Zeitliche Variabilität von internen Wellen und vertikaler Vermischung im Nordatlantik
Zusammenfassung der Projektergebnisse
The energy in the internal wave field can vary significantly on many temporal and spatial scales. Important factors that cause this variability include strong currents, their interaction with seafloor topography, and the wind stress. Wind-generated inertial motions within the mixed layer can radiate near-inertial waves into the interior of the ocean and induce a seasonal cycle in nearinertial internal wave energy largely in phase with storm intensity. In the tropical North Atlantic, mean winds introduce relatively little energy into the internal wave field, but hurricanes act as very energetic sources for near-inertial waves. In this project the farfield influence of hurricanes on the internal wave field was addressed using a five-year mooring time series from the Meridional Overturning Variability Experiment (MOVE) array at 16°N in the interior of the Atlantic. The mooring was located in an area with distinctly calm oceanographic and meteorological conditions characterized by weak and constant trade winds. Here, the temporal variability of internal wave energy south of the main hurricane track in different frequency bands was studied, and the magnitude of its variability, along with possible energy transfer mechanisms, was analyzed. The results show that changes in near-inertial energy are dominated by the passage of internal waves generated by hurricanes centered several hundred kilometers north of the mooring. During all deployment periods, hurricanes passed over or to the north of the mooring, with the strongest winds occurring in 2003 and 2004. In the 2003 hurricane season, two very strong hurricanes passed over the tropical Atlantic within several days of each other. With a time lag of approximately 2 to 3 weeks after the passage of the two hurricanes, an increase in depth-mean near-inertial wave available potential energy by a factor of 3.8 is observed in the mooring area. The major role of hurricanes in the generation of near-inertial waves was also seen in an extended slab model that takes the horizontal divergence of the near-inertial current field at the mixed layer base into account. It showed that hurricanes induced a factor of 5 increase in the energy input into near-inertial waves compared to the relatively low values during the rest of the year. Energy flux estimates based on the 2003-2004 mooring data show a corresponding increase in the southward energy flux induced by the two major hurricanes in September and October 2003 (up to 5 kW m^-1). In the zonal flux component, individual flux estimates are enhanced during September and October, but as no direction prevails, the time-integrated energy flux is much weaker than in the meridional component (ca.1x10^9 to 5x10^9Jm^-1). In addition to the hurricane-induced changes in near-inertial energy a seasonal cycle was also observed in the energy at the high-frequency end (frequencies above 6 cpd) of the internal wave spectrum. It is not in phase with the near-inertial energy variability but covaries with changes in the local surface waves. These high-frequency internal waves are most energetic at times when large-amplitude surface swell with long periods and correspondingly long wavelengths was observed. As the swell observed at the mooring location originates more than 1000 km to the north, it will be very narrowband due to dispersion, and, hence, an effective energy transfer into internal waves could be possible. This 5-yr mooring time series shows that the variability on both the low- and high-frequency ends of the internal wave spectrum can be dominantly influenced by changes in the wind field at distances of O(1000) km.
Projektbezogene Publikationen (Auswahl)
- 2016. Impact of the North Atlantic subpolar front on near inertial wave propagation, Ocean Sciences Meeting in New Orleans, USA
Köhler, J., C. Mertens, M. Walter, M. Rhein, B. Klein
- 2017. Temporal variability of the internal wave spectrum in the interior tropical North Atlantic, IAPSO-IAMAS-IAGA Joint Assembly, Capetown, South Africa
Köhler, J., G.S. Völker, M. Walter
- Process studies on the generation of near-inertial internal gravity waves by wind, Dissertation, Universität Bremen
G. S. Völker
- 2018. Response of the internal wave field to remote wind forcing by tropical cyclones, J. Phys. Oceanogr., 48(2), 317-328
Köhler, J. Köhler, G.S. Völker, and M. Walter
(Siehe online unter https://doi.org/10.1175/JPO-D-17-0112.1)
