Indian Ocean Temperature Research Articles

Dynamics of the Barrier Layer Dipole in the Equatorial Indian Ocean

AbstractThe barrier layer (BL) significantly impacts the upper ocean circulation and thermodynamic structure by inhibiting the heat and momentum exchange between the mixed layer (ML) and the subsurface layer. There exist sea surface temperature and salinity dipole modes in the tropical Indian Ocean, however, a BL dipole mode has not yet been identified. Using the latest observations and ocean reanalysis, here we show a robust BL dipole mode in the central and eastern equatorial Indian Ocean, which is highly correlated with the Indian Ocean Dipole (IOD) events. Composite analysis shows that the BL thickness anomalies peak in autumn and are much larger during positive IOD events than during negative IOD events. We show that a positive BL dipole phase is characterized by positive BL thickness anomalies in the central equatorial Indian Ocean and negative BL thickness anomalies in the eastern equatorial Indian Ocean, and vice versa for a negative BL dipole phase. During positive IOD events, negative surface salinity anomalies slightly affect the ML depth along the equatorial Indian Ocean. Positive subsurface temperature anomalies deepen the isothermal layer (IL) in the central equatorial Indian Ocean and strong negative subsurface temperature anomalies significantly lift the IL in the eastern equatorial Indian Ocean, controlling the BL thickness anomalies and forming a positive BL dipole pattern. This operates in an opposite direction during negative IOD events. Our study shows a close relationship between the BL dipole and the IOD and has far‐reaching implications for better understanding and predicting the IOD events.

The Effect of Indian Ocean Temperature on the Pacific Trade Winds and ENSO

AbstractA notable shift in the El Niño‐Southern Oscillation (ENSO) has been observed in the early 21st century, characterized by an increased prevalence of Central Pacific (CP) events and strengthened Pacific trade winds. This shift may be attributed to the warming tropical Indian Ocean (TIO). To investigate this, we conduct perturbation experiments using the Insitut Pierre Simon Laplace climate model and nudge TIO surface temperatures to induce warming or cooling effects. Our findings reveal that TIO warming (or cooling) leads to amplified (weakened) mean trade winds and surface warming (cooling) in the Pacific region. Surprisingly, ENSO variability increases in both TIO cooling and warming scenarios. This result is linked to stronger positive feedbacks and a less stable Bjerknes index for either TIO forcing. Additionally, we find that TIO warming leads to more frequent CP events, meridional widening of wind anomalies, and broadening of the ENSO power spectrum toward lower frequencies.

Spatial Correlations of Regional Tropical Cyclone– and Nontropical Cyclone–Induced Severe Rainstorms during 2000–19

Abstract Severe rainstorms are one of the most devastating disasters in southeast China (SEC). A deep and comprehensive understanding of the spatial correlations of severe rainstorms is important for preventing rainstorm-induced hazards. In this study, tropical cyclone– and nontropical cyclone–induced severe rainstorms (TCSRs and NTCSRs, respectively) over SEC during 2000–19 are discussed. Co-occurrence probability and range values calculated using the semivariogram method are used to measure the spatial correlations of severe rainstorms. The extent to which potential factors [El Niño/La Niña, Indian Ocean dipole (IOD), latitudes, longitudes, temperature, elevation, and radius of maximum wind] affect the spatial structure of severe rainstorms is discussed. The spatial correlation distances for TCSRs (300–700 km) in typhoon season (July, August, and September) are longer than most of those for NTCSRs (150–300 km) in mei-yu season (June and July). The range values of TCSRs at each percentile (except for the minimum range values) tend to be omnidirectional. While NTCSRs tend to have a major direction of northeast–southwest. El Niño tends to increase the average spatial correlation distance of TCSRs in the northeast–southwest and NTCSRs in the north–northeast. La Niña tends to decrease the spatial correlation distance of TCSRs in the northeast–southwest. The occurrence of positive IOD (+IOD) and negative IOD (−IOD) events may increase the spatial correlation distance of TCSRs in the northwest–southeast, and −IOD events may decrease the distance in the northeast–southwest. IOD events, especially −IOD, may change the spatial correlation distance of NTCSRs in the east–northeast. Latitudes, longitudes, temperature, elevation, and radius of maximum wind significantly affect the spatial correlation distance of TCSRs in various directions. Significance Statement Spatial correlation distances of rainfall events, especially severe rainstorms induced by different weather systems such as tropical and nontropical cyclones (TCSR and NTCSR), can provide important information for rain-induced hazard risk mitigation. Moreover, factors that affect the variability of the spatial correlation distance of TCSRs and NTCSRs are also of interest. To this end, co-occurrence probability and semivariogram methods are used to obtain the spatial correlation distances. The spatial correlation distance of TCSRs varies between 300 and 700 km in typhoon season (July, August, and September) while it ranges between 150 and 300 km in mei-yu season (June and July) for NTCSRs over southeast China (SEC). El Niño/La Niña, Indian Ocean dipole (IOD), latitudes, longitudes, temperature, elevation, and radius of maximum wind have significant impacts on the spatial correlation distances of TCSRs. These findings can provide useful information for forecasting the spatial correlation distance of severe rainstorms over SEC.

