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Yangyang Xu and Aixue Hu

Abstract

Decadal climate variability of sea surface temperature (SST) over the Pacific Ocean can be characterized by interdecadal Pacific oscillation (IPO) or Pacific decadal oscillation (PDO) based on empirical orthogonal function (EOF) analysis. Although the procedures to derive the IPO and PDO indices differ in their regional focuses and filtering methods to remove interannual variability, the IPO and PDO are highly correlated in time and are often used interchangeably. Studies have shown that the IPO and PDO conjointly (IPO/PDO for conciseness) play a vital role in modulating the pace of global warming. It is less clear, however, how externally forced global warming may, in turn, affect the IPO/PDO. One obstacle to revealing this effect is that the conventional definitions of the IPO/PDO fail to account for the spatial heterogeneity of the background warming trend, which causes the IPO/PDO to be conflated with the warming trend, especially for the twenty-first-century simulation when the forced change is likely to be more dominant. Using a large-ensemble simulation in the Community Earth System Model, version 1 (CESM1), it is shown here that a better practice of detrending prior to EOF analysis is to remove the local and nonlinear trend, defined as the ensemble-mean time series at each grid box (or simply as the quadratic fit of the local time series if such an ensemble is not available). The revised IPO/PDO index is purely indicative of internal decadal variability. In the twenty-first-century warmer climate, the IPO/PDO has a weaker amplitude in space, a higher frequency in time, and a muted impact on global and North American temperature and rainfall.

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Dong Si and Aixue Hu

Abstract

Interdecadal oceanic variabilities can be generated from both internal and external processes, and these variabilities can significantly modulate climate on global and regional scales, including the warming slowdown in the early twenty-first century and rainfall in East Asia. By analyzing simulations from a unique Community Earth System Model (CESM) Large Ensemble (CESM-LE) project, it is shown that the interdecadal Pacific oscillation (IPO) is primarily an internally generated oceanic variability, while the Atlantic multidecadal oscillation (AMO) may be an oceanic variability generated by internal oceanic processes and modulated by external forcing in the twentieth century. Although the observed relationship between IPO and the Yangtze–Huaihe River valley (YHRV) summer rainfall in China is well simulated in both the preindustrial control and the twentieth-century ensemble simulation, none of the twentieth-century ensemble members can reproduce the observed time evolution of both the IPO and YHRV rainfall because of the unpredictable nature of IPO on multidecadal time scales. On the other hand, although CESM-LE cannot reproduce the observed relationship between the AMO and Huanghe River valley (HRV) summer rainfall of China in the preindustrial control simulation, this relationship in the twentieth-century simulations is well reproduced, and the chance of reproducing the observed time evolution of both AMO and HRV rainfall is about 30%, indicating the important role of the interaction between the internal processes and the external forcing to realistically simulate the AMO and HRV rainfall.

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Zeyuan Hu, Aixue Hu, and Yongyun Hu

Abstract

Regional sea surface temperature (SST) mode variabilities, especially the La Niña–like Pacific Ocean temperature pattern known as the negative phase of the interdecadal Pacific oscillation (IPO) and the associated heat redistribution within the ocean, are the leading mechanisms explaining the recent global warming hiatus. Here version 1 of the Community Earth System Model (CESM) is used to examine how different phases of two leading decadal time scale SST modes, namely the IPO and the Atlantic multidecadal oscillation (AMO), contribute to heat redistribution in the global ocean in the absence of time-evolving external forcings. The results show that both the IPO and AMO contribute a similar magnitude to global mean surface temperature and ocean heat redistribution. Both modes contribute warmer surface temperature and higher upper ocean heat content in their positive phase, and the reverse in their negative phase. Regionally, patterns of ocean heat distribution in the upper few hundred meters of the tropical and subtropical Pacific Ocean depend highly on the IPO phase via the IPO-associated changes in the subtropical cell. In the Atlantic, ocean heat content is primarily associated with the state of the AMO. The interconnections between the IPO, AMO, and global ocean heat distribution are established through the atmospheric bridge and the Atlantic meridional overturning circulation. An in-phase variant of the IPO and AMO can lead to much higher surface temperatures and heat content changes than an out-of-phase variation. This result suggests that changes in the IPO and AMO are potentially capable of modulating externally forced SST and heat content trends.

