Takeshi Izumo

Influence of Indian Ocean Dipole and Pacific recharge on following year’s El Niño: interdecadal robustness

Climate Dynamics, 2013

The Indian Ocean Dipole (IOD) can affect the El Niño-Southern Oscillation (ENSO) state of the fol... more The Indian Ocean Dipole (IOD) can affect the El Niño-Southern Oscillation (ENSO) state of the following year, in addition to the well-known preconditioning by equatorial Pacific Warm Water Volume (WWV), as suggested by a study based on observations over the recent satellite era . The present paper explores the interdecadal robustness of this result over the 1872-2008 period. To this end, we develop a robust IOD index, which well exploits sparse historical observations in the tropical Indian Ocean, and an efficient proxy of WWV interannual variations based on the temporal integral of Pacific wind stress of the 20 th century reanalysis. A linear regression hindcast model based on these two indices in boreal fall explains 50% of ENSO peak variance 14 months later, with significant contributions from both the IOD and WWV over most of the historical period and a similar skill for El Niño and La Niña events. Our results further reveal that, when combined with WWV, the IOD index provides a larger ENSO hindcast skill improvement than the Indian Ocean basin-wide mode, the Indian Monsoon or ENSO itself. Based on these results, we propose a revised scheme of Indo-Pacific interactions. In this scheme, the IOD-ENSO interactions favour a biennial timescale and interact with the slower recharge-discharge cycle intrinsic to the Pacific Ocean.

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The Hybrid Recharge Delayed Oscillator: A More Realistic El Niño Conceptual Model

Journal of climate, May 1, 2024

The El Niño Southern Oscillation (ENSO) is the leading mode of climate interannual variability, w... more The El Niño Southern Oscillation (ENSO) is the leading mode of climate interannual variability, with large socioeconomical and environmental impacts, potentially increasing with climate change. Improving its understanding may shed further light about its predictability. Here we revisit the two main conceptual models for explaining ENSO cyclic nature, namely, the Recharge Oscillator (RO) and the advective-reflective Delayed Oscillator (DO). Some previous studies have argued that these two models capture similar physical processes. Yet, we show here that they actually capture two distinct roles of ocean wave dynamics in ENSO's temperature tendency equation, using observations, reanalyses, and Climate Model Intercomparison Project (CMIP) models. The slow recharge/discharge process mostly influences central-eastern Pacific by favoring warmer equatorial undercurrent and equatorial upwelling, while the 6-month delayed advective-reflective feedback process dominates in the western-central Pacific. We thus propose a hybrid Recharge Delayed Oscillator (RDO) that combines these two distinct processes into one conceptual model, more realistic than the RO or DO alone. The RDO eigenvalues (frequency and growth rate) are highly sensitive to the relative strengths of the recharge/discharge and delayed negative feedbacks, which have distinct dependencies to mean state. Combining these two feedbacks explains most of ENSO frequency diversity among models. Thanks to the two different spatial patterns involved, the RDO can even capture ENSO spatiotemporal diversity and complexity. We also develop a fully nonlinear and seasonal RDO, even more robust and realistic, investigating each nonlinear term. The great RDO sensitivity may explain the observed and simulated richness in ENSO's characteristics and predictability. El Niño and La Niña events, and related southern oscillation, cause the largest year-to-year variations of earth's climate. Yet the theories behind them are still debated, with two main conceptual models, the recharge oscillator and the delayed oscillator. Our purpose is here to address this debate by developing a more realistic theory, a hybrid Recharge Delayed Oscillator. We show how simple yet realistic it is, with equivalent contributions from the slow recharge process and from the faster delayed feedback. It even captures the observed El Niño and La Niña diversity in space and in frequency. Future studies could use the simple theoretical framework provided here to investigate the El Niño Southern Oscillation in observations, theories, climate models diagnostics and forecasts, and global warming projections.

