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    Daily (1200 UTC) values showing the 10-hPa evolution for (a) 80°N zonally averaged temperature (K), (b) 60°N zonally averaged zonal wind (m s−1), and (c) 60°N meridional wind amplitudes (m s−1) for zonal wavenumbers 1 (red), 2 (green), and 3 (black). The solid lines are based on the analyses. The plus symbols denote the corresponding 5-day forecasted values. The heavy vertical line denotes 6 Jan 2013, the SSW date.

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    Zonal mean (a) temperature (K) and (b) zonal wind (m s−1) at 10 hPa as a function of latitude for the 5-day forecast (blue curves) and analysis (green curves) from the 1200 UTC 2 Jan 2013 initial time analysis (red curves).

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    EPV [×102 potential vorticity unit (PVU, 1 PVU = 10−6 K kg−1 m2 s−1), gray shading, values greater than 8 are shaded white] on the 840-K potential temperature surface for (a) 28 Dec 2012, (b) 2 Jan 2013, (c) 7 Jan 2013, and (d) 12 Jan 2013. Also plotted are the 30.25- and 31.25-km (red and yellow, respectively) 10-hPa geopotential height contours. The polar Lambert projection of the Northern Hemisphere has 90°W at the bottom with blue circles at 30° and 60°N.

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    Vertical EP flux at 100 hPa for (a) all waves, (b) wave 1, and (c) wave 2 as a function of latitude and time with a contour interval of 0.5 × 105 kg s−2, and (d) the area weighted latitudinal average of all waves (black), wave 1 (red), wave 2 (blue), and wave 3 (green).

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    Contour plots of zonal wind (m s−1) as a function of latitude (degrees) and pressure (hPa) at 2-day intervals: (all at 1200 UTC) (a) 28 Dec 2012, (b) 30 Dec 2012, (c) 1 Jan 2013, (d) 3 Jan 2013, (e) 5 Jan 2013, and (f) 7 Jan 2013. The zonal winds are hemispheric averages over longitude with the left (right) half of each panel centered on 180° (0°). Shading denotes winds that are into the page. Dark blue arrows denote the hemispheric averaged wave activity flux vectors, scaled at each pressure by the maximum vertical component over December 2012 to January 2013. Note that with this scaling the arrows do not visually illustrate the divergence, only the relative amplitudes at each pressure level as a function of latitude and the six times shown. The main cyclonic (anticyclonic) circulations, as identified by regions of strong meridional wind shear that reverse sign, are highlighted by large red (blue) arrows.

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    Wind speed at 300 hPa (contoured at 40, 60, and 80 m s−1; yellow, brown, and red filled contours, respectively) and geopotential heights at 50 hPa (contour interval of 0.2 km, white contours) for 0000 UTC (a) 20, (b) 21, (c) 22, and (d) 23 Dec 2012. The thick red curve denotes the 9-km geopotential height contour at 300 hPa. The fields are plotted on an equator to North Pole Lambert equal area projection oriented with 90°W at the bottom and dashed lines at 10° latitude intervals.

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    As in Fig. 6, but for 0000 UTC (a) 24, (b) 25, (c) 26, and (d) 27 Dec 2012.

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    As in Fig. 6, but for 0000 UTC (a) 3, (b) 4, (c) 5, and (d) 6 Jan 2013.

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    The 5-day forecasts plotted as in Fig. 6 for 0000 UTC (a) 3, (b) 4, (c) 5, and (d) 6 Jan 2013.

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    The difference between the 0000 UTC 6 Jan 2013 analysis and its corresponding 5-day forecast for (a) 300-hPa geopotential height (100-m contour interval) and (b) 50-hPa geopotential height (50-m contour interval).

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    Time–altitude contour plot of the vertically scaled, vertical component of the wave activity flux (×10−4 p−1, black contours), and zero zonal wind contour (red) averaged over latitudes 30°–90°N and (a) the hemisphere centered on 180°, (b) the hemisphere centered on 0°, and (c) zonally averaged. The averages are area weighted. The black arrows suggest times of vertical wave propagation. The red “E” denotes regions of mean easterly winds.

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    Vertical wave activity flux at 100 hPa averaged over 30°–90°N as a function of longitude and time. Contour interval of 0.02 m2 s−2.

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    Sea level pressure (black contours, contour interval: 8 hPa) for 0000 UTC (a) 28 and (b) 29 Dec 2012. Surface pressure contours are filled as yellow (980–996 hPa), green (964–980 hPa), and light purple (below 964 hPa). The red cross denotes the location of the minimum surface pressure found in the North Atlantic: (a) 970.6 hPa at 59.5°N, 31.3°W and (b) 943.4 hPa at 62.5°N, 16.9°W. Also plotted are the corresponding 50-hPa geopotential heights (red curves labeled in km). The fields are plotted on an equator to North Pole Lambert equal area projection oriented with 90°W at the bottom and dashed lines at 10° latitude intervals. Some smoothing has been applied to the contoured fields.

  • View in gallery

    Polar plots of 360-K potential temperature surface height perturbations with respect to a 7-day running average (contour intervals of 0.25 km starting from ±0.5 km, colors are positive; grays are negative), 50-hPa geopotential heights (red contours, labeled in km), and 200-hPa heights (blue curves) at 11.25 and 11.5 km for (a) 27, (b) 28, (c) 29, and (d) 30 Dec 2012. The fields are plotted on an equator to North Pole Lambert equal area projection oriented with 90°W at the bottom and dashed lines at 10° latitude intervals.

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    Longitude–height cross section at 57°–63°N of the 24-h change in geopotential height (m) ending at 0000 UTC 29 Dec 2012 (color filled contours, contour interval: 50 m). Also plotted is potential temperature (red contours, contour interval: 20 K). The 360-K contour is highlighted in bold red. The arrows (plotted above 150 hPa) depict the 24-h change in the wave activity flux.

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    As in Fig. 15, but for a 5-day forecast ending at 0000 UTC 29 Dec 2012.

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The Major Stratospheric Sudden Warming of January 2013: Analyses and Forecasts in the GEOS-5 Data Assimilation System

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  • 1 Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, and Science Systems and Applications, Inc., Lanham, Maryland
  • | 2 Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, Maryland
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Abstract

The major stratospheric sudden warming (SSW) of 6 January 2013 is examined using output from the NASA Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System version 5 (GEOS-5) near-real-time data assimilation system (DAS). GEOS-5 analyses showed that the SSW of January 2013 was a major warming by 1200 UTC 6 January, with a wave-2 vortex-splitting pattern. Upward wave activity flux from the upper troposphere (~23 December 2012) displaced the ~10-hPa polar vortex off the pole in a wave-1 pattern, enabling the poleward advection of subtropical values of Ertel potential vorticity (EPV) into a developing anticyclonic circulation region. While the polar vortex subsequently split (wave-2 pattern) the wave-2 forcing [upward Eliassen–Palm (EP) flux] was smaller than what was found in recent wave-2, SSW events, with most of the forcing located in the Pacific hemisphere. Investigation of a rapidly developing tropospheric weather system over the North Atlantic on 28–29 December 2012 showed strong transient upward wave activity flux from the storm with influences up to 10 hPa; however, the Pacific hemisphere wave forcing remained dominate at this time. Results from the GEOS-5 five-day forecasts showed that the forecasts accurately predicted the major SSW of January 2013. The overall success of the 5-day forecasts provides motivation to produce regular 10-day forecasts with GEOS-5, to better support studies of stratosphere–troposphere interaction.