Plio‐Pleistocene Southwest African Hydroclimate Modulated by Benguela and Indian Ocean Temperatures

AbstractFuture projections of southwestern African hydroclimate are highly uncertain. However, insights from past warm climates, like the Pliocene, can reveal mechanisms of future change and help benchmark models. Using leaf wax hydrogen isotopes to reconstruct precipitation (δDp) from Namibia over the past 5 million years, we find a long‐term depletion trend (−50‰). Empirical mode decomposition indicates this trend is linked to sea surface temperatures (SSTs) within the Benguela Upwelling System, but modulated by Indian Ocean SSTs on shorter timescales. The influence of SSTs on reconstructed regional hydroclimate is similar to that observed during modern Benguela Nio events, which bring extreme flooding to the region. Isotope‐enabled simulations and PlioMIP2 results suggest that capturing a Benguela Nio‐like state is key to accurately simulating Pliocene, and future, regional hydroclimate. This has implications for future regional climate, since an increased frequency of Benguela Nios poses risk to the ecosystems and industries in the region.

Connection between the Tropical Pacific and Indian Ocean and Temperature Anomaly across West Antarctic

West Antarctic and the Antarctic Peninsula have experienced dramatic warming in austral spring since the 1970s. Using observations and the Community Atmosphere Model version 4 (CAM4), this study explores the physical mechanism by which the tropical Pacific and Indian Ocean temperature anomaly mode (PIM) affects the dipolar surface air temperature (SAT) anomalies across the West Antarctic in austral spring. The positive phase of the PIM, characterized by positive sea surface temperature anomalies (SSTAs) in the tropical central-eastern Pacific and western Indian Ocean and negative SSTAs in the Maritime Continent, can generate two branches of stationary Rossby wave trains propagating from the tropical central Pacific and southeastern Indian Ocean to the West Antarctic, with an anticyclonic anomaly appearing over the Amundsen Sea. The northerlies advect warmer air to the Ross–Amundsen Seas, but southerlies advect colder air to the Antarctic Peninsula–Weddell Sea, resulting in the dipole of SAT anomalies over the West Antarctic. In this process, the role of tropical central-eastern Pacific SSTAs dominate, and it is amplified by the SSTAs around the Maritime Continent. The SSTAs in the western Indian Ocean combined with the SSTAs over the Maritime Continent further contribute to the western pole of the SAT. Only simulation that includes a prescribed PIM forcing can exactly reproduce the observations of the dipolar SAT response across the West Antarctic, indicating the need to treat the tropical Pacific and Indian Oceans as a unified whole.

