Southward Shift Research Articles

The effect of cross-scale modulation of synoptic systems by the cold air pool on autumn nighttime rainfall over Hainan Island

This study uses numerical simulations to investigate the impact of cross-scale interaction between the cold air pool (CAP) and the synoptic systems on autumnal nighttime rain over Hainan Island. The results show that the precipitation formed initially via the convergence of the southerly wind from a mesoscale vortex located around the Beibu Gulf and the sea breeze along the northwest coast of Hainan Island in the afternoon. Subsequently, this precipitation propagated eastward due to the weakening and southward movement of the vortex, which was promoted by the CAP that formed in the lower troposphere within 2 h after rainfall. The CAP weakened the vortex by forming a sub-2-km divergent wind and accelerating upward propagation of the positive potential vorticity. And the CAP promoted the southward movement of the vortex by increasing the sea level pressure on its northeast side and stimulating a cyclonic disturbance on its southeast side, which was also conducive to the southward shift of cold air from the Chinese mainland. When the cold air arrived at Hainan Island at night, it was deepened by the blocking effect of the sub-2-km divergent wind. The superposition of the blocked cold air and the CAP stimulated a strong northerly land breeze, which merged with the southerly wind to reinvigorate rainfall in the eastern coastal area. This paper reveals the mechanism by which the CAP affects the heavy rainfall at night on Hainan Island in autumn under the combined influence of the tropical disturbance-related vortex and the cold air from the north.

The role of SST forcing in the interdecadal variations of the Pacific‐Japan pattern in the late 1990s

AbstractThe Pacific‐Japan (PJ) pattern is the dominant teleconnection over East Asia, closely related to the summer precipitation on various time scales. However, studies on the connection between the interdecadal variations of the PJ pattern and sea surface temperature (SST) forcing are still limited. This study indicates that the relationship between the PJ pattern and ENSO presents a pronounced interdecadal change around 1997/1998. The PJ pattern shows a baroclinic structure over the northwest Pacific and a barotropic structure near Japan during 1979–1997, while its baroclinic structure in the tropics weakens remarkably during 1998–2021, with a southward shift of its midlatitude barotropic structure. As a result, the connection between the PJ pattern and summer precipitation over East Asia exhibits an interdecadal transition in the late 1990s. In the slow La Niña decaying summer of 1979–1997, the cold SST anomalies in the tropical Indian Ocean (TIO) excite an eastward‐propagating Kelvin wave that enhances convection over the northwest Pacific, resulting in the formation of the PJ pattern. In contrast, the cold SST anomalies in the Maritime Continent rather than the TIO yield a zonally confined Kelvin wave in the rapid La Niña decaying summer of 1998–2021. The elongated Kelvin wave penetrating the central Pacific weakens convective anomalies over the northwest Pacific, thereby weakening the baroclinic structure of the PJ pattern in the tropics. Meanwhile, the warm SST anomalies in the Kuroshio Extension (KE) induce a northward shift of the East Asian subtropical westerly jet, with the anomalous anticyclonic circulation over the KE significantly affecting summer precipitation along the Meiyu belt. Therefore, the midlatitude barotropic structure of the PJ pattern shifts southward after the late 1990s.

Kalimantan hydroclimate variability since the last glacial period

Changes in convective activity and hydroclimate over Northeastern Kalimantan are key features to understand glacial to interglacial climate evolution in the center of the West Pacific Warm Pool during the Late Pleistocene to Holocene. We use high-resolution X-ray fluorescence (XRF) scanner-derived elemental ratios in sediment Core SO217-18522 (1º 24.106’ N, 119º 4.701’E, 975 m water depth) recovered from the northern Makassar Strait to reconstruct changes in precipitation-related weathering and erosion over Northeastern Kalimantan over the last 50 kyr. Enhanced seasonality of rainfall and an extended dry season during Heinrich Stadials (HS4 to HS1) and the Younger Dryas (YD) suggest weakening of the tropical convection associated with a southward shift of the tropical rain belt and the annual mean position of the Intertropical Tropical Convection Zone. Increasing sediment discharge and intensification of convective activity occurred during the early to mid-Holocene during an interval of high Northern Hemisphere insolation, elevated atmospheric pCO2 levels and global warming. Our reconstructions in comparison with regional terrestrial and marine records highlight the high spatial variability of Kalimantan hydroclimate on millennial to glacial-interglacial timescales.

