B757 flight crew report TCAS RA at 10300 FT approaching SSCOT intersection on the SSCOT1 RNAV to MIA. Controller work load is cited as possible contributing factor.

Narrative

B757 on the RNAV arrival into Miami had to push over aggressively in response to an RA. We were vectored off the SSCOT 1 arrival near Cypress (CYY) to aid the descent to 10;000 FT. The clearance after the vector was direct to SSCOT to cross the fix at 250 KTS and 10;000 FT MSL. That was the expected clearance for landing east at MIA International Airport. At approximately 10;500 FT MSL and descending a few miles outside of SSCOT; ZMA called the Baron as traffic. Near simultaneously with that call from Miami; the TCAS sounded 'Traffic; Traffic' and the RA called for a 2;000-2;500 VVI descent to resolve the conflict. The red arc that displayed on the VSI was wrapped around the bottom of the gauge at about the 5 o'clock position. I looked in the radar and saw a 'red box' conflict at an altitude of 10;300 FT overtop of our airplane symbology. If you don't know anything about the TCAS symbology of a 757; red is BAD! We were basically co-altitude with him. I also saw the negative sign by his altitude readout indicating that he was in a descent as well. I believe we were in a 20-40 miles scope range so it is hard to determine the lateral spacing between our aircraft and the Baron. The autopilot was engaged and beginning the roundout to level off at 10;000 FT MSL by the SSCOT fix. The pilot flying quickly disconnected the autopilot and commenced the pushover to comply with the RA. He completed the maneuver at 10;000 FT and I responded that we had to honor the RA and push over to resolve the conflict. With Miami's concurrence; we leveled off at 9;000 FT MSL and continued on the arrival. ZMA handed us off to Approach Control right after this happened so I didn't get a chance to ask why or how the conflict arose. I did ask Approach Control to inquire and he gave us the Watch Supervisor's phone number to call. The whole maneuver was very disorienting. I was frantically searching for the traffic visually while crosschecking his location electronically to make sure we were clear. There were scattered clouds at 10;000 FT MSL which made finding the Baron visually more difficult. We also went through the cloud deck during the pushover maneuver. After deplaning in Miami and apologizing to the passengers for the bumpy approach; we were able to talk to the Supervisor at ZMA. As it turned out; the Baron was flying VFR and he was in a descent like our TCAS indicated. Contributing factors: 1) Our Controller was working a lot of airplanes into several airports. I heard him vectoring traffic into Ft. Lauderdale; Miami; Ft. Myers; and West Palm Beach. A jet was holding and eventually asked for vectors to St. Petersburg from Ft. Myers. He had all of the VFR traffic to contend with as well. He may have been task saturated and lost track of the VFR traffic until it was a problem. 2) VFR traffic unaware of the descent profiles of the arrivals into Miami and the other East Coast airports. Most of the traffic into those airports comes down the west coast of Florida and crosses the peninsula in a descending profile. The Baron flying VFR was descending over top of a STAR fix within a couple hundred feet the published 'expect' altitude of the arrival. Factors affecting the quality of human performance: 1) When the conflict arose; I looked into the radar and saw the 'red box' conflict but I was unable to determine exactly how close we were laterally. In a 20 or 40 mile scope; the symbols are too big to be useful. The 'red box' is about 2 miles wide I the scope. There is a tradeoff between useful information and being able to see the symbols from the pilot seats. I think it would be useful for the scope to automatically go into a 5 or 10 mile range for an RA regardless of switch position. Or whatever range puts the traffic at the top of the scope. I guess; in theory; the radar can resolve a conflict at a greater range with a lot of closure. The traffic does appear on the VSI if the pilot has that feature selected but most; if not all; pilots look first to the radar for traffic and not the VSI. The RA commands a climb or descent. I thought a turn in this case would have also been prudent since I perceived us to be at the same altitude and the conflict popped up out of nowhere with little time to react. Supplemental information from ACN 828778: After landing; I checked on the Flight Attendants (who were not yet seated) to see if any of them were hurt by the pushover maneuver. Fortunately; they were all OK. I then called ZMA and talked to the Supervisor who saw the events unfold. He said that the Baron (who they were talking to) was in a slow descent to some nearby airport. So the Baron was in a descent also; and the Controller wanted us to level at 10;000 FT. Why? 2 questions pertaining to ZMA came to mind. First; why were they so late giving us the traffic call; especially when they were talking to the Baron? Second; why would they tell us to level at 10;000 FT? They should have told us to level at 11;000 FT (earlier). Also; I believe the Baron was VFR; but I have doubts that he was maintaining cloud clearance requirements. There was a cloud where I believe he was. Indeed; we entered the cloud for a short time; and never saw the Baron.