A Century of Observed Temperature Change in the Indian Ocean

AbstractThe Indian Ocean is warming rapidly, with widespread effects on regional weather and global climate. Sea‐surface temperature records indicate this warming trend extends back to the beginning of the 20th century, however the lack of a similarly long instrumental record of interior ocean temperatures leaves uncertainty around the subsurface trends. Here we utilize unique temperature observations from three historical German oceanographic expeditions of the late 19th and early 20th centuries: SMS Gazelle (1874–1876), Valdivia (1898–1899), and SMS Planet (1906–1907). These observations reveal a mean 20th century ocean warming that extends over the upper 750 m, and a spatial pattern of subsurface warming and cooling consistent with a 1°–2° southward shift of the southern subtropical gyre. These interior changes occurred largely over the last half of the 20th century, providing observational evidence for the acceleration of a multidecadal trend in subsurface Indian Ocean temperature.

Indian Ocean Dipole : Assessing its impacts on the Indian Summer Monsoon Rainfall (ISMR) across North East India

The Indian Ocean Dipole (IOD), a climatic anomaly, results in sustained sea surface temperature (SST) variations between tropical western and eastern Indian Ocean temperatures. In this study, we studied the variations to inculcate the teleconnections between IOD and Indian summer monsoon rainfall (ISMR) distribution across the country for the period 1960-2020 for all the three phases of ISMR. We analyzed rainfall, SST and low-level wind circulation anomalies for the above mentioned time horizon. Positive IOD events noticeably resulted in increase in summer monsoon rainfall distribution across the country respectively while its negative counterpart led to decrease in rainfall except for the commencement phase of ISMR. The variations in SST, wind circulation and moisture movement processes across the Indian Ocean characterize significant changes in rainfall during the positive and negative phases of IOD especially during the recent decades (1991-2020). The recent time horizon also witnesses enhanced low-level equatorial jets (LEJ) across the equatorial Indian Ocean and the Arabian Sea during the positive IOD events as compared to the prior decades (1960-1990). The effect of moisture convergence zone is also analyzed which results in above rainfall conditions across northeastern and central India. Conversely, negative IOD events were found to subdue any such moisture movement mechanisms. Furthermore, and additional investigation to analyze the effect of IOD on the retreating/withdrawal monsoon across northeast India has been done and it has been observed that a stronger positive IOD is detrimental to the seasonal rainfall (May- September) over North East India (-0.7 one month lag correlation). Furthermore, the DMI index of April-May presented a clear indication of monsoon activity over the area during the withdrawal or retreating phase of the summer monsoon, i.e., during September.

Indian Ocean Dipole : Assessing its impacts on the Indian Summer Monsoon Rainfall (ISMR) across North East India

The Indian Ocean Dipole (IOD), a climatic anomaly, results in sustained sea surface temperature (SST) variations between tropical western and eastern Indian Ocean temperatures. In this study, we studied the variations to inculcate the teleconnections between IOD and Indian summer monsoon rainfall (ISMR) distribution across the country for the period 1960-2020 for all the three phases of ISMR. We analyzed rainfall, SST and low-level wind circulation anomalies for the above mentioned time horizon. Positive IOD events noticeably resulted in increase in summer monsoon rainfall distribution across the country respectively while its negative counterpart led to decrease in rainfall except for the commencement phase of ISMR. The variations in SST, wind circulation and moisture movement processes across the Indian Ocean characterize significant changes in rainfall during the positive and negative phases of IOD especially during the recent decades (1991-2020). The recent time horizon also witnesses enhanced low-level equatorial jets (LEJ) across the equatorial Indian Ocean and the Arabian Sea during the positive IOD events as compared to the prior decades (1960-1990). The effect of moisture convergence zone is also analyzed which results in above rainfall conditions across northeastern and central India. Conversely, negative IOD events were found to subdue any such moisture movement mechanisms. Furthermore, and additional investigation to analyze the effect of IOD on the retreating/withdrawal monsoon across northeast India has been done and it has been observed that a stronger positive IOD is detrimental to the seasonal rainfall (May- September) over North East India (-0.7 one month lag correlation). Furthermore, the DMI index of April-May presented a clear indication of monsoon activity over the area during the withdrawal or retreating phase of the summer monsoon, i.e., during September.

