7: Ocean Circulation - Geosciences

7: Ocean Circulation - Geosciences

An ocean current is a continuous, directed movement of seawater generated by forces acting upon this mean flow, such as breaking waves, wind, the Coriolis effect, cabbeling, temperature and salinity differences, while tides are caused by the gravitational pull of the Sun and Moon. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength.

Section Outline

I. Effect of atmospheric circulation on water, and coriolis effect

Eckman transport

II. Major Gyres

III. Major Currents within Gyres

Western Boundary Currents

Eastern Boundary Currents


Equatorial Upwelling

IV. Density Differences - Salinity and Temperature of Ocean

V. Thermohaline Circulation

Locations of Deepwater formation

Locations of Intermediate Water formation

Transit of water through the ocean

Accumulation of nutrients, carbon & depletion of oxygen

VI. Meridional Overturning Circulation + Wind Driven Circulation - the global movement of heat and salt

Some sources for intro to wind driven circulation, mostly coriolis effect and ekman transport. Also includes some multimedia stuff.

Coriolis Effect and Ekman Transport. Short, but has links to other good information, including: Nice explanation of Coriolis Effect with good images, but appears to be possibly illegally copy-pasted from Encyclopedia Brittanica really excellent animations of coriolis effect showing expected/true paths when intending to move along a line of longitude good history of Ekman transport discovery, but explanation relatively poor very nice gif of upwelling! Also briefly explains downwelling. Downloadable video about circulation. Long, didn’t watch through all the way but looks like good material. Not sure if this counts as open source, but has some cool math stuff that explains Ekman transport. Also compares the theory behind it to what actually happens. Mostly images and math, may not be a good source. More math and nice explanations of calculations. Has links to more explanation and to history.

Study pinpoints key causes of ocean circulation change

Variability in ocean currents is influenced by multiple factors. Credit: Prof Helen Johnson

Researchers have identified the key factors that influence a vital pattern of ocean currents.

The Atlantic meridional overturning circulation (AMOC) carries warm water from the tropics northward.

Many scientists think that this heat transport makes areas including north-west Europe and the UK warmer than they would otherwise be.

Climate models suggest the AMOC is likely to weaken over the coming decades, with widespread implications for regional and global climate.

The new study—led by the universities of Exeter and Oxford, and published in Nature Geoscience—pinpoints the causes of monthly and annual AMOC variation and finds a differing picture at two key locations.

Observational data came from large arrays of monitoring equipment—off the coasts of Florida and Africa, and in the North Atlantic between Greenland and Scotland—run by the international RAPID and OSNAP projects.

"Understanding AMOC variability is challenging because the circulation is influenced by multiple factors that all vary and whose overlapping impacts persist for years," said lead author Dr. Yavor Kostov, of the Department of Geography at the University of Exeter.

"Our findings reveal the vital role of winds in driving changes in this ocean circulation.

"Winds were a key factor both in the sub-tropical and sub-polar locations we examined.

"As the climate continues to change, more efforts should be concentrated on monitoring those winds—especially in key regions on continental boundaries and the eastern coast of Greenland—and understanding what drives changes in them."

While AMOC variability off the southern U.S. is dominated by the impact of winds, variability in the North Atlantic is generated by the combined effects of winds, heat and freshwater anomalies."Our reconstruction suggests that, compared to the subtropics, the overturning circulation in the subpolar North Atlantic is more sensitive to changes in the background ocean state such as shifts in the sites of deep convection," Dr. Kostov said.

"This implies that future climate change may alter annual AMOC variability in this region. It emphasizes the need for continued observations of the subpolar North Atlantic ocean."

The study also finds that changes in the surface temperature and salinity near Canada and Greenland can trigger a delayed remote impact on the Atlantic circulation as far south as Florida.

Ocean Circulation

The oceans swirl and twirl under the influence of the winds, Coriolis, salinity differences, the edges of the continents, and the shape of the deep ocean floor. We will discuss ocean circulation in detail in Module 6, but since ocean currents are critical agents of heat transport, we must include them here as well. In general, the surface currents of the oceans are driven by winds, Coriolis, and the edges of continents, and the deep currents that mix the oceans are driven by density changes related to temperature and salinity as well as the shape of the deep ocean floor.

The pattern of circulation is shown in the figure below, which represents the average paths of flow on a shorter term, the flow is dominated by eddies that spin around.

In this map, the different colors correspond to the warm currents (red), cold currents (blue), and currents that move mostly along lines of latitude and thus do not transport waters across a temperature gradient (black). These latter currents may involve warm or cold water, but they do not move that water to warmer or colder places. As mentioned earlier, these arrows depict average flow paths, but on a shorter timescale, the water is involved in eddies that move along the directions indicated by these arrows. These ubiquitous eddies are important since they mix up the surface of the oceans just as swirling a spoon in a coffee cup mixes the coffee. There are several ways of forming eddies, including intermittent winds combining with the Coriolis effect, opposing currents interacting with each other, and currents interacting with coastlines. As this pattern of currents indicates, surface ocean circulation moves a lot of warm water to colder portions of the Earth it also moves cold water back down to warmer regions — the net effect is to exchange heat and bring the tropics and the poles a little closer to each other in terms of temperature. Or, in other words, this (along with the winds) moves surplus energy from the tropics to the regions of energy deficit near the poles.

It is important to realize that these currents, by themselves, would eventually homogenize the temperature on the surface, were it not for the huge difference in solar energy between the tropics and the poles. In addition, the strength of these air and ocean currents is sensitive to the temperature difference between the poles and the equator — the greater the temperature difference, the stronger the currents.

