Thanks to the recent advancements in climate observation methods and numerical simulation performance, there has been a significant increase in the availability climate datasets, which offer finer resolutions and broader coverage of variables than ever before. In contrast to the past, when scientists faced challenges due to limited data, the challenge now lies in extracting meaningful information from high-dimensional climate data. In climate analyses, each timestep of data provides a snapshot of atmospheric/oceanic conditions, analogous to a photograph. In such sense, techniques from the field of computer vision can serve as valuable tools for analyzing these climate snapshots.

This presentation aims to introduce the concept of the Bootstrapping Image Histogram, a fundamental idea in computer vision, and demonstrate its usefulness in simplifying climate snapshots and reducing the dimensionality of climate data. Additionally, given the crucial role of the Indo-Pacific warm pool (IPWP) in driving the global climate system, this presentation also showcases two applications of the Bootstrapping Image Histogram approach to IPWP expansion research, as recently published.

(1) Recent observed weakening of IPWP seasonality: We find that the amplitude of seasonal cycle of the IPWP size has decreased significantly since 1950, despite the sea surface warming being rather uniform across seasons. Analysis results suggest that the climatological spatial pattern of sea surface temperature (SST) over the Indo-Pacific Ocean is the primary factor contributing to the weakening IPWP seasonality. (https://doi.org/10.1088/1748-9326/acabd5)

(2) Overestimated IPWP expansion under greenhouse warming: The IPWP drives the global climate system by consistently supporting and maintaining atmospheric deep convection. For this reason, the IPWP is defined as the region where the SST exceeds a pre-condition necessary to favor deep convection (σ_conv). Previous conclusions regarding the rapid expansion of the IPWP were based on a constant σ_conv (typically 28°C). However, our analysis results reveal that σ_conv is indeed increasing under climate change, which corresponds to a slower IPWP expansion speed. This highlights the necessity of considering the response of the relationship between deep convection and SST to climate change when studying the long-term variability of the IPWP and its impacts on the climate system. (https://doi.org/10.1038/s41612-022-00315-w)

How to cite: Leung, J. C.-H., Gan, Q., Liu, S., Kong, H., and Zhang, B.: Bootstrapping Image Histogram for Simplifying Climate Snapshots: Exploring the Application to Indo-Pacific Warm Pool Expansion Research, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6517, https://doi.org/10.5194/egusphere-egu24-6517, 2024.

The Indo-Pacific warm pool (IPWP) expansion under global warming has huge impacts on global climate. While recent studies have revealed the seasonal diversity of IPWP surface expansion and its impacts under greenhouse warming, understanding the changes in seasonality of the IPWP volume is of greater importance, especially given the crucial role of subsurface ocean temperature in climate systems. This poster presents the seasonal diversity of Indo-Pacific warm pool volume expansion.

In this study, we find a significant difference of IPWP volume expansion rate across seasons from 1950–2020, although the oceanic warming is rather seasonally uniform. The expansions of IPWP volume during boreal autumn and winter are faster compared to boreal spring and summer. This consequently weakens the seasonality of IPWP volume, particularly in the upper-layer, with a significant decreasing trend of -0.54×10⁷ km³/decade. Further analyses suggest that this seasonal diversity in IPWP volume expansion is primarily caused by the seasonality of capacity for IPWP volume change, which is determined by the seasonal climatological subsurface temperature pattern over the Indo-Pacific Ocean. Furthermore, these variations may exert varied impacts on the troposphere and East Africa precipitation in rainy seasons. Namely that the larger expansion of IPWP in short rains is more closely related to the enhanced ascending motion and increased precipitation over East Africa, comparing with the long rains. This study emphasizes the crucial impacts of climatic subsurface Indo-Pacific Ocean temperature properties on the change of IPWP volume seasonality, which may have crucial effects on the precipitation in East Africa rainy seasons, and may hold important clues about how greenhouse warming affect oceanic seasonal cycle.

