Air carrier flight crew reported NMAC with another aircraft on approach. Flight crew followed TCAS commands and landed uneventfully.

Reporter 1

Flew an uneventful trip up the coast to SFO on a clear VMC morning. I was PM and CA (Captain) was PF (Pilot Flying). We did a full arrival briefing to include all threats—personal, environmental and technical. We line selected, briefed, and flew the RNAV-T 28L which constitutes the Tipp Toe Charted Visual for 28L. Prior to top of descent during our arrival briefing we talked extensively about using TCAS in the terminal area of SFO and that my choice would be to leave it in TA/RA, so as to be alerted to any threats. I briefed the First Officer that if we received an RA on the closely spaced parallel approach we would respond and execute a go-around. Joining the Tipp Toe from the SERFR 4 arrival, ATC began slowing us—first to 250, then 210, then 180. We were alerted to approaching parallel traffic from the east who would join final for 28R near us. We called the traffic in sight. We were told to maintain visual, cleared Tipp Toe 28L, 180 knots. I was hand flying the aircraft, with the autothrottles on. I called for the landing gear and flaps 15 in order to slow to 180 knots as I slowly rolled into a left turn to join the final approach segment. LNAV and VNAV were both giving us guidance and the speed window was open. I looked at the wind vector as I was watching the parallel traffic join their final for 28R. I noticed they had a tailwind. It appeared visually to me they overshot their final approach course, right as we received a TCAS RA “Descend Descend―. CA was hand flying and also disconnected the autothrottles and told the Tower that we were responding to an RA. CA lowered the nose to remain clear of the RA pitch command area. The TCAS traffic target turned to a solid red box and showed a distance of +01. As we descended that red box and the +01 stayed with us, as the target aircraft continued to descend over the top of us. We continued to descend with the red target box descending with us, but while following the guidance of the TCAS. We maintained the assigned lateral course. As we were descending through 1100 ft. I briefly tried to level off slightly and the TCAS immediately said “monitor vertical speed― so CA continued the descent. I began trying to figure out if we could safely execute a slight left turn, exit the danger area and commence a go-around. We had the airplane in sight the whole time but they were just so close and slightly above us, I continued following the guidance of the RA. At roughly 900 ft. the SFO Tower gave us a “low altitude warning―. Right around that same time the RA pitch command area disappeared, and the TCAS traffic target turned yellow with a distance of +02 then +03. We slowed to target speed, extended the flaps to 30 degrees, and completed the Before Landing Checklist. We were stable by 500 ft. and executed an uneventful landing and taxied to the gate. The aircraft was never in an undesirable state. The CA and I communicated well throughout the event. We met each of the stability gates. The CA and I debriefed the experience from beginning to end. We are grateful for the direction the company had given us on the use of TA/RA on closely spaced parallel approaches. Had we been in TA ONLY we would have lacked clear direction which could have been catastrophic. What the CA and I found troubling once we had time to think about it, was—being sandwiched between an airplane and the water with no clear exit path. We don't practice those types of low altitude RAs in the training. I know there has always been an issue with the closely spaced parallel approaches, but it seems to have risen to a new level of threat. We used to be more concerned with encountering wake turbulence from the preceding aircraft on the other final approach course. Now we are more concerned about actually impacting the airplane on the other final approach course. Turning the TCAS to TA ONLY seems like the worst possible thing we could do. Why would we remove that particular protection in a threatening situation? In our case today the RA response made all the difference.

Reporter 2

[Report provided no additional narrative.]