Southward Shift Research Articles

Evidence of a South Asian Proto‐Monsoon During the Oligocene‐Miocene Transition

AbstractThe geological history of the South Asian monsoon (SAM) before the Pleistocene is not well‐constrained, primarily due to a lack of available continuous sediment archives. Previous studies have noted an intensification of SAM precipitation and atmospheric circulation during the middle Miocene (∼14 Ma), but no records are available to test how the monsoon changed prior to this. In order to improve our understanding of monsoonal evolution, geochemical and sedimentological data were generated for the Oligocene‐early Miocene (30–20 Ma) from Indian National Gas Hydrate Expedition 01 Site NGHP‐01‐01A in the eastern Arabian Sea, at 2,674 m water depth. We find the initial glaciation phase (23.7–23.0 Ma) of the Oligocene‐Miocene transition (OMT) to be associated with an increase in water column ventilation and water mass mixing, suggesting an increase in winter monsoon type atmospheric circulation, possibly driven by a relative southward shift of the intertropical convergence zone. During the latter part of the OMT, or “deglaciation” phase (23.0–22.7 Ma), a long‐term decrease in Mn (suggestive of deoxygenation), increase in Ti/Ca and dissolution of the biogenic carbonate fraction suggest an intensification of a proto‐summer SAM system, characterized by the formation of an oxygen minimum zone in the eastern Arabian Sea and a relative increase of terrigenous material delivered by runoff to the site. With no evidence at this site for an active SAM prior to the OMT we suggest that changes in orbital parameters, as well as possibly changing Tethyan/Himalayan tectonics, caused this step change in the proto‐monsoon system at this intermediate‐depth site.

Paleoceanography and dinoflagellate cyst stratigraphy across the Lower\u2013Middle Pleistocene Subseries (Calabrian\u2013Chibanian Stage) boundary at the Chiba composite section, Japan

A dinoflagellate cyst record from the highly resolved Chiba composite section in Japan has been used to reconstruct sea-surface paleoceanographic changes across the Lower–Middle Pleistocene Subseries (Calabrian–Chibanian Stage) boundary at the global stratotype, constituting the first detailed study of this microfossil group from the Pleistocene of the Japanese Pacific margin. Cold, subarctic water masses from 794.2 ka gave way to warming and rapid retreat of the Subpolar Front at 789.3 ka, ~ 2000 years before the end of Marine Isotope Stage (MIS) 20. Throughout the fully interglacial conditions of MIS 19c, assemblages are consistent with warm sea surface temperatures but also reveal instability and latitudinal shifts in the Kuroshio Extension system. The abrupt dominance of Protoceratium reticulatum cysts between 772.9 and 770.4 ka (MIS 19b) registers the influence of cooler, mixed, nutrient-rich waters of the Kuroshio–Oyashio Interfrontal Zone resulting from a southward shift of the Kuroshio Extension. Its onset at 772.9 ka serves as a local ecostratigraphic marker for the Chibanian Stage Global Boundary Stratotype Section and Point (GSSP) which occurs just 1.15 m (= 1300 years) below it. An interval from 770.1 ka to the top of the examined succession at 765.8 ka (MIS 19a) represents warm, presumably stratified but still nutrient-elevated surface waters, indicating a northward shift of the Kuroshio Extension ~ 5 kyrs after the termination of full interglacial conditions on land.

Hot, unpredictable weather interacts with land use to restrict the distribution of the Yellow-tailed Black-Cockatoo