How to cite: Gan, Q., Leung, J. C.-H., Dong, W., Wang, L., Li, W., Qian, W., and Zhang, B.: Seasonal Diversity of Indo-Pacific Warm Pool Volume Expansion: The Role of Climatological Subsurface Temperature Patterns, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5301, https://doi.org/10.5194/egusphere-egu24-5301, 2024.

Sea surface salinity and its hydrological influences are important variables in global ocean and atmospheric circulation. In the Indian Ocean, particularly the Southwestern region, observational records of salinity are not well constrained before the early 2000’s making any understanding of decadal to interdecadal changes in sea surface salinity difficult. Current reconstructions of hydroclimate variability associated with ocean currents and salinity variability in the Southwestern Indian Ocean region are limited mainly to the Aguhlas current region, with a limited number of reconstructions capturing wider open ocean variability. Where reconstructions of this nature do exist across the wider southwest Indian Ocean, these records have limited ground-truthing due to short observational records and lack of replication.

Here we present a paired Sr/Ca and δ¹⁸O, bimonthly resolved record of a shallow water coral from the southwest Indian Ocean (Mauritius Island, 20.34°S, 57.55°E), extending from 1882 to 1989 to provide invaluable information about hydroclimate in the region. The reconstructed coral Sr/Ca-temperature proxy tracks the SST of the region very well, providing additional confidence to the current coral temperature reconstructions. Our record highlights the strong increasing SST trend across the southwest Indian Ocean, with an increase of +0.55°C since 1883. The paired analysis of Sr/Ca and δ¹⁸Oallows for the calculation of δ¹⁸O_sw (hydrology) at bimonthly resolution, developing the first high-resolution hydroclimate record which extends past the start of the 20th century and captures wider open ocean variability. The coral δ¹⁸O_sw record captures Mauritius rainy season (austral summer) precipitation, with a strong relationship between austral summer precipitation at stations on the island during the short period of observation. It is suggested that this relationship to Mauritius's rainy season captures wider-scale precipitation variability associated with the tropical rainfall belt. It is also suggested the non-rainy season (austral winter) δ¹⁸O_sw variability is controlled by oceanic processes as Mauritius lies along the South Equatorial Current, one of the major oceanic currents in the Indian Ocean and an important connection between the Pacific and Indian Ocean basins.

By using a network of current coral reconstructions from the wider southwest Indian Ocean, and the newly developed Mauritius coral record we hope to reveal variability in both ocean current variability and precipitation across this important region. Extending these coral records beyond the satellite era will further improve our understanding of the complex interaction between ocean and atmosphere variability in this region under past, present, and future climate change. This study uses legacy data as part of the DFG-Priority Programme “ Tropical Climate Variability & Coral Reefs” (SPP 2299).

How to cite: Hargreaves, J. A., Felis, T., and Kölling, M.: Hydrologic variability in the southwest Indian Ocean from Mauritius corals since the late 19th century (and connections to the Indo-Pacific throughflow), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18898, https://doi.org/10.5194/egusphere-egu24-18898, 2024.

The Indonesian Throughflow (ITF) is the primary tropical current, which transfers heat and salinity to the tropics and extratropics region. Crucial to the global ocean circulation system, the ITF is a major component of the tropical and global climate pattern. Re-analysis of instrumented data together with the results of coupled ocean-atmosphere model experiments, provide an understanding of the linkages between ITF variability and inter-annual modes of climate variability such as the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). However, the lack of longer climatic records of ITF variability makes it challenging to isolate the anthropogenic signal from natural variability. Here we present preliminary new, absolutely dated, seasonally resolved proxy records of sea surface salinity and temperature from the Timor Sea, northwest coast of Australia using cores taken from long-lived corals (~240 years) on the Hibernia-Ashmore reef. The Timor Sea is a proximal location for the ITF outlet into the eastern Indian Ocean, where temperature and salinity anomalies are greatest and ITF control is unequivocal. Herein we report sea surface temperature and the oxygen isotopic composition of seawater reconstructed using paired analyses of skeletal Sr/Ca and oxygen isotope composition for the last 40 yr. The resulting bimonthly coral record aids in understanding the linkage between ENSO, IOD and ITF strength. Comparison of the long ITF records with the marine and terrestrial records from around the region and world further reveals the relationship between ITF variability and Austral-Asian-African monsoon rainfall changes.

