B737 EXPERIENCED TCAS RA WITH SUCCEEDING TFC ON FINAL WHILE ON VISUAL APCH AT BUF.

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WHILE ON L DOWNWIND AND ABEAM THE TOUCHDOWN POINT; BUF APCH CTL CLRED US FOR THE VISUAL APCH TO RWY 23. ATC TOLD US TO TURN BASE AT OUR DISCRETION AND THAT THE TFC ON FINAL WAS NO FACTOR. APCH CTL THEN SAID TO SWITCH TO THE TWR. WHILE THE FO BEGAN CHANGING FREQS AND I STARTED TO TURN BASE; WE RECEIVED AN RA THAT DIRECTED US TO CLB. THE AUTOPLT HAD ALREADY BEEN DISCONNECTED AND THE LNDG GEAR WAS DOWN WITH FLAPS 15 DEGS SELECTED. I REMAINED ON OUR HDG AND BEGAN A CLB. AN ACFT SYMBOL WAS ON THE TCAS AT APPROX THE 2 O'CLOCK POS AND I COULD NOT DETERMINE IF ANY OTHER ACFT WERE IN OUR VICINITY. IT WAS NIGHT WITH NO HORIZON OVER AN UNPOPULATED AREA WITH LITTLE LIGHTING FROM THE GND. AFTER RECEIVING A CLR OF CONFLICT MESSAGE; I LEVELED OFF. THEN ONLY SECONDS LATER WE RECEIVED ANOTHER RA TELLING US TO CLB. I PAUSED FOR A BRIEF MOMENT; THE TCAS SHOWED AN ACFT 600 FT ABOVE US AND ABOUT 3 MI. WE KEPT RECEIVING THE CLB COMMAND FROM THE TCAS; SO I BEGAN A SHALLOW CLB. THE FO THEN HAD A VISUAL ON AN ACFT AND SAID WE SHOULD TURN L. WHILE IN THE TURN; THE TCAS GAVE US ANOTHER COMMAND TO DSND. WE COMPLIED AND THEN RECEIVED THE CLR OF CONFLICT MESSAGE. WE ORIENTED OURSELVES IN THE PATTERN; CONTACTED THE TWR AND MADE A LNDG ON RWY 23. I SPOKE WITH APCH CTL AFTER ARRIVING AT THE JETWAY. ATC SAID THEY EXPECTED US TO TURN BASE SOONER; THUS RESULTING IN THE CONFLICT. I BELIEVED WE WERE IN THE PROCESS OF TURNING BASE ABEAM THE MARKER. ANOTHER FACTOR WAS A LARGE TAILWIND ON DOWNWIND. SUPPLEMENTAL INFO FROM ACN 677840: #1 ACFT (B737) DIRECT WAYPOINT 5 MI NE OF BUF ARPT FOR VISUAL APCH RWY 23. 4000 FT DSNDING TO 2300 FT. #2 ACFT (DH8) BEING PROVIDED VECTORS FOR THE SEQUENCE TO FOLLOW B737 6 MI IN TRAIL OFFSET TO THE R. WHEN #1 ACFT RPTED FIELD IN SIGHT; I PROVIDED VISUAL APCH CLRNC AND XFERRED COMS TO TWR. I CONTINUED TO PROVIDE TA'S TO ACFT #2 IN ORDER THAT THEY WOULD FOLLOW ACFT #1. ACFT #1 THEN WAS OBSERVED TO EXECUTE A R TURN ESTIMATED 50 DEGS. I IMMEDIATELY ISSUED A SAFETY ALERT WITH A CLB INSTRUCTION TO ACFT #2 WHICH DID NOT HAVE ACFT #1 IN SIGHT. ACFT #1 RPTED AN INITIAL RA TO MONITOR VERT DSCNT; THEN A SECOND RA TO CLB; AND THEN DSND. ACFT #2 REPORTED AN RA TO CLB. I IN NO WAY ANTICIPATED ACFT #1 TO EXECUTE A TURN AWAY FROM THE ARPT AND CLB. I CLBED ACFT #2 IMMEDIATELY WITH A SAFETY ALERT TO INCREASE VERT SEPARATION.