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Dominant role of winds near Sri Lanka in driving seasonal sea level variations along the west coast of India

Geophysical Research Letters, Jul 2, 2016

The strong seasonal cycle of sea level along the west coast of India (WCI) has important conseque... more The strong seasonal cycle of sea level along the west coast of India (WCI) has important consequences for ecosystem and fisheries, and the Lakshadweep high/low in the southeast Arabian Sea is important for fisheries and the Indian summer monsoon. Previous studies suggested that WCI sea level variability is primarily driven by remote wind forcing from the Bay of Bengal and equatorial Indian Ocean through coastal Kelvin wave propagation. Using a linear ocean model, we demonstrate that wind forcing in a relatively small region around the southern tip of India and east of Sri Lanka contribute to~60% of this variability. Wind variations from the rest of the Bay and the equator only account respectively for~20% and 10%. Sea level signals forced by the "southern tip" winds extend westward into the eastern Arabian Sea through Rossby wave propagation, with more than 50% contribution in the Lakshadweep high/low region.

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Tidal Mixing in the Indonesian Archipelago and Its Impact on Climate

2014 AGU Fall Meeting, Dec 15, 2014

Improving and Harmonizing El Niño Recharge Indices

Geophysical Research Letters, Dec 9, 2022

While ENSO predictability skill is now rather good at short leads, it has to be improved at longe... more While ENSO predictability skill is now rather good at short leads, it has to be improved at longer leads (e.g., Barnston et al., 2012, 2019). One key element for long-lead seasonal forecasts is the recharge state of the tropical Pacific (be it in terms of Oceanic Heat Content (OHC), thermocline depth h or sea level anomaly (SLA)), as it brings long oceanic memory across ENSO phases. Its role is formalized in the Recharge Oscillator (RO) conceptual model of ENSO (e.g., Clarke, 2010; Clarke et al., 2007; Jin, 1997a, 1997b; Wyrtki, 1985). For example, during a La Niña, easterlies favor a slow accumulation of OHC in the western and equatorial Pacific. This recharge will progressively favor positive Sea Surface Temperature (SST) anomalies in the central-eastern equatorial Pacific (T E) and thus El Niño onset through the Bjerkness positive feedback. The El Niño event will in turn lead to a discharge favoring a reversal to La Niña conditions. The RO can thus well explain ENSO cyclic nature (cf. Section 3 for RO equations). Yet there is a debate on the best recharge metric, for example, western or equatorial Pacific, sea level or thermocline depth. In the RO, the equatorial Pacific basin adjustment is separated into two independent modes: (a) the fast adjustment mode associated to a zonal tilt of the thermocline, in phase with zonal equatorial wind stress τ x and T E (Figure 1a); (b) the slow adjustment recharge mode in phase quadrature with the fast mode (and thus with τ x and T E ; Figure 1b). Several indices have been developed for this slow recharge mode that all bring predictability Abstract El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability, with large socioeconomical and environmental impacts. The main conceptual model for ENSO, the Recharge Oscillator (RO), considers two independent modes: the fast zonal tilt mode in phase with central-eastern Pacific Temperature (T e), and the slow recharge mode in phase quadrature. However, usual indices (western or equatorial sea level/thermocline depth h) do not orthogonally isolate the slow recharge mode, leaving it correlated with T e. Furthermore the optimal index is currently debated. Here, we develop an improved recharge index by objectively optimizing the RO equations fit to observations. (a) T e-variability is regressed out, to build h ind statistically-independent from T e. Capturing the pure recharge, h ind reconciles usual indices. (b) The optimum is equatorial plus southwestern Pacific h ind_eq+sw (because of ENSO Ekman pumping meridional asymmetry). Using h ind_eq+sw , the RO becomes more consistent with observations. h ind_eq+sw is more relevant for ENSO operational diagnostics. Plain Language Summary El Niño and La Niña events have important impacts globally. A key element for long-lead forecasts is the recharge state of the tropical Pacific Ocean, as captured in the Recharge Oscillator (RO) conceptual model. The RO considers two independent modes of oceanic variability, a fast adjustment process and a slow recharge/discharge process. However, usual recharge indices mix these two modes of variability, and can thus lead to ambiguous operational diagnostics of the actual oceanic recharge state. Here we develop a better recharge index, independent of the fast mode, which reconciles typical indices and allows us to go beyond the current geographical debate on the optimal metrics. We use an objective approach optimizing the RO resemblance to observations to find the optimal index: the independent sea level (or equivalently thermocline depth) averaged over the equatorial and southwestern tropical Pacific. We recommend this simple and unambiguous index for El Niño operational forecasts diagnostics.

