South Indian Ocean Research Articles

Contrasting influence of the 1997 and 2015 El Niño on the Indian Summer Monsoon Rainfall: Role of the Southern Hemisphere

This study provides comprehensive analysis on the contrasting effects caused by the 1997 and 2015 historical El Niño events linked with the Indian Summer Monsoon (ISM) rainfall. The presence of strong southeast-northwest oriented cold Sea Surface Temperature (SST) anomalies during 1997, that spatially extended from southwest Pacific to Southeast Indian Ocean (SEIO) in comparison to the 2015 event is the robust feature. Evidently, these anomalies are closely related to interaction between cyclonic (anticyclonic) circulation over South Pacific Convergence Zone (south Australia). During 2015, the conjunction of Modoki II and classical El Niño triggered an asymmetrical equatorial circulation throughout the Indo-western Pacific (IWP) and thereby stimulated the Southern Annular Mode (SAM) through troposphere and stratospheric pathway mechanism. In addition, in 2015, SAM impacted the Indian Ocean, which intern affected ISM rainfall. Positive SAM associated with westward shift of anticyclone over south of Australia alters the circulation by inducing westerly winds over the South Indian Ocean, thereby suppressing Indian Ocean Dipole (IOD), and inducing drought conditions over India during 2015. Moreover, the AUS index, an indicator for IOD strength in boreal summer, is a bridging factor prevalent over mid-latitude regions in the southern hemisphere. Results from this study indicate the complex interaction of southern hemisphere atmospheric flow and its role in modulating the Indian Ocean region thereby ISM rainfall. A better understanding of these underlying mechanism can significantly enhance the predictive skills and projections of monsoon variability and extremes.

The economic benefit of spearfishing as an impure public good: A case study of invasive Lionfish in Florida

Lionfish (Pterois miles and P. volitans) is a highly invasive species originally from the South Pacific and Indian Oceans. As an effort to control its exponentially growing population, the Florida Fish and Wildlife Conservation Commission has launched a program called “Lionfish Challenge” to promote harvesting of lionfish from Florida waters. The program provides anglers with a unique recreational opportunity because they would need to dive and spearfish to harvest lionfish. Furthermore, participants in the program not only gain utility from spearfishing as a private good but also as a public good, knowing that they are contributing to the ecosystem in Florida waters. This paper contributes to the literature by measuring the economic benefit for participants from spearfishing as an impure public good. Using data provided by the Florida Fish and Wildlife Conservation Commission, a travel cost method was applied to model the demand for lionfish fishing trips. Data included information about counties where lionfish were harvested at the trip-level, but only if participants took a trip and harvested lionfish. Therefore, those who are good at catching lionfish had a higher probability of being included in the data. To account for potential sample selection bias, a probability weight was applied, using the average number of lionfish harvested as a proxy. Results indicate that the per-trip consumer surplus per participant was between $965.89 and $1,597.36, depending on model specifications, with the preferred specification indicating a mean CS of $1,117.72 per trip. The seasonal consumer surplus per participant was indicated to be between $4,550.45 and $3487.91, again, depending on model specification.

Decreasing trend in destructive potential of tropical cyclones in the South Indian Ocean since the mid-1990s

Tropical cyclone activity often leads to many adverse impacts and assessing their destructiveness is a crucial scientific concern. Here we investigated changes in the destructiveness of tropical cyclones worldwide using the power dissipation index and found that there is no clear trend in most basins, but a significant decrease in power dissipation index has been detected in the South Indian Ocean basin since 1994, which is almost entirely due to a decrease in both tropical cyclone frequency and duration in this basin. The decrease in tropical cyclone frequency is influenced by increased atmospheric stability. The decrease in tropical cyclone duration can be attributed to the changes in tropical cyclone locations. In addition, the weakened subtropical high is only observed in the South Indian Ocean (i.e., the Mascarene High), which is also associated with the decrease in tropical cyclone destructiveness. These findings have implications for assessing the destructive potential of future tropical cyclones.

