SDP and LP fluctuations observed along the coast of Sagami Bay

Abstract

Episodic and subtidal variations of current velocity and temperature are found in the mooring data obtained at seven stations in Sagami Bay during the summer-fall period, 1991. A detailed description of the episodic variation, which occurred from late July to 1 August, shows the intrusion of warm and saline water into Sagami Bay through Oshima east-channel, which causes the rise of sea level along the coast. The power spectra of the current and temperature records in the subtidal frequency range show that significant energy is found at the period of 2–7 days (SDP) and longer than twenty days (LP). The phase relation between eastward current and temperature fluctuations for SDP is different from that for LP. At the bay head, the temperature rise for LP is accompanied by a westward current in the surface layer. For SDP fluctuation, the phase relation between the near-surface, alongshore current and subsurface temperature implies the westward propagation of internal gravity waves. The SDP temperature fluctuations are larger along the bay coast than in the center of the bay, which is a characteristic of internal Kelvin waves. The SDP fluctuations propagate from east to west with the speeds of 0.32–1.16 m s−1, the propagation speed being larger at the eastern corner of the bay head than in the other area. A cross-correlation analysis between the local wind and temperature and/or current at the bay head show that the SDP fluctuations of temperature and current are induced by the local wind. The wind leads the temperature and/or current by about 12 h for the 2–3 day period (SDP2) fluctuations, and by about 2.5 days for the 5–6 day period (SDP5) fluctuations. A theoretical analysis using a two-layer model has been conducted to clarify the response of these SDP fluctuations to the periodic wind forcing. The SDP2 fluctuations observed in Sagami Bay are considered to be mostly generated along the east coast of the bay as a forced internal Kelvin wave by the local wind. In contrast, most of SDP5 fluctuations are caused by internal Kelvin waves propagating into Sagami Bay from outside, having an amplitude 2.5 times that of the forced internal Kelvin wave under local wind forcing.