Download The climatology, meteorology, and boundary layer structure of

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The climatology, meteorology, and boundary layer structure of marine cold
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air outbreaks in both hemispheres
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Jennifer Fletcher∗ , Shannon Mason, and Christian Jakob
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School of Earth, Atmosphere, and Environment, Monash University, Clayton, VIC, Australia
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∗ Corresponding author address: Jennifer Fletcher, Monash University, 9 Rainforest Walk, Clayton,
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VIC, Australia 3800.
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E-mail: [email protected]
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ABSTRACT
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A comparison of marine cold air outbreaks (MCAOs) in the Northern and
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Southern Hemispheres is presented, with attention to their seasonality, fre-
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quency of occurence, and strength as measured by a cold air outbreak index.
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When considered on a gridpoint-by-gridpoint basis, MCAOs are more severe
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and more frequent in the Northern Hemisphere than the Southern Hemisphere
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in winter. However, when MCAOs are viewed as individual events regardless
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of horizontal extent, they occur more frequently in the Southern Hemisphere.
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MCAOs occur throughout the year, but in warm seasons and in the South-
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ern Hemisphere they are smaller and weaker than in cold seasons and in the
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Northern Hemisphere. Strong MCAOs have similar meteorological contexts
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in both hemispheres: occupying the cold air sector of mid-latitude cyclones
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which generally appear to be in their growth phase. Weak MCAOs in the
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Southern Hemisphere are dynamically more similar to strong events in either
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hemisphere than they are to weak events in the Northern Hemisphere.
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1. Introduction
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Marine cold air outbreaks (MCAOs) are high-impact weather events in which air masses of polar
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or cold continental origin are advected over relatively warm open ocean. The resulting instability
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can lead to strong boundary layer turbulence and surface heat loss from the ocean (Brümmer 1996),
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as well as severe weather such as polar lows and boundary layer fronts (Businger 1985; Carleton
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and Song 1997; Rasmussen and Turner 2003).
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The intense heat loss from the sea surface in MCAOs suggests that they play a role in high
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latitude ocean-atmosphere heat exchange, including in regions of deep water formation (Condron
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et al. 2008; Isachsen et al. 2013). Their role in ocean-atmosphere heat exchange is disproportionate
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to their frequency of occurrence, and in the Southern Ocean Pacific sector they dominate seasonal
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extremes in surface sensible heat flux (Papritz et al. 2015).
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Intense cold air outbreaks are associated with roll convection leading to organized cellular con-
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vection downstream (Etling and Brown 1993; Muhlbauer et al. 2014). These have been studied in
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the Northern Hemisphere though remote sensing (Brümmer and Pohlmann 2000), in situ obser-
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vations (Hein and Brown 1988; Chou and Ferguson 1991; Brümmer 1999; Renfrew and Moore
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1999), and modeling studies (Stage and Businger 1981; Müller and Chlond 1996; Schröter et al.
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2005). An extensive body of knowledge on the relationship between meteorology and clouds in
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these environments now exists for the Northern Hemisphere.
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General circulation models tend to simulate both Northern Hemisphere MCAOs and Southern
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Hemisphere midlatitude cyclones in general with too little stratiform cloud cover (Bodas-Salcedo
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et al. 2014; Field et al. 2013; Williams et al. 2013). For this reason, field experiments on North-
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ern Hemisphere MCAOs have been used for climate model evaluation and development (Bodas-
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Salcedo et al. 2012), particularly in addressing the Southern Hemisphere radiation bias (Trenberth
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and Fasullo 2010). If one assumes that some of this bias in climate models is due to cold air
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outbreaks, case studies of Northern Hemisphere MCAOs can be used to test model changes aimed
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at reducing biases in Southern Ocean clouds. Such work has proven useful for this goal (Bodas-
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Salcedo et al. 2012); nonetheless it remains unknown how similar Northern Hemisphere MCAOs
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are to those in the Southern Hemisphere.
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There have already been several studies of Southern Hemisphere MCAOs. These first required
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a definition of an MCAO, and all used variations on that of Kolstad and Bracegirdle (2007), who
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proposed an MCAO index suitable for coarse resolution data sets such as reanalysis or climate
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models. This index was based on the potential temperature difference between the surface and
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700 hPa, divided by the pressure difference between 700 hPa and sea level. The 700 hPa pressure
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level was chosen primarily based on what was available from reanalysis at the time.