The surface currents described above are generally confined to the upper hundred meters or so of the oceans, and considering that the average depth of the oceans is about 4000 meters, the surface currents represent a very small part of the ocean system. The rest of the oceans are also in motion, moving much more slowly under the influence of density differences caused by temperature and salinity changes. Cold, salty water is dense, while warm, fresh water is light, and the resulting density differences drive a system of flows sometimes referred to as the thermohaline circulation. In today’s world, there are two principal places where deep waters form — the North Atlantic and Antarctica, as shown below:

In the North Atlantic, warm, salty water from the Gulf Stream comes into contact with cold Arctic air, and as the water cools it becomes very dense and sinks to the bottom of the ocean — this is called the North Atlantic Deep Water (NADW). When NADW forms, a tremendous amount of heat is transferred from the water to the air this heat is equivalent to about 30% of the thermal energy received by the whole polar region, so it can influence the Arctic climate in a major way. In the Antarctic, as sea ice forms at the edge of the ice sheet, pure water is removed from seawater, thus increasing the salinity of the remaining water the resulting density increase makes this the densest water in oceans, and it sinks to the bottom — this water mass is called the Antarctic Bottom Water (ABW). Of these two deep water flows, the NADW is much greater and it flows in a complex path, hugging the bottom of the ocean as it moves through the Atlantic and into the Indian and Pacific Oceans, by which point it has warmed and mixed with the surrounding water to rise back up into the surface, where it starts its return path back into the North Atlantic, completing the loop in something like a thousand years. This flow is sometimes called the Global Conveyor Belt (we will talk a lot more about this in Module 6), and it represents an important means of mixing the global oceans.

These deep currents are very important to the global climate system in a couple of ways. One of these ways, described above, is the way that NADW formation influences the Arctic climate this, in turn, can influence the formation or melting of ice in the polar region, which can trigger the ice-albedo feedback mechanism (see below). Another way these deep currents influence the global climate is by transporting CO2 to the deep waters of the oceans. The CO2 is dissolved into the seawater at the surface, so when deep waters form, they bring that CO2 with them, thus removing it from the atmosphere. What this does is to effectively increase the volume of ocean water that can hold CO2, which increases the total mass of carbon the oceans can hold. Indeed, these deep currents are already transporting anthropogenic CO2 and other gases such as CFCs into the deep ocean (we will talk a lot more about this in Modules 5 and 7).

Chapter 4: Section 8 - El Niño and Ocean Circulation

In this section you will find materials that support the implementation of EarthComm, Section 8: El Niño and the Ocean Circulation.

Learning Outcomes

  • Analyze data by comparing and contrasting a data set of sea surface temperatures during a normal year with a data set of an El Niño year.
  • Interpret data on maps of the Pacific Ocean to explain how sea surface temperatures vary during an El Niño event.
  • Analyze and interpret data from a remote-sensing satellite to determine the extent and duration of the 1997–1998 El Niño event.

Inquiring Further

  1. To learn more about the technology used to study oceanic-atmospheric interactions, specifically TOPEX/Poseidon and TAO, visit the following web sites:

Ocean Surface Topography From Space, NASA
Understand how continuous data from satellites like TOPEX/Poseidon help scientists understand and foresee the effects of the changing oceans on climate and on catastrophic climate events such as El Niño.

TOPEX/Poseidon Educational Outreach, University of Texas at Austin, Center for Space Research
Learn more about the TOPEX/Poseidon satellite, how it collects data, and what has been learned from this important mission.

La Niña Page, NOAA
Includes links to general La Niña information, such as climate data for past La Niña events, forecasts, and the impacts of La Niña.

What is La Niña, NOAA
Includes color animations comparing La Niña, El Niño, and "normal" conditions, a review of the impacts that La Niña has had on the global climate, and links to further information.


To learn more about this topic, visit the following web sites:

El Niño and non- El Niño Conditions

El Niño Theme Page, NOAA
Contains links to cover a wide range of topics, including definitions of El Niño and La Niña, impacts of El Niño, predicting El Niño, 3-D animations of El Niño temperatures, and information on finding El Niño data.

NOAA’s El Niño Page, NOAA
Contains an image of current sea-surface temperature anomalies which is updated regularly. Site also contains links to El Niño images and information, including a list of El Niño-related web sites.

Past El Niño (ENSO) Events, NOAA
ENSO events differ in their strength, coverage, and seasonality, there isn't unanimous agreement on what constitutes and ENSO event. NOAA contains a list of years broadly agreed on by researchers.

The 2015-2016 El Niño, World Meteorological Organization
Animation during an El Nino event about what to expect.

The 2015-2016 El Niño a Historical Perspective, NOAA
Learn about the temperature, precipitation, and drought outlooks from this El Nino event.

The Southern Oscillation

El Niño/La Niña Home, NOAA
Includes links to information on ENSO forecasts, an "expert assessment" (updated weekly), ENSO figures (updated weekly), and a list of FAQ. Also includes an online tutorial which contains numerous color images.

El Niño/Southern Oscillation (ENSO), NOAA
Information on the science behind ENSO, current ocean/atmosphere conditions, impacts of ENSO on climate, forecasts, current research, and links to educational resources.

The Equatorial Pacific and El Niño

TOGA-TAO and the 1991-93 El Niño-Southern Oscillation Event, McPhaden, M.J., NOAA/PMEL
Examines data collected by the TOGA-TAO program. Includes color data images.

