4.2. SOM of the Wind at the 5th Lowest Model Level
 The method of SOMs can be used to further analyze and characterize the RAS at the 5th lowest model level. Figure 7 is a SOM of the horizontal wind for the 5th lowest model level from the AMPS 2001–2005 30 km archive. The same process in creating the SOM for the lowest model level was used in creating this SOM. The most pronounced feature used to identify the RAS in the SOM is the presence of a corridor of atmospheric mass transported from the continental interior northward. The characteristics of the RAS are based on a composite of atmospheric forcing features in the region including synoptic-scale cyclones, mesocyclones on the Ross Ice Shelf and over the Ross Sea, katabatic winds from West Antarctica and the East Antarctic Plateau, and barrier winds. The synoptic influences associated with the different SOM wind patterns can be analyzed by evaluating the node-averaged sea level pressure (Figure 8) for each node of the SOM. Unfortunately, the small scale and transitory movement of the regional mesocyclones makes them almost impossible to identify in this synoptic-climatology study, and the influence of these systems on the RAS likely has to be analyzed at the case-study level. Parish et al.  identifies the presence of blocking, and hence barrier wind dynamics, to be when the largest pressure gradient is located adjacent and parallel to the Transantarctic Mountains. A barrier wind can be identified in this manner in the node-averaged sea level pressure (Figure 8) and verified with the SOM of the horizontal wind at the 5th lowest model level (Figure 7).
Figure 7. Self-organizing map of the horizontal wind for the 5th lowest model level (approximately 150–160 m AGL) from the AMPS 30 km domain for 2001–2005. Contour shading indicates the average vector wind speed and the arrows indicate the wind direction. Contour lines are in intervals of 2.5 m s−1.
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Figure 8. Sea level pressure averaged for each node of the SOM of horizontal wind for the 5th lowest model level from the AMPS 30 km 2001–2005 archive. Isobars are in intervals of 2 hPa.
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 Five distinct patters in the horizontal wind at the 5th lowest model level can be identified through an examination of the SOM (Figure 7). The 2001–2005 mean wind speed (Figure 6) is subtracted from each node-averaged value of wind speed (Figure 7) to create the wind speed differential plot in Figure 9. Figure 9 helps highlight the features that make the horizontal wind pattern of each node unique. The 2001–2005 mean (Figure 6) is subtracted from each node-averaged value of wind speed to create the wind speed differential plot. Nodes [2,1], [3,1], [2,2], and [3,2] are dominated by light wind conditions across the entire SOM domain. The node-averaged wind speed differences (Figure 9) show lighter wind speeds on the range of 2–6 m s−1 with basically no regions with higher than average wind speeds. The node-averaged sea level pressure (Figure 8) lacks any defined synoptic pattern for these nodes. These four nodes will be referred to as light wind (LW) and have a cyan background color on the SOM. Nodes along the left column ([1,1] to [1,4]) and part of the bottom row ([1,4] to [3,4]) have a horizontal wind pattern that is dominated by the movement of a cyclone from Cape Adare to Cape Colbeck across the Ross Sea. The node-averaged sea level pressure (Figure 8) shows the presence and progression of the large cyclone across the Ross Sea. Such a cyclone track is a common feature of the Ross Sea [King and Turner, 1997]. These six nodes will be collectively referred to as the Ross Sea (RS) cyclone nodes and are indicated with a yellow background color on the SOM. The four nodes making up the upper-right corner ([4,1], [5,1], [4,2], and [5,2]) are characterized by katabatic winds on the East Antarctic Plateau. The differential node-averaged wind speeds (Figure 9) show lighter wind speeds across the Ross Ice Shelf and most of West Antarctica and higher wind speeds across the East Antarctic Plateau. The amount of katabatic activity increases diagonally from node [4,1] to node [5,2]. The node averaged sea level pressure shows minimal synoptic activity in nodes [4,1] and [5,1] and the slight presence of a cyclone in the Ross Sea in nodes [4,2] and [5,2]. These four nodes will be referred to as katabatic (K) and highlighted with a red background. Nodes [4,3], [5,3], [4,4], and [5,4], in the lower-right corner of the SOM, indicate the influence of a significant cyclone near Cape Colbeck and over West Antarctica in addition to strong katabatic winds. The differential node-averaged wind speeds show high wind speed values for these four nodes across the East Antarctica Plateau, West Antarctica, and the Ross Ice Shelf. The intensity of the features increases diagonally from node [4,3] to [5,4]. These four nodes will be referred to as hybrid (H), as they have significant katabatic and synoptic influences, and they have a background color of magenta on the SOM. The remaining two nodes ([2,3] and [3,3]) do not have any pronounced defining characteristics. The node-averaged sea level pressure shows the presence of a weaker cyclone in the Ross Sea than what is seen in the Ross Sea cyclone nodes. Below average wind speeds on the East Antarctic Plateau shows that there is minimal katabatic activity for these nodes. The presence of such moderately defined nodes makes sense given that oftentimes, especially during seasonal transitions, weather patterns are less extreme. These two nodes are referred to as the moderate (M) nodes and are indicated with a blue background color.