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Previous studies of Southern Hemisphere MCAOs used variations on this index, some using
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700 hPa geopotential height instead of the 700 hPa - sea level pressure difference to account for
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pressure variations (Bracegirdle and Kolstad 2010), others using simply the potential temperature
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difference with no other reference to pressure or geopotential height (see Papritz et al. 2015, who
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used 850 hPa rather than 700 hPa to define the lower tropospheric potential temperature).
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Bracegirdle and Kolstad (2010) found that daily and monthly variability in Southern Ocean
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MCAOs was largely associated with lower tropospheric temperature variability, while on seasonal
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timescales it was also due to sea surface temperature (SST) variability. They also found that
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wintertime interannual variability in MCAOs was associated with the Southern Annular Mode in
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high latitude ice free regions, but not in the mid-latitudes.
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Kolstad (2011) used an MCAO index along with the dynamic tropopause pressure to define
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regions throughout the globe favorable for polar low development. The MCAO index generally
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limited where polar lows could form, with most Southern Hemisphere polar lows forming in the
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Pacific sector. Monthly variability in the MCAO index largely determined the frequency of occur-
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rence of polar lows.
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Papritz et al. (2015) used an MCAO index to show that wintertime Southern Hemisphere Pacific
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sector MCAOs were largely associated with extratropical cyclones and with air masses that are
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advected off of sea ice or the Antarctic continent. They also showed that MCAOs contribute to
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one third of the wintertime surface sensible heat flux in those regions, despite occurring 9% of the
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time.
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We build on previous work in order to deepen the understanding of Southern Hemisphere
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MCAOs, their similarities to and differences from their Northern Hemisphere counterparts, their
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seasonal variability, and their meteorological context. We use an MCAO index similar to those
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used in the above studies and confirm that its seasonality and inter-hemispheric contrasts are con-
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sistent with previous results. We add to those previous studies by using the MCAO index to
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identify individual MCAO events and study their composite thermodynamic and dynamic struc-
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ture.
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This paper is organized as follows: Section 2 describes the data used, defines the MCAO index,
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and presents the climatological features of that index. Section 3 defines MCAO events and shows
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their meteorological and thermodynamic context. Section 4 summarizes the results and indicates
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future work motivated by this study.
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2. The climatology of MCAOs
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a. Data description
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We use meteorological and thermodynamic output from the European Centre for Medium-Range
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Weather Forecasting Interim Reanalysis (ERA-Interim, Dee et al. (2011)) twice-daily analysis, at
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0Z and 12Z, from September 1979 to August 2014. The data is on a 1-degree equally-spaced
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latitude-longitude grid. We also use ERA-Interim twice-daily twelve hour forecasts of surface
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sensible heat flux (SHF) at 0Z and 12Z. All variables are instantaneous except fluxes, which are
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12 hour means.
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b. MCAO index definition
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We define an MCAO index as M = θSKT − θ800 , i.e., the potential temperature difference be-
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tween the surface skin and 800 hPa. Using the surface skin temperature rather than SST prevents
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spurious MCAO diagnosis in areas of high sea ice cover. Positive M defines an absolutely unstable
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lower troposphere and is our bare minimum to define a cold air outbreak (Kolstad et al. 2008).
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M is similar to indices used in other work on cold air outbreaks in the Southern and Northern
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Hemispheres, with slight differences in definition producing quantitative differences in the index
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but qualitative similarities in its global structure. For example, Kolstad et al. (2008) and subsequent
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work (Bracegirdle and Kolstad 2010; Kolstad 2011) used the potential temperature at 700 hPa,
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while Papritz et al. (2015) used 850 hPa potential temperature. We test all of these indices and find
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that using the 800 hPa level produces more high latitude MCAOs than using the 700 hPa level,
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while using 850 hPa produces a similar climatology to 800 hPa at middle and high latitudes but
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identifies more MCAOs in the subtropics (not shown).
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To give the reader a greater intuition for what the MCAO index captures, we include as sup-
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plementary material a 1-year animation of the twice-daily M in the extra-tropics. To emphasize
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MCAO events, we exclude values of M ≤ 0.
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c. Climatology of the MCAO index
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Figure 1 shows the seasonal relative frequency of occurrence (RFO) with which grid points
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have M > 0 for a) area-weighted hemispheric means and b) zonal means. The relative frequency
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is determined by dividing the number of hits of M > 0 by the number of opportunities; i.e. the
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number of oceanic grid points in a hemisphere or latitude band over the length of the time series.
