Atmospheric Blocking in an Aquaplanet and the Impact of Orography

Atmospheric Blocking in an Aquaplanet and the Impact of Orography 1 Veeshan Narinesingh, James F. Booth, Spencer K. Clark, Yi Ming 2 Department of Physics, City University of New York – The Graduate Center, New York, New York, 10016, United States of 3 America 4 Department of Earth and Atmospheric Sciences and NOAA-CESSRST, City University of New York – City College, New 5 York, New York, 10031, United States of America 6 Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey, 08544, United States of 7 America 8 4 Atmospheric Physics Division, NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, 08540, United 9 States of America 10 Correspondence to: Veeshan Narinesingh (veenarinesingh@gmail.com) 11

and thus more blocking. As the prescribed mountain height is increased, so does the magnitude and size of climatological

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This suggests the extratropical cyclones (i.e., synoptic-scale eddies) that occur upstream of the blocking regions may 49 be key. Colucci (1985) and Pfahl et al. (2015) show that extratropical cyclones can impact blocks downstream of the storm 50 track exit region. In a related theory, blocks are linked to Rossby wave-breaking ( Hu. et al. (2008) presents case studies that show blocks in an aquaplanet model behave in a realistic manner. They 53 also find that blocks in their aquaplanet model occur more frequently than what is observed in natureregardless of 54 hemisphere, which is contradictory to the idea that stationary waves facilitate blocking episodes. The results of Hu et al. (2008) 55 however, are complicated by known discrepancies within the community regarding the identification (e.g. Barnes et al., 2012)  blocking occurs more frequently in the northern hemisphere than the southern. This difference in blocking frequency is 64 assumed to related to the stronger stationary wave in the NH (Nakamura and Huang, 2018), often attributed to more prominent 4 midlatitude topography and land-sea contrasts, e.g., Held et al. (2002). However, to our knowledge, no study has confirmed 66 this assumption.

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Previous work suggests that the spatial distribution of blocking frequency (hereafter, the blocking climatology) is 68 dependent on the behaviour of the stationary waves, jet streams, and storm tracks. Nakamura and Huang (2018) (Frierson, 2007), boundary layer turbulence (Troen and Mahrt, 1986), and surface fluxes.

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There is no treatment of cloud radiative effects or condensed water in the atmosphere.

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Here we use a 500 hPa geopotential height (Z500) hybrid metric that utilizes the Z500 anomaly and meridional 129 gradient. This metric was chosen for its robustness in terms of capturing high amplitude events involving wave-breaking 134 Sausen et al. (1995). This algorithm searches for large, contiguous regions of persistent, high amplitude, positive anomalies in 135 the Z500 field. Within these regions, Z500 must satisfy a meridional gradient reversal condition. What follows is an overview 136 of the block identification algorithm, but specific details can be found in DS13:   In case studies using ERAI and the idealized configurations described here, it was observed that two existing blocks 152 sometimes merged with one another to form a single, larger block. We objectively identified this merging process based on 153 extreme shifts in the location of the block centroid (defined as the gridpoint that is the centroid of the anomalous area associated 154 with the block). If the centroid shifted by more than 1500 km from one 6-hourly snapshot to the next, we labelled the block as 155 a merged event. These merged events represented 23-27 percent of the total initial blocks found in the idealized model 156 integrations. We judge these events to be unique in terms of their relationship between block duration. Furthermore, the 157 merger-blocks create uncertainty in terms of defining a block centre for the sake of our block-centered composite analysis.
158 Therefore, we have excluded the merged events from our block-centered compositing and block duration analyses. The This calculation is performed on variables on the 250-hPa pressure surface. For each point is the pressure and is latitude.

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→ is the 30-day low-pass filtered horizontal wind vector with zonal and meridional components and , respectively. The 202 anomalous zonal wind, meridional wind, and geopotential are given by ′ , ′ , and ′, respectively. Derivatives are computed 203 using finite-differencing, where zonal derivatives are weighted by latitude.

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The composites presented in this paper, only include midlatitude-blocks whose centroid are always south of 65˚ N.