The Initial Errors in the Tropical Indian Ocean that Can Induce a Significant “Spring Predictability Barrier” for La Niña Events and Their Implication for Targeted Observations

Initial errors in the tropical Indian Ocean (IO-related initial errors) that are most likely to yield Spring Prediction Barrier (SPB) for the La Nina forecasts are explored by using the CESM model. These initial errors can be classified into two types. Type-1 initial error consists of positive sea temperature in the western Indian Ocean and negative sea temperature errors in the eastern Indian Ocean, while the spatial structure of Type-2 initial error is nearly opposite. Both kinds of IO-related initial errors induce positive prediction errors of sea temperature in the Pacific Ocean, leading to under-prediction of La Nina events. Type-1 initial error in the tropical Indian Ocean mainly influence the SSTA in the tropical Pacific Ocean via atmospheric bridge, leading to the developments of localized sea temperature error in the eastern Pacific Ocean. While, for Type-2 initial error, its positive sea temperature error in the eastern Indian Ocean can induce downwelling error and influence the La Nina predictions through oceanic channel Indonesian ThroughFlow. Based on the large value area of these SPB-related initial errors, the sensitive area in the tropical Indian Ocean for the La Nina predictions is identified. Furthermore, applying targeted observations in the sensitive area are shown to be very useful in decreasing prediction errors of La Nina by sensitivity experiments. It is therefore promising to adopt targeted observation strategy in the tropical Indian Ocean to increase the ENSO prediction skills.

Plio‐Pleistocene Continental Hydroclimate and Indian Ocean Sea Surface Temperatures at the Southeast African Margin

AbstractEfforts to understand long‐term Indian Ocean dynamics and land‐sea linkages in southeast Africa during periods of significant global and regional climate change have been inhibited by a lack of high‐resolution climate records, particularly during the Plio‐Pleistocene. Here we present new biomarker and pollen records from International Ocean Discovery Program (IODP) Site U1478, located at the Upper Agulhas Confluence near the Limpopo River mouth, to establish environmental conditions at the southeast African margin between 4 and 1.8 Ma and address this spatiotemporal gap. Compound‐specific hydrogen isotopes of terrestrial leaf waxes (δDwax) and TEX86, using marine archaeal lipids, document hydroclimate variability and sea surface temperature (SST), respectively, permitting an onshore‐offshore climate comparison. The U1478 records establish the Limpopo catchment response to the switch in Indonesian Throughflow source waters, the mid‐Pliocene Warm Period, and intensification of Northern Hemisphere glaciations at ∼2.7 Ma. Broad coherence between the δDwax and SST records supports a linkage between Indian Ocean temperatures and southeast African hydroclimate. We hypothesize that additional mechanisms including Indian Ocean cross‐basin SST gradients (ΔSST) and high latitude glaciation acted as hydroclimate controls during the Plio‐Pleistocene. We use ΔSST to evaluate ocean‐atmosphere patterns similar to the Indian Ocean Dipole (IOD) and establish generally wetter conditions in the region associated with positive IOD‐like phases. Additionally, an obliquity signal evident in the δDwax record indicates that glacial‐interglacial variability likely influenced the tropical rain belt position and also controlled rainfall. Hydroclimate and environmental conditions across the Plio‐Pleistocene in southeast Africa may have important implications for regional hominin evolution.

Observed rainfall changes in the past century (1901–2019) over the wettest place on Earth