Assessing South Indian Ocean tropical cyclone characteristics in HighResMIP simulations

Assessing South Indian Ocean tropical cyclone characteristics in <scp>HighResMIP</scp> simulations

Indian Ocean Dipole (IOD) forecasts based on convolutional neural network with sea level pressure precursor

Abstract Forecasting the Indian Ocean Dipole (IOD) is crucial because of its significant impact on regional and global climates. While traditional dynamic and empirical models suffer from systematic errors due to nonlinear processes, convolutional neural networks (CNN) are nonlinear in nature and have demonstrated remarkable El Niño Southern Oscillation (ENSO) and IOD forecasting skills based on oceanic predictors, particularly sea surface temperature and heat content. However, it is difficult to measure heat content and easily introduces uncertainties, prompting the need to explore atmospheric predictors for IOD forecasts. Based on sensitivity prediction experiments, we identified the sea level pressure (SLP) signal as a crucial predictor, which forecasts IOD at a 7 month lead. In addition, the CNN model improves monthly forecasting accuracy while reducing errors by 13.43%. Utilizing the heatmap analysis, we elucidated that the multi-seasonal predictability of the IOD primarily originates from mid-latitude climate variability. Besides ENSO signals in the Pacific Ocean, our study highlights the significant impact of remote climate forcing in the South Indian Ocean, tropical North Indian Ocean, and Northwest Pacific Ocean on IOD forecasts. By introducing the SLP precursor and extratropical zones into IOD forecasts, our study offers fresh insights into the underlying dynamics of IOD evolution.

Statistical seasonal forecasting of tropical cyclone landfalls on Taiwan Island

Forecasting tropical cyclone (TC) activities has been a topic of great interest and research. Taiwan Island (TW) is one of the key regions that is highly exposed to TCs originated from the western North Pacific. Here, the authors utilize two mainstream reanalysis datasets for the period 1979–2013 and propose an effective statistical seasonal forecasting model—namely, the Sun Yat-sen University (SYSU) Model—for predicting the number of TC landfalls on TW based on the environmental factors in the preseason. The comprehensive predictor sampling and multiple linear regression show that the 850-hPa meridional wind over the west of the Antarctic Peninsula in January, the 300-hPa specific humidity over the open ocean southwest of Australia in January, the 300-hPa relative vorticity over the west of the Sea of Okhotsk in March, and the sea surface temperature in the South Indian Ocean in April, are the most significant predictors. The correlation coefficient between the modeled results and observations reaches 0.87. The model is validated by the leave-one-out and nine-fold cross-validation methods, and recent 9-yr observations (2014–2022). The Antarctic Oscillation, variabilities of the western Pacific subtropical high, Asian summer monsoon, and oceanic tunnel are the possible physical linkages or mechanisms behind the model result. The SYSU Model exhibits a 98% hit rate in 1979–2022 (43 out of 44), suggesting an operational potential in the seasonal forecasting of TC landfalls on TW.摘要本文利用1979–2013年的两个主流再分析数据集, 提出了一个中山大学 (SYSU) 热带气旋统计季节预报模型, 基于4个季前环境因子对登陆台湾岛的热带气旋数量进行预报. 模型通过了留一法, 九折交叉验证法和近9年观测数据 (2014–2022) 的验证, 模型结果与实际观测的相关系数达0.87. 南极涛动, 西太平洋副热带高压变化, 亚洲夏季风和海洋通道是模型潜在的物理联系或机制. SYSU模型在1979–2022年期间的预报准确率为98%, 表现出其业务应用价值.

Late Quaternary responses to glacial-interglacial climate cycles of offshore Benguela region and associated foraminifera in the southeast Atlantic along Namibia and western South Africa

