Migration Of Intertropical Convergence Zone Research Articles

Contrasting fast and slow intertropical convergence zone migrations linked to delayed Southern Ocean warming

Contrasting fast and slow intertropical convergence zone migrations linked to delayed Southern Ocean warming

The 4.2 ka event: A review of palaeoclimate literature and directions for future research

In recent years, much evidence has been presented on the 4.2 ka event. A review of 317 palaeoclimate papers shows that dry conditions were common during the event, especially from Eastern Mediterranean to India. The 4.2 ka event was not, however, a global drought event. Wet conditions were reported especially for central/northern Europe and sub-Saharan Africa. The 4.2 ka event is typically characterized either as short (4.2–4.0 ka) or long (4.4–3.8 ka) episode, possibly developing over an extended interval of time, in keeping with the North Atlantic forcing and correlating with the Bond 3 event of ice-rafted debris. This forcing is understood to drive a southward migration of the Inter-Tropical Convergence Zone (ITCZ), resulting in decreased rainfall over most of the Asian monsoon region, with possibility that an interplay of El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) has modulated the global circulation. Cold conditions were also reported but less frequently, in comparison to other Bond events such as the 8.2 ka event, Dark Ages Cold Period and Little Ice Age. Some high-resolution records show a double peak structure of which two anomalies are tree-ring dated to 4.14–4.05 ka and 3.97 ka. Accurately and precisely dated high-resolution records indicative of various climatic variables, especially outside of the traditional study region (Mediterranean–Middle East–India–China), including reconstructions of the ENSO and NAO histories and ITCZ migrations, are crucially needed for rigorous examination of the global scale characteristics of the 4.2 ka event and its forcings. Such research seems to be just beginning.

Decoupling Warming and Monsoon Precipitation Over the Last 3110 Years in Northwestern Yunnan Province, Southwestern China

AbstractUnderstanding the variability and related dynamic mechanisms of the Indian summer monsoon evolution over the past millenniums is crucial for disaster prevention and mitigation in the context of strengthening global warming. Here, we present a high‐resolution hydroclimate reconstruction over the past 3110 years in northwestern Yunnan Province, southwestern China, based on grain size analysis using the End‐member modeling algorithm and other environmental proxies. The results indicate that the temperature in northwestern Yunnan Province generally decreased since 3110 cal. yr BP, aligning with other temperature records on regional and global scales. Precipitation in the study area remained relatively stable before 1000 cal. yr BP, but exhibited an obvious out‐phase change compared with the temperature records over multidecadal to centennial timescale. While after 1000 cal. yr BP, the precipitation steadily increased, revealing the hydroclimate pattern of notably dry Medieval Warm Period (MWP) and relatively wet Little Ice Age (LIA) in the study area. The decoupling combination of temperature and precipitation spanning the MWP and LIA in southwestern China contrasts with that in the East Asian summer monsoon area and the Indian subcontinent which is also controlled by the Indian summer monsoon, indicating the spatial heterogeneity of precipitation in the Asian summer monsoon region. The discussion about dynamic mechanisms points out that solar activity emerges as a significant factor influencing regional temperature changes, and El Niño‐Southern Oscillation and Intertropical Convergence Zone migration jointly impact the precipitation change in the study area and the hydroclimate discrepancy in the Asian summer monsoon region.

A multi-proxy lacustrine sedimentary record of sub-decadal to decadal scale variability of monsoon during the late Holocene in southern India

The present study is aimed at understanding the sub-decadal to decadal scale variability of rainfall in southern India based on a sedimentary record (dated using AMS 14C, 210Pb, 137Cs methods) from Madagadakere lake (MK) situated in the foothills of the Western Ghats. A strong linkage between sediment magnetic signature and the instrumental rainfall record for the past 115 years demonstrates that the environmental magnetic proxies are useful and have greater applicability in the paleomonsoonal reconstructions. Fluctuations in the mineral magnetic, grain size, and geochemical parameters of the studied sediment profile revealed that pedogenic activity and terrigenous sediment flux to the lake varied cyclically during the Late Holocene period in response to varying intensities of rainfall. The region experienced higher or increasing trend of rainfall during periods 2100–1900, 1600–1250, 1050–925, 680–600, and 350–270 cal. years B.P., with low rainfall/decreasing trend during intervening periods. The spectral analysis of magnetic parameters revealed significant periodicities of 291, 164, 101, 54, 48, 44, 41, 39, 105, 48, and 41 years, which are very well documented in other paleoclimatic records, and whose origin is ascribed unequivocally to variation in Total Solar Irradiance (TSI). We propose that the periods of higher values for concentration-dependent magnetic parameters (higher rainfall) can be attributed to the timings of the increased TSI, positive mode of Indian Ocean Dipole (IOD), northward migration of Intertropical convergence zone (ITCZ) and weakened El-Niño southern oscillation (ENSO) activity in the region, establishing the teleconnection between different climatic systems.

