The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle
. The AMOC has ...been shown to be weakening in recent years
; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC
. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD 1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA-sourced from melting glaciers and thickened sea ice that developed earlier in the LIA-weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet
. Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.
The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is ...thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells, but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well-dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells.
The last interglacial interval was terminated by the inception of a long, progressive glaciation that is attributed to astronomically influenced changes in the seasonal distribution of sunlight over ...the earth. However, the feedbacks, internal dynamics, and global teleconnections associated with declining northern summer insolation remain incompletely understood. Here we show that a crucial early step in glacial inception involves the weakening of the subpolar gyre (SPG) circulation of the North Atlantic Ocean. Detailed new records of microfossil foraminifera abundance and stable isotope ratios in deep sea sediments from Ocean Drilling Program site 984 south of Iceland reveal repeated, progressive cold water-mass expansions into subpolar latitudes during the last peak interglacial interval, marine isotope substage 5e. These movements are expressed as a sequence of progressively extensive southward advances and subsequent retreats of a hydrographic boundary that may have been analogous to the modern Arctic front, and associated with rapid changes in the strength of the SPG. This persistent millennial-scale oceanographic oscillation accompanied a long-term cooling trend at a time of slowly declining northern summer insolation, providing an early link in the propagation of those insolation changes globally, and resulting in a rapid transition from extensive regional warmth to the dramatic instability of the subsequent ∼100 ka.
The Nd isotope composition of seawater has been used to reconstruct past changes in the contribution of different water masses to the deep ocean. In the absence of contrary information, the Nd ...isotope compositions of endmember water masses are usually assumed constant during the Quaternary. Here we show that the Nd isotope composition of North Atlantic Deep Water (NADW), a major component of the global overturning ocean circulation, was significantly more radiogenic than modern during the Last Glacial Maximum (LGM), and shifted towards modern values during the deglaciation. We propose that weathering contributions of unradiogenic Nd modulated by the North American Ice Sheet dominated the evolution of the NADW Nd isotope endmember. If water mass mixing dominated the distribution of deep glacial Atlantic Nd isotopes, our results would imply a larger fraction of NADW in the deep Atlantic during the LGM and deglaciation than reconstructed with a constant northern endmember.
Antarctic Intermediate Water (AAIW) is a key player in the global ocean circulation, contributing to the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), and influencing ...interhemispheric heat exchange and the distribution of salinity, nutrients and carbon. However, the deglacial history of AAIW flow into the North Atlantic is controversial. Here we present a multicore-top neodymium isotope calibration, which confirms the ability of unclean foraminifera to faithfully record bottom water neodymium isotopic composition (εNd) values in their authigenic coatings. We then present the first foraminifera-based reconstruction of εNd from three sediment cores retrieved from within modern AAIW, in the western tropical North Atlantic. Our records reveal similar glacial and interglacial contributions of AAIW, and a pronounced decrease in the AAIW fraction during North Atlantic deglacial cold episodes, Heinrich Stadial 1 (HS1) and Younger Dryas (YD). Our results suggest two separate phases of reduced fraction of AAIW in the tropical Atlantic during HS1, with a greater reduction during early HS1. If a reduction in AAIW fraction also reflects reduced AMOC strength, this finding may explain why, in many regions, there are two phases of hydrologic change within HS1, and why atmospheric CO2 rose more rapidly during early than late HS1. Our result suggesting less flow of AAIW into the Atlantic during North Atlantic cold events contrasts with evidence from the Pacific, where intermediate-depth εNd records may indicate increased flow of AAIW into the Pacific during the these same events. Antiphased εNd behavior between intermediate depths of the North Atlantic and Pacific implies that the flow of AAIW into Atlantic and Pacific seesawed during the last deglaciation.
•Unclean foraminifera εNd serve as a reliable proxy for bottom water εNd value.•A pronounced decrease in the fraction of AAIW in the equatorial Atlantic during Heinrich Stadial 1 (HS1) and Younger Dryas.•Two stages of AMOC reduction during HS1, larger reduction during early HS1.•The flow of AAIW into Atlantic and Pacific may seesaw on millennial timescales.
Pacific Ocean Heat Content During the Past 10,000 Years Rosenthal, Yair; Linsley, Braddock K.; Oppo, Delia W.
Science (American Association for the Advancement of Science),
11/2013, Letnik:
342, Številka:
6158
Journal Article
Recenzirano
Observed increases in ocean heat content (OHC) and temperature are robust indicators of global warming during the past several decades. We used high-resolution proxy records from sediment cores to ...extend these observations in the Pacific 10,000 years beyond the instrumental record. We show that water masses linked to North Pacific and Antarctic intermediate waters were warmer by 2.1 ± 0.4°C and 1.5 ± 0.4°C, respectively, during the middle Holocene Thermal Maximum than over the past century. Both water masses were ~0.9°C warmer during the Medieval Warm period than during the Little Ice Age and ~0.65° warmer than in recent decades. Although documented changes in global surface temperatures during the Holocene and Common era are relatively small, the concomitant changes in OHC are large.