Cause and Effect - El Niño

Coastal Impacts of El Niño, USGS
Focuses on the impacts of El Niño, including coastal erosion, flooding, landslides, storms, and climate change. Click on link to open the article of interest.

The Story of El Niño - Science on a Sphere, NOAA
Learn about this phenomenon and its impacts in this movie created by the Aquarium of the Pacific.

Impacts, Predictions, and Regional Benefits, NOAA
Further information on the impacts of El Niño, such as El Niño and tornado occurrences, El Niño and hurricane frequency, coral reef bleaching, El Niño and marine fish and birds, and more.

El Niño and La Niña: Effect on Phytoplankton and Fish- Science on a Sphere, NOAA
Learn how satellite data are being used to understand fish distributions and why some fisheries suddenly collapse.

El Niño Sea-Level Rise Wreaks Havoc in California's San Fransico Bay Region, USGS
Looks at the impact of the 1997-1998 El Niño event on the Bay area. Site includes damage photos, sea level data, upwelling images, and an explanation of Kelvin waves and their impact.

All Science Journal Classification (ASJC) codes

  • APA
  • Author
  • Harvard
  • Standard
  • RIS
  • Vancouver

Connecting changing ocean circulation with changing climate. / Winton, Michael Griffies, Stephen M. Samuels, Bonita L. Sarmiento, Jorge Louis Licher, Thomas L.Frö.

In: Journal of Climate , Vol. 26, No. 7, 04.2013, p. 2268-2278.

Research output : Contribution to journal › Article › peer-review

T1 - Connecting changing ocean circulation with changing climate

AU - Sarmiento, Jorge Louis

N1 - Copyright: Copyright 2013 Elsevier B.V., All rights reserved.

N2 - The influence of changing ocean currents on climate change is evaluated by comparing an earth system model's response to increased CO2 with and without an ocean circulation response. Inhibiting the ocean circulation response, by specifying a seasonally varying preindustrial climatology of currents, has a much larger influence on the heat storage pattern than on the carbon storage pattern. The heat storage pattern without circulation changes resembles carbon storage (either with or without circulation changes) more than it resembles the heat storage when currents are allowed to respond. This is shown to be due to the larger magnitude of the redistribution transport-the change in transport due to circulation anomalies acting on control climate gradients-for heat than for carbon. The net ocean heat and carbon uptake are slightly reduced when currents are allowed to respond. Hence, ocean circulation changes potentially act to warm the surface climate. However, the impact of the reduced carbon uptake on radiative forcing is estimated to be small while the redistribution heat transport shifts ocean heat uptake from low to high latitudes, increasing its cooling power. Consequently, global surface warming is significantly reduced by circulation changes. Circulation changes also shift the pattern of warming from broad Northern Hemisphere amplification to a more structured pattern with reduced warming at subpolar latitudes in both hemispheres and enhanced warming near the equator.

AB - The influence of changing ocean currents on climate change is evaluated by comparing an earth system model's response to increased CO2 with and without an ocean circulation response. Inhibiting the ocean circulation response, by specifying a seasonally varying preindustrial climatology of currents, has a much larger influence on the heat storage pattern than on the carbon storage pattern. The heat storage pattern without circulation changes resembles carbon storage (either with or without circulation changes) more than it resembles the heat storage when currents are allowed to respond. This is shown to be due to the larger magnitude of the redistribution transport-the change in transport due to circulation anomalies acting on control climate gradients-for heat than for carbon. The net ocean heat and carbon uptake are slightly reduced when currents are allowed to respond. Hence, ocean circulation changes potentially act to warm the surface climate. However, the impact of the reduced carbon uptake on radiative forcing is estimated to be small while the redistribution heat transport shifts ocean heat uptake from low to high latitudes, increasing its cooling power. Consequently, global surface warming is significantly reduced by circulation changes. Circulation changes also shift the pattern of warming from broad Northern Hemisphere amplification to a more structured pattern with reduced warming at subpolar latitudes in both hemispheres and enhanced warming near the equator.


The oceanic circulation south of Africa is characterised by a complex dynamics with a strong variability due to the presence of the Agulhas current and numerous mesoscale eddies from the Mozambique Channel (Penven et al. 2006 Halo et al. 2014). More recently, high resolution modeling study by Tedesco et al. (2019) has highlighted the existence of numerous submesoscale eddies along the Agulhas cyclonic front.

Lutjeharms et al. (2003) observed the presence of cyclonic eddies embedded in the landward border of the southern Agulhas Current. These eddies have a diameter of about 50 km and are associated with a surface warm signature. Simulations suggest that those eddies remain trapped in the Agulhas Bank shelf bight and that eddies that travel downstream of the current represent leakages from the resident shear eddy. This occurs at a roughly 20 days occurrence frequency. The intensity of the meso-scale activity in this key region for the retro-flexion modulate the exchanges of heat and salt between oceans (Lutjeharms 1981 Reason et al. 2003 Van-Aken et al. 2013 Guerra et al. 2018) as well as towards the atmosphere (Messager and Stuart 2016).

This region exhibits furthermore a dynamical upwelling induced by the Agulhas Currents (Arnone et al. 2017) as observed by Goschen et al. (2015) during Natal Pulses. This upwelling, as been shown by Lutjeharms et al. (2000) to occurs on the landward side of the Agulhas Current and have an effect on the nutrient availability, stratification and primary productivity in the eastern Agulhas Bank. It as also been shown by Meyer and Niekerk (2016) that implementing an ocean current power plant in this region would outperforms onshore wind power plants and could increase the load carrying capacity of the country.