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Gerald A. Meehl and Aixue Hu

Abstract

A 1360-yr control run from a global coupled climate model (the Parallel Climate Model) is analyzed. It simulates “megadroughts” in the southwestern United States and Indian monsoon regions. The megadroughts represent extreme events of naturally occurring multidecadal precipitation variations linked to the dominant pattern of multidecadal SST variability in the Indian and Pacific Oceans. Gaining insight into the occurrence of megadroughts thus requires an understanding of the mechanism that is producing this multidecadal SST variability. Analysis of the model variability shows that the mechanism involves atmosphere–ocean and tropical–midlatitude interactions, with a crucial element being wind-forced ocean Rossby waves near 20°N and 25°S in the Pacific whose transit times set the decadal time scale. At the western boundary, the Rossby waves reflect into the equatorial Pacific to affect thermocline depth. The resulting feedbacks, involving surface temperature, winds, and the strength of the subtropical cells, produce SST anomalies and associated precipitation and convective heating anomalies. These anomalies are associated with atmospheric Rossby waves and resulting anomalous atmospheric circulation patterns in the midlatitude North and South Pacific. Consequent surface wind stress anomalies extend equatorward into the Tropics and help force ocean Rossby waves near 20°N and 25°S, and so on. Though there are some common elements with various ENSO processes, this decadal mechanism is physically distinct mainly because the surface wind stress anomalies near 20°N and 25°S supplement the wave reflections at the eastern boundary to force the ocean Rossby waves that provide the decadal time scale. These wind anomalies are closely tied to the anomalous midlatitude atmospheric circulation that is a product of teleconnections from the multidecadal SST and tropical convective heating anomalies, themselves linked to precipitation anomalies in the southwestern United States and south Asia associated with megadroughts.

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Zeyuan Hu, Aixue Hu, Yongyun Hu, and Nan Rosenbloom

Abstract

A slowdown in the rate of surface warming in the early 2000s led to renewed interest in the redistribution of ocean heat content (OHC) and its relationship with internal climate variability. We use the Community Earth System Model version 1 to study the relationship between OHC and the interdecadal Pacific oscillation (IPO), a major mode of decadal sea surface temperature variability in the Pacific Ocean. By comparing the relative contributions of surface heat flux and ocean dynamics to changes in OHC for different phases of the IPO, we try to identify the underlying physical processes involved. Our results suggest that during IPO phase transitions, changes of 0–300-m OHC across the northern extratropical Pacific are positively contributed by both surface heat flux and oceanic heat transport. By contrast, oceanic heat transport appears to drive the OHC changes in equatorial Pacific whereas surface heat flux acts as a damping term. During a positive IPO phase, weakened wind-driven circulation acts to increase the OHC in the equatorial Pacific while the enhanced evaporation acts to damp OHC anomalies. In the Kuroshio–Oyashio Extension region, a dipole anomaly of zonal heat advection amplifies an OHC dipole anomaly that moves eastward, while strong turbulent heat fluxes act to dampen this OHC anomaly. In the northern subtropical Pacific, both the wind-driven evaporation change and the change of zonal heat advection along Kuroshio Extension contribute to the OHC change during phase transition. For the northern subpolar Pacific, both surface heat flux and enhanced meridional advection contribute to the positive OHC anomalies during the positive IPO phase.