The Physical Processes Dominating the Impact of the Summer North Atlantic Oscillation on the Eastern Tibetan Plateau Summer Rainfall

Abstract The summer North Atlantic Oscillation (SNAO), an important climate signal in regulating the interannual variability of Tibetan Plateau (TP) summer rainfall, is closely related to a meridional precipitation dipole pattern between the southeastern and northeastern TP. In this study, based on diagnoses of observations and multiple realizations of the CESM2 historical simulation, we find that there are fundamental differences between the formation processes dominating the SNAO-related summer rainfall anomaly in the southeastern and northeastern TP. An atmospheric moisture budget analysis reveals that the anomalous vertical (horizontal) moisture advection makes the largest contribution to the southeastern (northeastern) TP summer rainfall anomaly. During the negative phase of SNAO, the increased precipitation in the southeastern TP is related to the anomalous ascending flows, which are driven by two processes according to the moist static energy budget. The first is the southward shift of the subtropical westerly jet stream, which produces positive anomalous zonal advection of the climatological moist enthalpy in the upper-middle troposphere over the southeastern TP. The second is related to the enhanced transport of anomalous warm moist air to the south of TP, which produces positive anomalous meridional advection of anomalous moist enthalpy into the lower troposphere over the southeastern TP under the control of climatological monsoonal meridional circulations. This positive moist enthalpy advection enhances the atmospheric moist static energy and facilitate enhanced local convection. For the northeastern TP, the decreased precipitation is dominated by negative anomalous horizontal moisture advections due to the SNAO-induced equivalent-barotropic anomalous cyclone near the eastern edge of the TP.

Wind‐Driven Seasonal Intrusion of Modified Circumpolar Deep Water Onto the Continental Shelf in Prydz Bay, East Antarctica

AbstractIntrusion of warm modified Circumpolar Deep Water (mCDW) is critical to heat transport onto the continental shelf around Antarctica, with substantial impacts on basal melting of ice shelves and sea ice production. We use hydrographic data collected in Prydz Bay, East Antarctica, and atmospheric and ocean reanalyses to investigate the seasonality of mCDW intrusions onto the East Antarctic continental shelf. Mooring measurements confirm that there is significant intrusion of mCDW over the inner continental shelf of Prydz Bay in March–July (austral autumn and early winter). The warming signal of intruded mCDW over the inner shelf is correlated with wind changes north of the shelf break, with a significant lag. This suggests that the autumn‐winter mCDW intrusions over the inner shelf are significantly affected by the wind regime north of the shelf break in January–May, when a southward displacement of the westerly winds occurs. The southward shift of westerly winds drives the shallowest depth of upwelled mCDW to move poleward and contributes more warm water to the shelf break. This results in the shoaling of mCDW near the shelf break in January–May and promotes mCDW intrusions onto the continental shelf. The dynamic barrier imposed on mCDW intrusions by the strong Antarctic Slope Front in January–May is offset by this shoaling of mCDW. This study indicates the strong sensitivity of continental shelves in East Antarctica to atmospheric forcing changes over the Southern Ocean.

Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2

The Oceanic Anoxic Event 2 (OAE 2; ca. ∼94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO2 drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (△εHfclay) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (∼62°S) during the late Cretaceous. The reliability of △εHfclay as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions. At Site U1516, the early stage of OAE 2 was characterized by relatively low △εHfclay values (−5.9 ± 2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in △εHfclay points towards accelerated export of poorly weathered sediments resulting from the abrupt reactivation of river systems in southwest Australia. This period was followed by a pronounced △εHfclay shift towards positive values, indicative of intensifying chemical weathering conditions during the OAE 2 interval showing the highest δ13C anomaly. Based on these results, we posit that enhanced hydrological cycle, most likely caused by a southward shift of the westerlies, led to a large increase in weatherability at southern high latitudes during peak OAE 2 warmth. This finding provides empirical support for the potential role played by high-latitude weathering systems in driving the termination of OAE 2, via weathering-driven consumption of atmospheric CO2 and accelerated riverine fluxes of nutrients leading to enhanced organic carbon burial in marine sediments.

