Indian Ocean SSTs Research Articles

The Relative Role of Indian and Pacific Tropical Heating as Seasonal Predictability Drivers for the North Atlantic Oscillation

AbstractUnderstanding the predictability drivers for the North Atlantic Oscillation (NAO) during boreal winter at seasonal time scales remains challenging. This study uses large ensembles with the ECMWF seasonal forecasting system to investigate the relative impact of tropical Indian and Pacific heating on NAO predictability by examining the tropical forcing, teleconnection pathways, and sources of uncertainty. We select three case studies ‐ 1997/98, 2015/16 and 2019/20 ‐ with strong Indian Ocean heating anomalies, but with different El Niño conditions. We show that in 2019/20, with neutral ENSO conditions, Indian Ocean SSTs favor a positive NAO response via stratospheric and tropospheric pathways. In the cases with strong El Niño, we find contrasting results: in 1997/98, the Pacific forcing dominates, producing a negative NAO. In 2015/16, despite the strong El Niño, the Indian Ocean forcing dominates, leading to a positive NAO via intensification of the stratospheric polar vortex (SPV). While the stratospheric pathway exhibits varying responses to Indian Ocean forcing ‐ being weaker in 1997/98 and strongest in 2015/16, the Indian Ocean‐related tropospheric pathway remains robust along the Pacific subtropical jet across years. However, there is destructive interference between teleconnections from Indian and Pacific SST anomalies in both the tropospheric and stratospheric pathways. The competing effects of tropical heating in both basins, uncertainties in the Rossby wave response to tropical heating and SPV variability contribute to uncertainty in seasonal NAO predictions. The flow‐dependent nature of the stratospheric pathway underscores the complexity of seasonal forecast predictability, and the existence of windows of opportunity.

Prediction of dominant daily modes of the Indian summer monsoon in the NCEP GFS model

The prediction capability of dominant daily monsoon modes of Indian summer monsoon in the forecast of the Global Forecast System Version 2 (GFSv2) model is scrutinized. The dominant monsoon modes are procured by performing the multichannel singular spectrum analysis (MSSA) on daily precipitation anomalies of the Indian summer monsoon region (60–100°E, Eq.-35°N) during 2001–2014. The observation has one seasonally persistent mode and two intraseasonal oscillations with periods around 42 and 26 days, and the model has correctly simulated these modes. The spatial structure of the phase composites of the precipitation anomalies of the intraseasonal modes of the model is almost similar to the observed spatial pattern with slightly less magnitude of the precipitation anomalies over Western India and the core monsoon zone. The spatial structure of the 26-day mode is similar to the spatial structure of the 42-day mode with less magnitude of the precipitation anomalies all over the study domain. The lead forecast of the model demonstrates the robust predictive skill of intraseasonal modes. The variation of the active and break spells of monsoon precipitation over the Indian subcontinent is captured accurately by the contribution of both intraseasonal modes. The observed eastward and northward propagation features of the Indian summer monsoon have been accurately simulated by the model. The model has weak seasonally persistent signals over Western India, northeast India, and eastern land regions adjacent to the Western Ghats. The seasonally persistent mode shows a strong relationship with the equatorial central Pacific Ocean SSTs and a moderate correlation with the Indian and Atlantic Ocean SSTs. The seasonally persistent mode contributes largely to the seasonal precipitation anomalies over the Indian monsoon region.

A Review of Research on Tropical Air-Sea Interaction, ENSO Dynamics, and ENSO Prediction in China

Remarkable progress has been made in observations, theories, and simulations of the ocean-atmosphere system, laying a solid foundation for the improvement of short-term climate prediction, among which Chinese scientists have made important contributions. This paper reviews Chinese research on tropical air-sea interaction, ENSO dynamics, and ENSO prediction in the past 70 years. Review of the tropical air-sea interaction mainly focuses on four aspects: characteristics of the tropical Pacific climate system and ENSO; main modes of tropical Indian Ocean SSTs and their interactions with the tropical Pacific; main modes of tropical Atlantic SSTs and inter-basin interactions; and influences of the mid-high-latitude air-sea system on ENSO. Review of the ENSO dynamics involves seven aspects: fundamental theories of ENSO; diagnosis and simulation of ENSO; the two types of ENSO; mechanisms of ENSO initiation; the interactions between ENSO and other phenomena; external forcings and teleconnections; and climate change and the ENSO response. The ENSO prediction part briefly summarizes the dynamical-statistical methods used in ENSO prediction, as well as the operational ENSO prediction systems and their applications. Lastly, we discuss some of the issues in these areas that are in need of further study.

