scholarly journals Northward shifts of Canadian late deglacial drainage routes caused a stronger and warmer Canaries Current that enabled Holocene monsoons and a savanna on the western Sahara

2020 ◽  
Vol 7 ◽  
pp. 11-18
Author(s):  
Robert G. Johnson
Keyword(s):  
Gulf Stream ◽  
Cold Water ◽  
Return Flow ◽  
Trade Winds ◽  
Western Sahara ◽  
Humidity Index ◽  
Westward Movement ◽  
Meridional Overturning ◽  
The Many ◽  
Current Return

This paper proposes an explanation for the well-watered savanna on the presently barren western Sahara Desert during the mid-Holocene and near the ends of other earlier Canadian deglaciations. Between 7,500 and 4,000 years before present, a humidity index indicates moist conditions and a savanna in the western Sahara. During this interval, a stronger and warmer Canaries Current return flow of the Gulf Stream nullified the effect of cold water that upwells off the northwest African coast due to the westward movement of surface water by trade winds. The return flow was stronger than today because less eastward Gulf Stream flow was lost to the northward flow of the Atlantic Meridional Overturning Current. The Canaries Current was warmer than today because cold southern Canadian meltwater no longer entered the Gulf Stream, and the high latitude climate was warmer then. Similar conditions probably prevailed at the ends of many other deglaciations, which were separated by 20,000 to 70,000 years due to orbital factors and variable glaciation. The savanna connections across the Sahara would have allowed each Hominin population to evolve in the isolated Moroccan-Algerian coastal zone to extend its range into the larger Africa. The intermittent savannas could therefore have played a significant role in the evolution of the many Hominin species found in the African fossil record over the last three million years.

scholarly journals An extraordinary breach of the Gulf Stream north wall by a cold water intrusion

2002 ◽  
Vol 29 (15) ◽  
pp. 8-1-8-4 ◽  
Author(s):  
Xiaofeng Li ◽  
Timothy F. Donato ◽  
Quanan Zheng ◽  
William G. Pichel ◽  
Pablo Clemente-Colón
Keyword(s):  
Gulf Stream ◽  
Cold Water ◽  
Water Intrusion ◽  
North Wall

scholarly journals Intrinsic Variability of the Atlantic Meridional Overturning Circulation at Interannual-to-Multidecadal Time Scales

2015 ◽  
Vol 45 (7) ◽  
pp. 1929-1946 ◽  
Author(s):  
Sandy Grégorio ◽  
Thierry Penduff ◽  
Guillaume Sérazin ◽  
Jean-Marc Molines ◽  
Bernard Barnier ◽  
...  
Keyword(s):  
Time Scales ◽  
Gulf Stream ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Intrinsic Variability ◽  
Meridional Heat Transport ◽  
Overturning Circulation ◽  
Spectral Peaks ◽  
Meridional Overturning

AbstractThe low-frequency variability of the Atlantic meridional overturning circulation (AMOC) is investigated from 2, ¼°, and ° global ocean–sea ice simulations, with a specific focus on its internally generated (i.e., “intrinsic”) component. A 327-yr climatological ¼° simulation, driven by a repeated seasonal cycle (i.e., a forcing devoid of interannual time scales), is shown to spontaneously generate a significant fraction R of the interannual-to-decadal AMOC variance obtained in a 50-yr “fully forced” hindcast (with reanalyzed atmospheric forcing including interannual time scales). This intrinsic variance fraction R slightly depends on whether AMOCs are computed in geopotential or density coordinates, and on the period considered in the climatological simulation, but the following features are quite robust when mesoscale eddies are simulated (at both ¼° and ° resolutions); R barely exceeds 5%–10% in the subpolar gyre but reaches 30%–50% at 34°S, up to 20%–40% near 25°N, and 40%–60% near the Gulf Stream. About 25% of the meridional heat transport interannual variability is attributed to intrinsic processes at 34°S and near the Gulf Stream. Fourier and wavelet spectra, built from the 327-yr ¼° climatological simulation, further indicate that spectral peaks of intrinsic AMOC variability (i) are found at specific frequencies ranging from interannual to multidecadal, (ii) often extend over the whole meridional scale of gyres, (iii) stochastically change throughout these 327 yr, and (iv) sometimes match the spectral peaks found in the fully forced hindcast in the North Atlantic. Intrinsic AMOC variability is also detected at multidecadal time scales, with a marked meridional coherence between 35°S and 25°N (15–30 yr periods) and throughout the whole basin (50–90-yr periods).

scholarly journals Overflow of cold water across the Iceland-Faroe Ridge through the Western Valley