Figure 9. Difference between node-average wind speed and the 2001–2005 average wind speed (node average minus 2001–2005 average) for the SOM of horizontal wind for the 5th lowest model level. Contour lines are in intervals of 2 m s−1 and contour shading is in intervals of 1 m s−1. The non-shaded region covers values in the −1 m s−1 to +1 m s−1 range.
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 The defining characteristic of the RAS is a pronounced corridor of atmospheric mass transport northward across the Ross Ice Shelf and Ross Sea from the continental interior. The identification of a RAS in the node patterns is ultimately a subjective classification due to the distinction of when there is a continual stream of air flowing across the Ross Ice Shelf and Ross Sea. An additional factor to evaluate is if the stream of air is being maintained by the three sources feeding air from the continental interior. A review of the SOM for the 5th lowest model level (Figure 7) shows that the light wind nodes do not have RAS signatures due to the minimal amount of air flowing onto the Ross Ice Shelf from the continental interior. The katabatic nodes (K) also do not indicate the presence of a RAS. These nodes show a katabatic flow onto the Ross Ice Shelf and Ross Sea but the airflow lacks continuous strong flow across the entire north-south extent of the Ross Ice Shelf. For these nodes an ambient pressure gradient to drive the katabatic air northward in a continual stream of air is lacking. Node [1,1] of the Ross Sea (RS) cyclone nodes has a similar pattern on the Ross Ice Shelf and the southern Ross Sea as the light wind nodes, with a lighter than average winds across much of this area (Figure 9), and therefore does not indicate the presence of a RAS. Node [1,2] of the Ross Sea (RS) cyclone nodes does show a continual stream of air over the western Ross Sea fed by air from the continental interior at the Byrd and Reeves Glacier valleys. The node can be classified as a partial RAS because it is basically only present over the western Ross Sea. All other nodes ([1,3] to [5,4]), including the remaining Ross Sea cyclone nodes, the moderate (M) nodes, and the hybrid (H) nodes, show a continual northward corridor of atmospheric mass transport across the western Ross Ice Shelf and the western Ross Sea. In some nodes (e.g., [3,3] to [5,3]) the RAS is weak in intensity. In other nodes (e.g., [1,3], [1,4] to [5,4]) the RAS is strong in intensity. Similarly, in some nodes the RAS is fed through all three source regions and in other nodes one of the source regions is weak (e.g., Reeves Glacier in nodes [3,3] to [5,3]). The nodes classified as having a RAS present are indicated by ‘RAS’ being placed above the node in Figure 7.