Dominant Modes of China Summer Heat Waves Driven by Global Sea Surface Temperature and Atmospheric Internal Variability

AbstractThis study applies the maximum temperatures at more than 2000 Chinese stations to investigate the dominant modes of China summer heat waves (HWs). The first empirical orthogonal function (EOF) mode of the HW days reflects an increased frequency of HWs in northern China (NC), while the second and third modes represent two distinct interannual modes, with key regions over the Yangtze River valley (YRV) and southern China (SC), respectively. The NC HWs are possibly associated with the Atlantic–Eurasian teleconnection, showing zonally propagating wave trains over the North Atlantic and Eurasian continent. The YRV HWs are proposed to be linked to the North Atlantic Oscillation, which may trigger a southeastward-propagating wave train over northern Russia and East Asia that results in a high pressure anomaly over the YRV. The SC HWs are obviously dominated by the Indian Ocean and northwest Pacific warm SSTs owing to the transition from the preceding El Niño to La Niña, which excites above-normal highs over SC. The anomalously high pressures over NC, the YRV, and SC are usually accompanied by descending air motions, clear skies, decreased precipitation, and increased solar radiation, which jointly cause a drier and hotter soil condition that favors the emergence of HWs. The GFDL HiRAM experiments are able to reproduce the historical evolution of NC and SC HWs, but fail to capture the YRV HWs. The correlation coefficient between model PC1 (PC2) and observed PC1 (PC3) for the period of 1979–2008 is 0.65 (0.38), which significantly exceeds the 95% (90%) confidence level, indicating that this model has a more faithful representation for the SST-forced HWs.

A Process-Based Assessment of CMIP5 Rainfall in the Congo Basin: The September–November Rainy Season

AbstractCongo basin September–November rainfall varies by up to a factor of 3 across CMIP5 coupled models. The severe lack of observational data in this region makes model evaluation difficult using standard techniques. This study uses a process-based assessment to evaluate the plausibility of mechanisms related to coupled model rainfall in September–November. Models tend to simulate a rainfall maximum in either the west or east of the basin. In most months, western Congo rainfall is positively correlated with eastern Congo rainfall across models, as relatively wet models are often wetter everywhere; however, in August–November this correlation becomes insignificant, suggesting that processes relating to rainfall differences in each subdomain are distinct. Composite analysis of wet and dry models in each subdomain suggests that the tropical eastern Atlantic SST bias helps differentiate between models in the west: wetter models tend to have a larger (~1°–2°C) and more equatorward SST bias, higher evaporation over the ocean, and higher local convection. In the east, rainfall differences between models are related to more remote SST differences, including cold South Atlantic and warm eastern Indian Ocean SSTs. Wetter models exhibit stronger westerly flow across the tropical eastern Atlantic, as part of an enhanced equatorial zonal overturning cell. Dry models in the east feature a stronger and more equatorward northerly component of the midlevel African easterly jet, which may contribute to suppressed convection over the domain. This assessment casts doubt on the credibility of models that are very wet in the west of the Congo basin and have a large Atlantic SST bias.

Impacts of El Nino Southern oscillation and the Indian Ocean SSTs on inflow to the Roseires Dam in the Sudan

In this study, potential impacts of ENSO and Indian Ocean SSTs indices on variability of inflow to the Roseires Dam are investigated. The objective is to investigate the relationship between these climatic indices and the river flow to enable building a forecasting tool to give a lead time prediction of inflow to the dam using lead time information of these indices. Ninety one years of annual flow (1914–2004) were used in the analysis. Correlation results showed that the annual inflow to the dam has negative relation with the ENSO-SST index and SST indices from two regions in the Indian Ocean. Two probabilistic prediction models were developed using the concept of conditional probability to forecast the inflow to the dam. Modelling results showed significant improvements in predictability of inflow when these indices are used as predictors. The models developed could contribute to better water management and operation of the dam reservoir.