ABSTRACT Conserving nomadic species is challenging due to the difficulty in monitoring their characteristically transient populations, and thereby detecting range-wide declines. An example is the Yellow-tailed Black-Cockatoo (YTBC; Zanda funerea), which disperses widely in search of food and is regularly – but sporadically – observed across eastern Australia. Under climate warming, a general southward shift in species distributions is expected in the southern hemisphere, with the extreme southern margins being truncated by an ocean barrier. Given these constraints, we ask whether sufficient refugia will exist for the YTBC in the future, by: (i) modelling habitat relationships within the current geographic range of the YTBC based on weather, climate, vegetation, and land use, and (ii) using this framework, coupled with climate-model projections, to forecast 21st century impacts. Intensive land use and high variability in temperature and rainfall seem to most limit YTBC occurrence. In contrast, areas with a cooler, stable climate and a network of old-growth forests, such as those in parts of south-eastern Australia and Tasmania, are most suitable for the species. As Australia becomes progressively hotter under climate change, the preferred bioclimatic envelope of the YTBC is forecast to contract poleward (as a general pattern) and to fragment within the existing range. However, despite an extensive loss of climatically suitable regions, the YTBC might find stable refugia at the southern margins of its geographic range, although continued loss of old-growth forests undermines their nesting potential. Therefore, beyond habitat conservation, creating nesting opportunities within plantation forests would likely be an effective conservation strategy to preserve habitat quality in climate refugia.

Mechanism for Southward Shift of Zonal Wind Anomalies During the Mature Phase of ENSO

AbstractThe cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.

“Beyond Weather Regimes”: Descriptors Monitoring Atmospheric Centers of Action. A case study for Aotearoa New Zealand

AbstractThis paper introduces a set of descriptors applied to weather regimes, that allow for a detailed monitoring of the location and intensity of their atmospheric centers of action (e.g. troughs and ridges) and the gradients between them, when applicable. Descriptors are designed to document the effect of climate variability and change in modulating the character of daily weather regimes, rather than merely their occurrence statistics.As a case study, the methodology is applied to Aotearoa New Zealand (ANZ), using ERA5 ensemble reanalysis data for the period 1979-2019. Here, we analyze teleconnections between the regimes and their descriptors, and large-scale climate variability. Results show a significant modulation of centers of action by the phase of the Southern Annular Mode, with a strong relationship identified with the latitude of atmospheric ridges. Significant associations with El Niño Southern Oscillation are also identified. Modes of large-scale variability have a stronger influence on the regimes’ intrinsic features than their occurrence. This demonstrates the usefulness of such descriptors, which help understand the relationship between mid-latitude transient perturbations and large-scale modes of climate variability.In future research, this methodological framework will be applied to analyze (i) low-frequency changes in weather regimes under climate change, in line with the southward shift of storm tracks, and (ii) regional-scale effects on the climate of ANZ, resulting from interaction with its topography.

Holocene glacier history of northeastern Cordillera Darwin, southernmost South America (55°S)

AbstractIn the Cordillera Darwin, southernmost South America, we used 10Be and 14C dating, dendrochronology, and historical observations to reconstruct the glacial history of the Dalla Vedova valley from deglacial time to the present. After deglacial recession into northeastern Darwin and Dalla Vedova, by ~16 ka, evidence indicates a glacial advance at ~13 ka coeval with the Antarctic Cold Reversal. The next robustly dated glacial expansion occurred at 870 ± 60 calendar yr ago (approximately AD 1150), followed by less-extensive dendrochronologically constrained advances from shortly before AD 1836 to the mid-twentieth century. Our record is consistent with most studies within the Cordillera Darwin that show that the Holocene glacial maximum occurred during the last millennium. This pattern contrasts with the extensive early- and mid-Holocene glacier expansions farther north in Patagonia; furthermore, an advance at 870 ± 60 yr ago may suggest out-of-phase glacial advances occurred within the Cordillera Darwin relative to Patagonia. We speculate that a southward shift of westerlies and associated climate regimes toward the southernmost tip of the continent, about 900–800 yr ago, provides a mechanism by which some glaciers advanced in the Cordillera Darwin during what is generally considered a warm and dry period to the north in Patagonia.