Investigating the effect of the Tibetan Plateau on the ITCZ using a coupled Earth system model

The effect of the Tibetan Plateau (TP) on the Intertropical Convergence Zone (ITCZ) was investigated using a coupled Earth system model. The location of the ITCZ (in this work represented by the center of the tropical precipitation maximum) over the tropical Atlantic was found to be sensitive to the existence of the TP. Removing the TP led to a remarkable sea surface temperature (SST) cooling (warming) in the Northern (Southern) Hemisphere, which manifested clearly in the Atlantic rather than the Pacific. The locations of maximum precipitation and SST moved southwards clearly in the tropical Atlantic, forcing a southward shift of the atmospheric convection center, and thus the ITCZ. The shift in the ITCZ was also supported by the latitudinal change in the ascending branch of the tropical Hadley Cell, which moved southwards by about 2° in the boreal summer in response to the TP's removal. From the viewpoint of the energy balance between the two hemispheres, the cooling (warming) in the Northern (Southern) Hemisphere requires an enhanced northward atmospheric heat transport across the equator, which can be realized by the southward displacement of the ITCZ. This study suggests that the presence of the TP may have played an important role in the climatology of the ITCZ, particularly its location over the tropical Atlantic.摘要本文利用耦合地球气候系统模式研究了青藏高原对热带辐合带 (ITCZ) 的影响. 我们研究发现热带大西洋ITCZ的位置对青藏高原存在与否有明显的敏感性. 与目前真实情况相比, 移除青藏高原会导致北半球海面降温, 南半球海面升温. 这种海面温度变化在大西洋表现得尤为明显, 导致热带大西洋最大海温中心向南移动, 从而迫使大气对流中心向南移动, 即表现为ITCZ的南移. 相应地, 夏季热带大气Hadley环流的上升支也发生明显南移. 北 (南) 半球海洋变冷 (变暖) 这种态势要求增强跨赤道向北的大气经向热量输送, 从而维持各个半球的能量平衡, 而这需要ITCZ位置的南移才能实现. 本文研究表明, 青藏高原的存在在现今ITCZ气候态的形成中可能扮演了重要角色.

Multi-decadal changes in the at-sea distribution and abundance of black-browed and light-mantled sooty albatrosses in the southwest Pacific Ocean

Abstract Many long-term studies have reported changes in seabird abundance and distribution in response to climate change and various anthropogenic activities. However, a greater understanding of how species are responding to change over large spatial and temporal scales are required—particularly at high latitudes such as the Southern Ocean. We examined black-browed Thalassarche melanophris (BBAL) and light-mantled sooty Phoebetria palpebrata albatross (LMSA) observations spanning over 50 years. Both species have a wide-ranging distribution in a rapidly changing Southern Ocean. We used generalized additive models (GAMs) to investigate environmental drivers of their abundance and occurrence. Our results show that climate indices, sea surface temperature and sea surface height are the main drivers influencing the distribution and abundance of both species. The abundance of BBAL southeast of Australia was observed to be decreased substantially whereas no significant change was observed in the abundance of LMSA. Both species demonstrated contrasting distributions along their latitudinal gradient with BBAL showing early stages of a southward range shift. Our analyses suggest that responses to climate change are species-specific. These rare, long-term data have provided an understanding of species’ responses to past changes in the marine environment and can provide critical information for future conservation and management.

Responses of Global Atmospheric Energy Transport to Idealized Groundwater Conditions in a General Circulation Model

Abstract The representation of groundwater dynamics in land surface models and their roles in global precipitation variations has received attention in recent years. Studies have revealed the overall higher soil moisture but rather diverse precipitation changes after incorporating the groundwater component in climate models. However, groundwater effects on large-scale atmospheric energy transport, the fundamental atmospheric variable regulating Earth’s climate, have not been explored thoroughly. In this study, a pair of idealized experiments corresponding to contrast globally fixed water table depths by AMIP-type simulations in the Community Earth System Model was conducted. In the wet (shallow water table) experiments, an increased meridional surface temperature gradient makes the mean meridional energy transports and Hadley circulation stronger than dry (deep water table) experiments over the tropics. Such energy transport changes are primarily attributed to the dynamic contribution (intensified Hadley circulation). The wet experiments make the simulated world be like an aquaplanet simulation with less land–sea temperature contrast and the enhancement (reduction) of mean meridional circulation (stationary eddies) energy transports. Furthermore, the South Asian monsoon circulation in the wet experiment shows a southward shift in the premonsoon season (April–June) and slight weakening in the mature phase (July and August). This study explores the impacts of the soil conditions caused by various water table depths on global energy transport and has further implications for climate model developments and experiment designs.

