signature feature

今年のUS Openでは強敵が隠れていました。赤字の解説をPくん，お願いします。（UG）
At Main Court, Wind Is Common Opponent
It was August 1997, shortly before the official opening of Arthur Ashe Stadium, the new center court at the United States Open, and the leading American players Pete Sampras and Jim Courier had just finished a practice session.
Sampras happily reported that the swirling winds that had been a bother in Louis Armstrong Stadium were not a problem in the new stadium.
“What’s not to like?” Courier said that day of the stadium itself.
Thirteen years later — numerologists take note — swirling winds have been the talk of the tournament and remain a signature feature of Ashe Stadium, which is considered the most challenging environment of the four center courts in Grand Slam tennis.
http://www.nytimes.com/2010/09/09/sports/tennis/09wind.html?_r=1&ref=global-home