The area of interest of this paper, represented on Fig. 1 is also the location of several natural gas fields under seafloor which are targeted for drilling and exploitation. The complex and powerful ocean currents induces significant issues for ship operations at the surface as well as under the surface for deep sea operations. Strong ocean currents can also modify the height and direction of ocean waves, causing dangerous sea states (Quilfen et al. 2018). The risk of extreme waves is an important hazard for the shipping activity and off shore industry when crossing the main current systems. Therefore, knowledge of the currents state and the ability to forecast it in a realistic manners could greatly enforce the safety of various marine operations.

Following this objective an array of HF radar was deployed along the coast to allow a detailed knowledge of the Agulhas currents and its associated eddy activity. The purpose of the present document is to present and evaluate the impact of the 4DVAR assimilation of those radar data on ocean model simulation and forecast of the sea surface currents.

Data used for assimilation and validation are described in the following section. The model setup and the assimilation procedure are described in a third section while results are presented in section four and further discuss in the conclusion.

Area of interest of this study. Main interest Focus on area 11b/12b and Brulpadda point. Credit:

To monitor the variability of the Agulhas currents during offshore operations, three WERA HF radars, manufactured by Helzel Messtechnik GmbH, were installed by ACTIMAR and LWANDLE companies on the south coast of South Africa. The location of the radar system and the averaged area of measurement during April 2020 is represented on Fig. 2. The radial velocities are estimated by using the conventional method of Beam Forming with an extra filtering of the residual artefacts. Then the radial velocities are combined on a Cartesian grid at 6km resolution using the method describe by Barth et al. (2010) and made available every 30 min.

Comparisons with mobile and fixed ADCP measurements have been performed (cf. Fig. 3). For the fixed ADCP, differences intensity are observed for weak current ( (le) 1.3 m/s) and a better matching is observed for stronger values. Current directions derived from radar are well correlated with ADCP measurements. The differences in intensity are explained by the low angular resolution of the BeamForming compared to the grid resolution (by a factor of about 4) at the position of the ADCP. For the mobile ADCP, the differences in intensity are lower compared to the mobile ADCP, while the differences in direction may be due to a poor calibration of the hull ADCP. Therefore, the currents provided with the Beam Forming method seems to be robust enough to be assimilated in ROMS simulations. Nevertheless, to overcome some inaccuracies, a hybrid Beam Forming/Direction Finding method developed by ACTIMAR called HYDDOA (cf. patent : FR 1562550) has been used for marine operations with better performances. Unfortunately, these data could not be used for this study.

Black squares represent the emplacement of the HF radar installed to monitor the area delimited by the black contour (cf Fig. 1). The colored area represents the intensity of the averaged current measured by the radar during April 2020 and the white arrows are representative of the averaged direction of the surface currents during the same period

Quantile-Quantile plot of speed and direction for comparison with fixed ADCP (a, c) and mobile ADCP (b, d)

In addition, Altimeters data were generated by a processing system including data from several altimeter missions: Sentinel-3A/B, Jason-3, HY-2A, Saral[-DP]/AltiKa, Cryosat-2, OSTM/Jason-2, Jason-1, Topex/Poseidon, Envisat, GFO, ERS-1/2 and delivered by E.U. Copernicus Marine Service Information. Being at a significant lower resolution than both model experiment those data were excluded from the assimilation process (although there are somehow assimilated in the Mercator ocean simulation used as boundary forcing) and may be considered as an independent source of observations for our validation process. Nonetheless an import bias in the representation of the Agulhas current by the altimeters data has been highlighted by Rouault et al. (2010).

Access options

Get full journal access for 1 year

All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.

Get time limited or full article access on ReadCube.

All prices are NET prices.

This test proves how many temperature/salinity measurements of one profile are identical. The test includes two tunable parameters: the minimal thickness of the layer within which all measurements shows exactly the same parameter value, and the number of such levels within the layer. The first parameter sets the threshold thickness of the thermostad and halostad, whereas the second parameter takes the typical observed level spacing into account, which differs between instrumentation types.

This test identifies profiles with unrealistically large numbers of local parameter extrema. For each triple of three neighbor observed levels the extremum is considered to be significant if | p k - p k + 1 | < d and | p k - p k - 1 | < d , where the parameter d is selected to be larger than the measurement precision and the typical amplitude of the microscale parameter inversions.

Code for all data analysis presented is available at and code for the Pyberg model is available at

Lisiecki, L. E. Atlantic overturning responses to obliquity and precession over the last 3 Myr. Paleoceanography 29, 71–86 (2014).

Hesse, T., Butzin, M., Bickert, T. & Lohmann, G. A model-data comparison of δ 13 C in the glacial Atlantic Ocean. Paleoceanography 26, PA3220 (2011).

Bower, A. et al. Lagrangian views of the pathways of the Atlantic meridional overturning circulation. JGR Oceans 124, 5313–5335 (2019).

Talley, L. D. Closure of the global overturning circulation through the Indian, Pacific, and southern oceans. Oceanography 26, 80–97 (2013).

Swingedouw, D., Braconnot, P., Delecluse, P., Guilyardi, E. & Marti, O. The impact of global freshwater forcing on the thermohaline circulation: adjustment of North Atlantic convection sites in a CGCM. Clim. Dyn. 28, 291–305 (2007).

Caley, T., Giraudeau, J., Malaizé, B., Rossignol, L. & Pierre, C. Agulhas leakage as a key process in the modes of Quaternary climate changes. Proc. Natl Acad. Sci. USA 109, 6835–6839 (2012).

