Interdecadal Mode Research Articles

The Frequency of Extreme Cold Events in North China and Their Relationship with Sea Surface Temperature Anomalies

This study investigated the interdecadal characteristics of the frequency of the winter single station extreme cold events (SSECEs) in North China and their relationship with sea surface temperature (SST). The results showed the following: (a) The SSECEs occurred frequently before 1991, but less thereafter, with an increase after 2018. The first two interdecadal modes of the SSECE frequency were east–west inverse and “n” patterns. (b) The interdecadal abrupt change of the “n” pattern occurred around 1997/1998. Before 1997/1998, the synergistic effects between the positive Interdecadal Pacific Oscillation (+IPO) and the negative North Atlantic Multidecadal Oscillation (−AMO) triggered the “two troughs and one ridge” anomalous circulation in Eurasia. The Rossby wave energy propagated downstream from the Atlantic, strengthening the Lake Baikal ridge. Furthermore, the Siberian High (SH) became weaker in the north and stronger in the south. With the favorable jet conditions, the cold air invaded North China along the northerly airflow in front of the Lake Baikal ridge, resulting in the frequent SSECE occurrence in central North China. Afterwards, the opposite occurred. (c) The cooperation of SST anomalies (SSTAs) led to the east–west inverse anomaly of the SSECE frequency. Before 1991, the high SSTAs in the central North Atlantic and low SSTAs in the equatorial Indian Ocean and the southwest Pacific triggered “+”, “−”, “+”, and “−” wave trains at mid-latitudes from the Atlantic to the North Pacific. The Rossby wave energy propagated eastward from the Atlantic, resulting in the SH and Urals ridge strengthening, and the Aleutian Low and East Asian trough deepening. The northwestern airflow in front of the Urals ridge guided the cold air into North China, leading to frequent SSECEs in central and eastern North China before 1991. The opposite occurred between 1992 and 2018.

Teleconnections between ocean-atmosphere circulations and historical integrated drought in the Middle East and North Africa.

In the Middle East and North Africa (MENA) with extreme water scarcity worldwide, drought significantly impacts various aspects of the local people. This study uses the empirical multivariate standardized drought index (MSDI), wavelet power spectrum, and a blind source septation technique of independent component analysis (ICA) to unravel linkages between Atlantic- and Pacific-based climate indices and MENA's drought between Jan 1979 and Dec 2016. It is found that drought in the eastern MENA is more than its western linked to climate indices. PDO, ENSO, and AMO, on the whole, have statistically stronger correlations with the MENA's drought than NAO (absolute R2 is less than 0.17, 0.22, and 0.21 for interannual, decadal, and interdecadal modes, respectively). The blind sources extracted by ICA confirm the higher impacts of PDO (R2 = 0.39), ENSO (R2 = 0.30), and AMO (R2 = 0.38) on the Middle East's drought. PDO's modulation linkage to MENA's drought occurs from interannual to decadal, and interdecadal variability cycles. ENSO largely impacts the 2-7-year interannual (R2 = 0.40-0.48 for the Middle East) and 8-12-year decadal (R2 = 0.18-0.30) drought cycles, while the PDO more affects 8-12-year decadal (R2 = 0.29-0.58) modes. The interdecadal (30-50-year) variability of MENA's drought mostly correlates with AMO and PDO. Overall, the interannual droughts are mostly controlled by ENSO while the decadal by PDO. For the interdecadal droughts, PDO and AMO are the most important in the Middle East and the western MENA, respectively. This study highlights that an integrated drought signal is not solely derived from a single climate index since each index may modulate a specific drought variability mode that may differ from other indices. The physical mechanisms behind these observations are further described.

Variability modes of September Arctic sea ice: drivers and their contributions to sea ice trend and extremes