Impacts of El Nino Southern oscillation and the Indian Ocean SSTs on inflow to the Roseires Dam in the Sudan

In this study, potential impacts of ENSO and Indian Ocean SSTs indices on variability of inflow to the Roseires Dam are investigated. The objective is to investigate the relationship between these climatic indices and the river flow to enable building a forecasting tool to give a lead time prediction of inflow to the dam using lead time information of these indices. Ninety one years of annual flow (1914–2004) were used in the analysis. Correlation results showed that the annual inflow to the dam has negative relation with the ENSO-SST index and SST indices from two regions in the Indian Ocean. Two probabilistic prediction models were developed using the concept of conditional probability to forecast the inflow to the dam. Modelling results showed significant improvements in predictability of inflow when these indices are used as predictors. The models developed could contribute to better water management and operation of the dam reservoir.

Seasonal rainfall variability in southeast Africa during the nineteenth century reconstructed from documentary sources

Analyses of historical patterns of rainfall variability are essential for understanding long-term changes in precipitation timing and distribution. Focussing on former Natal and Zululand (now KwaZulu-Natal, South Africa), this study presents the first combined annual and seasonal reconstruction of rainfall variability over southeast Africa for the 19th century. Analyses of documentary sources, including newspapers and colonial and missionary materials, indicate that the region was affected by severe or multi-year drought on eight occasions between 1836 and 1900 (the rainy seasons of 1836–38, 1861–63, 1865–66, 1868–70, 1876–79, 1883–85, 1886–90 and 1895–1900). Six severe or multi-year wet periods are also identified (1847–49, 1854–57, 1863–65, 1879–81, 1890–91 and 1892–94). The timing of these events agrees well with independent reconstructions of 19th century rainfall for other parts of the southern African summer rainfall zone (SRZ), suggesting subcontinental scale variability. Our results indicate that the relationship between El Nino and rainfall in southeast Africa was relatively stable, at least for the latter half of the 19th century. El Nino conditions appear to have had a more consistent modulating effect upon rainfall during the 19th century than La Nina. The rainfall chronology from this study is combined with other annually-resolved palaeoclimate records from mainland southern Africa and surrounding oceans as part of a multi-proxy rainfall reconstruction for the SRZ. This reconstruction confirms (i) the long-term importance of ENSO and Indian Ocean SSTs for modulating regional rainfall; and (ii) that summer precipitation has been declining progressively over the last 200 years.

Decadal Rainfall Dipole Oscillation over Southern Africa Modulated by Variation of Austral Summer Land–Sea Contrast along the East Coast of Africa

Abstract A rainfall dipole mode characterized by negative correlation between subtropical southern Africa and equatorial eastern Africa is identified in instrumental observation data in the recent 100 years. The dipole mode shows a pronounced oscillation signal at a time scale of about 18 years. This study investigates the underlying dynamical mechanisms responsible for this dipole pattern. It is found that the southern African rainfall dipole index is highly correlated to the land–sea contrast along the east coast of Africa. When the land–sea thermal contrast strengthens, the easterly flow toward the continent becomes stronger. The stronger easterly flow, via its response to east coast topography and surface heating, leads to a low pressure circulation anomaly over land south of the maximum easterly flow anomalies and thus causes more rainfall in the south. On a decadal time scale, an ENSO-like SST pattern acts to modulate this land–sea contrast and the consequent rainfall dipole. During a “wet in the south and dry in the north” dipole, there are warm SSTs over the central Indian Ocean and cold SSTs over the western Indian Ocean. The cold SSTs over the western Indian Ocean further enhance the land–sea contrast during austral summer. Moreover, these cold western Indian Ocean SSTs also play an important role in regulating land temperature, thereby suppressing clouds and warming the land via increased shortwave radiation over the less-cloudy land. This cloud–SST coupling acts to further strengthen the land–sea contrast.