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The equatorial undercurrent, meridional overturning circulation, and their roles in mass and heat exchanges during El Niño events in the tropical Pacific

HAL (Le Centre pour la Communication Scientifique Directe), 2005

When do regional tropical climate signals become detectable in CMIP5/6 simulations?

&amp;amp;lt;p&amp;amp;gt;Estimating when anthropogenically-forced signals emerge from amb... more &amp;amp;lt;p&amp;amp;gt;Estimating when anthropogenically-forced signals emerge from ambient natural climate variability is crucial for climate-change detection. Here we propose a new method for estimating the emergence time of climate signals based on a significance test on the nonlinear trend rather than the commonly-used method based on a signal-to-noise ratio. This method is less sensitive to the choice of a confidence level (0.1, 0.05 or 0.01) than the previous method to commonly-used signal to noise ratios (0.5, 1 or 2). For signals that tend to emerge in the late 21&amp;amp;lt;sup&amp;amp;gt;st&amp;amp;lt;/sup&amp;amp;gt; century, our method tends to yield earlier detection dates, by taking the large number of degrees of freedom into account. Here, we apply this method to relative SST (RSST, SST minus its tropical mean) changes, which tend to emerge much later than SST change signals. RSST is an indicator of changes in the atmosphere vertical stability and thus of changes in tropical cyclones intensity and precipitation. By 2100, CMIP5/6 projections indicate greater than tropical average warming (positive RSST signal) in the central and eastern equatorial Pacific, equatorial Atlantic, and Arabian Sea, and reduced warming (negative RSST signal) in the three southern hemisphere subtropical gyres. In general agreement with observations, relative warming in the Arabian Sea and relative cooling in the South-Eastern Pacific are already detectable in a majority of models (median emergence time &amp;amp;lt; 2020), making these regions suitable for testing a model&amp;#39;s ability to predict a regional SST trend. In contrast, RSST signals in other regions do not become detectable until after 2050. Tropical precipitation projections indicate more (less) rainfall in regions of positive (negative) RSST change that typically emerge one or two decades later than RSST signals. This lack of currently-detectable regional rainfall trends in CMIP models makes it difficult to evaluate their ability to predict tropical regional rainfall trends. In general, signals tend to emerge later in CMIP6 than in CMIP5 due to both weaker signals and larger climate noise. The only exception is Sahel, where CMIP6 models already display a detectable rainfall increase, that is not yet detectable in CMIP5.&amp;amp;lt;/p&amp;amp;gt;

VASCO-Cir ENE MEASUr EMENtS

This document is a supplement to “Cirene: Air–Sea Interactions in the Seychelles–Chagos Thermocli... more This document is a supplement to “Cirene: Air–Sea Interactions in the Seychelles–Chagos Thermocline Ridge Region,” by J. Vialard, J. P. Duvel, M. J. McPhaden, P. Bouruet-Aubertot, B. Ward, E. Key, D. Bourras, R. Weller, P. Minnett, A. Weill, C. Cassou, L. Eymard, T. Fristedt, C. Basdevant, Y. Dandonneau, O. Duteil, T. Izumo, C. de Boyer Montegut, S. Masson, F. Marsac, C. Menkes, and S. Kennan (

The self-organizing map, a new approach to apprehend the Madden–Julian Oscillation influence on the intraseasonal variability of rainfall in the southern African region

Climate Dynamics, Nov 5, 2013

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Natural decadal sea-level variability in the Indian Ocean: lessons from CMIP models