The 2019 Autumn Hot Drought Over the Middle‐Lower Reaches of the Yangtze River in China: Early Propagation, Process Evolution, and Concurrence

AbstractHot droughts (i.e., concurrent droughts and hot anomalies) have drawn much attention recently, but current knowledge about early propagation and concurrence remains limited. The 2019 autumn severe hot drought (2019AHD) over the middle‐lower reaches of the Yangtze River (MLYR) is employed for process‐based analysis, while water vapor transport, heating budgets, and atmospheric dynamics are comprehensively investigated. The main achievements are as follows: (a) The 2019AHD over MLYR is not isolated and is related to early propagation from adjacent North China (NC) two months earlier. (b) Direct physical causes are explored. Drought evolution is related to anomalous water vapor transport dominated by low‐level cyclonic anomalies over the East China Sea. Concurrently, local low‐level hot anomalies over MLYR are related to amplified horizontal advection heating, while near‐surface warming is favored by decreased low cloud cover and enhanced downward shortwave radiation. (c) Regional‐scale concurrence of droughts and hot extremes might be understood by the combined effects of amplified subsidence (suppressing precipitation formation) and intensified low‐level divergence (reducing low cloud cover). (d) From a large‐scale dynamic perspective, early propagation and development of the 2019AHD are related to the southward shift and enhancement of the subsidence branch of meridional circulations. Meanwhile, intensity variation in 200 hPa cyclonic anomalies over MLYR corresponds well with local drought development, and the enhanced intensity at the time of drought occurrence is probably related to Silk Road Pattern–like wave train propagation over Eurasia. These insights can help us understand the mechanisms and detect early signals of hot droughts.

Impacts of the Madden-Julian Oscillation (MJO) on rainfall in Sri Lanka

The influence of the Madden Julian Oscillation (MJO) on rainfall in Sri Lanka (SL) is examined based on 30 years of daily station data from 1981-2010. Composites are constructed for each of the eight phases of the MJO defined with the Real-time Multivariate MJO (RMM) index, using daily rainfall data from 44 stations over SL for four climatic seasons and comparing to similar results from a satellite-based rainfall product. Composites of lower tropospheric wind and convective anomaly are also investigated in order to examine how the local rainfall anomalies are associated with large-scale circulations. The greatest impact of the MJO on rainfall over SL occurs in the Second Inter-Monsoon (SIM) and Southwest Monsoon (SWM) seasons. Enhanced rainfall generally occurs over SL during RMM phases 2 and 3 when the MJO convective envelop is located in the Indian Ocean and conversely suppressed rainfall in phases 6 and 7. This rainfall impact is due to the direct influence of the MJO’s tropical convective anomalies and associated low-level circulations in the vicinity of SL. In contrast, the MJO influence during the Northeast Monsoon (NEM) season is slightly less than during the SWM and SIM seasons as a result of the southward shift of the MJO convective envelop during boreal winter. Occurrence of extreme rainfall events is most frequent during phase 2 in First Inter-Monsoon (FIM) phases 2 and 3 in SWM, phases 1, 2 and 3 in SIM and phases 2 and 3 in NEM seasons. The analysis of this study provides a useful reference of when and where the MJO has significant impacts on rainfall as well as extreme rainfall events during four climatic seasons in SL. This information can be used along with accurately predicted MJO phase by dynamical or statistical models, to improve extended range forecasting in SL.

Distinct Holocene precipitation trends over arid Central Asia and linkages to westerlies and Asian monsoon

The response of precipitation over arid Central Asia since the mid Holocene is long considered to be controlled by astronomical insolation-induced changes in the westerlies. However, it is still controversial in proxy and modeling studies whether the trend of Central Asian precipitation is in accordance with Asian monsoon or not. Compared to the monsoon, the westerlies-associated precipitation changes are complicated and also rely on regional mountain complex. In this study, we conducted a high-resolution transient experiment by a coupled climate model to evaluate the response of Central Asian precipitation. The simulated precipitation changes are distinctly different over regions although they are mainly associated with the westerlies. Since mid Holocene, the annual precipitation increases over Balkhash-Altai region but decreases over Turan Plain, Mongolia-Baikal and Tarim Basin, in all of which the summer precipitation contributes most. Following the summer insolation, the southward shift of westerly jet is accompanied by wave-like anomalies in the upper-tropospheric meridional winds across mid-latitude Eurasia (the Silk Road pattern), which results in distinct summer precipitation changes. The decreased precipitation over Tarim is mainly explained by weakened low-level easterly wind anomaly and less moisture supply from East Asia. In contrast, the responses of winter precipitation are limited even in the winter-rainfall-dominated regime due to the small insolation change. Such simulated complicated precipitation trends during the Holocene are in qualitative agreement with existing proxies, highlighting the modulation of Silk Road pattern on Central Asian rainfall on the orbital timescale.