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Gerald A. Meehl, Aixue Hu, and Claudia Tebaldi

Abstract

A “perfect model” configuration with a global coupled climate model 30-member ensemble is used to address decadal prediction of Pacific SSTs. All model data are low-pass filtered to focus on the low-frequency decadal component. The first three EOFs in the twentieth-century simulation, representing nearly 80% of the total variance, are used as the basis for early twenty-first-century predictions. The first two EOFs represent the forced trend and the interdecadal Pacific oscillation (IPO), respectively, as noted in previous studies, and the third has elements of both trend and IPO patterns. The perfect model reference simulation, the target for the prediction, is taken as the experiment that ran continuously from the twentieth to twenty-first century using anthropogenic and natural forcings for the twentieth century and the A1B scenario for the twenty-first century. The other 29 members use a perturbation in the atmosphere at year 2000 and are run until 2061. Since the IPO has been recognized as a dominant contributor to decadal variability in the Pacific, information late in the twentieth century and early in the twenty-first century is used to select a subset of ensemble members that are more skillful in tracking the time evolution of the IPO (EOF2) in relation to a notional start date of 2010. Predictions for the 19-yr period centered on the year 2020 use that subset of ensemble members to construct Pacific SST patterns based on the predicted evolution of the first three EOFs. Compared to the perfect model reference simulation, the predictions show some skill for Pacific SST predictions with anomaly pattern correlations greater than +0.5. An application of the Pacific SST prediction is made to precipitation over North America and Australia. Even though there are additional far-field influences on Pacific SSTs and North American and Australian precipitation involving the Atlantic multidecadal oscillation (AMO) in the Atlantic, and Indian Ocean and South Asian monsoon variability, there is qualitative skill for the pattern of predicted precipitation over North America and Australia using predicted Pacific SSTs. This exercise shows that, in the presence of a large forced trend like that in the large ensemble, much of Pacific region decadal predictability about 20 years into the future arises from increasing greenhouse gases.

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Dong Si, Aixue Hu, Huijun Wang, and Qingchen Chao

Abstract

In contrast to dominant interannual time-scale variability in other ocean basins, the leading observed mode variability in the Atlantic is characterized as a basinwide seesaw-like sea surface temperature variability between the North and South Atlantic on a multidecadal time scale (approximately 60–80 years), known as the Atlantic multidecadal variability (AMV). AMV has been identified as a key driver for climate shifts that occurred in the mid-1960s and late 1990s. Here we attempt to predict the summer AMV by analyzing decadal prediction experiments from two climate models. Results show that these climate models with proper initialization do a better job than uninitialized historical runs, and are capable of predicting the observed AMV time evolution. Our models predict that the AMV will be in a neutral to slightly negative phase, leading to a warm–dry trend over western Europe and North Africa and a cold–wet trend (cold relative to the warming trend) over southeastern China and Indochina in the next few years.

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ZhongDa Lin, Yun Li, Yong Liu, and AiXue Hu

Abstract

Rainfall in southeastern Australia (SEA) decreased substantially in the austral autumn (March–May) of the 1990s and 2000s. The observed autumn rainfall reduction has been linked to the climate change–induced poleward shift of the subtropical dry zone across SEA and natural multidecadal variations. However, the underlying physical processes responsible for the SEA drought are still not fully understood. This study highlights the role of sea surface temperature (SST) warming in the subtropical South Pacific (SSP) in the autumn rainfall reduction in SEA since the early 1990s. The warmer SSP SST enhances rainfall to the northwest in the southern South Pacific convergence zone (SPCZ); the latter triggers a divergent overturning circulation with the subsidence branch over the eastern coast of Australia. As such, the subsidence increases the surface pressure over Australia, intensifies the subtropical ridge, and reduces the rainfall in SEA. This mechanism is further confirmed by the result of a sensitivity experiment using an atmospheric general circulation model. Moreover, this study further indicates that global warming and natural multidecadal variability contribute approximately 44% and 56%, respectively, of the SST warming in the SSP since the early 1990s.

