Tides are the rhythmic rising and falling of the ocean’s water level, primarily driven by the gravitational forces exerted by the Moon and the Sun. Water levels are measured against a long-term average. A “negative tide” describes an extreme fluctuation where the water drops significantly lower than what is typically considered a low tide. This unusual phenomenon reveals parts of the seabed that are rarely exposed to the air.
Defining the Negative Tide
A negative tide is a water level measurement that falls below the official zero reference point used on tide charts for a specific location. This phenomenon is also referred to as a “minus tide.” It is an extremely low tide that dips below the established baseline for charting and predicting water depths. A negative tide is assigned a negative number, such as -0.5 feet or -1.0 meters, indicating the actual water level is physically lower than the average lowest point.
This occurrence is unusual because the zero line is purposefully set to represent the average of the lowest daily tides over many years. Therefore, the water level must drop below the historical average low mark to be officially designated as negative. When a negative tide occurs, it exposes large sections of the foreshore and seabed that are typically submerged even at the lowest predicted astronomical tides.
The Role of Astronomical and Meteorological Forces
The most significant astronomical factor contributing to a negative tide is the alignment of the Earth, Moon, and Sun, which produces what are known as spring tides. Spring tides occur twice a month during the New Moon and Full Moon phases, when the gravitational pulls of the Sun and Moon combine to create the largest tidal bulge and, consequently, the largest range between high and low water. This alignment sets the stage for the lowest possible low tides.
However, the most severe negative tides are often caused by the combination of these astronomical forces and powerful meteorological factors. Strong, sustained offshore winds can physically push water away from the coastline, effectively draining the shallow coastal waters and exacerbating a predicted low tide. This wind-driven effect can significantly lower the observed water level beyond what the gravitational forces alone can achieve.
Another influential factor is atmospheric pressure, specifically a high-pressure weather system. A strong high-pressure cell pressing down on the ocean’s surface will depress the water level, causing it to sink. The combination of an astronomically low spring tide, strong offshore winds, and a high-pressure system results in the most extreme negative tide events.
Understanding Tidal Datums
A negative tide is defined by its relationship to a specific reference point called a tidal datum, which acts as the official zero elevation for water-level measurements. For most coastal regions in the United States, the primary chart datum used for navigation and tide prediction is Mean Lower Low Water, or MLLW. MLLW is calculated as the average height of the lowest low water recorded each day over a specific 19-year period.
This 19-year cycle, known as the National Tidal Datum Epoch, accounts for long-term changes in the Moon’s orbital path and other influences on sea level. Setting the zero line at MLLW establishes a dependable minimum depth for mariners. Therefore, a negative tide reading means the water level has dropped below this established, long-term average low point.
The existence of a negative reading confirms that the actual water surface is below the calculated mean of the lower daily low waters. This technical baseline is necessary for generating accurate tide tables and nautical charts.
Practical Implications of Extreme Low Water
The sudden and dramatic drop in water level associated with a negative tide has direct consequences for human activities and the coastal environment. For navigation and maritime infrastructure, extreme low water creates a serious hazard. Boats, particularly those with deep drafts, risk grounding in shallow ports, channels, and marinas where predicted depths are calculated relative to the MLLW datum. This can necessitate temporary closures or restrictions for large vessels.
Infrastructure near the coast can also be affected. Water intake pipes for power plants or desalination facilities may become exposed or operate inefficiently when the water level drops too low. Extreme low water can also concentrate sediment in channels, sometimes requiring dredging to maintain safe passage once the tide returns.
Ecologically, negative tides expose the sensitive intertidal zone to air for extended periods, putting significant stress on marine organisms. Seaweed, barnacles, mussels, and tide pool inhabitants are suddenly subjected to desiccation, increased air temperatures, and predation. This exposure is particularly harmful to sedentary organisms that rely on the rhythmic coverage of the tide for survival. The phenomenon also offers a rare opportunity for researchers and beachcombers to view otherwise submerged life and geological features.