7 o 25 8 6 N 5 a) b) 80 o Recent Amplification of the Summer Beaufort Sea High and its Impact on Chukchi Sea Ice Extent and

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N 3 70 o Wrangel Island 15 N G.W.K. Moore 7 o 2 N 65 o 80 N Department of Physics, University of Toronto 10 o 80 60 70 o N 1011 N 80 o R.S. Pickart Polynya Area (10 5 150 50 N 1010 68 o W Department of Physical Oceanography,Woods Hole Oceanographic Institution N Siberia Alaska o o 1009 0 E 1975 1980 1985 1990 1995 2000 2005 2010 2015 Year 70 The Beaufort Sea High (BSH), a closed anti-cyclone over the Beaufort Sea, is an important feature of the sea-level 6 o 160 1013 6 N 170 o o W In recent years, the size of the early-summer Wrangel E o 150 o W 175 o W o 1010 1012 pressure field over the Arctic Ocean. It plays an important role in the export of Arctic sea ice. Years characterized E o o 155 Polynya has been increasing. 1011 180 W o o o W E H 175 W 170 W 165 W 160 80 by low Arctic sea ice extent are associated with a reduction in summer cyclones over the Arctic Ocean and the pres- 170 Figure 7) (a) The monthly mean sea ice concentration (%) over the Chukchi Sea during o o W 3 2 E ence of a stronger BSH with the opposite occurring during years with high sea ice extent. This was especially true June 1979 to 2011. (b) The polynya size (10 km ) in the region north of Wrangel Island for 180 90 in 2007, the record minimum sea ice extent, when the BSH was anomalously high throughout the summer. Although June 1979 to 2011. The polynya size was determined as the area for which the sea ice con- o 1012 o W W centration was less than 80%. 170 o 100 there has been a trend towards a loss of Arctic sea ice over the entire spaced-based observational record, i.e. from o W 85 W o The early-summer Wrangel Island Polynya is associat- 160 o o 110 1979 onwards, an acceleration occurred in the late 1990s. We show this acceleration coincided with a trend towards N W 80 ed with a more intense BSH and enhanced easterly flow W o o o 150 o W 120 N W 140 W 130 a stronger BSH as well as a trend towards a reduction in cyclogenesis over much of the Arctic Ocean including the 75 across the Beaufort and Chukchi Seas. The trend towards o 4 N 2 a more intense BSH would also lead to enhanced easterly Beaufort Sea. We argue that these trends are the result of a warming of the Arctic troposphere and the concomitant 70 3 o 1 N flow in this region. 65 2 reduction in baroclincity. o 0 N 1 60 We also show that a consequence of this amplification of the BSH is an increase in the frequency of easterly winds o N 55 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 o over the Chukchi Sea shelf break that are of sufficient magnitude to result in a upwelling as well as a reversal in N 50 Figure 1) Climatological summer (JJA) sea-level pressure (mb) from the NCEP Re- o the direction of the oceanic shelf break jet. The impact of this increased frequency of upwelling on phytoplankton 160N o o W analysis for the period 1948-2011. The climatological center of the Beaufort Sea High E blooms in the region will be discussed. 110 170 −2 −2 o is indicated by the ‘H’. 8 8 o W a) 5 b) 5 E −1 80 o 80 o 180 −1 0 120 N N o o W o o W 75 75 170 o o 130 −1 N N W o o 140 2m s 70 o 70 o 160 W 150 W N N 65 o 65 o N 1013 N 60 o 60 o 1011 The role of the Beaufort N N 1010 o o 15 15 −5 −4 −3 −2 −1 0 1 2 3 4 5 Sea High in transporting N N 1014 1010 0 1011 o W 0 o W sea ice out of the western o 1008 o 1010 1011 Figure 8) The difference in the monthly mean sea-level pressure (contours and shading - E 1009 1013 1009 E 1008 Arctic is evident. 70 70 mb) and 10m wind field (vectors - m/s) for June between those years in which there is ei- 160 1015H 160 1012 o W 1009 o W o 1010 o H E 1011 E ther a large or small polynya north of Wrangel Island. 170 80 170 80 o o W o o W E 1012 E 1011 1012 180 90 180 90 o 1012 o W o 1011 o W W W 170 100 170 100 o o W o 1012 o W W 1013 W 160 o o 110 160 o 1013 o 110 W W W W 150o o o 120 150o o o 120 W 140 W 130 W W 140 W 130 W Figure 4) Composite of the sea-level pressure (mb) during high (a) and low (b) index summers, based on the Singular Spectrum Anal- ysis reconstruction of the variability on the decadal time-scale (Figure 3b). The BSH alternates between a deeper anti-cyclone centered Both transects show evidence of high phytoplankton bio- near 75oN, 145oW and a weaker trough-like circulation centered near 70oN, 120oW. mass (POC=Particulate Organic Carbon) under the sea ice along the Chukchi shelf break. The phytoplankton 8 5 80 o bloom is the result of a thinning of the ice cover that al- N 75 o lows sunlight through as well as upwelling of nutrient rich N bottom water. 