Anticyclones Cause Weather Too: An Understanding of Worldwide Strong Anticyclones and Anticyclogenesis Matthew L. Doody, Lance Bosart and Daniel Keyser Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York NROW VIII 1-2 November, 2006 NSF ATM

Purposes Examine global regions where strong anticyclones and anticyclogenesis occur. Determine various thresholds and timescales to enhance the climatology. Find any possible interannual variability. Link strong anticyclones to geographical features. Lead into a study of predictability of strong anticyclones and anticyclogenesis.

Data and Methodology Data used consisted of: NCEP/NCAR Global Reanalysis at 2.5° resolution. ECMWF ERA-40 Global Reanalysis at 2.5° resolution. Both data sets were used from 1958– 2000 Thresholds were set for the MSLP. At each gridpoint a counter was used to sum the number of times the MSLP ≥ the threshold. Counts were only done at 0000 and 1200 UTC to mitigate double counting.

Data and Methodology After tabulation of the data it was then contoured objectively to show the regions where the threshold was met or exceeded. It is important to note that the figures that follow do not count coherent closed anticyclones, but simply the number of times the MSLP met or exceeded the defined threshold.

Outline Compare and contrast the two datasets. Examine the Northern Hemisphere (NH) at various thresholds as well as at monthly timescales. Same as above for the Southern Hemisphere (SH). Interannual variability. Time series of maximum count for ≥ 1050 hPa for each year throughout the datasets.

ERA-40 MSLP ≥ 1050 hPa from 1958–2000

NCEP/NCAR MSLP ≥ 1050 hPa from 1958–2000

ERA-40 MSLP ≥ 1040 hPa from 1958–2000

ERA-40 MSLP ≥ 1060 hPa from 1958–2000

Northern Hemisphere Winter Threshold: ≥ 1050 hPa

ERA-40 MSLP ≥ 1050 hPa for October

ERA-40 MSLP ≥ 1050 hPa for November

ERA-40 MSLP ≥ 1050 hPa for December

ERA-40 MSLP ≥ 1050 hPa for January

ERA-40 MSLP ≥ 1050 hPa for February

ERA-40 MSLP ≥ 1050 hPa for March

ERA-40 MSLP ≥ 1050 hPa for April

Northern Hemisphere Summer Threshold: ≥ 1035 hPa

ERA-40 MSLP ≥ 1035 hPa for May

ERA-40 MSLP ≥ 1035 hPa for June

ERA-40 MSLP ≥ 1035 hPa for July

ERA-40 MSLP ≥ 1035 hPa for August

ERA-40 MSLP ≥ 1035 hPa for September

Southern Hemisphere

ERA-40 MSLP ≥ 1040 hPa from 1958–2000

Hoskins and Hodges (2005)

ERA-40 MSLP ≥ 1050 hPa from 1958–2000

Southern Hemisphere Winter Threshold: ≥ 1040 hPa

ERA-40 MSLP ≥ 1040 hPa for May

ERA-40 MSLP ≥ 1040 hPa for June

ERA-40 MSLP ≥ 1040 hPa for July

ERA-40 MSLP ≥ 1040 hPa for August

ERA-40 MSLP ≥ 1040 hPa for September

ERA-40 MSLP ≥ 1040 hPa for October

Southern Hemisphere Summer Threshold: ≥ 1035 hPa

ERA-40 MSLP ≥ 1035 hPa for November

ERA-40 MSLP ≥ 1035 hPa for December

ERA-40 MSLP ≥ 1035 hPa for January

ERA-40 MSLP ≥ 1035 hPa for February

ERA-40 MSLP ≥ 1035 hPa for March

ERA-40 MSLP ≥ 1035 hPa for April

Interannual Variability

ERA-40 MSLP ≥ 1050 hPa for 1962

ERA-40 MSLP ≥ 1050 hPa for 1963

ERA for 1983 Western Hemisphere ERA-40 MSLP ≥ 1050 hPa for 1983

ERA-40 MSLP ≥ 1050 hPa for 1984

Time Series Of Maximum Count In The NH For Each Year

Conclusions ERA-40 was better at masking terrain than NCEP/NCAR. Cool season maxima occurred primarily over the continents while warm season maxima occurred primarily over the oceans in the NH. Distinct variability on the interannual timescale between the eastern and western halves of the NH. SH strong anticyclones tend to occur along time-mean storm track. Seasonal threshold contrast much smaller in SH due to oceans. SH contintental maxima tend to occur on lee of higher terrain. Both data sets support a decline in high threshold count during latter half of twentieth century.

Future Work Understand dynamical reasons for SH strong anticyclones remaining in time-mean storm track. Relate interannual variability to global teleconnections. Learn role of arctic-extratropical interactions to strong anticyclones and anticyclogenesis. Discuss key predictability issues associated with these anticyclones and their associated cold surges.

Thank You

NCEP/NCAR 1000hPa Heights 375+ m