The variability of September Arctic sea ice at interannual to multidecadal time scales in the midst of anthropogenically forced sea ice decline is not fully understood. Understanding Arctic sea ice variability at different time scales is crucial for better predicting future sea ice conditions and separating the externally forced signal from internal variability. Here, we study modes of variability, extreme events and trend in September Arctic sea ice in 100–150 year datasets by using time-frequency analysis. We extract the non-linear trend for sea ice area and provide an estimate for the sea ice loss driven by anthropogenic warming with a rate of ∼−0.25 million km2 per decade in the 1980s and accelerating to ∼−0.47 million km2 per decade in 2010s. Assuming the same accelerating rate for sea ice loss in the future and excluding the contributions of internal variability and feedbacks, a September ice-free Arctic could occur around 2060. Results also show that changes in sea ice due to internal variability can be almost as large as forced changes. We find dominant modes of sea ice variability with approximated periods of around 3, 6, 18, 27 and 55 years and show their contributions to sea ice variability and extremes. The main atmospheric and oceanic drivers of sea ice modes include the Arctic Oscillation and Arctic dipole anomaly for the 3 year mode, variability of sea surface temperature (SST) in the Gulf Stream region for the 6-year mode, decadal SST variability in the northern North Atlantic Ocean for the 18-year mode, Pacific Decadal Oscillation for the 27 year mode, and Atlantic Multidecadal Oscillation for the 55 year mode. Finally, our analysis suggests that over 70% of the sea ice area loss between the two extreme cases of 1996 (extreme high) and 2007 (extreme low) is caused by internal variability, with half of this variability being related to interdecadal modes.

Internal Ocean Dynamics Contribution to North Atlantic Interdecadal Variability Strengthened by Ocean–Atmosphere Thermal Coupling

Abstract Identifying the primary drivers of North Atlantic interdecadal climate variability is crucial for improving climatic prediction over the coming decades. Here the effect of thermal coupling on the leading energy sources of the interdecadal variability of the ocean–atmosphere system is examined by means of a stochastically forced idealized coupled model. The effect of coupling is quantified from a comparison of the buoyancy variance budget of coupled and uncoupled model configurations. The simplicity of the model allows us to contrast the effect of coupling between a supercritical regime where the deterministic ocean dynamics drive the variability and a damped regime where noise forcing is central to its existence. The results show that changes in surface buoyancy fluxes act as a sink of temperature variance in the supercritical regime, and only become a source in the strongly damped regime. By contrast, internal ocean dynamics associated with the interaction of transient buoyancy fluxes with mean buoyancy gradients always act as a source of interdecadal variability. In addition to the reduced thermal damping effect in coupled integrations, thermal coupling with the atmosphere is shown to significantly increase the role of internal ocean dynamics in the variability, particularly in the regime where interdecadal modes are damped. Only for oceanic background states in the strongly damped regime do changes in surface buoyancy fluxes play a leading role in the upper-ocean variability. A stochastically forced coupled box model is proposed that captures the basic effect of thermal coupling on atmospheric and oceanic energy sources of variability. Significance Statement The purpose of this study is to better understand the impact of ocean–atmosphere thermal coupling on the leading energy sources of Atlantic interdecadal variability. Increasing our understanding of the physical mechanisms driving climate variability at interdecadal time scales is important to improve climate prediction. We show that the effect of ocean–atmosphere thermal coupling, as measured by the atmospheric feedback on sea surface temperature anomalies, is to substantially increase the role of internal ocean dynamics in the low-frequency variability of the upper-ocean heat content and sea surface temperature. Atmospheric stochastic forcing only becomes the primary driver of the oceanic temperature variability in the large dissipative limit, when internal ocean modes are strongly damped.

A Multiscale‐Model‐Based Near‐Term Prediction of Tropical Cyclone Genesis Frequency in the Northern Hemisphere

AbstractNear‐term prediction of tropical cyclone (TC) activity is of great importance to formulate the long‐term plans for mitigating the potential TC‐induced damages. Here, we introduce a multi‐timescale regression model of TC genesis frequency (TCGF), which includes contributions by three interannual modes (El Niño‐Southern Oscillation, sea surface temperature anomalies over the eastern Indian Ocean and tropical North Atlantic), two interdecadal modes (the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation), and a global warming mode. The model is shown to be able to capture well the present‐day multi‐timescale changes in TCGF in the major TC basins in the Northern Hemisphere. By combining the model and 100‐member simulations by the Max Planck Institute Earth System Model, we predict a robust increase in TCGF over the eastern North Pacific, an insignificant increase over the western North Pacific, and little change over the North Atlantic during 2020–2030 relative to 2009–2019.