Rapid humidity changes across the Northern South China Sea during the last ~40 kyrs

A key aspect of East Asian climate is its summer monsoonal system which influences nearly one-third of the world's population. Recent results indicate that the primary response of the East Asian summer monsoon (EASM) to anthropogenic forced climate warming may be a shift in geographical range instead of an intensity change, which would lead to spatial coexistence of floods and droughts over southeastern Asia. The predicted EASM variability in the future has made it paramount to study its past changes and the associated tempo-spatial pattern of aridity and humidity in its purview. In order to decipher past changes in EASM, we applied a multi-proxy geochemical approach to the sediment core ORI-891-16-P1 located in the northern South China Sea. The position of this sediment core on top of a seamount makes it uniquely sensitive to changes in the terrigenous input into northern South China Sea unbiased by sea level-induced downslope transport processes. Utilizing the ln(Ti/Ca) ratio throughout the sediment sequence we trace terrigenous influx changes reflecting EASM prevalence during the last ~40 kyrs. Based on the comparison of our results to previous studies we infer that the Last Glacial Maximum (LGM; ~ 20 ka BP) was characterized by a steep N-S humidity gradient. This spatial pattern was in line with a southward shift or contraction of the summer monsoonal trough of 10–15° from its current position toward the centre of the South China Sea. Superimposed on orbital time scale fluctuations we also find strong indication of millennial-scale variability related to Heinrich Stadials. The impact of Heinrich Stadials on the EASM seems amplified during insolation minima, while high summer insolation seems to buffer the monsoonal system to such perturbations. We infer that (i) the humidity-aridity distribution during the LGM mimics predictions of the proposed future EASM configuration, and (ii) that the sensitivity of the EASM to weakening in the Atlantic Meridional Overturning Circulation is the strongest since the last glacial.

Reconstructing ocean surface dynamics over the last ̴ 25 kyr at “Shackleton Site” IODP - U1385

The main objective of this study is to reconstruct surface water dynamics occurring along the Western Iberian Margin (WIM) relating them to the position of the Subpolar Front and to the dynamics affecting the Subtropical gyre. The study, centred on changes in calcareous nannoplankton paleo-assemblages at IODP Site U1385 (37°34.285′N; 10°7.562′W), allows to reconstruct the regional surface water circulation patterns, as well as the surface paleoproductivity fluctuations and the water column stratification between ̴ 25 and 0 ka. Abrupt changes in calcareous nannoplankton paleo-assemblages occurr at the transition from glacial to interglacial conditions. The glacial conditions (25.3–11.5 ka) are characterized by dominance, of cold calcareous nannoplankton species, suggesting prevalent Portugal Current (PC) influence and a shift southward of the Intertropical Convergence Zone (ITCZ), as consequence of Northern Hemisphere (NH) insolation minima. Whereas opposite conditions verified during the interglacial interval (11.5–0.84 ka), favour the proliferation of warm species because of a strong Iberian Poleward Current (IPC) influence in the entire area. The Last Glacial Maximum (LGM) is characterized by a moderate productivity together with a decreasing mixing and the establishment of a medium-deep nutricline. Subpolar and Subtropical Fronts migration southward during Heinrich stadial 1 (HS1) show low productivity conditions and a water column stratification. The Bølling/Allerød (B/A) is characterized by the northward extension of the Subtropical gyre favouring an increasing productivity that decreases during the Younger Dryas stadial (YD). The latter is characterized by a general cooling although less pronounced of that recorded for HS1. During the entire period, other than the major abundance of warm taxa suggesting a general prevalence of the IPC, a variable productivity and a water column stratification in particular at the beginning of the Holocene is shown.

