scholarly journals Influence of the North Atlantic subpolar gyre circulation on the 4.2 ka BP event

2019 ◽  
Vol 15 (2) ◽  
pp. 701-711 ◽  
Author(s):  
Bassem Jalali ◽  
Marie-Alexandrine Sicre ◽  
Julien Azuara ◽  
Violaine Pellichero ◽  
Nathalie Combourieu-Nebout
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Polar Front ◽  
Western Mediterranean ◽  
Cold Climate ◽  
Subpolar Gyre ◽  
The North ◽  
Western Mediterranean Sea ◽  
Temperature And Precipitation ◽  
The North Atlantic

Abstract. The 4.2 ka BP event, spanning from ca 4200 to 3900 cal BP, has been documented in numerous archaeological data and continental archives across the Northern Hemisphere as an abrupt shift to dry and cold climate. However, data on synchronous ocean circulation changes are notably lacking, thus preventing us from getting a full insight into the physical mechanisms responsible for this climate deterioration. Here, we present two high-resolution (5–20 years) sea surface temperature (SST) records from the subpolar gyre and off north Iceland in the vicinity of the polar front obtained from alkenone paleo-thermometry and compare them with proxy data from the western Mediterranean Sea to gain information on regional temperature and precipitation patterns. Our results are evidence of a temperature dipole pattern which, combined with other paleo-oceanographic records of the North Atlantic, suggests a weakening of the subpolar gyre possibly associated with atmospheric blocked regimes.

scholarly journals Influence of the North Atlantic subpolar gyre circulation on the 4.2 ka BP event

2018 ◽  
Author(s):  
Bassem Jalali ◽  
Marie-Alexandrine Sicre ◽  
Julien Azuara ◽  
Violaine Pellichero ◽  
Nathalie Combourieu-Nebout
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Polar Front ◽  
Cold Climate ◽  
Subpolar Gyre ◽  
The North ◽  
Dipole Pattern ◽  
The North Atlantic ◽  
Temperature Records ◽  
Gyre Circulation

Abstract. The 4.2 ka BP event, spanning from ca 4200 to 3900 cal yr BP, has been documented in numerous archaeological data and continental archives across the northern hemisphere as an abrupt shift to dry and cold climate. However, data on synchronous ocean circulation changes are notably lacking thus preventing from getting a full insight into the physical mechanisms responsible for this climate deterioration. Here, we present two high-resolution sea surface temperature records from key locations in the subpolar gyre and off North Iceland in the vicinity of the polar front obtained from alkenone paleothermometry. Our data evidence a temperature dipole pattern in the subpolar North Atlantic between 4400−4100 yr BP, which combined with other paleoclimatic records from the North Atlantic/Euro-Mediterranean suggests a significant reduction of the subpolar gyre circulation possibly associated with atmospheric blocked regimes.

North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

2021 ◽  
Author(s):  
Shenjie Zhou ◽  
Xiaoming Zhai ◽  
Ian Renfrew
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Ensemble Prediction ◽  
Subpolar Gyre ◽  
Atmospheric Forcing ◽  
The North ◽  
The North Atlantic

<p>The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization – based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems<sup> </sup>– to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges – an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.</p>

scholarly journals Wintertime Atmospheric Response to North Atlantic Ocean Circulation Variability in a Climate Model

2015 ◽  
Vol 28 (19) ◽  
pp. 7659-7677 ◽  
Author(s):  
Claude Frankignoul ◽  
Guillaume Gastineau ◽  
Young-Oh Kwon
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Atmospheric Response ◽  
Subpolar Gyre ◽  
Climate System Model ◽  
The North ◽  
The North Atlantic

Abstract Maximum covariance analysis of a preindustrial control simulation of the NCAR Community Climate System Model, version 4 (CCSM4), shows that a barotropic signal in winter broadly resembling a negative phase of the North Atlantic Oscillation (NAO) follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 7 yr. The delay is due to the cyclonic propagation along the North Atlantic Current (NAC) and the subpolar gyre of a SST warming linked to a northward shift and intensification of the NAC, together with an increasing SST cooling linked to increasing southward advection of subpolar water along the western boundary and a southward shift of the Gulf Stream (GS). These changes result in a meridional SST dipole, which follows the AMOC intensification after 6 or 7 yr. The SST changes were initiated by the strengthening of the western subpolar gyre and by bottom torque at the crossover of the deep branches of the AMOC with the NAC on the western flank of the Mid-Atlantic Ridge and the GS near the Tail of the Grand Banks, respectively. The heat flux damping of the SST dipole shifts the region of maximum atmospheric transient eddy growth southward, leading to a negative NAO-like response. No significant atmospheric response is found to the Atlantic multidecadal oscillation (AMO), which is broadly realistic but shifted south and associated with a much weaker meridional SST gradient than the AMOC fingerprint. Nonetheless, the wintertime atmospheric response to the AMOC shows some similarity with the observed response to the AMO, suggesting that the ocean–atmosphere interactions are broadly realistic in CCSM4.