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Weiqing Han, Peter Webster, Roger Lukas, Peter Hacker, and Aixue Hu

Abstract

An ocean general circulation model (OGCM) is used to investigate the low-frequency (period longer than 90 days) rectification of atmospheric intraseasonal variability (10–90-day periods) in zonal surface current and transport of the equatorial Indian Ocean. A hierarchy of OGCM solutions is found in an actual tropical Indian Ocean basin for the period of 1988–2001. To help to identify and isolate nonlinear processes, a linear continuously stratified model and a 4-layer intermediate ocean model are also used. Results from the OGCM solutions suggest that intraseasonal atmospheric forcing acts to weaken the equatorial seasonal surface currents. Amplitudes of the spring and autumn eastward surface jets, the Wyrtki jets (WJ), and the westward surface current during January–March are reduced by as much as 15–25 cm s−1 by intraseasonal atmospheric forcing, and strengths of the rectification exhibit a significant interannual variability. Important processes that cause the low-frequency rectification are asymmetric response of mixed layer depth to easterly and westerly winds, entrainment, and upwelling of momentum. During spring and autumn, the westerly (easterly) phase of an intraseasonal event enhances (weakens or even reverses) the seasonal westerly winds, increases (decreases) equatorial convergence and entrainment, and thus deepens (thins) the mixed layer. A net, westward current is generated over an event mean because easterly wind acts on a thinner surface mixed layer whereas westerly wind acts on a thicker one. In contrast, during January–March when the seasonal winds are equatorial easterlies, surface currents are westward and equatorial undercurrents (EUC) develop. The rectified surface currents are eastward, which reduces the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. The seasonal-to-interannual variability of the rectified surface flow is determined primarily from the seasonal cycle and interannual variability of the background state. Seasonal-to-interannual variability of the intraseasonal wind forcing also contributes. The rectified low-frequency zonal volume (heat) transports integrated over the entire water column along the Indian Ocean equator are persistently eastward with an amplitude of 0– 15 × 106 m3 s−1 (0–1.2 pW). This is because westerly winds generate equatorial downwelling, advecting the surface eastward momentum downward and giving an eastward subsurface current. Easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current. This eastward subsurface current is advected upward by upwelling. The mean effect over an intraseasonal event at the equator is to increase the eastward transport in the water column. In the layers above the thermocline, the rectified zonal volume (heat) transports are in the same direction as the rectified surface currents. Results from this paper may have important implications for understanding climate variability because modification of WJ strength and transports can affect the SST and heat storage in the equatorial Indian Ocean warm pool.

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Bingyi Wu, Dörthe Handorf, Klaus Dethloff, Annette Rinke, and Aixue Hu

Abstract

Using NCEP–NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) winter daily (1 December–28 February) data for the period 1979–2012, this paper reveals the leading pattern of winter daily 850-hPa wind variability over northern Eurasia from a dynamic perspective. The results show that the leading pattern accounts for 18% of the total anomalous kinetic energy and consists of two subpatterns: the dipole and the tripole wind patterns. The dipole wind pattern does not exhibit any apparent trend. The tripole wind pattern, however, has displayed significant trends since the late 1980s. The negative phase of the tripole wind pattern corresponds to an anomalous anticyclone over northern Eurasia during winter, as well as two anomalous cyclones occurring over southern Europe and in the mid- to high latitudes of East Asia. These anomalous cyclones in turn lead to enhanced winter precipitation in these two regions, as well as negative surface temperature anomalies over the mid- to high latitudes of Asia. The intensity of the tripole wind pattern and the frequency of its extreme negative phase are significantly correlated with autumn Arctic sea ice anomalies. Simulation experiments further demonstrate that the winter atmospheric response to Arctic sea ice decrease is dynamically consistent with the observed trend in the tripole wind pattern over the past 24 winters, which is one of the causes of the observed declining winter surface air temperature trend over Central and East Asia. The results of this study also imply that East Asia may experience more frequent and/or intense winter extreme weather events in association with the loss of Arctic sea ice.

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