OCEANS AND THE WEATHER
The ocean plays an important role in shaping our climate and weather patterns. Warm ocean waters provide the energy to fuel storm systems that provide fresh water vital to all living things. Hurricanes originate over the tropical regions of the ocean under conditions where high humidity, light winds, and warm sea surface temperatures combine. [Source: NOAA]
Understanding and predicting precipitation is critical to farmers who decide which crops to plant, and how deep, based in part on soil moisture levels. Crop and food prices may increase when weather that is too wet or too dry adversely affects crops. Like precipitation, extreme heat and cold also affect livestock management.
Weather prediction can be a life-saving tool. Aside from helping people prepare for catastrophic storms, prediction can help citizens and governments anticipate extreme hot and cold temperatures, which may cause death among the elderly. Water management experts study how much rainfall to anticipate so they can manage reservoir levels and water usage, to ensure everyone has abundant water supplies.
Websites and Resources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org
Atmospheric Influences on the Ocean
Robert Stewart wrote in the “Introduction to Physical Oceanography”: The sun and the atmosphere drive directly or indirectly almost all dynamical processes in the ocean. The dominant external sources and sinks of energy are sunlight, evaporation, infrared emissions from the sea surface, and sensible heating of the sea by warm or cold winds. Winds drive the ocean’s surface circulation down to depths of around a kilometer. Wind and tidal mixing drive the deeper currents in the ocean.
The ocean, in turn, is the dominant source of heat that drives the atmospheric circulation. The uneven distribution of heat loss and gain by the ocean leads to winds in the atmosphere. Sunlight warms the tropical ocean, which evaporate, transferring heat in the form of water vapor to the atmosphere. The heat is released when the vapor condenses as rain. Winds and ocean currents carry heat poleward, where it is lost to space. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]
Because the atmosphere drives the ocean, and the ocean drives the atmosphere, we must consider the ocean and the atmosphere as a coupled dynamic system. In this chapter we will look mainly at the exchange of momentum between the atmosphere and the ocean. In the next chapter, we will look at heat exchanges. In chapter 14 we will look at how the ocean and the atmosphere interact in the Pacific to produce El Niño.
A heat dome occurs when the atmosphere traps hot ocean air like a lid or cap. High-pressure circulation in the atmosphere acts like a dome or cap, trapping heat at the surface and favoring the formation of a heat wave. As prevailing winds move the hot air east, the northern shifts of the jet stream trap the air and move it toward land, where it sinks, resulting in heat waves.[Source: NOAA]
Summertime means hot weather — sometimes dangerously hot — and extreme heat waves have become more frequent in recent decades. Sometimes, the scorching heat is ensnared in what is called a heat dome. This happens when strong, high-pressure atmospheric conditions combine with influences from La Niña, creating vast areas of sweltering heat that gets trapped under the high-pressure "dome."
A team of scientists funded by the NOAA MAPP Program investigated what triggers heat domes and found the main cause was a strong change (or gradient) in ocean temperatures from west to east in the tropical Pacific Ocean during the preceding winter. Imagine a swimming pool when the heater is turned on — temperatures rise quickly in the areas surrounding the heater jets, while the rest of the pool takes longer to warm up. If one thinks of the Pacific as a very large pool, the western Pacific’s temperatures have risen over the past few decades as compared to the eastern Pacific, creating a strong temperature gradient, or pressure differences that drive wind, across the entire ocean in winter. In a process known as convection, the gradient causes more warm air, heated by the ocean surface, to rise over the western Pacific, and decreases convection over the central and eastern Pacific.
Wind is simply the air in motion. Usually when we are talking about the wind it is the horizontal motion we are concerned about. If you hear a forecast of west winds of 10 to 20 mph that means the horizontal winds will be 10 to 20 mph FROM the west. Sunlight is the primary energy source driving the atmosphere and ocean. It is also the power behind winds. There is a boundary layer at the bottom of the atmosphere where wind speed decreases as the boundary is approached, and in which fluxes of heat and momentum are constant in the lower 10–20 meters.
