Positive Indian Ocean Dipole Events Research Articles

Oceanic Rossby Waves Induced Two Types of Ocean–Atmosphere Response and Opposite Indian Ocean Dipole Phases

AbstractThis study analyzed the downwelling Rossby waves in the south Indian Ocean (IO)-induced spring asymmetric mode and the relationship with the Indian Ocean dipole (IOD) event based on observations and reanalysis datasets. The westward downwelling Rossby waves favor significant sea surface temperature (SST) warming in the Seychelles thermocline dome that triggers atmosphere response and the asymmetric mode in spring. The zonal sea level pressure gradient causes anomalous easterly winds in the central and eastern equatorial IO, cooling the SST off Sumatra–Java. Meanwhile, the remainder of the downwelling Rossby waves reach the west coast, transform to northward coastal-trapped waves, and then reflect as eastward downwelling Kelvin waves along the equator. The downwelling Kelvin waves reach the Sumatra–Java coast during late spring to early summer, favoring SST warming in the southeastern tropical Indian Ocean. Thus, there are two types of ocean–atmosphere response almost at the same time along the equator. The final SST status depends on which process is stronger and, as a consequence, triggers a negative or a positive phase of the IOD event in the fall season. The results show four positive and three negative IOD events related to the above processes from 1960 to 2019. The strong downwelling Rossby waves are easier to induce an intense asymmetric mode and negative IOD event, usually associated with preceding strong El Niño in the Pacific. In contrast, the weak downwelling Rossby waves tend to induce a weak asymmetric mode and positive IOD event, usually associated with preceding weak El Niño or anomalous anticyclonic atmospheric circulation in the southeastern IO.

Combined role of ENSO and IOD on compound drought and heatwaves in Australia using two CMIP6 large ensembles

Compound drought and heatwaves (CDHWs) can cause significant socio-economic and ecological impacts. A better understanding of historical spatiotemporal changes of Australian CDHWs and their underlying physical mechanisms can help improve their predictability. We analyse changes in Australian CDHW metrics in the extended summer season (November to March) during the period 1958–2020. Our results suggest that CDHWs increased significantly in terms of their frequency, duration, amplitude, and severity in the recent period (1989/90–2019/20) relative to a historical period (1958/59–1988/89), particularly in eastern Australia. We further analysed the influence of co-occurring modes of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on the frequency and severity of Australian CDHWs using two Coupled Model Intercomparison Project phase 6 (CMIP6) single model initial-condition large ensembles. We found that frequency, duration, and severity of CDHWs are significantly increased during strong El Niño phases across northeast Australia compared to neutral ENSO and IOD conditions. This increase is widespread over northeast and southeast Australia during the concurrence of strong El Niños and moderate-strong positive IOD events. Our results show that an extreme CDHW season in terms of their frequency, duration, and severity occurs one out of every two seasons of strong El Niño over the northeast Australia. Moreover, the same applies for the co-occurring strong El Niño and positive IOD over the northeast and southeast Australia as well. Our results highlight the importance of ENSO and IOD combinations for Australian CDHW events. Our findings provide insights into the importance of climate variability in driving Australian CDHWs.

The increased frequency of combined El Niño and positive IOD events since 1965s and its impacts on maritime continent hydroclimates

The Indian and Pacific Oceans surround the Maritime Continent (MC). Major modes of sea surface temperature variability in both oceans, including the Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO), can strongly affect precipitation on the MC. The prevalence of fires in the MC is closely associated with precipitation amount and terrestrial water storage in September and October. Precipitation and terrestrial water storage, which is a measurement of hydrological drought conditions, are significantly modulated by Indian Ocean Dipole (IOD) and El Niño events. We utilize long-term datasets to study the combined effects of ENSO and the IOD on MC precipitation during the past 100 years (1900–2019) and find that the reductions in MC precipitation and terrestrial water storage are more pronounced during years when El Niño and a positive phase of the IOD (pIOD) coincided. The combined negative effects are produced mainly through an enhanced reduction of upward motion over the MC. Coincident El Niño-pIOD events have occurred more frequently after 1965. However, climate models do not project a higher occurrence of coincident El Niño-pIOD events in a severely warming condition, implying that not the global warming but the natural variability might be the leading cause of this phenomenon.