Climate Dynamics, Jul 5, 2019

Indian Ocean decadal sea-level variability is an active research area, with many unresolved quest... more Indian Ocean decadal sea-level variability is an active research area, with many unresolved questions due to inadequate observational coverage. In this study, we analyse 26 Coupled Model Intercomparison Project (CMIP) pre-industrial simulations and isolate two consistent modes of Indian Ocean variability, which collectively explain about 50% of the total decadal sea-level variance. With opposite sea-level signals in the southwestern and eastern Indian Ocean, the first mode is related to decadal modulation of the Indian Ocean Dipole (DIOD) and equatorial wind-driven dynamics. Though IOD is more independent of the El Niño-Southern Oscillation (ENSO) at decadal (r ~ 0.4) than interannual (r ~ 0.6) timescales, the DIOD-ENSO covariability yields sea-level signals along the west coast of Australia, transmitted from the western Pacific via the Indonesian Throughflow. The second mode encompasses variability in the south Indian Ocean (SIODV), exhibiting a broad monopolar sea-level pattern east of Madagascar. In about half of the models, the SIODV is largely independent from DIOD (and decadal ENSO) and driven by south Indian Ocean wind-stress curl associated with meridional shifts in the Mascarene High (MH). In the other models, the SIODV lags the DIOD about 3 years. In those models, in addition to MH forcing, the DIOD-related alongshore wind stress off the northwest Australian coast triggers Rossby waves that also contribute to the SIODV, further west. The DIOD and MH forcing are mutually independent (r ~ 0.2). The results are broadly consistent with sea-level variations derived from the short altimeter data, despite an underestimation of the Oceanic bridge signals in CMIP models.

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Can Ningaloo Niño/Niña Develop Without El Niño–Southern Oscillation?

Geophysical Research Letters, Jul 16, 2018

Ningaloo Niño/Niña is the dominant climate mode in the southeastern Indian Ocean with its center ... more Ningaloo Niño/Niña is the dominant climate mode in the southeastern Indian Ocean with its center of positive/negative sea surface temperature anomalies attached to Australia. Ningaloo Niño is the major cause of marine heatwaves in the region. Although oceanic variability in this region has long been considered mainly as a response to the El Niño-Southern Oscillation (ENSO), some recent studies have suggested the possible existence of local air-sea feedback processes. Using a state-of-the-art ocean-atmosphere coupled model that realistically simulates Ningaloo Niño/Niña, whether Ningaloo Niño/Niña can occur independently of ENSO is examined. Even in an experiment in which ENSO is suppressed by strongly nudging tropical Pacific sea surface temperatures toward the model climatology, Ningaloo Niño/Niña with a similar magnitude and seasonality still develops, likely through an air-sea interaction off Western Australia amplifying atmospheric stochastic forcing. This study is the first to show that Ningaloo Niño/Niña can develop even without ENSO. Plain Language Summary Ningaloo Niño/Niña is the major cause of extreme warming/cooling in the southeastern Indian Ocean in austral summer and affects the marine environment and precipitation over the Western Australia. Although it has long been considered that El Niño and La Niña in the Pacific drive ocean extreme events in this region, some recent studies have suggested a possibility that warming/cooling off Western Australia would add to itself by changing the atmospheric condition and creating further heating/cooling (i.e., positive air-sea feedback). Using climate model simulations, we show that, even without El Niño and La Niña, Ningaloo Niño/Niña with a similar magnitude occurs in summer, likely through the positive air-sea feedback off Western Australia amplifying atmospheric random forcing. This study is the first to show that Ningaloo Niño/Niña can develop even without El Niño and La Niña.

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Progress in the 3-D circulation of the eastern equatorial Pacific in a climate ocean model

Ocean Modelling, 2007

... observations. The sensitivity to vertical and lateral eddy mixing coefficients is explored, a... more

The equatorial undercurrent, meridional overturning circulation, and their roles in mass and heat exchanges during El Niño events in the tropical Pacific ocean

Ocean Dynamics, Jul 21, 2005

The equatorial undercurrent (EUC), the shallow meridional overturning cells feeding it, and their... more The equatorial undercurrent (EUC), the shallow meridional overturning cells feeding it, and their role in El Niño and decadal variability in the equatorial Pacific are studied using both in situ data and an ocean general circulation model. Using temperature and current data from the TAO/TRITON moorings at the equator, their data gaps are filled and it was shown that continuous

