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1 The climatology, meteorology, and boundary layer structure of marine cold 2 air outbreaks in both hemispheres 3 Jennifer Fletcher∗ , Shannon Mason, and Christian Jakob 4 School of Earth, Atmosphere, and Environment, Monash University, Clayton, VIC, Australia 5 ∗ Corresponding author address: Jennifer Fletcher, Monash University, 9 Rainforest Walk, Clayton, 6 VIC, Australia 3800. 7 E-mail: [email protected] Generated using v4.3.2 of the AMS LATEX template 1 ABSTRACT 8 A comparison of marine cold air outbreaks (MCAOs) in the Northern and 9 Southern Hemispheres is presented, with attention to their seasonality, fre- 10 quency of occurence, and strength as measured by a cold air outbreak index. 11 When considered on a gridpoint-by-gridpoint basis, MCAOs are more severe 12 and more frequent in the Northern Hemisphere than the Southern Hemisphere 13 in winter. However, when MCAOs are viewed as individual events regardless 14 of horizontal extent, they occur more frequently in the Southern Hemisphere. 15 MCAOs occur throughout the year, but in warm seasons and in the South- 16 ern Hemisphere they are smaller and weaker than in cold seasons and in the 17 Northern Hemisphere. Strong MCAOs have similar meteorological contexts 18 in both hemispheres: occupying the cold air sector of mid-latitude cyclones 19 which generally appear to be in their growth phase. Weak MCAOs in the 20 Southern Hemisphere are dynamically more similar to strong events in either 21 hemisphere than they are to weak events in the Northern Hemisphere. 2 22 1. Introduction 23 Marine cold air outbreaks (MCAOs) are high-impact weather events in which air masses of polar 24 or cold continental origin are advected over relatively warm open ocean. The resulting instability 25 can lead to strong boundary layer turbulence and surface heat loss from the ocean (Brümmer 1996), 26 as well as severe weather such as polar lows and boundary layer fronts (Businger 1985; Carleton 27 and Song 1997; Rasmussen and Turner 2003). 28 The intense heat loss from the sea surface in MCAOs suggests that they play a role in high 29 latitude ocean-atmosphere heat exchange, including in regions of deep water formation (Condron 30 et al. 2008; Isachsen et al. 2013). Their role in ocean-atmosphere heat exchange is disproportionate 31 to their frequency of occurrence, and in the Southern Ocean Pacific sector they dominate seasonal 32 extremes in surface sensible heat flux (Papritz et al. 2015). 33 Intense cold air outbreaks are associated with roll convection leading to organized cellular con- 34 vection downstream (Etling and Brown 1993; Muhlbauer et al. 2014). These have been studied in 35 the Northern Hemisphere though remote sensing (Brümmer and Pohlmann 2000), in situ obser- 36 vations (Hein and Brown 1988; Chou and Ferguson 1991; Brümmer 1999; Renfrew and Moore 37 1999), and modeling studies (Stage and Businger 1981; Müller and Chlond 1996; Schröter et al. 38 2005). An extensive body of knowledge on the relationship between meteorology and clouds in 39 these environments now exists for the Northern Hemisphere. 40 General circulation models tend to simulate both Northern Hemisphere MCAOs and Southern 41 Hemisphere midlatitude cyclones in general with too little stratiform cloud cover (Bodas-Salcedo 42 et al. 2014; Field et al. 2013; Williams et al. 2013). For this reason, field experiments on North- 43 ern Hemisphere MCAOs have been used for climate model evaluation and development (Bodas- 44 Salcedo et al. 2012), particularly in addressing the Southern Hemisphere radiation bias (Trenberth 3 45 and Fasullo 2010). If one assumes that some of this bias in climate models is due to cold air 46 outbreaks, case studies of Northern Hemisphere MCAOs can be used to test model changes aimed 47 at reducing biases in Southern Ocean clouds. Such work has proven useful for this goal (Bodas- 48 Salcedo et al. 2012); nonetheless it remains unknown how similar Northern Hemisphere MCAOs 49 are to those in the Southern Hemisphere. 