The Benguela Upwelling System (BUS) exerts a major oceanographic and sedimentary influence on the Namibian and western South African continental margin. Previous literature has distinguished between the Northern Benguela Region (NBR) and Southern Benguela Region (SBR) due to the varying sedimentary, oceanographic, upwelling and faunal nature of the two areas. A comprehensive understanding of how upwelling and palaeoceanography between these two subsystems relate to, or differ between each other, is limited. Late Quaternary foraminifera are extremely useful in recording and tracking past oceanographic and climatic changes in the region. Planktic foraminifera from two cores recovered from the western continental slope of South Africa, at a water depth of ∼3500 m, were analysed and compared to previous studies from the region to determine any variations in the water mass, oxygenation, upwelling and productivity history between the NBR and SBR, and during glacial-interglacial periods. Results from this study indicate that influence from subpolar, subtropical and upwelled surface waters play a major role in the faunal distributions and oceanographic history of the area. Abundances of subpolar species are lowest off western South Africa during interglacial periods, indicating obstruction from the subtropical convergence and restriction of the polar fronts during those periods. Benthic and planktic foraminifera indicate strengthened upwelling and the extension of upwelling cells further slopeward during glacial periods. Increased productivity associated with stronger upwelling during glacial periods led to increased nutrient and organic matter delivery to the seafloor, resulting in eutrophic conditions. These environments are further amplified by nutrient-rich bottom water masses that are strengthened during these periods. Upwelling intensity and productivity also exert controls on the benthic foraminifera, where epifaunal taxa abundances and epifaunal-infaunal ratios increase from the NBR to SBR, and glacial epifaunal-infaunal ratios decrease compared to interglacial periods. Higher abundances of tropical-subtropical species were recorded in the NBR and Agulhas retroflection relative to western South Africa, indicative of the influence of the Angola-Benguela Front to the north of upwelling zones and the inflow of warm Agulhas Current waters from the South Indian Ocean to the southwest of South Africa. This study therefore reveals that the NBR and SBR vary in their palaeoceanographic record as a result of their position to important southeast Atlantic oceanographic features such as the Angola-Benguela Front, the Agulhas Current, the Benguela Current and polar fronts.

Unveiling the global influence of tropical cyclones on extreme waves approaching coastal areas

Tropical and extra-tropical storms generate extreme waves, impacting both nearby and remote regions through swell propagation. Despite their devastating effects in tropical areas, the contribution of tropical cyclones (TCs) to global wave-induced coastal risk remains unknown. Here, we enable a quantitative assessment of TC’s role in extreme waves approaching global coastlines, by designing twin oceanic wave simulations with and without realistic TC wind forcing. We find that TCs substantially contribute to extreme breaking heights in tropical regions (35-50% on average), reaching 100% in high-density TC areas like the North Pacific. TCs also impact remote TC-free regions, such as the equatorial Pacific experiencing in average 30% of its extreme wave events due to TCs. Interannual variability amplifies TC-induced wave hazards, notably during El Niño in the Central Pacific, and La Niña in the South China Sea, Caribbean Arc, and South Indian Ocean coastlines. This research offers critical insights for global risk management and preparedness.

Decreasing global tropical cyclone frequency in CMIP6 historical simulations.

The impact of anthropogenic global warming on tropical cyclone (TC) frequency remains a challenging issue, partly due to a relatively short period of reliable observational TC records and inconsistencies in climate model simulations. Using TC detection from 20 CMIP6 historical simulations, we show that the majority (75%) of these models show a decrease in global-scale TC frequency from 1850 to 2014. We demonstrated that this result is largely explained by weakened mid-tropospheric upward motion in CMIP6 models over the Pacific and Atlantic main development regions. The reduced upward motion is due to a zonal circulation adjustment and shifts in Intertropical Convergence Zone in response to global warming. In the South Indian Ocean, reduced TC frequency is mainly due to the decreased survival rate of TC seeds because of an increased saturation deficit in a warming climate. Our analysis highlights global warming's potential impact on the historical decrease in global TC frequency.