Evaluation and projection of the AMO and PDO variabilities in the CMIP5 models under different warming scenarios part1: Evaluation

The capabilities of the CMIP5 models to simulate the two most important interdecadal modes, the Atlantic Multi-decadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO), are evaluated by comparing the observed sea surface temperature data from 1880 to 2009 and the CMIP5 simulations under the historical scenario. The multi-model ensemble (MME) historical simulations show deficiency in reproducing characteristics of the PDO mode in the tropics but well simulated the spatial patterns of the AMO and PDO in the high latitudes. The amplitude of the two modes is obviously weaker in the historical simulations than the observation. In terms of periodic simulations, the MME can simulate the 60–70 year period of the AMO and the double periods, namely 20 and 60–70 years, of the PDO. The MME show more realistic simulations of observed spatial and temporal features of AMO and PDO than most models. The CESM1-CAM5, FGOALS-g2, GISS-E2-R-CC, GISS-E2-H-CC and MIROC-ESM-CHEM are the top-performing models. By comparing the individual models, the variances of the AMO and PDO modes decrease when the simulated global mean sea surface temperatures are higher.

El Niño–Southern Oscillations and Pacific Decadal Oscillation as Drivers of the Decadal Dynamics of Benthic Macrofauna in Two Subtropical Estuaries (Southern Brazil)

The El Nino–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are large-scale climatic phenomena affecting atmospheric and oceanic teleconnections, and thus triggering weather events at different temporal and spatial scales, both in tropical and extratropical areas. During 11 years (2007–2017), the influence of ENSO events (El Nino, La Nina and neutral), ENSO types (canonical and Modoki) and PDO phases (cold and warm) on meteorological conditions and macrobenthic invertebrates were analyzed in two distinct (tide-dominated and river-dominated) subtropical estuaries of Southern Brazil. The estuarine macrobenthic communities differed markedly, with higher species richness in the tide-dominated estuary (146 taxa) than river-dominated (44). We showed that long-term variability of benthic macrofauna in both estuaries was strongly influenced by the PDO and ENSO events. However, the signs of PDO and ENSO effects differed between the estuaries. While the main modulating force of the decadal variability of the macrofauna of the tide-dominated estuary was the PDO (19%), the river-dominated estuary was primarily influenced by the ENSO (9%). Besides, PDO and ENSO teleconnections have combined effects, with higher macrofauna dissimilarities during constructive periods (PDO Positive/EL Nino vs. PDO Negative/La Nina). Our findings indicate that: (1) The effects of the ENSO on coastal areas are dependent of the spatial scale and the intrinsic characteristics of each estuary; (2) Studies relating ENSO and biodiversity should consider the ENSO type (Modoki and canonical) and interdecadal modes, such as PDO, because the location of the warm pools and the intensity of induced heating affect the atmospheric teleconnections.

Asian Summer Precipitation over the Past 544 Years Reconstructed by Merging Tree Rings and Historical Documentary Records

Sparse long-term Asian monsoon (AM) records have limited our ability to understand and accurately model low-frequency AM variability. Here we present a gridded 544-yr (from 1470 to 2013) reconstructed Asian summer precipitation (RAP) dataset by weighted merging of two complementary proxies including 453 tree-ring-width chronologies and 71 historical documentary records. The RAP dataset provides substantially improved data quality when compared with single-proxy-type reconstructions. Skillful reconstructions are obtained in East and North China, northern India and Pakistan, the Indochina Peninsula, midlatitude Asia, the Maritime Continent, and southern Japan. The RAP faithfully illustrates large-scale regional rainfall variability but has more uncertainties in representing small-scale local rainfall anomalies. The RAP reproduces a realistic climatology and captures well the year-to-year rainfall variability averaged over monsoon Asia, arid central Asia, and all of Asia during the twentieth century. It also shows a general agreement with other proxies (speleothems and ice cores) during the period of 1470–1920. The RAP captures the remarkably abrupt change during the 1600s recorded in the upwelling proxy over the Arabian Sea. Four major modes of variability of the Asian summer precipitation are identified with the long record of the RAP, including a biennial El Niño–Southern Oscillation (ENSO) mode, a low-frequency ENSO mode, a central Pacific El Niño–like decadal mode, and an interdecadal mode. In sum, the RAP provides a valuable dataset for study of the large-scale Asian summer precipitation variability, especially the decadal–centennial variability that is caused by external forcing and internal feedback processes within the Earth climate system.