Surface and thermocline conditions of the Western Pacific Warm Pool (WPWP) reflect changes in regional and basin scale ocean and atmosphere circulations and in turn may affect climate globally. ...Previous studies suggest that a range of factors influences the WPWP on different timescales, however the precise forcings and mechanisms are unclear. Combining surface and thermocline records from sediment cores offshore Papua New Guinea we explore the influence of local and remote processes on the WPWP in response to astronomical forcing and changing glacial-interglacial boundary conditions over the past 110 kyr. We find that thermocline temperatures change with variations in Earth's obliquity with higher temperatures coinciding with high obliquity, which is attributed to variations in subduction and advection of the South Pacific Tropical Water. In contrast, rainfall variations associated with meridional migrations of the Intertropical Convergence Zone are primarily driven by changes in insolation due to precession. Records of bulk sedimentary Ti/Ca and foraminiferal Nd/Ca indicate an additional influence of obliquity, which, however, cannot unambiguously be related to changes in precipitation. Finally, our results suggest a thermocline deepening during the Last Glacial Maximum (LGM). A compilation of available proxy records illustrates a dipole-like pattern of LGM thermocline depth anomalies with a shoaling (deepening) in the northern (southern) WPWP. A comparison of the proxy compilation with an ensemble of Paleoclimate Model Intercomparison Project (PMIP) climate model simulations reveals that the spatial pattern of LGM thermocline depth anomalies is mainly attributable to a contraction of the Pacific Walker circulation on its western side.
•Orbital control on precipitation over/off Papua New Guinea.•Thermocline off Papua New Guinea is influenced by obliquity-driven changes in SPTW advection.•LGM thermocline deepening associated with a contraction of the Pacific Walker cell.
Northern Hemisphere surface temperature reconstructions suggest that the late twentieth century was warmer than any other time during the past 500 years and possibly any time during the past 1,300 ...years (refs 1, 2). These temperature reconstructions are based largely on terrestrial records from extra-tropical or high-elevation sites; however, global average surface temperature changes closely follow those of the global tropics, which are 75% ocean. In particular, the tropical Indo-Pacific warm pool (IPWP) represents a major heat reservoir that both influences global atmospheric circulation and responds to remote northern high-latitude forcings. Here we present a decadally resolved continuous sea surface temperature (SST) reconstruction from the IPWP that spans the past two millennia and overlaps the instrumental record, enabling both a direct comparison of proxy data to the instrumental record and an evaluation of past changes in the context of twentieth century trends. Our record from the Makassar Strait, Indonesia, exhibits trends that are similar to a recent Northern Hemisphere temperature reconstruction. Reconstructed SST was, however, within error of modern values from about ad 1000 to ad 1250, towards the end of the Medieval Warm Period. SSTs during the Little Ice Age (approximately ad 1550-1850) were variable, and ∼0.5 to 1 °C colder than modern values during the coldest intervals. A companion reconstruction of 18O of sea water-a sea surface salinity and hydrology indicator-indicates a tight coupling with the East Asian monsoon system and remote control of IPWP hydrology on centennial-millennial timescales, rather than a dominant influence from local SST variation.
The Makassar Strait, the main passageway of the Indonesian Throughflow (ITF), is an important component of Indo‐Pacific climate through its inter‐basin redistribution of heat and freshwater. ...Observational studies suggest that wind‐driven freshwater advection from the marginal seas into the Makassar Strait modulates the strait's surface transport. However, direct observations are too short (<15 years) to resolve variability on decadal timescales. Here we use a series of global ocean simulations to assess the advected freshwater contributions to ITF transport across a range of timescales. The simulated seasonal and interannual freshwater dynamics are consistent with previous studies. On decadal timescales, we find that wind‐driven advection of South China Sea (SCS) waters into the Makassar Strait modulates upper‐ocean ITF transport. Atmospheric circulation changes associated with Pacific decadal variability appear to drive this mechanism via Pacific lower‐latitude western boundary current interactions that affect the SCS circulation.
Plain Language Summary
The Makassar Strait plays an important role in regional climate as the main pathway in the Indonesian Seas through which tropical Pacific waters flow into the Indian Ocean. Observations from moorings in Makassar Strait indicate that the flow of fresher waters from adjacent shallow seas can impact surface transport through the Strait on seasonal and interannual timescales. However, because these direct measurements are sparse, they cannot resolve Makassar Strait transport on decadal timescales. Using global ocean model simulations, we explore what factors affect surface transport on these longer timescales. We first show that the ocean model skillfully reproduces seasonal and interannual freshwater impacts on surface transport. On decadal timescales, we find that freshwater input indeed influences Makassar transport. Specifically, fresh surface water from the South China Sea (SCS) plays an important role. This freshwater influence is likely driven by variability from the Pacific Ocean which impacts SCS currents through large‐scale, surface wind changes.
Key Points
A global ocean model is used to show how freshwater impacts the decadal variability of transport through the main Indonesian Throughflow pathway
Wind‐driven advection of South China Sea freshwater induces an upstream pressure gradient that reduces transport
Freshwater input is modulated by atmospheric circulation changes associated with Pacific decadal variability
PDF
