It is known that background mesoscale flows (BMFs) modify the local inertial frequency and affect the energy and vertical propagation speeds of near‐inertial waves (NIWs), but their effects have not been quantified, especially using observations. This study aimed to quantify BMF effects on NIWs energy, group velocity, and the downward energy flux (Fz) of NIWs using 2 years of mooring data (November 2017–October 2019) from the Kuroshio Extension. By dividing the data into 11 days segments, the temporal variability of the effective near‐inertial frequency and group velocity owing to the BMFs was considered. During winter, when NIWs are active—on a temporal average in anticyclonic flows—Fz increased by 50%, whereas in cyclonic flows, Fz decreased by 45% when the varying effective near‐inertial frequency was considered. Because cyclonic circulations were twice as frequent, Fz decreased by ∼17%, to 0.37 × 10^-3 W m^{- 2}. Even so, this amount is ∼1.8 times greater than that in the Northeastern Pacific and accounts for ∼28% of the wind work rate, similar to eddy‐resolving high‐resolution numerical model results. This high efficiency suggests that NIWs could play a more important role in deep mixing than previously considered. To overcome spatiotemporal limitations of Fz estimation from our data, long‐term eddy statistics were used to estimate overall Fz over the Kuroshio Extension. Anticyclonic circulations lasted longer, leading to an ∼12% increase in Fz. Thus, the Kuroshio Extension is an important region for downward NIW energy propagation, and BMFs should be considered for accurate NIWs energetics.

effective inertial frequency,wind work,mooring observation