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A new perspective on ocean and lake currents

Global wind belts (orange) and actual average wind pattern due to land AND ocean effects.
Global wind belts (orange) and actual average wind pattern due to land AND ocean effects.

For the past few days, Hurricane (now extra-tropical storm) Arthur brought concerns to East Coast residents about storm surge and rip currents. Today, Jul. 6, 2014, the focus shifts to the eastern and western shores of Lake Michigan where an array of wind-driven currents could create problems for boaters, swimmers and lifeguards.

So, it’s probably a good idea to get caught up on so-called, “current events,” or some of the things that winds can do as they interact with water bodies.

Persistent winds, such as those typically experienced as part of global wind patterns, help to establish and maintain large-scale ocean currents. It shouldn’t be surprising that global wind belts (Fig. 1) and global ocean currents (Fig. 2) match so closely. Hurricane paths (Fig. 3) also closely match both the wind and ocean current maps.

On a more transitory basis, high and low pressure systems may contribute to multi-day, persistent wind regimes. These often evolve into swells or a persistent, building wave pattern coming from a particular direction.

When swells interact with high wind speed weather systems (e.g., storms like Sandy in 2012), water can be pushed up on coastal shores leading to “storm surge” flooding. This doesn’t involve a “wall of water,” such as a tsunami. Instead more water pushes up on a beach or coastal area than can be removed by gravity during a succession of waves. This scenario can quickly lead to building water levels and flooding.

On a more localized and short-term basis, winds, typically above about 20 miles per hour (17 knots / nautical miles per hour), can generate white caps on the surface of water bodies. Often white caps are seen a short distance offshore (rather than nearshore) due to how the frictional effects of land slow down winds on and near the coast. However, even without the presence of whitecaps, winds can be sufficiently strong along coastal areas to impact water motion in the form of longshore, structural and rip currents. All three of these are problematical along the eastern shore of Lake Michigan today, where gusty southerly winds will be at work.

Early this Jul. 6, 2014 morning, the National Weather Service (NWS) in Grand Rapids, MI posted detailed information about possible lakeshore and beach hazards (Fig. 4) as people in the area try to capture yet another day of relaxation near the water. With winds expected to be blowing from the south at speeds of 15 to 25 miles per hour with higher gusts, the NWS has flagged the potential for high wave action (waves at least in the 3 to 6 foot range) and dangerous swimming conditions. With winds blowing so strongly, longshore currents are anticipated. While these typically mean that people in the water find themselves moving along the shore, even without noticing their movement. This is especially dangerous for children who can be carried significant distances from where parental eyes are watching.

Wave action on beaches, interacting with underwater sand bars, can generate rip currents. These strong, localized, underwater currents typically extend at right angles to the shoreline, and can pull even experienced swimmers away from shore. According to the U.S. Lifesaving Association (USLA), some 80% of rescues by USLA affiliated lifeguards at ocean beaches are caused by rip currents. The USLA recommends that swimmers not try to fight these currents. Rather, they suggest swimming parallel to shore until one feels the current relax, and then swim to shore. Since most rip currents are narrow, a short swim parallel to the shore should allow one to quickly escape the current.

Another watery risk involves structural currents. Here, waves, combined with the longshore current, can interact with piers, pier pilings, docks and other structures in the water. As the water flows past these (at higher than usual velocity), eddies and enhanced current flow can develop. These can quickly sweep people off their feet and possibly even drive them into a structure.

Not surprisingly, many piers have posted signage (Fig. 5) advising against swimming near a pier.

Whatever one’s beach destination, a few simple, common sense safety thoughts can help to ensure a safe day in the water.

© 2014 H. Michael Mogil

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