How to cite: Behera, P., Zinke, J., Boom, A., Wilson, P. A., and Hambach, B.: Reconstructing the Indonesian Throughflow variability and its climatology using long corals from the North-western coast of Australia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15391, https://doi.org/10.5194/egusphere-egu24-15391, 2024.

Identifying the processes that control tropical Pacific climate variations on long timescales is a pressing problem in climate research, given the outsized impacts of the El Niño/Southern Oscillation (ENSO) on global climate and the uncertainty in future ENSO behavior under anthropogenic climate change. By studying the characteristics of tropical Pacific climate under different climate states in the past, we can better assess its sensitivity to external forcing. Such paleoclimate constraints can serve as critically important test beds for coupled climate models that underlie future climate projections. In this talk, I will present a new set of climate reconstructions from the central equatorial Pacific spanning a range of timescales from seasonal to interannual to millennial, based on a large ensemble of coral oxygen isotope measurements from Kiritimati (aka Christmas Island) that span the past 7,000 years. Each of these timescales yields unique and complementary information about the climate of this region. We implement several new techniques to minimize the uncertainty in the climate reconstructions, which show a trend toward cooler and/or drier conditions and a reduced annual cycle going back in time that provide much needed context for understanding low-frequency changes in ENSO variability over the Holocene.

How to cite: Atwood, A. R., Cobb, K. M., Grothe, P. M., Sayani, H. R., Garber, S., Southon, J. R., and Edwards, R. L.: Central equatorial Pacific climate change over the last 7,000 years using a coral ensemble approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13006, https://doi.org/10.5194/egusphere-egu24-13006, 2024.

It is widely believed that anomalies in the state of tropical Sea Surface Temperature (SST), manifested as intensified ENSO variability, triggered the collapse of the Northern Hemisphere monsoon that occurred from 4500 to 3900a BP, known as the 4.2ka event. However, explicit records of ENSO variability, including events and variance, during the onset of 4.2ka event were still lacking to show how it changed and related to this climatic event. Here, we present a century-length (104-year) monthly coral record from the South China Sea (SCS) combined with modeling data to show that reduced ENSO variability was associated with an intensified Pacific Walker circulation ranging from 4400 to 4300 years BP, precisely corresponding to the onset of 4.2ka event. We hypothesize that such a mean state of tropical Pacific climate might not have triggered the development of the 4.2ka event, but rather responded to it.

How to cite: Dang, S., Yu, K., and Liu, Z.: Reduced ENSO Variability During the Onset of 4.2ka Event, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7665, https://doi.org/10.5194/egusphere-egu24-7665, 2024.

Hydroclimate in the tropical South Pacific is dominated by the South Pacific Convergence Zone (SPCZ), a region of low-level atmospheric convergence responsible for providing fresh water to 11 million people. The SPCZ is known to change in orientation and intensity in response to interannual climate phenomena, including El Niño Southern Oscillation (ENSO) and the interdecadal Pacific Oscillation (IPO), principally through modulation of trade wind strength (i.e., Walker circulation strength), and the resultant moisture inflow. Understanding how the orientation and intensity of the SPCZ changed under past climate states is important to predict future SPCZ changes, currently poorly represented in existing GCM’s. However, our knowledge of the dynamics of the SPCZ beyond the last 1000 years is limited by a lack of proxy archives and a large spread in climate model ensembles. We present a 60 ka plant wax record of paleoprecipitation collected from a peat sediment core from the island of Nuku Hiva, French Polynesia, located in the northeastern margin of the SPCZ. We demonstrate that Nuku Hiva was drier during the last glacial maximum (LGM) and wetter during the early Holocene compared to modern conditions. This indicates that the SPCZ was located further to the south during the LGM and further to the north during the early Holocene. We find a strong correlation between our SPCZ precipitation record and foraminifera based reconstructions of western Pacific warm pool thermocline depth. Given that both modern western Pacific thermocline depth and Nuku Hiva precipitation are influenced by easterly trade wind speed, we deduce that trade wind speeds were likely lower during the LGM and higher during the early Holocene, highlighting the long term dependence of SPCZ orientation on Walker circulation strength. This study, will help constrain future predictions of SPCZ precipitation change.