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Seasons are defined as follows: winter is DJF in the Northern Hemisphere and JJA in the Southern
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Hemisphere; spring is MAM in the Northern Hemisphere and SON in the Southern Hemisphere,
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and so forth.
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Fig. 1 shows that M is greater than zero most often in winter in both hemispheres, but also
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shows that this occurs almost as often in shoulder seasons as it does in winter, particularly in the
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Southern Hemisphere. The Northern Hemisphere annual cycle in M is much greater than that of
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the Southern Hemisphere.
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There is an apparent discrepancy between Fig. 1a and 1b: while 1a shows both hemispheres
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having similar RFO in spring and autumn, 1b generally shows the RFO being greater in the North-
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ern Hemisphere during those two seasons. This is because the RFO is calculated with respect to
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the number of grid points available for an MCAO, i.e., oceanic grid points. In the mid-latitudes,
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the Northern Hemisphere has far fewer of these than the Southern Hemisphere, while the oppo-
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site is true at high latitudes. This means that the Northern Hemisphere-average RFO places less
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weight on the mid-latitude grid points than would appear from Fig. 1b, while the Southern Hemi-
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sphere average places less weight on the high latitude grid points. Both of these act to increase the
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hemisphere-mean RFO in the Southern Hemisphere relative to the Northern Hemisphere.
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Figure 1b also shows that at Southern Hemisphere high latitudes, the MCAO index is greater
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than zero more often in autumn than in winter. This is because in winter the high latitudes have a
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high sea ice cover and therefore a low surface skin temperature, reducing the MCAO index.
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Figure 2 compares the wintertime RFO of all MCAOs to the magnitude of very strong MCAOs,
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i.e., the 95th percentile of the index. The top two panels show that in both hemispheres M > 0 most
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often in regions downstream of cold continents or areas of high sea ice cover, as well as midlatitude
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regions with a strong SST gradient, most notably the Gulf Stream and Kuroshio currents but
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also the Southern Brazil current, the Argulas current near South Africa, and the Indian Ocean
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subtropical front.
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The differences between the two hemispheres are even more apparent when the strongest 5%
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of MCAOs are compared, as in the bottom two panels of Fig. 2. Northern Hemisphere MCAOs
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are not only more frequent but also much stronger than those in the Southern Hemisphere. Fig.
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2d also highlights the Pacific sector as the main area where strong MCAOs occur in the Southern
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Hemisphere. These results are consistent with those of Bracegirdle and Kolstad (2010) and Kolstad
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(2011).
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Comparing panels b) and d) of Fig. 2, we see that MCAOs occur about as frequently in the
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Indian Ocean sector of the Southern Ocean as they do in the Pacific sector, but they are weaker
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in the former than the latter. The local RFO maximum in the high latitude Indian Ocean sector
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coincides with regions of maximum extra-tropical cyclone frequency (Simmonds et al. 2003). As
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these storms move poleward (Hoskins and Hodges 2005) they advect very cold air masses, often
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originating over sea ice, equatorward. In the Pacific sector, the Antarctic topography, with the Ross
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and Amundsen Seas and associated ice shelves, particularly the Ross Ice Shelf corridor (Parish and
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Bromwich 2007), provides the most favorable conditions for equatorward advection of polar air
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masses.
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3. The horizontal and vertical structure of MCAO events
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a. MCAO event definition and size distribution
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Having established some of the climatological features of our MCAO index, we now use this
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index to define MCAO events, to study the horizontal and vertical structure of typical MCAO
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events, and to compare events between different regions, in particular the two hemispheres. We
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define an MCAO event as an instantaneous closed contour encircling regions with M > 0; events
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that occupy fewer than eight grid points are excluded. Apart from the average event size, our
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results are not sensitive to the eight-gridpoint choice.
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The strength of the event is defined by Mmax , the maximum value of M within the closed contour.
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Weak events have 0 < Mmax ≤ 3 K, moderate events have 3 < Mmax ≤ 6 K, and strong events have
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Mmax > 6 K. This range of strengths distinguishes weak events, which can occur year-round, from
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strong events, which occur mostly in winter. The particular numbers are chosen such that the total
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number of Southern Hemisphere events decreases with each successive category.
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As previously mentioned, Fig. 2 implies that some MCAOs are associated with advection of
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polar air masses over high latitude oceans, while others involve the advection of continental or
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cold marine air over SST gradients associated with western boundary currents. This distinction
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applies more in the Northern Hemisphere than in the Southern Hemisphere. In the former, MCAO
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events appear to be mostly stationary or to travel eastward and die out over the open ocean. In
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the latter, an MCAO originating at high latitudes often travels northward into the midlatitudes; see
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supplementary material for an example of this.
