Seasonal and intraseasonal modulation of near-inertial wind power associated with fluctuations in unidirectional wind speed in the Bay of Bengal

Abstract

This study examines the seasonal and intraseasonal modulation of near-inertial wind power associated with fluctuations in unidirectional wind speed in the Bay of Bengal (BoB). For that purpose, we use concurrent measurements of high-resolution in situ near-surface current and wind speed from six moorings in the BoB. It is found that the annual mean of near-inertial wind power in the BoB shows roughly similar magnitude (0.25–0.35 mW m⁻²) at all the mooring locations. However, in response to the seasonal evolution of monsoonal wind forcing, near-inertial wind power shows significant annual variability, with a maximum during summer (~ 0.4–0.5 mW m⁻²) and fall (~ 0.3–0.4 mW m⁻²) and a minimum during winter (~ 0.1 mW m⁻²) and spring (~ 0.2 mW m⁻²). In addition, it is also found that modulation of near-inertial wind power due to summer monsoon intraseasonal oscillation (MISO), such as its magnitude, reaches as large as ~ 1 mW m⁻² at the mooring in the northern BoB during phases 3–4 of MISO. Using a high vertical resolution of current profile data, the near-inertial kinetic energy (NIKE) budget in the mixed layer in the northern BoB shows good temporal correspondence with the magnitude of the rate of change of NIKE and near-inertial wind power, with a maximum magnitude of the rate of change of NIKE lags the wind power by 24 hr. The NIKE budget also indicates that a significant portion of near-inertial wind power dissipates in the mixed layer and rarely energises the depth regime underneath the mixed layer.

Appendix. Sensitivity of the results

Note that, at the RAMA mooring locations, the current measurement at 10 m is used in contrast to measurements at 1 m at OMNI moorings for wind power estimates. Though the current measurement at 10 m is close to the sea surface, the MLD at the mooring location may occasionally be shallower than the mooring current meter deployment depth. These conditions are more common in the spring (where 50% of MLD are shallower than 10 m) and less common in the summer (where 10% of MLD are shallower than 10 m) (Table S1). In this case, the current measurement below the shallow MLD cannot be considered as mixed layer currents. However, after eliminating data points when MLD is shallower than 10 m, the assessment of seasonal and intraseasonal variability of wind power at RAMA mooring locations indicates that the result will not change significantly (Figures S4 and S5). Hence, we followed an assumption, such as the measurements at 10 m were representative of mixed layer current.

Following Liu et al. (2019), a band-pass filter with a cutoff ± 0.25 f₀ (0.75 f₀ and 1.25 f₀) from theoretical inertial frequency is adopted in the present study. As depicted in Fig. 3, these bands are either slightly broad or narrow at particular mooring locations. However, previous studies have also used a slightly narrow band (± 0.2 f₀) to filter the data in near-inertial frequency (Lin et al. 2022; Liu et al. 2018; Majumder et al. 2015; von Storch and Lüschow 2023) (Figure S7). The seasonal and intraseasonal average of near-inertial wind power estimated using our default choice (0.75 f₀ and 1.25 f₀) is compared to an estimation based on the narrow band and arbitrarily selected band to examine how sensitive our results are to the choice of the frequency band (Figures S4-S6; Table S2). The seasonal and intraseasonal variability of wind power estimated using the narrow-band and arbitrary bands is shown to be insignificantly different from our default choice (Figures S4-S6). As a result, we focused on the wind power estimation based on the frequency band 0.75 f₀–1.25 f₀ in the present study.

Previous research in the BoB used a higher value of density increment (0.2 kg m⁻³; density change equivalent to a temperature change of magnitude 0.8 °C) for MLD calculation (Thangaprakash et al. 2016; Vijith et al. 2020). However, MLD estimation by this criterion is deeper compared to the base of the surface isopycnal layer (Figure S2). However, it is found that the NIKE budget estimation based on this higher threshold for MLD does not differ considerably from our default decision (0.125 kg m⁻³) (Figure S7).