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Oceanic deserts are regions of the oceans characterized by low annual precipitation, comparable to that of continental deserts.[1] These areas typically overlap with subtropical gyres - large systems of circular ocean currents formed by the global wind patterns.[2] These gyres are characterized by semi-permanent high-pressure systems, which inhibit the formation of deep precipitating clouds.[3][4] [5] Unlike continental deserts, oceanic deserts maintain a relatively high cloud fraction throughout the year. [6] Despite the pronounced cloud cover, these low level clouds produce very little precipitation, distinguishing these areas as oceanic deserts.[7]

The term "desert" in this context not only refers to the low precipitation but also to the low biodiversity found in these regions. The oceanic circulation in these regions significantly impacts marine life, leading to lower productivity and biodiversity compared to other parts of the ocean.[8]

Ocean gyres and currents from Atlas of World Maps. (Army Service Forces Manual M-101, 1943)
Figure 1: Global average annual precipitation. Dark blue areas indicating regions with low precipitation amount. The eastern basin of subtropical oceans are distinguished with low precipitation. (NASA SVS, 2023)

Geographic distribution

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Oceanic deserts are primarily found in the subtropical regions of the world's major ocean basins. The corresponding subtropical gyres are the North Atlantic Gyre extends from the eastern coast of North America to the western coast of Europe and Africa. The South Atlantic Gyre is located off the coast of South America, stretching towards Africa. The North Pacific Gyre spans from the western coast of North America to the eastern coast of Asia. The South Pacific Gyre is found off the coast of South America, extending to the western Pacific. The Indian Ocean Gyre is positioned between the eastern coast of Southern Africa and the western coast of Australia. [9]

Climatic and atmospheric conditions

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Figure 2: Average of all of the satellite’s cloud observations between July 2002 and April 2015. Colors range from dark blue (no clouds) to light blue (some clouds) to white (frequent clouds). (Earth Observatory NASA, 2015)

Oceanic deserts are influenced by several key atmospheric and climatic factors. Persistent high-pressure systems, known as subtropical highs, dominate these regions, leading to stable atmospheric conditions.[10] The stable high-pressure zones prevent the development of deep convective clouds, which are necessary for rainfall. As a result, annual precipitation in oceanic deserts is minimal, comparable to the aridity observed in terrestrial deserts.

Despite the low precipitation, oceanic deserts maintain a relatively high cloud fraction throughout the year (see figure 2). Coastal regions typically see stratus clouds, while offshore areas are characterized by stratocumulus clouds.[6] Over the relatively warm ocean to the west, trade-wind cumuli are common.[11] The cloud cover in these regions is largely non-precipitating, contributing to the persistent dry conditions despite the presence of clouds.[7] This is in contrast to continental deserts, which generally have clear skies with occasional, but often intense, rainfall events.[12][13]

A key feature of the atmospheric profile in oceanic deserts is the presence of the trade wind inversion.[11] This inversion layer, found at an altitude of about 1 to 2 kilometers, acts as a cap that limits vertical mixing and the development of deep convective clouds and thus contributes to the suppression of precipitation. The vertical profile of meteorological parameters such as temperature, humidity, and wind speed in oceanic deserts reveals a sharp gradient at the inversion layer.[14]

Oceanographic features

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Figure 3: This composite image gives an indication of the magnitude and distribution of global primary production, both oceanic (mg/m3 chlorophyll a) and terrestrial (normalized difference land vegetation index). (Visible Earth NASA, 2009)

Oceanic deserts are depicted in dark blue on maps produced by NASA (see figures 1 and 3), indicating areas with low precipitation and low chlorophyll concentrations, highlighting their status as nutrient-starved oceanic regions.[15] [16]However, the coastal zones in these oceanic deserts have a relatively high productivity and biodiversity due to deposition of nutrient rich continental sediment by surface runoff as well as upwelling induced by prevailing winds and rising sea floors brings iron and other essential nutrients to the surface. This includes the equator zone.[17] [18]

The physical characteristics of oceanic deserts significantly influence their ecological dynamics. Strong stratification in these regions prevents the mixing of nutrient-rich deep waters with surface waters, maintaining nutrient-poor conditions at the surface. This stable stratification is a result of the warm, saline surface waters overlaying cooler, denser deep waters, which inhibits vertical mixing.[19]

Nutrient depletion in subtropical gyres is primarily due to strong downwelling and particle sinking.[20]In contrast, regions with the highest chlorophyll concentrations are found in cold waters, where nutrient-rich upwelling occurs, allowing phytoplankton to thrive.

