Geophysical Research Letters

The Novaya Zemlya Bora and its impact on Barents Sea air-sea interaction

Authors


Abstract

[1] Novaya Zemlya is a mountainous archipelago in the Eastern Arctic. Weather station data indicates that southeasterly high-speed winds are frequent, especially during the winter. Although it has been proposed that a bora is responsible for these winds, there has been no quantitative analysis of the flow in the region. Here we present the first high-resolution climatology of the three-dimensional wind field near Novaya Zemlya. The high-speed wind events are shown to share many characteristics with bora as well as models of downslope windstorms. The highest wind speeds occur where Barents Sea dense water is observed to form, and we show that there is a doubling in the turbulent heat transfer from the ocean to the atmosphere during bora. It is argued that the bora plays an important role in the transformation of the Atlantic water that passes by the archipelago en route to the central Arctic Ocean.

1 Introduction

[2] Novaya Zemlya is an archipelago in the Eastern Arctic that is situated approximately 500 km north of the Arctic Circle. The two major islands in the archipelago are separated by the Matochkin Strait that is at its maximum 3 km wide [Matzko, 1993], and so for all but the smallest scales of atmospheric motion, the two main islands can be considered as one (Figure 1 and supporting information Figure S1). The archipelago is an extension of the Ural Mountains and has maximum heights in excess of 400 m.

Figure 1.

Topography and mean surface atmospheric circulation in the Novaya Zemlya region. (a) The topography (shading and contours - km). (b) The winter mean (DJF) sea-level pressure (contours - mb), 10 m wind field (vectors - m/s) and the 10 m wind speed (shading - m/s). In Figure 1a, the region with winter mean sea ice concentrations in excess of 50% is indicated by the grey shading, while in Figure 1b, the winter mean 50% sea ice concentration contour is indicated by the thick solid line.

[3] The archipelago also lies along the boundary between the Kara and Barents Seas (Figures 1 and S1). During winter the Kara Sea is typically ice covered; while the Barents Sea has ice concentrations of 15% or less. A wind-driven polynya has been observed to form during the winter along the western coastline of the archipelago [Martin and Cavalieri, 1989]. The surface cooling and the concomitant brine rejection during sea ice formation in this polynya contribute to dense water formation that occurs in the Barents Sea [Arthun et al., 2011].

[4] Weather station data from the southwest coast of the archipelago indicates that the period from December through February is the windiest with monthly mean wind speeds of 10 m/s with a prevailing southeasterly wind direction [Lydolph, 1977]. In addition, there are approximately 125 days each year in which a wind speed in excess of 15 m/s is observed [Lydolph, 1977].

[5] The analysis of the archive of polar orbiting satellite imagery identified the frequent occurrence during the winter of anomalous cloud features in the vicinity of Novaya Zemlya [Matson, 1986] that were subsequently proposed to be orographic clouds associated with flow over the high topography of the islands [Parmenter-Holt, 1987]. Fett [1992] considered several case studies based on satellite imagery and likened the phenomenon to downslope windstorms. A QuikSCAT surface wind speed climatology of the Nordic Seas identified the Barents Sea just to the west of Novaya Zemlya as a region where high speed southeasterly flow is common [Kolstad, 2008]. However, no connection was made with the previous observations of presumed orographic flow in the region.

[6] Although a number of authors have referred to these high-speed winds as a bora [Matzko, 1993; Zeeberg, 2002], there has been no systematic study of their structure, the role that Novaya Zemlya plays in their formation, nor their impact on the ocean. In this paper, we will use the recently released Interim Arctic System Reanalysis (ASRI) [Bromwich et al., 2010] to investigate this phenomenon highlighting the commonality with observations of the Yugoslavian Bora [Smith, 1987] as well as idealized studies of stratified flow over topography in the presence of an environmental critical layer [Clark and Peltier, 1984; Bacmeister and Pierrehumbert, 1988].

2 Data and Methods

[7] Atmospheric reanalyses offer the possibility of developing a climatology of high-speed wind events near Novaya Zemlya as has been done for Greenland barrier flow and tip jets [Harden and Renfrew, 2012; Moore, 2012]. However, until recently, reanalyses with sufficient resolution to capture these events near Novaya Zemlya have not been available. The recent completion of the ASRI with its 30 km resolution offers the possibility of generating such a climatology. The ASRI is based on the “polar” version of the Weather Research and Forecasting model [Bromwich et al., 2010]. The reanalysis covers the entire Arctic at a 3 h temporal resolution for the period 2000–2010. A comparison with the Interim Reanalysis from the European Centre for Medium-Range Weather Forecasts shows comparable RMS errors for surface fields [Bromwich et al., 2012].