Tidal variations associated with sea level changes in the Northern Bay of Bengal

Changing tides due to non-astronomical factors have been reported globally, with particular emphasis on estuaries and tidal rivers. Changes in tidal amplitude and range, coupled with altered mean sea level (MSL) has the potential to raise high water levels and aggravate the impacts of coastal flooding events. The current study used water level observations to analyze tidal changes over three decades and report its relationship to sea level variations along estuarine zones in the northern Bay of Bengal. The study highlights on the seasonal, inter-annual and long-term variations in tides, with special emphasis on the Ganga-Brahmaputra-Meghna (GBM) delta. The long-term trend analysis of M2 amplitude showed unusual shifts in magnitude and change in pattern after 1993, associated with similar changes in the MSL. This accelerated M2 amplitude (0.27 mm/y2) implied an association to accelerated RWL (Relative Water Level) changes in the Ganges floodplain, due to delta subsidence. These tidal changes were found to be inconsistent with astronomical forcing and the analysis of coherence between M2 and MSL showed 50–70% correlation, which was primarily due to a covarying seasonal signal. The coherence between sea level and tides varied regionally and is influenced by different processes at different timescales. The positive annual coherence between M2 and MSL directly stems from river forcing. The co-occurrence of strong El Niño and strong positive Indian Ocean Dipole events caused perturbations in both the water level and M2, but in a negatively coherent relationship. The study establishes a delayed response of M2 to inter-annual climate variability indices. The differential behavior of tidal constituents along the Chittagong coastal plain and the opposite nature of coherence between M2 and MSL there, relative to that at the GBM delta, imply the prevalence of spatially non-uniform response of tides to sea level fluctuations. The study emphasizes on the need to understand changing tides, for prediction of extreme sea levels and coastal flooding.

The 1994 Positive Indian Ocean Dipole Event as Investigated by the Transfer Routes of Oceanic Wave Energy

Abstract The present study investigates the interannual variability of the tropical Indian Ocean (IO) based on the transfer routes of wave energy in a set of 61-yr hindcast experiments using a linear ocean model. To understand the basic feature of the IO dipole mode, this paper focuses on the 1994 pure positive event. Two sets of westward transfer episodes in the energy flux associated with Rossby waves (RWs) are identified along the equator during 1994. One set represents the same phase speed as the linear theory of equatorial RWs, while the other set is slightly slower than the theoretical phase speed. The first set originates from the reflection of equatorial Kelvin waves at the eastern boundary of the IO. On the other hand, the second set is found to be associated with off-equatorial RWs generated by southeasterly winds in the southeastern IO, which may account for the appearance of the slower group velocity. A combined empirical orthogonal function (EOF) analysis of energy-flux streamfunction and potential reveals the intense westward signals of energy flux are attributed to off-equatorial RWs associated with predominant wind input in the southeastern IO corresponding to the positive IO dipole event. Significance Statement The present study gains a new insight into the mechanism of the Indian Ocean dipole events using a new diagnostic scheme for wave energy based on 61-yr hindcast experiments. The results have shown the existence of two sets of westward transfer of wave energy at the equator during 1994. One set of westward signals shows the same group velocity with theoretical equatorial Rossby waves that appear reasonably along the equator. The other set of westward signals at the equator represents a slightly slower group velocity than the theoretical equatorial Rossby waves, which is associated with abnormally extended southeasterly winds during the Indian Ocean dipole event.

Impact of equatorial wind stress on Ekman transport during the mature phase of the Indian Ocean Dipole

This paper investigates the impact of equatorial wind stress on the equatorial Ekman transport during the Indian Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper levels. In normal years, the zonal wind stress south of the equator is easterly and that north of the equator is westerly, which contributes to southward Ekman transport at the upper levels to form the climatological Indian Ocean shallow meridional overturning circulation. During the years of positive IOD events, abnormal easterly winds near the equator bring southward Ekman transport south of the equator while they bring northward Ekman transport north of the equator. This causes the seawater to move away from the equator and hence induces upwelling near the equator, which forms a pair of small circulation cells that are symmetric about the equator at the upper levels (approximately 100 m deep). The abnormal circulation cell south (north) of the equator strengthens (weakens) the southward (southward) motion south (north) of the equator. During years with negative IOD events, the opposite occurs. In addition, during the mature period of IOD, the remote sea surface temperature anomaly (SSTA) such as El Niño–Southern Oscillation (ENSO) may exert some influence on Ekman transport anomaly near the equator during the mature period of IOD.