Knorr, G. & Lohmann, G. Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglaciation. Nature 424, 532–536 (2003).

Watson, A. J., Vallis, G. K. & Nikurashin, M. Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO2. Nat. Geosci. 8, 861–864 (2015).

Ferrari, R. et al. Antarctic sea ice control on ocean circulation in present and glacial climates. Proc. Natl Acad. Sci. USA 111, 8753–8758 (2014).

Govin, A. et al. Evidence for northward expansion of Antarctic Bottom Water mass in the Southern Ocean during the last glacial inception. Paleoceanography 24, PA1202 (2009).

Marino, G. et al. Agulhas salt-leakage oscillations during abrupt climate changes of the Late Pleistocene. Paleoceanography 28, 599–606 (2013).

Simon, M. H. et al. Millennial-scale Agulhas Current variability and its implications for salt-leakage through the Indian–Atlantic Ocean Gateway. Earth Planet. Sci. Lett. 383, 101–112 (2013).

Ziegler, M., Diz, P., Hall, I. R. & Zahn, R. Millennial-scale changes in atmospheric CO2 levels linked to the Southern Ocean carbon isotope gradient and dust flux. Nat. Geosci. 6, 457–461 (2013).

Rackow, T. et al. A simulation of small to giant Antarctic iceberg evolution: differential impact on climatology estimates. JGR Oceans 122, 3170–3190 (2017).

Zhang, X., Lohmann, G., Knorr, G. & Xu, X. Different ocean states and transient characteristics in last glacial maximum simulations and implications for deglaciation. Clim. Past 9, 2319–2333 (2013).

Bigg, G. R. The impact of icebergs of sub-Antarctic origin on Southern Ocean ice-rafted debris distributions. Quat. Sci. Rev. 232, 106204 (2020).

Schmittner, A. et al. Calibration of the carbon isotope composition (δ 13 C) of benthic foraminifera. Paleoceanography 32, 512–530 (2017).

Mackensen, A. & Licari, L. Carbon isotopes of live benthic foraminifera from the South Atlantic: sensitivity to bottom water carbonate saturation state and organic matter rain rates. In The South Atlantic in the Late Quaternary (eds Wefer, G. et al.) 623–644 (Springer, 2003).

Lear, C. H. et al. Breathing more deeply: deep ocean carbon storage during the mid-Pleistocene climate transition. Geology 44, 1035–1038 (2016).

Howe, J. N. & Piotrowski, A. M. Atlantic deep water provenance decoupled from atmospheric CO2 concentration during the lukewarm interglacials. Nat. Commun. 8, 2003 (2017).

Merino, N. et al. Antarctic icebergs melt over the Southern Ocean: climatology and impact on sea ice. Ocean Model. 104, 99–110 (2016).

Keany, J., Ledbetter, M., Watkins, N. & Huang, T. C. Diachronous deposition of ice-rafted debris in sub-Antarctic deep-sea sediments. Bull. Geol. Soc. Am. 87, 873–882 (1976).

Diekmann, B. et al. Terrigenous sediment supply in the polar to temperate South Atlantic: land–ocean links of environmental changes during the Late Quaternary. In The South Atlantic in the Late Quaternary (eds Wefer, G. et al.) 375–399 (Springer, 2003).

Weber, M. E. et al. Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation. Nature 510, 134–138 (2014).

Teitler, L. et al. Determination of Antarctic Ice Sheet stability over the last

500 ka through a study of iceberg-rafted debris. Paleoceanography 25, PA1202 (2010).

Gersonde, R., Crosta, X., Abelmann, A. & Armand, L. Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum – a circum-Antarctic view based on siliceous microfossil records. Quat. Sci. Rev. 24, 869–896 (2005).

Wolff, E. W. et al. Southern Ocean sea-ice extent, productivity and iron flux over the past eight glacial cycles. Nature 440, 491–496 (2006).

Wagner, T. J. et al. Wave inhibition by sea ice enables trans-Atlantic ice rafting of debris during Heinrich events. Earth Planet. Sci. Lett. 495, 157–163 (2018).

Schodlok, M. P., Hellmer, H. H., Rohardt, G. & Fahrbach, E. Weddell Sea iceberg drift: five years of observations. JGR Oceans 111, C06018 (2006).

Tournadre, J., Bouhier, N., Girard-Ardhuin, F. & Rémy, F. Antarctic icebergs distributions 1992–2014. JGR Oceans 121, 327–349 (2016).

Teitler, L. et al. Antarctic Ice Sheet response to a long warm interval across Marine Isotope Stage 31: a cross-latitudinal study of iceberg-rafted debris. Earth Planet. Sci. Lett. 409, 109–119 (2015).

Pollard, D. & DeConto, R. M. Modelling West Antarctic ice sheet growth and collapse through the past five million years. Nature 458, 329–332 (2009).

Silva, T. A., Bigg, G. R. & Nicholls, K. W. Contribution of giant icebergs to the Southern Ocean freshwater flux. JGR Oceans 111, C03004 (2006).

Sun, S., Eisenman, I. & Stewart, A. L. Does Southern Ocean surface forcing shape the global ocean overturning circulation? Geophys. Res. Lett. 45, 2413–2423 (2018).

Gordon, A. L. Interocean exchange of thermocline water. J. Geophys. Res. 91, 5037–5046 (1986).

Scussolini, P., Marino, G., Brummer, G. J. A. & Peeters, F. J. Saline Indian Ocean waters invaded the South Atlantic thermocline during glacial termination II. Geology 43, 139–142 (2015).