Remarkable Impacts of Indian Ocean Sea Surface Temperature on Interdecadal Variability of Summer Rainfall in Southwestern China

During the boreal summer from June to August, rainfall in Southwestern China shows substantial interdecadal variabilities on timescales longer than 10 years. Based on observational analyses and numerical modeling, we investigated the characteristics of interdecadal Southwestern China summer rainfall (SWCSR) and its dynamic drivers. We find that the SWCSR is markedly impacted by the interdecadal Indian Ocean basin mode (ID-IOBM) of the sea surface temperature (SST), which may induce anomalous inter-hemispheric vertical circulation. During the cold phase of the ID-IOBM, an enhanced lower-level divergence and upper-level convergence exist over the tropical Indian Ocean. The simultaneous lower-level outflow anomalies further converge over the Indo-China peninsula, resulting in an anomalous ascending motion and a lower-level cyclone that contribute to strengthening the eastward moisture transport from the Bay of Bengal to Southwestern China. The joint effects of the anomalous ascending motion and the above-normal moisture transport play a key role in increasing the SWCSR. In summers during the warm phase of the ID-IOBM, the situation is approximately the same, but with opposite polarity. After the beginning of the 1970s, the impacts of interdecadal Indian Ocean dipole (ID-IOD) on SWCSR is strengthening. The anomalous vertical circulation associated with the positive (negative) phase of ID-IOD is in favor of decreased (increased) rainfall in SWC. However, the impacts of ID-IOD on SWCSR is relatively weak before the 1970s, indicating that the ID-IOD is the secondary driver of the interdecadal variability of SWCSR. Modeling results also indicate that the ID-IOBM of SST anomalies is the main driver of interdecadal variability of SWCSR.

Heat Waves in China: Definitions, Leading Patterns, and Connections to Large‐Scale Atmospheric Circulation and SSTs

AbstractBased on the daily maximum temperatures (Tmax) from 587 surface observation stations in China during 1959–2013, heat waves are detected using both absolute and relative definitions. The spatiotemporal variations of heat wave occurrence/duration/amplitude are compared between the two definitions. Considering the significant differences in regional climatology, relative threshold is more meaningful to detect the local extremes. By utilizing the empirical orthogonal function, the integral index heat wave total intensity is decomposed into three dominant modes: interdecadal (ID), interannual‐tripole (IA‐TR), and interannual‐dipole (IA‐DP) modes. The ID mode shows uniform anomalies over the whole China, with the maximum in north, and its corresponding time series depict notable interdecadal variations with a turning point around mid‐1990s. The IA‐DP mode exhibits opposite‐signed anomalies over north and south China. The IA‐TR mode shows an anomalous tripole pattern with negative anomalies over central China and positive anomalies over north and south China in its positive phase. Both the IA‐DP and IA‐TR patterns are more obvious since mid‐1990s with mainly year‐to‐year variations before that. All the three modes are controlled by anomalous high‐pressure systems, which are accompanied by local‐scale dry land conditions. The diabatic heating associated with anomalous convective activities over tropical western Pacific triggers Rossby wave trains propagating northward along the East Asia, which causes abnormal heat waves through descending motion over the high‐pressure nodes. In turn, the severe convections are generated by enhanced Walker circulation in the tropical Pacific due to warming and/or cooling sea surface temperature (SST) anomalies in the tropical western and eastern Pacific, respectively.

Simultaneous Evolution of Gyre and Atlantic Meridional Overturning Circulation Anomalies as an Eigenmode of the North Atlantic System

The authors identify an interdecadal oscillatory mode of the North Atlantic atmosphere–ocean system in a general circulation model (GFDL CM2.1) via a linear inverse model (LIM). The oscillation mechanism is mostly embedded in the subpolar gyre: anomalous advection generates density anomalies in the eastern subpolar gyre, which propagate along the mean gyre circulation and reach the subpolar gyre center around 10 years later, when associated anomalous advection of the opposite sign starts the other half cycle. As density anomalies reach the Labrador Sea deep convection region, Atlantic meridional overturning circulation (AMOC) anomalies are also induced. Both the gyre and AMOC anomalies then propagate equatorward slowly, following the advection of density anomalies. The oscillation is further demonstrated to be more likely an ocean-only mode while excited by the atmospheric forcing; in particular, it can be approximated as a linearly driven damped oscillator that is partly excited by the North Atlantic Oscillation (NAO). The slowly evolving interdecadal oscillation significantly improves and prolongs the LIM’s prediction skill of sea surface temperature (SST) evolution.