 The associated katabatic influences on the wind regimes can be evaluated by analyzing node-averaged vertical profiles of potential temperature upstream of Byrd and Reeves Glaciers and over the Siple Coast confluence zone of West Antarctica (Figure 10). Katabatic winds are dependent on the strength of the near-surface inversion and the terrain slope. A stronger near-surface inversion, over a given terrain slope, provides more katabatic forcing. Parish and Cassano  estimate the inversion strength by using a linear interpolation of the ambient temperature profile (the profile at and above the 7th lowest model level for this study) and extending the interpolation toward the surface. The difference between the interpolated temperature profile and the actual temperature indicates the inversion strength. Higher inversion strengths indicate a larger amount of katabatic forcing on the plateaus. The regions used to calculate the average potential temperature profiles in Figure 10 are indicated in Figure 1 and are located upstream of Byrd Glacier, upstream of Reeves Glacier, and over West Antarctica. Node-averaged values for six nodes are provided in Figure 10. The selected nodes were chosen to represent the five different dominant SOM patterns. A second node was selected for the Ross Sea cyclone nodes given the variation in conditions across that dominant pattern. A dashed line has been added to the profiles of potential temperature representing a linear interpolation of the potential temperature profile from the 7th to the 12th lowest model levels. The difference between the dashed line (the linear interpolation) and the surface temperature is referred to as the inversion strength. A value indicating the inversion strength (K) is included on each profile next to the dashed line. The light-wind node shows an inversion strength around 6 K over the selected East Antarctic Plateau regions. This value is at least half that of all other selected nodes which is in agreement with the minimal katabatic winds indicated on the SOM. The vertical profile representing the moderate nodes has the next lowest values for the East Antarctic Plateau regions. The selected nodes for the katabatic, Ross Sea cyclone, and hybrid domains indicate an inversion strength of 14–16 K over the East Antarctic Plateaus regions. The high values of inversion strength correspond favorably to the above average winds, mostly katabatic, on the East Antarctic Plateau for these selected nodes. The region over West Antarctica is more complicated. For example in the hybrid node the considerable mixing associated with the nearby cyclone results in a relatively weak inversion strength.
Figure 10. Node-averaged vertical profile of potential temperature (K; left side) and wind speed (m s−1; right side) for selected regions and selected nodes representing the different dominant patterns. Wind barbs are plotted at the height of each model level. The number at the base of each temperature profile is the strength of the inversion (K). See text for a description of calculating this value. The selected regions are indicated in Figure 1.
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 The ambient forcing associated with the synoptic-scale cyclones is the dominant forcing mechanism for determining the presence of a RAS (Figure 7 and 8). Without the ambient pressure gradient there is not sufficient forcing to transport the atmospheric mass northward over large distances across the Ross Ice Shelf and Ross Sea. In the katabatic nodes there is katabatic winds across the East Antarctic Plateau and descending over the Ross Ice Shelf and Ross Sea. Yet the katabatic winds lose the katabatic forcing when it reaches the Ross Ice Shelf and there is not a cyclone present to drive this flow across the Ross Ice Shelf (Figures 8 and 9). In all of the RAS nodes there is a cyclone present in the Ross Sea (Figure 7 and 8). The one Ross Sea cyclone node ([1,1]) that is not classified as a RAS has the cyclone positioned north of Cape Adare. The intensity of the RAS is dependent on the position and associated pressure gradient of the Ross Sea cyclone. The nodes with the largest pressure gradient (see Figure 7) are [1,3], [1,4], [2,4], and [5,4]. These nodes also show the strongest wind speeds across the western Ross Ice Shelf and/or Ross Sea within the RAS.
 There is a significant katabatic component to the RAS, particularly near southern Ross Ice Shelf and the Siple Coast. However, this katabatic component must be aided by the presence of the Ross Sea cyclone to drive the air of katabatic origin across the Ross Ice Shelf. Node [5,4], part of the hybrid dominant pattern, shows the additional intensity of the RAS due to a strong katabatic flow from the Byrd Glacier with high node differential wind speed values downstream of Byrd Glacier. The contributions of the katabatic winds for node [5,4] is also indicated by node [5,4] having one of the largest values of inversion strength, as indicated in Figure 10. The amount of katabatic activity on the East Antarctic Plateau is also influenced by the position of the Ross Sea cyclone. The differential of node-averaged wind speed (Figure 9) can be used as a quick estimate of the amount of katabatic activity. As discussed previously, the node-averaged vertical profile of potential temperature and inversion strength can be used to approximate the amount of katabatic forcing. When the cyclone is positioned over the western Ross Sea (e.g., nodes [1,1], [1,2], and [1,3]) the amount of katabatic activity appears to be about that of the 5-year average or slightly less. As the cyclone moves to the eastern Ross Sea the amount of katabatic activity is increased. The katabatic winds are enhanced the most when the cyclone is over Cape Colbeck / West Antarctica (e.g., nodes [4,4] and [5,4]). The inversion strength upstream of Byrd Glacier is 16.1 K for node [1,4] and 15.9 K for node [5,4]. Meanwhile the wind speed values on the East Antarctic Plateau are approximately 4–6 m s−1 higher for node [5,4] than node [1,4]. The inversion strength upstream of Reeves Glacier is larger (16.3 K) for node [5,4] than node [1,4] (15.0 K). The wind speed for node [5,4] is also 6–8 m s−1 higher than node [1,4] indicating that the larger inversion strength and the synoptic-scale cyclone are both enhancing the wind flow in the Reeves Glacier region.
