Deglacial Indian monsoon failure and North Atlantic stadials linked by Indian Ocean surface cooling

2015 ◽  
Vol 9 (1) ◽  
pp. 46-50 ◽  
Author(s):  
Jessica E. Tierney ◽  
Francesco S. R. Pausata ◽  
Peter deMenocal
Keyword(s):  
Indian Ocean ◽  
North Atlantic ◽  
Indian Monsoon ◽  
Ocean Surface ◽  
Surface Cooling ◽  
Monsoon Failure

scholarly journals Glacial fluctuations of the Indian monsoon and their relationship with North Atlantic climate: new data and modelling experiments

2013 ◽  
Vol 9 (5) ◽  
pp. 2135-2151 ◽  
Author(s):  
C. Marzin ◽  
N. Kallel ◽  
M. Kageyama ◽  
J.-C. Duplessy ◽  
P. Braconnot
Keyword(s):  
North Atlantic ◽  
Bay Of Bengal ◽  
Indian Monsoon ◽  
Hydrological Cycle ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The North ◽  
North Atlantic Climate ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. Several paleoclimate records such as from Chinese loess, speleothems or upwelling indicators in marine sediments present large variations of the Asian monsoon system during the last glaciation. Here, we present a new record from the northern Andaman Sea (core MD77-176) which shows the variations of the hydrological cycle of the Bay of Bengal. The high-resolution record of surface water δ18O dominantly reflects salinity changes and displays large millennial-scale oscillations over the period 40 000 to 11 000 yr BP. Their timing and sequence suggests that events of high (resp. low) salinity in the Bay of Bengal, i.e. weak (resp. strong) Indian monsoon, correspond to cold (resp. warm) events in the North Atlantic and Arctic, as documented by the Greenland ice core record. We use the IPSL_CM4 Atmosphere-Ocean coupled General Circulation Model to study the processes that could explain the teleconnection between the Indian monsoon and the North Atlantic climate. We first analyse a numerical experiment in which such a rapid event in the North Atlantic is obtained under glacial conditions by increasing the freshwater flux in the North Atlantic, which results in a reduction of the intensity of the Atlantic meridional overturning circulation. This freshwater hosing results in a weakening of the Indian monsoon rainfall and circulation. The changes in the continental runoff and local hydrological cycle are responsible for an increase in salinity in the Bay of Bengal. This therefore compares favourably with the new sea water δ18O record presented here and the hypothesis of synchronous cold North Atlantic and weak Indian monsoon events. Additional sensitivity experiments are produced with the LMDZ atmospheric model to analyse the teleconnection mechanisms between the North Atlantic and the Indian monsoon. The changes over the tropical Atlantic are shown to be essential in triggering perturbations of the subtropical jet over Africa and Eurasia, that in turn affect the intensity of the Indian monsoon. These relationships are also found to be valid in additional coupled model simulations in which the Atlantic meridional overturning circulation (AMOC) is forced to resume.

scholarly journals The Meridional Shift of the Midlatitude Westerlies over Arid Central Asia during the Past 21 000 Years Based on the TraCE-21ka Simulations

2020 ◽  
Vol 33 (17) ◽  
pp. 7455-7478
Author(s):  
Nanxuan Jiang ◽  
Qing Yan ◽  
Zhiqing Xu ◽  
Jian Shi ◽  
Ran Zhang
Keyword(s):  
Indian Ocean ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Last Deglaciation ◽  
The Past ◽  
The North ◽  
External Forcings ◽  
The Indian Ocean ◽  
The Last Deglaciation ◽  
The North Atlantic

AbstractTo advance our knowledge of the response of midlatitude westerlies to various external forcings, we investigate the meridional shift of midlatitude westerlies over arid central Asia (ACA) during the past 21 000 years, which experienced more varied forcings than the present day based on a set of transient simulations. Our results suggest that the evolution of midlatitude westerlies over ACA and driving factors vary with time and across seasons. In spring, the location of midlatitude westerlies over ACA oscillates largely during the last deglaciation, driven by meltwater fluxes and continental ice sheets, and then shows a long-term equatorward shift during the Holocene controlled by orbital insolation. In summer, orbital insolation dominates the meridional shift of midlatitude westerlies, with poleward and equatorward migration during the last deglaciation and the Holocene, respectively. From a thermodynamic perspective, variations in zonal winds are linked with the meridional temperature gradient based on the thermal wind relationship. From a dynamic perspective, variations in midlatitude westerlies are mainly induced by anomalous sea surface temperatures over the Indian Ocean through the Matsuno–Gill response and over the North Atlantic Ocean by the propagation of Rossby waves, or both, but their relative importance varies across forcings. Additionally, the modeled meridional shift of midlatitude westerlies is broadly consistent with geological evidence, although model–data discrepancies still exist. Overall, our study provides a possible scenario for a meridional shift of midlatitude westerlies over ACA in response to various external forcings during the past 21 000 years and highlights important roles of both the Indian Ocean and the North Atlantic Ocean in regulating Asian westerlies, which may shed light on the behavior of westerlies in the future.