[8] For this paper, 3 hourly surface and pressure-level data were analyzed for the available 11 winters (DJF) on a domain centered on Novaya Zemlya (Figure 1). Winter mean fields were calculated as well as the directional characteristics of the high-speed 10 m wind events over this domain using a technique developed by Moore [2012].

3 Results

[9] Figure 1 shows the ASRI topography in the region of interest as well as the winter mean ASRI sea ice field. The topography of Novaya Zemlya is oriented in an approximate north-south direction with elevations in excess of 400 m occurring on the northern island. Figure 1 also shows the winter mean sea-level pressure and 10 m wind fields and indicates that the circulation in the region is dominated by the Lofoten Low [Jahnke-Bornemann and Bruemmer, 2009]. Wind speeds over the Barents Sea are significantly higher than those over the Kara Sea, and this is most likely the result of the change in surface roughness between the ice-covered Kara Sea and the ice free Barents Sea [Petersen and Renfrew, 2009] as well as the synoptic-scale sea-level pressure gradient. Along the west coast of Novaya Zemlya, there is a pronounced cross-isobar southeasterly flow.

[10] The frequency of occurrence of high-speed wind events has been shown to be a useful diagnostic for the identification of regions where topographically forced flow occurs [Moore and Renfrew, 2005; Kolstad, 2008]. Moore [2012] introduced the concept of partitioning this occurrence frequency by wind direction to further elucidate the characteristics of these systems. In Figure 2, the wintertime occurrence frequency of northerly and southerly flow with 10 m wind speeds in excess of 14 m/s is shown. Other cut-offs produced similar results. The occurrence frequency for high-speed northerly flow (Figure 2a) was much lower than that for southerly flow (Figure 2b) and is a reflection of the winter mean flow over the region of interest (Figure 1b). There are two regions, one along the west coast of Novaya Zemlya and the other over the central Barents Sea, where the occurrence frequency of high-speed southerly flow is elevated. The further partitioning into southeasterly (Figure 2c) and southwesterly (Figure 2d) flows shows that the maximum along the west coast of Novaya Zemlya is the result of the southeasterly flow. In contrast, that over the central Barents Sea is the result of both wind orientations.

Figure 2.

The frequency of occurrence (%) of 10 m wind speeds in excess of 14 m/s during the winter months (DJF) with the following: (a) a northerly wind direction; (b) a southerly wind direction; (c) a southeasterly wind direction; and (d) a southwesterly wind direction.

[11] From the results presented in Figure 2, high-speed southeasterly flow occurs in excess of 10% of the time during the winter along the west coast of Novaya Zemlya. To identify the characteristics of the high-speed winds at this location, 90 individual events over the 11 winters that met the criteria for at least 6 h were identified using a technique employed in similar studies of near Greenland [Moore and Renfrew, 2005] and the Siberian coast of the Bering Sea [Moore and Pickart, 2012]. Figure 3 shows the composite sea-level pressure, 10 m wind and sea ice fields for these events. These events tend to be associated with deep low-pressure systems centered near 73°N, 25°E. The pronounced mesoscale pressure gradient across Novaya Zemlya with a magnitude of approximately 4 mb is evident as is the cross-isobar acceleration that leads to the southeasterly flow on the lee shore of the archipelago.

Figure 3.

Composite sea-level pressure (contours - mb), 10 m wind field (vectors - m/s), and the 10 m wind speed (shading - m/s) associated with high-speed southeasterly wind events along the west coast of Novaya Zemlya. The composite 50% sea ice concentration contour is indicated by the thick black line.