Asymmetric Relationship between Indian Ocean SST and the Western North Pacific Summer Monsoon

Abstract The summer precipitation anomalies over the tropical western North Pacific (WNP), which greatly affect East Asian climate, are closely related to Indian Ocean (IO) SST anomalies, and this WNP–IO relationship is widely assumed to be linear. This study indicates that the IO SST–WNP precipitation relationship is generally linear only when the IO SST anomalies are positive and not when the IO SST anomalies are negative, that is, a strongly cooler IO, in comparison with a moderately cooler IO, does not correspond to stronger precipitation enhancement over the WNP. Further analysis suggests that the phases of ENSO play a crucial role in modifying the impacts of IO SSTs on WNP anomalies. The reverse IO SST–WNP precipitation relationship, which exists without apparent ENSO development/decay, is intensified by El Niño decay through the enhancement of IO SST anomalies, but weakened by El Niño development and La Niña decay through the concurrence of SST anomalies in the tropical central and eastern Pacific. After removing El Niño developing and La Niña decaying cases, the IO SST and WNP precipitation anomalies show a clear linear relationship. Because of the effects of the phases of ENSO, the years of negative precipitation or anticyclonic anomalies over the WNP are highly concentrated over strongly warmer IO and El Niño decaying years, which is consistent with previous studies. However, the years of positive precipitation anomalies are scattered over cooler IO and moderately warmer IO years, implying a complexity of tropical SST forcing on positive WNP precipitation anomalies.

The Indian Ocean Dipole: A Monopole in SST

Abstract The claim for a zonal-dipole structure in interannual variations of the tropical Indian Ocean (IO) SSTs—the Indian Ocean dipole (IOD)—is reexamined after accounting for El Niño–Southern Oscillation’s (ENSO) influence. The authors seek an a priori accounting of ENSO’s seasonally stratified influence on IO SSTs and evaluate the basis of the related dipole mode index, instead of seeking a posteriori adjustments to this index, as common. Scant observational evidence is found for zonal-dipole SST variations after removal of ENSO’s influence from IO SSTs: The IOD poles are essentially uncorrelated in the ENSO-filtered SSTs in both recent (1958–98) and century-long (1900–2007) periods, leading to the breakdown of zonal-dipole structure in surface temperature variability; this finding does not depend on the subtleties in estimation of ENSO’s influence. Deconstruction of the fall 1994 and 1997 SST anomalies led to their reclassification, with a weak IOD in 1994 and none in 1997. Regressions of the eastern IOD pole on upper-ocean heat content, however, do exhibit a zonal-dipole structure but with the western pole in the central-equatorial IO, suggesting that internally generated basin variability can have zonal-dipole structure at the subsurface. The IO SST variability was analyzed using the extended-EOF technique, after removing the influence of Pacific SSTs; the technique targets spatial and temporal recurrence and extracts modes (rather than patterns) of variability. This spatiotemporal analysis also does not support the existence of zonal-dipole variability at the surface. However, the analysis did yield a dipole-like structure in the meridional direction in boreal fall/winter, when it resembles the subtropical IOD pattern (but not the evolution time scale).

ENSO, the IOD and the intraseasonal prediction of heat extremes across Australia using POAMA-2

The simulation and prediction of extreme heat over Australia on intraseasonal timescales in association with the El Nino–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) is assessed using the Bureau of Meteorology’s Predictive Ocean Atmosphere Model for Australia (POAMA). The analysis is based on hindcasts over 1981–2010 and focuses on weeks 2 and 3 of the forecasts, i.e. beyond a typical weather forecast. POAMA simulates the observed increased probabilities of extreme heat during El Nino events, focussed over south eastern and southern Australia in SON and over northern Australia in DJF, and the decreased probabilities of extreme heat during La Nina events, although the magnitude of these relationships is smaller than observed. POAMA also captures the signal of increased probabilities of extreme heat during positive phases of the IOD across southern Australia in SON and over Western Australia in JJA, but again underestimates the strength of the relationship. Shortcomings in the simulation of extreme heat in association with ENSO and the IOD over southern Australia may be linked to deficiencies in the teleconnection with Indian Ocean SSTs. Forecast skill for intraseasonal episodes of extreme heat is assessed using the Symmetric Extremal Dependence Index. Skill is highest over northern Australia in MAM and JJA and over south-eastern and eastern Australia in JJA and SON, whereas skill is generally poor over south-west Western Australia. Results show there are windows of forecast opportunity related to the state of ENSO and the IOD, where the skill in predicting extreme temperatures over certain regions is increased.