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This is because we find that the high-latitude blocks exhibit distinct physical behavior. From reanalysis data, high-latitude

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Significance testing for mean block duration also utilizes a u-test to compare differences between the various 265 configurations and regions. A 95% confidence interval is imposed as the significance threshold for all significance testing.  Block-centered compositing analysis is used to confirm that, on average, the blocks identified in the aquaplanet model 284 evolve in a dynamically similar manner to models with zonally asymmetric forcing. Figure 3 shows block centered composites 285 of 500′, → , and ⋅ → for blocks over the NH oceans, and for the SH as well (Fig. 3 rows 1 and 2, respectively). In both 286 panels only blocks anchored in the midlatitudes are considered (i.e., occurring between 30˚ and 65˚ of latitude). For the sake 287 of comparison with the aquaplanet, blocks over land are excluded. For the idealized model, we show blocks from the 288 11 aquaplanet (Fig. 3, row 3) and the East region (see table 1 and Fig. 1) of the 3 km single mountain configuration (3 km   289 SingleMtn East, Fig. 3, row 4). The East region of the 3 km SingleMtn was chosen to isolate blocks generated in the model 290 that form near the high-pressure anomaly of stationary waves. However, block-centered composites for all orographic 291 configurations (i.e. 1 km, 2 km, 3 km, and TwoMtn), and each of their respective regions yielded similar results (not shown), 292 with little to no regional variationthis result is discussed again below.

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The onset of blocking in the composites (Fig. 3, column 1) is qualitatively similar to that found in the case study ( Fig.   294 2). The Z500 anomalies all show a positive anomaly at the center of the composite and negative anomalies upstream. In the 295 NH, this upstream anomaly has two closed centers (Fig 3a), whereas the SH and the idealized configurations each have only 296 one. We have subset the NH observations for the North Atlantic and North Pacific (not shown), and this difference is mainly 297 due to the blocks in the North Atlantic.

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The reanalysis and idealized model results all show → convergence (i.e., blue shading) on the downstream-299 equatorward flanks of composite blocks during onset (shading in Fig. 3, column 1). The

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On the final day of the block life cycles (Fig 3., third column), each respective composite block's Z500 anomaly 312 weakens, and low-pressure is concentrated downstream from the block. Weak values of → exit the block downstream of the 313 high-pressure maximum during this time (Fig. 3c, 3f, 3i). A net divergence of  Block-centered composites for the aquaplanet are qualitatively similar to composites for reanalysis, and the 318 similarities are strongest between SH and aquaplanet (Fig. 3). This is consistent with the fact that the SH has less orography 12 than the NH. However, we remind the reader that surface forcing in the SH is still asymmetric, as discussed in Berrisford et  the blocking climatology is not zonally symmetric after 30 years (Fig. 4b). We find that it takes 250 years for the aquaplanet 343 blocking climatology to approach zonal symmetry (Figs. 4c and 4d). However, for the models with orography, the time to 344 reach convergence is likely not as large. We deduced this from the following analysis: we generate 20-year climatologies using 345 randomly sampled years from our 30-year integrations and compare them. For the for the configurations with orography, the 346 blocking climatology is spatially consistent, whereas, for the aquaplanet, each climatology has a unique spatial distribution 347 (not shown). Therefore, we believe that 30-years of model runs provides a usable level of convergence of the spatial 348 climatology of blocking in the integrations with mountains.

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The different orographic configurations of the northern and southern hemispheres produce distinct spatial 352 distributions of general circulation features and atmospheric blocking (Fig. 5). Stationary wave patterns can emerge due to 353 13 land-sea heating contrasts, drag, and flow deflection by topography (e.g. Held et al., 2002). The two strongest regions of 354 anomalous high-pressure in the NH are located on the windward side of the Rocky Mountains, and near the western edge of 355 Europe (Fig. 5a). In the SH, the high-pressure maximum is southwest of South America, and a secondary maximum can be 356 found southeast of Australia (Fig 5b). These results are consistent with previous work (

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Focusing next on storm tracks, we see that the entrance of the storm tracks occurs on the northeast edge of the 250 367 maxima (Fig. 5a, 5c) (Colucci 1985) and maintenance of blocks (Shutts, 1983;Nakamura et al. 1997 Wang and Kuang, 2019). This region is also where the stationary wave and blocking maxima occur (Fig. 5). There is one 372 exception in the SH however: the SH storm track exit at the eastern terminus of the Indian Ocean (i.e., 90˚ E) does not coincide 373 with a maxima in blocking or the stationary wavebut it is a region of locally weak 250.

