A319 FLT HAS AIRFRAME OVERSPD DURING CRUISE FLT.

Narrative

AT XA55Z; WE WERE APPROX 40 MI W OF FLM AT FL370 AND TALKING TO ZID. THE PURSER HAD JUST DELIVERED THE MEALS AND THE FO HAD SECURED THE DOOR AND WAS SITTING DOWN. THE WINDS HAD BEEN ABOUT 225 DEGS AT 125 KTS FOR A WHILE. WE ENCOUNTERED A RAPIDLY DECREASING TAILWIND DIMINISHING TO 225 DEGS AT 100 KTS OVER A VERY SHORT SPAN OF TIME. THE ACFT WAS ON AUTOPLT AND MACH .80 SELECTED. I NOTICED A LARGE TREND ARROW ON THE SPD TAPE AND IMMEDIATELY DIALED DOWN THE MACH NUMBER. THE AIRSPD INCREASED BEYOND VMO; THE AUTOPLT DISCONNECTED; AND THE AIRBUS 'PULL-UP' BEGAN. I TOOK CTL OF THE SIDESTICK; SELECTED IDLE THRUST WHILE THE FO THOUGHTFULLY DEPLOYED THE SPD BRAKES. MAX SPD ATTAINED WAS ABOUT VMO +10 KTS AND WE CLBED TO FL375 BEFORE REGAINING SIDESTICK CTL TO BRING US DOWN TO OUR ASSIGNED ALT. THERE WERE NO OTHER ACFT ON TCASII IN OUR VICINITY AT FL390 AND NO ATC CONFLICT OCCURRED. AUTOFLT WAS RE-ENGAGED AND MAINT NOTIFIED OF THE OVERSPD. WE LANDED AT IAD WITHOUT FURTHER INCIDENT. LESSONS LEARNED: THE AIRBUS OVERSPD PROTECTION IS NOT JUST COMMONLY ENCOUNTERED NEAR MOUNTAIN WAVE; RAPID JETSTREAM SHEARS CAN CAUSE AIRSPD FLUCTUATIONS SUFFICIENT TO OVERSPD THE ACFT. THE BIGGEST CLUE ON THE FPF WAS THE CHANGE IN THE TROPOPAUSE FROM FL290 AT STL TO FL430 AT FLM. I'LL PAY AS MUCH ATTN TO XING FRONTAL BOUNDARIES AND JETSTREAM GRADIENTS AS I DO WHEN IN AN AREA OF MOUNTAIN WAVE.