Changes in anthropogenic particulate matters and resulting global climate effects since the Industrial Revolution

AbstractIn order to quantify air pollution effects on climate change, we investigated the climate response associated with anthropogenic particulate matters (PMs) by dividing fine PM (PM2.5, particle size ≤2.5 μm) and coarse particulate matter (CPM, particle size >2.5 μm) in great detail in this work, with an aerosol‐climate coupled model. We find that the changes in PM2.5 and CPM are very different and thus result in different, even opposite effects on climate, especially on a regional scale. The column burden of PM2.5 increases globally from 1850 to the present, especially over Asia's southern and eastern parts, whereas the column concentration of CPM increases over high‐latitude regions and decreases over South Asia. The resulted global annual mean effective radiative forcing (ERF) values due to PM2.5 and CPM changes are −1.21 W·m−2 and −0.24 W·m−2, respectively. Increases in PM2.5 result in significant cooling effects on the climate, whereas changes in CPM produce small and even opposite effects. The global annual mean surface air temperature (SAT) decreases by 0.94 K due to PM2.5 increase. Coolings caused by increased PM2.5 are more apparent over Northern Hemisphere (NH) terrain and ocean at mid‐ and high latitudes. Increases in SATs caused by increased CPM are identified over high latitudes in the NH, whereas decreases are identified over mid‐latitude regions. Strong cooling due to increased PM2.5 causes a southward shift of the Intertropical Convergence Zone (ITCZ), whereas the Hadley circulation associated with CPM is enhanced slightly over both hemispheres, along with the weak movement of corresponding ITCZ. The global annual mean precipitation decreases by approximately 0.11 mm day−1 due to the increased PM2.5. Generally, PM2.5 concentration changes contribute more than 80% of the variation caused by all anthropogenic aerosols in ERF, SAT, cloud fraction, and precipitation.

On the contribution of fast and slow responses to precipitation changes caused by aerosol perturbations

Abstract. Changes in global-mean precipitation are strongly constrained by global radiative cooling, while regional rainfall changes are less constrained because energy can be transported. Absorbing and non-absorbing aerosols have different effects on both global-mean and regional precipitation, due to the distinct effects on energetics. This study analyses the precipitation responses to large perturbations in black carbon (BC) and sulfate (SUL) by examining the changes in atmospheric energy budget terms on global and regional scales, in terms of fast (independent of changes in sea surface temperature, SST) and slow responses (mediated by changes in SST). Changes in atmospheric radiative cooling/heating are further decomposed into contributions from clouds, aerosols, and clear–clean sky (without clouds or aerosols). Both cases show a decrease in global-mean precipitation, which is dominated by fast responses in the BC case and slow responses in the SUL case. The geographical patterns are distinct too. The intertropical convergence zone (ITCZ), accompanied by tropical rainfall, shifts northward in the BC case, while it shifts southward in the SUL case. For both cases, energy transport terms from the slow response dominate the changes in tropical rainfall, which are associated with the northward (southward) shift of the Hadley cell in response to the enhanced southward (northward) cross-equatorial energy flux caused by increased BC (SUL) emission. The extra-tropical precipitation decreases in both cases. For the BC case, fast responses to increased atmospheric radiative heating contribute most to the reduced rainfall, in which absorbing aerosols directly heat the mid-troposphere, stabilise the column, and suppress precipitation. Unlike BC, non-absorbing aerosols decrease surface temperatures through slow processes, cool the whole atmospheric column, and reduce specific humidity, which leads to decreased radiative cooling from the clear–clean sky, which is consistent with the reduced rainfall. Examining the changes in large-scale circulation and local thermodynamics qualitatively explains the responses of precipitation to aerosol perturbations, whereas the energetic perspective provides a method to quantify their contributions.

Greenland Blocking Promotes Subtropical North Atlantic Spring Blooms

AbstractClimate patterns drive broad‐scale patterns of ocean ecosystem productivity. However, the mechanistic connections between regional climate variability and ocean ecosystem responses are not well understood for much of the ocean. By analyzing multi‐decadal atmospheric and oceanic data over the North Atlantic Ocean, we show that the subtropical spring bloom variability is connected to winter Greenland blocking events. The blocking enforces a southward shift of the eddy‐driven jet stream, inducing higher surface westerly winds, causing deeper vertical mixing in the subtropical North Atlantic Ocean in winter and promotes larger spring blooms. These findings reveal the physical mechanisms connecting a predominant climate variability mode in the Arctic with variability in the subtropical spring bloom and highlight the importance of ocean‐atmosphere coupling in shaping the response of oceanic ecosystems to climate variability. Our results also imply a potential teleconnection between rapid Arctic warming over Greenland and phytoplankton variability in the subtropical North Atlantic.