Southward Shift and Intensification of the Intertropical Convergence Zone in the North Pacific Across the Mid‐Pleistocene Transition

AbstractThe Hadley Circulation and associated westerlies strengthened and moved equatorward across the mid‐Pleistocene transition (MPT). However, the evolution of the intertropical convergence zone (ITCZ) is still elusive due to the scarcity of long‐term hydrological records from regions sensitive to the ITCZ change. Here, high‐resolution sea surface salinity estimates derived from surface‐dwelling planktic foraminiferal δ18O and Mg/Ca in Ocean Drilling Program Site 871 reveal a long‐term freshening trend in the central equatorial Pacific across the MPT. We attribute this secular reorganization of the precipitation‐evaporation balance to the gradual southward migration and intensification of ITCZ in the North Pacific. It is inferred that the long‐term evolution of the ITCZ was modulated by the increased meridional sea surface temperature gradients and the enhancements of trade winds across the MPT.

Changes of bottom water oxygenation during the last half millennium in the Gulf of Tehuantepec (Eastern Tropical North Pacific): A multiproxy approach

The Gulf of Tehuantepec (GoT), in the Eastern Tropical North Pacific Ocean, is home to one of the largest and more intense Oxygen Minimum Zones (OMZ). To identify the OMZ variability during the last five hundred years in the GoT, we analyzed the temporal variations of benthic foraminiferal (BF) assemblages, enrichment of redox-sensitive elements (Mo, V, Cd, U, and Re) and δ15Nsed, on a laminated sediment core. During the Little Ice Age (LIA; ∼1500 to ∼1860 CE), the BF assemblages showed high dominance, and the most abundant BF species in the sediments were Epistominella sandiegoensis, Takayanagia delicata, and Buliminella tenuata. This assemblage can withstand the lowest dissolved oxygen conditions, indicating an intensified OMZ consistent with the enrichment of Mo, Re, Cd, U, and enhanced denitrification. During the Current Warm Period (CWP; ∼1860 CE to present), the BF assemblages were more diverse, and the most abundant species were Bolivina seminuda, Epistominella sp.1, Gyroidina nitidula, and Suggrunda eckisi, an assemblage less tolerant to low dissolved oxygen conditions, together with lower concentrations of redox-sensitive elements and δ15Nsed, suggest weakening of the reducing conditions within the OMZ. Additionally, our BF data showed evidence of the influence of the Pacific Decadal Oscillation on bottom-water oxygenation. Bottom-water oxygenation changes appear to respond to increased solar forcing caused by Intertropical Convergence Zone migration and strengthened Pacific Walker Circulation, which in turn modulated productivity, organic matter flux to the seabed, and oxygen consumption in the bottom water. Our findings agree with previous studies further north from the study site, indicating these changes' regional scale.

The link between intertropical convergence zone stagnation and bias in local shortwave cloud radiative forcing over tropical Africa in climate models

The northward migration of the intertropical convergence zone (ITCZ) is a significant feature of the West African (WA) monsoon. An accurate simulation of ITCZ migration is essential for the realistic representation of WA precipitation in global coupled models. In this study, we employ the energetics and dynamics framework with a subset of CMIP6 models to investigate the bias in the simulated WA precipitation. Models were found to simulate more local positive (negative) shortwave cloud radiative forcing (SWCRF) in the Southeastern Atlantic Ocean (over the African continent). The effect of the excess local SWCRF is linked to the stagnation of the ITCZ latitudinal migration and the associated biases in the asymmetry index of precipitation. In the models, there is more (less) moist static energy in the lower (mid and upper) troposphere than in the reanalysis. The worst models have a stronger bias, especially over land. The vertical transport of moisture is confined to the boundary layer in the worst model ensemble. In most cases, the high-resolution coupled models show substantial northward migration of the ITCZ compared to the low-resolution models. Furthermore, the best-performing models capture local circulation and energetic processes more accurately than the worst-performing models.