The relationship between El Niño and the western North Pacific summer climate in a coupled GCM: Role of the transition of El Niño decaying phases

This study investigates the impacts of the transition of El Niño decaying phases on the western North Pacific anticyclone (WNPAC) anomalies in the subsequent summer with a coupled GCM. The modeling results suggest that the El Niños with short decaying phases lead to significant WNPAC anomalies in the following summer, which are contributed to mainly by the El Niños followed by La Niñas, in comparison with those not followed by La Niñas. In contrast, the long decaying cases are associated with the disappearance of WNPAC anomalies in the summer. These differences in the WNP circulation anomalies can be explained by the different configurations of simultaneous SSTs in the Indian Ocean and in the central and eastern tropical Pacific: positive SSTs in the former region and negative ones in the latter region constructively induce significant WNPAC anomalies for the short decaying cases, while the roles of positive SSTs in both regions for the long decaying cases work destructively and lead to weak WNP circulation anomalies. Further analysis indicates that the different lengths of El Niño decaying phases are predicted by the strength of Indian Ocean SSTs in the mature winter. The warmer wintertime Indian Ocean SSTs favor the anomalous easterly wind over the western and central equatorial Pacific in the subsequent summer, leading to a short decaying of El Niño. Thus, the strength of wintertime Indian Ocean SSTs is one of the important factors that affect the length of El Niño decaying phase and resultant WNPAC anomalies in the following summer.

Decadal Variability of Asian–Australian Monsoon–ENSO–TBO Relationships

Abstract A set of dynamically coupled ocean–atmosphere mechanisms has previously been proposed for the Asia–Pacific tropics to produce a dominant biennial component of interannual variability [the tropospheric biennial oscillation (TBO)]. Namely, a strong Asian–Australian monsoon is often associated with negative SST anomalies in the equatorial eastern Pacific and a negative Indian Ocean dipole in northern fall between the strong Indian monsoon and strong Australian monsoon, and tends to be followed by a weak monsoon and positive SST anomalies in the Pacific the following year and so on. These connections are communicated through the large-scale east–west (Walker) circulation that involves the full depth of the troposphere. However, the Asia–Pacific climate system is characterized by intermittent decadal fluctuations whereby the TBO during some time periods is more pronounced than others. Observations and models are analyzed to identify processes that make the system less biennial at certain times due to one or some combination of the following:increased latitudinal extent of Pacific trade winds and wider cold tongue;warmer tropical Pacific compared to tropical Indian Ocean that weakens trade winds and reduces coupling strength;eastward shift of the Walker circulation;reduced interannual variability of Pacific and/or Indian Ocean SSTs. Decadal time-scale SST variability associated with the interdecadal Pacific oscillation (IPO) has been shown to alter the TBO over the Indo-Pacific region by contributing changes in either some or all of the four factors listed above. Analysis of a multicentury control run of the Community Climate System Model, version 4 (CCSM4), shows that this decadal modulation of interannual variability is transferred via the Walker circulation to the Asian–Australian monsoon region, thus affecting the TBO and monsoon–Pacific connections. Understanding these processes is important to be able to evaluate decadal predictions and longer-term climate change in the Asia–Pacific region.

Teleconnections of the tropical Atlantic to the tropical Indian and Pacific Oceans: A review of recent findings

Abstract Recent studies found that the tropical Atlantic may exert a considerable teleconnection to both the tropical Pacific and Indian Ocean basins, possibly modulating the Indian summer monsoon and Pacific ENSO events. A warm (cold) tropical Atlantic Ocean forces a Gill-Matsuno-type quadrupole response with a low-level anticyclone (cyclone) located over India that weakens (strengthens) the Indian monsoon circulation. A further analysis shows that the tropical Atlantic Ocean can induce a change in the Indian Ocean SSTs, especially along the coast of Africa and in the western side of the Indian basin. The tropical Atlantic can also influence on the tropical Pacific Ocean via the inter-basin SST gradient variability that is associated with the Atlantic Walker circulation. Although the Pacific El Nino does not correlate with the Atlantic Nino, anomalous warming or cooling of the two equatorial oceans can form an inter-basin SST gradient variability that induces surface zonal wind anomalies over equatorial South America and over some regions of both ocean basins. The zonal wind anomalies act to bridge the interaction of the two ocean basins, reinforcing the inter-basin SST gradient through atmospheric Walker circulations and oceanic dynamics. Thus, a positive feedback seems to exist for climate variability of the tropical Pacific-Atlantic Oceans and atmosphere system, in which the inter-basin SST gradient is coupled to the overlying atmospheric wind.

