Low-frequency Modulation Of Turbulent Diapycnal Mixing By Wind Stress And Mesoscale Eddies Inferred From The HOT Time Series

Turbulent diapycnal mixing in the ocean controls the transport of heat,freshwater, dissolved gases, nutrients, and pollutants. It has great influences on thevariability of the global climate and meridional overturning circulation.Understanding the spatial and temporal variation of diapycnal diffusivity is importantfor improving models’ representation and prediction of large-scale ocean circulationand for understanding the role of the ocean in global climate changes.Since October1988, the HOT (Hawaii Ocean Time-Series) program has beenrepeating cruises monthly to the Station ALOHA, which is to the north of the island ofOahu, Hawaii. Profiles of potential density obtained from CTD measurements areused to evaluate low-frequency variability of turbulent diapycnal mixing based on afinescale parameterization method. In the upper300-600m, the diapycnal mixingexhibits a distinct seasonal variation with the turbulent kinetic dissipation rate inwinter and spring about40%larger than that in summer and autumn. Besides, there isalso a positive trend in dissipation rate. But the trend is mainly due to the muchweaker diapycnal mixing in the first four years i.e.1988-1992. There is no low-frequency variability in the dissipation rate of the deeper segment i.e.600-900m.In the upper300-600m, diapycnal mixing is enhanced under anticycloniceddies with the dissipation rate about50%larger than that under eddy-free conditions.The modulation by anticyclonic eddies becomes more evident with increasing eddystrength. The role of cyclonic eddies in modulating diapycnal mixing is much lesspronounced than that of anticyclonic eddies.The wind work on near-inertial motions in the mixed layer plays an importantrole in maintaining diapycnal mixing in the upper ocean. The dissipation rate in theupper300-600m is significantly correlated to the wind work. The stronger (weaker)diapycnal mixing corresponds to the larger (smaller) wind work. There is nosignificant correlation between the wind work and dissipation rate in600-900m.Seasonality of the dissipation rate in the upper300-600m probably arises fromthe seasonal variation of both the wind-work on near-inertial motions in the mixedlayer and the mesoscale eddy field. The lack of the seasonality in deeper regions isprobably due to the much weaker wind work and mesoscale eddy variability in thesubtropical East Pacific.2TW energy is needed to maintain the diapycnal mixing in the deep ocean.However, it is estimated that the total tidal dissipation rate in the open ocean is on theorder of0.7–0.9TW, which may only contribute50%of the energy required forsustaining diapycnal mixing. It has been argued whether a substantial portion of thewind work on near-inertial motions in the mixed layer can propagate downward togreat depth to support deep ocean mixing. So far both modeling and observational studies have not reached consensus. Based on the results here, it seems that asignificant role of the wind work on near-inertial motions in maintaining diapycnalmixing is only confined to the upper600m. Therefore, the wind work on near-inertialmotions is unlikely to make considerable contribution to the deep ocean mixing atleast in the subtropical East Pacific.