ENSO skewness hysteresis and associated changes in strong El Niño under a CO2 removal scenario

El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomaly skewness encapsulates the nonlinear processes of strong ENSO events and affects future climate projections. Yet, its response to CO2 forcing remains not well understood. Here, we find ENSO skewness hysteresis in a large ensemble CO2 removal simulation. The positive SST skewness in the central-to-eastern tropical Pacific gradually weakens (most pronounced near the dateline) in response to increasing CO2, but weakens even further once CO2 is ramped down. Further analyses reveal that hysteresis of the Intertropical Convergence Zone migration leads to more active and farther eastward-located strong eastern Pacific El Niño events, thus decreasing central Pacific ENSO skewness by reducing the amplitude of the central Pacific positive SST anomalies and increasing the scaling effect of the eastern Pacific skewness denominator, i.e., ENSO intensity, respectively. The reduction of eastern Pacific El Niño maximum intensity, which is constrained by the SST zonal gradient of the projected background El Niño-like warming pattern, also contributes to a reduction of eastern Pacific SST skewness around the CO2 peak phase. This study highlights the divergent responses of different strong El Niño regimes in response to climate change.

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

The scientific explanation of cave δ18O in the East Asian Summer Monsoon (EASM) region is a crucial issue restricting stalagmite-inferred paleoclimate research. The connection between El Niño Southern Oscillation (ENSO) and monsoonal precipitation/cave δ18O and the mechanism involved also remain contentious. Long-term cave monitoring is an effective solution to deal with these issues. Here, we present 11-year-long precipitation, dripwater, and modern calcite δ18O data from Yuhua Cave in Southeast China, located in the frontal zone affected by the EASM. The upstream convection and rainout processes mainly regulate seasonal and interannual variations in precipitation δ18O (δ18Op). The δ18Op series shows a strong response to EASM activities associated with the Intertropical Convergence Zone migrations and the activities of the West Pacific Subtropical High affected by ENSO activities. Due to the influence of the epikarst reservoir mixing effect, the seasonal variation of dripwater δ18O (δ18Od) is irregular, and its amplitude is much smaller than that of δ18Op. The δ18Od can reflect integrated δ18Op changes within the residence time of rainwater in the karst reservoir and can inherit the ENSO signal in δ18Op. The inferred recharge models for the Yuhua drip sites reveal differences in the response to ENSO in terms of both timing and amplitude, which can be attributed to variations in the residence time of rainwater and the mixing ratio of new and old water within the karst aquifer. Higher rainfall would lead to a more sensitive response of δ18Od to ENSO activities, with a shorter time lag and greater amplitude. The modern calcite δ18O (δ18Oc) series show significant seasonal variations controlled by the fractionation coefficient varied with cave temperature. On the inter-annul time scale, the change of δ18Oc is mainly controlled by δ18Od and can record the ENSO-related variations. Our findings imply that high-resolution stalagmite δ18O reconstructions from Yuhua Cave may be able to identify historical ENSO-related variability in the EASM on annual to decade time scales.

Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part II: δ18Op Response

Abstract Understanding the hydroclimate representations of precipitation δ18O (δ18Op) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), characterized by a northwest–southeast (NW–SE) dipole in both rainfall and δ18Op, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of such δ18Op dipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the intertropical convergence zone (ITCZ) migrates southward, causing a moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to the δ18Op depletion in eastern Brazil (SE pole). In contrast, the moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated subcloud layers) contribute nearly equally to the δ18Op enrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship between δ18Op and rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennial δ18Op change in the central-western Amazon, while δ18Op does not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when using δ18Op as a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennial δ18Op dipole response and pose a challenge in reconciling the discrepancy. Significance Statement We want to comprehensively understand the hydroclimate footprints of δ18Op and the mechanisms of the millennial variability of δ18Op over tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennial δ18Op change in eastern Brazil mainly documents the moisture source location change associated with ITCZ migration and the change of the isotopic value of the incoming water vapor, instead of the local rainfall amount. In contrast, the central-western Amazon partially documents the moisture source location shift and local precipitation change. Our study cautions that one should not simply resort to the isotopic “amount effect” to reconstruct past precipitation in tropical regions without studying the mechanisms behind it.