50 There have already been several studies of Southern Hemisphere MCAOs. These first required 51 a definition of an MCAO, and all used variations on that of Kolstad and Bracegirdle (2007), who 52 proposed an MCAO index suitable for coarse resolution data sets such as reanalysis or climate 53 models. This index was based on the potential temperature difference between the surface and 54 700 hPa, divided by the pressure difference between 700 hPa and sea level. The 700 hPa pressure 55 level was chosen primarily based on what was available from reanalysis at the time. 56 Previous studies of Southern Hemisphere MCAOs used variations on this index, some using 57 700 hPa geopotential height instead of the 700 hPa - sea level pressure difference to account for 58 pressure variations (Bracegirdle and Kolstad 2010), others using simply the potential temperature 59 difference with no other reference to pressure or geopotential height (see Papritz et al. 2015, who 60 used 850 hPa rather than 700 hPa to define the lower tropospheric potential temperature). 61 Bracegirdle and Kolstad (2010) found that daily and monthly variability in Southern Ocean 62 MCAOs was largely associated with lower tropospheric temperature variability, while on seasonal 63 timescales it was also due to sea surface temperature (SST) variability. They also found that 64 wintertime interannual variability in MCAOs was associated with the Southern Annular Mode in 65 high latitude ice free regions, but not in the mid-latitudes. 66 Kolstad (2011) used an MCAO index along with the dynamic tropopause pressure to define 67 regions throughout the globe favorable for polar low development. The MCAO index generally 68 limited where polar lows could form, with most Southern Hemisphere polar lows forming in the 4 69 Pacific sector. Monthly variability in the MCAO index largely determined the frequency of occur- 70 rence of polar lows. 71 Papritz et al. (2015) used an MCAO index to show that wintertime Southern Hemisphere Pacific 72 sector MCAOs were largely associated with extratropical cyclones and with air masses that are 73 advected off of sea ice or the Antarctic continent. They also showed that MCAOs contribute to 74 one third of the wintertime surface sensible heat flux in those regions, despite occurring 9% of the 75 time. 76 We build on previous work in order to deepen the understanding of Southern Hemisphere 77 MCAOs, their similarities to and differences from their Northern Hemisphere counterparts, their 78 seasonal variability, and their meteorological context. We use an MCAO index similar to those 79 used in the above studies and confirm that its seasonality and inter-hemispheric contrasts are con- 80 sistent with previous results. We add to those previous studies by using the MCAO index to 81 identify individual MCAO events and study their composite thermodynamic and dynamic struc- 82 ture. 83 This paper is organized as follows: Section 2 describes the data used, defines the MCAO index, 84 and presents the climatological features of that index. Section 3 defines MCAO events and shows 85 their meteorological and thermodynamic context. Section 4 summarizes the results and indicates 86 future work motivated by this study. 87 2. The climatology of MCAOs 88 a. Data description 89 We use meteorological and thermodynamic output from the European Centre for Medium-Range 90 Weather Forecasting Interim Reanalysis (ERA-Interim, Dee et al. (2011)) twice-daily analysis, at 5 91 0Z and 12Z, from September 1979 to August 2014. The data is on a 1-degree equally-spaced 92 latitude-longitude grid. We also use ERA-Interim twice-daily twelve hour forecasts of surface 93 sensible heat flux (SHF) at 0Z and 12Z. All variables are instantaneous except fluxes, which are 94 12 hour means. 95 b. MCAO index definition 96 We define an MCAO index as M = θSKT − θ800 , i.e., the potential temperature difference be- 97 tween the surface skin and 800 hPa. Using the surface skin temperature rather than SST prevents 98 spurious MCAO diagnosis in areas of high sea ice cover. Positive M defines an absolutely unstable 99 lower troposphere and is our bare minimum to define a cold air outbreak (Kolstad et al. 2008). 100 M is similar to indices used in other work on cold air outbreaks in the Southern and Northern 101 Hemispheres, with slight differences in definition producing quantitative differences in the index 102 but qualitative similarities in its global structure. For example, Kolstad et al. (2008) and subsequent 103 work (Bracegirdle and Kolstad 2010; Kolstad 2011) used the potential temperature at 700 hPa, 104 while Papritz et al. (2015) used 850 hPa potential temperature. We test all of these indices and find 105 that using the 800 hPa level produces more high latitude MCAOs than using the 700 hPa level, 106 while using 850 hPa produces a similar climatology to 800 hPa at middle and high latitudes but 107 identifies more MCAOs in the subtropics (not shown). 