How to cite: Peaple, M., Inglis, G., Langdon, P., Joshi, M., Skinner, D., Mattews, A., Osborn, T., Roberts, W., and Sear, D.: Reconstructing South Pacific Convergence Zone precipitation over the past 60 ka using plant wax biomarkers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8966, https://doi.org/10.5194/egusphere-egu24-8966, 2024.

The Indo-Pacific Warm Pool (IPWP) holds the largest warm water body on Earth. With sea surface temperatures (SST) above 28°C, it promotes deep atmospheric convection in the rising limb of the Hadley and Walker circulation cells, and is a major source of heat and moisture to the global atmosphere with far-reaching climate impacts. Spatiotemporal changes in SST influence the location and strength of atmospheric convection and thus the atmospheric circulation. Despite its importance for the global climate, long-term SST variability across the IPWP is not well understood yet. Compilations of proxy-based reconstructions of SST during previous interglacials combined with climate models provide ideal means to study the SST variability in response to varying astronomical forcing. Hitherto, global compilations of interglacial SST anomalies and data-model comparisons have mostly focused on the Holocene and the Last Interglacial (LIG) period. The available studies reveal a striking mismatch between proxy-derived and modelled SST anomalies across low latitudes. However, the data coverage across the low-latitude Indo-Pacific is poor with little to no data from the IPWP. Here, we compare a proxy network of SST anomalies from the low-latitude Indo-Pacific during Holocene, LIG and MIS 11 time slices to the output of CESM 1.2 climate model simulations. We find large discrepancies between the proxy network and CESM output, concerning the magnitude and pattern of SST change including zonal gradients across the tropical Pacific. For instance, proxy data indicate highest SSTs during the LIG, with a slight warming as compared to the preindustrial reference period, while CESM indicates lowest SSTs during the LIG. By performing individual forcing experiments with CESM, we disentangle the roles of astronomical forcing, greenhouse gas concentration and vegetation cover in shaping interglacial tropical SST patterns. In particular, we find that an expanded Northern Hemisphere vegetation cover during interglacials mitigates model-data discrepancies in IPWP temperatures. 

How to cite: Hollstein, M., Prange, M., Jonkers, L., and Mohtadi, M.: Sea surface temperatures across the low-latitude Indo-Pacific Ocean during the Holocene, Last Interglacial and Marine Isotope Stage 11, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9887, https://doi.org/10.5194/egusphere-egu24-9887, 2024.

The tropical eastern Indian Ocean is part of Earth´s largest warm pool and its surface and thermocline temperatures exert strong control on deep atmospheric convection and play a critical role for the development of basin-wide, zonal climate anomalies across the Indian Ocean. However, the nature, timing, and mechanisms of changes in the Walker circulation in the tropical Indian Ocean since the Last Glacial Maximum (LGM) are poorly constrained owing to a lack of suitable proxy records and proxy-model disagreements (DiNezio et al. 2016; Mohtadi et al., 2017). Here we reconstruct surface and thermocline temperature and hydrographic changes in the tropical eastern Indian Ocean based on high-resolution, planktic foraminiferal (G. ruber and P. obliquiloculata) d18O and Mg/Ca records from three sediment cores retrieved offshore west Sumatra along a latitudinal transect across the equator. These records are put into a chronological framework based on numerous radiocarbon ages of surface-dwelling planktic foraminifera and cover the last 22 ka.