Chemical Weathering Intensity as a Reliable Indicator for Southwest Summer Monsoon Reconstruction: Evidence From Clay Minerals of Qionghai Lake Sediments Since the Last Glacial Maximum

AbstractStudying the evolution history of the southwest summer monsoon (SSM) throughout geological time, particularly during its strongest period in the Holocene, can improve our understanding of its variation and driving mechanisms, and even help predict future climate changes, due to its significant social and economic implications. Here, we reconstructed the history of chemical weathering intensity since the Last Glacial Maximum (LGM) based on clay mineral proxies [(illite/smectite)/(illite + chlorite) and illite crystallinity] obtained from Qionghai Lake sediments and examined its response to paleoclimate and SSM. Our findings indicate that the intensity of chemical weathering generally aligned with changes in paleoclimate, exhibiting strong chemical weathering intensity during warm and humid climate conditions. In addition, the intensity of chemical weathering basically tracks the evolution of the SSM since the LGM. Our results support the view that the highest SSM intensity occurred during the early‐middle Holocene, followed by gradual weakening during the late Holocene, with Northern Hemisphere summer insolation being the primary driver of the SSM evolution. The variations of the SSM and the corresponding intensity of chemical weathering were also influenced by the cumulative effects of glacier boundary conditions, North Atlantic climate fluctuations, and Intertropical Convergence Zone migrations.

Late Holocene change in South Asian monsoons and their influences on human activities in the southern Tibetan Plateau

Prominent and well studied is the South Asian summer monsoon (SASM), but much less is known about changes in South Asian winter monsoon (SAWM). In this study, we analyzed a 3.0-m aeolian sedimentary sequence from the Yarlung Tsangpo Valley in the southern Tibetan Plateau. To establish the sequence chronology, fourteen sediment samples are dated using quartz optically stimulated luminescence dating technique, and five charcoal samples are dated using accelerator mass spectrometry 14C. The frequency-dependent susceptibility in the percentage and redness are used to reflect the SASM, and the recently proposed sorting coefficient of grain size data was used to reflect the SAWM for the past ∼4.5 kyr. Our results suggest an obvious anti-phase relationship between the SASM and SAWM during the late Holocene, and the SASM (SAWM) strengthened (weakened) significantly at ∼2 ka and was the strongest (weakest) at ∼1.3 ka. Such inverse monsoonal behavior may be attributed to the migration of Intertropical Convergence Zone. Moreover, a strong SASM and a weak SAWM could have facilitated human activities in the southern Tibetan Plateau after ∼1.3 ka, resulting in the significantly enhanced dust activity in Yarlung Tsangpo Valley.

The new Kr-86 excess ice core proxy for synoptic activity: West Antarctic storminess possibly linked to Intertropical Convergence Zone (ITCZ) movement through the last deglaciation

Abstract. Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure changes weakly disrupt gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (86Krxs). The 86Krxs may therefore reflect the time-averaged synoptic pressure variability over several years (site “storminess”), but it likely cannot record individual synoptic events as ice core gas samples typically average over several years. We validate 86Krxs using late Holocene ice samples from 11 Antarctic ice cores and 1 Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong spatial correlation (r=-0.94, p<0.01) between site average 86Krxs and time-averaged synoptic variability from reanalysis data. The main uncertainties in the analysis are the corrections for gas loss and thermal fractionation and the relatively large scatter in the data. Limited scientific understanding of the firn physics and potential biases of 86Krxs require caution in interpreting this proxy at present. We show that Antarctic 86Krxs appears to be linked to the position of the Southern Hemisphere eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability. We present a 86Krxs record covering the last 24 kyr from the West Antarctic Ice Sheet (WAIS) Divide ice core. Based on the empirical spatial correlation of synoptic activity and 86Krxs at various Antarctic sites, we interpret this record to show that West Antarctic synoptic activity is slightly below modern levels during the Last Glacial Maximum (LGM), increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods, weakens abruptly at the Holocene onset, remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide 86Krxs record resembles records of monsoon intensity thought to reflect changes in the meridional position of the Intertropical Convergence Zone (ITCZ) on orbital and millennial timescales such that West Antarctic storminess is weaker when the ITCZ is displaced northward and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Niño–Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling. We emphasize that 86Krxs is a new proxy, and more work is called for to confirm, replicate, and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain speculative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in 86Krxs. A list of suggested future studies is provided.

