Wind and Currents Push Stormwater Bacteria Offshore – or into the Swimming Zone

Stormwater plumes carry high levels of enterococcus bacteria that can close ocean bathing beaches when it rains. Especially when the beach is near a stormwater outfall (map on page 20).

Because stormwater contains less dissolved solids than ocean water, it is less saline, and lighter - much lighter than water that sinks to the ocean floor.

Wind can blow a stormwater plume offshore or concentrate it in the swimming zone. Along NJ, persistent southwest winds create upwelling currents that blow lighter, lower-saline surface water offshore. Northeast winds cause downwelling currents that move surface waters towards the beach (see slide 1).

Southwest Winds and Upwellings

Winds from the south predominate during the summer and form a nearshore current that flows to the north. This creates the littoral drift that deposits sand on the south side of jetties in Monmouth.

Sustained southwest winds create upwellings. Cold, higher-saline bottom water flows towards the shoreline as warmer, lighter water disperses offshore.

In July of 2013, ocean water temperatures dropped to 55 degrees after 2 weeks of southwest winds caused bottom waters to upwell along the coast. The Rutgers University Coastal Ocean Observation Lab (@rutgers_cool) announced in a tweet that the first upwelling of the 2015 season occurred on May 26th.

The water at the beach may be colder during an upwelling, but that deeper, higher-saline water moving into the swimming area contains less stormwater bacteria than the surface water that is being pushed offshore. Upwellings move the plume from Raritan Bay – and its bacteria and floatables - towards Long Island instead of beaches in northern Monmouth, as it discharges into the ocean by Sandy Hook.

Northeast Winds and Downwellings

While southerly winds create northerly currents during most of the summer, the general flow along the NJ coastline is actually to the south.

This southerly flow is driven by the Labrador Current in the region from Massachusetts to North Carolina known as the Mid-Atlantic Bight.

Ocean water over the continental shelf in the Mid-Atlantic Bight flows towards the southwest until reaching Cape Hatteras, NC, where the shelf is so narrow that it mixes with the nearby Gulf Stream (see page 6).

Winds that blow from the northeast are aligned with the Labrador Current, creating stronger southwesterly currents that flow towards the beach. The opposite of an upwelling - a downwelling – occurs with sustained northeast winds. Lighter surface water – including stormwater - remains inshore while cold, dense bottom water is displaced offshore.

During a downwelling, the plume from Raritan Bay narrows and extends to the bottom of the water column as it is compressed along the NJ coast instead of Long Island. In this satellite photo of a downwelling, the muddy plume from Raritan Bay is still visible along the beach past the Barnegat Inlet (slide 1).

Forecasting Beach Water Quality

Wind and current direction is just one variable affecting water quality at the beach. Like: tidal and lunar stages, water temperature, wave height and intensity, and how sunny it is, since the ultraviolet light in sunlight inactivates bacteria.

Just analyzing rain data is complicated. You need to look beyond the amount, intensity, and timing of just that one, most recent rainfall. You also need to determine how wet or dry it was before it rained, as described in a previous blog.

Predicting ocean water quality is as complicated as predicting the weather – and as doable. That's why the EPA is now asking states to use forecasting models to supplement their weekly water sampling.

Some states along the Great Lakes, and California, are already using models to predict bacterial water quality at beaches on days when they don't collect water samples.

You can view a map of currents along NJ and NY at the website for the New York Harbor Observing and Prediction System (NYHOPS) that is maintained by the Stevens Institute of Technology.