Meridional Overturning Circulation

Meridional Overturning Circulation
Robert Simmon, NASA. Minor modifications by Robert A. Rohde also released to th · Public domain · source
Name	Meridional Overturning Circulation
Type	Ocean current system
Location	Atlantic Ocean, Pacific Ocean, Southern Ocean, global oceans
Components	Thermohaline circulation, wind-driven currents, deep water formation
Significance	Climate regulation, carbon sequestration, heat transport

Meridional Overturning Circulation The Meridional Overturning Circulation is a global system of surface and deep currents that transports heat, salt, and biogeochemical tracers between basins and latitudes, linking regions such as the North Atlantic Ocean, Southern Ocean, Arctic Ocean, and Pacific Ocean. Its pathways and strength influence regional climates including those of Europe, North America, and West Africa, and interact with phenomena like the El Niño–Southern Oscillation, North Atlantic Oscillation, and events recorded in the Paleocene–Eocene Thermal Maximum. Observations and models by institutions such as National Oceanic and Atmospheric Administration, Woods Hole Oceanographic Institution, and Scripps Institution of Oceanography underpin assessments from bodies like the Intergovernmental Panel on Climate Change.

Overview

The circulation comprises linked surface currents and deep return flows driven by buoyancy and wind forcing, connecting formation sites in the Greenland Sea, Norwegian Sea, and Weddell Sea with abyssal pathways across the Mid-Atlantic Ridge and global basins, and redistributing heat between the Tropics and polar regions. Historical synthesis draws on voyages by explorers like James Cook and measurements inaugurated by programs including the International Geophysical Year and the World Ocean Circulation Experiment. The conceptual framework integrates work from scientists affiliated with Lamont–Doherty Earth Observatory, National Centre for Atmospheric Research, and universities such as University of Southampton and University of Cambridge.

Mechanisms and Dynamics

Buoyancy-driven convection associated with cooling and salinification at high latitudes produces dense water masses (e.g., North Atlantic Deep Water and Antarctic Bottom Water) that sink and flow equatorward, while wind-driven Ekman transport and gyre circulations in the Atlantic Ocean and Pacific Ocean modulate surface return pathways. Thermodynamic processes interact with dynamics influenced by features such as the Gulf Stream, Kuroshio Current, Antarctic Circumpolar Current, and bathymetric structures like the Mid-Atlantic Ridge and Drake Passage, and are affected by sea-ice processes around Greenland and Antarctica. Internal variability includes eddies, boundary currents, and modes described by theories from Lewis Fry Richardson and numerical frameworks developed at Princeton University and MIT.

Observations and Measurement Methods

Direct measurement networks include moored arrays such as the RAPID Climate Change-Meridional Overturning Circulation and Heatflux Array across the Strait of Gibraltar and across 26°N in the North Atlantic Ocean, autonomous platforms like Argo floats, gliders developed at Ocean Observatories Initiative, and ship-based hydrographic surveys standardized by International Council for the Exploration of the Sea. Remote sensing from satellites operated by European Space Agency, National Aeronautics and Space Administration, and Japan Aerospace Exploration Agency provides sea-surface temperature and altimetry that constrain surface transports, while chemical tracers measured by teams at Scripps Institution of Oceanography and GEOTRACES help trace pathways and ventilation ages.

Regional Components (e.g., Atlantic, Pacific, Southern Ocean)

In the Atlantic Ocean a pronounced overturning cell links subtropical surface inflow via the Gulf Stream to deep outflow of North Atlantic Deep Water; the Pacific Ocean exhibits weaker abyssal export but significant intermediate overturning influenced by the Kuroshio Current and marginal seas like the Bering Sea and Sea of Okhotsk. The Southern Ocean and the Antarctic Circumpolar Current form a critical conduit for inter-basin exchange and formation of Antarctic Bottom Water, with interactions at the Drake Passage and the Rosenstiel School of Marine and Atmospheric Science research sites. Regional variability is modulated by atmospheric teleconnections such as the Southern Annular Mode and by freshwater inputs from glaciers like those of Greenland and West Antarctica.

Role in Climate and Biogeochemical Cycles

The circulation transports large heat anomalies that affect surface climate patterns across Europe and North America, shapes regional precipitation through ocean–atmosphere coupling with features like the Atlantic Multidecadal Oscillation, and controls uptake and storage of carbon and nutrients by ventilating deep waters documented by programs such as GO-SHIP and GEOTRACES. Biogeochemical impacts include regulation of oceanic oxygen minimum zones linked to regions like the Eastern Tropical Pacific and modulation of marine ecosystems that sustain fisheries off Iceland, Norway, and the Northwest Atlantic Fisheries Organization jurisdictions. Paleoceanographic records from cores at IODP and studies of ice cores from Greenland and Antarctica document past circulation shifts during events such as the Younger Dryas.

Variability, Trends, and Climate Change Impacts

Instrumental and paleoclimate evidence analyzed by groups at National Centre for Atmospheric Science and in assessments by the Intergovernmental Panel on Climate Change indicate multi-decadal variability and potential long-term weakening linked to increased freshwater from Greenland ice sheet melt and altered wind patterns caused by greenhouse forcing. Projections from ensembles run at Met Office Hadley Centre, NOAA Geophysical Fluid Dynamics Laboratory, and European Centre for Medium-Range Weather Forecasts differ in magnitude and timing, but many show reduced overturning strength under high-emission scenarios, with implications for regional sea-level change observed by Jason (satellite) altimetry and for abrupt climate risks discussed in reports by United Nations Framework Convention on Climate Change.

Modeling and Predictive Challenges

Representation of small-scale processes such as eddies, overflows across sills like the Denmark Strait, and mixing over rough topography requires high-resolution models developed at Los Alamos National Laboratory and National Center for Atmospheric Research, coupled with data assimilation systems used by European Centre for Medium-Range Weather Forecasts. Uncertainties arise from model parameterizations, scenario forcing differences among CMIP6 ensembles, and limited observational constraints in key regions such as the Southern Ocean and high latitudes; bridging these gaps involves coordinated field programs, multi-model intercomparisons, and advances in numerical methods from institutions like Princeton University and Massachusetts Institute of Technology.

Category:Ocean currents