black line (Figure 4a), indicating that the movement of the STJ was not associated with the low.On 30 June the STJ and PFJ were superposed, so the movement of the STJ and PFJ is expected to be the same.Thus, it is not surprising to see the coincidence of the black solid line and the blue dashed line in the jet-streak on 30 June (Figure 4d).
STJ generation is closely associated with tropical convection.Gillett et al. (2021) indicated that an intensification of the STJ is associated with enhanced divergent outflow from diabatic heating over the equatorial Pacific Ocean.For the superposed jet during the 1-3 May 2010 Nashville Flood, Winters and Martin (2016) demonstrated that the convection over the southeastern United States acted substantially to superpose the jets through restructuring the tropopause, and advection of the STJ axis towards the polar jet via its associated divergent outflow.
CAPE is a good indicator of convection.On 29 June up to 1000 J/kg of CAPE was found around the cold front (or the convergence zone between the northerly winds and the westerly/southwesterly winds) to the north of the STJ (Figure 5a).The cold front lifting or rising motion due to low-level convergence would release CAPE for the development of convection.On 30 June (Figure 5b), up to 500 J/kg of CAPE was also found associated with the cold front north of 30 S but smaller in magnitude and area than that on 29 June, implying a large proportion of CAPE associated with the cold front in and near the tropical region has been released as diabatic heating to sustain convection from 29 to 30 June.As described earlier, the movement of the PFJ from 29 to 30 June was associated with the movement of the low over the Tasman Sea, but the STJ tended to move towards the PFJ relative to the movement of the low.Because STJ generation and development are closely associated with convection and its associated diabatic heating (Gillett et al., 2021;Winters & Martin, 2016), it is likely that the convection in and near the tropical region and its associated diabatic heating affected the movement of the STJ from 29 to 30 June.However, testing this hypothesis needs more deep investigation, which is out of the scope of this paper and will be conducted in future.

| CONCLUSION
In the southwest Pacific, a single meandering jetstream can sometimes be found at $30 S during austral winters.These jets are usually treated as STJs due to their low latitudes.In this study, a specific case near the end of June 2014 was examined.In significant contrast to a horizontally well-separated STJ and PFJ on 18 June 2015, we presented evidence from two perspectives to show that the meandering jet-stream observed on June 29 and 302,014 was a merged and even superposed STJ and PFJ.From the merged STJ and PFJ on 29 June to the superposition of the two jets on 30 June, the movement of PFJ was closely associated with the movement of a low over the Tasman Sea.However, the movement of STJ was not associated with the movement of the low.AUTHOR CONTRIBUTIONS Y. Yang: Formal analysis; investigation; methodology; writingoriginal draft.T. Carey-Smith: Funding acquisition; project administration; resources; writingreview and editing.R. Turner: Resources; supervision; writingreview and editing.

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I G U R E 1 Simulated surface winds, surface air temperature (shading) and sea-level pressure (hPa, blue contours) at (a) 00:00 UTC on 18 June 2015, (b) 12:00 UTC on 29 June and (c) 12:00 UTC on 30 June 2014.Geo-potential height (blue contours), wind speed (shading), and air temperature (black dotted contours) on 300 hPa for at same three times are shown in (d), (e) and (f).The thick solid lines (T1 to T6) are the transects used for Figure 2. F I G U R E 2 Legend on next page.

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I G U R E 3 The 2 PVU contours at 300 hPa (dotted lines) and at 200 hPa (solid lines) (a) at 1200 UTC on 29 June (black), on 30 June 2014 (blue) and (b) at 00:00 UTC on 18 June 2015 (green).

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I G U R E 4 Ageostrophic wind vectors and the total wind speed (shading) on 300 hPa at 1200 UTC (a) on 29 June and (b) on 30 June, and on 200 hPa at 1200 UTC (c) on 29 June and (d) on 30 June 2014.Black dotted (solid) lines are the jet-stream axes at 0600 UTC (1800 UTC).Blue dashed lines are the new position of jet-stream at 1800 UTC if moving with the low over the Tasman Sea.

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Based on the qualitative criteria/attributes of the STJ and PFJ summarized by previous publications, the PFJ and STJ cores were identified in the meandering jet-stream.Both cores in the jet-streaks were separated by only $100 km (merged jets) on 29 June and became vertically superposed on 30 June.•The jet-stream axes of the PFJ and STJ identified by the 2-PVU curves in the jet-streams at 300 hPa and 200 hPa, respectively, clearly showed the merged PFJ and STJ on 29 June, the superposed PFJ and STJ in the jet-streak on 30 June, and the splitting of the PFJ and STJ downstream.• The vertical structure of the superposed jet in the jetstreak was quite different from that of well-separated PFJ or STJ regarding the shapes of jet-zone and tropopause.For the superposed jet, the wind speed of the jet-core has larger downward extension from the level of STJ core than that of the STJ in well-separated jets.

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I G U R E 5 Surface winds and convective available potential energy (CAPE, J/kg, shading) at (a) 1200 UTC 29 June and (b) 1200 UTC 30 June 2014.Thick black dashed lines show the jet-stream axes of STJ at 200 hPa.