Tropical explosive volcanic eruptions can trigger El Niño by cooling tropical Africa

Nature Communications, Oct 3, 2017

Stratospheric aerosols from large tropical explosive volcanic eruptions backscatter shortwave rad... more Stratospheric aerosols from large tropical explosive volcanic eruptions backscatter shortwave radiation and reduce the global mean surface temperature. Observations suggest that they also favour an El Niño within 2 years following the eruption. Modelling studies have, however, so far reached no consensus on either the sign or physical mechanism of El Niño response to volcanism. Here we show that an El Niño tends to peak during the year following large eruptions in simulations of the Fifth Coupled Model Intercomparison Project (CMIP5). Targeted climate model simulations further emphasize that Pinatubo-like eruptions tend to shorten La Niñas, lengthen El Niños and induce anomalous warming when occurring during neutral states. Volcanically induced cooling in tropical Africa weakens the West African monsoon, and the resulting atmospheric Kelvin wave drives equatorial westerly wind anomalies over the western Pacific. This wind anomaly is further amplified by air-sea interactions in the Pacific, favouring an El Niño-like response.

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Improving the oceanic metric of the El Ni&ntilde;o recharge oscillator model

Asymmetric influence of oceanic heat content on ENSO prediction

AGU Fall Meeting Abstracts, Dec 1, 2019

Robustness of observation-based decadal sea level variability in the Indo-Pacific Ocean

Geophysical Research Letters, Jul 28, 2017

We examine the consistency of Indo-Pacific decadal sea level variability in 10 gridded, observati... more We examine the consistency of Indo-Pacific decadal sea level variability in 10 gridded, observation-based sea level products for the 1960-2010 period. Decadal sea level variations are robust in the Pacific, with more than 50% of variance explained by decadal modulation of two flavors of El Niño-Southern Oscillation (classical ENSO and Modoki). Amplitude of decadal sea level variability is weaker in the Indian Ocean than in the Pacific. All data sets indicate a transmission of decadal sea level signals from the western Pacific to the northwest Australian coast through the Indonesian throughflow. The southern tropical Indian Ocean sea level variability is associated with decadal modulations of ENSO in reconstructions but not in reanalyses or in situ data set. The Pacific-independent Indian Ocean decadal sea level variability is not robust but tends to be maximum in the southwestern tropical Indian Ocean. The inconsistency of Indian Ocean decadal variability across the sea level products calls for caution in making definitive conclusions on decadal sea level variability in this basin.

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Relevance of Relative Sea Surface Temperature for Tropical Rainfall Interannual Variability

Geophysical Research Letters, Feb 3, 2020

The coupling between sea surface temperature (SST) anomalies and rainfall is an important driver ... more The coupling between sea surface temperature (SST) anomalies and rainfall is an important driver of tropical climate variability. Observations however reveal inconsistencies, such as decreased convective rainfall in regions with positive SST anomalies in the tropical Indian and Atlantic Oceans during and after El Niño-Southern Oscillation (ENSO) events. The upper troposphere warms during El Niño events, stabilizing the atmosphere. SST anomalies only account for the influence of the surface. Using theoretical arguments, we show that relative SST (RSST; defined as SST minus its tropical mean) anomalies, by also accounting for the influence of upper tropospheric temperature anomalies on gross moist stability, explain tropical convection interannual variations better than SST anomalies in observations and climate models. This relation further improves when RSST anomalies are weighted by the precipitation climatology to account for low and high-precipitation regimes. Using RSST is thus essential to understand El Niño-Southern Oscillation teleconnections and interactions between modes of tropical climate variability. Plain Language Summary Rainfall is an important driver of coupled air-sea climate variability in tropical regions, through its influence on atmospheric motions. Sea surface temperature anomalies influence tropical rainfall, as a warm anomaly is expected to destabilize the atmosphere, yielding a rising motion and increased rainfall. Decreased rainfall however sometimes occurs over positive sea surface temperature anomalies, notably during El Niño events, because these events tend to warm the upper atmosphere in the entire tropics, hence stabilizing the atmosphere and inhibiting rainfall. We show that the relative sea surface temperature, defined as the sea surface temperature minus its average over the tropical oceans, explains precipitation anomalies better than sea surface temperature. Relative sea surface temperature is thus a simple and powerful indicator to better understand year-to-year climate variability, notably El Niño-Southern Oscillation influence on rainfall in the Atlantic and Indian Oceans and its interactions with other tropical ocean-atmosphere coupled modes.

