B737-900
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
ATC Issue All Types; Inflight Event / Encounter Wake Vortex Encounter
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Environment - Non Weather Related; Procedure
SEA was utilizing a south flow configuration even though surface winds were northerly 5-10 kt. We were turned in tight approximately 3-4 miles behind a B757 and instructed to maintain 190 KIAS to the marker for Runway 16R. I told the Controller unable due risk of unstable approach with tailwinds not to mention wake turbulence hazard behind 757. As our final speed in the B737-900 this day would easily be 25 kt more than a B757 there would be a significant chance of a wake turbulence encounter as our spacing tightened up. Approach Control countered with an instruction of 180 KIAS and we elected to continue. We soon encountered several sharp jolts of wake turbulence and my First Officer did a fine job of maneuvering above glide slope to get out of the wake. Our spacing with the B757 was quickly eroding as we were given instructions to contact the Tower. The Tower Controller quickly approved our request to sidestep to Runway 16L and we landed without further incident.The PIC is the final authority as to how the aircraft will be flown. The Approach Controller seemed incredulous that I did not wish to comply with her first request; but the reality of the situation was this was an unworkable plan. We elected to continue this approach with the assumption that we would either get to sidestep to 16L or go around. Lesson learned: be the PIC and speak up when you don't like what is happening. Recommendations: we had 13 kt of tailwind to about 500 feet AGL; why does SEATAC insist on the south flow configuration in these conditions? Also; 3 miles behind a B757; especially with a tailwind is not safe.
B737-900 Captain reported encountering wake turbulence on approach to SEA in trail of a B757.