Negative surge
From BAWiki
Definition
A compound or reacting wave action - usually a continuation of the vessel's wash, acting and reacting in opposite directions; the name applied to wave motion with a period intermediate between that of the ordinary wind wave and that of the tide, say from 1/2 to 60 minutes. It is of low height; usually less than 0.3 foot. See also SEICHE; in fluid flow, long interval variations in velocity and pressure, not necessarily periodic, perhaps even transient in nature.