Asian summer monsoon responses under RCP4.5 and RCP8.5 scenarios in CESM large ensemble simulations

The response of the Asian Summer Monsoon (ASM) circulation to the Representative Concentration Pathway 4.5 and 8.5 (RCP4.5 and RCP8.5) forcing scenarios is examined using the CESM1 state-of-the-art global circulation model from 2021 to 2050. The projections show that monsoon precipitation will increase over East Asia, the North Pacific Ocean, the Indian Peninsula, and the Bay of Bengal under the RCP4.5 scenario. Conversely, the South Indian Ocean, West Asia, the Middle East, and the Central Pacific Ocean exhibit a decreasing trend in precipitation. Under the RCP8.5 scenario, precipitation is projected to increase over a wider swath of the Indian Ocean and the Middle East Asia. In the RCP4.5 scenario, the low-level wind circulation is likely to strengthen over the entire northern Indian Ocean, extending to the South China Sea, thereby increasing moisture transport from the Indian Ocean to peninsular India and the South China Sea. Conversely, in the RCP8.5 scenario, easterly winds strengthen over the South Indian Ocean, leading to an increase in moisture transport from the equatorial West Pacific Ocean to the Indian Ocean. A weak (strong) cyclonic circulation in response to the east-centered (west-centered) low sea level pressure trend over the North Pacific in RCP4.5 (RCP8.5) scenario is projected to help maintaining a strong (weak) ASM circulation from the India to east Asia. Internal climate variability is also calculated, revealing that the North Pacific Ocean near the Bering Sea is likely to play a dominating role and contribute significantly to the future ASM dynamics. In both scenarios, internal variability is found to substantially contribute to changes in monsoon circulation over the Indian Ocean.

Interannual variability of the East African Coastal Current associated with the El Niño-Southern Oscillation

Abstract The East African Coastal Current (EACC) is an important western boundary current of the tropical South Indian Ocean and plays an important role in the ocean circulation and biogeochemical cycles in the Indian Ocean. This study investigates the interannual variability of the EACC and its dynamical mechanisms. The result shows that the EACC has interannual variability associated with the El Niño-Southern Oscillation (ENSO) during 1993-2017. The EACC shows a significantly positive correlation with the Niño3.4 index with a correlation coefficient of 0.65, lagging the Niño3.4 index by 18 months. During the decaying phases of El Niño (La Niña) events, the negative (positive) sea level anomaly (SLA) propagates westward as upwelling (downwelling) Rossby waves from the southeast Indian Ocean to the southwest Indian Ocean, and then strengthens (weakens) the EACC due to zonal SLA gradient off the East African coast under geostrophic equilibrium. The SLA gradually weakens in the southeast Indian Ocean during its westward propagation but strengthens in the southwest Indian Ocean promoted by local wind stress curl anomaly. This study can improve our understanding of the relationship between the western boundary current of the tropical South Indian Ocean and large-scale ENSO air-sea processes, and is important for managing marine fisheries and ecosystems on the East African coast.

The distribution and generation of carbonatites

The physio-chemical framework that generates carbonatites and, ultimately, the associated rare earth element deposits remains contentious. This primarily reflects the diverse tectonic settings in which carbonatites occur: large igneous provinces, continental rifts and major extensional terranes, syn- to post-collisional settings, or ocean islands. There is, however, a broad consensus that carbonatites (or their parental melts) originate in the mantle. These exotic melts have small volumes that make them ideal probes of conditions in their underlying source regions. We combine the carbonatite locations with global maps of lithospheric thickness, derived from seismic tomography, and show that post-Neoproterozoic carbonatites occur preferentially above the margins of thick cratonic lithosphere (e.g., adjacent to the South Atlantic and Indian Oceans or in North America, Greenland, and Asia) and where once thick lithosphere has undergone stretching (e.g., eastern Asia). Our thermal modeling reveals that lateral and vertical heat conduction on rifted craton margins, or rapid stretching of cratonic lithosphere, can mobilize carbonated peridotite at the temperatures (950−1250 °C) and pressures (2−3 GPa) required to form primary carbonatites or their parental alkali silicate melts. Importantly, our models show that heat conduction from upwelling mantle plumes or ambient mantle on rifted cratonic margins may sufficiently modify the temperature of the lithospheric mantle to cause melting of carbonated peridotite, settling the long-standing debate on the role of rifting and heating in the generation of carbonatites.