Although we cannot actually see the air moving we can measure its motion by the force that it applies on objects. For example, on a windy day leaves rustling or trees swaying indicate that the wind is blowing. Officially, a wind vane measures the wind direction and an anemometer measures the wind speed. The vertical component of the wind is typically very small (except in thunderstorm updrafts) compared to the horizontal component, but is very important for determining the day to day weather. Rising air will cool, often to saturation, and can lead to clouds and precipitation. Sinking air warms causing evaporation of clouds and thus fair weather.
Wind Systems in the Oceans
According to the “Introduction to Physical Oceanography”: If solar heat was rapidly redistributed over earth, maximum temperature would occur in January. Conversely, if heat were poorly redistributed, maximum temperature in the northern hemisphere would occur in summer. So it is clear that heat is not rapidly redistributed by winds and currents. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]
Instead the strength and direction of winds in the atmosphere is the result of uneven distribution of solar heating and continental land masses and the circulation of winds in a vertical plane in the atmosphere under influences like the temperatures of the sea and land topography and forces like strong winds from the west between 40 degrees to 60 degrees latitude, the roaring forties, weak winds in the subtropics near 30 degrees latitude, trade winds from the east in the tropics, and weaker winds from the east along the Equator,.
The atmosphere within 100 meters of the sea surface is influenced by the turbulent drag of the wind on the sea and the fluxes of heat through the surface. This is the atmospheric boundary layer. It’s thickness varies from a few tens of meters for weak winds blowing over water colder than the air to around a kilometer for stronger winds blowing over water warmer than the air. In addition to things near the Earth’s surface, surface winds are also influenced by equatorial convection and other processes higher in the atmosphere.
Surface winds can change with the seasons. The largest changes are in the Indian Ocean and the western Pacific Ocean. In East Asia air blows southeastward across Japan and on across the hot Kuroshio, extracting heat from the ocean. In summer, the thermal low over Tibet draws warm, moist air from the Indian Ocean leading to the rainy season over India.
Leeward and Windward
"Windward" and "leeward" refer to the prevailing winds on opposite sides of an island. In sailing terminology, windward means "upwind," or the direction from which the wind is blowing. A windward vessel refers to one that is upwind of another vessel; a leeward vessel is downwind. In naval warfare during the Age of Sail, windward ships had the advantage due to much greater maneuverability than their leeward (downwind) foes. [Source: NOAA]
An island’s windward side faces the prevailing, or trade, winds, whereas the island’s leeward side faces away from the wind, sheltered from prevailing winds by hills and mountains. As trade winds blow across the ocean, they pick up moist air from the water.
Once the damp air makes landfall on an island, it ascends hills and mountains to form condensation, clouds, and precipitation. As the air moves to the other side of the island, it warms up and dries out. Thus, an island’s windward side is wetter and more verdant than its drier leeward side. Meteorologists call this contrast the orographic effect.
As an example, the Hawaiian Islands have damp windward sides and drier leeward sides most of the time as a result of the Pacific Ocean’s northeasterly trade winds. Windward locations are generally lush and green. Famously sunny beaches like Oahu’s Waikiki and Maui’s Wailea are found on the islands’ more sheltered leeward sides.
A waterspout is a whirling column of air and water mist. According to NOAA's National Weather Service, the best way to avoid a waterspout is to move at a 90-degree angle to its apparent movement. Never move closer to investigate a waterspout. Some can be just as dangerous as tornadoes. [Source: NOAA]
Waterspouts fall into two categories: fair weather waterspouts and tornadic waterspouts. Tornadic waterspouts are tornadoes that form over water, or move from land to water. They have the same characteristics as a land tornado. They are associated with severe thunderstorms, and are often accompanied by high winds and seas, large hail, and frequent dangerous lightning.
Fair weather waterspouts usually form along the dark flat base of a line of developing cumulus clouds. This type of waterspout is generally not associated with thunderstorms. While tornadic waterspouts develop downward in a thunderstorm, a fair weather waterspout develops on the surface of the water and works its way upward. By the time the funnel is visible, a fair weather waterspout is near maturity. Fair weather waterspouts form in light wind conditions so they normally move very little.