scholarly journals LONG-TERM VARIABILITY OF CURRENTS IN THE SUBARCTIC FRONT OF THE ATLANTIC OCEAN

2019 ◽  
Vol 47 (2) ◽  
pp. 246-265
Author(s):  
A. K. Ambrosimov ◽  
N. A. Diansky ◽  
A. A. Kluvitkin ◽  
V. A. Melnikov
Keyword(s):  
North Atlantic ◽  
Ocean Surface ◽  
North Atlantic Current ◽  
Reykjanes Ridge ◽  
The North ◽  
Subarctic Front ◽  
Average Water ◽  
The North Atlantic ◽  
Atlantic Current

Based on time series of near-bottom current velocities and temperatures obtained in the period June, 2016 to July, 2017, at three points in the Atlantic Subarctic Front, along with the use of multi-year (since 1993 up to now) satellite ocean surface sounding data, multi-scale fluctuations of ocean surface and near-bottom flows over the western and eastern flanks of the Reykjanes ridge, as well as near Hatton Rise, on the Rokoll plateau, are studied. Hydrological profiles were carried out from the ocean surface to the bottom with readings every 10 m, when setting and retrieving the buoy stations. Using data from the Bank of hydrological stations (WOD13), SST satellite arrays (Pathfinder), long-term sea level and geostrophic velocities time series (AVISO), and bottom topography (model ETOPO-1), features of longterm cyclical fluctuations of SST, sea level, geostrophic currents on the ocean surface were defined in the sub-polar North Atlantic. It is shown that, in accordance with the large-scale thermohaline structure of the Subarctic front, two branches of the North Atlantic Current are detected on the ocean surface.One is directed from the Hatton towards the Icelandic-Faroese Rise, and the other – alomg the western flank of the Reykjanes Ridge toward Iceland. For the first branch, which is the main continuation of the North Atlantic Current, the average (for 25 years) water drift at a speed of 9.1±0.1 cm/s is determined to the northeast. The second branch, which forms the eastern part of the Subarctic cyclonic gyre, has the average water drift at a speed of 4.0±0.1 cm/s is directed north-northeast, along the western flank of the Reykjanes Ridge. In the intermediate waters of the frontal zone, an average water flow is observed at a speed of 2.7±0.1 cm/s to the north-northeast, along the eastern slope of the Reykjanes ridge.Due to the multy-scale components of the total variability, the average kinetic energy densities(KED) of total currents (109, 45, 97, (±3) erg/cm3, at station points from east to west) are much greater than the mean drift KED. The near-bottom flows on the Reykjanes ridge flanks are opposite to the direction of the North Atlantic Current. Outside the Subarctic gyre, the direction of average transport is maintained from the ocean surface to the bottom. The average (per year) KED of near-bottom currents are 31, 143, 27 (±3 erg/cm3), for three stations from east to west, respectively. In the intermediate waters of the frontal zone, above the eastern slope of the Reykjanes Ridge, there is a powerful reverse (relative to the North Atlantic Current) near-bottom water flow to the south-west, with a high average speed of ~ 15 cm/s. The KED of the currents during the year varies widely from zero to ~ 600 erg/cm3. The overall variability is due to cyclical variations and intermittency (“flashes”) of currents. Perennial cycles, seasonal variations, synoptic fluctuations with periods in the range of 30–300 days, as well as inertial oscillations and semi-diurnal tidal waves are distinguished. The intermittency of oscillations is partly due to changes in low-frequency flows, which can lead to a dopler frequency shift in the cyclic components of the spectrum. The amplitude of temperature fluctuations in the bottom layer for the year was (0.07–0.10) ± 0.01°C by the standard deviation. The seasonal changes of the bottom temperature are not detected. However, a linear trend with a warming of ~ (0.10–0.15) ± 0.01°С per year is noticeable.

scholarly journals Impact of mid-glacial ice sheets on deep ocean circulation and global climate: Role of surface cooling on the AMOC

2020 ◽  
Author(s):  
Sam Sherriff-Tadano ◽  
Ayako Abe-Ouchi ◽  
Akira Oka
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Global Climate ◽  
Surface Wind ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Strengthening Effect ◽  
Surface Cooling ◽  
Glacial Ice