Modelling interdecadal climate variability and the role of the ocean

To estimate the anthropogenic contribution to climate signals in the recent past and future decades implies a certain degree of confidence in both understanding and simulating natural internal variability at interdecadal time scales. If we are to embark on the challenge of decadal prediction, we must be able to mechanistically attribute events to known processes and phenomena, and reproduce their features and statistics within our models. To date, models have succeeded in reproducing only partially spatial patterns, statistics and climatic impacts of interdecadal modes of variability. Reasons for the partial success and agreement among models are to be attributed to the short observational record, the different and complex flavours of coupling between the many subcomponents of the climate system, and the present inability to resolve all climate processes. At an even more fundamental level, this difficulty is aggravated by the limited understanding of the physical mechanisms involved. Here, we review the proposed mechanisms giving rise to interdecadal climate variability, we discuss the hypotheses explaining the main interdecadal modes of variability, and present an overview on the ability and level of agreement in their simulation by the latest generation of coupled climate models. To achieve any progress, the modeling community should focus on both improving the representation and parameterization of key ocean physical processes and obtaining a firmer grasp on the physical mechanisms generating the variability. Both goals can benefit from process studies, intercomparisons with perturbation experiments to study model's sensitivities, and the use of a hierarchy of climate models. WIREs Clim Change 2017, 8:e441. doi: 10.1002/wcc.441This article is categorized under: Climate Models and Modeling > Earth System Models

Relationship over southern China between the summer rainfall induced by tropical cyclones and that by monsoon

This study investigates the relationship between tropical cyclone–induced precipitation (P TC) and summer monsoon–induced precipitation (P SM) in southern China (SC) during June–August. The spatial patterns of the first interannual mode are uniform in sign over SC, with positive anomalies for P TC and negative anomalies for P SM. The background of an increase in cyclonic vorticity, an increase in RH, and a decrease in vertical wind shear over the South China Sea (SCS)–western north Pacific (WNP) provides favorable conditions for more TC genesis. The positive equatorial central Pacific SST anomaly and negative North Indian Ocean SST anomaly contribute to the anomalous cyclone over the SCS–WNP, which causes decreasing P SM in SC together with an anomalous anticyclone over eastern China–Japan. By contrast, whilst the spatial patterns of the first interdecadal mode are also uniform in sign over SC, there are positive anomalies for both P TC and P SM. The first interdecadal principal component features significant positive correlation with the number of TCs forming in the SCS. There is a significant increase in P TC and P SM after early 1990s. A positive tropical Indian Ocean (TIO) SST anomaly persists from the preceding winter to summer. During the preceding winter and spring after the early 1990s, a positive western Pacific SST anomaly can result in TIO SST warming through vertical circulation. Then, the positive TIO SST anomaly triggers an anomalous WNP anticyclone and contributes to the interdecadal increase in SC P SM in the succeeding summer. The persistent heating source over SC from May to summer related to an earlier onset of the SCS monsoon may strengthen the East Asian subtropical summer monsoon.

Western Pacific hydroclimate linked to global climate variability over the past two millennia.

Interdecadal modes of tropical Pacific ocean-atmosphere circulation have a strong influence on global temperature, yet the extent to which these phenomena influence global climate on multicentury timescales is still poorly known. Here we present a 2,000-year, multiproxy reconstruction of western Pacific hydroclimate from two speleothem records for southeastern Indonesia. The composite record shows pronounced shifts in monsoon rainfall that are antiphased with precipitation records for East Asia and the central-eastern equatorial Pacific. These meridional and zonal patterns are best explained by a poleward expansion of the Australasian Intertropical Convergence Zone and weakening of the Pacific Walker circulation (PWC) between ∼1000 and 1500 CE Conversely, an equatorward contraction of the Intertropical Convergence Zone and strengthened PWC occurred between ∼1500 and 1900 CE. Our findings, together with climate model simulations, highlight the likelihood that century-scale variations in tropical Pacific climate modes can significantly modulate radiatively forced shifts in global temperature.