Seidov, D., Stouffer, R. J. & Haupt, B. J. Is there a simple bi-polar ocean seesaw? Glob. Planet. Change 49, 19–27 (2005).

Stouffer, R. J., Seidov, D. & Haupt, B. J. Climate response to external sources of freshwater: North Atlantic versus the Southern Ocean. J. Clim. 20, 436–448 (2007).

Schmittner, A. Southern Ocean sea ice and radiocarbon ages of glacial bottom waters. Earth Planet. Sci. Lett. 213, 53–62 (2003).

Menviel, L. et al. Poorly ventilated deep ocean at the Last Glacial Maximum inferred from carbon isotopes: a data-model comparison study. Paleoceanography 32, 2–17 (2017).

Molyneux, E. G., Hall, I. R., Zahn, R. & Diz, P. Deep water variability on the southern Agulhas Plateau: interhemispheric links over the past 170 ka. Paleoceanography 22, PA4209 (2007).

Imbrie, J. et al. On the structure and origin of major glaciation cycles 1. Linear responses to Milankovitch forcing. Paleoceanography 7, 701–738 (1992).

Brathauer, U. & Abelmann, A. Late quaternary variations in sea surface temperatures and their relationship to orbital forcing recorded in the Southern Ocean (Atlantic sector). Paleoceanography 14, 135–148 (1999).

Venz, K. A., Hodell, D. A., Stanton, C. & Warnke, D. A. A 1.0 Myr record of Glacial North Atlantic Intermediate Water variability from ODP site 982 in the northeast Atlantic. Paleoceanography 14, 42–52 (1999).

Timmermann, A. et al. Modeling obliquity and CO2 effects on Southern Hemisphere climate during the past 408 ka. J. Clim. 27, 1863–1875 (2014).

Romero, O. E. et al. High-latitude forcing of diatom productivity in the southern Agulhas Plateau during the past 350 kyr. Paleoceanography 30, 118–132 (2015).

Willeit, M., Ganopolski, A., Calov, R. & Brovkin, V. Mid-Pleistocene transition in glacial cycles explained by declining CO2 and regolith removal. Sci. Adv. 5, eaav7337 (2019).

Rodríguez-Sanz, L., Graham Mortyn, P., Martínez-Garcia, A., Rosell-Melé, A. & Hall, I. R. Glacial Southern Ocean freshening at the onset of the Middle Pleistocene Climate Transition. Earth Planet. Sci. Lett. 345–348, 194–202 (2012).

Raymo, M. E., Lisiecki, L. E. & Nisancioglu, K. H. Plio-pleistocene ice volume, Antarctic climate, and the global δ 18 O record. Science 313, 492–495 (2006).

Pena, L. D. & Goldstein, S. L. Thermohaline circulation crisis and impacts during the mid-Pleistocene transition. Science 345, 318–322 (2014).

Graham, R. M. & De Boer, A. M. The dynamical subtropical front. JGR Oceans 118, 5676–5685 (2013).

Laskar, J. et al. A long-term numerical solution for the insolation quantities of the Earth. Astron. Astrophys. 428, 261–285 (2004).

Ahn, S., Khider, D., Lisiecki, L. E. & Lawrence, C. E. A probabilistic Pliocene–Pleistocene stack of benthic δ 18 O using a profile hidden Markov model. Dyn. Stat. Clim. Syst. 2, dzx002 (2017).

Lisiecki, L. E. & Raymo, M. E. A Pliocene–Pleistocene stack of 57 globally distributed benthic δ 18 O records. Paleoceanography 20, PA1003 (2005) correction 20, PA2007 (2005).

Lougheed, B. C. & Obrochta, S. P. A. Rapid, deterministic age-depth modeling routine for geological sequences with inherent depth uncertainty. Paleoceanogr. Paleoclimatol. 34, 122–133 (2019).

Hall, I. R. et al. Site U1475. In Proc. Int. Ocean Discovery Program Vol. 361 (eds Hall, I. R. et al.) (International Ocean Discovery Program, 2017).

Gruetzner, J. et al. A new seismic stratigraphy in the Indian–Atlantic Ocean Gateway resembles major paleo-oceanographic changes of the last 7 Ma. Geochem. Geophys. Geosyst. 20, 339–358 (2019).

Kanfoush, S. L. et al. Millennial-scale instability of the Antarctic Ice Sheet during the last glaciation. Science 288, 1815–1819 (2000).

Nielsen, S. H., Hodell, D. A., Kamenov, G., Guilderson, T. & Perfit, M. R. Origin and significance of ice-rafted detritus in the Atlantic sector of the Southern Ocean. Geochem. Geophys. Geosyst. 8, Q12005 (2007).

Diekmann, B. & Kuhn, G. Provenance and dispersal of glacial–marine surface sediments in the Weddell Sea and adjoining areas, Antarctica: ice-rafting versus current transport. Mar. Geol. 158, 209–231 (1999).

St John, K., Passchier, S., Tantillo, B., Darby, D. & Kearns, L. Microfeatures of modern sea-ice-rafted sediment and implications for paleo-sea-ice reconstructions. Ann. Glaciol. 56, 83–93 (2015).

Roeckner, E. et al. The Atmospheric General Circulation Model ECHAM 5. Part I: Model Description. Report no. 439 (Max-Planck-Institut für Meteorologie, 2003).

Brovkin, V., Raddatz, T., Reick, C. H., Claussen, M. & Gayler, V. Global biogeophysical interactions between forest and climate. Geophys. Res. Lett. 36, L07405 (2009).