The influence of variability in meridional over turning on global ocean circulation

AbstractThe Atlantic Meridional Overturning circulation (AMOC) is an important global scale circulation and changes in AMOC can induce significant regional and global climate impacts. Here we study the stability of AMOC and its influence on global ocean circulation and the surface climate though analyzing a set of sensitivity experiments using the Community Climate System Model version 1 (CESM1). Results show that a collapsed AMOC can induce changes in global ocean circulation, such as reduced (or reversed) Bering Strait transport, weakened Indonesia throughflow and Agulhas current, but strengthened Drake Passage transport. It also changes the global wind pattern and surface temperature, such as a seesaw-like surface temperature change between Northern and Southern Hemispheres, a weakening of Indian-Australia summer monsoon, a southward shift of the Southern ocean westerlies, etc. We also found that AMOC and the Pacific deep meridional overturning circulation (or PMOC) do not form a natural seesaw under modern day climate and geography. A collapsed AMOC (active PMOC) is not stable under modern conditions if there is no additional freshwater (salt) input in the subpolar North Atlantic (Pacific), suggesting that the modern mean state of AMOC (PMOC) does not depend on local haline forcing although its variability and changes do.

Data‐Model Comparisons of Tropical Hydroclimate Changes Over the Common Era

AbstractWe examine the evidence for large‐scale tropical hydroclimate changes over the Common Era based on a compilation of 67 tropical hydroclimate records from 55 sites and assess the consistency between the reconstructed hydroclimate changes and those simulated by transient model simulations of the last millennium. Our synthesis of the proxy records reveals several regionally coherent patterns on centennial time scales. From 800 to 1000 CE, records from the eastern Pacific and parts of Mesoamerica indicate a pronounced drying event relative to background conditions of the Common Era. In addition, 1400–1700 CE is marked by pronounced hydroclimate changes across the tropics, including dry and/or isotopically enriched conditions in South and East Asia, wet and/or isotopically depleted conditions in the central Andes and southern Amazon in South America, and fresher and/or isotopically depleted conditions in the Maritime Continent. We find notable dissimilarities between the regional hydroclimate changes and global‐scale and hemispheric‐scale temperature reconstructions, indicating that more work needs to be done to understand the mechanisms of the widespread tropical hydroclimate changes during the LIA. Apropos to previous interpretations of large‐scale reorganization of tropical Pacific climate during the LIA, we do not find support for a large‐scale southward shift of the Pacific Intertropical Convergence Zone, while evidence for a strengthened Pacific Walker Circulation and/or an equatorward contraction of the monsoonal Asian‐Australian rain belt exists from limited geographic regions but require additional paleoclimate constraints. Transient climate model simulations exhibit weak forced long‐term tropical rainfall changes over the last millennium but provide several important insights to the proxy reconstructions.

Impact of an accelerated melting of Greenland on malaria distribution over Africa

Studies about the impact of future climate change on diseases have mostly focused on standard Representative Concentration Pathway climate change scenarios. These scenarios do not account for the non-linear dynamics of the climate system. A rapid ice-sheet melting could occur, impacting climate and consequently societies. Here, we investigate the additional impact of a rapid ice-sheet melting of Greenland on climate and malaria transmission in Africa using several malaria models driven by Institute Pierre Simon Laplace climate simulations. Results reveal that our melting scenario could moderate the simulated increase in malaria risk over East Africa, due to cooling and drying effects, cause a largest decrease in malaria transmission risk over West Africa and drive malaria emergence in southern Africa associated with a significant southward shift of the African rain-belt. We argue that the effect of such ice-sheet melting should be investigated further in future public health and agriculture climate change risk assessments.