2018 ◽  
Author(s):  
Bogi Hansen ◽  
Karin Margretha Húsgarð Larsen ◽  
Steffen Malskær Olsen ◽  
Detlef Quadfasel ◽  
Kerstin Jochumsen ◽  
...  
Keyword(s):  
Satellite Altimetry ◽  
Cold Water ◽  
Large Fraction ◽  
Acoustic Doppler Current Profiler ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Direct Measurements ◽  
Current Profiler ◽  
Meridional Overturning

Abstract. The Iceland-Faroe Ridge (IFR) is considered to be the third-most important passage for dense overflow water from the Nordic Seas feeding into the lower limb of the Atlantic Meridional Overturning Circulation with a volume transport on the order of 1 Sv (106 m3 s−1). The Western Valley, which is the northernmost deep passage across the IFR, has been presumed to supply a strong and persistent overflow (WV-overflow), contributing a large fraction of the total overflow across the IFR. However, prolonged measurements of this transport are so far missing. In order to quantify the flow by direct measurements, three instrumental packages were deployed close to the sill of the Western Valley for 278 days (2016–2017) including an Acoustic Doppler Current Profiler at the expected location of the overflow core. The average volume transport of WV-overflow during this field experiment was found to be less than 0.03 Sv. Aided by the observations and a two-layer hydraulic model, we argue that the reason for this low value is the inflow of warm Atlantic Water to the Norwegian Sea in the upper layers suppressing the deep overflow. The link between deep and surface flows explains an observed relationship between overflow and sea level slope as measured by satellite altimetry. This relationship, combined with historical hydrographic measurements allows us to conclude that the volume transport of WV-overflow most likely has been less than 0.1 Sv on average since the beginning of regular satellite altimetry in 1993. Our new direct measurements do not allow us to present an updated estimate of the total overflow across the IFR, but they indicate that it may well be considerably less than 1 Sv.

scholarly journals Impact of the Atlantic Meridional Mode on Gulf Stream North Wall Position

2018 ◽  
Vol 31 (21) ◽  
pp. 8875-8894 ◽  
Author(s):  
Sultan Hameed ◽  
Christopher L. P. Wolfe ◽  
Lequan Chi
Keyword(s):  
Gulf Stream ◽  
Cold Water ◽  
Meridional Overturning Circulation ◽  
Major Influence ◽  
Tropical Atlantic ◽  
The North ◽  
Meridional Mode ◽  
North Wall ◽  
Atlantic Meridional Mode ◽  
Wall Position

The path of the Gulf Stream as it leaves the continental shelf near Cape Hatteras is marked by a sharp gradient in ocean temperature known as the North Wall. Previous work in the literature has considered processes related to the North Atlantic Oscillation (NAO) in triggering latitudinal displacements of the North Wall position. This paper presents evidence that the Atlantic meridional mode (AMM) also impacts interannual variations of the North Wall position. The AMM signal from the tropics propagates to the Gulf Stream near the 200-m depth, and there are two time scales for this interaction. Anomalous Ekman suction induced by AMM cools the tropical Atlantic. The cold water in the Caribbean Sea is entrained into the currents feeding the Gulf Stream, and this cooling signal reaches the North Wall within a year. A second mechanism involves cold anomalies in the western tropical Atlantic, which initially propagate westward as baroclinic planetary waves, reaching the Gulf Stream and resulting in a southward shift in the North Wall position after a delay of about one year. In an analysis for the period 1961–2015, AMM’s signal dominates North Wall fluctuations in the upper 300 m, while NAO is the major influence below ~500 m; the influence of both the teleconnections is seen between 300 and 500 m. The relationship between the Atlantic meridional overturning circulation (AMOC) and the North Wall is investigated for the 2005–15 period and found to be statistically significant only at the sea surface in one of the three North Wall indices used.

Caught in a Booby Trap

2021 ◽  
pp. 175-211
Author(s):  
William H. Durham
Keyword(s):  
South America ◽  
Genetic Analysis ◽  
Common Ancestor ◽  
Cold Water ◽  
Recent Common Ancestor ◽  
Close Relative ◽  
Trade Winds ◽  
Coloring Agents ◽  
New Evidence

How did the blue-footed booby evolve those lovely blue feet? Genetic analysis shows that the blue-footed booby shared a recent common ancestor with the grey-footed Peruvian booby of coastal South America. Unlike the latter, whose diet includes anchovies and sardines, the blue-footed booby is a sardine specialist that feeds by plunge diving exclusively into cold-water upwellings, like those of the Cromwell current. Sardines normally abound in Galápagos, but not anchovies—the strong currents quickly carry away their young. But sardines are also rich in blue-green coloring agents (carotenes) that show up in the feet of successful booby divers. Hence it’s an uncanny “fit” of the blue-footed booby to island conditions is uncanny: did it evolve in Galápagos from continental ancestors who followed trade winds west? The species also regulates brood sizes in relation to sardine supply via flexible siblicide, which surely helps in coping with ENSO. It shares siblicide with its close Galápagos relative, the Nazca booby, as well as aggressive NAV behavior in which nonparental adult visitors peck at innocent, unrelated chicks until they bleed or drown. New evidence from brown boobies—another close relative—suggests that this behavior evolved as a result of benefits from survival cannibalism during extreme El Niños.

