We investigate a mechanism for producing double-lows and double-highs in the semi-diurnal tide by selective amplification of higher harmonics in a resonant gulf. A double low water is observed at Providence, RI, near the head of Narragansett Bay on days when there is a flattening of the low water tidal curve at Newport, at the mouth of the bay. The flattening is caused by an unusually large quarter-diurnal component to the tide at Newport. The quarter diurnal component has the right phase (a maximum close to the time of the minimum in the semi-diurnal tide) to produce a prolonged flattening of the tidal curve around low water. The natural period of Narragansett Bay (for quarter-wavelength resonance) is close to 4 h and the quarter diurnal tide is amplified, relative to the semi-diurnal tide, within the bay. The selective amplification of the higher harmonic further prolongs the flattening effect at Providence and, occasionally, is sufficient to create a double low water at the head of the bay from quarter and semi-diurnal tides alone. More often, though, a sixth-diurnal harmonic produced within the bay, added to the flattened low water at Providence, creates the double low water. This mechanism of selective amplification of tidal harmonics could be relevant to double tides elsewhere.

我们研究了通过在共振海湾中选择性放大高次谐波在半日潮中产生双低和双高的机制。在罗纳普罗维登斯（Providence）的纳拉甘塞特湾（Narragansett Bay）头附近，当海湾口的纽波特（Newport）的低潮汐曲线趋于平坦时，观测到双低水位。扁平化是由纽波特潮中异常大的四分之一昼夜分量引起的。四分之一昼夜分量具有正确的相位（最大值接近半昼夜潮汐中的最小值的时间），以使低水位附近的潮汐曲线长时间展平。Narragansett湾的自然周期（用于四分之一波长共振）接近4小时，并且该海湾内的四分之一日潮相对于半日潮是放大的。对高次谐波的选择性放大进一步延长了普罗维登斯（Providence）的平坦化效果，偶尔足以单独从四分之一和半日潮汐在海湾顶部形成双倍的低水位。不过，更常见的是，海湾中产生的第六个昼夜谐波被添加到普罗维登斯的平坦低水位上，从而产生了双重低水位。潮汐谐波选择性放大的这种机制可能与其他地方的双潮有关。