70 o N Figure 9) During ICESCAPE 2011, an under ice phytoplankton bloom was observed be- Sea-level pressures over the west- 65 o Figure 2) Annual mean sea-level pressure over the period 1979–2008 from the N 5 ern Arctic have been increasing at a 60 o tween July 4-8 along two transects (Arrigo et al 2012 in press Science). NCEP reanalysis with overlay of mean sea ice velocity vectors for 1979–2006 based N 50 o 150 Recent increase in upwelling favour- -1 statistically significant rate since the N 4 W 45 on a combination of satellite and buoy data. Ice motion is cm s . Figure from Serreze o late 1990s. o able winds that coincided with recent E 3 40 70 trend towards a more intense BSH. and Barrett J. Climate 2011. 160 2 W 35 Correlation coefficient between the two a) 1022 o o E 1 30 80 is 0.59. 1020 170 0 o o W 25 E −1 mean + 180 90 20 1σ 1018 o o W W Frequency (%) 15 170 100 The central pres- o o W mean 1016 W 10 sure of the BSH 160 o −1 o 110 W W 5 150o o o 120 began to increase 1014 W 130 W 140 W 0 in the late 1990s. Figure 5)The trend in the summer (JJA) sea-level pressure (mb/decade) from the NCEP Reanalysis for the period 1996-2011. The 1975 1980 1985 1990 1995 2000 2005 2010 2015 1012 Year trend is statistically significant at the 95% level in the shaded regions using a test that uses the red-noise characteristics of the sea-

Sea−Level Pressure (mb) Figure 10) Frequency of upwelling favourable, i.e. easterly, winds along the Chukchi shelf 1010 level pressure field. −6 −4 −2 0 2 4 6 break during early July 1979-2011. 8 8 5 b) 5 a) 80 o 80 o 1008 N N 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 75 o 75 o Conclusions: N N Year 70 o 70 o N N 0 0 • The summer BSH exhibits variability on the decadal time scale as well as a trend towards b) 1020 65 o 65 o N 0.2 N higher central pressures that began in the late 1990s. 60 o 60 o N 0.25 N o o 15 15 • The decadal scale variability is manifested by a meridional shift in the center of the BSH. 1018 N N The central pres- 0.35 0 o W 0 o W o o • Since the late 1990s, there has been a statistically significant increase in sea-level pressure sure of the BSH E 0.3 E 0 −0.1 1016 70 70 exhibits variabil- 160 160 0.05 across the western Arctic. 0.35 o W o W ity on the decadal o o −0.05 −0.15 E E • This trend is coincident with a reduction in Eady Growth Rate in the region that is the 1014 0.1 time scale 170 80 170 80 0.4 result of a reduction in baroclinicity associated with differential warming of the western o o W o −0.05 o W E 0.2 0.4 E 1012 180 90 180 90 Arctic’s troposphere. o 0.25 o W o 0 0 o W W W • The increase in the central pressure of the BSH has resulted in enhanced easterly flow 170 0.35 100 170 0 100 Sea−Level Pressure (mb) o 0.3 o W o o W 1010 W W across the Beaufort and Chukchi Seas. 160 o o 110 160 o o 110 W W W W 150o o o 120 150o o o 120 • This has lead to an increased frequency of occurrence of the summer Wrangel Island Po- 1008 W 140 W 130 W W 140 W 130 W 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 -1 lynya. Year Figure 6) (a) The summer (JJA) Eady Growth Rate (days ) from the NCEP Reanalysis during the period 1948-2011. (b) The trend -1 • The enhanced easterly flow has also resulted in an increase in upwelling along the Chuk- Figure 3) Time series of the summer (JJA) sea-level pressure (mb) in the center of the in the summer (JJA) Eady Growth Rate (days /decade) during the period 1996-2011. In (b), the trend is statistically significant at the 95% level in the shaded regions. In (a) the region of large Eady Growth Rate that extends southeastwards from the Beaufort Sea chi self break. Beaufort Sea High from the NCEP Reanalysis during the period 1948-2011. The mean 0 0.05 0.11 0.16 0.22 0.27 0.33 0.38 0.44 0.49 0.55 0.6 −0.2 −0.1 0 0.1 0.2 represents the storm track associated with the Arctic Front. In (b), there is a statistically significant trend towards lower Eady Growth • This has increased the nutrient levels near the surface that contributed to the existence of over this period is indicated by the thin dashed line. In (a), the low-frequency recon- an under ice phytoplankton bloom that was observed in early July 2011. struction of the time series is indicated by the thick red line. In (b), the reconstruction Rates that is consistent with the trend towards higher sea-level pressures in the region (Figure 5) and a more intense BSH (Figure 3a). that captures variability in the time series on the decadal time scale is indicated. Both The trend towards lower Eady Growth Rates is the result of a reduction in the meridional temperature gradient resulting from a differen- were computed using the Singular Spectrum Analysis (SSA) technique. tial warming of the western Arctic’s troposphere.