Corresponding author address: Lawrence Coy, Science Systems and Applications, Inc., 10210 Greenbelt Rd., Lanham, MD 20706. E-mail: lawrence.coy@nasa.gov

Abstract

The major stratospheric sudden warming (SSW) of 6 January 2013 is examined using output from the NASA Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System version 5 (GEOS-5) near-real-time data assimilation system (DAS). GEOS-5 analyses showed that the SSW of January 2013 was a major warming by 1200 UTC 6 January, with a wave-2 vortex-splitting pattern. Upward wave activity flux from the upper troposphere (~23 December 2012) displaced the ~10-hPa polar vortex off the pole in a wave-1 pattern, enabling the poleward advection of subtropical values of Ertel potential vorticity (EPV) into a developing anticyclonic circulation region. While the polar vortex subsequently split (wave-2 pattern) the wave-2 forcing [upward Eliassen–Palm (EP) flux] was smaller than what was found in recent wave-2, SSW events, with most of the forcing located in the Pacific hemisphere. Investigation of a rapidly developing tropospheric weather system over the North Atlantic on 28–29 December 2012 showed strong transient upward wave activity flux from the storm with influences up to 10 hPa; however, the Pacific hemisphere wave forcing remained dominate at this time. Results from the GEOS-5 five-day forecasts showed that the forecasts accurately predicted the major SSW of January 2013. The overall success of the 5-day forecasts provides motivation to produce regular 10-day forecasts with GEOS-5, to better support studies of stratosphere–troposphere interaction.

Corresponding author address: Lawrence Coy, Science Systems and Applications, Inc., 10210 Greenbelt Rd., Lanham, MD 20706. E-mail: lawrence.coy@nasa.gov

1. Introduction

Modern global numerical weather prediction (NWP) systems are capable of providing accurate 5-day forecasts and analyses of stratospheric circulations, including stratospheric sudden warming (SSW) events at state-of-the-art horizontal and vertical resolutions (Dörnbrack et al. 2012). Stratospheric forecasts are of interest because of the stratosphere’s role as an upper boundary to the tropospheric weather forecasts and possible influence on global modes, such as the Arctic Oscillation (Baldwin and Dunkerton 2001) and Pacific blocking (Kodera et al. 2013). Stratospheric forecasts are especially intriguing as the stratosphere (with dynamics dominated by global-scale vorticity advection) tends to be more predictable than the troposphere (Hoppel et al. 2008) so that, if the stratosphere has a significant influence on global modes, a realistic stratosphere may enhance their predictability. Stratospheric and tropospheric analyses are useful for dynamical studies of coupling between the troposphere and stratosphere, including the forcing of the stratospheric planetary waves by the troposphere and their subsequent vertical propagation and breaking (e.g., Harada et al. 2010). In addition, the higher horizontal resolution typically found in NWP systems allows for studies of resolved gravity wave coupling between the tropospheric and stratosphere.

Past studies have examined individual SSW events (e.g., Kuttippurath and Nikulin 2012; Harada et al. 2010; Coy et al. 2009) as well as composites of SSW events (e.g., Sjoberg and Birner 2012; Limpasuvan et al. 2004; Charlton and Polvani 2007). SSW events are characterized by enhanced planetary wave forcing by upper-tropospheric weather disturbances (see, e.g., Coy et al. 2009) and blocking ridges that act to generate planetary waves that propagate into the stratosphere (Martius et al. 2009; Castanheira and Barriopedro 2010; Woolings et al. 2010; Nishii et al. 2011). These upward-propagating waves increase in amplitude (as density decreases) and interact strongly with the background flow creating the potential for “wave breaking,” an irreversible mixing of Ertel potential vorticity (EPV) between low and high latitudes (McIntyre and Palmer 1983). If the planetary waves advect sufficient low EPV air poleward, the conservation of EPV will create a strong enough anticyclonic circulation in that air mass to displace or split the climatological cyclonic wintertime polar vortex. The warming results from the strong descent in the polar regions needed to maintain quasigeostrophic balance in response to the reduced vorticity (and corresponding weaker westerlies) of the anticyclone. These dynamical changes inhibit the further upward propagation of planetary waves, causing them to break at lower levels than the initial wave breaking, and hence result in the descending pattern of wind and temperature changes characteristic of a SSW event (Matsuno 1971). A major SSW occurs when the 10-hPa 60°N zonal mean zonal wind reverses from westerly to easterly and the 10-hPa zonal mean temperature gradient increases poleward of 60°N. If only the temperature gradient increases while the winds remain westerly then the SSW is considered minor (see Andrews et al. 1987, p. 259).

Blocking events in the troposphere have been investigated for their role in forcing SSW events. While different definitions of blocking exist, most identify blocking patterns as quasi-stationary, mid- to upper-tropospheric ridges with lifetimes greater than 5 days. Martius et al. (2009) showed that 25 of 27 SSW events identified in the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) were preceded by blocking patterns in the upper troposphere (200 hPa or above). Both Martius et al. (2009) and Castanheira and Barriopedro (2010) showed that the longitude of the blocking determines the type of SSW generated, where Atlantic blocking tends to lead to a wave-1 SSW and Pacific blocking to a wave-2 SSW event. Nishii et al. (2011) showed that the relation between upper-tropospheric blocking and vertical planetary wave propagation into the stratosphere is complex, with blocks in some regions of the Pacific (western Pacific and Far East) leading to suppression rather that growth of stratospheric wave activity. Thus, while upper-tropospheric blocking ridges are undoubtedly important in forcing the majority of SSW event, questions remain about the significance of specific tropospheric blocks (which may act to increase or decrease vertical planetary wave propagation) prior to specific SSW events.

While recent studies of specific SSW events have focused on identifying tropospheric weather features such the large upper-tropospheric ridge over the U.S. West Coast preceding the SSW of January 2009 (Harada et al. 2010), there has been less investigation of transient tropospheric disturbances prior to SSW events. Coy et al. (2009) showed that transient upper-tropospheric disturbances over the North Atlantic lead directly to the wave breaking associated with the SSW events of January 2003 and January 2006. The response of the stratospheric polar vortex to transient waves forced in the upper troposphere has been investigated by Esler and Scott (2005) using models of vortex wave propagation. These waves are modeled as trapped in latitude at the strong potential vorticity (PV) gradient found at the vortex edge and have some similar features to the disturbances observed by Coy et al. (2009). Since most of the wave forcing of the troposphere on the stratosphere is known to be from the larger-scale, quasi-stationary waves (blocking), these transient waves are only expected to provide a small additional forcing that may or may not lead to large stratospheric changes. How, or if, these transient upper-tropospheric disturbances affect the polar vortex is an open question (Gerber et al. 2012).