Evidence of the AMOC interdecadal mode related to westward propagation of temperature anomalies in CMIP5 models

Climate models show a broad diversity in the simulations of the Atlantic meridional overturning circulation (AMOC) with its leading modes of variability having different amplitudes, periods and driving mechanisms. Theoretical considerations and computations using ocean GCMs suggest that on interdecadal timescales this variability can be controlled by an internal weakly-damped oceanic mode associated with westward propagation of large-scale density anomalies in the North Atlantic Ocean. These anomalies are dominated by temperature with some compensation from salinity. The quadrature phases of this mode include the strengthening of the AMOC, followed a quarter-period later by the development of a broad warm temperature anomaly in the northern Atlantic extending to about 1000 m, then followed by the weakening of the AMOC, and then the upper-ocean cooling. Here, we investigate whether this mode is present in the simulations of Coupled Model Intercomparison Project 5 (CMIP5). Out of the 25 models investigated, we find that more than half of the models exhibit variability consistent with this mode. Some of the relevant modal features includes statistically significant spectral peaks in the band between 15 and 35 years, the westward propagation of density anomalies in the $$40^{\circ }$$ N– $$60^{\circ }$$ N latitudinal band, which sets the period of the mode, the existence of the distinct quadrature phases of the AMOC variability, and the predominant effect of temperature on density anomalies.

Unveiling the multiscale teleconnection between Pacific Decadal Oscillation and global surface temperature using time-dependent intrinsic correlation analysis

Global surface temperature is significantly influenced by different climate forcings operating at specific time scales. This study investigates the association between global surface temperature and the Pacific Decadal Oscillation (PDO) in a multiscaling framework in terms of both time scale of variability and non-stationarity. First, the ensemble empirical mode decomposition (EEMD) is used for multiscale disintegration of the Global Surface Temperature Anomaly (GSTA) and PDO datasets. A close matching of the periodicity of different modes of PDO and GSTA is noticed, and the subsequent cross-correlation analysis of the modes showed that their linear association is the most perceptible at the slowly varying trend component. The correlation between the different modes is further analyzed using a multiscale dynamic correlation method namely, time-dependent intrinsic correlation (TDIC). This study found a strong long-range positive correlation between the time series pairs in decadal and inter-decadal modes exceeding 20-year periodicity. Further it is found that, the multiscale teleconnection between PDO and GSTA is not always of unique character but associated with localized reversals in the nature of correlation in the time domain. The study further observed a similar pattern of correlation for both cold phases of the 20th century (1901–1924 and 1947–1976); whereas the pattern of correlation is different for the warm phases of PDO (1925–1946 and 1977–1995) in different process scales.

Possible impacts of mega‐El Niño/Southern Oscillation and Atlantic Multidecadal Oscillation on Eurasian heatwave frequency variability

Identifying predictability sources of heatwave variations is a scientific challenge and of practical importance. This study investigates the summertime heatwave frequency (HWF) over Eurasia for 1950–2014. The Eurasian HWF is dominated by two distinct modes: the interdecadal (ID) mode, featured by an increasing pattern overall, centred around eastern Europe–central Asia and Mongolia–southwestern China; and the interannual (IA) mode, resembling a tripole anomaly pattern with three centres over western–northern Europe, northeastern Asia and East Asia. The ID mode is found to be influenced by mega‐El Niño/Southern Oscillation (mega‐ENSO) and the Atlantic Multidecadal Oscillation (AMO), and the latter has far more effect, whereas the IA mode is connected with mega‐ENSO.Further analysis suggests that mega‐ENSO variations can incite a Gill‐type response spreading to Eurasia, while the AMO changes cause eastward‐propagating Rossby wave trains toward Eurasia. These two teleconnection patterns together contribute to the large‐scale circulation anomalies of the ID mode, and those related to the IA mode arise from the teleconnection pattern excited by mega‐ENSO. A strong mega‐ENSO triggers subsidence with high pressure anomalies, warms the surface and increases the HWF significantly over northeastern Asia particularly. Likewise, the warm AMO‐induced circulation anomalies engender surface radiative heating and HWF growth in most of the Eurasian continent except some localized Siberian and Asian regions. The situation is opposite for a weak mega‐ENSO and AMO. Those models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) which realistically capture the features of the ID mode can reproduce the AMO‐like sea‐surface temperature anomalies (SSTAs), while signals resembling mega‐ENSO are found in those with favourable capability of simulating the IA mode. On the contrary, these relevant SSTAs linked to the respective modes vanish in the models with little skills. Thus, mega‐ENSO and the AMO might provide two critical predictability sources for heat waves over Eurasia.