Marsland, S. J., Haak, H., Jungclaus, J. H., Latif, M. & Röske, F. The Max-Planck-Institute global ocean/sea ice model with orthogonal curvilinear coordinates. Ocean Model. 5, 91–127 (2003).

Hibler, W. D. A dynamic thermodynamic sea ice model. J. Phys. Oceanogr. 9, 815–846 (1979).

Wei, W. & Lohmann, G. Simulated Atlantic multidecadal oscillation during the Holocene. J. Clim. 25, 6989–7002 (2012).

Stärz, M., Jokat, W., Knorr, G. & Lohmann, G. Threshold in North Atlantic–Arctic Ocean circulation controlled by the subsidence of the Greenland–Scotland Ridge. Nat. Commun. 8, 15681 (2017).

Bigg, G. R., Wadley, M. R., Stevens, D. P. & Johnson, J. A. Modelling the dynamics and thermodynamics of icebergs. Cold Reg. Sci. Technol. 26, 113–135 (1997).

Gladstone, R. M., Bigg, G. R. & Nicholls, K. W. Iceberg trajectory modeling and meltwater injection in the Southern Ocean. JGR Oceans 106, 19903–19915 (2001).

Lichey, C. & Hellmer, H. H. H. Modeling giant-iceberg drift under the influence of sea ice in the Weddell Sea, Antarctica. J. Glaciol. 47, 452–460 (2001).

Wagner, T. J., Stern, A. A., Dell, R. W. & Eisenman, I. On the representation of capsizing in iceberg models. Ocean Model. 117, 88–96 (2017).

Weeks, W. F. & Mellor, M. Some elements of iceberg technology. In Iceberg Utilization: Proc. First Int. Conf. Worksh. on Iceberg Utilization for Fresh Water Production, Weather Modification and Other Applications (ed. Husseiny, A. A.) 45–98 (Pergamon, 1977).

Bouhier, N., Tournadre, J., Rémy, F. & Gourves-Cousin, R. Melting and fragmentation laws from the evolution of two large Southern Ocean icebergs estimated from satellite data. Cryosphere 12, 2267–2285 (2018).

Wagner, T. J. et al. The “footloose” mechanism: iceberg decay from hydrostatic stresses. Geophys. Res. Lett. 41, 5522–5529 (2014).

Wesche, C. & Dierking, W. Near-coastal circum-Antarctic iceberg size distributions determined from synthetic aperture radar images. Remote Sens. Environ. 156, 561–569 (2015).

Barbat, M. M., Rackow, T., Hellmer, H. H., Wesche, C. & Mata, M. M. Three years of near-coastal Antarctic iceberg distribution from a machine learning approach applied to SAR imagery. JGR Oceans 124, 6658–6672 (2019).

Cooke, D. W. & Hays, J. D. Estimates of Antarctic Ocean seasonal sea-ice cover during glacial intervals. In Antarctic Geoscience: Symposium on Antarctic Geology and Geophysics (ed. Craddock, C.) 1017–1025 (1982).

Grobe, H. & Mackensen, A. Late Quaternary climatic cycles as recorded in sediments from the Antarctic continental margin. In The Antarctic Paleoenvironment: A Perspective on Global Change (eds Kennett, J. P. & Warkne, D. A.) 349–376 (1992).

Oppo, D. W. & Fairbanks, R. G. Variability in the deep and intermediate water circulation of the Atlantic Ocean during the past 25,000 years: Northern Hemisphere modulation of the Southern Ocean. Earth Planet. Sci. Lett. 86, 1–15 (1987).

Raymo, M. E. et al. Stability of North Atlantic water masses in face of pronounced climate variability during the Pleistocene. Paleoceanography 19, PA2008 (2004).

Hodell, D. et al. Data report: oxygen isotope stratigraphy of ODP leg 177 sites 1088, 1089, 1090, 1093, and 1094. In Proc. ODP Sci. Res. Vol. 177 (eds Gersonde, R. et al.) Ch. 9 (2003).

Raymo, M. E., Oppo, D. W. & Curry, W. The mid-Pleistocene climate transition: a deep sea carbon isotopic perspective records from the deep ocean, extending back examined in order to constrain decrease in mean. Paleoceanography 12, 546–559 (1997).

Lang, D. C. et al. Incursions of southern-sourced water into the deep North Atlantic during late Pliocene glacial intensification. Nat. Geosci. 9, 375–379 (2016).

Venz, K. A. & Hodell, D. A. New evidence for changes in Plio–Pleistocene deep water circulation from Southern Ocean ODP leg 177 site 1090. Palaeogeogr. Palaeoclimatol. Palaeoecol. 182, 197–220 (2002).

Yu, J. et al. Last glacial atmospheric CO2 decline due to widespread Pacific deep-water expansion. Nat. Geosci. 13, 628–633 (2020).

Rehfeld, K., Marwan, N., Heitzig, J. & Kurths, J. Comparison of correlation analysis techniques for irregularly sampled time series. Nonlinear Process. Geophys. 18, 389–404 (2011).

Barker, S. et al. Icebergs not the trigger for North Atlantic cold events. Nature 520, 333–336 (2015).

Past Interglacials Working Group of PAGES. Interglacials of the last 800,000 years. Rev. Geophys. 54, 162–219 (2016).

Paillard, D., Labeyrie, L. & Yiou, P. Macintosh program performs time-series analysis. Eos 77, 379 (1996).