108 To give the reader a greater intuition for what the MCAO index captures, we include as sup- 109 plementary material a 1-year animation of the twice-daily M in the extra-tropics. To emphasize 110 MCAO events, we exclude values of M ≤ 0. 6 111 c. Climatology of the MCAO index 112 Figure 1 shows the seasonal relative frequency of occurrence (RFO) with which grid points 113 have M > 0 for a) area-weighted hemispheric means and b) zonal means. The relative frequency 114 is determined by dividing the number of hits of M > 0 by the number of opportunities; i.e. the 115 number of oceanic grid points in a hemisphere or latitude band over the length of the time series. 116 Seasons are defined as follows: winter is DJF in the Northern Hemisphere and JJA in the Southern 117 Hemisphere; spring is MAM in the Northern Hemisphere and SON in the Southern Hemisphere, 118 and so forth. 119 Fig. 1 shows that M is greater than zero most often in winter in both hemispheres, but also 120 shows that this occurs almost as often in shoulder seasons as it does in winter, particularly in the 121 Southern Hemisphere. The Northern Hemisphere annual cycle in M is much greater than that of 122 the Southern Hemisphere. 123 There is an apparent discrepancy between Fig. 1a and 1b: while 1a shows both hemispheres 124 having similar RFO in spring and autumn, 1b generally shows the RFO being greater in the North- 125 ern Hemisphere during those two seasons. This is because the RFO is calculated with respect to 126 the number of grid points available for an MCAO, i.e., oceanic grid points. In the mid-latitudes, 127 the Northern Hemisphere has far fewer of these than the Southern Hemisphere, while the oppo- 128 site is true at high latitudes. This means that the Northern Hemisphere-average RFO places less 129 weight on the mid-latitude grid points than would appear from Fig. 1b, while the Southern Hemi- 130 sphere average places less weight on the high latitude grid points. Both of these act to increase the 131 hemisphere-mean RFO in the Southern Hemisphere relative to the Northern Hemisphere. 7 132 Figure 1b also shows that at Southern Hemisphere high latitudes, the MCAO index is greater 133 than zero more often in autumn than in winter. This is because in winter the high latitudes have a 134 high sea ice cover and therefore a low surface skin temperature, reducing the MCAO index. 135 Figure 2 compares the wintertime RFO of all MCAOs to the magnitude of very strong MCAOs, 136 i.e., the 95th percentile of the index. The top two panels show that in both hemispheres M > 0 most 137 often in regions downstream of cold continents or areas of high sea ice cover, as well as midlatitude 138 regions with a strong SST gradient, most notably the Gulf Stream and Kuroshio currents but 139 also the Southern Brazil current, the Argulas current near South Africa, and the Indian Ocean 140 subtropical front. 141 The differences between the two hemispheres are even more apparent when the strongest 5% 142 of MCAOs are compared, as in the bottom two panels of Fig. 2. Northern Hemisphere MCAOs 143 are not only more frequent but also much stronger than those in the Southern Hemisphere. Fig. 144 2d also highlights the Pacific sector as the main area where strong MCAOs occur in the Southern 145 Hemisphere. These results are consistent with those of Bracegirdle and Kolstad (2010) and Kolstad 146 (2011). 147 Comparing panels b) and d) of Fig. 2, we see that MCAOs occur about as frequently in the 148 Indian Ocean sector of the Southern Ocean as they do in the Pacific sector, but they are weaker 149 in the former than the latter. The local RFO maximum in the high latitude Indian Ocean sector 150 coincides with regions of maximum extra-tropical cyclone frequency (Simmonds et al. 2003). As 151 these storms move poleward (Hoskins and Hodges 2005) they advect very cold air masses, often 152 originating over sea ice, equatorward. In the Pacific sector, the Antarctic topography, with the Ross 153 and Amundsen Seas and associated ice shelves, particularly the Ross Ice Shelf corridor (Parish and 154 Bromwich 2007), provides the most favorable conditions for equatorward advection of polar air 155 masses. 