The goal of this paper is to examine the ability of a weather forecasting data assimilation system (DAS) to predict an SSW, and to explore the tropospheric forcing of the stratosphere by both tropospheric ridges and transient tropospheric systems. To this end a case study of the major SSW that occurred on 6 January 2013 is examined using the National Aeronautics and Space Administration (NASA) Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System version 5 (GEOS-5) near-real-time analyses and forecasts. First, the ability of the near-real-time GEOS-5 five-day forecasts to predict the SSW is evaluated. Next, the overall evolution of the SSW is characterized in terms of EPV redistribution in the middle stratosphere along with the zonal wavenumber 1–3 decomposition of the vertical Eliassen–Palm (EP) flux [see Andrews et al. (1987), p. 128], a measure of the vertical wave forcing of the stratosphere by the troposphere. This is followed by an examination of the longitudinally varying vertical forcing of the stratosphere using the three-dimensional wave activity flux formulation of Plumb (1985) averaged over the Pacific and Atlantic hemispheres to evaluate the location and strength of blocking ridges prior to and during the January 2013 SSW event. The detailed evolution of a Pacific hemisphere ridge (prior to the SSW) and an Atlantic hemisphere ridge (during the SSW) in the upper troposphere and lower stratosphere is then shown. An example is also given that evaluates the ability of the GEOS-5 DAS to accurately forecast these blocking ridge events, a prerequisite to forecasting a SSW event. Finally, a detailed analysis is presented of a rapidly developing transient system over the North Atlantic during 28–29 December 2012 to determine whether that system may have played a role in initiating the SSW.

The plan of this paper is as follows: section 2 gives a brief description of the GEOS-5 DAS. Section 3a presents the forecast and overview of the January 2013 SSW event. Section 3b discusses the Plumb (1985) three-dimensional wave activity flux formulation and its results during this SSW event. Section 3c shows the Pacific and Atlantic blocking ridges while section 3d presents results of the transient North Atlantic system and section 4 provides a discussion and summary.

2. Data assimilation system description

For this study the near-real-time GMAO GEOS-5.7.2 system was used. The GEOS-5.7.2 system is updated from the version of GEOS-5 used in the Modern-Era Retrospective Analysis for Research and Applications (MERRA) project, which is described in detail in Rienecker et al. (2011, 2008) and Molod et al. (2012). One of the main differences between MERRA and the GEOS-5.7.2 system is the increased horizontal resolution used in the near-real-time system—a 0.3125° × 0.25° longitude–latitude grid was used. The analysis increments are calculated on a 0.625° × 0.5° longitude–latitude horizontal grid that are then interpolated onto the higher (0.25°) resolution as part of the assimilation cycle. The radiative transfer package and the model layers remain unchanged from MERRA.

The GEOS-5 DAS forecast model is based on a finite-volume dynamical core (Lin 2004). Relevant physics for stratospheric studies include orographic (McFarlane 1987) and nonorographic (Garcia and Boville 1994) gravity wave drag, and shortwave (Chou and Suarez 1999) and longwave (Chou et al. 2001) radiative transfer models valid up to ~80 km. The three-dimensional variational analysis is done every 6 h using the GMAO implementation of the gridpoint statistical interpolation (GSI) scheme (Wu et al. 2002; Purser et al. 2003a,b). Observational data include both conventional (radiosondes, aircraft, etc.) and available satellite radiances, with the Advanced Microwave Sounding Unit (AMSU-A) radiance channels 11–14 providing a major constraint in the stratosphere. An incremental analysis update (IAU; Bloom et al. 1996) procedure gradually adds the analysis to the model as a dynamical forcing. The final three-dimensional output fields (winds and temperature) are saved every 3 h. The GEOS-5 DAS has been successfully used for many studies including driving chemistry transport models (e.g., Pawson et al. 2007) and observation impact experiments (e.g., Gelaro et al. 2010).

3. Results

a. SSW overview

This section examines the time evolution of the 2013 major SSW. Figure 1 shows an overview of 10-hPa wind, temperature, and planetary waves 1–3 during the SSW, as analyzed in GEOS-5 for 11 December 2012–10 February 2013, along with GEOS-5 daily 5-day forecast output.

Fig. 1.
Fig. 1.

Daily (1200 UTC) values showing the 10-hPa evolution for (a) 80°N zonally averaged temperature (K), (b) 60°N zonally averaged zonal wind (m s−1), and (c) 60°N meridional wind amplitudes (m s−1) for zonal wavenumbers 1 (red), 2 (green), and 3 (black). The solid lines are based on the analyses. The plus symbols denote the corresponding 5-day forecasted values. The heavy vertical line denotes 6 Jan 2013, the SSW date.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

The 10-hPa temperature near the North Pole (zonally averaged at 80°N, Fig. 1a) is 200 K on 1200 UTC 1 January increasing up to 240 K by 1200 UTC 6 January for a 40-K change in 5 days. After the rapid rise, the North Pole temperature remains warm until ~18 January, followed by a slower decay back to near 200 K by 10 February. The 1200 UTC 5-day forecasts of 10-hPa temperature near the North Pole closely follow the analysis temperatures during this time period, including the rapid rise in North Pole temperature characteristic of the major SSW.

The 60°N zonal mean of the zonal wind (Fig. 1b) decreases as the 10-hPa polar temperature increases, changing from westerly to easterly at 1200 UTC 6 January. Coupled with the reversed 60°N to pole 10-hPa temperature gradient (Fig. 2a) this change in sign of the 10-hPa zonal mean zonal wind determines the time of the SSW event, 1200 UTC 6 January 2013. These winds, after coming close to zero on 10 January, remain easterly until 28 January. The forecasted values of the 10-hPa, 60°N, zonal mean zonal wind tracks the analysis, closely following the westerly wind decrease and the change to easterly winds associated with the SSW.

Fig. 2.
Fig. 2.

Zonal mean (a) temperature (K) and (b) zonal wind (m s−1) at 10 hPa as a function of latitude for the 5-day forecast (blue curves) and analysis (green curves) from the 1200 UTC 2 Jan 2013 initial time analysis (red curves).