[12] The composite zonal cross-section of the flow associated with these high-speed wind events is shown in Figure 4. The potential temperature and Brunt-Väisälä frequency cross-sections (Figures 4a and 4b) show the marked difference in static stability between the Kara and Barents Seas that is the result of the change in ice cover. In particular, the Brunt-Väisälä frequencies upwind of Novaya Zemlya are in excess of 3 × 10−2 s−1; while those immediately downwind of the archipelago are less than 5 × 10−3 s−1. There is also a pronounced lowering of the isentropes as the obstacle is crossed. The cross-section of the zonal component of the wind (Figure 4c) indicates that upwind of the barrier; there is weak easterly flow at the surface that is approximately 1 km deep. Over the obstacle, there is a dramatic descent of the 0 m/s isocontour with a pronounced acceleration on its lee side. The cross-section of the vertical velocity field (Figure 4d) has a wave-like structure in the vertical; while that of the meridional component of the wind (Figure 4e) has a jet-like core of southerly flow situated just downwind of the obstacle. Finally, the cross-section of the horizontal wind speed (Figure 4f) indicates that the highest wind speeds associated with these events is in excess of 18 m/s and occurs at a height of approximately 500 m above the surface.

Figure 4.

Composite height-zonal cross-sections along 74.5°N for high-speed southeasterly wind events along the west coast of Novaya Zemlya: (a) the potential temperature field (K); (b) the Brunt-Väisälä frequency (10−2 s−1); (c) the zonal component of the wind (m/s); (d) the vertical component of the wind (cm/s); (e) the meridional component of the wind (m/s); and (f) the horizontal wind speed (m/s). The topography along the cross-section is indicated.

[13] The zonal cross-section of the flow during these high-speed wind events indicated the presence of a region of low static stability just downwind of Novaya Zemlya (Figure 4b). At high latitudes, such regions typically form as a result of intense air-sea interaction arising from large ocean-atmosphere temperature differences and high wind speeds [Renfrew and Moore, 1999]. Figure 5 shows the winter mean climatological total turbulent heat flux, i.e., the sum of the sensible and latent heat fluxes, over the region as well as the composite total turbulent heat flux during the high-speed wind events. The sensible and latent heat fluxes were calculated from the 3 hourly surface fields using a bulk parameterization that has been validated against observed fluxes over the Labrador Sea during the winter [Renfrew et al., 2002]. The winter mean indicates that the total turbulent heat flux is in excess of 100 W/m2 over the Barents Sea with two local maxima that correspond to the regions where the occurrence frequency of southerly high-speed wind is elevated (Figure 2b). There is also a pronounced gradient across the transition from ice-covered to ice free regions. During the Novaya Zemlya high-speed wind events, the total turbulent heat flux in the vicinity of the maxima in the occurrence frequency of high-speed southeasterly flow is doubled as compared to the winter mean, i.e., 300 W/m2 as compared to 150 W/m2. This is primarily the result of the higher wind speeds. However the increased advection of cold air across Novaya Zemlya during these events was also a factor.

Figure 5.

The total turbulent heat flux (W/m2) in the Novaya Zemlya region. (a) The winter mean (DJF) climatological field with the winter mean 50% sea ice concentration contour indicated by the thick solid line. (b) The composite field associated with high-speed southeasterly wind events along the west coast of Novaya Zemlya with the composite 50% sea ice concentration isocontour indicated by the thick solid line.

4 Discussion

[14] In this paper, the structures of the high-speed wind events that occur along the west coast of the Novaya Zemlya archipelago have been investigated with the ASRI. Novaya Zemlya is approximately 120–140 km wide, and so the ASRI with its horizontal resolution of 30 km is able to resolve the resulting topographic flow distortion. For example, the sea-level pressure field for both the winter mean (Figure 1b) and the composite high-speed wind event (Figure 3) both show clear evidence of a dipolar pressure anomaly across the topography that is predicted to occur in such situations [Smith, 1982]. In addition, the winter mean ASRI 10 m wind speed and direction along the west coast of Novaya Zemlya are in good agreement with station data, although it appears that the ASRI underestimates the occurrence frequency of high-speed winds [Lydolph, 1977].

[15] The location along the west coast of Novaya Zemlya where the frequency of occurrence of high-speed wind events is elevated agrees with that found by Kolstad [2008] as well as being in the region where a polynya has been observed to form [Martin and Cavalieri, 1989]. A case study of polynya formation indicated that it developed in southeasterly flow associated with a low-pressure system with a center over the western Barents Sea [Martin and Cavalieri, 1989]. The sea-level pressure and 10 m wind fields during the composite high-speed wind event shows a similar circulation (Figure 3). It should be noted that the sea ice field in the ASRI does not contain a representation of the polynya either in the winter mean climatology or in the high-speed wind event composite. The reasons for this are not clear, but it may reflect the recent retreat of sea ice in the region [Arthun et al., 2012] or difficulties in resolving sea ice concentration in the marginal ice zone [Meier, 2005].