Decadal changes in the basin-wide heat budget of the mid-latitude North Pacific Ocean

Research has indicated that the meridional heat transport (MHT) in the North Pacific Ocean (NP) across 24°N increased in the 1980s and 1990s, resulting in different heat distributions: the ocean heat content (OHC) increased in the 1980s, while the net surface heat release was strengthened in the 1990s; however, the reasons for these differences remained unclear. The authors revisited the investigation of the heat budget in the NP mainly using hydrographic observations to understand why the heat distribution was different between these 2 decades and extend the analysis to the 2010s. The OHC in the upper 700 m north of 24°N and east of 137°E exhibited sharp increases around 1990 as well as in the 2010s, while it was nearly stable in between. The northward retreat of the subarctic gyre boundary coincided with the spin-up of the subtropical gyre in the late 1980s, thereby allowing warm anomaly from the subtropics to propagate northeastward. Meanwhile, the concurrent weakening of the wintertime westerlies resulted in the suppression of the surface heat loss in the western NP. In contrast, the southward shift of the subarctic front suppressed the OHC rise, despite the MHT increase in the late 1990s. In the 2010s, unprecedented warming occurred in the eastern NP. The MHT estimation based on hydrographic observations indicates that the net surface release must have been suppressed since the MHT did not increase; however, the latest atmospheric reanalysis datasets failed to reproduce this.

On the temporal clustering of European extreme precipitation events and its relationship to persistent and transient large-scale atmospheric drivers

Extreme precipitation events that occur in close succession can have important societal and economic repercussions. Here we use 42 years of reanalysis data (ERA-5) to investigate the link between Euro-Atlantic large-scale pattern of weather and climate variability and the temporal clustering of extreme rainfall events over Europe. We implicitly model the seasonal rate of extreme occurrences as part of a Poisson General Additive Model (GAM) using cyclic regression cubic splines. The smoothed seasonal rate of extreme rainfall occurrences is used to (i) infer the frequency of significant temporal clustering and (ii) implicitly serves as the baseline rate when modeling the effects of atmospheric drivers on extreme rainfall clustering. We use GAMs to model the association between the temporal clustering of extreme rainfall events and seven predominant year-round weather regimes in the Euro-Atlantic sector as well as a measure of synoptic-scale transient recurrent Rossby wave packets. Sub-seasonal clustering of precipitation events is significant at all grid-points over Europe; the proportion of extreme rainfall events that cluster in time ranges between 2% to 27%. The most relevant weather regime is the Atlantic Trough (corresponding to NAO+ with a southward shift of the jet) explaining most of the significant increase in clustering probability over Europe. The Greenland Blocking regime explains most of the clustering over the Iberian Peninsula. The Scandinavian Blocking regime is associated with a significant increase in clustering probability over the western Mediterranean, with a northwards shift in the signal to central Europe in summer.

Inferring probable distributional gaps and climate change impacts on the medically important viper Echis leucogaster in the western Sahara-Sahel: An ecological niche modeling approach

Abstract. Bouam I, Khelfaoui F, Saoudi M. 2022. Inferring probable distributional gaps and climate change impacts on the medically important viper Echis leucogaster in the western Sahara-Sahel: An ecological niche modeling approach. Biodiversitas 23: 5175-5183. Knowledge of biodiversity distribution and how climate change may affect species across the Sahara-Sahel is scarce despite it harboring both high biodiversity and a high rate of endemism. As ectotherms, snakes are particularly vulnerable to climate change and susceptible to range shifts and demographic changes driven by climate change. Ecological niche models are a common method for predicting the probability of the occurrence of species and future range shifts induced by climate change. This study examines the probable gaps in the distribution of the white-bellied saw-scaled viper, Echis leucogaster, and the potential influence of climate change on its future geographic range in the western Sahara-Sahel. The currently predicted environmentally suitable areas fitted well with the known geographical range of the species showed relative congruence with the Sahara-Sahel ecoregion delineations and identified areas without known occurrences. In the future, the environmental conditions for the occurrence of E. leucogaster are predicted to increase, as the environmentally suitable areas will potentially experience an increase in their proportion. Future projections also showed that the potentially suitable areas might undergo moderate southward shifts during the late twenty-first century. The results of the present study significantly expand our knowledge on the potential distribution of E. leucogaster and provide valuable insights to guide future sampling efforts and conservation planning for the species.