References

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  1. ^ "Precipitation Climatology | NASA Global Precipitation Measurement Mission". gpm.nasa.gov. Retrieved 2024-05-31.
  2. ^ Talley, Lynne D.; Pickard, George L.; Emery, William J.; Swift, James H. (2011), "Introduction to Descriptive Physical Oceanography", Descriptive Physical Oceanography, Elsevier, pp. 142–145, doi:10.1016/C2009-0-24322-4, ISBN 978-0-7506-4552-2
  3. ^ US Department of Commerce, National Oceanic and Atmospheric Administration. "What is a gyre?". oceanservice.noaa.gov. Retrieved 2024-05-31.
  4. ^ Nugent, Alison; DeCou, David. "Chapter 11: General Circulation". {{cite journal}}: Cite journal requires |journal= (help)
  5. ^ Thompson, J. Dana (1978-09-01). "Ocean deserts and ocean oases". Climatic Change. 1 (3): 205–230. doi:10.1007/BF00166175. ISSN 1573-1480.
  6. ^ a b "Cloud Fraction". earthobservatory.nasa.gov. 2024-03-31. Retrieved 2024-05-31.
  7. ^ a b Vial, Jessica; Vogel, Raphaela; Bony, Sandrine; Stevens, Bjorn; Winker, David M.; Cai, Xia; Hohenegger, Cathy; Naumann, Ann Kristin; Brogniez, Hélène (2019-10). "A New Look at the Daily Cycle of Trade Wind Cumuli". Journal of Advances in Modeling Earth Systems. 11 (10): 3148–3166. doi:10.1029/2019MS001746. ISSN 1942-2466. PMC 6919927. PMID 31894190. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
  8. ^ Voiland, Gene Feldman and Adam P (2011-01-27). "NASA Scientific Visualization Studio | Marine Deserts On The Move". NASA Scientific Visualization Studio. Retrieved 2024-06-05.
  9. ^ McClain, Charles R; Signorini, Sergio R; Christian, James R (2004-01-01). "Subtropical gyre variability observed by ocean-color satellites". Deep Sea Research Part II: Topical Studies in Oceanography. Views of Ocean Processes from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Mission: Volume 1. 51 (1): 281–301. doi:10.1016/j.dsr2.2003.08.002. ISSN 0967-0645.
  10. ^ "Did You Know? | National Centers for Environmental Information (NCEI)". www.ncei.noaa.gov. Retrieved 2024-07-02.
  11. ^ a b "MetEd » Sign In". www.meted.ucar.edu. Retrieved 2024-07-02.
  12. ^ "Map of the Week: Does it Rain? Does it Pour? | UBIQUE". Retrieved 2024-07-30.
  13. ^ "Extreme Precipitation in Arid Regions: Observation, Mechanisms, and Simulations". www.frontiersin.org. Retrieved 2024-07-30.
  14. ^ "The Marine Layer | National Oceanic and Atmospheric Administration". www.noaa.gov. Retrieved 2024-07-02.
  15. ^ Huffman, Alex Kekesi and George (2024-03-29). "NASA Scientific Visualization Studio | Grand Average Precipitation Climatology (2000-2023)". NASA Scientific Visualization Studio. Retrieved 2024-07-02.
  16. ^ Voiland, Gene Feldman and Adam P. (2011-01-27). "NASA Scientific Visualization Studio | Marine Deserts On The Move". NASA Scientific Visualization Studio. Retrieved 2024-07-02.
  17. ^ "Sediment Plume off the Louisiana Coast". earthobservatory.nasa.gov. 2018-03-09. Retrieved 2024-06-25.
  18. ^ "The Earth's Biosphere". earthobservatory.nasa.gov. 2002-08-03. Retrieved 2024-06-25.
  19. ^ Talley, Lynne D.; Pickard, George L.; Emery, William J., eds. (2011). Descriptive physical oceanography: an introduction (6th ed ed.). Amsterdam ; Boston: Academic Press. ISBN 978-0-7506-4552-2. OCLC 720651296. {{cite book}}: |edition= has extra text (help)
  20. ^ Renfrow, Stephanie (2009-02-06). "An Ocean full of Deserts | Earthdata". {{cite journal}}: Cite journal requires |journal= (help)
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