Changes in rainfall affect drinking water, river and surface runoff, soil moisture, groundwater reserve, electricity generation, agriculture production and ultimately the economy of a country. Trends in rainfall, therefore, are important for examining the impact of climate change on water resources for its planning and management. Here, as analysed from 119 years of rainfall measurements at 16 different rain gauge stations across northeast India, a significant change in the rainfall pattern is evident after the year 1973, with a decreasing trend in rainfall of about 0.42 ± 0.024 mm dec−1. The wettest place of the world has shifted from Cherrapunji (CHE) to Mawsynram (MAW) (separated by 15 km) in recent decades, consistent with long-term rainfall changes in the region. The annual mean accumulated rainfall was about 12 550 mm at MAW and 11 963 mm at CHE for the period 1989–2010, as deduced from the available measurements at MAW. The changes in the Indian Ocean temperature have a profound effect on the rainfall in the region, and the contribution from the Arabian Sea temperature and moisture is remarkable in this respect, as analysed with a multivariate regression procedure for the period 1973–2019. The changes in land cover are another important aspect of this shift in rainfall pattern, as we find a noticeable reduction in vegetation area in northeast India in the past two decades, implying the human influence on recent climate change.

Vertical Structure of the Upper–Indian Ocean Thermal Variability

AbstractMulti-time-scale variabilities of the Indian Ocean (IO) temperature over 0–700 m are revisited from the perspective of vertical structure. Analysis of historical data for 1955–2018 identifies two dominant types of vertical structures that account for respectively 70.5% and 21.2% of the total variance on interannual-to-interdecadal time scales with the linear trend and seasonal cycle removed. The leading type manifests as vertically coherent warming/cooling with the maximal amplitude at ~100 m and exhibits evident interdecadal variations. The second type shows a vertical dipole structure between the surface (0–60 m) and subsurface (60–400 m) layers and interannual-to-decadal fluctuations. Ocean model experiments were performed to gain insights into underlying processes. The vertically coherent, basinwide warming/cooling of the IO on an interdecadal time scale is caused by changes of the Indonesian Throughflow (ITF) controlled by Pacific climate and anomalous surface heat fluxes partly originating from external forcing. Enhanced changes in the subtropical southern IO arise from positive air–sea feedback among sea surface temperature, winds, turbulent heat flux, cloud cover, and shortwave radiation. Regarding dipole-type variability, the basinwide surface warming is induced by surface heat flux forcing, and the subsurface cooling occurs only in the eastern IO. The cooling in the southeast IO is generated by the weakened ITF, whereas that in the northeast IO is caused by equatorial easterly winds through upwelling oceanic waves. Both El Niño–Southern Oscillation (ENSO) and IO dipole (IOD) events are favorable for the generation of such vertical dipole anomalies.

Warming Trends in the Central Equatorial Indian Ocean and the Associated Coupled Feedback Processes

Mohapatra, S. and Gnanaseelan, C., 2020. Warming trends in the central equatorial Indian Ocean and the associated coupled feedback processes. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 39-45. Coconut Creek (Florida), ISSN 0749-0208.The recent increasing trends in the tropical Indian Ocean (TIO) temperature have strong impact on the regional as well as global climate variability. In this work we studied the changes in the boreal summer monsoon winds and the associated monsoon currents on modulating the surface and subsurface temperature of the equatorial Indian Ocean. The reduction in the meridional sea surface temperature (SST) gradient and strengthening of zonal SST gradient over the Indian Ocean reduced the northward propagation of south-westerly wind, thereby enhancing the surface westerlies over the equator. The strengthened surface westerlies increased the equatorial downwelling Kelvin waves and deepened the thermocline. The present study unravels the causes for the observed confinement of strong SST trend over the central equatorial Indian Ocean (CEIO) even though surface westerlies and the associated downwelling extend upto the eastern equatorial Indian Ocean (EEIO). In contrast to the CEIO surface warming, EEIO experienced strong subsurface warming at thermocline. Our analysis suggests that the upper ocean stratification plays an important role in the observed warming trend of both CEIO surface and EEIO subsurface. The eastward equatorial currents bring warm salty water from Arabian Sea and WEIO towards east and warms CEIO, however the high saline surface water is found to sink in the EEIO, thereby warming the subsurface instead of surface. The composite of temperature anomaly for the period 1978-2002, the period of warm Pacific Decadal Oscillation, shows patterns similar to temperature trends, which clearly suggests the dominance of decadal variability on the TIO temperature trends.