Deforestation Drives Desiccation in Global Monsoon Region

AbstractGlobal monsoon system is a major component of the Earth's climate system and carries about two‐thirds of the world's population. How large‐scale deforestation induced changes in water cycle and energy balance may modify global monsoon system and its underlying mechanisms are not well understood. Here, we use simulations of 11 models participating in the Land Use Model Intercomparison Project in the framework of the Coupled Model Intercomparison Project, phase 6 to study the response of global monsoon precipitation and atmospheric circulation to large‐scale deforestation. Global deforestation of about 38% induces annual precipitation reduction of about −15.01 mm yr−1 (Multi‐Model Ensemble) in global monsoon regions and −26.15 mm yr−1 in land monsoon regions. The precipitation reduction is particularly pronounced in the Northern Hemisphere Summer Monsoon (NHSM) regions with decreased precipitation by 16.83 mm in the summer monsoon season from May to September along with weakened circulation intensity. Deforestation causes asymmetrical temperature response in the northern high latitudes and Southern Hemisphere Tropics, leading to decreased hemispheric thermal contrast, which reduces the meridional pressure gradient and weakens the convergence of cross‐equatorial flows into the NHSM trough regions, thereby decreasing the NHSM circulation and precipitation intensity. The deforestation‐induced hemispheric temperature asymmetry also increases the northward cross‐equatorial atmosphere heat transport (c‐eq AHT) that drives southward shift of Intertropical Convergence Zone, which suppresses the NHSM precipitation. Our results show that the drying monsoon region in response to deforestation is fairly consistent among the models implying the adverse effects of large‐scale deforestation beyond the region of land use change.

Large-scale climate response to regionally confined extratropical cooling: effect of ocean dynamics

This study investigates the effect of ocean dynamics on the tropical climate response to localized radiative cooling over three northern extratropical land regions using hierarchical model simulations that vary in the degree of ocean coupling. Without ocean dynamics, the tropical climate response is independent of the extratropical forcing location, characterized by a southward tropical precipitation shift with a high degree of zonal symmetry, a reduced zonal sea surface temperature gradient along the equatorial Pacific, and the eastward-shifted Walker circulation. When ocean dynamical adjustments are allowed, the zonal-mean tropical precipitation shift is damped primarily via Eulerian-mean ocean heat transport. The oceanic damping effect is strongest (weakest) for North Asian (American) cooling, associated with the largest (smallest) Eulerian-mean ocean heat transport across the equatorial Pacific. The cross-equatorial ocean heat transport in the Pacific is anchored to the North Pacific subtropical high, the response of which can be inferred from the corresponding slab ocean simulations. Hence, the slab ocean simulations provide useful a priori prediction for oceanic damping efficiency. Ocean dynamics also modulates the spatial pattern of climate response in a distinct manner depending on the zonal distribution of imposed forcing. North Asian forcing induces a pronounced eastern equatorial Pacific cooling extending to the western basin, accompanying the westward shifted Walker circulation. European forcing causes cooling confined to the eastern equatorial Pacific and strengthens the Walker circulation. The tropical precipitation response in these two cases exhibits large zonal variations with a high degree of equatorial symmetry, being essentially uncorrelated with the corresponding slab ocean simulations. By contrast, North American forcing induces a sufficiently strong inter-hemispheric contrast in the tropical Pacific SST response, due to the relatively weak oceanic damping effect, producing a weaker but spatially similar tropical response to that in the slab ocean simulation. This study demonstrates that the effect of ocean dynamics in modulating the tropical climate response depends on the extratropical forcing location. The results are relevant for understanding the distinct climate response induced by aerosols from different continental sites.

