The Saharan air layer (SAL) is an extremely hot, dry, and sometimes dust-laden layer of the atmosphere that often overlies the cooler, more humid surface air of the Atlantic Ocean. It carries upwards of 60 million tons of dust annually over the ocean and the Americas. [1] This annual phenomenon sometimes cools the ocean and suppresses Atlantic tropical cyclogenesis. [2]
The SAL is a subject of ongoing study and research. Its existence was first postulated in 1972. [3] : 1330
The dust cloud originates in Saharan Africa and extends from the surface upwards several kilometers. As the dust drives, or is driven out over the Atlantic ocean, it is lifted above the denser marine air. This atmospheric arrangement is an inversion where the temperature actually increases with height, as the boundary between the SAL and the marine layer suppresses or "caps" any convection originating in the marine layer. Since it is dry air, the lapse rate within the SAL itself is steep, that is, the temperature falls rapidly with height. [4]
Disturbances such as large thunderstorm complexes over North Africa periodically result in vast dust and sand storms, some of which extend as high as 6,000 metres (20,000 ft). The layer is transported westwards cross the Atlantic by a series of broad anticyclonic eddies that are typically found 1,500–4,500 metres (4,900–14,800 ft) above sea level. [5] An estimated 60-200 million tons of dust particles are carried from the Sahara Desert region of North Africa, where it originates, and moves westward annually. [1]
Sometimes a depression to the southwest of the Canary Islands increases the wind speed and intensity of the SAL, which can lift the dust around 5,000 metres (16,000 ft) into the air and often carries the dust as far as the Caribbean. [6]
The SAL passes over the Canary Islands where the phenomenon is named " calima " (English: "haze") and manifests as a fog that reduces visibility and deposits a layer of dust over everything. It is especially prevalent in the winter months. Canary islanders suffer respiratory problems during this weather event, and sometimes the dust is so bad that public life and transport halt completely. On January 8, 2002, the dust was so heavy over the Tenerife South Airport, dropping the visibility to 50 metres (160 ft), that it was forced to close. [6]
From northern Africa, winds blow twenty percent of dust from a Saharan storm out over the Atlantic Ocean, and twenty percent of that, or four percent of a single storm's dust, reaches all the way to the western Atlantic. The remainder settles out into the ocean or washes out of the air with rainfall.[ citation needed ] Scientists believe the July 2000 measurements made in Puerto Rico, nearly 8 million tonnes, equaled about one-fifth of the year's total dust deposits.[ citation needed ]
The clouds of dust SAL creates are visible in satellite photos as a milky white to gray shade, similar to haze.[ citation needed ]
Findings to date indicate that the iron-rich dust particles that often occur within the SAL reflect solar radiation, thus cooling the atmosphere. The particles also reduce the amount of sunlight reaching the ocean, thus reducing the amount of heating of the ocean. [2] They also tend to increase condensation as they drift into the marine layer below, but not precipitation as the drops formed are too small to fall and tend not to readily coalesce. These tiny drops are subsequently more easily evaporated as they move into drier air laterally or dry air mixes down from the SAL aloft. [7] Research on aerosols also shows that the presence of small particles in air tends to suppress winds. The SAL has also been observed to suppress the development and intensifying of tropical cyclones, which may be related directly to these factors. [8]
A dust storm, also called a sandstorm, is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Fine particles are transported by saltation and suspension, a process that moves soil from one place and deposits it in another.
The trade winds or easterlies are permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries. They enabled European colonization of the Americas, and trade routes to become established across the Atlantic Ocean and the Pacific Ocean.
A tropical wave, in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equatorial side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.
The Pacific decadal oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20°N. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California.
Sea spray consists of aerosol particles formed from the ocean, primarily by ejection into Earth's atmosphere through bursting bubbles at the air-sea interface Sea spray contains both organic matter and inorganic salts that form sea salt aerosol (SSA). SSA has the ability to form cloud condensation nuclei (CCN) and remove anthropogenic aerosol pollutants from the atmosphere. Coarse sea spray has also been found to inhibit the development of lightning in storm clouds.
The North Brazil Current (NBC) is a warm water ocean current that is part of the southwestern North Atlantic Gyre. It begins when the westward moving Atlantic South Equatorial Current splits in half and flows northwestward, following the coastline of north Brazil. It ends at the border of Brazil and Guiana, where it is renamed the Guiana Current. It is predominantly a salt water current, but it does help transport fresh water from the Amazon River northward.
The African easterly jet is a region of the lower troposphere over West Africa where the seasonal mean wind speed is at a maximum and the wind is easterly. The temperature contrast between the Sahara Desert and the Gulf of Guinea causes the jet to form to the north of the monsoon trough. The jet's maximum wind speeds are at a height of 3 kilometres (1.9 mi). The jet moves northward from its south-most location in January, reaching its most northerly latitude in August. Its strongest winds are in September while it begins shifting back towards the equator. Within the easterly jet, tropical waves form. Convective complexes associated with these waves can form tropical cyclones. If the jet is south of its normal location during August and September, tropical cyclogenesis is suppressed. If desertification continues across Sub-Saharan Africa, the strength of the jet could increase, although tropical wave generation probably would decrease, which would decrease the number of tropical cyclones in the Atlantic basin.