If a waterspout moves onshore, the National Weather Service issues a tornado warning, as some of them can cause significant damage and injuries to people. Typically, fair weather waterspouts dissipate rapidly when they make landfall, and rarely penetrate far inland.
Trade winds are the prevailing easterly winds that circle the Earth near the equator. Known to sailors around the world, the trade winds and associated ocean currents helped early sailing ships from European and African ports make their journeys to the Americas. Likewise, the trade winds also drive sailing vessels from the Americas toward Asia. Even now, commercial ships use "the trades" and the currents the winds produce to hasten their oceanic voyages.
How do these commerce-friendly winds form? Between about 30 degrees north and 30 degrees south of the equator, in a region called the horse latitudes, the Earth's rotation causes air to slant toward the equator in a southwesterly direction in the northern hemisphere and in a northwesterly direction in the southern hemisphere. This is called the Coriolis Effect.
The Coriolis Effect, in combination with an area of high pressure, causes the prevailing winds — the trade winds — to move from east to west on both sides of the equator across this 60-degree "belt." As the wind blows to about five degrees north and south of the equator, both air and ocean currents come to a halt in a band of hot, dry air. This 10-degree belt around Earth's midsection is called the Inter-Tropical Convergence Zone, more commonly known as the doldrums.
Intense solar heat in the doldrums warms and moistens the trade winds, thrusting air upwards into the atmosphere like a hot air balloon. As the air rises, it cools, causing persistent bands of showers and storms in the tropics and rainforests. The rising air masses move toward the poles, then sink back toward Earth's surface near the horse latitudes. The sinking air triggers the calm trade winds and little precipitation, completing the cycle.
The horse latitudes are subtropical regions known for calm winds and little precipitation. horse latitudes They are located at about 30 degrees north and south of the equator. These latitudes are characterized by calm winds and little precipitation. [Source: NOAA]
It is common in the region of the subtropics 30 degrees north and south of the equator to diverge and either flow toward the poles (known as the prevailing westerlies) or toward the equator (known as the trade winds). These diverging winds are the result of an area of high pressure, which is characterized by calm winds, sunny skies, and little or no precipitation.
According to legend, the term Horse Latitudes comes from ships sailing to the New World that would often become stalled for days or even weeks when they encountered areas of high pressure and calm winds. Many of these ships carried horses to the Americas as part of their cargo. Unable to sail and resupply due to lack of wind, crews often ran out of drinking water. To conserve scarce water, sailors on these ships would sometimes throw the horses they were transporting overboard. Thus, the phrase 'horse latitudes' was born.
The "doldrums" is a popular nautical term that refers to the belt around the Earth near the equator where sailing ships sometimes get stuck on windless waters. The scientific name of the doldrums is the Inter-Tropical Convergence Zone, (ITCZ, pronounced and sometimes referred to as the “itch”), This belt around the Earth extending approximately five degrees north and south of the equator. Here, the prevailing trade winds of the northern hemisphere blow to the southwest and collide with the southern hemisphere’s driving northwest trade winds.
Due to intense solar heating near the equator, the warm, moist air is forced up into the atmosphere like a hot air balloon. As the air rises, it cools, causing persistent bands of showers and storms around the Earth’s midsection. The rising air mass finally subsides in what is known as the horse latitudes, where the air moves downward toward Earth’s surface.
Because the air circulates in an upward direction, there is often little surface wind in the ITCZ. That is why sailors well know that the area can becalm sailing ships for weeks. And that’s why they call it the doldrums.