Pacific North Equatorial Current bifurcation latitude and Kuroshio Current shifts since the Last Glacial Maximum inferred from a Sulu Sea thermocline reconstruction

The meridional migration of the bifurcation latitude of the Pacific North Equatorial Current (NEC) in the western boundary of the tropical Pacific modulates the strength of the Kuroshio Current. Using salinity reanalysis data, we show the NEC bifurcation latitude also acts as the dominant control on thermocline salinity of the Sulu Sea, just west of the Philippine archipelago, by regulating influx of western Pacific thermocline water via the Luzon Strait. We used oxygen isotopes (δ18O) and Mg/Ca in the thermocline-dwelling foraminifera Globorotalia tumida from Sulu Sea sediment core MD97-2141 to determine past thermocline δ18Ow and salinity variability spanning ∼20–5 ka with an average sampling interval of ∼50 years and infer past changes in the NEC bifurcation latitude. Our Sulu Sea thermocline reconstruction reveals high salinity from ∼18.8–15.5 ka, ∼12.2–11.5 ka, and from ∼9.5–8.5 ka indicating the NEC bifurcation latitude was shifted north and the Kuroshio was weak at those times. Low Sulu Sea thermocline salinity from ∼13.0–12.4 ka, ∼11.5–10.9 ka, and from ∼8.5 ka until the end of the record at ∼5.6 ka indicates the NEC bifurcation latitude was shifted south and the Kuroshio Current was relatively strong. Comparison to other paleoclimate records suggests the observed northward (southward) shifts of the NEC bifurcation latitude were driven by southward (northward) shifts of the Indo-Pacific ITCZ, consistent with modern mechanisms controlling interannual NEC bifurcation variability. The NEC bifurcation latitude shifts likely modulated northward energy transport via the Kuroshio Current and the mean temperature and salinity of the Indonesian Throughflow.

Influence of atmospheric circulation on glacier mass balance in western Tibet: an analysis based on observations and modeling

AbstractGlacier changes on the Tibetan Plateau (TP) have been spatially heterogeneous in recent decades. The understanding of glacier mass changes in western Tibet, a transitional area between the monsoon-dominated region and the westerlies-dominated region, is still incomplete. For this study, we used an energy-mass balance model to reconstruct annual mass balances from October 1967 to September 2019 to explore the effects of local climate and large-scale atmospheric circulation on glacier mass changes in western Tibet. The results showed Xiao Anglong Glacier is close to a balanced condition, with an average value of -53±185 mm w.e. a-1 for 1968-2019. The interannual mass balance variability during 1968-2019 was primary driven by ablation-season precipitation, which determined changes in the snow accumulation and strongly influenced melt processes. The interannual mass balance variability during 1968-2019 was less affected by ablation-season air temperature, which only weakly affected snowfall and melt energy. Further analysis suggests that the southward (or northward) shift of the westerlies caused low (or high) ablation-season precipitation, and therefore low (or high) annual mass balance for glaciers in western Tibet. In addition, the average mass balance for Xiao Anglong Glacier was 83±185, -210±185, and -10±185 mm w.e. a-1 for 1968-1990, 1991-2012, and 2013-2019, respectively. These mass changes were associated with the variations in precipitation and air temperature during the ablation season on interdecadal time scales.

Early Holocene greening of the Sahara requires Mediterranean winter rainfall

The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 y ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in Early to Middle Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. Here, we present an 18,500-y pollen and leaf-wax δD record from Lake Tislit (32° N) in Morocco, which provides quantitative reconstruction of winter and summer precipitation in northern Africa. The record from Lake Tislit shows that the northern Sahara and the Mediterranean region were wetter in the AHP because of increased winter precipitation and were not influenced by the monsoon. The increased seasonal contrast of insolation led to an intensification and southward shift of the Mediterranean winter precipitation system in addition to the intensified summer monsoon. Therefore, a winter rainfall zone must have met and possibly overlapped the monsoonal zone in the Sahara. Using a mechanistic vegetation model in Early Holocene conditions, we show that this seasonal distribution of rainfall is more efficient than the increased monsoon alone in generating a green Sahara vegetation cover, in agreement with observed vegetation. This conceptual framework should be taken into consideration in Earth system paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.