Trend and interannual variability of summer marine heatwaves in the tropical Indian ocean: Patterns, mixed layer heat budget, and seasonal prediction

Marine heatwaves (MHWs) are extreme sea surface temperature (SST) events in all ocean basins, with far-reaching impacts on marine ecosystems and socio-economy. The leading patterns, trend, and interannual variability of summer MHWs in the tropical Indian Ocean (TIO) are investigated in this study. The first empirical orthogonal function (EOF) mode of frequency of MHWs exhibits a monopole pattern over the entire basin. This mode is highly associated with the concurrent Indian Ocean Basin warming, indicating a remarkable trend over the past four decades. The linear trend in MHW properties largely relates to increased summer Indian Ocean mean SST. The second EOF mode exhibits a zonal dipole with the MHW numbers increasing in the west and decreasing in the east. On the interannual timescale, the first two EOF modes are remotely affected by antecedent and concurrent El Niño events, respectively. The ocean mixed layer budget is utilized for examining the formation of different summer MHW patterns. During the preceding spring, the surface heat flux is important for the development of MHWs, while the ocean advections play a secondary role in the South Indian Ocean for the MHW monopole. Once the SST anomaly rises in summer, the ocean advections play a dominant role in maintaining the SST. Last, we assess the prediction skill of summer TIO MHWs by performing a bilinear seasonal statistical prediction model. Our results suggest the frequency of summer MHWs in the TIO could be predicted one season in advance. This study has great implications for understanding and predicting ocean extreme events in the TIO.

Insights into the origin of precipitation moisture for tropical cyclones during rapid intensification process

In this study, we identified the moisture sources for the precipitation associated with tropical cyclones (TCs) during the rapid intensification (RI) process from 1980 to 2018 by applying a Lagrangian moisture source diagnostic method. We detected sixteen regions on a global scale for RI events distributed as follows: four in the North Atlantic (NATL), two in the Central and East Pacific Ocean (NEPAC), the North Indian Ocean (NIO) and South Indian Ocean (SIO), and three in the South Pacific Ocean (SPO) and the Western North Pacific Ocean (WNP). The moisture uptake (MU) mostly was from the regions where TCs underwent RI. The Western NATL, tropical NATL, Caribbean Sea, the Gulf of Mexico and the Central America and Mexico landmass supported ∼85.4% of the precipitating moisture in the NATL, while the latter source and the eastern North Pacific Ocean provided the higher amount of moisture in NEPAC (∼84.3%). The Arabian Sea, the Bay of Bengal and the Indian Peninsula were the major moisture sources in NIO, contributing approximately 81.3%. The eastern and western parts of the Indian Ocean supplied most of the atmospheric humidity in SIO (∼83.8%). The combined contributions (∼87.9%) from the western and central SPO and the Coral Sea were notably higher in SPO. Meanwhile, TCs in the WNP basin mostly received moisture from the western North Pacific Ocean, the Philippine Sea and the China Sea, accounting for 80.1%. The remaining moisture support in each basin came from the summed contributions of the remote sources. Overall, RI TCs gained more moisture up to 2500 km from the cyclone centre than those slow intensification (SI) and the total MU was approximately three times higher during RI than SI. Finally, the patterns of the MU differences respond to the typical pathways of moisture transport in each basin.

Characterizing the stable oxygen isotopic composition of the southeast Indian Ocean

New seawater stable oxygen isotope (δ18O) samples were collected from the southeast Indian Ocean as part of the Coring to Reconstruct Ocean Circulation and Carbon dioxide Across 2 Seas (CROCCA-2S) expedition in November – December of 2018. These data fill a gap in the δ18O sampling coverage of the southern Indian basin, providing new insights into the hydrologic and oceanographic processes influencing the δ18O distribution of the region and its relationship to salinity in the upper ocean. Our surface ocean data (<100 m)—in combination with decades of observations from the broader south Indian Ocean—show distinct δ18O – salinity characteristics on either side of ∼85°E. The balance between evaporation and precipitation yields a strong, robust δ18O – salinity relationship west of 85°E (δ18O = 0.50(±0.01) * S – 17.2(±0.22)). However, within the mesoscale eddy field initiated by the Leeuwin Current further east (∼85–120°E), our observations fall along a mixing line between the southwest Indian Ocean and data collected from the Australian coastal margin, illustrating for the first time how the unique eastern boundary system of the south Indian Ocean drives regional-scale variability in the δ18O – salinity relationship of the surface ocean. A comparison between our observations in the shallow subsurface (100–1000 m) and those from neighboring surveys reinforces this upper ocean connection across the Indo-Australian basin. Antarctic Intermediate Water from the Indian Ocean can be isotopically distinguished from the more regional Tasman Intermediate Water occupying the South Australian Bight, suggesting exchange between the two regions is most prevalent at surface and mode water depths. In deeper waters (> ∼1500 m), we observe a notable 0.87‰ spread in δ18O. This variability may represent interactions between distinct deep water masses in the region, although additional data are needed to confirm. Overall, our data provide a new look at the hydrography and isotopic chemistry of the southeast Indian Ocean, emphasizing the impact of the region's mesoscale eddy field and its interconnectivity with neighboring basins.