Abstract. This study explores the effect of southward expansion of mid-glacial ice sheets on the global climate and the Atlantic meridional overturning circulation (AMOC), as well as the processes by which the ice sheets modify the AMOC. For this purpose, simulations of Marine Isotope Stage (MIS) 3 and 5a are performed with an atmosphere-ocean general circulation model. In the MIS3 and MIS5a simulations, the global average temperature decreases by 5.0 °C and 2.2 °C, respectively, compared with the preindustrial climate simulation. The AMOC weakens by 3 % in MIS3, whereas it is enhanced by 16 % in MIS5a, both of which are consistent with a reconstruction. Sensitivity experiments extracting the effect of the expansion of glacial ice sheets from MIS5a to MIS3 show a global cooling of 1.1 °C, contributing to about 40 % of the total surface cooling from MIS5a to MIS3. These experiments also demonstrate that the ice sheet expansion leads to a surface cooling of 2 °C over the Southern Ocean as a result of colder North Atlantic deep water. We find that the southward expansion of the mid-glacial ice sheet exerts a small impact on the AMOC. Partially coupled experiments reveal that the global surface cooling by the glacial ice sheet tends to reduce the AMOC by increasing the sea ice at both poles, and hence compensates for the strengthening effect of the enhanced surface wind over the North Atlantic. Our results show that the total effect of glacial ice sheets on the AMOC is determined by the two competing effects, surface wind and surface cooling. The relative strength of surface wind and surface cooling depends on the ice sheet configuration, and the strength of the surface cooling can be comparable to that of surface wind when changes in the extent of ice sheet are prominent.

scholarly journals Role of air–sea fluxes and ocean surface density in the production of deep waters in the eastern subpolar gyre of the North Atlantic

Ocean Science ◽  
2021 ◽  
Vol 17 (5) ◽  
pp. 1353-1365
Author(s):  
Tillys Petit ◽  
M. Susan Lozier ◽  
Simon A. Josey ◽  
Stuart A. Cunningham
Keyword(s):  
North Atlantic ◽  
Surface Density ◽  
Ocean Surface ◽  
Density Field ◽  
The North ◽  
Iceland Basin ◽  
Deep Waters ◽  
Water Formation ◽  
Dense Water Formation

Abstract. Wintertime convection in the North Atlantic Ocean is a key component of the global climate as it produces dense waters at high latitudes that flow equatorward as part of the Atlantic Meridional Overturning Circulation (AMOC). Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of North Atlantic Deep Water. Dense water formation in these basins is mainly explained by buoyancy forcing that transforms surface waters to the deep waters of the AMOC lower limb. Air–sea fluxes and the ocean surface density field are both key determinants of the buoyancy-driven transformation. We analyze these contributions to the transformation in order to better understand the connection between atmospheric forcing and the densification of surface water. More precisely, we study the impact of air–sea fluxes and the ocean surface density field on the transformation of subpolar mode water (SPMW) in the Iceland Basin, a water mass that “pre-conditions” dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that the variance in SPMW transformation is mainly influenced by the variance in density at the ocean surface. This surface density is set by a combination of advection, wind-driven upwelling and surface fluxes. Our study shows that the latter explains ∼ 30 % of the variance in outcrop area as expressed by the surface area between the outcropped SPMW isopycnals. The key role of the surface density in SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–2015 over the Iceland Basin.

scholarly journals The Linear Sensitivity of the North Atlantic Oscillation and Eddy-Driven Jet to SSTs

2019 ◽  
Vol 32 (19) ◽  
pp. 6491-6511 ◽  
Author(s):  
Hugh S. Baker ◽  
Tim Woollings ◽  
Chris E. Forest ◽  
Myles R. Allen
Keyword(s):  
Indian Ocean ◽  
North Atlantic Oscillation ◽  
North Atlantic ◽  
High Sensitivity ◽  
Internal Variability ◽  
The North ◽  
The Indian Ocean ◽  
The North Atlantic ◽  
The North Atlantic Oscillation ◽  
The Impact

Abstract The North Atlantic Oscillation (NAO) and eddy-driven jet contain a forced component arising from sea surface temperature (SST) variations. Due to large amounts of internal variability, it is not trivial to determine where and to what extent SSTs force the NAO and jet. A linear statistical–dynamic method is employed with a large climate ensemble to compute the sensitivities of the winter and summer NAO and jet speed and latitude to the SSTs. Key regions of sensitivity are identified in the Indian and Pacific basins, and the North Atlantic tripole. Using the sensitivity maps and a long observational SST dataset, skillful reconstructions of the NAO and jet time series are made. The ability to skillfully forecast both the winter and summer NAO using only SST anomalies is also demonstrated. The linear approach used here allows precise attribution of model forecast signals to SSTs in particular regions. Skill comes from the Atlantic and Pacific basins on short lead times, while the Indian Ocean SSTs may contribute to the longer-term NAO trend. However, despite the region of high sensitivity in the Indian Ocean, SSTs here do not provide significant skill on interannual time scales, which highlights the limitations of the imposed SST approach. Given the impact of the NAO and jet on Northern Hemisphere weather and climate, these results provide useful information that could be used for improved attribution and forecasting.

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