8 156 3. The horizontal and vertical structure of MCAO events 157 a. MCAO event definition and size distribution 158 Having established some of the climatological features of our MCAO index, we now use this 159 index to define MCAO events, to study the horizontal and vertical structure of typical MCAO 160 events, and to compare events between different regions, in particular the two hemispheres. We 161 define an MCAO event as an instantaneous closed contour encircling regions with M > 0; events 162 that occupy fewer than eight grid points are excluded. Apart from the average event size, our 163 results are not sensitive to the eight-gridpoint choice. 164 The strength of the event is defined by Mmax , the maximum value of M within the closed contour. 165 Weak events have 0 < Mmax ≤ 3 K, moderate events have 3 < Mmax ≤ 6 K, and strong events have 166 Mmax > 6 K. This range of strengths distinguishes weak events, which can occur year-round, from 167 strong events, which occur mostly in winter. The particular numbers are chosen such that the total 168 number of Southern Hemisphere events decreases with each successive category. 169 As previously mentioned, Fig. 2 implies that some MCAOs are associated with advection of 170 polar air masses over high latitude oceans, while others involve the advection of continental or 171 cold marine air over SST gradients associated with western boundary currents. This distinction 172 applies more in the Northern Hemisphere than in the Southern Hemisphere. In the former, MCAO 173 events appear to be mostly stationary or to travel eastward and die out over the open ocean. In 174 the latter, an MCAO originating at high latitudes often travels northward into the midlatitudes; see 175 supplementary material for an example of this. 176 To account for the different origins of high latitude and mid-latitude MCAOs, we further divide 177 MCAO events into those existing between 35 and 55 degrees north or south (mid-latitude events) 9 178 and those existing between 55 and 75 north or south (high latitude events). Events that overlap are 179 classified by the location of the maximum in the MCAO index. 180 Fig. 3 shows the number of MCAO events per season and latitude band, comparing the Northern 181 and Southern Hemisphere as well as middle and high latitudes. Because events are counted at 182 single instances in time, the same ”weather event” will lead to multiple MCAO events by our 183 counting; this should be kept in mind when considering raw numbers of events. Since the data is 184 twice-daily, if we assume that an event typically lasts around three days, then a count of 60 per 185 season corresponds to roughly 10 independent cold air outbreaks. 186 Fig. 3 shows that when MCAOs are viewed as events, rather than on a gridpoint-by-gridpoint 187 basis as in Sec. 2c, they appear more frequent in the Southern Hemisphere than in the Northern 188 Hemisphere. There are two plausible, mutually independent reasons for this: MCAOs are smaller 189 in the Southern Hemisphere and so occupy fewer gridpoints, and the greater ocean coverage in 190 the Southern Hemisphere provides more opportunities for separate MCAO events. The latter is 191 certainly true, to investigate the former we will explore the sizes of MCAO events. 192 We first note that all latitude belts in Fig. 3 show a weaker annual cycle than that shown in 193 Fig. 1, implying that in warmer seasons MCAOs are smaller and so occupy fewer grid points 194 than in colder seasons. There is almost no annual cycle in the number of events in the Southern 195 Hemisphere mid-latitudes, with small and likely weak events occurring throughout the year, con- 196 sistent with the weak annual cycle in both extratropical cyclones and sea surface temperatures in 197 the Southern Ocean (Trenberth 1991). This further suggests that weaker MCAOs are smaller than 198 stronger ones no matter when or where they occur. 199 To investigate the size of MCAO events, we could simply calculate their areal extent. However, 200 it’s also interesting to know how they tend to be oriented, e.g., if they are usually circular or tend 201 to be elongated in a particular direction. We define a zonal (meridional) length scale for all events: 10 202 the maximum length of the closed contour in the east-west (north-south) direction. We then define 203 the horizontal scale of events by their zonal length scale. We define their orientation by their aspect 204 ratio: the ratio of zonal length scale to meridional length scale. 