 The west-to-east extent of the RAS is dependent on the position of the Ross Sea cyclone. The RAS extends toward the center of the Ross Ice Shelf in nodes [1,3], [2,4], and especially in [1,4]. The node-averaged sea level pressure patterns show the position of the center of the cyclone to be over the central to eastern Ross Sea for these nodes. When the cyclone is located near Cape Adare and near Cape Colbeck / West Antarctica the RAS remains in close proximity of the Transantarctic Mountains.
 Seasonal node frequencies are able to describe the SOM patterns and the variations in the RAS over the course of the year. The seasons for this analysis are defined as December–January (DJ), February–March–April (FMA), May–June–July–August (MJJA), and September–October–November (SON) for the summer, fall, winter, and spring seasons. A two-month summer and a four-month winter were chosen as this distribution corresponds more closely to the annual progression of temperature and radiation in the Antarctic. This phenomenon has been referred to as the coreless winter [Schwerdtfeger, 1984]. There is a strong seasonal dependency in the occurrence of the different nodes for the SOM of the 5th lowest model-level. The light wind nodes dominate the summer months. Nearly 60% of the summer (DJ) time slices map to the light wind nodes ([2,1]: 14.7% + [3,1]: 25.5% + [2,2]: 11.4% + [3,2]: 8.1% = 59.7%). An additional 22% of the summer (DJ) time slices map to the katabatic node with the weakest winds ([4,1]), and the Ross Sea cyclone near Cape Adare ([1,1] and [1,2]). All but one of these nodes ([1,2]) has been classified as not being a RAS node. The remaining summer time slices (∼18%) are distributed to the other 13 nodes with less than 1% frequency of occurrence for most of the nodes with high winds speeds along the bottom row and right column. All but three of these nodes, the remaining katabatic nodes, have been classified as nodes with the RAS present. This evaluation of the summer months indicates that the RAS is rarely present during the summer months. 71% of the winter (MJJA) time slices map to the nodes along the right column ([5,1]–[5,4]), bottom row ([1,4]–[5,4]), and node [1,3]. All of these nodes are classified as having a RAS present. The six nodes making up the light wind nodes and the moderate nodes occur with a total of less than 10% frequency during the winter months. Thus the RAS is a nearly permanent feature for the region during the winter months. The RAS nodes are present approximately half of the time during the spring and fall months. The fall patterns are reasonably distributed across all nodes with frequency of occurrence between 2.5% and 6.9%, except for node [1,1] at 7.8%. The frequency of occurrence is relatively evenly distributed across the different dominant patterns. The frequency of occurrences are 19.9% for the four light wind nodes, 8.7% for the two moderate nodes, 24.8% for the four katabatic nodes, 27.1% for the six Ross Sea cyclone nodes, and 19.5% for the four hybrid nodes. The distribution of frequency of occurrence for the spring months is somewhere between that of the fall and the winter. The Ross Sea cyclone patterns occur 37.8% of the time, close to that of the winter. Meanwhile the light wind nodes and moderate nodes occur a combined 25.4% of the time which is considerably more than the 9.1% for the winter months. This makes it consistent with the spring season transitions from the strong baroclinic regions over the southern ocean during the winter months to the more synoptically weak summer months.