[16] The upwind conditions over the ice-covered Kara Sea during these high-speed wind events are characterized by a stable surface layer (Figure 4b) as well as surface easterly flow with westerly flow aloft (Figure 4c). Vertical profiles of these two parameters in the undisturbed upwind region are shown in Figure S2. The reversal of the cross-obstacle wind with height indicates the presence of a critical level that can result in the formation of severe downslope winds [Clark and Peltier, 1984; Bacmeister and Pierrehumbert, 1988]. Using an analogy with hydraulic theory, the downslope acceleration can be viewed as a transition from subcritical to supercritical flow resulting from absorption of gravity waves [Smith, 1989]. Assuming that the obstacle is two-dimensional, its evolution is a function of the nondimensional barrier height:

display math

and the nondimensional critical layer height:

display math

where h is the barrier height, zc is the critical layer height, N is the upwind Brunt-Väisälä frequency and U0 is the upwind surface velocity [Smith, 1989]. For the present case: h ≈ 500 m, zc ≈ 1 km, N ≈ 2.5 × 10− 2s− 1 , and U0 ≈ 2 m/s resulting in inline image and inline image For cases in which a severe downslope windstorm develops in the presence of a critical layer, there exists a relationship between inline image and inline image [Smith and Sun, 1987] that in the supporting information is shown to reduce to inline image in the limit of large inline image. So in this case, inline image is predicted to be on the order of 15. As shown in the supporting information, the level of agreement in this instance is similar to that obtained in other cases of severe downslope windstorms in the presence of a critical layer [Smith, 1987]. However, it should be noted that all previous examples of downslope windstorms had inline image [Smith, 1987; Bacmeister and Pierrehumbert, 1988], and so the Novaya Zemlya Bora appears to develop in a previously unexplored region of phase space.

[17] In addition, the vertical structure of the composite bora event is similar to that of bora observed in the Yugoslavian region [Smith, 1987] as well as that predicted to occur in idealized models of supercritical flow over topography [Bacmeister and Pierrehumbert, 1988]. Most importantly, the descent of the isentropes and the critical level across the obstacle (Figures 4a and 4c) are common features of both bora and idealized models of downslope windstorms. It is this descent that is, through the mass conservation, ultimately responsible for the downslope acceleration. In agreement with observations, this acceleration begins upstream of the obstacle. There is also a region of relatively stagnant cross-obstacle flow downstream of the obstacle. However, unlike observations of the Yugoslavian Bora, this region is characterized by a higher static stability as compared to the surface layer. This is most likely the result of the advection of high static stability air over the barrier as well as the air-sea interaction that is occurring in the lee. Again, unlike the cases described by Smith [1987], there was pronounced flow perpendicular to the obstacle downwind of the barrier. This appears to be the result of barrier flow along the west coast of Novaya Zemlya. The colocation of enhanced downslope and alongslope flow has also been identified to be a characteristic of barrier flow along the southeast coast of Greenland [Harden and Renfrew, 2012].

[18] Finally, the high winds associated with the Novaya Zemlya Bora have been shown to result in a doubling of the total turbulent heat flux in the region where, as previously mentioned, a polynya has been observed to form. Such a polynya requires strong winds to advect away the sea ice that forms as a result of the oceanic heat loss to the atmosphere. The oceanic cooling and brine rejection that occurs during sea ice formation densifies the surface waters. Indeed, the polynya forms in a region where dense water is observed to form in the Barents Sea, a process that contributes to the modification of the inflowing Atlantic Water as it enters the central Arctic Ocean north of Novaya Zemlya [Arthun et al., 2011]. It is therefore likely that the Novaya Zemlya Bora is responsible for this polynya and contributes to the Arctic Ocean thermohaline circulation.

Acknowledgments

[19] The author would like to thank the Polar Meteorology Group at the Ohio State University and National Center for Atmospheric Research for access to the ASRI data as well as the reviewers for their comments. The author was supported by the Natural Sciences and Engineering Research Council of Canada.

[20] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.

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