On the weakening of northward propagation of intraseasonal oscillations during positive Indian Ocean Dipole events

The northward propagation of intraseasonal oscillations (ISO) is one of the major modes of variability in the tropics during boreal summers, associated with active and break spells of monsoon rainfall over the Indian region. These northward propagations modulate the Indian summer monsoon rainfall. The northward march starts close to the equator over warm waters of the Indian Ocean and continues till the foothills of the Himalayas. In this study, we investigate the influence of Indian Ocean Dipole (IOD) on northward propagations. We show that northward propagations tend to be weaker during positive Indian Ocean Dipole (pIOD) events. The “moisture mode” framework is used to understand the processes responsible for the weakening of northward propagations during pIOD years. Our analyses show that moistening caused by the horizontal advection is the major contributor for the northward propagations during negative IOD (nIOD) years, and its amplitude is much smaller during pIOD years. Further analyses revealed that the reduction in the advection of the background entropy/moisture by zonal wind perturbations during pIOD is primarily responsible for the reduction in the horizontal advection. The mean structure of entropy between 925 and 500 hpa levels remained similar over most of the Asian monsoon region across the contrasting IOD years, and the reason for weaker northward propagations can be attributed to the weaker zonal wind perturbations at intraseasonal timescales. These weaker zonal wind perturbations during ISO events in pIOD years results from weak Rossby Vortex lobes. The weakening of Rossby wave response owing to cooler than average sea surface temperatures in the South-East Equatorial Indian Ocean and warmer than average West Equatorial Indian Ocean is proposed to be the possible reason for the weakening of northward propagations during pIOD years.

Strengthening impacts of spring sea surface temperature in the north tropical Atlantic on Indian Ocean dipole after the mid-1980s

It is well known that the Indian Ocean dipole (IOD) is closely related to El Niño-Southern Oscillation (ENSO). In this study, it is found that spring–summer north tropical Atlantic (NTA) sea surface temperature (SST) anomalies can contribute to the development of the IOD since the mid-1980s as well as ENSO. After the mid-1980s, an anticyclonic circulation over the subtropical northeastern Pacific could be excited by cool NTA SST anomalies in spring and summer due to a Gill-type Rossby-wave response. A low-level cyclonic circulation appears in the west Pacific in turn. The anomalous lower-level southwesterlies on the southeastern flank of the cyclonic circulation reduce the climatological wind speed, and thus, warm SST anomalies appear and extend to the central tropical Pacific via the wind-evaporation-SST (WES) feedback. As a result, Walker circulation over the Indo-Pacific region is changed, with anomalous descending in the eastern tropical Indian Ocean and ascending in the central tropical Pacific. The descending branch of anomalous Walker circulation generates surface southeasterly wind anomalies along the coast off Sumatra and lifts the thermocline in the eastern tropical Indian Ocean, which produces a positive IOD event in autumn. Such a cross-basin mechanism is supported by a coupled model experiment with warm SST perturbations over the NTA. In contrast, although cold NTA SST anomalies can induce warm SST anomalies in the eastern equatorial Pacific through interhemispheric meridional circulation before the mid-1980s, the responses in the tropical Indian Ocean are rather weak. In addition, the enhancement in the NTA-IOD relationship after the mid-1980s is suggested to result from the changes in the SST mean state under the context of global warming, which is confirmed by two coupled model experiments with different mean SST backgrounds.

Extreme Indian Ocean dipole and rainfall variability over Central Africa

AbstractUsing both observational and reanalysis datasets, this study examines the influence of extreme Indian Ocean Dipole (IOD) events on Central Africa (CA) rainfall from September to December over the period 1980–2016. It is shown that during extreme positive IOD events (hereafter pIOD), CA experiences enhanced moisture supply from the Indian Ocean that results in increased rainfall while the opposite process is recorded during the negative phase (hereafter nIOD), with the magnitude of pIOD events anomalies stronger than those of nIOD events. Furthermore, the inflow is strongest in the lower troposphere over the ocean while it is strongest in the mid‐troposphere over CA because much of the lower‐level inflow is blocked by the topography to the east of CA. The intensification of mid‐tropospheric moisture transport over CA is associated with the African Easterly Jet which is present during these months. From 1980 to 2016 the IOD and the El Niño‐Southern Oscillation (ENSO) showed a strong positive relationship so that extreme pIOD (nIOD) are mostly coincident with El Niño (La Niña). To assess the possible role of non‐ENSO influences on the IOD influence over CA, the linear relationship between the Niño‐3.4 and dipole mode index (DMI) was removed from the DMI. It appears that the influence of the pIOD on CA rainfall is significant with the non‐El Niño effect and this is characterized by an increase in moisture advection toward CA that contributes to an enhancement of rainfall intensity. However, the IOD seems to contribute strongly to the rainfall variability in the CA regions south of 5°N, especially over the eastern Democratic Republic of Congo, and over the area from Gabon to the Republic of Congo where about 20–32% of the total rainfall variance is associated with the IOD independently of ENSO from November to December.

