at a low ebb 落ち込んでいる状態

at a low ebb は落ち込んでいる状態、不振、の意味です。 0;33 in recent years, relations between China and Japan have experienced twists and turns and been at a low ebb. the reason, in my opinion, is quite clear to Japan. 日中関係は冷え込んでいる。

Chinese, Japanese FMs Hold Talks in Beijing
https://www.youtube.com/watch?v=O96KNNd_w0Q  1:10 relations between the two are already at a low ebb given the current territorial dispute between China and Japan over islands in the East China Sea.
Japanese PM's war shrine visit angers China
https://www.youtube.com/watch?v=0h2gGnwGIDE  5:05 they are in a race with the ebbing tide. 車の積み込みは引き潮（干潮時刻）との競争だ。
The Worlds Biggest/Super Ships: Faust - Documentary

参考リンク
http://bridge-english.blogspot.jp/2013/09/blog-post_13.html  暑さがぶり返すを英語でなんと言うか？

練習問題解答
b-2851 at their lowest ebb
b-4930 ebbed away