Sailors call the latitudes between 40 and 50 degrees south of the equator the Roaring Forties. During the Age of Sail (circa 15th to 19th centuries), these strong prevailing winds propelled ships across the Pacific, often at breakneck speed. Nevertheless, sailing west into heavy seas and strong headwinds could take weeks, especially around Cape Horn at the southern tip of South America, making it one of the most treacherous sailing passages in the world. [Source: NOAA]
The Roaring Forties take shape as warm air near the equator rises and moves toward the poles. Warm air moving poleward (on both sides of the equator) is the result of nature trying to reduce the temperature difference between the equator and at the poles created by uneven heating from the sun.This process sets up global circulation cells, which are mainly responsible for global-scale wind patterns. The air descends back to Earth’s surface at about 30 degrees’ latitude north and south of the equator. This is known as the high-pressure subtropical ridge, also known as the horse latitudes. Here, as the temperature gradient decreases, air is deflected toward the poles by the Earth’s rotation, causing strong westerly and prevailing winds at approximately 40 degrees. These winds are the Roaring Forties.
The Roaring Forties in the Northern Hemisphere don’t pack the same punch that they do in the Southern Hemisphere. This is because the large land masses of North America, Europe, and Asia obstruct the airstream, whereas, in the southern hemisphere, there is less land to break the wind in South America, Australia, and New Zealand.
While the Roaring Forties may be fierce, 10 degrees south are even stronger gale-force winds called the Furious Fifties. And 10 degrees south of the Furious Fifties lie the Screaming Sixties! We can thank the intrepid sailors of yore for these wildly descriptive terms.
Measurement of Wind in the Ocean
According to the “Introduction to Physical Oceanography”: Wind at sea has been measured for centuries. Maury (1855) was the first to systematically collect and map wind reports. Now the US National Atmospheric and Oceanic Administration (NOAA) has collected, edited, and digitized millions of observations going back over a century. The resulting International Comprehensive Ocean, Atmosphere Data Set is widely used for studying atmospheric forcing of the ocean. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]
Our knowledge of winds at the sea surface come from many sources. Here are the more important, listed in a crude order of relative importance: Beaufort Scale By far the most common source of wind data up to 1991 have been reports of speed based on the Beaufort scale. The scale is based on features, such as foam coverage and wave shape, seen by an observer on a ship. The scale was originally proposed by Admiral Sir F. Beaufort in 1806 to give the force of the wind on a ship’s sails. It was adopted by the British Admiralty in 1838 and it soon came into general use.
The International Meteorological Committee adopted the force scale for international use in 1874. In 1926 they adopted a revised scale giving the wind speed at a height of 6 meters corresponding to the Beaufort Number. The scale was revised again in 1946 to extend the scale to higher wind speeds and to give the equivalent wind speed at a height of 10 meters. The 1946 scale was based on the equation U10 = 0.836B3/2, where B = Beaufort Number and U10 is the wind speed in meters per second at a height of 10 meters. More recently, various groups have revised the Beaufort scale by comparing Beaufort force with ship measurements of winds. Kent and Taylor (1997) compared the various revisions of the scale with winds measured by ships having anemometers at known heights.
Beaufort Wind Scale (Type of Wind, Speed, and Appearance)
1) Light Air: 1.2 meters per second (4.3 kilometers per hour, 2.7 miles per hour); Ripples with appearance of scales; no foam crests.
2) Light Breeze: 2.8 meters per second (10 kilometers per hour, 6.3 miles per hour); Small wavelets; crests of glassy appearance, not breaking.
3) Gentle Breeze: 4.9 meters per second (17.6 kilometers per hour, 11 miles per hour); Large wavelets; crests begin to break; scattered whitecaps.
4) Moderate Breeze: 7.7 meters per second (28 kilometers per hour, 17.2 miles per hour); Small waves, becoming longer; numerous whitecaps.
5) Fresh Breeze: 10.5 meters per second (38 kilometers per hour, 23.5 miles per hour); Moderate waves, taking longer to form; many whitecaps; some spray.
6) Strong Breeze: 13.1 meters per second (47 kilometers per hour, 29 miles per hour); Large waves forming; whitecaps everywhere; more spray.
7) Near Gale: 15.8 meters per second (57 kilometers per hour, miles per hour); Sea heaps up; white foam from breaking waves begins to be blown into streaks.
8) Gale: 18.8 meters per second (67.7 kilometers per hour, 35.3 miles per hour); Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks.