Mixed layer heat budget in the eastern equatorial Indian Ocean during the two consecutive positive Indian Ocean dipole events in 2018 and 2019

Mixed layer heat budget in the eastern equatorial Indian Ocean during the two consecutive positive Indian Ocean dipole events in 2018 and 2019

The IOD–ENSO Interaction: The Role of the Indian Ocean Current’s System

The Indian Ocean dipole (IOD) is one of the main modes characterizing the interannual variability of the large-scale ocean–atmosphere interaction in the equatorial zone of the World Ocean. A dipole manifests itself as an out-of-phase interannual fluctuation of the ocean–atmosphere characteristics in the western and eastern parts of the equatorial–tropical zone of the Indian Ocean. IOD can be a consequence of the ENSO (El Niño–Southern Oscillation) events in the Pacific Ocean, or it can be independent of them and arise due to the Indian Ocean inherent processes. Earlier, it was suggested that the generation of the long planetary waves in the Indian Ocean by the ENSO events is one of the mechanisms of the ENSO impact on the IOD. However, quite often, such a mechanism is not the case and IOD is generated itself as an independent Indian Ocean mode. We hypothesized that this generation is due to the growing oceanic disturbances, as a result of instability of the system of Indian Ocean zonal currents in the vicinity of the critical layer, in which the phase velocity of Rossby waves is equal to the average velocity of the zonal currents. In the present work, the study of the features of the formation of the critical layer in the equatorial–tropical zone of the Indian Ocean is continued using different oceanic re-analyses and standard theory of the Rossby waves. As a result of comparison of different re-analyses data with the RAMA (The Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction) measurements, the operative re-analysis ORAS5 output of European Centre for Medium-Range Weather Forecasts (ECMWF) on potential temperature, salinity, and the zonal component of the currents’ velocity for the period 1979–2018 was used. Monthly profiles of potential temperature, salinity, and the zonal component of the currents’ velocity were selected from the ORAS5 archive for the sections situated between 7.5–15.5° S and 50–100° E. From these data and for each month, using the standard theory of planetary waves, the phase velocity of the lowest baroclinic mode of the Rossby long waves was calculated and the critical layers were determined. For each critical layer, its length was calculated. The obtained time series of the length of the critical layers were compared to the variability of dipole mode index (DMI). It is shown that the majority of the cases of the IOD generation as inherent (independent on the Pacific processes) mode were accompanied by the critical layer formation in the region of interest. Usually, the critical layers occur in spring, one to two months before the onset of the positive IOD events. This indicates that the presence of instability in the system of the zonal currents can be a reason for the generation of IOD and the asymmetry of the amplitude of the dipole mode index between positive and negative events. During the extremely intense ENSO event of 1997–1998, which was accompanied by the strong IOD event, the critical layer in the equatorial–tropical zone of the Indian Ocean was absent. This ENSO event generated the oceanic planetary waves at the eastern edge of the Indian Ocean. Therefore, it is shown that the above mechanism of the ENSO–IOD interaction is a reality.

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.

Long‐Lasting Marine Heatwaves Instigated by Ocean Planetary Waves in the Tropical Indian Ocean During 2015–2016 and 2019–2020

AbstractMarine heatwaves (MHWs) in the tropical Indian Ocean (TIO) showed remarkable increases in duration and frequency during the satellite observing era, responding to rising sea surface temperature. Long‐lasting MHWs were found in three upwelling regions of the TIO in 2015–2016 and 2019–2020, closely related to persistent downwelling oceanic planetary waves. In 2015, a prolonged MHW (149 days) in the western TIO was initiated by the downwelling Rossby waves associated with the co‐occurring super El Niño and positive Indian Ocean dipole (IOD) events. In the following year, the negative IOD sustained the longest MHW (372 days) in the southeastern TIO, prompted by the eastward‐propagating equatorial Kelvin waves. In 2019–2020, the two longest MHWs recorded in the southwestern TIO (275 days in 2019 and 149 days in 2020) were maintained by the downwelling Rossby waves associated with the 2019 extreme IOD. This study revealed the importance of ocean dynamics in long‐lasting MHWs in the TIO.

