Strong Polar Anticyclone Activity over Strong Polar Anticyclone Activity over the Northern Hemisphere and an the Northern Hemisphere and an

Examination of the Alaskan AnticycloneExamination of the Alaskan Anticyclone

Justin E. Jones, Lance F. Bosart, and Daniel KeyserJustin E. Jones, Lance F. Bosart, and Daniel KeyserDepartment of Earth and Atmospheric SciencesDepartment of Earth and Atmospheric Sciences

University at AlbanyUniversity at AlbanyState University of New YorkState University of New York

10th Northeast Regional Operational WorkshopAlbany, NY

5 November 2008

Research supported by NSF Grant ATM-0434189

GoalsGoals

• Perform an analysis of strong anticyclone activity over the Northern Hemisphere (NH).

• Compare the variability in strong anticyclone activity using the NCEP–NCAR and ERA-40 reanalysis datasets.

• Create a composite Alaskan anticyclone of individual events to examine large-scale dynamical and thermodynamical processes relevant to its formation and evolution.

Data SourcesData Sources

• ECMWF ERA-40 global reanalysis at 2.5° horizontal resolution

- 1 January 1958 – 31 December 2001

• NCEP–NCAR global reanalysis at 2.5° horizontal resolution

- 1 January 1948 – 31 December 2007

Methodology: Anticyclone Methodology: Anticyclone ClimatologyClimatology

• An MSLP threshold of 1050 hPa was set at each grid point and a counter was used to sum the number of times the threshold was met or exceeded.

• Frequency of occurrence was then contoured objectively based on the number of counts at each grid point.

• Full climatology stratified by 10-year periods in the ERA-40 to examine temporal variability on decadal timescales.

Strong Anticyclone Distribution over the NHMSLP MSLP ≥ 1050 hPa≥ 1050 hPa Max Count: 2785

CountCount

NCEPNCEP––NCARNCAR 1948 – 2007 1948 – 2007

Strong Anticyclone Distribution over the NHMSLP MSLP ≥ 1050 hPa≥ 1050 hPa

CountCount

NCEPNCEP––NCARNCAR 1948 – 2007 1948 – 2007 Max Count: 2785

Strong Anticyclone Distribution over the NHMSLP MSLP ≥ 1050 hPa≥ 1050 hPa Max Count: 852

CountCount

ERA-40 ERA-40 1958 – 20011958 – 2001

Strong Anticyclone Distribution over the NHERA-40 ERA-40 1958 – 19671958 – 1967 MSLP MSLP ≥ 1050 hPa≥ 1050 hPa

CountCount

Max Count: 246

Strong Anticyclone Distribution over the NHERA-40 ERA-40 1968 – 19771968 – 1977 MSLP MSLP ≥ 1050 hPa≥ 1050 hPa

CountCount

Max Count: 235

Strong Anticyclone Distribution over the NHERA-40 ERA-40 1978 – 19871978 – 1987 MSLP MSLP ≥ 1050 hPa≥ 1050 hPa

CountCount

Max Count: 166

MSLP MSLP ≥ 1050 hPa≥ 1050 hPa

CountCount

Strong Anticyclone Distribution over the NHERA-40 ERA-40 1988 – 19971988 – 1997 Max Count: 136

NCEP–NCAR Anticyclone Maximum Counts by Year for MSLP threshold of 1050 hPa (1948–2007)

y = -0.2501x + 57.13

R2 = 0.0829

0

20

40

60

80

100

1948 1958 1968 1978 1988 1998 2008

Year

Ma

xim

um

Co

un

t

ERA-40 Anticyclone Maximum Counts by Year for MSLP threshold of 1050 hPa (1958–2001)

y = -0.294x + 28.89

R2 = 0.1369

0

10

20

30

40

50

60

1958 1968 1978 1988 1998

Year

Max

imu

m C

ou

nt

Climatology SummaryClimatology Summary

• Significant interannual and interdecadal variability in both reanalysis datasets

• NCEP–NCAR overestimates the number of strong anticyclones over higher terrain features (e.g., Greenland, Tibetan plateau)

• Steady decrease in annual maximum 1050 hPa or greater count concentrated over central Asia

Methodology: Alaskan Anticyclone Methodology: Alaskan Anticyclone CompositeComposite

• Subjectively-defined domain over Alaska and extreme northwestern Canada.

• Composite of 22 individual events using the ERA-40 dataset which exceeded 1050 hPa within the domain.

• Centered on each individual event attaining a 1050 hPa pressure (t = −48 h to t = +48 h).

• Examine the development, evolution, and maintenance of the composite anticyclone.

