Stratospheric Forcing of Mid-tropospheric Blocking Anticyclogenesis Stephen J. Colucci Cornell University

Motivation Sudden Stratospheric Warmings (SSWs) are occasionally preceded by tropospheric blocking (e.g. O’Neill and Taylor, QJRMS,1979; Quiroz, JGR, 1986). However, there is no meaningful relationship between SSWs and antecedent or eventual blocking (Taguchi, JAS, 2008). Nevertheless, can stratospheric processes influence the onset of tropospheric blocking anticyclogenesis?

Procedure Study a tropospheric blocking episode during January 1987 when there was a downward propagating stratospheric warming event (Baldwin and Dunkerton, JGR, 1999). Use ERA-40 data (Z, T, u, v, ) on 23 pressure levels (1000 mb through 1 mb) at 6-h time resolution and 2.5-degree horizontal resolution. Supplement these data with NNRP diabatic heating fields (long- and short-wave heating, condensational warming) on 28 sigma surfaces (0.995 to 0.0027) at 6-h time resolution, every 1.875 degrees in longitudinal direction and on a 94-point Gaussian grid from pole to pole.

Evolution of January 1987 blocking episode

Analyzed 72-h 500-mb height change during block onset

Diagnostic Model for 500-mb Height Tendency Vertically integrate hydrostatic, thermodynamic energy equation between bottom pressure Pb and top pressure PT : ∂Zb/∂t = ∂Zt/∂t Pb - (Rd/g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P Pt [ similar to Hirschberg and Fritsch (JAS, 1991) ]

(Diagnostic Model continued) Set Pb = 1000 mb and PT = 500 mb and rearrange to get 500-mb height tendency equation in terms of lower tropospheric processes: ∂Z500/∂t = ∂Z1000/∂t 1000 + (Rd/g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P 500

(Diagnostic Model continued) Set Pb = 500 mb and PT = 20 mb* to get a 500-mb height tendency equation in terms of upper-level (including stratospheric) processes: ∂Z500/∂t = ∂Z20/∂t 500 - (Rd/g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P 20 *vertical motion data are unreliable in the upper stratosphere, according to Wohltmann and Rex (Atm. Chem. Phys., 2008)

(Diagnostic Model continued) Combine previous two equations for 500-mb height tendency to obtain: ∂Z500/∂t = [ ∂Z20/∂t + ∂Z1000/∂t ]/2 1000 + (Rd/2g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P 500 - (Rd/2g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P 20

Total contribution to 500-mb height tendencies from lower tropospheric (left) and upper-level (right) processes during block onset

Analyzed and predicted 500-mb anticyclogenesis from lower-level (left) and upper-level (right) processes

Area-averaged (10 deg X 10 deg) total calculated 500-mb height tendencies (upper forced versus lower forced) following maximum analyzed 500-mb height rise

Area-averaged advective versus adiabatic 500-mb height tendencies, following maximum analyzed 500-mb height rise

Area-averaged radiatively versus condensationally forced 500-mb height tendencies, following maximum analyzed 500-mb height rise

Conclusions The 500-mb anticyclogenesis associated with the January 1987 blocking case critically depended upon upper-level (mostly stratospheric) adiabatic cooling. Lower tropospheric adiabatic and advective warmings assisted the 500-mb anticyclogenesis. Condensational warming, mostly from the lower troposphere and associated with a sea-level cyclone (not shown), assisted with the maintenance of the 500-mb anticyclone, once it was established.

Question: Is there a stratospheric influence on sea-level cyclones and anticyclones? Answer: Yes. See manuscript by Colucci (in review by JAS, 2008) for details (available upon request). Briefly: stratospheric warm-air advection helped force sea-level cyclogenesis in two cases; stratospheric adiabatic cooling initiated a spectacular sea-level anticyclogenesis case. Important question: What is the typical thermodynamic structure of the stratosphere above tropospheric weather systems?