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

The evolution of the 10-hPa 60°N meridional wind amplitude of zonal waves 1–3 during the major SSW is shown in Fig. 1c. Wave 1 dominates over waves 2 and 3 prior to the SSW with a varying amplitude near ~25 m s−1. This wave-1 amplitude rapidly decreases to ~10 m s−1 or less during the SSW and remains relatively low thereafter. The wave-2 amplitude increases before the SSW, however, it is still less than 20 m s−1 at 1200 UTC 4 January. During the SSW the wave-2 amplitude increases rapidly up to 38 m s−1 at 1200 UTC 8 January, nearly doubling in amplitude over 4 days. Following the SSW, the wave-2 meridional wind amplitude continues being large (<20 m s−1) out to 15 January, after that time it decays, becoming less than ~10 m s−1 on 22 January. The wave-3 amplitude peaks on the date of the SSW wind reversal (6 January) and is relatively small at other times, though it is smaller after the SSW than before. The 5-day forecast of the 10-hPa 60°N meridional wind wave 1–3 amplitude (plus symbols) shows fair agreement with the analysis amplitudes.

Note that using the meridional wind in the zonal wavenumber decomposition emphasizes the higher wavenumbers more than a geopotential height decomposition would. This is expected as, through geostrophy, the meridional wind is closely related to the longitudinal gradient of the geopotential height field, |υgeo| ∝ k|Φ|, where k is the zonal wavenumber. Because advection of PV by the meridional wind is an important dynamical component during SSW vortex breakup, meridional wind wave amplitudes are plotted in Fig. 1c rather than the more traditional geopotential height wavenumber amplitudes.

The ability of the GEOS-5 data assimilation system to forecast the dramatic circulation changes in 10-hPa zonal averaged temperature and zonal wind characteristic of SSW events is shown in Fig. 2. At 1200 UTC 2 January 2013, the zonal average analysis temperature is over 20 K cooler at 90°N compared with 60°N (Fig. 2a, red curve) while the 5-day forecast (blue curve) has reversed this zonal mean temperature gradient with the polar temperature on 7 January predicted to be over 15 K warmer than the 60°N temperature. The 10-hPa zonal averaged zonal wind 5-day forecast (Fig. 2b, blue curve) shows a change of ~65 m s−1 from westerly to easterly winds when compared to the initial 2 January analysis winds (red curve). The temperature and wind verifying analyses at 1200 UTC 7 January (green curves) show good agreement with the predicted 5-day changes. This forecast of the January 2013 major SSW was identified on 3 January 2013 as part of the routine monitoring of the GEOS-5 system.

An overview of the middle stratosphere vortex breakdown during the SSW is shown at four times (5-day intervals) in Fig. 3, with Figs. 3b and 3c corresponding to the analyses associated with the initial and final times (2 and 7 January) of the 5-day forecast results shown in Fig. 2. The EPV fields on the 840-K potential temperature surface (~10 hPa) along with the 10-hPa geopotential height fields show the polar vortex (high EPV values) displaced off the pole in a mainly wave-1 pattern (Fig. 3a, 28 December) followed by the advection of low EPV air (accompanied by high geopotential heights, yellow contour) from low latitudes (<30°N) toward 180° (Fig. 3b, 2 January), the development of a substantial low EPV region near 180° and the near splitting of the polar vortex (Fig. 3c, 7 January), and the vortex fully split (Fig. 3d, 12 January). In summary, the 850-K EPV and 10-hPa geopotential height patterns both show the vortex displaced off the pole followed by a splitting of the displaced vortex.

Fig. 3.
Fig. 3.

EPV [×102 potential vorticity unit (PVU, 1 PVU = 10−6 K kg−1 m2 s−1), gray shading, values greater than 8 are shaded white] on the 840-K potential temperature surface for (a) 28 Dec 2012, (b) 2 Jan 2013, (c) 7 Jan 2013, and (d) 12 Jan 2013. Also plotted are the 30.25- and 31.25-km (red and yellow, respectively) 10-hPa geopotential height contours. The polar Lambert projection of the Northern Hemisphere has 90°W at the bottom with blue circles at 30° and 60°N.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

The zonally averaged forcing of the SSW from the troposphere can be characterized by an examination of the upward EP flux [see Andrews et al. (1987), p. 128] near the tropopause (~100 hPa). Figure 4 shows the upward EP flux at 100 hPa from 1 December 2012 to 31 March 2013, as a function of time and latitude, and decomposed in terms of zonal waves 1 and 2. For calculating the quadratic quantities the output fields of meridional wind, zonal wind, and temperatures were first decomposed into Fourier longitudinal wavenumber components and then the corresponding wavenumber fields were multiplied together. The total upward EP flux (Fig. 4a) shows relatively high values from the end of December through the beginning of February before declining in the rest of February and March. Note the high-latitude peaks near 23 December and 6 January that were associated with the preconditioning of the polar vortex and the date the SSW criteria were met, respectively. The wave-1 contribution (Fig. 4b) occurs mainly before the SSW, while the wave-2 contribution (Fig. 4c) occurs mainly during and after the SSW. The time series of the 30°–90°N averages of the upward EP fluxes are shown in Fig. 4d for comparison with similar figures in Harada et al. (2010) for the Northern Hemisphere winters of 1984/85, 1988/89, and 2008/09. The latitudinally averaged wave-1 upward EP flux (red curve) decreases during the SSW as the wave-2 EP flux (blue curve) increases. The wave-3 forcing (green curve) increases somewhat during the SSW but remains relatively small. The wave-2 EP flux maxima found before and during the SSW are less than one (×105 kg s−2), smaller than the maximum values found during any of the three winters examined by Harada et al. (2010). Thus, the major vortex-splitting SSW of 2013 had a relatively weak forcing contribution from the wave-2 component of the vertical EP flux.

Fig. 4.
Fig. 4.

Vertical EP flux at 100 hPa for (a) all waves, (b) wave 1, and (c) wave 2 as a function of latitude and time with a contour interval of 0.5 × 105 kg s−2, and (d) the area weighted latitudinal average of all waves (black), wave 1 (red), wave 2 (blue), and wave 3 (green).

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

b. Wave activity flux

Up to this point, we have focused on the SSW evolution in the middle stratosphere (10 hPa) and the zonally averaged EP flux forcing near the tropopause (~100 hPa). To better understand the vertical and horizontal dependence of the SSW evolution we have calculated the quasigeostrophic three-dimensional wave activity flux developed by Plumb (1985):
e1
where p is normalized pressure (1 at the surface); u and υ are the zonal and meridional wind components, respectively; T is temperature; Φ is geopotential height; λ and ϕ are longitude and latitude, respectively; a is Earth’s radius; Ω is the frequency of Earth’s rotation; and S is a measure of average static stability (taken to be constant here). The primes denote deviations from a zonal average. Plumb (1985) showed this flux is nondivergent for conservative, steady waves; parallel to the wave group velocity (in the limit of almost-plane waves); and that the convergence of this flux implies a local increase of wave activity when the winds are westerly. In addition, when Eq. (1) is zonally averaged the meridional and zonal components reduce to the corresponding components of the quasigeostrophic EP flux (Andrews et al. 1987), a quantity whose divergence is proportional to the change in the zonal mean zonal wind. The original application of this wave activity flux in Plumb (1985) examined planetary waves propagating on a time averaged basic state that minimized wave transient effects. In this study of a rapidly changing SSW event, wave transient effects are large so that wave activity flux convergence is expected in regions without significant wave dissipation. This wave activity flux formulation was used by Harada et al. (2010) in their study of the major SSW of January 2009 to identify tropospheric source regions and the subsequent vertical and horizontal propagation of wave activity. In particular, the localization in longitude given by the wave activity flux formation, enables a different perspective of the SSW event than that given by a Fourier analysis of the planetary waves. Here we investigate some aspects of the vertical/horizontal evolution of the major SSW of January 2013 based on averages of zonal wind and wave activity flux over limited longitudinal ranges.