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For the NH (SH) in this dataset, 485 (336) blocking events are found yielding a hemispherically-averaged blocking 375 frequency of 2.7 % (1.6 %). We find the differences in hemispherically averaged blocking frequency between the hemispheres 376 to be statistically significant. The greater amount of blocking in the NH is typically assumed to be a result of the relative 377 abundance of topographic features. Therefore, we will use configurations of the model to explore the effects of mountains on 378 the spatial distribution and hemispherically averaged statistics of blocking frequency.

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Here, a single mountain is added to the aquaplanet to study the response of the idealized model blocking climatology 382 to the presence of orography. Figure 6 shows the stationary waves, storm tracks, blocking climatologies, and 250 in the 383 SingleMtn integrations. In each integration, a stationary wave is induced (Figs. 6a-6d) with a high-pressure anomaly generated 384 near the coastline on the windward side of the mountain, and a low-pressure anomaly on the leeward side (Fig. 6a-d). This 385 results in a meridionally tilted stationary wave pattern that extends into the subtropics leeward of the mountain. This pattern 14 has been explained in previous idealized modeling work (Grose and Hoskins, 1979;Cook and Held, 1992;Lutsko 2016). The 387 intensity and zonal extent of the stationary wave extrema increases with mountain height (Figs. 6a-d).

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In the SingleMtn integrations, as the height of the mountain is increased, the local maximum in the 250 increases 389 as well (right column, Fig 6). This relationship between the strength of the local jet maxima and mountain height follows from

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The zonal extent of the blocking climatology maximum increases when mountain height is increased (Figs. 6e-h).

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This agrees with the response of the stationary wave (Figs. 6a-d). The overall hemispherically averaged statistics of blocking 412 frequency yields an increase in blocking when mountain height is increased (See Table 2). These increases for the 2k-4k 413 configurations are modest however and should be taken with some degree of caution. Still, it is clear that as mountain height 414 increases, there is a greater area of significantly more blocking compared to the aquaplanet (Figs. 6e-h). Also worth noting is 415 hemispherically-averaged blocking frequency is significantly greater in the 2k, 3k, and 4k mountain runs when compared with 416 aquaplanet. Next, we investigate the response of adding an additional mountain.

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The addition of a second mountain induces a second trough and ridge in the stationary wave, and a second maxima 425 for the blocking climatology, storm track, and 250 (Fig. 7). The intensity and zonal extent of these features, however, varies

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The TwoMtn configuration has a greater hemispherically-averaged blocking frequency than the other configurations 429 (Table 2) and is also significantly greater than the aquaplanet. This is despite the TwoMtn configuration having a lower total 430 number of blocks than the 3 and 4 km SingleMtn configurations, respectivelymeaning the blocks have a longer average 431 duration in the 2-mountain configuration (Table 3). Each mountain also creates regions of enhanced and suppressed blocking 432 frequency (Fig. 7b). However, just like the general circulation features, there are differences in the blocking climatology for 433 the two ocean basins.

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Next, we examine the blocking climatology within each of the two ocean basins in the TwoMtn simulation (Wide 435 Basin and Short Basin, respectively, see Fig. 1 and Table 1). In the Wide Basin, there is close to a basinwide enhancement of 436 blocking frequency when compared to the single mountain cases (Figs. 6e-h, and 7b). Consistent with this enhancement, the 437 overall midlatitude 250 climatology is much weaker in the wide basin compared to the other ocean basin and SingleMtn 438 integrations. In the Short Basin, a separate blocking maximum exists near the high-pressure stationary wave anomaly. This 439 maximum, albeit much weaker than its wide basin counterpart, is still significantly more than what occurs in the same region 440 for the aquaplanet.

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One of the characteristics that allows blocks to influence midlatitude weather is their persistence. As such, we examine 450 the influence of mountains on block persistence using our duration metric. First, we find that adding mountains leads to at 451 least a modest increase in the average midlatitude block duration (Table 3). All topographic configurations aside from 1 km 452 16 SingleMtn, also have 7-39 more blocks than the aquaplanet (Table 3). This helps to explain some of the climatological 453 differences in block frequency between the idealized model configurations (Table 2), particularly for the 1 km SingleMtn case.