Tropical Air–Sea Interactions Accelerate the Recovery of the Atlantic Meridional Overturning Circulation after a Major Shutdown

2007 ◽  
Vol 20 (19) ◽  
pp. 4940-4956 ◽  
Author(s):  
Uta Krebs ◽  
A. Timmermann
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Tropical Atlantic ◽  
Trade Winds ◽  
The North ◽  
Salinity Anomaly ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract Using a coupled ocean–sea ice–atmosphere model of intermediate complexity, the authors study the influence of air–sea interactions on the stability of the Atlantic Meridional Overturning Circulation (AMOC). Mimicking glacial Heinrich events, a complete shutdown of the AMOC is triggered by the delivery of anomalous freshwater forcing to the northern North Atlantic. Analysis of fully and partially coupled freshwater perturbation experiments under glacial conditions shows that associated changes of the heat transport in the North Atlantic lead to a cooling north of the thermal equator and an associated strengthening of the northeasterly trade winds. Because of advection of cold air and an intensification of the trade winds, the intertropical convergence zone (ITCZ) is shifted southward. Changes of the accumulated precipitation lead to the generation of a positive salinity anomaly in the northern tropical Atlantic and a negative anomaly in the southern tropical Atlantic. During the shutdown phase of the AMOC, cross-equatorial oceanic surface flow is halted, preventing dilution of the positive salinity anomaly in the North Atlantic. Advected northward by the wind-driven ocean circulation, the positive salinity anomaly increases the upper-ocean density in the deep-water formation regions, thereby accelerating the recovery of the AMOC considerably. Partially coupled experiments that neglect tropical air–sea coupling reveal that the recovery time of the AMOC is almost twice as long as in the fully coupled case. The impact of a shutdown of the AMOC on the Indian and Pacific Oceans can be decomposed into atmospheric and oceanic contributions. Temperature anomalies in the Northern Hemisphere are largely controlled by atmospheric circulation anomalies, whereas those in the Southern Hemisphere are strongly determined by ocean dynamical changes and exhibit a time lag of several decades. An intensification of the Pacific meridional overturning cell in the northern North Pacific during the AMOC shutdown can be explained in terms of wind-driven ocean circulation changes acting in concert with global ocean adjustment processes.

The spiralling North Atlantic Subtropical Gyre

2020 ◽  
Author(s):  
Kristofer Döös ◽  
Sara Berglund ◽  
Trevor Mcdougall ◽  
Sjoerd Groeskamp
Keyword(s):  
North Atlantic ◽  
Gulf Stream ◽  
Subtropical Gyre ◽  
The South ◽  
Spiral Flow ◽  
The North ◽  
Saline Waters ◽  
Downward Spiral ◽  
Meridional Overturning ◽  
The North Atlantic

<p>The North Atlantic Subtropical Gyre is shown to have a downward spiral flow beneath the mixed layer, where the water slowly gets denser, colder and fresher as it spins around the gyre. This path is traced with Lagrangian trajectories as they enter the Gyre in the Gulf Stream from the south until they exit through the North Atlantic Drift. The preliminary results indicate that these warm, saline waters from the south gradually becomes fresher, colder and denser due to mixing with waters originating from the North Atlantic. There are indications that there is also a diapycnal mixing, in the eastern part of the gyre due to mixing with the saline Mediterranean Waters, which would then be crucial for the Atlantic Meridional Overturning. The mixing in the rest of the gyre is dominated by isopycnic mixing, which transforms gradually the water into colder and fresher water as it spins down the gyre into the abyssal ocean before heading north.</p>

A Simple Linear Model of the Depth Dependence of the Wind-Driven Variability of the Meridional Overturning Circulation

2008 ◽  
Vol 38 (2) ◽  
pp. 492-502 ◽  
Author(s):  
Peter D. Killworth
Keyword(s):  
Heat Flux ◽  
Spatial Distribution ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Return Flow ◽  
Vertical Position ◽  
Time Varying ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
East West

Abstract The part of the meridional overturning circulation driven by time-varying winds is usually assumed to be an Ekman flux within a mixed layer, and a depth- and laterally independent return flow beneath. For a simple linear frictional ocean model, the return flow is studied for a range of frequencies from several days to decades. It is shown that while the east–west integral of the return flow is usually, but not always, almost independent of depth, the spatial distribution of the return flow varies strongly with both horizontal and vertical position. This can have important consequences for calculations of the northward heat flux, which traditionally assumes a spatially uniform return flow.

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