To examine in more detail the development of the high 10-hPa geopotential heights, low 850-K EPV region, near 180° longitude seen in Fig. 3, the zonal wind and wave activity flux are averaged over hemispheric domains centered on 0° and 180° (hereafter referred to as the Atlantic and Pacific hemispheres, respectively) and the vertical and meridional components of the wave activity flux are then plotted as latitude versus altitude cross sections for each hemisphere (Fig. 5). This division into hemispheres is based on an initial examination of the vertical wave activity flux that showed generally larger values in the Pacific region than in the Atlantic region along with the development of the SSW associated stratospheric anticyclone in the Pacific region. While some wave activity flux for these planetary-scale waves will cross hemispheres, vertical wave propagation, as denoted by the wave activity flux, from the tropopause up to the middle stratosphere can take place within a single hemispheric domain (e.g., Harada et al. 2010).

Fig. 5.
Fig. 5.

Contour plots of zonal wind (m s−1) as a function of latitude (degrees) and pressure (hPa) at 2-day intervals: (all at 1200 UTC) (a) 28 Dec 2012, (b) 30 Dec 2012, (c) 1 Jan 2013, (d) 3 Jan 2013, (e) 5 Jan 2013, and (f) 7 Jan 2013. The zonal winds are hemispheric averages over longitude with the left (right) half of each panel centered on 180° (0°). Shading denotes winds that are into the page. Dark blue arrows denote the hemispheric averaged wave activity flux vectors, scaled at each pressure by the maximum vertical component over December 2012 to January 2013. Note that with this scaling the arrows do not visually illustrate the divergence, only the relative amplitudes at each pressure level as a function of latitude and the six times shown. The main cyclonic (anticyclonic) circulations, as identified by regions of strong meridional wind shear that reverse sign, are highlighted by large red (blue) arrows.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

The anticyclone initially develops at ~10-hPa altitude, 40°–50°N (Figs. 5a and 5b, 28 and 30 December) as can be seen in the growing easterly (shaded) and westerly wind couplet in the Pacific hemisphere (left side of the panels in Fig. 5). This anticyclone strengthens considerably by 1 January (Fig. 5c), as seen by the stronger winds in the Pacific hemisphere between 10 and 1 hPa, and the axis of the anticyclone (thick blue arrow) is tilted slightly poleward at this time. By 3 January (Fig. 5d) the anticyclone has continued to increase in strength, has moved poleward to ~60°N, and now extends above 1 hPa into the mesosphere as well as down into the lower stratosphere. By 5 January (Fig. 5e) the anticyclone continues to move poleward, especially in the mesosphere, associated with strong winds across the pole and by 7 January (Fig. 5d) the anticyclone is nearly over the pole as the vortex splits at this time. In the Atlantic hemisphere (0°, right side of panels in Fig. 5) the westerlies (shaded) gradually decrease in strength and shift equatorward, especially from 3 to 5 January (Figs. 5d and 5e).

The wave activity flux vectors (Fig. 5) are generally larger in the Pacific than the Atlantic hemisphere. Strong poleward focusing of the wave activity flux vectors is found on 5 January (Fig. 5e) in the lower stratosphere, Pacific hemisphere, when the anticyclone moves over the pole. Note that, from Eq. (1), the wave activity vectors tend to zero toward the pole, as the wave perturbations are defined with respect to a zonal average, so it is not possible to follow wave propagation across the pole in this formulation. On 7 January (Fig. 5f) the Pacific hemisphere wave activity flux vectors remain large, extending well into the mesosphere, denoting strong wave propagation continuing in this hemisphere at this time. Though not always focused into the polar region, the strong upward wave activity flux acts to decelerate the zonal wind in the mesosphere and upper stratosphere even during times when the horizontal wave activity flux is persistently toward the equator (the direction of the mean climatological horizontal component of the wave activity flux). The wave activity vectors in the Atlantic hemisphere are largest on 3–5 January (Figs. 5d and 5e), the time when the vortex is moving away from the pole.

c. Upper-tropospheric systems

As noted in the introduction, we expect the stratosphere to be most directly influenced by both blocking events and eastward-propagating synoptic-scale events with significant amplitude in the upper troposphere (500–100 hPa). For example, the upper-tropospheric blocking ridge over the U.S. West Coast was a major source of wave activity flux during the January 2009 SSW event (Harada et al. 2010). For simplicity we refer to these events as upper-tropospheric systems and in this section we examine some of the upper-tropospheric systems that occurred before and during the January 2013 SSW event. These include high-latitude, ridge events over the Pacific hemisphere prior to the SSW (24 December) and over the Atlantic hemisphere during the SSW event (6 January). A rapidly developing tropospheric system over the North Atlantic will be examined in section 3d.

In Figs. 6, 7, and 8 the relation between the troposphere jet at 300 hPa and the lower-stratosphere vortex (50-hPa geopotential heights) is explored. We chose to focus on these levels as we expect the lower stratosphere to respond more directly to the upper-troposphere perturbations than mid- or upper-stratospheric levels. The tropospheric ridge associated with the relatively early (24 December) vertical wave activity flux at high latitudes initially formed near 180° on 20 December (Fig. 6a), moved eastward, increased in meridional amplitude (Fig. 6b), extended under the lower-stratospheric jet (Fig. 6c), and formed a high-latitude cutoff high that reached the pole by 23 December. The lower-latitude (40°–60°N) vertical wave activity flux (not shown) peaked on the west side of the ridge on 21 December. The upper-tropospheric ridge remained strong on 24 December (Fig. 7a); however, by 25 December (Fig. 7b), it had decayed substantially as the large-scale, lower-stratospheric ridge above (at 50 hPa) the upper-tropospheric ridge continued to increase in amplitude. Though the upper-tropospheric flow on 26–27 December (Figs. 7c and 7d) undulated without a major ridge over the United States and the eastern Pacific, the lower-stratosphere high persisted in that region. In summary, an upper-tropospheric cutoff high develops nearly concurrently with the amplification of a large-scale ridge in the lower stratosphere. The wave activity flux connection between the upper troposphere and the lower stratosphere for the Pacific hemisphere during this pre-SSW time will be discussed below (see Fig. 11).