Characteristics and Possible Sources of the Intraseasonal South Asian Jet Wave Train in Boreal Winter

2020 ◽  
Vol 33 (24) ◽  
pp. 10523-10537
Author(s):  
Sijie Huang ◽  
Xiuzhen Li ◽  
Zhiping Wen
Keyword(s):  
Mediterranean Sea ◽  
South Asian ◽  
North Atlantic ◽  
Wave Train ◽  
Western Mediterranean ◽  
Upper Troposphere ◽  
The North ◽  
Western Mediterranean Sea ◽  
Activity Center ◽  
The North Atlantic

AbstractThe characteristics and possible energy sources of the South Asian jet wave train in winter are analyzed, with the intraseasonal signal emphasized. The wave train is equivalently barotropic and strongest in the upper troposphere, with its daily evolution dominated by the intraseasonal (10–30 day) time scale. Along the wave train, the propagation of disturbances from the North Atlantic to the western North Pacific takes around 8 days, which is much faster than the eastward migration of activity centers. The energy sources of the intraseasonal wave train are complicated and can be separated into three categories depending on the role of the North Atlantic Oscillation (NAO). When NAO− precedes the wave train, it is northwest–southeast oriented. The energy is rooted in the lower troposphere over the high-latitude North Atlantic, and excites the Rossby wave source (RWS) over the western Mediterranean Sea via vortex stretching by abnormal divergence. When NAO+ precedes the wave train, it is southwest–northeast oriented. The energy rooted in the northeastern activity center excites RWS over the eastern Mediterranean Sea. Additionally, disturbances from the western North Atlantic and southwestern activity center of NAO+ excite the RWS over the western Mediterranean Sea. Hence, both NAO− and NAO+ can excite the same wave train, but with different orientation and via different paths. Without the NAO, the wave train can also be stimulated by enhanced disturbances over the midlatitude central North Atlantic. The signal lies mainly in the middle-upper troposphere, which might be related to atmospheric internal dynamic processes, such as kinetic energy conversion from synoptic disturbances.

Mid-Holocene emergence of a low-frequency millennial oscillation in western Mediterranean climate: Implications for past dynamics of the North Atlantic atmospheric westerlies

The Holocene ◽  
2012 ◽  
Vol 23 (2) ◽  
pp. 153-166 ◽  
Author(s):  
William J Fletcher ◽  
Maxime Debret ◽  
Maria Fernanda Sanchez Goñi
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Late Holocene ◽  
Mediterranean Climate ◽  
Zonal Flow ◽  
Western Mediterranean ◽  
Early Holocene ◽  
Internal Oscillation ◽  
The North ◽  
The North Atlantic

The nature and tempo of Holocene climate variability is examined in the record of forest vegetation from western Mediterranean marine core MD95-2043. Episodes of forest decline occurred at 10.1, 9.2, 8.3, 7.4, 5.4–4.5 and 3.7–2.9 cal. ka BP, and between 1.9 cal. ka BP and the top of the record (1.3 cal. ka BP). Wavelet analysis confirms a ~900 yr periodicity prior to and during the early Holocene and the dominance of a ~1750 yr periodicity after 6 cal. ka BP. The ~900 yr periodicity has counterparts in numerous North Atlantic and Northern Hemisphere palaeoclimate records, and in solar irradiance proxies (Δ14C and 10Be), and may relate to a Sun–climate connection during the early Holocene. Comparisons between the MD95-2043 forest record and strategically located records from Morocco, Iceland, Norway and Israel suggest that the ~1750 yr mid- to late-Holocene oscillation reflects shifts between a prevailing strong and weak state of the zonal flow, with impacts similar to the positive and negative modes of the present-day North Atlantic Oscillation (NAO). The mid- to late-Holocene millennial oscillation in zonal flow appears closely coupled to North Atlantic surface ocean circulation dynamics, and may have been driven by an internal oscillation in deep-water convection strength. The findings suggest that the mid-Holocene transition in western Mediterranean climate was accompanied by a shift in the fundamental tempo of millennial-scale variability, reflecting contrasting sensitivity of the North Atlantic climate system to different forcing factors (solar versus oceanic) under deglacial and fully interglacial conditions.

scholarly journals Mechanisms Governing the Development of the North Atlantic Warming Hole in the CESM-LE Future Climate Simulations

2018 ◽  
Vol 31 (15) ◽  
pp. 5927-5946 ◽  
Author(s):  
Melissa Gervais ◽  
Jeffrey Shaman ◽  
Yochanan Kushnir
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Sea Surface ◽  
The Arctic ◽  
Labrador Sea ◽  
Sea Surface Temperatures ◽  
Subpolar Gyre ◽  
The North ◽  
Surface Temperatures ◽  
The North Atlantic

A warming deficit in North Atlantic sea surface temperatures is a striking feature in global climate model future projections. This North Atlantic warming hole has been related to a slowing of the Atlantic meridional overturning circulation (AMOC); however, the detailed mechanisms involved in its generation remain an open question. An analysis of the Community Earth System Model Large Ensemble simulations is conducted to obtain further insight into the development of the warming hole and its relationship to the AMOC. It is shown that increasing freshwater fluxes through the Arctic gates lead to surface freshening and reduced Labrador Sea deep convection, which in turn act to cool Labrador Sea sea surface temperatures. Furthermore, the resulting changes in surface ocean circulation lead to enhanced transport of cooled Labrador Sea surface waters into the interior of the subpolar gyre and a more zonal orientation of the North Atlantic Current. As a result, there is an increase in ocean advective heat flux divergence within the center of the subpolar gyre, causing this warming deficit in North Atlantic sea surface temperatures. These local changes to the ocean circulation affect the AMOC and lead to its slowdown.

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