The Initial Condition Errors Occurring in the Indian Ocean Temperature That Cause “Spring Predictability Barrier” for El Niño in the Pacific Ocean

AbstractThe Community Earth System Model is used to explore the influences of the initial condition errors in sea temperatures in the Indian Ocean (related initial condition errors) on the uncertainties in the Pacific El Niño predictions. Two categories of Indian Ocean‐related initial condition errors are shown to frequently induce the “spring predictability barrier” for El Niño events. The category‐1 error consists of a positive Indian Ocean Dipole‐like sea temperature pattern over the Indian Ocean while the category‐2 initial condition error has a negative Indian Ocean Dipole‐like sea temperature structure and is nearly opposite to that of the category‐1. Both categories of errors underestimate the Pacific El Niño in terms of its amplitude, although they present different spatial structures and undergo different mechanisms. Specifically, the category‐1 error induces negative errors in El Niño predictions in terms of its amplitude by tropical oceanic channel Indonesian Throughflow while the category‐2 error exerts its impact on El Niño prediction and makes it underestimate through the atmospheric bridge. Both categories of initial condition errors emphasize the sensitivity of El Niño predictions to the initial uncertainties in sea temperatures in the Indian Ocean. They may therefore provide useful information on how initialization in the Indian Ocean can be utilized to improve El Niño predictions.

末次冰消期内印度洋海温对冰盖融水响应的模拟研究

末次冰消期内印度洋海温对冰盖融水响应的模拟研究

Indian Ocean corals reveal crucial role of World War II bias for twentieth century warming estimates

The western Indian Ocean has been warming faster than any other tropical ocean during the 20th century, and is the largest contributor to the global mean sea surface temperature (SST) rise. However, the temporal pattern of Indian Ocean warming is poorly constrained and depends on the historical SST product. As all SST products are derived from the International Comprehensive Ocean-Atmosphere dataset (ICOADS), it is challenging to evaluate which product is superior. Here, we present a new, independent SST reconstruction from a set of Porites coral geochemical records from the western Indian Ocean. Our coral reconstruction shows that the World War II bias in the historical sea surface temperature record is the main reason for the differences between the SST products, and affects western Indian Ocean and global mean temperature trends. The 20th century Indian Ocean warming pattern portrayed by the corals is consistent with the SST product from the Hadley Centre (HadSST3), and suggests that the latter should be used in climate studies that include Indian Ocean SSTs. Our data shows that multi-core coral temperature reconstructions help to evaluate the SST products. Proxy records can provide estimates of 20th century SST that are truly independent from the ICOADS data base.

Multidecadal Indian Ocean Variability Linked to the Pacific and Implications for Preconditioning Indian Ocean Dipole Events

Abstract The Indian Ocean has sustained robust surface warming in recent decades, but the role of multidecadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature variations are explored. Simulated upper-ocean temperature changes across the Indian Ocean in the hindcast are consistent with those recorded in observational products and ocean reanalyses. Indian Ocean temperatures exhibit strong warming trends since the 1950s limited to the surface and south of 30°S, while extensive subsurface cooling occurs over much of the tropical Indian Ocean. Previous work focused on diagnosing causes of these long-term trends in the Indian Ocean over the second half of the twentieth century. Instead, the temporal evolution of Indian Ocean subsurface heat content is shown here to reveal distinct multidecadal variations associated with the Pacific decadal oscillation, and the long-term trends are thus interpreted to result from aliasing of the low-frequency variability. Transmission of the multidecadal signal occurs via an oceanic pathway through the Indonesian Throughflow and is manifest across the Indian Ocean centered along 12°S as westward-propagating Rossby waves modulating thermocline and subsurface heat content variations. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline depth in the 1970s and 1980s is associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin’s leading mode of variability, with implications for regional climate and vulnerable societies in surrounding countries.