Fig. 6.
Fig. 6.

Wind speed at 300 hPa (contoured at 40, 60, and 80 m s−1; yellow, brown, and red filled contours, respectively) and geopotential heights at 50 hPa (contour interval of 0.2 km, white contours) for 0000 UTC (a) 20, (b) 21, (c) 22, and (d) 23 Dec 2012. The thick red curve denotes the 9-km geopotential height contour at 300 hPa. The fields are plotted on an equator to North Pole Lambert equal area projection oriented with 90°W at the bottom and dashed lines at 10° latitude intervals.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

Fig. 7.
Fig. 7.

As in Fig. 6, but for 0000 UTC (a) 24, (b) 25, (c) 26, and (d) 27 Dec 2012.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

Fig. 8.
Fig. 8.

As in Fig. 6, but for 0000 UTC (a) 3, (b) 4, (c) 5, and (d) 6 Jan 2013.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

The development of the upper-tropospheric and lower-stratospheric circulation during the warming is shown in Fig. 8. On 3 January (Fig. 8a) the lower-stratospheric ridge over the United States has decayed and the lower-stratospheric vortex shows a wave-3 shape combined with nonzero wave-1 and 2 components. Upper-tropospheric ridges are prominent over the western United States and over the North Atlantic on 3 January; however, only the ridge over the North Atlantic strengthens (Fig. 8b), extending under the stratospheric vortex by 5 January (Fig. 8c) and persisting through 6 January (Fig. 8d), a time when the lower-stratospheric vortex begins to split as part of the SSW with strong 50-hPa ridges over both the eastern Pacific and the North Atlantic.

Accurate forecasting of upper-tropospheric ridge development is likely important in forecasting the SSW events. Figure 9 shows the GEOS-5 five-day forecasts for the same times and quantities as plotted in Fig. 8. The overall agreement between the 5-day forecasts and the analyses are good. Specifically, development of the upper-tropospheric ridge near 0° to high latitudes is captured by the 5-day forecast. The poorest agreement occurs on 3 January (Fig. 9a) where the two upper-tropospheric ridges in the forecasts have less eastward tilt with latitude than those in the analysis. This corresponds to less horizontal momentum flux and, hence, an underestimate of latitudinal wave propagation in the forecasts [the meridional term when Eq. (1) is zonally averaged]. The geopotential height difference patterns (analysis minus forecast) for 0000 UTC 6 January at 300 and 50 hPa are shown in Fig. 10. The largest differences at 300 hPa (~200 m) occur near the ridge at 0° as well as over the United States and North Atlantic, while the largest differences at 50 hPa (~150 m) occur at high latitudes. Taking as a normalized measure of overall fit the ratio of the standard deviation (poleward of 30°N, area weighted) of the analysis minus forecast to the standard deviation of analysis field at this time, we find ratio values of 22% at 300 hPa and 13% at 50 hPa. Thus, the forecasted differences from the analysis are relatively small for this 5-day forecast, particularly in the stratosphere.

Fig. 9.
Fig. 9.

The 5-day forecasts plotted as in Fig. 6 for 0000 UTC (a) 3, (b) 4, (c) 5, and (d) 6 Jan 2013.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

Fig. 10.
Fig. 10.

The difference between the 0000 UTC 6 Jan 2013 analysis and its corresponding 5-day forecast for (a) 300-hPa geopotential height (100-m contour interval) and (b) 50-hPa geopotential height (50-m contour interval).

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

Figure 11 shows the vertical component of the wave activity flux averaged over 30°–90°N for the two hemispheres examined above over the 16 December 2012–20 January 2013 period. The vertical propagation near the tropopause is larger in the Pacific hemisphere (Fig. 11a) than in the Atlantic hemisphere (Fig. 11b). There are three main forcing events identifiable, with 100-hPa peak values on 23 December, 3 January, and 14 January. Note that there is a consistent time lag of ~4 days between the upward flux maxima in the midtroposphere and the delayed upward flux maxima at 100 hPa. This may reflect the time it takes for the planetary waves to propagate vertically from the middle troposphere to the tropopause/lower stratosphere, with limitations of the quasigeostrophic vertical wave activity flux creating the separate peaks; however, further investigation is needed to determine the precise mechanism for this delay.

Fig. 11.
Fig. 11.

Time–altitude contour plot of the vertically scaled, vertical component of the wave activity flux (×10−4 p−1, black contours), and zero zonal wind contour (red) averaged over latitudes 30°–90°N and (a) the hemisphere centered on 180°, (b) the hemisphere centered on 0°, and (c) zonally averaged. The averages are area weighted. The black arrows suggest times of vertical wave propagation. The red “E” denotes regions of mean easterly winds.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

Before 23 December the upward wave activity flux at 100 hPa is weak. The 23 December upward flux event is evident in both hemispheres but stronger in the Pacific hemisphere. Moreover, the upward flux at this time propagates vertically more rapidly in the Atlantic hemisphere than in the Pacific hemisphere, as shown by the black arrows. The strong upward wave activity flux across the tropopause on 3 January only occurs in the Pacific hemisphere, implying that wave-2 forcing at these latitudes is not especially large at this time. There is no evidence of an upward flux peak on 6 January associated with the high-latitude ridge seen at that time, indicating that the 6 January ridge, while large and extending under the stratospheric vortex was not a major contributor to the hemispheric vertical wave forcing. The upward wave activity flux after the SSW on 13 January is once again largest in the Pacific hemisphere and shows that the lower stratosphere still supports significant wave activity at this time. Another feature of the Pacific hemisphere that is missing in the Atlantic hemisphere is the strong upward flux in the upper stratosphere and lower mesosphere that occurs on ~9 January after SSW has satisfied the major warming criteria.

Figure 12 summarizes the upward wave activity flux at 100 hPa, averaged over 30°–90°N and presented as a function of time and longitude. Averaging over this large latitude region will include all the planetary wave forcing during the time period, including times when the vortex jet is located far south of the pole. The upward wave activity flux should be small when the vortex edge (with its strong PV gradient) is not present, as the stratosphere outside the polar vortex lacks the strong PV gradient needed for planetary wave propagation in those regions, so that Fig. 12 encompasses all the extratropical wave forcing of the stratospheric polar vortex from the troposphere during this time period. The regions of strong upward wave activity flux (red shaded contours) are generally located in the Pacific hemisphere before and during the SSW, with the exception of a small region near 0° on ~23 January and a weak (yellow shaded contours) region near 30°W on ~1–10 January. While these exceptions are associated with a wave-2 contribution to the SSW forcing, the main upward wave activity flux is confined to the Pacific hemisphere, and is thus predominately a wave-1 signal. Comparing the longitudes of strong upward wave activity flux in Fig. 12 with the location of the ridges and troughs in Fig. 6 shows that the main upward wave activity flux occurs upstream of the upper-tropospheric high pressure noted near 180°. The relatively small vertical wave activity flux contribution from the region of the high pressure system results from the definition of the wave activity flux vector that causes it to tend toward zero near the pole and also because, in this case, most of the vertical wave activity flux is emanating from the broad trough region upstream of the ridge.

