Subtropical Gyre

Subtropical Gyre
Name	Subtropical Gyre
Caption	Schematic of subtropical gyres in the world's oceans
Type	Oceanic circulation
Location	Atlantic Ocean; Pacific Ocean; Indian Ocean; Southern Ocean
Area	Global subtropical basins

Subtropical Gyre Subtropical gyres are large, persistent, wind-driven ocean circulation systems centered in the subtropical basins of the Earth's oceans. They organize sea surface currents, influence heat and salt transport, and interact with atmospheric systems to modulate regional climate and weather patterns. These gyres are bounded by western boundary currents, eastern boundary currents, and subtropical convergence zones that link to major basins and coastal regions.

Overview

Subtropical gyres occur in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean basins and are crucial to basins such as the Mediterranean Sea and marginal seas linked to the Gulf of Mexico, Bay of Bengal, and South China Sea. The gyres arise where persistent trade winds and westerlies drive surface currents that form clockwise circulations in the Northern Hemisphere and counterclockwise circulations in the Southern Hemisphere, connecting features like the Gulf Stream, Kuroshio Current, Benguela Current, and California Current. These circulations interact with atmospheric phenomena including the Hadley cell, Walker circulation, El Niño–Southern Oscillation, and the North Atlantic Oscillation, influencing heat transport to regions such as the European continent, West Africa, and the East Asian coast. Studies by institutions including the Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, NOAA, and National Oceanography Centre have characterized gyre structure through observations, theory from Ekman, and theories developed by researchers at Princeton University and WHOI.

Formation and Dynamics

Gyre formation depends on wind stress curl from the trade winds, subtropical highs such as the Bermuda High and Pacific High, and planetary vorticity linked to the Coriolis effect and Earth rotation. Ekman transport drives convergence that produces a subtropical gyre's high-pressure dome and sea surface height gradients measured by altimeters from missions like TOPEX/Poseidon, Jason-1, Jason-2, and Sentinel-3. Western intensification produces strong western boundary currents exemplified by the North Brazil Current feeding into the Gulf Stream and the East Australian Current connected to the Tasman Sea. Mesoscale eddies, rings, and jets such as the Agulhas rings and Loop Current rings modulate momentum and tracer transport; eddy shedding at the Leeuwin Current and instability along fronts influences nutrient fluxes near the Humboldt Current System. Thermohaline gradients, buoyancy forcing from the North Atlantic Drift and freshwater inputs from the Amazon River and Ganges River also affect gyre stratification and subtropical mode waters like the Mode Water in the North Pacific Subtropical Countercurrent.

Major Subtropical Gyres

Major gyres include the North Atlantic Gyre bounded by the Gulf Stream, North Atlantic Current, and Canary Current; the North Pacific Gyre with the Kuroshio Current, North Pacific Current, and California Current; the South Atlantic Gyre involving the Brazil Current and Benguela Current; the South Pacific Gyre connected to the East Australian Current and Peru Current; and the Indian Ocean Gyre influenced by the Agulhas Current and seasonal monsoonal reversals near the Arabian Sea and Bay of Bengal. Each gyre interacts with climatic teleconnections such as the Pacific Decadal Oscillation, Atlantic Multidecadal Oscillation, and regional indices used by groups like the IPCC and WCRP to assess variability. Peripheral features include the subtropical convergence zones near the Azores and Hawaiian Islands and large-scale garbage patches noted within the North Pacific and North Atlantic basins documented by organizations such as the Ocean Cleanup and researchers at University of Hawaii.

Ecological and Climate Impacts

Subtropical gyres shape biogeography by creating oligotrophic gyre cores contrasted with productive eastern boundary upwelling regions like the Peru Current and California Current System, affecting ecosystems from picophytoplankton to large pelagic species including tuna, swordfish, and sea turtles that migrate along currents to areas like the Sargasso Sea. Gyres influence carbon cycling through the biological pump and air-sea CO2 fluxes monitored at time-series stations such as Bermuda Atlantic Time-series Study and HOT (Hawaii Ocean Time-series). Climate links include modulation of heat uptake by the ocean, interactions with the Atlantic Meridional Overturning Circulation, and impacts on extreme events such as heatwaves affecting regions like California, Mediterranean Basin, and Australia. Gyre-associated stratification affects oxygen minimum zones found near the Arabian Sea and Bay of Bengal and contributes to habitat compression documented by the IUCN and national agencies like NOAA Fisheries.

Human Interactions and Impacts

Humans affect and are affected by subtropical gyres via shipping routes connecting ports such as New York City, Los Angeles, Shanghai, and Rotterdam that rely on currents for transit, and fisheries managed by bodies like the International Commission for the Conservation of Atlantic Tunas and regional commissions. Pollution accumulates in gyre centers forming large marine debris accumulations documented in the Great Pacific Garbage Patch, prompting actions by NGOs including The Ocean Cleanup and policy discussions at the United Nations and IMO. Climate change driven by emissions from nations and industries assessed by the IPCC alters wind patterns, potentially shifting gyre positions and intensities with implications for coastal flooding in areas like Florida, Netherlands, and Bangladesh and for desalination and coastal infrastructure projects in cities such as Dubai and Singapore.

Observation and Modeling Methods

Observation relies on in situ platforms like Argo floats, drifting buoys from Global Drifter Program, current meter moorings deployed by NOAA and Ifremer, and biological surveys by institutions including CSIRO and MBARI. Remote sensing uses satellite altimetry (e.g., Jason-3, CryoSat-2), sea surface temperature from MODIS and AVHRR, and ocean color sensors on SeaWiFS and Sentinel-2 to infer productivity and gyre boundaries. Numerical modeling employs general circulation models such as those developed at GFDL, MITgcm, and the UK Met Office and Earth system models used in CMIP6 to simulate gyre responses to forcing from reanalysis products like ERA5 and datasets from NOAA and NASA. Emerging techniques include autonomous gliders used by Scripps and data-assimilative frameworks at ECMWF that integrate observations to improve forecasts for stakeholders including coastal managers and fisheries managers.

Category:Oceanography