9) Strong Gale: 22.1 meters per second (80 kilometers per hour, 49.3 miles per hour); High waves; sea begins to roll; dense streaks of foam; spray may reduce visibility.
10) Storm: 25.9 meters per second (93 kilometers per hour, 58 miles per hour); Very high waves with overhanging crests; sea takes white appearance as foam is blown in very dense streaks; rolling is heavy and visibility reduced.
11) Violent Storm: 30.2 meters per second (109 kilometers per hour, 67.5 miles per hour); Exceptionally high waves; sea covered with white foam patches; visibility still more reduced.
12) Hurricane 35.2 meters per second (123 kilometers per hour, 78.7 miles per hour); Air is filled with foam; sea completely white with driving spray; visibility greatly reduced.
Observers on ships everywhere in the world usually report weather observations, including Beaufort force, at the same four times every day. The times are at 0000Z, 0600Z, 1200Z and 1800Z, where Z indicates Greenwich Mean Time. The reports are coded and reported by radio to national meteorological agencies. The biggest error in the reports is the sampling error. Ships are unevenly distributed over the ocean. They tend to avoid high latitudes in winter and hurricanes in summer, and few ships cross the southern hemisphere. Overall, the accuracy is around 10 percent.
Wind Measuring Satellites and Tools
Scatterometers: According to the “Introduction to Physical Oceanography”: Observations of winds at sea now come mostly from scatterometers on satellites (Liu, 2002). The scatterometer is a instrument very much like a radar that measures the scatter of centimeter-wavelength radio waves from small, centimeter-wavelength waves on the sea surface. The area of the sea covered by small waves, their amplitude, and their orientation, depend on wind speed and direction. The scatterometer measures scatter from 2–4 directions, from which wind speed and direction are calculated. The scatterometers on ERS-1 and 2 have made global measurements of winds from space since 1991. The NASA scatterometer on ADEOS measured winds for a six-month period beginning November 1996 and ending with the premature failure of the satellite. It was replaced by another scatterometer on QuikScat, launched on 19 June 1999. Quikscat views 93 percent of the ocean every 24 hours with a resolution of 25 kilometers. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]
Freilich and Dunbar (1999) report that, overall, the nasa scatterometer on adeos measured wind speed with an accuracy of ±1.3 meters per second. The error in wind direction was ±17 degrees . Spatial resolution was 25 kilometers. Data from QuikScat has an accuracy of ±1 meters per second. Because scatterometers view a specific oceanic area only once a day, the data must be used with numerical weather models to obtain 6-hourly wind maps required for some studies.
Windsat is an experimental, polarimetric, microwave radiometer developed by the US Navy that measures the amount and polarization of microwave radiation emitted from the sea at angles between 50 degrees to 55 degrees relative to the vertical and at five radio frequencies. It was launched on 6 January 2003 on the Coriolis satellite. The received radio signal is a function of wind speed, seasurface temperature, water vapor in the atmosphere, rain rate, and the amount of water in cloud drops. By observing several frequencies simultaneously, data from the instrument are used for calculating the surface wind speed and direction, sea-surface temperature, total precipitable water, integrated cloud liquid water, and rain rate over the ocean regardless of time of day or cloudiness. Winds are calculated over most of the ocean on a 25- kilometers grid once a day. Winds measured by Windsat have an accuracy of ±2 meters per second in speed and ±20 degrees in direction over the range of 5–25 meters per second.
Special Sensor Microwave SSM/I is another satellite instrument that is used to measure wind speed is the Special-Sensor Microwave/Imager (ssm/i) carried since 1987 on the satellites of the U.S. Defense Meteorological Satellite Program in orbits similar to the noaa polar-orbiting meteorological satellites. The instrument measures the microwave radiation emitted from the sea at an angle near 60 degrees from the vertical. The radio signal is a function of wind speed, water vapor in the atmosphere, and the amount of water in cloud drops. By observing several frequencies simultaneously, data from the instrument are used for calculating the surface wind speed, water vapor, cloud water, and rain rate. Winds measured by ssm/i have an accuracy of ± 2 meters per second in speed. When combined with ECMWF (European Centre for Medium-Range Weather Forecasts) 1000 mb wind analyses, wind direction can be calculated with an accuracy of ±22 degrees. Global, gridded data are available since July 1987 on a 0.25 degrees grid every 6 hours. But remember, the instrument views a specific oceanic area only once a day, and the gridded, 6-hourly maps have big gaps.