Sea surface temperatures at all three sites show a stepwise warming of ~3°C with an ‘Antarctic timing’ between 18 ka and 11 ka. The thermocline temperature variability is also consistent among the three core sites but distinctly different from the sea surface temperature variability. Thermocline temperatures show a major warming of 2–3°C between ~13 ka and ~10 ka, while differences between LGM and Holocene temperatures are rather small. The resulting surface-thermocline temperature gradient reveals not only a difference between LGM and Holocene thermocline depth levels, but also a major breakdown pointing at a rapid deepening of the thermocline at ~12 ka. This thermocline deepening might have been associated with a strengthened convective activity and Walker circulation, with its timing suggesting a connection to feedbacks related to inundation of the large Sahul Shelf during deglacial sea level rise. Supplemented by deuterium isotope analyses of leaf waxes, our new set of proxy records will provide unprecedented insights into sea surface and thermocline dynamics in the tropical eastern Indian Ocean since the LGM, their relationship to local rainfall, and whether and how basin-wide circulation and rainfall anomalies were shaped by sea level rise and deglacial climate change.

DiNezio et al., 2016, Paleoceanography 31, 866–894, doi:10.1002/2015PA002890

Mohtadi et al., 2017, Nature Communications 8, 1015, doi:10.1038/s41467-017-00855-3

How to cite: Sadatzki, H., Mohtadi, M., Hollstein, M., Lückge, A., Yokoyama, Y., and Oppo, D.: Surface and thermocline variability in the tropical eastern Indian Ocean since the Last Glacial Maximum, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18460, https://doi.org/10.5194/egusphere-egu24-18460, 2024.

Dansgaard-Oeschger oscillations and Heinrich events described in Greenland ice cores are also visible in the climate of the monsoon realm as documented in Arabian Sea sediments. However, little is known about these millennial scale fluctuations beyond the reach of the Greenland ice cores. Here, we present high-resolution geochemical and micropaleontological data from two sediment cores located offshore Pakistan, extending the monsoon record to the past 250,000 years in millennial scale resolution.

The stable oxygen isotope (d¹⁸O) record of the planktic foraminifera G. ruber shows a strong correspondence to Greenland ice core d¹⁸O, whereas the d¹⁸O signal of benthic foraminifera (U. peregrina and G. affinis) reflects patterns similar to those observed in Antarctic ice core records. Distinct shifts in benthic d¹⁸O during stadials are interpreted to show frequent injections of oxygen-rich intermediate water masses of Southern Ocean origin into the Arabian Sea. Alkenone SSTs show variations between 23 and 28°C. Millennial scale SST changes of 2°C are modulated by long-term SST fluctuations. Interstadials and the cold phases of interglacials are characterized by sediments enriched in organic carbon (TOC) whereas sediments with low TOC contents appear during stadials. Abrupt shifts (50-60 year duration) at climate transitions, such as interstadial inceptions, correlate with changes in productivity-related and anoxia-indicating proxies. Interstadial inorganic data consistently show that enhanced fluxes of terrestrial-derived sediments are paralleled by productivity maxima, and are characterized by an increased fluvial contribution from the Indus River. The hydrogen isotopic composition of terrigenous plant waxes indicates that stadials are dry phases whereas humid conditions seem to have prevailed during interstadials. In contrast, stadials are characterized by an increased contribution of aeolian dust probably from the Arabian Peninsula. Heinrich events are especially dry and dusty, indicating a dramatically weakened Indian summer monsoon and increased continental aridity.

These results strengthen the evidence that North Atlantic temperature changes and shifts in the hydrological cycle of the Indian monsoon system are closely coupled, and had a massive impact on regional environmental conditions such as river discharge and ocean margin anoxia. These shifts were modulated by changes in the supply of water masses from the Southern Hemisphere.

How to cite: Lückge, A., Hollstein, M., Mohtadi, M., Schefuß, E., and Steinke, S.: Millennial scale monsoon variability over the last 250,000 years in the Arabian Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14611, https://doi.org/10.5194/egusphere-egu24-14611, 2024.