Continental drift shifts tropical rainfall by altering radiation and ocean heat transport.

Shifts in the position of the intertropical convergence zone (ITCZ) have great importance for weather, climate, and society. The ITCZ shifts have been extensively studied in current and future warmer climate; however, little is known for its migration in the past on geological time scales. Using an ensemble of climate simulations over the past 540 million years, we show that ITCZ migrations are controlled primarily by continental configuration through two competing pathways: hemispheric radiation asymmetry and cross-equatorial ocean heat transport. The hemispheric asymmetry of absorbed solar radiation is produced mainly by land-ocean albedo contrast, which can be predicted using only the landmass distribution. The cross-equatorial ocean heat transport is strongly associated with the hemispheric asymmetry of surface wind stress, which is, in turn, controlled by the hemispheric asymmetry of ocean surface area. These results allow the influence of continental evolution on global ocean-atmosphere circulations to be understood through simple mechanisms that depend primarily on the latitudinal distribution of land.

Sensitivity of northwest Australian tropical cyclone activity to ITCZ migration since 500 CE.

Tropical cyclones (TCs) regularly form in association with the intertropical convergence zone (ITCZ), and thus, its positioning has implications for global TC activity. While the poleward extent of the ITCZ has varied markedly over past centuries, the sensitivity with which TCs responded remains poorly understood from the proxy record, particularly in the Southern Hemisphere. Here, we present a high-resolution, composite stalagmite record of ITCZ migrations over tropical Australia for the past 1500 years. When integrated with a TC reconstruction from the Australian subtropics, this time series, along with downscaled climate model simulations, provides an unprecedented examination of the dependence of subtropical TC activity on meridional shifts in the ITCZ. TCs tracked the ITCZ at multidecadal to centennial scales, with a more southward position enhancing TC-derived rainfall in the subtropics. TCs may play an increasingly important role in Western Australia's moisture budgets as subtropical aridity increases due to anthropogenic warming.

Phytoplankton productivity and community structure changes in the middle Okinawa Trough since the last deglaciation

Marine phytoplankton plays an important role in the ocean biological pump, whereas our understandings of phytoplankton growth in response to global climate and enviromment changes are still debated. Due to the forcing from the strong land-ocean interaction between the East Asia and the Indo-Pacific Warm Pool, the Okinawa Trough is characterized by high phytoplankton productivity and fast sediment accumulation, therefore it provides ideal archives to study marine productivity changes on millennial-to-orbital timescales. In this study, lipid biomarkers of C37 alkenones, dinosterol and brassicasterol, were analyzed in a sediment core M063–05 retrieved from the middle Okinawa Trough to reconstruct phytoplankton productivity and community structure changes over the last 15.8 kyr. We find that temporal variations of these lipid biomarker contents are parallel with precipitation changes in the Changjiang drainage basin during the last deglaciation, with high values during increased precipitation interval (14.5–12.7 ka) and low values during decreased precipitation intervals (15.8–15.0 ka and 12.7–11.5 ka). We propose that precipitation-induced terrestrial nutrient supply controlled phytoplankton growth in the middle Okinawa Trough during the last deglaciation. Lipid biomarker ratios reveal high (low) contributions of diatoms and dinoflagellates to total phytoplankton productivity during increased (decreased) precipitation intervals. The phytoplankton community structure record is also consistent with precipitation-induced terrestrial nutrient forcing during the deglacial stage, with higher nutrient condition favored diatom and dinoflagellate growth. Precipitation in the Changjiang drainage basin was mainly controlled by the East Asian summer monsoon change and the Intertropical Convergence Zone migration, however the deglacial sea level rise also modulated the forcing of precipitation on phytoplankton growth by affecting the delivery distance of terrestrial nutrients to our site. From the last deglaciation to the Holocene, the biomarker content decreased and the contribution of coccolithophorids to total phytoplankton productivity increased at the expense of dinoflagellates and diatoms. The most likely explanation is the regime shift of major nutrient sources supporting phytoplankton growth, from terrestrial riverine nutrients during the deglaciation to the Kuroshio subsurface water nutrients during the Holocene. Our results provide new insight of past phytoplankton responses to climate and environment changes.