205 We hypothesize that the size and/or shape of MCAO events may vary according to a) location, 206 b) season, and c) strength. We find that the strength of MCAO events is most related to their 207 size, with stronger events being much larger than weaker ones, as is shown in Figure 4. However, 208 for the same strength category, Southern Hemisphere events are generally larger than Northern 209 Hemisphere events, with the exception of strong events in the Northern Hemisphere mid-latitudes. 210 We find that the seasonality of event sizes is largely explained by the seasonality of their strength 211 (not shown). 212 In Section 3b, we will show that MCAOs are associated with mid-latitude cyclones, and the 213 size and shape of MCAOs are set primarily by this large-scale meteorology as well as the size 214 and shape of the surface temperature gradient. High latitude MCAOs are smaller than their mid- 215 latitude counterparts partly because the extratropical cyclones they are embedded in are smaller, 216 but in the Northern Hemisphere in particular their size is also limited by the ocean basins they tend 217 to occur in (e.g., the Labrador or Norwegian Seas, see Fig. 2). 218 The second panel in Fig 4 shows the aspect ratio of MCAOs. Most events are fairly round 219 — they have similar zonal and meridional scales. However, Southern Hemisphere high latitude 220 events have a larger zonal than meridional length scale because they occur along a temperature 221 gradient that is mainly north-south. Northern Hemisphere mid-latitude events occur on tempera- 222 ture gradients with both north-south and east-west components (mainly the tilted Gulf Stream and 223 Kuroshio currents), making them less zonally elongated than Southern Hemisphere high latitude 224 events. In the Northern Hemisphere high latitudes, basin sizes and geometries generally result in 225 MCAO events that are somewhat longer meridionally than zonally. 11 226 b. Composite meteorology of MCAOs 227 In order to study the synoptic conditions of MCAOs, we composite meteorological and thermo- 228 dynamic variables on similar events. In order to be considered similar, events have to be of the 229 same classification of strength and they have to be roughly the same size. Events are selected by 230 size according to the strength category being composited. Weak events are included in their com- 231 posite if they have both zonal and meridional length scales between 350 and 750 km; strong events 232 are included in their composite if they have length scales between 1750 and 2250 km. Intermediate 233 strength events are not shown, but are more similar to strong events than weak ones. 234 Additionally, composited events must be in the same geographical region, as different geome- 235 tries of surface temperature gradient lead to a different large-scale flow pattern most conducive to 236 lower tropospheric cold advection (this applies mainly in the Northern Hemisphere high latitudes 237 but is used globally). Composites are performed in six regions of both the Northern Hemisphere: 238 the Bering Sea, the Norwegian Sea, the Labrador Sea, the Kuroshio, the Gulf Stream, and the 239 North Atlantic; and the Southern Hemisphere: the Indian polar front, the north Ross Sea, the north 240 Bellingshausen Sea, the Indian subtropical front, the Brazil Current, and the Argulas current. 241 Composites are calculated separately for all seasons, but we find little difference in the compos- 242 ites between seasons that is not attributable to MCAO strength – e.g., strong events in spring are 243 similar to strong events in winter, but occur less often. We show results for winter only. 244 Prior to compositing, each event is interpolated to a 4000 by 2000 km grid, with 100 km grid 245 spacing, using nearest-neighbor interpolation. This grid is centered on the location of the maxi- 246 mum value of M within the closed contour of each event; this point is the origin in figures below. 12 247 Figure 5 shows the composited sea level pressure, 10 m horizontal winds, sea ice cover, and sur- 248 face sensible heat flux surrounding the MCAO. Within each hemisphere and latitude belt, results 249 are similar across different ocean basins, so only one example per latitude belt is shown. 