Future Slower Reduction of Anthropogenic Aerosols Enhances Extratropical Ocean Surface Warming Trends

AbstractGlobal surface temperature short‐term trends fluctuate between cooling and fast‐warming under the combined action of external forcing and internal variability, significantly influencing the detectability of near‐term climate change. A key driver of these variations is anthropogenic aerosols (AAs), which have undergone a non‐monotonic evolution with rapid reduction in recent decades. However, their reduction is projected to decelerate under a high carbon emission scenario, yet the impact on surface temperature trends remains unknown. Here, using initial‐perturbation large ensembles, we find that future slowdown in AA reduction over Europe and North America expedites the subpolar North Atlantic surface warming by intensifying the Atlantic meridional overturning circulation. Further, it accelerates the South Indian Ocean and Southern Ocean surface warming through positive low‐cloud feedback and oceanic dynamical adjustment, triggered by the poleward migration of westerlies under interhemispheric energy constraint. These AA‐driven warmings exacerbate greenhouse warming, significantly enhancing the detectability of local decadal warming trends.

Lagged effect of Southern Annular Mode on chlorophyll-a in the mid-latitude South Pacific and Indian Oceans

This study investigates the influence of the Southern Annular Mode (SAM) on chlorophyll-a (Chl-a) concentrations and the underlying mechanisms governing their associated environmental variations in the mid-latitude (35–50° S) ocean from 1998 to 2021. The intensification of westerly winds during positive SAM phases influences meridional water transport and mixed layer depth (MLD), which are both critical factors that affect surface nutrient availability. A marked contrast in the relationship between the meridional current anomaly and the SAM was observed, with reduced northward transport of nutrient-rich water in regions north of 50° S during positive SAM phases. This reduction could be attributed to the poleward migration of the westerly winds, which impeded the meridional current from reaching the mid-latitudes. The relationship between SAM and MLD south of 50° S was positive whereas that in the mid-latitude eastern (60–110° E) South Indian Ocean and eastern (90–140° W) South Pacific Ocean was negative or weak. The immediate effect of a more positive SAM on Chl-a in the mid-latitude ocean was reduced productivity caused by enhanced nutrient depletion. However, in the mid-latitude eastern South Pacific Ocean, the northward migration of the zonal mean meridional current anomaly closely aligned with the lagged correlation pattern between SAM variability and Chl-a over time, suggesting that the delayed northward transport of nutrient-rich waters may partially counterbalance the immediate effects of the SAM on ocean productivity. This mechanism was not present in the mid-latitude eastern South Indian Ocean, implying that future climate change may variably affect these regions. Our findings emphasize the importance of considering regional differences and temporal lags when evaluating the influence of SAM variability on ocean productivity and nutrient dynamics in the context of climate change.