Orbital forcing of tropical climate dynamics in the Early Cambrian

According to modern atmospheric circulation models, the Intertropical Convergence Zone (ITCZ), as the Earth's meteorological equator, plays an essential role in the low-latitude hydrologic cycles. The limited availability of high-resolution tropical climate archives, especially from the Early Paleozoic Era, severely limits our understanding of ITCZ migration dynamics in deep time. Here we present high-resolution climate-proxy records (i.e., magnetic susceptibility (MS) and Zirconium/Aluminum (Zr/Al)) from tropical marine sediments of the ~526-million-year-old Qiongzhusi Formation in South China to investigate the link between orbitally forced insolation changes, ITCZ migration dynamics, and low-latitude climate processes. These orbital-scale variations in MS and Zr/Al series are interpreted as alternations between wet and dry cycles, controlled by monsoon intensity under the orbitally forced ITCZ-related paleo-Hadley Cell dynamics. Our results show that combined precession and obliquity orbital cycles had an impact on the Early Cambrian ITCZ migration. Specifically, the precession and obliquity forcing shift the mean position of the ITCZ latitudinally by changing the interhemispheric pressure contrasts, thus affecting the low latitude hydroclimate cycle. We report semi-precession cycles of 8.3–7.9 kyr, which were probably associated with the twice-annual passage of the ITCZ across the intertropical zone, consistent with the paleogeographical location of South China near the equator during the Early Cambrian. Observed ~1.1 – ~1.5 Myr eccentricity amplitude modulation (AM) cycles and ~ 1.0 – ~1.2 Myr obliquity AM cycles may provide geological evidence for the chaotic motion between Earth and Mars in the Early Cambrian. • Combined precession and obliquity cycles regulated the Early Cambrian ITCZ migration. • We report the semi-precession cycles of 7.9–8.3 kyr in the Early Cambrian. • Our study provides geological evidence for the chaotic Solar System in the Early Cambrian.

Intertropical Convergence Zone as the Possible Source Mechanism for Southward Propagating Medium-Scale Traveling Ionospheric Disturbances over South American Low-Latitude and Equatorial Region

This paper presents the Intertropical Convergence Zone (ITCZ) as the possible source mechanism of the medium-scale traveling ionospheric disturbances (MSTIDs) propagating to the southeast direction over the South American region. Using the data collected by the GNSS dual-frequency receivers network from January 2014 to December 2019, detrended TEC maps were generated to identify and characterize 144 MSTIDs propagating southeastward over the South American low-latitude and equatorial region. We also used images from the Geostationary Operational Environmental Satellite (GOES) 13 and 16 in the infrared (IR) and water vapor (WV) channel, and reanalisys data from the National Centers for Environmental Prediction (NCEP) of the National Oceanic and Atmospheric Administration (NOAA) to study the daily features and seasonal migration of ITCZ. In the winter, when ITCZ migrates to the northern hemisphere around 10–15° N, 20 MSTIDs propagated southeastward. During summer, when the ITCZ lies within the continent, around 0–5° S 80 MSTIDs were observed to propagate southeastward; in the equinoxes (spring and fall), 44 MSTIDs were observed. Again, the MSTIDs propagating southeastward showed a clear seasonality of their local time dependence; in summer, the MSTIDs occurred frequently in the evening hours, whereas those in winter occurred during the daytime. We also found for the first time that the day-to-day observation of ITCZ position and MSTIDs propagation directions were consistent. With regard to these new findings, we report that the MSTIDs propagating southeastward over the South American region are possibly induced by the atmospheric gravity waves, which are proposed as being generated by the ITCZ in the troposphere. The mean distribution of the horizontal wavelength, period, and phase velocity are 698 ± 124 km, 38 ± 8 min, and 299 ± 89 m s−1, respectively. For the first time, we were able to use MSTID propagation directions as a proxy to study the source region.