Observed of Equatorial Currents in the Indian Ocean during Two Contrasting IOD Events: 2006 and 2010

The evolution of Indian Ocean Dipole (IOD) events in 2006 and 2010 is investigated using observational data products that are made to understand several processes in the positive (negative) phase of IOD events. Two Acoustic Doppler Current Profiler (ADCP) moorings mounted at 90°E and 80.5°E along the equator were used to evaluate the zonal current variation during two contrasting Indian Ocean Dipole (IO) events. Westward anomalies of the zonal current were observed at 0°, 80.5°E during the peak phase of the positive IOD event from October to December 2006. Meanwhile, the observed zonal currents at 0°, 90°E only showed the short-term westward anomalies during October 2006. On the other hand, during the negative IOD event in 2010, the observed zonal current at both mooring locations indicated strong intraseasonal variations of the eastward anomalies from August to December 2010. Strong easterly (westerly) anomalies of the surface zonal winds were observed during the peak phase of the positive (negative) IOD event in 2006 (2010). These easterly (westerly) anomalies forced upwelling (downwelling) equatorial Kelvin waves indicated by the negative (positive) sea surface height anomalies. Strengthening (weakening) of upwelling (downwelling) along the equatorial Indian Ocean would be a significant factor for further understanding of IOD evolution.

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.

Influence of variability to coastline dynamic in the small uninhabited reef islands of Kepulauan Seribu

Small reef islands provide habitable land for coastal communities in many parts of the world. However, the small, low lying reef islands are commonly considered among the most geomorphically sensitive landforms to changes in sea level, wave processes, sediment supply and anthropogenic impacts. Kepulauan Seribu in the Java Sea comprise of numerous reef islands. By 2019, the islands chain is host to more than 24 thousand people. Kepulauan Seribu is affected by monsoon wind cycle. The monsoon wind also known to interact with an interannual phenomenon such as Indian Ocean Dipole (IOD) which affecting regional and local wind circulation. This study aims to examine the reef shoreline response to seasonal and interannual climate variability using satellite data that encompasses yearly monsoon cycle and IOD event. Strengthens (weakens) of winds speed in the study area during the East (West) Monsoon, which in some year also coincides with a positive (negative) IOD event, are observed from 2009 to 2018 ERA - Interim by The European Centre for Medium - Range Weather Forecasts (ECMWF) data. This variability influences the shoreline shifting in the uninhabited reef islands of Kepulauan Seribu as identified based on satellite imagery analysis. More pronounce shifted of large sediment flux are perceptible on opposing monsoon which coincides with positive/negative IOD event. Small uninhabited reef islands have ecological and economical value. Hence, enhancing coastal resilience from erosion by using conservation-types approach should be taking into consideration. Ultimately, a good understanding of climate variability that controlled changes in beach systems of reef islands is important for adequate coastal management decisions.

How Does El Niño Affect Predictability Barrier of Its Accompanied Positive Indian Ocean Dipole Event?

The effects of El Niño on the predictability of positive Indian Ocean dipole (pIOD) events are investigated by using the GFDL CM2p1 coupled model from the perspective of error growth. The results show that, under the influence of El Niño, the summer predictability barrier (SPB) for pIOD tends to intensify and the winter predictability barrier (WPB) is weakened. Since the reason for the weakening of WPB has been explained in a previous study, the present study attempts to explore why the SPB is enhanced. The results demonstrate that the initial sea temperature errors, which are most likely to induce SPB for pIOD with El Niño, possess patterns similar to those for pIOD without El Niño, whose dominant errors concentrate in the tropical Pacific Ocean (PO), with a pattern of negative SST errors occurring in the eastern and central PO and subsurface sea temperature errors being negative in the eastern PO and positive in the western PO. By tracking the development of such initial errors, it is found that the initial errors over PO lead to anomalous westerlies in the southeastern Indian Ocean (IO) through the effect of double-cell Walker circulation. Such westerly anomalies are inhibited by the strongest climatological easterly wind and the southeasterlies related to the pIOD event itself in summer, while they are enhanced by El Niño. This competing effect causes the intensified seasonal variation in latent heat flux, with much less loss in summer under the effect of El Niño. The greater suppression of the loss of latent heat flux favors the positive sea surface temperature (SST) errors developing much faster in the eastern Indian Ocean in summer, and eventually induces an enhanced SPB for pIOD due to El Niño.