250 We find that globally MCAOs are associated with the cold air sector of mid-latitude cyclones, 251 as was also found by Kolstad et al. (2008) for the North Atlantic and Papritz et al. (2015) for the 252 South Pacific. In both hemispheres, high latitude strong events are near well-developed closed 253 surface lows, with winds optimal for cold air advection. High latitude weak events are associated 254 with weaker average surface lows. High latitude strong events show similar surface wind speeds 255 in both hemispheres, but Southern hemiphere weak events have much greater wind speeds than 256 Northern Hemisphere weak events. 257 Weak event composites, particularly in the Northern Hemisphere, likely represent a mix of cy- 258 clones in various stages of growth or decay, and may also include large-scale flow around a weak 259 low that is not part of an extratropical cyclone (Papritz et al. (2015) found that roughly 25% of 260 Pacific sector Southern Ocean MCAOs were not associated with mid-latitude cyclones, and weak 261 events are likely to be over-represented in this category.) 262 Fig. 5 also shows the composite surface sensible heat flux (SHF, only values greater than 50 263 W m−2 are shown) and sea ice cover. Average SHF ranges from over 200 W m−2 at the center 264 of strong events to 50-100 W m−2 in weak events and at the edges of strong ones. Northern 265 Hemisphere events have greater SHF than Southern Hemisphere events despite comparable or 266 weaker surface wind speeds, implying that even within similar strength classifications the average 267 air-sea temperature difference is greater in Northern Hemisphere events. 268 Figure 6 shows geopotential height anomalies in a west-to-east cross-section through the com- 269 posites shown in Fig. 5. Consistent with Fig. 5, severe events are associated with a strong upper- 270 level trough that exhibits a westward tilt with height, implying that many of these cyclones are 13 271 still in the growth phase. Southern Hemisphere weak events are also associated with upper-level 272 troughs, again weaker on average than for the strong events. However, Northern Hemisphere weak 273 events exhibit an upper-level ridge to the east (in the Labrador Sea) and to the west in the Gulf 274 Stream. These are likely associated with a range of meteorological conditions, all of which allow 275 weak lower tropospheric cold advection. 276 Next we examine similarities and differences in boundary layer structure in the composite 277 MCAOs. Figure 7 shows composites of sea level pressure, the difference in horizontal winds 278 between 10 m and 800 hPa (u800 − u10m ), and boundary layer height. 279 The change in wind speed from the surface to 800 hPa is fairly weak at the center of the MCAO, 280 but increases in magnitude downstream. It is likely that the surface-driven convection is mixing 281 momentum effectively within the boundary layer near the center of the MCAO. Outside of this 282 region the increase in wind speed ranges from 0-10 m s−1 . 283 The ERA-Interim boundary layer height is diagnosed with a dry Richardson number, and so 284 indicates the base of boundary layer clouds rather than their top (von Engeln and Teixeira 2013). 285 The PBL deepens from a few hundred meters to 1-2 km downstream of the MCAO maximum. The 286 location of the maximum boundary layer depth is downstream of that of the maximum surface heat 287 flux shown in Fig. 5. Boundary layer deepening is driven both by surface heating and large scale 288 dynamics, as air masses pass from regions of large scale subsidence to regions of large scale ascent. 289 Fig 8 shows cross-section composites of thermodynamic variables in the lower troposphere for 290 severe events in both hemispheres. Instead of running from west to east as in Fig. 6, cross sections 291 run along the direction of the 800 hPa winds at the location of the maximum MCAO index (the 292 origin of the composites). The locations of the cross sections are shown schematically as black 293 thick lines in Fig 5. We composite liquid water potential temperature, θl = θ − Lv /C p ∗ ql and total 294 water specific humidity, qT = qv + ql + qi , from 1000 to 700 hPa. 14 295 Although the along-wind cross section is not a perfect trajectory, it depicts the change in bound- 296 ary layer structure as cold air masses pass over warm water. The Northern Hemisphere cross 297 sections qualitatively match the drop-sonde measurements of a cold air outbreak case near Sval- 298 bard in Hartmann et al. (1997). Upwind of the MCAO maximum, the boundary layer is in a regime 299 of large-scale subsidence and is strongly stable. As it is advected over warm water and toward a 300 region of large-scale ascent, it is heated from below and destabilized. Both surface fluxes and 301 entrainment warm the boundary layer until it evolves into a well-mixed or weakly stable layer 302 downstream. This is more effective in the Northern Hemisphere systems than in the Southern 303 Hemisphere due to higher sensible heat fluxes. In fact, Southern Hemisphere events begin to cool 304 and dry downstream of the MCAO maximum. Qualitatively similar features are seen in PBL 305 structure of weak MCAO composites (not shown). 