Buoyancy Forcing Dominates the Cross-Equatorial Ocean Heat Transport Response to Northern Hemisphere Extratropical Cooling

Abstract Cross-equatorial ocean heat transport (OHT) changes have been found to damp meridional shifts of the intertropical convergence zone (ITCZ) induced by hemispheric asymmetries in radiative forcing. Zonal-mean energy transport theories and idealized model simulations have suggested that these OHT changes occur primarily due to wind-driven changes in the Indo-Pacific’s shallow subtropical cells (STCs) and buoyancy-driven changes in the deep Atlantic meridional overturning circulation (AMOC). In this study we explore the partitioning between buoyancy and momentum forcing in the ocean’s response. We adjust the top-of-atmosphere solar forcing to cool the Northern Hemisphere (NH) extratropics in a novel set of comprehensive climate model simulations designed to isolate buoyancy-forced and momentum-forced changes. In this case of NH high-latitude forcing, we confirm that buoyancy-driven changes in the AMOC dominate in the Atlantic. However, in contrast with prior expectations, buoyancy-driven changes in the STCs are the primary driver of the heat transport changes in the Indo-Pacific. We find that buoyancy-forced Indo-Pacific STC changes transport nearly 4 times the amount of heat across the equator as the shallower wind-driven STC changes. This buoyancy-forced STC response arises from extratropical density perturbations that are amplified by the low cloud feedback and communicated to the tropics by the ventilated thermocline. While the ocean’s specific response is dependent on the forcing scheme, our results suggest that partitioning the ocean’s total response to energy perturbations into buoyancy and momentum forcing provides basin-specific insight into key aspects of how the ocean damps ITCZ migrations that previous zonal-mean frameworks omit.

Climate and sea level forcing of terrigenous sediments input to the eastern Arabian Sea since the last glacial period

Analyses of mass accumulation rate (MAR), laser grain-size, clay mineralogy, and SrNd isotopic compositions of <2 μm and < 63 μm grain-size fractions have been applied to sediments from the International Ocean Discovery Program (IODP) Site U1456, which span the last 31 ka, in order to constrain (i) past changes in the relative proportions of the fine-grained detrital sediments derived from the Indus River and the Deccan Traps and the dynamics of sediment transport to the Laxmi Basin and (ii) past changes in Indian monsoon rainfall and latitudinal migration of Intertropical Convergence Zone since the last glacial period. Sr and Nd isotopes results combined with clay mineral assemblages suggest that clay-sized sediments at IODP Site U1456 derive mainly from the Indus River (70–90%), followed by a minor contribution from rivers draining the volcanic province of the Deccan Traps (10–30%). During the Last Glacial Maximum, clay mineralogy was dominated by illite and chlorite primarily derived from the highlands of the Indus River basin. The combination of lower temperatures and lower monsoonal rainfall during glacial times led to a reduction of smectite formation rates in the floodplains, while glacial erosion accelerated in the Himalayan and Tibetan highlands leading to the release of large quantities of illite and chlorite minerals into the Arabian Sea. In addition, glacial low sea-level stands allowed a connection between river mouths and the heads of deep-sea channels favoring the southward transfer of sediments to IODP Site U1456 by turbidity currents. Each maximum of the summer insolation curve, associated with a northward shift of the ITCZ and an intensification of summer monsoon rainfall (from 31 to 25 ka BP and from 12 to 8 cal ka BP), was associated with enhanced rainfall in the Indus floodplain which in turn led to an increase in the physical erosion and weathering of plain soils, which were rich in pedogenic minerals (such as smectite), and/or a higher production rate of smectite in the soils of the Indus floodplain. A compilation of three Nd isotopic composition records obtained at sites located on a N-S latitudinal transect in the eastern Arabian Sea have allowed us to establish the impact of sea level variations and latitudinal migration of the ITCZ over time on the sedimentation of the eastern Arabian Sea.