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To account for the different origins of high latitude and mid-latitude MCAOs, we further divide
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MCAO events into those existing between 35 and 55 degrees north or south (mid-latitude events)
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and those existing between 55 and 75 north or south (high latitude events). Events that overlap are
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classified by the location of the maximum in the MCAO index.
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Fig. 3 shows the number of MCAO events per season and latitude band, comparing the Northern
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and Southern Hemisphere as well as middle and high latitudes. Because events are counted at
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single instances in time, the same ”weather event” will lead to multiple MCAO events by our
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counting; this should be kept in mind when considering raw numbers of events. Since the data is
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twice-daily, if we assume that an event typically lasts around three days, then a count of 60 per
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season corresponds to roughly 10 independent cold air outbreaks.
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Fig. 3 shows that when MCAOs are viewed as events, rather than on a gridpoint-by-gridpoint
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basis as in Sec. 2c, they appear more frequent in the Southern Hemisphere than in the Northern
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Hemisphere. There are two plausible, mutually independent reasons for this: MCAOs are smaller
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in the Southern Hemisphere and so occupy fewer gridpoints, and the greater ocean coverage in
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the Southern Hemisphere provides more opportunities for separate MCAO events. The latter is
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certainly true, to investigate the former we will explore the sizes of MCAO events.
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We first note that all latitude belts in Fig. 3 show a weaker annual cycle than that shown in
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Fig. 1, implying that in warmer seasons MCAOs are smaller and so occupy fewer grid points
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than in colder seasons. There is almost no annual cycle in the number of events in the Southern
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Hemisphere mid-latitudes, with small and likely weak events occurring throughout the year, con-
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sistent with the weak annual cycle in both extratropical cyclones and sea surface temperatures in
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the Southern Ocean (Trenberth 1991). This further suggests that weaker MCAOs are smaller than
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stronger ones no matter when or where they occur.
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To investigate the size of MCAO events, we could simply calculate their areal extent. However,
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it’s also interesting to know how they tend to be oriented, e.g., if they are usually circular or tend
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to be elongated in a particular direction. We define a zonal (meridional) length scale for all events:
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the maximum length of the closed contour in the east-west (north-south) direction. We then define
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the horizontal scale of events by their zonal length scale. We define their orientation by their aspect
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ratio: the ratio of zonal length scale to meridional length scale.
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We hypothesize that the size and/or shape of MCAO events may vary according to a) location,
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b) season, and c) strength. We find that the strength of MCAO events is most related to their
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size, with stronger events being much larger than weaker ones, as is shown in Figure 4. However,
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for the same strength category, Southern Hemisphere events are generally larger than Northern
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Hemisphere events, with the exception of strong events in the Northern Hemisphere mid-latitudes.
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We find that the seasonality of event sizes is largely explained by the seasonality of their strength
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(not shown).
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In Section 3b, we will show that MCAOs are associated with mid-latitude cyclones, and the
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size and shape of MCAOs are set primarily by this large-scale meteorology as well as the size
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and shape of the surface temperature gradient. High latitude MCAOs are smaller than their mid-
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latitude counterparts partly because the extratropical cyclones they are embedded in are smaller,
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but in the Northern Hemisphere in particular their size is also limited by the ocean basins they tend
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to occur in (e.g., the Labrador or Norwegian Seas, see Fig. 2).
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The second panel in Fig 4 shows the aspect ratio of MCAOs. Most events are fairly round
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— they have similar zonal and meridional scales. However, Southern Hemisphere high latitude
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events have a larger zonal than meridional length scale because they occur along a temperature
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gradient that is mainly north-south. Northern Hemisphere mid-latitude events occur on tempera-
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ture gradients with both north-south and east-west components (mainly the tilted Gulf Stream and
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Kuroshio currents), making them less zonally elongated than Southern Hemisphere high latitude
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events. In the Northern Hemisphere high latitudes, basin sizes and geometries generally result in
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MCAO events that are somewhat longer meridionally than zonally.
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b. Composite meteorology of MCAOs
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In order to study the synoptic conditions of MCAOs, we composite meteorological and thermo-
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dynamic variables on similar events. In order to be considered similar, events have to be of the
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same classification of strength and they have to be roughly the same size. Events are selected by
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size according to the strength category being composited. Weak events are included in their com-
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posite if they have both zonal and meridional length scales between 350 and 750 km; strong events
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are included in their composite if they have length scales between 1750 and 2250 km. Intermediate
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strength events are not shown, but are more similar to strong events than weak ones.
