Weather Watch

On northerly winds and northward currents

Special to The TribuneOctober 6, 2012 

I watched a large boat preparing to dock at Morro Bay. It gradually made its way to the T-pier, delicately balanced by the winds blowing out of the northwest that pushed the boat toward the southeast. The ebb tide flowing out to the Pacific pushed the vessel in the opposite direction.

I’ve wondered why winds are described as where they are blowing from and ocean currents are described as where they flow to. In other words, wind and current directions are based on opposite conventions. A “northerly” wind is a wind that blows from the north and goes to the south; a “northward” current is a current that comes from the south and flows toward the north.

Obviously, this convention can cause confusion, especially for landlubbers who don’t often venture out to sea and stumble on the concept of “current direction.”

Through all my research, I’ve never found an explanation for why wind and current directions are reported opposite from each other. Since wind vanes and sailing ships have been around for thousands of years, this convention has probably been around since ancient Rome, if not longer.

On land, I suspect it may be rooted in the importance of where the wind blows from: for instance, a northeasterly (offshore) wind often means dry, clear and warm weather along the Central Coast, while a southerly wind can indicate an approaching storm. 

Perhaps it could be the downwind smell of an approaching fire or possible food. If you built a windbreak, it must be erected in the direction of where the wind is blowing from. Where the wind goes afterward is of little consequence, except if it you were trying sneak up on animals.

It may be even more basic than the above reasons. The best way to determine where the wind is blowing is by feeling it on your face. Even a slight turn to the left or right causes a change in sensation on your eyes and cheeks. If you ever noticed, cows and other grazing animals will turn into the wind during storms.

On the ocean, where the current is heading is the most important factor. The current pushes ships toward obstacles downstream and “sets” your vessel while docking; where the water or currents come from, unlike the winds, is less important.

When we determine the direction of the current, we often use drifting seaweeds, ocean foam and other floating items on the sea surface to tell us where the current is going. 

Many types of currents exist along our coastline: near-surface currents produced by the local winds; tidal currents; the California current (a cold, southerly flowing current that usually occurs during the late winter and continues through the summer months); the Davison current (a warm, northerly flowing current that usually occurs during the fall and early winter months); and the gyres and eddies that derive from these.

Waves are created by the friction or the dragging motion of the wind over the water. One possible cause of rogue waves is when wind-driven waves run into strong ocean currents that are running counter to the direction of the swell. When waves hit these currents they can suddenly increase in height, producing steep forward wave faces and deep troughs. Old salts often refer to these waves as “holes in the sea.”

At Diablo Canyon Power Plant, the Center for Coastal Marine Sciences at Cal Poly installed a network of sensors that provide real-time measurements and informative computer displays of surface currents. The sensors are manufactured by CODAR Ocean Sensors of Mountain View.

There are 46 stations along the California coastline. These stations, working together, have given all of us a detailed and informative mosaic of surface currents that flow by California. Real-time ocean surface data from these stations can be found at www.cocmp.org.

One convention that is constant in reporting directions of wind and current: The wind and current direction are always indicated as “true directions” and not “magnetic directions.” Magnetic declination is the angle between magnetic north and true north. At this time, the magnetic variance along the Central Coast is around 15 degrees.


Today’s forecast
Night and morning northerly winds have kept the marine layer to a bare minimum along the beaches and coastal valleys over the last few days. Avila Beach reached 84 degrees Saturday afternoon. 
Today will be mostly clear and sunny throughout the Central Coast with above-normal temperatures along the westerly and southwesterly facing beaches.

Today’s high temperatures will range from the low 80s in the North County to the mid-70s in the coastal valleys. Maximum temperatures along the shoreline will range from the mid-70s along the southwesterly facing beaches (Cayucos, Avila Beach and Shell Beach) and the low 70s along the westerly facing beaches (Pismo Beach, Grover Beach and Cambria). The northwesterly facing areas (Morro Bay, Los Osos, Montaña De Oro and the Nipomo Mesa) will range from the low to mid-60s.

An almost stationary upper-level low pressure system will continue to spin west of the California coast today, and will gradually move toward the Central Coast by Monday. This low system and will produce cooler temperatures, variable cloudiness on Monday along with the return of the marine layer with areas of night and morning fog and drizzle.

As this system moves down the coast and moves closer to the Central Coast Tuesday night into Wednesday morning, it will produce a few widely scattered rain showers. At this time, rainfall amounts appeared to be light, with only a few hundredths of an inch expected, if any rain falls at all.

This slow-moving system will finally track inland into the Central Coast on Thursday and bring increasing chances for showers. The system is also expected to produce moderate to heavy showers and thundershowers in the Sierra.

Clouds will decrease across the Central Coast on Friday with high temperatures well below normal through next weekend.

Today’s surf report
Strong to gale force (25- to 38-mph) afternoon northwesterly winds will generate 3 to 5 foot northwesterly (320-degree deep-water) sea and swell (with a 4- to 16-second period) today through Monday.

Former Tropical Storm Maliksi has become extra-tropical. This system intensified to 965 millibars with hurricane-force winds.

A long period west-northwesterly (290-degree deep-water) swell from this storm will arrive along the Central Coast shoreline later Tuesday at 2 to 3 feet (with a 22- to 24-second period) and peak on Wednesday at 4 to 6 feet (with an 18- to 20-second period). This swell will slowly decrease Thursday through Friday.

Seawater temperatures
Seawater temperature will range between 56 and 58 degrees through Tuesday, increasing on Wednesday and Thursday.


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John Lindsey’s column is special to The Tribune. He is a media relations representative for Pacific Gas and Electric Company and longtime local meteorologist. If you have a question, send him an email at pgeweather@pge.com.

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