Tectonic and climate controls on Neogene environmental change in the Zhada Basin, southwestern Tibetan Plateau

Abstract Evidence for a long-lived, high-elevation plateau contradicts the hypothesis that uplift of the southern Tibetan Plateau was a driver of late Miocene–Pliocene changes in ecology and monsoon strength. We illuminate the mechanisms underlying late Miocene–Pliocene environmental changes using a multi-proxy record from the Zhada Basin, southwestern Tibetan Plateau, between ca. 9.2 and 2.3 Ma. An increase in mean carbonate δ18Oc and δ13Cc values, decrease in grain size, and onset of lacustrine deposition at 6.0 Ma is attributed to local tectonic damming and transition from a through-flowing fluvial system to a terminal lake. This is followed at 3.5 Ma by a decrease in lake size indicated by synchronous increase in grain size, progradation of lake-margin depositional systems, increase in δ18Oc values at lake-central locations, and divergence in δ18Oc values between inflowing and lake water. We attribute lake shrinking to decreasing Indian summer monsoon precipitation due to the combined effects of decreased southern Tibetan Plateau elevations and cooler Indian Ocean temperatures. We attribute increased grain size and δ18Oc value variability to changes in fluvial discharge due to increased sensitivity to orbital forcing, itself possibly coupled with onset of glacial conditions at high elevations. These mechanisms link regional tectonic events to Pliocene changes in climate and environmental conditions, including erosion, granularity, sediment accumulation rates, and potentially biological turnover on and around the Tibetan Plateau.

Evolution of Indian Ocean dipole and its forcing mechanisms in the absence of ENSO

The evolution of Indian Ocean dipole (IOD) and its forcing mechanisms are examined based on the analysis of coupled model simulations that allow or suppress the El Nino-Southern Oscillation (ENSO) mode of variability. The model can reproduce the most salient observed features of IOD even without ENSO, including the relationships between the eastern and western poles at both the surface and subsurface, as well as their seasonality. This suggests that ENSO is not fundamental for the existence of IOD. It is demonstrated that cold (warm) sea surface temperature (SST) anomalies in the eastern Indian Ocean associated with IOD can be initiated by springtime Indonesian rainfall deficit (surplus) through local surface wind response. The growth of the SST anomalies depends on the initial local subsurface condition. Both the air–sea interaction and surface–subsurface interaction contribute to the development of IOD. The evolution of IOD can be represented by two leading extended empirical orthogonal function (EEOF) modes of tropical surface–subsurface Indian Ocean temperatures; one stationary and the other non-stationary. The onset, growth, and termination of IOD, as well as the transition to an opposite phase, can be interpreted as alternations between the non-propagating mode (EEOF1) and the eastward-propagating Kelvin wave (EEOF2). The evolution of IOD is also accompanied by a westward-propagating Rossby wave which is captured in the EEOF1 of the subtropical surface–subsurface Indian Ocean temperatures. Therefore, both Bjerknes feedback and a delayed oscillator operate during the evolution of IOD in the absence of ENSO also.

A 117-year long index of the Pacific-Japan pattern with application to interdecadal variability

The Pacific-Japan (PJ) pattern affects interannual variability in the East Asian and western North Pacific (WNP) summer monsoons. This teleconnection pattern is characterized by a meridional dipole of anomalous circulation and precipitation between the tropical WNP and the midlatitudes. This study develops a long index of the PJ pattern using station-based atmospheric pressure data to track the PJ variability from 1897 to 2013. This index is correlated with a wide array of climate variables including air temperature, precipitation, Yangtze River flow, Japanese rice yield and the occurrence of tropical cyclones over the WNP (especially those that make landfall on the Chinese and Korean coast). For the recent three decades, the PJ index reproduces well-known correlations with El Nino-Southern Oscillation (ENSO) in the preceding boreal winter and Indian Ocean temperature in the concurrent summer. For the 117-year period, this ENSO-PJ relationship varies on interdecadal time scales, with low correlations in the 1920s and from the 1940s to 1970s, and recurrences of significant correlations at the beginning of the 20th century and the 1930s. In accordance with the modulation, the magnitude and regional climate effect of the PJ variability have changed. These results highlight the importance of interdecadal modulations of climate anomalies in the summer WNP and the need of long-term observations to study such modulations.