Grinsted, A., Moore, J. C. & Jevrejeva, S. Application of the cross wavelet transform and wavelet coherence to geophysical time series. Nonlinear Process. Geophys. 11, 561–566 (2004).

Fogwill, C. J., Turney, C. S., Hutchinson, D. K., Taschetto, A. S. & England, M. H. Obliquity control on Southern Hemisphere climate during the last glacial. Sci. Rep. 5, 11673 (2015).

Wu, Z., Yin, Q., Guo, Z. & Berger, A. Hemisphere differences in response of sea surface temperature and sea ice to precession and obliquity. Glob. Planet. Change 192, 103223 (2020).

Marshall, J. & Speer, K. Closure of the meridional overturning circulation through Southern Ocean upwelling. Nat. Geosci. 5, 171–180 (2012).

Eide, M., Olsen, A., Ninnemann, U. S. & Johannessen, T. A global ocean climatology of preindustrial and modern ocean δ 13 C. Glob. Biogeochem. Cycles 31, 515–534 (2017).

Budge, J. S. & Long, D. G. A comprehensive database for Antarctic iceberg tracking using scatterometer data. IEEE J. Sel. Top. Appl. Earth Obs. Remote Sens. 11, 434–442 (2018).

Ocean circulation change: Sea level spiked for two years along Northeastern North America

A four-inch increase in sea levels from New York to Newfoundland occurred in 2009 and 2010 because ocean circulation changed, reports a UA-led team of geoscientists.

Sea levels from New York to Newfoundland jumped up about four inches in 2009 and 2010 because ocean circulation changed, a University of Arizona-led team reports in an upcoming issue of Nature Communications.

The team was the first to document that the extreme increase in sea level lasted two years, not just a few months.

"The thing that stands out is the time extent of this event as well as the spatial extent of the event," said first author Paul Goddard, a UA doctoral candidate in geosciences.

Independent of any hurricanes or winter storms, the event caused flooding along the northeast coast of North America. Some of the sea level rise and the resulting flooding extended as far south as Cape Hatteras.

The paper is also the first to show that the unusual spike in sea level was a result of changes in ocean circulation.

Co-author Jianjun Yin, UA assistant professor of geosciences, said, "We are the first to establish the extreme sea level rise event and its connection with ocean circulation."

Goddard detected the two-year-long spike in sea level by reviewing monthly tide-gauge records, some of which went back to the early 1900s, for the entire Eastern Seaboard. No other two-year period from those records showed such a marked increase.

The team linked the spike to a change in the ocean's Atlantic Meridional Overturning Circulation and also a change in part of the climate system known as the North Atlantic Oscillation.

The researchers then used computer climate models to project the probability of future spikes in sea level.

The team found that, at the current rate that atmospheric carbon dioxide is increasing, such extreme events are likely to occur more frequently, Goddard said.

Goddard's and Yin's research paper, "An Extreme Event of Sea Level Rise along the Northeast Coast of North America in 2009-10," is scheduled for online publication in Nature Communications today. Stephen Griffies and Shaoqing Zhang of the National Oceanographic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, are also co-authors. NOAA funded the research.

Yin's previous work on climate models suggests that weakening of the Atlantic Meridional Overturning Circulation could cause sea levels to rise faster along the northeast coast of North America.

Yin wondered whether such sea level rise had actually been observed, so he asked Goddard to compile the tide-gauge records for the east coast of North America. The 40 gauges, spanning the coast from Key West, Florida, north to Newfoundland, have been recording sea levels as far back as the 1920s.

Goddard's work revealed a surprise -- that during 2009 and 2010, sea level between New York and Newfoundland rose an average of four inches. Sea level from Cape Hatteras to New York also had a notable spike, though not as dramatic.

"The sea level rise of 2009-10 sticks out like a sore thumb for the Northeast," Goddard said.

His research also confirmed that, as others have reported, sea level has been gradually rising since the 1920s and that there is some year-to-year variation.

About the time Goddard finished analyzing the tide-gauge records, another group of researchers reported that the Atlantic Meridional Overturning Circulation, or AMOC, had a 30 percent decline in strength in 2009-10. Those researchers reported the decline started just two months before the tide gauges started recording the spike in sea level.

"To me, it was like putting together a puzzle," Goddard said.

The more he and his colleagues examined the timing of the AMOC downturn and the subsequent increase in sea level, the more it fit together, he said.

The AMOC brings warm water from the tropics and the southern Atlantic Ocean to the North Atlantic and the polar regions. The water then cools and sinks, eventually flowing south in the deep ocean. Yin's climate model predicted that when the AMOC weakened, sea level in northeastern North America would rise.

In addition to the weakening AMOC, during 2009-10 the region's atmosphere was in a very negative phase of the climate mode called the North Atlantic Oscillation. The NAO flip-flops between negative and positive phases.

"The negative North Atlantic Oscillation changes the wind patterns along the northeast coast, so during the negative NAO the winds push water onto the northeast coast," Goddard said.

Although the NAO has resumed flipping between positive and negative states, observations show that the AMOC, while somewhat stronger, has still not recovered its previous strength.

Even now, sea level is still higher than before 2009, Yin said. He's not surprised, because most of the climate models predict a weakening of the AMOC over the 21 st century.

Yin said that at the current rate of increase in greenhouse gases, most climate models predict a weakening of the AMOC over the 21 st century. Therefore, such extreme sea level rise events and coastal flooding are quite likely to occur along the densely populated northeast coast of North America more often.

Watch the video: How do ocean currents work? - Jennifer Verduin