Role of TBO and ENSO scale ocean–atmosphere interaction in the Indo-Pacific region on Asian summer monsoon variability

Interannual variability of the Indian summer monsoon rainfall has two dominant periodicities, one on the quasi-biennial (2–3 year) time scale corresponding to tropospheric biennial oscillation (TBO) and the other on low frequency (3–7 year) corresponding to El Nino Southern Oscillation (ENSO). In the present study, the spatial and temporal patterns of various atmospheric and oceanic parameters associated with the Indian summer monsoon on the above two periodicities were investigated using NCEP/NCAR reanalysis data sets for the period 1950–2005. Influences of Indian and Pacific Ocean SSTs on the monsoon season rainfall are different for both of the time scales. Seasonal evolution and movement of SST and Walker circulation are also different. SST and velocity potential anomalies are southeast propagating on the TBO scale, while they are stationary on the ENSO scale. Latent heat flux and relative humidity anomalies over the Indian Ocean and local Hadley circulation between the Indian monsoon region and adjacent oceans have interannual variability only on the TBO time scale. Local processes over the Indian Ocean determine the Indian Ocean SST in biennial periodicity, while the effect of equatorial east Pacific SST is significant in the ENSO periodicity. TBO scale variability is dependent on the local factors of the Indian Ocean and the Indian summer monsoon, while the ENSO scale processes are remotely controlled by the Pacific Ocean.

Pacific Sea Surface Temperatures in the Twentieth Century: An Evolution-Centric Analysis of Variability and Trend

Abstract A consistent analysis of natural variability and secular trend in Pacific SSTs in the twentieth century is presented. By focusing on spatial and temporal recurrence, but without imposition of periodicity constraints, this single analysis discriminates between biennial, ENSO, and decadal variabilities, leading to refined evolutionary descriptions, and between these natural variability modes and secular trend, all without advance filtering (and potential aliasing) of the SST record. SST anomalies of all four seasons are analyzed together using the extended-EOF technique. Canonical ENSO variability is encapsulated in two modes that depict the growth (east-to-west along the equator) and decay (near-simultaneous amplitude loss across the basin) phases. Another interannual mode, energetic in recent decades, is shown linked to the west-to-east SST development seen in post–climate shift ENSOs: the noncanonical ENSO mode. The mode is closely related to Chiang and Vimont’s meridional mode, and leads to some reduction in canonical ENSO’s oscillatory tendency. Pacific decadal variability is characterized by two modes: the Pan-Pacific mode has a horseshoe structure with the closed end skirting the North American coast, and a quiescent eastern equatorial Pacific. The mode exhibits surprising connections to the tropical/subtropical Atlantic, with correlations there resembling the Atlantic multidecadal oscillation. The second decadal mode—the North Pacific mode—captures the 1976/77 climate shift and is closer to Mantua’s Pacific decadal oscillation. This analysis shows, perhaps for the first time, the striking links of the North Pacific mode to the western tropical Pacific and Indian Ocean SSTs. The physicality of both modes is assessed from correlations with the Pacific biological time series. Finally, the secular trend is characterized: implicit accommodation of natural variability leads to a nonstationary SST trend, including midcentury cooling. The SST trend is remarkably similar to the global surface air temperature trend. Geographically, a sliver of cooling is found in the central equatorial Pacific in the midst of widespread but nonuniform warming in all basins. An extensive suite of sensitivity tests, including counts of the number of observational analogs of the modes in test analyses, supports the robustness of this analysis.

