Short-Period Temperature Oscillations in the Coastal Summer Water. II

Abstract

In the preceding paper⁽¹⁾, the writer has reported many types of variations of water tem-perature observed by a resistance thermometer at a station 2 miles north-west off Osyoro on the west coast of Hokkaid ??, in the summer of 1937. Certain groups of these variations are remarkable having very short periods such as 5-15 minutes. They had been noticed by few earlier observers^(2)(3)(4) and considered to be interesting and of quite unknown origin. In the summer of 1938 and 1939, some observations were repeated at the same station. The electric thermometers employed in those observations were the resistance thermometers and the S_UN-DERS' thereto-electric apparatus⁽⁵⁾; and the “printing dots” type self-recorder was also used. The mechanism of the recorder is illustrated in Fig. 1, and some examples of the records actually obtained are shown in Fig. 4, I, and 5, K.
The results of these observations are summarised as follows :
1. A considerably regular variation manifests itself through a day in the vertical distri-butions of water temperature in harmony with the tidal current, though irregular and con-spicuous changes happen on certain days, as shown in Fig. 2. In many cases, the disconti-nuities of the vertical temperature gradient are remarkable in the middle and lower layers.
2. Rapid oscillations of temperture are observed at the layers where the temperature gradient is steep, and, in many cases, the steeper the gradient, the greater the amplitudes of the oscillations. These oscillations can be classified into two groups; periodic temperature oscillations of regular forms (Fig. 3, A, B, D, F, G, H, I, J, L, Fig. 4). and temperature fluctua-tions of quite irregular nature (Fig. 3, C, E, K, M, Fig. 5).
3. About the regular periodic oscillations, the following facts should be remarked.
a) Periods of various kinds can be traced, but each one is greater than 5 minutes (Tab. 1). The oscillations of periods of 5-6 minutes often appear with considerable amplitudes.
b) In many cases, the phases of oscillations at the two or three adjacent layers coincide with one another (Fig. 4, D, G.).
c) Sometimes, however, the oscillation is conspicuous in one layer, while no oscillation can be found in the adjacent layers, and the temperature gradients are equally remark-able (Fig. 3, B, Fig. 5, M).
d) The sinking and rising of the fishing net situated near the observational station were investigated by means of pressure gauge (Fig. 3, No. 4). The motion of the net seems to be fairly coincident with the temperature variations observed at the same time.
e) If it is supposed that these temperature oscillations result from the wave motions of sea water, the wave heights estimated from their amplitudes often reach 2-4 meters.
f) It is not seldom that these rapid temperature oscillations appear in the course of the temperature variations of long periods (Fig. 3, D, G, I, J).
4. The undulations of periods of 15-20 min. are often traced in the tide gauge record of Yoiti Wan (Bay).
5. The characteristic of the temperature fluctuations belonging to the second group is a rapid, discontinuous, and irregular change of temperature as shown in Fig. 5. Some of these fluctuations appear just before or after the “Siome” or current rip passes the station.
From these results, the following conclusions may be deduced. In the coastal summer water, the temperature gradient is conspicuous and its discontinuity is also remarkable. The discontinuity is often caused by the displacement of wedge-shaped, colder water mass on the bottom. The temperature oscillations of the first group result from the undulations that may occur in such coastal water. There are instances which seem to indicate that these undulations are of long wave nature, but in other instances they seem to be of internal wave nature.