Strengthened connection between meridional location of winter polar front jet and surface air temperature since the mid-1990s

This study reveals a marked enhancement in the relationship between the variations in location of the winter East Asian Polar Front Jet (EAPJ) and the surface air temperature (SAT) in Eurasia since the mid-1990s. Before the mid-1990s, an evident wave train related to the meridional location of the EAPJ exhibited an anticyclonic anomaly over northern Europe and a cyclonic anomaly in northwestern Asia. With an equatorward shift of the EAPJ after the mid-1990s, the wave train experiences a notable adjustment that is conducive to East Asian cooling, displaying an anticyclonic anomaly around the Kara-Laptev Seas and a cyclonic anomaly near northeastern Asia. Arctic warming anomalies and sea ice loss contributed significantly to these decadal changes. Simulation experiments forced by observed Arctic sea-ice variability further confirm this result. Since the mid-1990s, Arctic sea ice loss (or Arctic warming anomaly) has contributed to a reduction in westerly winds in high latitudes by modulating the meridional temperature gradient. The deaccelerated winds intensify the Arctic cold air propagating to the south, enhancing the atmospheric baroclinicity and the westerly flow in the upper level at the south side of the EAPJ, favoring the southward shift of the EAPJ. With the equatorward shift of the EAPJ, the corresponding SAT anomalies in East Asia are more salient.

Impact of an acceleration of ice sheet melting on monsoon systems

Abstract. The study of past climates has demonstrated the occurrence of Heinrich events during which major ice discharges occurred at the polar ice sheet, leading to significant additional sea level rise. Heinrich events strongly influenced the oceanic circulation and global climate. However, standard climate change scenarios (Representative Concentration Pathways or RCPs) do not consider such potential rapid ice sheet collapse; RCPs only consider the dynamic evolution of greenhouse gas emissions. We carried out water-hosing simulations using the Institute Pierre Simon Laplace global Climate Model (IPSL-CM5A) to simulate a rapid melting of the Greenland and Antarctic ice sheets, equivalent to +1 and +3 m additional sea level rise (SLR). Freshwater inputs were added to the standard RCP8.5 emission scenario over the 21st century. The contribution to the SLR from Greenland or from Antarctic ice sheets has differentiated impacts. The freshwater input in the Antarctic is diluted by the circumpolar current, and its global impact is moderate. Conversely, a rapid melting of the ice sheet in the North Atlantic slows down the Atlantic Meridional Overturning Circulation. This slowdown leads to changes in winds, inter-hemispheric temperature and pressure gradients, resulting in a southward shift of the tropical rain belt over the Atlantic and eastern Pacific region. The American and African monsoons are strongly affected and shift to the south. Changes in the North American monsoon occur later, while changes in the South American monsoon start earlier. The North African monsoon is drier during boreal summer, while the southern African monsoon intensifies during austral summer. Simulated changes were not significant for the Asian and Australian monsoons.

Evaluating uncertainty in aerosol forcing of tropical precipitation shifts

Abstract. An observed southward shift in tropical rainfall over land between 1950 and 1985, followed by a weaker recovery post-1985, has been attributed to anthropogenic aerosol radiative forcing and cooling of the Northern Hemisphere relative to the Southern Hemisphere. We might therefore expect models that have a strong historic hemispheric contrast in aerosol forcing to simulate a further northward tropical rainfall shift in the near-term future when anthropogenic aerosol emission reductions will predominantly warm the Northern Hemisphere. We investigate this paradigm using a perturbed parameter ensemble (PPE) of transient coupled ocean–atmosphere climate simulations that span a range of aerosol radiative forcing comparable to multi-model studies. In the 20th century, in our single-model ensemble, we find no relationship between the magnitude of pre-industrial to 1975 inter-hemispheric anthropogenic aerosol radiative forcing and tropical precipitation shifts. Instead, tropical precipitation shifts are associated with major volcanic eruptions and are strongly affected by internal variability. However, we do find a relationship between the magnitude of pre-industrial to 2005 inter-hemispheric anthropogenic aerosol radiative forcing and future tropical precipitation shifts over 2006 to 2060 under scenario RCP8.5. Our results suggest that projections of tropical precipitation shifts will be improved by reducing aerosol radiative forcing uncertainty, but predictive gains may be offset by temporary shifts in tropical precipitation caused by future major volcanic eruptions.