Fig. 12.
Fig. 12.

Vertical wave activity flux at 100 hPa averaged over 30°–90°N as a function of longitude and time. Contour interval of 0.02 m2 s−2.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

d. Tropospheric storm of 29 December 2012

As shown in Coy et al. (2009), rapidly developing upper-tropospheric systems over the North Atlantic appeared to play a role in the forcing of the major SSW events of January 2003 and January 2006. These weather systems developed most rapidly as they moved poleward under the strong stratospheric winds at which time their development extended into the lower stratosphere. To some extent the cold temperatures near the tropopause associated with these systems, and the corresponding lifting of potential temperature surfaces, acted like a moving orographic forcing under the stratospheric jet. Here we investigate a strong tropospheric storm that occurred several days before the major SSW of January 2013.

Examining first the surface pressure of this system, Fig. 13 shows that prior to the major SSW a low surface pressure system rapidly developed at high latitudes near 62°N, 17°W. The GEOS-5 analysis surface pressure at the center of the low decreased by 27.2 hPa in 24 h from 28 to 29 December 2012 (970.6–943.4 hPa) reaching the ~60°N “bomb” definition at this time (Sanders and Gyakum 1980). Note that the required 24-h pressure decrease for a bomb depends on latitude (24 hPa × sinϕ/sin60°). During 28–29 December the location of the storm’s minimum pressure ranged from 59.5° to 62.5°N, corresponding to a required bomb pressure change range of 23.9–24.6 hPa. Thus, this storm’s change of 27.2 hPa exceeded the required bomb definition for surface pressure decrease in 24 h. In addition, Fig. 13 shows that the 28–29 December surface pressure decrease associated with the storm was ~24 hPa (a three contour change) over a broad area of the developing low. This rapid surface development occurs under the strong lower-stratospheric vortex winds, identified by the strong gradient in the 50-hPa geopotential heights.

Fig. 13.
Fig. 13.

Sea level pressure (black contours, contour interval: 8 hPa) for 0000 UTC (a) 28 and (b) 29 Dec 2012. Surface pressure contours are filled as yellow (980–996 hPa), green (964–980 hPa), and light purple (below 964 hPa). The red cross denotes the location of the minimum surface pressure found in the North Atlantic: (a) 970.6 hPa at 59.5°N, 31.3°W and (b) 943.4 hPa at 62.5°N, 16.9°W. Also plotted are the corresponding 50-hPa geopotential heights (red curves labeled in km). The fields are plotted on an equator to North Pole Lambert equal area projection oriented with 90°W at the bottom and dashed lines at 10° latitude intervals. Some smoothing has been applied to the contoured fields.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

In additional to rapidly decreasing pressure at the surface, this system was associated with changes in the upper troposphere and lower stratosphere. In Coy et al. (2009) synoptic-scale disturbances in the upper troposphere, characterized by large fluctuations in the height of the 360-K potential temperature surface, were shown to precede the major SSW events of January 2003 and January 2006. The 360-K surface typically varies from ~9 to 18 km in December–January, closely mirroring upper-tropospheric (200 hPa) temperatures with cold (warm) temperatures corresponding to high (low) 360-K heights. The 360-K potential temperature surface spans both the troposphere (in the tropics) and the stratosphere (in polar regions) and is at relatively high heights in both of these regions. At midlatitudes the 360-K surface is relatively low in height with significant perturbations created by synoptic-scale weather systems. While other diagnostics (such as 200-hPa temperatures) can also reflect synoptic-scale systems, this study continues to use the 360-K surface height perturbations, allowing direct comparisons with the results of Coy et al. (2009). To the extent that the potential temperature surface resembles a material surface its height fluctuations may influence the atmosphere above. As noted in Coy et al. (2009), high 360-K potential temperature surface heights also coincide with low column ozone values, reinforcing the idea that these are regions where strong upper-troposphere uplift extends into the lower stratosphere.

Figure 14 shows snapshots of the upper-tropospheric 360-K surface deviations from a 7-day running average superimposed with the lower-stratospheric 50-hPa surface on 27–30 December 2012. Subtracting the 7-day time average removes the persistently high tropical and polar heights revealing the midlatitude, synoptic variations. During 27–28 December (Figs. 14a and 14b) the weather systems over the North Atlantic are propagating to the northeast, moving under the strong polar vortex winds, and increasing in amplitude. As the surface pressure decreases on 29 December, the high 360-K heights increase slightly, tracking under the stratospheric vortex winds (Fig. 14c). By 30 December the high and low 360-K perturbations decrease in amplitude (Fig. 14d). During the 15 December 2012–15 January 2013 time period this system produced the largest 360-K height perturbations in the midlatitudes, 45°–75°N, indicating the potential of this system to strongly influence the stratosphere prior to the SSW event. Note that there is also a distinct high 360-K surface increasing in amplitude near 140°E at the outer edge of the stratospheric polar vortex.

Fig. 14.
Fig. 14.

Polar plots of 360-K potential temperature surface height perturbations with respect to a 7-day running average (contour intervals of 0.25 km starting from ±0.5 km, colors are positive; grays are negative), 50-hPa geopotential heights (red contours, labeled in km), and 200-hPa heights (blue curves) at 11.25 and 11.5 km for (a) 27, (b) 28, (c) 29, and (d) 30 Dec 2012. The fields are plotted on an equator to North Pole Lambert equal area projection oriented with 90°W at the bottom and dashed lines at 10° latitude intervals.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

We next examine how the developing system is acting to change geopotential height patterns and vertical wave activity flux in the upper troposphere and lower stratosphere and focus on the transient (24 h) changes in these quantities. Note that these 24-h differences in geopotential heights and wave activity flux highlight changes in both location (generally eastward advection) as well as in amplitude, and so reflect motion as well development. A longitude–pressure cross section through the storm at 60°N on 29 December (Fig. 15) shows an increase in 24-h geopotential height change near 0° in the upper troposphere. Lower-stratospheric height changes are concentrated from 60°W to 90°E, above the tropospheric changes. The 24-h change in wave activity flux (arrows) shows an increase in the upward wave activity flux ahead of the increasing upper-tropospheric geopotential heights, suggesting that the transient tropospheric system is creating the 24-h changes in geopotential heights seen in the stratosphere.

Fig. 15.
Fig. 15.

