Background mesoscale flows (BMFs) modify the local inertial frequency and affect the energy and vertical propagation speeds of near-inertial waves (NIWs). Despite their significance, the effect of BMFs on the downward energy flux (Fz) of NIWs remains underexplored. This study aimed to estimate the downward energy flux (Fz) of NIWs while accounting for BMF effects using two years of mooring data (November 2017–October 2019) from the Kuroshio Extension. By dividing the data into 11 day 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 BMFs were considered. Because cyclonic circulations are twice as frequent, Fz decreased by ~17 %, to 0.37 x 10-3 W m-2. 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. Thus, the Kuroshio Extension is an important area for downward NIW energy propagation and BMFs should be considered for accurate NIW energetics.

This study uses mooring data from two years (November 2017 to October 2019) to estimate downward energy flux (Fz) of near-inertial waves (NIWs) in the Kuroshio Extension. By dividing the data into 11-day segments, the temporal variability of the effective near-inertial frequency and group velocity due to background mesoscale motions (BMMs) was considered. During winter when NIWs were active, on average in anticyclonic circulations, Fz increased by 50%, while in cyclonic circulations, Fz decreased by 45% when the BMMs were considered. Since cyclonic circulations are twice as frequent, Fz decreased by approximately 17%, or 0.37 x 10-3 W m-2. This amount is approximately 1.8 times greater than that in the northeastern Pacific and accounts for approximately 28% of the wind work rate, similar to high-resolution numerical model results. Thus, the Kuroshio Extension is an important area for NIWs energy transfer, and BMMs should be considered for accurate NIWs energetics.