Modulation of the Madden–Julian Oscillation Center Stagnation on Typhoon Genesis over the Western North Pacific

Madden–Julian Oscillation (MJO) modulates the generation of typhoons (TYs) in the western North Pacific (WNP). Using IBTrACS v04 tropical cyclone best path data, ERA5 reanalysis data, and the MJO index from the Climate Prediction Center (CPC), this paper defines an index to describe the persistent anomalies of the MJO and to examine the statistical characteristics of TYs over 44 years (1978–2021), focusing on the analysis of major differences in environmental conditions after the removal of the ENSO signal over the WNP. The results indicate that the persistent anomalous state of the MJO influences the change in large-scale environmental factors, which, in turn, affects the generation of TYs, as follows: (1) For the I high-value years, the center of the MJO stagnates in the Indian Ocean–South China Sea (SCS), the monsoon trough retreats westward, the warm pool becomes warmer, and the Walker circulation is enhanced. There is stronger upper-level divergence and low-level convergence, larger low-level relative vorticity, higher mid-level relative humidity, and smaller vertical wind shear in the SCS and the seas near the Philippines. Consequently, these conditions foster a conducive environment for TY genesis in the SCS and the seas near the Philippines. (2) For the I low-value years, the center of the MJO stagnates in the WNP–North America region, the monsoon trough extends eastward, the warm pool becomes colder, and the Walker circulation is weakened. Consequently, these conditions are more likely to facilitate TY genesis in the central–eastern WNP. The results show that persistent anomalies in MJO active centers can effectively improve the predictive ability of TY frequency.

Transitions in surface thermal signatures during the evolution of long-lived eddies in the global ocean

Mesoscale eddies are quasi-circular ocean currents accompanied by dynamic and thermodynamic variations, and are classified as anticyclonic (AEs) and cyclonic eddies (CEs) according to their rotation direction. Eddies transition between surface cold CEs/surface warm AEs (“conventional” eddies) and surface warm CEs/surface cold AEs (“unconventional” eddies) during their lifetimes. However, the characteristics and mechanisms of these transitions remain unclear. In this study, satellite and in-situ data were used to explore the spatiotemporal variation and vertical structures of “conventional” and “unconventional” eddies during the evolution of long-lived eddies (>1 year) in the global ocean from 1993 to 2015. On average, long-lived eddies are “unconventional” for about 40% of their lifetimes, and these “unconventional” eddies are concentrated in the South Atlantic Ocean and off the western and southern coasts of Australia. “Unconventional” eddies show distinct temporal variation and subsurface temperature structures. Generally, cold AEs and warm CEs last <3 months and are more active in summer than in winter. They have cold or warm cores within the mixed layer depth of the water column, which is affected by eddy–wind interactions via eddy-induced Ekman pumping. However, some cold AEs in the South Atlantic and warm CEs in the South Indian Ocean can last for >3 months and display weak seasonal variation. In addition, their cold and warm cores can extend to ∼200 and ∼300 m, respectively, and are related to subduction and the Leeuwin Current System.

Global marine gravity gradient tensor inverted from altimetry-derived deflections of the vertical: CUGB2023GRAD

Abstract. Geodetic applications of altimetry have largely been inversions of gravity anomaly. Previous studies of Earth's gravity gradient tensor mostly presented only the vertical gravity gradient (VGG). However, there are six unique signals that constitute the gravity gradient tensor. Gravity gradients are signals suitable for detecting short-wavelength topographic and tectonic features. They are derived from double differentiation of the disturbing potential and hence are susceptible to noise amplification which was exacerbated by low across-track resolution of altimetry data in the past. However, current generation of altimetry observations have improved spatial resolutions, with some better than 5 km. Therefore, this study takes advantage of current high-resolution altimetry datasets to present CUGB2023GRAD, a global (latitudinal limits of ±80°) 1 arcmin model of Earth's gravity gradient tensor over the oceans using deflections of the vertical as inputs in the wavenumber domain. The results are first assessed via Laplace's equation, whereby the resultant residual gradient is virtually zero everywhere. Further analysis at local regions in the Arctic and south Indian oceans showed that Txy, Txz and Tyz are the most dominant gravity gradients for bathymetric studies. This proves that bathymetric signatures in the non-diagonal tensor components are worth exploiting. Bathymetric coherence analysis of Tzz over the Tonga Trench showed strong correlation with multibeam shipboard depths. This study proves that current generation of altimetry geodetic missions can effectively resolve Earth's gravity gradient tensor. The CUGB2023GRAD model data can be freely accessed at https://doi.org/10.5281/zenodo.10511125 (Annan et al., 2024).