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Additionally, composited events must be in the same geographical region, as different geome-
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tries of surface temperature gradient lead to a different large-scale flow pattern most conducive to
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lower tropospheric cold advection (this applies mainly in the Northern Hemisphere high latitudes
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but is used globally). Composites are performed in six regions of both the Northern Hemisphere:
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the Bering Sea, the Norwegian Sea, the Labrador Sea, the Kuroshio, the Gulf Stream, and the
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North Atlantic; and the Southern Hemisphere: the Indian polar front, the north Ross Sea, the north
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Bellingshausen Sea, the Indian subtropical front, the Brazil Current, and the Argulas current.
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Composites are calculated separately for all seasons, but we find little difference in the compos-
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ites between seasons that is not attributable to MCAO strength – e.g., strong events in spring are
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similar to strong events in winter, but occur less often. We show results for winter only.
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Prior to compositing, each event is interpolated to a 4000 by 2000 km grid, with 100 km grid
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spacing, using nearest-neighbor interpolation. This grid is centered on the location of the maxi-
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mum value of M within the closed contour of each event; this point is the origin in figures below.
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Figure 5 shows the composited sea level pressure, 10 m horizontal winds, sea ice cover, and sur-
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face sensible heat flux surrounding the MCAO. Within each hemisphere and latitude belt, results
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are similar across different ocean basins, so only one example per latitude belt is shown.
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We find that globally MCAOs are associated with the cold air sector of mid-latitude cyclones,
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as was also found by Kolstad et al. (2008) for the North Atlantic and Papritz et al. (2015) for the
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South Pacific. In both hemispheres, high latitude strong events are near well-developed closed
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surface lows, with winds optimal for cold air advection. High latitude weak events are associated
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with weaker average surface lows. High latitude strong events show similar surface wind speeds
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in both hemispheres, but Southern hemiphere weak events have much greater wind speeds than
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Northern Hemisphere weak events.
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Weak event composites, particularly in the Northern Hemisphere, likely represent a mix of cy-
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clones in various stages of growth or decay, and may also include large-scale flow around a weak
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low that is not part of an extratropical cyclone (Papritz et al. (2015) found that roughly 25% of
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Pacific sector Southern Ocean MCAOs were not associated with mid-latitude cyclones, and weak
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events are likely to be over-represented in this category.)
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Fig. 5 also shows the composite surface sensible heat flux (SHF, only values greater than 50
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W m−2 are shown) and sea ice cover. Average SHF ranges from over 200 W m−2 at the center
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of strong events to 50-100 W m−2 in weak events and at the edges of strong ones. Northern
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Hemisphere events have greater SHF than Southern Hemisphere events despite comparable or
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weaker surface wind speeds, implying that even within similar strength classifications the average
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air-sea temperature difference is greater in Northern Hemisphere events.
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Figure 6 shows geopotential height anomalies in a west-to-east cross-section through the com-
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posites shown in Fig. 5. Consistent with Fig. 5, severe events are associated with a strong upper-
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level trough that exhibits a westward tilt with height, implying that many of these cyclones are
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still in the growth phase. Southern Hemisphere weak events are also associated with upper-level
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troughs, again weaker on average than for the strong events. However, Northern Hemisphere weak
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events exhibit an upper-level ridge to the east (in the Labrador Sea) and to the west in the Gulf
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Stream. These are likely associated with a range of meteorological conditions, all of which allow
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weak lower tropospheric cold advection.
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Next we examine similarities and differences in boundary layer structure in the composite
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MCAOs. Figure 7 shows composites of sea level pressure, the difference in horizontal winds
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between 10 m and 800 hPa (u800 − u10m ), and boundary layer height.
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The change in wind speed from the surface to 800 hPa is fairly weak at the center of the MCAO,
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but increases in magnitude downstream. It is likely that the surface-driven convection is mixing
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momentum effectively within the boundary layer near the center of the MCAO. Outside of this
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region the increase in wind speed ranges from 0-10 m s−1 .
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The ERA-Interim boundary layer height is diagnosed with a dry Richardson number, and so
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indicates the base of boundary layer clouds rather than their top (von Engeln and Teixeira 2013).