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Despite a 0.25 day greater mean block duration (Table 3), 1 km was found to have less hemispherically averaged blocking 455 than the aquaplanet (Table 2) due to 21 less events. The blocks in the topographic integrations were then put into subsets based 456 off those originating near the high-pressure stationary wave anomaly and those that were not.

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Regions used to subset blocks are denoted as "East", those originating at the eastern end of the ocean basin near the 458 high-pressure stationary anomaly, and "Other", those originating elsewhere in the midlatitudes ( Fig. 1a and Table 1). Figure 8 459 shows the probability density distributions for the aquaplanet and East blocks from each configuration. With the exception of 460 the 4 km run, the "East" regions of the single mountain integrations have relatively less shorter duration blocks (i.e. 5-11 days), 461 and relatively more longer duration blocks (11 days or more) compared to the aquaplanet (Fig. 8). Blocks from the "East" 462 regions last longer on average than aquaplanet blocks (Table 3), but the 3 km and 4 km enhancement of block duration are not 463 significant to the 95 th percentile. Mean block duration is greater for the "East" region compared to the "Other" in the single 464 mountain configurations (Table 3), with significant differences found in the 1 km and 2 km integrations. This leads to a 465 cautious suggestion that blocks that originate near mountains last longer on average than those that do not. However, the 466 modest differences found in the 3 km and 4 km integrations must be considered, and the nonlinear response of block duration 467 to linear changes in topography attests the systems own internal variability.

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The response of the TwoMtn configuration is much less straightforward. This integration is divided into 3 regions,

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Wide Basin East, Wide Basin Other, and Short Basin ( Fig. 1b and Table 1); Note the Short Basin does not have distinct "East" 470 and "Other" regions because of its shortened zonal extent. Average block duration in the "Other" region in the Wide Basin is 471 slightly longer than the "East", but both regions are significantly greater than the Short Basin. This coupled with more Wide 472 Basin East events (Table 3) is consistent with the weaker maximum in the blocking climatology for the Short Basin ( Figure   473 7b). Perhaps this is related to the inhibition of blocking by the nearby storm track and 250 maximum in the Short Basin, but 474 we do not seek to attribute a causal relationship here.

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Our results suggest that blocks starting near mountains last longer on average than those that do not (Table 3) To add some perspective on the role of mountains as compared to land masses with no orographic features, we analyze 486 the response of an idealized model configuration with a single flat land mass, herein referred to as 0 km (Fig. 9). The results 487 of 0 km are briefly mentioned here to primarily serve as a benchmark for this setup. This configuration is like the others that 488 include mountains in that it imposes zonally asymmetric forcing in land-sea contrast; The difference, however, is that that the

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The 0 km integration has a 3.42 % hemispherically averaged block frequency, which is greater than the aquaplanet and 1 km 497 configurations but less than the others with taller mountains (Table 2) Does orography affect the duration of blocking events?introduction, followed by concluding remarks.

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With regards to question 1, using the aquaplanet we confirm that blocks can be generated without any zonally

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Based on ERA-Interim reanalysis, these results mirror what is observed for the NH and SH, where the NH contains more 534 topography and blocking. In the idealized model, the enhancement of block frequency near the stationary wave maximum and 535 250 minimum is consistent with these regions being conducive to the convergence (or "traffic jamming") of wave activity 536 fluxes. These regions are found to be far from the storm track exit however, which is dissimilar to the NH in reanalysis. At the 537 storm track exit region, previous work has shown that extratropical cyclones can seed blocks (Colucci 1985) or maintain them, 538 Pfahl et al. (2015). However, in those studies the storm track exit coincides, or sits spatially close to the stationary wave 539 maxima. In our single mountain experiments, the storm track exit is far from the stationary wave maxima, and the result is that 540 the blocks preferentially occur at the stationary wave maxima region. This suggests that the role of the cyclones in nature may 541 be secondary to the role of the large-scale flow. That being said, secondary blocking maxima are found near the storm track 542 exit in the idealized model, suggesting that this location also plays a key role in anchoring where blocks most frequently occur.

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We note that the influence of differences in blocking for model configurations with and without mountains in our 544 model is not identical to the differences between the NH and SH in observations. First, from the block-centered composites 545 (Fig. 3), it wasis clear that the NH vsversus SH differences in observations for Z500 anomalies and wave activity flux are 546 larger than those found for the aquaplanet as compared to the idealized configurations with orography. This is true for the case 547 shown in Fig. 3 (3