El Niño and Indian Ocean influences on Indonesian drought: implications for forecasting rainfall and crop productivity

AbstractIndonesia is impacted by severe droughts that cause major food shortages over much of the country, and have long been linked with El Niño events in the tropical Pacific Ocean. Despite seasonal forecasts based on relatively complex climate models, the 1997–1998 ‘El Niño of the century’ was still followed by huge shortfalls in crop production (e.g. a loss of 3 million tons of rice in Java alone), reflecting the large gap that can exist between these forecasts and their actual utilization in agricultural planning. Alternatively, simple predictive models of rainfall and crop yields for Indonesia and other Indian Ocean rim countries have utilized an index of equatorial Pacific sea surface temperatures (Niño‐3.4 SST), and related this index directly to indices of rainfall and crop productivity. However, these latter models have not included climatic information from the Indian Ocean, also implicated as a likely cause of drought in western Indonesia. Here we show that an index of Indian Ocean SSTs, when combined with the previously utilized Niño‐3.4 SSTs, provides a significantly more accurate model of Sept‐Dec drought (PDSI) over Java, Indonesia, than Niño‐3.4 SSTs alone (ar2 64.5% vs 37.9%). Based solely on data for the month of August, this model provides the best tradeoff between model skill and adequate lead time for the Sept‐Dec rice planting season. Such simple models can be used to generate readily usable early warning forecasts of drought and crop failure risk in Indonesia, particularly in the western part of the country that is most influenced by Indian Ocean climate variability. Copyright © 2008 Royal Meteorological Society

Impact of southeast Indian Ocean sea surface temperature anomalies on monsoon-ENSO-dipole variability in a coupled ocean–atmosphere model

Recent studies show that SouthEast Indian Ocean (SEIO) SSTs are a highly significant precursor of transitions of the whole monsoon-El Nino-Southern Oscillation (ENSO) system during recent decades. However, the reasons for this specific interannual variability have not yet been identified unequivocally from the observations. Among these, the possibility of SEIO SST-driven variability in the monsoon-ENSO system is investigated here by inserting positive/negative SEIO temperature anomalies in the February’s restart files of a state-of-the-art coupled General Circulation Model (GCM) for 49 years of a control simulation. For each year of the control simulation, the model was then integrated for a 1-year period in fully coupled mode. These experiments show that Indian Summer Monsoon (ISM) and tropical Indian Ocean Dipole Mode (IODM) events are significantly influenced by the SEIO temperature perturbations inserted in the mixed layer of the coupled GCM several months before. A warm SEIO perturbation, inserted in late boreal winter, slowly propagates northward during the following seasons, implies enhanced ISM rainfall and finally triggers a negative IODM pattern during boreal fall in agreement with observations. A reversed evolution is simulated for a cold SEIO perturbation. It is shown that the life cycle of the simulated SEIO signal is driven by the positive wind-evaporation-SST, coastal upwelling and wind-thermocline-SST feedbacks. Further diagnosis of the sensitivity experiments suggests that stronger ISM and IODM variabilities are generated by excluding the El Nino years of the control simulation or when the initial background state in the SEIO is warmer. This finding confirms that IODM events may be triggered by multiple factors, other than ENSO, including subtropical SEIO SST anomalies. However, the ENSO mode does not react significantly to the SEIO temperature perturbation in the perturbed runs even though the simulated Pacific pattern agrees with the observations during boreal fall. These discrepancies with the observations may be linked to model biases in the Pacific and to the too strong ENSO simulated by this coupled GCM. These modeling evidences confirm that subtropical Indian Ocean SST anomalies generated by Mascarene high pulses during austral summer are a significant precursor of both ISM and IODM events occuring several months later.

A Study of Predictable Patterns for Seasonal Forecasting of New Zealand Rainfall

Abstract A recently developed variance decomposition approach is applied to study the causes of the predictability of New Zealand seasonal mean rainfall. In terms of predictability, the Southern Oscillation is identified as being the most important cause of variability for both the winter and summer New Zealand rainfall, especially for the North Island. Indian Ocean sea surface temperature variability and the Southern Hemisphere annular mode are the second most important causes of variability for winter and summer rainfall, respectively. Based on this study, a statistical prediction scheme has been developed. May Niño-3 (5°N–5°S, 150°–90°W) SSTs and March–May (MAM) central Indian Ocean SSTs are identified as being the most important predictors for the winter rainfall, while September–November (SON) Niño-3 SSTs, November local New Zealand SSTs, and the SON Southern Hemisphere annular mode index are the most important predictors for the summer rainfall. The predictive skill, in term of the percentage explained variance for the verification period (1993–2000) is nearly 20%, which is considerably higher than that achieved previously.