Impacts of the PMIP4 ice sheets on Northern Hemisphere climate during the last glacial period

This study comprehensively investigates the impacts on the mean state of the Last Glacial Maximum (LGM) climate, particularly atmospheric circulation over the Northern Hemisphere associated with the different Paleoclimate Modelling Intercomparison Project Phase 4 (PMIP4) ice sheets, ICE-6G_C, GLAC-1D, and PMIP3, using the coupled atmosphere–ocean–vegetation model HadCM3B-M2.1aD. The simulation with PMIP3 ice sheets is colder than either of the two PMIP4 ice sheets mainly because of the larger area of land ice impacting surface albedo. However, changes in the circulation impact sea ice cover resulting in the GLAC-1D simulation being almost as cold. Although the PMIP4 ice sheets also induce different responses in the atmospheric circulation, some common features are identified in all simulations, including strengthening and lateral expansion of the winter upper-level North Atlantic jet with a large southwest-northeast tilt and summertime North Pacific jet, a southward shift of the wintertime Icelandic Low and Azores High and the summertime Pacific High. Compared to terrestrial-ocean reconstructions, all the PMIP4 ice sheet experiments overestimate the LGM cooling and wet conditions. The simulation with the ICE-6G_C ice sheet provides the closest reproduction of LGM climate, while the simulation with the PMIP3 ice sheet shows the coldest LGM climate state. Our study shows that in order to "benchmark" the ability of climate models to realistically simulate the LGM climate, we need to have reliable boundary conditions to ensure that any model biases are caused by model limitations rather than uncertainty about the LGM boundary conditions.

Intensification of Asian dust storms during the Mid-Pliocene Warm Period (3.25–2.96 Ma) documented in a sediment core from the South China Sea

Dust storms are an important component of the global climate system. At the same time, they also bear a risk for human health by causing pulmonary diseases. Today, East Asian dust storms account for as much as half of the global dust emissions and temporarily affect highly populated areas. Therefore, understanding their mechanisms and predicting their evolution under warmer near-future climate conditions is of major interest. The mid-Pliocene Warm Period (mPWP; 3.264–3.025 Ma) is considered one of the best analogues from the past for anthropogenic climate change. Consequently, understanding the climate dynamics and associated environmental change during the mPWP can help with predicting the environmental effects of warmer-than-present climates. In order to reconstruct Asian dust storm evolution during the mPWP we have analyzed a sediment core from the northern South China Sea (SCS) for its elemental composition, grain-size variations and radiogenic isotope signature for the interval spanning from 3.69 to 2.96 Ma. We show that shortly after the first strong northern hemisphere glaciation (Marine Isotope Stage [MIS] M2; 3.25 Ma) atmospherically transported dust appeared in the northern SCS and this dust deposition prevailed throughout the mPWP. Atmospheric dust input further intensified with the onset of the MIS KM2 glaciation at 3.15 Ma, with distinct and strong dust storms occurring periodically from that time onwards. The increase in atmospherically transported dust can be attributed to the cooling and drying of interior Asia over the course of the mPWP along with an intensification of the East Asian Winter Monsoon and a potential southward shift of the westerlies.

Sea level variability along the Japanese coast forced by the Kuroshio and its extension

Sea level variability along the Japanese coast and its relation to the Kuroshio-Kuroshio Extension (KE) are investigated using ocean reanalysis data. The first mode of an empirical-orthogonal-function for the coastal sea-level represents a simultaneous sea-level change along the whole Japanese coast, which is synchronized with sea levels in the KE region , the Japan Sea and the East China Sea. The second mode is characterized by an east–west dipole pattern at the south coast. The first mode is correlated with the Kuroshio variations around the Izu–Ogasawara Ridge (IOR) and tends to be in a positive phase when the Kuroshio takes a nearshore path around IOR. The Kuroshio’s position around IOR is closely related to the KE dynamic state. When the KE jet is in a stable (unstable) state, a nearshore (meandering) Kuroshio path is formed around IOR. A composite analysis suggests that the sea level along the Japanese coast becomes high due to propagation of coastal trapped waves when the Kuroshio takes a nearshore path around IOR. That is why the first mode is synchronized with the KE decadal variability. The second mode has a close relation with the Kuroshio Large Meander (LM). The eastern positive anomaly at the coast between the Izu and Kii Peninsulas is formed by warm Kuroshio water brought by a westward branch flow along the coast. The western negative anomaly is attributed to a southward shift in the Kuroshio south of the Kii Peninsula associated with the LM.