New Metrics of the Tropospheric Biennial Oscillation in the East Asian Summer Monsoon and Its Interdecadal Shift

AbstractThis study designed a simple index for measuring irregular tropospheric biennial oscillation (TBO) activities, which was used to determine that the TBO in the East Asian Summer Monsoon (EASM), the most important summer precipitation system for China, has strengthened rather than weakened since the late 1970s. The lead/lag correlations between the EASM and tropical Indian-Pacific sea surface temperature (SST) suggest a relationship between interbasin SST and EASM coupling processes and that this alternative correlation pattern is likely related to TBO. Significant correlation occurred only in recent decades, which implies a reinforcement of TBO in the EASM. From records of representative points in the Indian-Pacific, the interdecadal intrinsic SST modes of the areas can be obtained with ensemble empirical mode decomposition owing to its good temporal locality. Statistical results show Indian-Pacific SST interdecadal trends that include out-of-phase and in-phase warming before and after the late 197...

Influence of extratropical sea‐surface temperature on the Indian summer monsoon: an unexplored source of seasonal predictability

Based on extensive analysis of observations and a series of climate model experiments, here we establish that slow variations of northern hemispheric extratropical sea‐surface temperature (SST) anomalies can augment seasonal predictability of the south Asian monsoon. The SST conditions and performance of the south Asian monsoon during 2013 boreal summer months (June–September) led us to hypothesize that the strong extratropical SST anomalies in the North Pacific and North Atlantic in conjunction with weak tropical SST anomalies (weak La Niña) were responsible for the above‐normal rainfall over India during 2013. We also argue that the 2013 SST pattern and above‐normal monsoon condition are not unique but occurred on several occasions in the past. Further, we show that there is a complementary pattern of strong extratropical SST and weak tropical SST that is associated with below‐normal south Asian monsoon rainfall. We also show that the extratropical SST pattern in the Northern Hemisphere is associated with a low‐frequency interdecadal mode of variability indicating potential predictability associated with such extratropical SST forcing. Extensive experiments with an atmospheric general circulation model forced by such SST conditions elucidate the mechanism through which the extratropical SSTs influence the Indian monsoon. The SST anomalies affect the north–south temperature gradient and lead to a local displacement of the jet stream, setting up a quasi‐stationary wave. Such a stationary wave, in turn, affects the tropospheric temperature (TT) over southern Eurasia, influencing the north–south TT gradient in the region and thereby the Indian monsoon. Our discovery of this additional source of potential predictability together with the fact that the new‐generation coupled ocean–atmosphere models are capable of predicting the extratropical SST anomalies brightens the prospect of south Asian monsoon prediction.

Summer temperature in the eastern part of southern South America: its variability in the twentieth century and a teleconnection with Oceania

The 1907–2001 summer-to-summer surface air temperature variability in the eastern part of southern South America (SSA, partly including Patagonia) is analysed. Based on records from instruments located next to the Atlantic Ocean (36°S–55°S), we define indices for the interannual and interdecadal timescales. The main interdecadal mode reflects the late-1970s cold-to-warm climate shift in the region and a warm-to-cold transition during early 1930s. Although it has been in phase with the Pacific Decadal Oscillation (PDO) index since the 1960s, they diverged in the preceding decades. The main interannual variability index exhibits high spectral power at ~3.4 years and is representative of temperature variability in a broad area in the southern half of the continent. Eleven-years running correlation coefficients between this index and December-to-February (DJF) Nino3.4 show significant decadal fluctuations, out-of-phase with the running correlation with a DJF index of the Southern Annular Mode. The main interannual variability index is associated with a barotropic wavetrain-like pattern extending over the South Pacific from Oceania to SSA. During warm (cold) summers in SSA, significant anticyclonic (cyclonic) anomalies tend to predominate over eastern Australia, to the north of the Ross Sea, and to the east of SSA, whereas anomalous cyclonic (anticyclonic) circulation is observed over New Zealand and west of SSA. This teleconnection links warm (cold) SSA anomalies with dry (wet) summers in eastern Australia. The covariability seems to be influenced by the characteristics of tropical forcing; indeed, a disruption has been observed since late 1970s, presumably due to the PDO warm phase.