Longitude–height cross section at 57°–63°N of the 24-h change in geopotential height (m) ending at 0000 UTC 29 Dec 2012 (color filled contours, contour interval: 50 m). Also plotted is potential temperature (red contours, contour interval: 20 K). The 360-K contour is highlighted in bold red. The arrows (plotted above 150 hPa) depict the 24-h change in the wave activity flux.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

If this tropospheric system is important in forcing the SSW, then realistically forecasting this system becomes important in forecasting the SSW. Figure 16 plots the same fields as in Fig. 15 for the corresponding 5-day forecast. The 5-day forecast picks up the main features seen in the analysis, including the increasing tropospheric high, the increasing perturbations in the lower stratosphere above the tropospheric high, the increasing vertical wave activity flux, and the perturbations in the 360-K potential temperature surface. The forecasted 24-h changes generally have larger amplitudes than those seen in the analysis.

Fig. 16.
Fig. 16.

As in Fig. 15, but for a 5-day forecast ending at 0000 UTC 29 Dec 2012.

Citation: Monthly Weather Review 143, 2; 10.1175/MWR-D-14-00023.1

Unlike the January 2003 and January 2006 major SSW events, this transient upper-tropospheric system is difficult to relate directly to the January 2013 SSW. There is a change in the 10-hPa geopotential heights from 28 December 2012 to 2 January 2013 (the red contour in Figs. 3a and 3d) that suggests an eastward stratospheric wave propagation near 0° coincident with the transient tropospheric development and propagation; however, the large wave-breaking signature is apparent on 28 December, before the transient development in the upper troposphere. This suggests that the transient system, while it may have helped augment the wave breaking, played only a minor role in the January 2013 SSW.

4. Discussion and summary

The GEOS-5 five-day forecasts accurately predicted the major SSW of January 2013 (Figs. 1 and 2). In an examination of European Centre of Medium-Range Weather Forecasts (ECMWF) products during the major SSW of 2010, Dörnbrack et al. (2012) showed that forecasting midstratosphere dynamics is most challenging after the SSW, a time when horizontal EPV gradients are small. In spite of an increased spread seen in the ensemble system, Dörnbrack et al. (2012) showed that the high-resolution 5-day ECMWF forecast accurately captured the evolution of the complete 2010 SSW, including the time after the warming. Similarly, the GEOS-5 five-day forecasts at 10 hPa (Fig. 1) are capable of accurately representing the post-SSW dynamics of the lower stratosphere.

The evolution of the 10-hPa geopotential height field (Fig. 3) showed that the major SSW of January 2013 falls into the vortex-splitting-type SSW, which is distinct from a vortex-displacement-type SSW (Charlton and Polvani 2007). On 6 January 2013 the 10-hPa 60°N zonal mean zonal wind reversed direction from westerly to easterly, accompanied by a change in the zonal mean, 60°–90°N, 10-hPa temperature gradient from negative to positive at that time, satisfying the criteria for a major SSW (Figs. 1 and 2). Unlike the January 2009 SSW, that was preceded by a large wave-2 EP flux into the stratosphere (Harada et al. 2010), the wave-2 vertical EP flux amplitude was relatively small before and during the January 2013 SSW, with the wave-1 EP flux showing the largest peak before the SSW (Fig. 4).

The wave breaking and concomitant increase in poleward advection of low EPV associated with the major SSW occurs first near 10 hPa, the level where the anticyclone (large blue arrow in Fig. 5) first increases in strength, thereby increasing the amplitude of the climatological Aleutian high (Harvey and Hitchman 1996) in the Pacific hemisphere (Fig. 5). The wave activity flux is also greatest in this hemisphere over the course of the SSW event (see also Fig. 11). The tilt of the upward-developing high toward the pole, consistent with Aleutian high climatology of Harvey and Hitchman (1996), causes the SSW associated changes in polar temperature to first appear at somewhat higher levels near the pole, even though the initial wave breaking was ~10 hPa. This may reflect the focusing of the wave activity flux toward the pole (Fig. 5e) indicating wave propagation from the midlatitudes to the pole in this SSW event.

Overall, the vertical flux of wave activity at the tropopause (~100 hPa) occurred mainly in the Pacific hemisphere prior to the SSW (Figs. 11 and 12), in agreement with Martius et al. (2009) and Castanheira and Barriopedro (2010) who found Pacific blocking associated with wave-2 major SSW events. This vertical wave activity flux was associated with an upper-tropospheric (300 hPa) ridge that cut off at high latitudes under the polar vortex. The lower-stratospheric (50 hPa) flow subsequently experienced ridging in this region leading to a displacement of the polar vortex, wave breaking, and the major SSW event. A later upper-tropospheric ridge (6 January, ~0°) may have aided in splitting the vortex based on its location; however, it had only a small signal in the Atlantic hemisphere vertical wave activity flux. Accurate forecasting of these upper-tropospheric ridges is important in forecasting SSW events. An examination of the GEOS-5, five-day forecasts of the Atlantic upper-tropospheric ridge development and lower-stratospheric geopotential height changes found generally good agreement with the analyses (Figs. 9 and 10).

The surface low pressure system that rapidly developed under the stratospheric polar vortex on 29 December 2012 was accompanied by a large disturbance in the 360-K potential temperature surface and wavelike changes in the stratosphere up to 10 hPa (Figs. 13, 14, and 15). This storm system produced a strong increase in upward wave activity flux as it developed and propagated under the strong vortex winds. In the January 2003 and January 2006 such strong upper-tropospheric development over the North Atlantic also occurred prior to the wave-breaking, poleward PV advection and SSW events (Coy et al. 2009). While not directly associated with persistent large vertical wave activity flux, the North Atlantic storm of 29 December 2012 may have played a role in perturbing the larger horizontal-scale stratospheric waves that led to the SSW. However, the transient system of 28–29 December 2012 was not found to lead directly to significant wave breaking, as in the January 2003 and January 2006 SSW events, as the wave breaking and poleward PV advection was occurring by 28 December 2012.

In summary, GEOS-5 analyses showed that the SSW of January 2013 was a major warming by 1200 UTC 6 January, with a wave-2 vortex splitting pattern. Upward wave activity flux from the upper troposphere (~23 December 2012) displaced the ~10-hPa polar vortex off the pole in a wave-1 pattern, enabling the poleward advection of subtropical values of EPV into a developing anticyclonic circulation region. While the polar vortex subsequently split (wave-2 pattern) the wave-2 forcing (upward EP flux) was seen to be smaller than what was found in recent wave-2, SSW events, with most of the forcing located in the Pacific hemisphere. Our results show that the SSW began at midlatitudes at ~10 hPa, developing poleward and upward in amplitude before descending over the polar region. The overall success of the 5-day forecasts provides motivation to produce regular 10-day forecasts with GEOS-5, to better support studies of stratosphere–troposphere interaction.

Acknowledgments

Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Center for Climate Simulation (NCCS) at Goddard Space Flight Center. This work was supported by the NASA Modeling, Analysis and Prediction (MAP) program. We strongly thank both anonymous reviewers for their detailed comments and ideas for improvements on this manuscript.

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