Mineral dust is atmospheric aerosol originated from the suspension of minerals constituting the soil, composed of various oxides and carbonates. Human activities lead to 30% of the airborne dust (particulates) load in the atmosphere. The Sahara Desert is the major source of mineral dust, which subsequently spreads across the Mediterranean and Caribbean seas into northern South America, Central America, and eastern North America, and Europe. Additionally, it plays a significant role in the nutrient inflow to the Amazon rainforest. The Gobi Desert is another source of dust in the atmosphere, which affects eastern Asia and western North America.
A tropical cyclone is a rapidly rotating storm system with a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is called a hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean. A typhoon occurs in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as "tropical cyclones". In modern times, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of 65 kn or more.
The Bodélé Depression, located at the southern edge of the Sahara Desert in north central Africa, is the lowest point in Chad. It is 500 km long, 150 km wide and around 160 m deep. Its bottom lies about 155 meters above sea level. The dry endorheic basin is a major source of fertile dust essential for the Amazon rainforest.
The Atlantic Multidecadal Oscillation (AMO), also known as Atlantic Multidecadal Variability (AMV), is the theorized variability of the sea surface temperature (SST) of the North Atlantic Ocean on the timescale of several decades.
Explosive cyclogenesis is the rapid deepening of an extratropical cyclonic low-pressure area. The change in pressure needed to classify something as explosive cyclogenesis is latitude dependent. For example, at 60° latitude, explosive cyclogenesis occurs if the central pressure decreases by 24 millibars (0.71 inHg) or more in 24 hours. This is a predominantly maritime, winter event, but also occurs in continental settings. This process is the extratropical equivalent of the tropical rapid deepening. Although their cyclogenesis is entirely different from that of tropical cyclones, bomb cyclones can produce winds of 74 to 95 mph, the same order as the first categories of the Saffir–Simpson scale, and yield heavy precipitation. Even though only a minority of bomb cyclones become this strong, some weaker ones can also cause significant damage.
In meteorology, eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones with maximum sustained winds greater than 119 km/h (74 mph), or hurricane strength, and particularly in major hurricanes of Saffir–Simpson category 3 to 5. In such storms, some of the outer rainbands may strengthen and organize into a ring of thunderstorms—a new, outer eyewall—that slowly moves inward and robs the original, inner eyewall of its needed moisture and angular momentum. Since the strongest winds are in a tropical cyclone's eyewall, the storm usually weakens during this phase, as the inner wall is "choked" by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm may re-intensify.
An atmospheric river (AR) is a narrow corridor or filament of concentrated moisture in the atmosphere. Other names for this phenomenon are tropical plume, tropical connection, moisture plume, water vapor surge, and cloud band.
Sreedharan Krishnakumari Satheesh is an Indian meteorologist and a professor at the Centre for Atmospheric and Oceanic Sciences of the Indian Institute of Science (IISc). He holds the chair of the Divecha Centre for Climate Change, a centre under the umbrella of the IISc for researches on climate variability, climate change and their impact on the environment. He is known for his studies on atmospheric aerosols and is an elected fellow of all the three major Indian science academies viz. Indian Academy of Sciences Indian National Science Academy and the National Academy of Sciences, India as well as The World Academy of Sciences. The Council of Scientific and Industrial Research, the apex agency of the Government of India for scientific research, awarded him the Shanti Swarup Bhatnagar Prize for Science and Technology, one of the highest Indian science awards for his contributions to Earth, Atmosphere, Ocean and Planetary Sciences in 2009. He received the TWAS Prize of The World Academy of Sciences in 2011. In 2018, he received the Infosys Prize, one of the highest monetary awards in India that recognize excellence in science and research, for his work in the field of climate change.
Saharan dust is an aeolian mineral dust from the Sahara, the largest hot desert in the world. The desert spans just over 9 million square kilometers, from the Atlantic Ocean to the Red Sea, from the Mediterranean Sea to the Niger River valley and the Sudan region in the south.
In atmospheric science, a cold pool (CP) is a cold pocket of dense air that forms when rain evaporates during intense precipitation e.g. underneath a thunderstorm cloud or a precipitating shallow cloud. Typically, cold pools spread at 10 m/s and last 2–3 hours. Cold pools are ubiquitous both over land and ocean.
Pacific Meridional Mode (PMM) is a climate mode in the North Pacific. In its positive state, it is characterized by the coupling of weaker trade winds in the northeast Pacific Ocean between Hawaii and Baja California with decreased evaporation over the ocean, thus increasing sea surface temperatures (SST); and the reverse during its negative state. This coupling develops during the winter months and spreads southwestward towards the equator and the central and western Pacific during spring, until it reaches the Intertropical Convergence Zone (ITCZ), which tends to shift north in response to a positive PMM.
Lorraine Remer is research professor at University of Maryland, Baltimore County known for her work on developing algorithms to study aerosol particles using satellites with a particular focus on how aerosols impact climate processes.
Calima or Kalima is a term used to describe a meteorological phenomenon that occurs when fine sand and dust particles from the Sahara are lifted into the atmosphere and transported by prevailing winds.