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Improving and harmonizing El Niño recharge indices

While ENSO predictability skill is now rather good at short leads, it has to be improved at longe... more While ENSO predictability skill is now rather good at short leads, it has to be improved at longer leads (e.g., Barnston et al., 2012, 2019). One key element for long-lead seasonal forecasts is the recharge state of the tropical Pacific (be it in terms of Oceanic Heat Content (OHC), thermocline depth h or sea level anomaly (SLA)), as it brings long oceanic memory across ENSO phases. Its role is formalized in the Recharge Oscillator (RO) conceptual model of ENSO (e.g., Clarke, 2010; Clarke et al., 2007; Jin, 1997a, 1997b; Wyrtki, 1985). For example, during a La Niña, easterlies favor a slow accumulation of OHC in the western and equatorial Pacific. This recharge will progressively favor positive Sea Surface Temperature (SST) anomalies in the central-eastern equatorial Pacific (T E) and thus El Niño onset through the Bjerkness positive feedback. The El Niño event will in turn lead to a discharge favoring a reversal to La Niña conditions. The RO can thus well explain ENSO cyclic nature (cf. Section 3 for RO equations). Yet there is a debate on the best recharge metric, for example, western or equatorial Pacific, sea level or thermocline depth. In the RO, the equatorial Pacific basin adjustment is separated into two independent modes: (a) the fast adjustment mode associated to a zonal tilt of the thermocline, in phase with zonal equatorial wind stress τ x and T E (Figure 1a); (b) the slow adjustment recharge mode in phase quadrature with the fast mode (and thus with τ x and T E ; Figure 1b). Several indices have been developed for this slow recharge mode that all bring predictability Abstract El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability, with large socioeconomical and environmental impacts. The main conceptual model for ENSO, the Recharge Oscillator (RO), considers two independent modes: the fast zonal tilt mode in phase with central-eastern Pacific Temperature (T e), and the slow recharge mode in phase quadrature. However, usual indices (western or equatorial sea level/thermocline depth h) do not orthogonally isolate the slow recharge mode, leaving it correlated with T e. Furthermore the optimal index is currently debated. Here, we develop an improved recharge index by objectively optimizing the RO equations fit to observations. (a) T e-variability is regressed out, to build h ind statistically-independent from T e. Capturing the pure recharge, h ind reconciles usual indices. (b) The optimum is equatorial plus southwestern Pacific h ind_eq+sw (because of ENSO Ekman pumping meridional asymmetry). Using h ind_eq+sw , the RO becomes more consistent with observations. h ind_eq+sw is more relevant for ENSO operational diagnostics. Plain Language Summary El Niño and La Niña events have important impacts globally. A key element for long-lead forecasts is the recharge state of the tropical Pacific Ocean, as captured in the Recharge Oscillator (RO) conceptual model. The RO considers two independent modes of oceanic variability, a fast adjustment process and a slow recharge/discharge process. However, usual recharge indices mix these two modes of variability, and can thus lead to ambiguous operational diagnostics of the actual oceanic recharge state. Here we develop a better recharge index, independent of the fast mode, which reconciles typical indices and allows us to go beyond the current geographical debate on the optimal metrics. We use an objective approach optimizing the RO resemblance to observations to find the optimal index: the independent sea level (or equivalently thermocline depth) averaged over the equatorial and southwestern tropical Pacific. We recommend this simple and unambiguous index for El Niño operational forecasts diagnostics.

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Aliasing of the Indian Ocean anthropogenic warming spatial pattern by natural climate variability

EGU General Assembly Conference Abstracts, Apr 1, 2019

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