Anemometers are device that measures wind speed and direction. They come in variety of types, with weather vanes being one of the most basic instruments used to determine wind direction and cup anemometers (devises with cups that go round and round a shaft) to measure wind speed. On the global oceanic scale, satellite observations are supplemented by winds reported to meteorological agencies by observers reading anemometers on ships. The anemometer is read four times a day at the standard Greenwich times and reported via radio to meteorological agencies. Again, the biggest error is the sampling error. Very few ships carry calibrated anemometers. The accuracy of wind measurements from these ships is about ±2 meters per second.
Calibrated Anemometers on Moored Weather Buoys make the most accurate measurements of winds at sea. Unfortunately there are few such buoys, perhaps only a hundred scattered around the world. Some, such as Tropical Atmosphere Ocean TAO array in the tropical Pacific provide data from remote areas rarely visited by ships, but most tend to be located just offshore of coastal areas. NOAA operates buoys offshore of the United States and the TAO array in the Pacific. Data from the coastal buoys are averaged for eight minutes before the hour, and the observations are transmitted to shore via satellite links. The best accuracy of anemometers on buoys operated by the us National Data Buoy Center is the greater of ±1 meters per second or 10 percent for wind speed and ±10 degrees for wind direction.
Wind Calculations and Models
According to the “Introduction to Physical Oceanography”: Satellites, ships, and buoys measure winds at various locations and times of the day. One source of gridded winds over the ocean is the surface analysis calculated by numerical weather models. The strategy used to produce the six-hourly gridded winds is called sequential estimation techniques or data assimilation. Usually, all available measurements are used in the analysis, including observations from weather stations on land, pressure and temperature reported by ships and buoys, winds from scatterometers in space, and data from meteorological satellites. These are incorporated into models that interpolate the measurements to produce analyses consistent with previous and present observations. Daley (1991) describes the techniques in considerable detail. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]
Perhaps the most widely used weather model is that run by the European Centre for Mediumrange Weather Forecasts (ECMWF). It calculates a surface analysis, including surface winds and heat fluxes every six hours on a 1 degree × 1 degree grid from an explicit boundary-layer model. Calculated values are archived on a 2.5 degrees grid. Thus the wind maps from the numerical weather models lack the detail seen in maps from scatterometer data, which have a 1/4 degrees grid.
ECMWF calculations of winds have relatively good accuracy. Freilich and Dunbar (1999) estimated that the accuracy for wind speed at 10 meters is ±1.5 meters per second, and ±18 degrees for direction. Accuracy in the southern hemisphere is probably as good as in the northern hemisphere because continents do not disrupt the flow as much as in the northern hemisphere, and because scatterometers give accurate positions of storms and fronts over the ocean. The NOAA National Centers for Environmental Prediction and the US Navy also produces global analyses and forecasts every six hours.
Reanalyzed Data from Numerical Weather Models Surface analyses of weather over some regions have been produced for more than a hundred years, and over the whole earth since about 1950. Surface analyses calculated by numerical models of the atmospheric circulation have been available for decades. The U.S. National Centers for Environmental Predictions, working with the National Center for Atmospheric Research have produced the NCEP/ NCAR reanalysis based on 51 years of weather data from 1948 to 2005. ECMWF has reanalyzed 45 years of weather data from September 1957 to August 2002 The reanalysis uses mostly the same surface and ship data used by the NCEP/NCAR reanalysis plus data from the ERS-1 and ERS-2 satellites and ssm/i. The ERA-40 full-resolution products are available every six hours on a N80 grid having 160 × 320 grid points with a spatial resolution of 1.125 degrees and with 60 vertical levels.
Image Sources: Wikimedia Commons; YouTube, NOAA
Text Sources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated March 2023