306 4. Summary and conclusions 307 We use an MCAO index to develop a comprehensive climatology of Southern Hemisphere 308 MCAOs, including their extremes and frequency of occurrence, and compare that to a similar 309 climatology of Northern Hemisphere MCAOs. We use this index to define MCAO events and 310 compare the frequency, size and shape, synoptic environment, and PBL structure among compos- 311 ite events for different hemispheres and regions. 312 Our most important results are: 313 1. The frequency of occurrence of MCAOs depends on how they are defined. From a gridpoint- 314 by-gridpoint perspective, MCAOs occur most often in Northern Hemisphere winter. But when 315 classified as discrete events, with each counting as a single MCAO regardless of how many grid- 316 points it occupies, MCAO frequency of occurrence and seasonality looks different. They are most 317 frequent in Southern Hemisphere autumn, and have only a weak annual cycle in frequency in the 15 318 Southern Hemisphere. Many MCAOs exist outside of winter, particularly in shoulder seasons and 319 Southern Hemisphere summer, but they are smaller and weaker than their winter counterparts. 320 2. Strong (Mmax > 6 K) MCAO events have similar meteorological contexts and boundary layer 321 structure in both hemispheres. They occur in the cold air sector of extratropical cyclones optimally 322 positioned to advect polar or very cold continental air masses over warm water. The high surface 323 heat fluxes within MCAOs contribute to the deepening of the boundary layer by 500-1800 m. 324 3. In the Southern Hemisphere, weak MCAOs (0 < Mmax < 3K) appear dynamically similar 325 to strong MCAOs. Weak Northern Hemisphere events do not have a characteristic meteorolog- 326 ical context, especially at mid-latitudes. This is likely because greater surface skin temperature 327 gradients exist in the Northern Hemisphere and occur with a greater range of geometries than in 328 the Southern Hemisphere, allowing for a greater range of synoptic environments conducive to the 329 advection of moderately cold air masses. 330 Northern Hemisphere MCAOs, for which there are numerous case studies with detailed observa- 331 tions, are often used as an analogue for the Southern Hemisphere systems that produce the positive 332 shortwave cloud forcing bias in most climate models. Although the frequency of MCAOs is such 333 that they are unlikely to be the source of this bias, we find that Southern Hemisphere MCAOs 334 are similar to those in the Northern Hemisphere, especially severe ones. Furthermore, simulated 335 MCAOs may contribute to the Southern Ocean cloud bias disproportionate to their frequency of 336 occurrence. A logical next step in this work is to examine the radiative impact of MCAOs and 337 their possible significance for shortwave radiation errors in climate models. 338 Acknowledgments. This research is supported by ARC Discovery Grant (DP130100869) and the 339 ARC Centre of Excellence for Climate System Science (CE110001028). 16 340 References 341 Bodas-Salcedo, A., K. D. Williams, P. R. Field, and A. P. Lock, 2012: The Surface Downwelling 342 Solar Radiation Surplus over the Southern Ocean in the Met Office Model: The Role of Mid- 343 latitude Cyclone Clouds. J. Climate, 25 (21), 7467–7486, doi:10.1175/jcli-d-11-00702.1, URL 344 http://dx.doi.org/10.1175/jcli-d-11-00702.1. Bodas-Salcedo, A., and Coauthors, 2014: Origins of the Solar Radiation Biases over the Southern 345 346 Ocean in CFMIP2 Models∗.J. 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D., and Coauthors, 2013: The Transpose-AMIP II Experiment and Its Application 442 to the Understanding of Southern Ocean Cloud Biases in Climate Models. J. Climate, 26 (10), 443 3258–3274, 10.1175/jcli-d-12-00429.1, http://dx.doi.org/10.1175/jcli-d-12-00429.1. Storm Tracks in the Southern Hemisphere. 