Assessing the representation of the Australian carbon cycle in global vegetation models

Abstract. Australia plays an important role in the global terrestrial carbon cycle on inter-annual timescales. While the Australian continent is included in global assessments of the carbon cycle such as the global carbon budget, the performance of dynamic global vegetation models (DGVMs) over Australia has rarely been evaluated. We assessed simulations of net biome production (NBP) and the carbon stored in vegetation between 1901 to 2018 from 13 DGVMs (TRENDY v8 ensemble). We focused our analysis on Australia's short-term (inter-annual) and long-term (decadal to centennial) terrestrial carbon dynamics. The TRENDY models simulated differing magnitudes of NBP on inter-annual timescales, and these differences resulted in significant differences in long-term vegetation carbon accumulation (−4.7 to 9.5 Pg C). We compared the TRENDY ensemble to several satellite-derived datasets and showed that the spread in the models' simulated carbon storage resulted from varying changes in carbon residence time rather than differences in net carbon uptake. Differences in simulated long-term accumulated NBP between models were mostly due to model responses to land-use change. The DGVMs also simulated different sensitivities to atmospheric carbon dioxide (CO2) concentration, although notably, the models with nutrient cycles did not simulate the smallest NBP response to CO2. Our results suggest that a change in the climate forcing did not have a large impact on the carbon cycle on long timescales. However, the inter-annual variability in precipitation drives the year-to-year variability in NBP. We analysed the impact of key modes of climate variability, including the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), on NBP. While the DGVMs agreed on sign of the response of NBP to El Niño and La Niña and to positive and negative IOD events, the magnitude of inter-annual variability in NBP differed strongly between models. In addition, we find that differences in the timing of simulated phenology and fire dynamics are associated with differences in simulated or prescribed vegetation cover and process representation. We further find model disagreement in simulated vegetation carbon, phenology, and apparent carbon residence time, indicating that the models have different types and coverage of vegetation across Australia (whether prescribed or emergent). Our study highlights the need to evaluate parameter assumptions and the key processes that drive vegetation dynamics, such as phenology, mortality, and fire, in an Australian context to reduce uncertainty across models.

Strengthened Relationship Between Tropical Indian Ocean Dipole and Subtropical Indian Ocean Dipole After the Late 2000s

AbstractThis study identifies a strengthened relationship between the subtropical Indian Ocean dipole (SIOD) during January–February–March (JFM) and the following autumn Indian Ocean dipole (IOD) after the late 2000s. Following the positive SIOD, the northwesterly wind anomalies help to maintain positive sea surface temperature (SST) anomalies in the southwestern tropical Indian Ocean by wind‐evaporation‐SST (WES) feedback during June–July–August (JJA). The anomalous northwesterly winds and the sustained positive SST anomalies promote the development of the zonal wavenumber‐3 pattern (ZW3P) and induce low‐level convergence in the mid‐latitude central southern Indian Ocean, leading to the anticyclone circulation over the northern Indian Ocean by the local Hadley circulation during JJA. The accompanying central equatorial easterly wind anomalies appear in JJA, which is the key to trigger a positive IOD event through Bjerknes feedback. This mechanism for the enhanced SIOD‐IOD relationship is independent from the El Niño‐Southern Oscillation (ENSO).

К механизму формирования Индоокеaнского диполя

The purpose of this work is to identify individual cases of the occurrence of a critical layer in the system of zonal currents of the Indian Ocean within certain months for the period 1979-2018 aimed at studying the impact of instability of these currents on the Indian Ocean Dipole (IOD) generation. It has been shown that in most cases, the critical layer occurrence coincides with the onset of positive IOD events and takes place one or two months before the onset of these events. This indicates that sporadic instability of the system of zonal currents is one of the main mechanisms for generating IOD events in the region.