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The PBL deepens from a few hundred meters to 1-2 km downstream of the MCAO maximum. The
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location of the maximum boundary layer depth is downstream of that of the maximum surface heat
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flux shown in Fig. 5. Boundary layer deepening is driven both by surface heating and large scale
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dynamics, as air masses pass from regions of large scale subsidence to regions of large scale ascent.
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Fig 8 shows cross-section composites of thermodynamic variables in the lower troposphere for
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severe events in both hemispheres. Instead of running from west to east as in Fig. 6, cross sections
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run along the direction of the 800 hPa winds at the location of the maximum MCAO index (the
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origin of the composites). The locations of the cross sections are shown schematically as black
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thick lines in Fig 5. We composite liquid water potential temperature, θl = θ − Lv /C p ∗ ql and total
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water specific humidity, qT = qv + ql + qi , from 1000 to 700 hPa.
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Although the along-wind cross section is not a perfect trajectory, it depicts the change in bound-
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ary layer structure as cold air masses pass over warm water. The Northern Hemisphere cross
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sections qualitatively match the drop-sonde measurements of a cold air outbreak case near Sval-
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bard in Hartmann et al. (1997). Upwind of the MCAO maximum, the boundary layer is in a regime
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of large-scale subsidence and is strongly stable. As it is advected over warm water and toward a
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region of large-scale ascent, it is heated from below and destabilized. Both surface fluxes and
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entrainment warm the boundary layer until it evolves into a well-mixed or weakly stable layer
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downstream. This is more effective in the Northern Hemisphere systems than in the Southern
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Hemisphere due to higher sensible heat fluxes. In fact, Southern Hemisphere events begin to cool
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and dry downstream of the MCAO maximum. Qualitatively similar features are seen in PBL
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structure of weak MCAO composites (not shown).
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4. Summary and conclusions
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We use an MCAO index to develop a comprehensive climatology of Southern Hemisphere
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MCAOs, including their extremes and frequency of occurrence, and compare that to a similar
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climatology of Northern Hemisphere MCAOs. We use this index to define MCAO events and
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compare the frequency, size and shape, synoptic environment, and PBL structure among compos-
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ite events for different hemispheres and regions.
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Our most important results are:
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1. The frequency of occurrence of MCAOs depends on how they are defined. From a gridpoint-
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by-gridpoint perspective, MCAOs occur most often in Northern Hemisphere winter. But when
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classified as discrete events, with each counting as a single MCAO regardless of how many grid-
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points it occupies, MCAO frequency of occurrence and seasonality looks different. They are most
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frequent in Southern Hemisphere autumn, and have only a weak annual cycle in frequency in the
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Southern Hemisphere. Many MCAOs exist outside of winter, particularly in shoulder seasons and
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Southern Hemisphere summer, but they are smaller and weaker than their winter counterparts.
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2. Strong (Mmax > 6 K) MCAO events have similar meteorological contexts and boundary layer
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structure in both hemispheres. They occur in the cold air sector of extratropical cyclones optimally
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positioned to advect polar or very cold continental air masses over warm water. The high surface
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heat fluxes within MCAOs contribute to the deepening of the boundary layer by 500-1800 m.
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3. In the Southern Hemisphere, weak MCAOs (0 < Mmax < 3K) appear dynamically similar
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to strong MCAOs. Weak Northern Hemisphere events do not have a characteristic meteorolog-
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ical context, especially at mid-latitudes. This is likely because greater surface skin temperature
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gradients exist in the Northern Hemisphere and occur with a greater range of geometries than in
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the Southern Hemisphere, allowing for a greater range of synoptic environments conducive to the
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advection of moderately cold air masses.
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Northern Hemisphere MCAOs, for which there are numerous case studies with detailed observa-
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tions, are often used as an analogue for the Southern Hemisphere systems that produce the positive
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shortwave cloud forcing bias in most climate models. Although the frequency of MCAOs is such
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that they are unlikely to be the source of this bias, we find that Southern Hemisphere MCAOs
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are similar to those in the Northern Hemisphere, especially severe ones. Furthermore, simulated
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MCAOs may contribute to the Southern Ocean cloud bias disproportionate to their frequency of
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occurrence. A logical next step in this work is to examine the radiative impact of MCAOs and
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their possible significance for shortwave radiation errors in climate models.
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Acknowledgments. This research is supported by ARC Discovery Grant (DP130100869) and the
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ARC Centre of Excellence for Climate System Science (CE110001028).
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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.
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456
457
Fig. 5.
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459
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Fig. 6.
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463
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Fig. 7.
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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