Composite Case ListComposite Case List

1200 UTC 8 Dec 1977 1200 UTC 30 Jan 1989

0000 UTC 18 Nov 1978 1200 UTC 27 Feb 1989

0000 UTC 3 Jan 1979 1200 UTC 18 Dec 1990

1200 UTC 8 Jan 1980 0000 UTC 3 Jan 1991

0000 UTC 25 Jan 1980 0000 UTC 27 Dec 1992

1200 UTC 17 Dec 1980 1200 UTC 11 Feb 1995

0000 UTC 23 Feb 1982 1200 UTC 7 Dec 1995

1200 UTC 19 Dec 1983 1200 UTC 11 Mar 1997

1200 UTC 17 Oct 1984 1200 UTC 17 Dec 1998

0000 UTC 9 Nov 1985 1200 UTC 21 Mar 2001

0000 UTC 21 Nov 1985 0000 UTC 4 Mar 2002

t = 0 ht = 0 h

300 hPa height (solid, dam), wind speed (shaded, m s−1), 300 hPa divergence (negative values dashed, 10−6 s−1)

60

50

40

30

n = 22n = 22

MSLP (solid, hPa), 700 hPa geostrophic relative vorticity (shaded, 10−5 s−1), 1000–500 hPa thickness (dashed, dam)

t = t = −−48 h48 h

60

50

40

30

n = 22n = 22

MSLP (solid, hPa), 700 hPa geostrophic relative vorticity (shaded, 10−5 s−1), 1000–500 hPa thickness (dashed, dam)

t = t = −−24 h24 h

300 hPa height (solid, dam), wind speed (shaded, m s−1), 300 hPa divergence (negative values dashed, 10−6 s−1)

60

50

40

30

MSLP (solid, hPa), 700 hPa geostrophic relative vorticity (shaded, 10−5 s−1), 1000–500 hPa thickness (dashed, dam)

n = 22n = 22t = 0 ht = 0 h

300 hPa height (solid, dam), wind speed (shaded, m s−1), 300 hPa divergence (negative values dashed, 10−6 s−1)

30

40

50

60

MSLP (solid, hPa), 700 hPa geostrophic relative vorticity (shaded, 10−5 s−1), 1000–500 hPa thickness (dashed, dam)

n = 22n = 22t = +24 ht = +24 h

300 hPa height (solid, dam), wind speed (shaded, m s−1), 300 hPa divergence (negative values dashed, 10−6 s−1)

40

50

60

30

t = +48 ht = +48 hn = 22n = 22

MSLP (solid, hPa), 700 hPa geostrophic relative vorticity (shaded, 10−5 s−1), 1000–500 hPa thickness (dashed, dam)

300 hPa height (solid, dam), wind speed (shaded, m s−1), 300 hPa divergence (negative values dashed, 10−6 s−1)

t = t = −−24 h24 h n = 22n = 22

A A’

A

A’

HH

Below: 700 hPa height (solid black,dam), 700 hPa temperature (dashed blue, K), and vertical motion (positivevalues shaded, 10−3 hPa s−1)

Above: Cross section of potential temperature (solid black, K), vertical motion (positive values solid red, 10−3 hPa s−1), and winds (knots)

t = 0 ht = 0 h

A A’

n = 22n = 22

A

A’

HH

Below: 700 hPa height (solid black,dam), 700 hPa temperature (dashed blue, K), and vertical motion (positivevalues shaded, 10−3 hPa s−1)

Above: Cross section of potential temperature (solid black, K), vertical motion (positive values solid red, 10−3 hPa s−1), and winds (knots)

t = +24 ht = +24 h n = 22n = 22

A A’

A

A’

HH

Below: 700 hPa height (solid black,dam), 700 hPa temperature (dashed blue, K), and vertical motion (positivevalues shaded, 10−3 hPa s−1)

Above: Cross section of potential temperature (solid black, K), vertical motion (positive values solid red, 10−3 hPa s−1), and winds (knots)

t = +48 ht = +48 h n = 22n = 22

A A’

A

A’

HH

Below: 700 hPa height (solid black,dam), 700 hPa temperature (dashed blue, K), and vertical motion (positivevalues shaded, 10−3 hPa s−1)

Above: Cross section of potential temperature (solid black, K), vertical motion (positive values solid red, 10−3 hPa s−1), and winds (knots)

Alaskan Composite SummaryAlaskan Composite Summary

• Strong amplification of the upper ridge upstream of the surface anticyclone between t = −24 h and t = +24h.

• Upper-level convergence and advection of anticyclonic geostrophic relative vorticity by the thermal wind (TW) over the center of the surface anticyclone act to rapidly intensify it between t = 0 h and t = +24h.

• Composite anticyclone reaches maximum intensity (1049 hPa) at t = +24 h.

• Anticyclone elongates southeastward along the eastern slope of the Rockies between t = +24 h and t = +48 h.

• ERA-40 outperforms NCEP–NCAR in representing the number of strong anticyclones over higher terrain.

• Strong anticyclone (1050 hPa or greater) frequency is declining at a rate of approximately 3–4 counts per decade over central Asia.

• The strong Alaskan anticyclone is tropospheric deep and dynamically forced.– Convergence in upper troposphere– Advection of geostrophic relative vorticity by TW in

the mid-troposphere

ConclusionsConclusions