The internal wave field away from topographies consists of the superposition of multiple waves coming from different directions. Even at 1200m, vertical amplitudes of several tens of meters are observed all year long.
In order to learn more about open-ocean internal wave motions and their variability on the longer “climatic” time scales, NIOZ3 sensors were deployed for 1.5 years while sampling at a rate of 1 Hz. This thermistor string consisted of 54 sensors at 2.5 m intervals and was mounted directly below the top-buoy of a 3800 m long mooring to the West of Madeira in the Canary Basin (Fig. 2). This area is under the known influence of Mediterranean outflow of warm, salty water, which sinks to depths between 1000 and 1500 m. This is partly in the range of NIOZ3.
The result of this first 1.5 years deployment of NIOZ3 sensors running at 1 Hz is not so bad (Fig. 3). Initially only one, from day 350 onwards two, sensors failed. Some 18 sensors continued accurate sampling the entire period of 530 days. The remaining sensors stopped, mainly in the second half of the record, but this is all attributable to a known battery problem. This problem is solved now. Over the 1.5 years, the temperature variation is large, some 5°C, with enhanced values between days 400 and 450. This is expected to be Mediterranean influence, in the form of typical sub-surface ‘M-eddies’, but some satellite observations show eddies approaching the mooring from the south during the above period. In addition to this large-scale structure are fine lines across almost the entire range of sensors that represent smaller-scale eddies and groups of internal waves.
In more detail of one day, the vertical coherence of motions is striking (Fig. 4, upper panel). On the larger internal wave scale, semidiurnal tidal motions are visible. But also much higher frequency motions show vertical coherency across some 60-100 m. Strongly stratified and near-homogeneous layers move up and down, more or less simultaneously. This layering is typical for the open ocean, but especially so below (M)eddies of warm salty water. Partially, these layers are due to convective mixing processes, partially they are due to internal wave deformation, the dominant process yet being unknown.
In contrast with the smooth wave form in the contours of upper panel of Fig. 4, a single temperature time series shows an extremely ‘steppy’ record (Fig. 4, lower panel). This is partially due to the passage of the strongly stratified layers and it justifies the fast sampling, to resolve these rapid variations with time. The noise level in these data is occasionally extremely low, down to about 50 K, or 5×10°C (Fig. 4, insert). These values confirm the instrumental noise level. The periods with higher apparent noise level contain high-frequency natural turbulent motions.
The above figures are just samples of a wealthy data-set that represents unprecedented detailed information of a non-quiescent deep-ocean. The rich variability of the ocean interior in permanent motion, with 10 m or more wave heights, is best appreciated in video.