21 J. Atmos. Sci., 444 LIST OF FIGURES 445 Fig. 1. 446 447 Fig. 2. 448 449 450 451 Fig. 3. 452 453 454 Fig. 4. 455 456 457 Fig. 5. 458 459 460 461 Fig. 6. 462 463 464 Fig. 7. 465 466 467 468 469 Fig. 8. Gridpoint-by-gridpoint area-weighted relative frequency of occurrence of M > 0 by a) hemisphere and b) zonal mean. . . . . . . . . . . . . . . . . . . . 23 wintertime M > 0 relative frequency of occurrence in a) the northern hemisphere and b) the southern hemisphere. The scale in a) and b) is quadratic to allow easier comparison between hemispheres. The 95th percentile of M in wintertime in c) the northern hemisphere and d) the southern hemisphere. . . . . . . . . . . . . . . . . . . . 24 Number of MCAO events per season, per latitude band. Events are defined from instantaneous snapshots; multiple events defined here may constitute a single, multi-day cold air outbreak episode. . . . . . . . . . . . . . . . . . . . . . 25 Size and shape of MCAO events according to location and strength: a) zonal length scale, and b) aspect ratio. The length of the colored bars indicates the 50th percentile of the data, while the black bars show the 25th and 75th percentiles. . . . . . . . . . . . 26 Composite sea level pressure (black lines), 10 m winds (arrows), sea ice cover (blue), and surface sensible heat flux (red) for MCAO events. Regions shown: a) and c) Labrador Sea; b) and d) Gulf Stream; e) and g) north Ross Sea; f) and h) Indian Ocean midlatitudes. Solid black lines show locations of cross sections used in Fig. 8. . . . . . . . . . . Composite geopotential height anomalies [dkm] in west-to-east cross section for the same regions shown in Fig 5. The origin is the location of the maximum MCAO index within each event used in the composite. . . . . . . . . . . . . . . . . 27 . 28 Composite sea level pressure (lines), shear in the 10m to 800 hPa horizontal winds [m s−1 , arrows], and boundary layer height [m, colors], as in Fig 5 . . . . . . . . . . 29 Cross section composites of boundary layer structure in example MCAOs. From left to right, the cross section runs along the direction of the 800 hPa wind at the location of maximum MCAO index (see text and Fig 5). Panels a, b, e, and f show liquid water potential temperature [K]; panels c, d, g, and h show total water specific humidity [g kg−1 ]. . . . . 30 22 470 471 F IG . 1. Gridpoint-by-gridpoint area-weighted relative frequency of occurrence of M > 0 by a) hemisphere and b) zonal mean. 23 472 F IG . 2. wintertime M > 0 relative frequency of occurrence in a) the northern hemisphere and b) the southern 473 hemisphere. The scale in a) and b) is quadratic to allow easier comparison between hemispheres. The 95th 474 percentile of M in wintertime in c) the northern hemisphere and d) the southern hemisphere. 24 475 476 F IG . 3. Number of MCAO events per season, per latitude band. Events are defined from instantaneous snapshots; multiple events defined here may constitute a single, multi-day cold air outbreak episode. 25 477 F IG . 4. Size and shape of MCAO events according to location and strength: a) zonal length scale, and b) 478 aspect ratio. The length of the colored bars indicates the 50th percentile of the data, while the black bars show 479 the 25th and 75th percentiles. 26 480 F IG . 5. Composite sea level pressure (black lines), 10 m winds (arrows), sea ice cover (blue), and surface 481 sensible heat flux (red) for MCAO events. Regions shown: a) and c) Labrador Sea; b) and d) Gulf Stream; e) 482 and g) north Ross Sea; f) and h) Indian Ocean midlatitudes. Solid black lines show locations of cross sections 483 used in Fig. 8. 27 484 F IG . 6. Composite geopotential height anomalies [dkm] in west-to-east cross section for the same regions 485 shown in Fig 5. The origin is the location of the maximum MCAO index within each event used in the composite. 28 486 487 F IG . 7. Composite sea level pressure (lines), shear in the 10m to 800 hPa horizontal winds [m s−1 , arrows], and boundary layer height [m, colors], as in Fig 5 29 488 F IG . 8. Cross section composites of boundary layer structure in example MCAOs. From left to right, the 489 cross section runs along the direction of the 800 hPa wind at the location of maximum MCAO index (see text 490 and Fig 5). Panels a, b, e, and f show liquid water potential temperature [K]; panels c, d, g, and h show total 491 water specific humidity [g kg−1 ]. 30