How to predict the year‐to‐year variation of the east Asian summer monsoon (EASM) is one of the most challenging and important tasks in climate prediction. It has been recognized that the EASM ...variations are intimately but not exclusively linked to the development and decay of El Niño or La Niña. Here we present observed evidence and numerical experiment results to show that anomalous North Atlantic Oscillation (NAO) in spring (April–May) can induce a tripole sea surface temperature pattern in the North Atlantic that persists into ensuing summer and excite downstream development of subpolar teleconnections across the northern Eurasia, which raises (or lowers) the pressure over the Ural Mountain and the Okhotsk Sea. The latter strengthens (or weakens) the east Asian subtropical front (Meiyu‐Baiu‐Changma), leading to a strong (or weak) EASM. An empirical model is established to predict the EASM strength by combination of the El Niño–Southern Oscillation (ENSO) and spring NAO. Hindcast is performed for the 1979–2006 period, which shows a hindcast prediction skill that is comparable to the 14 state‐of‐the‐art multimodel ensemble hindcast. Since all these predictors can be readily monitored in real time, this empirical model provides a real time forecast tool.
The twentieth century Northern Hemisphere mean surface temperature (NHT) is characterized by a multidecadal warming‐cooling‐warming pattern followed by a flat trend since about 2000 (recent warming ...hiatus). Here we demonstrate that the North Atlantic Oscillation (NAO) is implicated as a useful predictor of NHT multidecadal variability. Observational analysis shows that the NAO leads both the detrended NHT and oceanic Atlantic Multidecadal Oscillation (AMO) by 15–20 years. Theoretical analysis illuminates that the NAO precedes NHT multidecadal variability through its delayed effect on the AMO due to the large thermal inertia associated with slow oceanic processes. An NAO‐based linear model is therefore established to predict the NHT, which gives an excellent hindcast for NHT in 1971–2011 with the recent flat trend well predicted. NHT in 2012–2027 is predicted to fall slightly over the next decades, due to the recent NAO decadal weakening that temporarily offsets the anthropogenically induced warming.
Key Points
The NAO leads the DNHT by 15‐20 years in the observations
The slow oceanic processes may account for the time lead of NAO leading DNHT
A NAO‐based model is a useful predictive tool for the NHT
The El Niño-Southern Oscillation (ENSO) results from the instability of and also modulates the strength of the tropical-Pacific cold tongue. While climate models reproduce observed ENSO amplitude ...relatively well, the majority still simulates its asymmetry between warm (El Niño) and cold (La Niña) phases very poorly. The causes of this major deficiency and consequences thereof are so far not well understood. Analysing both reanalyses and climate models, we here show that simulated ENSO asymmetry is largely proportional to subsurface nonlinear dynamical heating (NDH) along the equatorial Pacific thermocline. Most climate models suffer from too-weak NDH and too-weak linear dynamical ocean-atmosphere coupling. Nevertheless, a sizeable subset (about 1/3) having relatively realistic NDH shows that El Niño-likeness of the equatorial-Pacific warming pattern is linearly related to ENSO amplitude change in response to greenhouse warming. Therefore, better simulating the dynamics of ENSO asymmetry potentially reduces uncertainty in future projections.
The El Niño–Southern Oscillation (ENSO) tends to behave arguably as two different “types” or “flavors” in recent decades. One is the canonical cold-tongue-type ENSO with major sea surface temperature ...anomalies (SSTA) positioned over the eastern Pacific. The other is a warm-pool-type ENSO with SSTA centered in the central Pacific near the edge of the warm pool. In this study, the basic features and main feedback processes of these two types of ENSO are examined. It is shown that the interannual variability of upper-ocean heat content exhibits recharge–discharge processes throughout the life cycles of both the cold tongue (CT) and warm pool (WP) ENSO types. Through a heat budget analysis with focus on the interannual frequency band, the authors further demonstrate that the thermocline feedback plays a dominant role in contributing to the growth and phase transitions of both ENSO types, whereas the zonal advective feedback contributes mainly to their phase transitions. The westward shift of the SSTA center of the WP ENSO and the presence of significant surface easterly wind anomalies over the far eastern equatorial Pacific during its mature warm phase are the two main factors that lead to a reduced positive feedback for the eastern Pacific SSTA. Nevertheless, both the WP and CT ENSO can be understood to a large extent by the recharge oscillator mechanism.
Nonlinear interactions between ENSO and the western Pacific warm pool annual cycle generate an atmospheric combination mode (C-mode) of wind variability. The authors demonstrate that C-mode dynamics ...are responsible for the development of an anomalous low-level northwest Pacific anticyclone (NWP-AC) during El Niño events. The NWP-AC is embedded in a large-scale meridionally antisymmetric Indo-Pacific atmospheric circulation response and has been shown to exhibit large impacts on precipitation in Asia. In contrast to previous studies, the authors find the role of air–sea coupling in the Indian Ocean and northwestern Pacific only of secondary importance for the NWP-AC genesis. Moreover, the NWP-AC is clearly marked in the frequency domain with near-annual combination tones, which have been overlooked in previous Indo-Pacific climate studies. Furthermore, the authors hypothesize a positive feedback loop involving the anomalous low-level NWP-AC through El Niño and C-mode interactions: the development of the NWP-AC as a result of the C-mode acts to rapidly terminate El Niño events. The subsequent phase shift from retreating El Niño conditions toward a developing La Niña phase terminates the low-level cyclonic circulation response in the central Pacific and thus indirectly enhances the NWP-AC and allows it to persist until boreal summer. Anomalous local circulation features in the Indo-Pacific (e.g., the NWP-AC) can be considered a superposition of the quasi-symmetric linear ENSO response and the meridionally antisymmetric annual cycle modulated ENSO response (C-mode). The authors emphasize that it is not adequate to assess ENSO impacts by considering only interannual time scales. C-mode dynamics are an essential (extended) part of ENSO and result in a wide range of deterministic high-frequency variability.
The operational dynamic subseasonal to seasonal (S2S) models for Madden‐Julian oscillation (MJO) forecasting mostly still suffer from systematic errors in capturing the MJO's key dynamic features, ...such as its growth rate and propagation speed. By deriving the linear dynamic operators using the linear inverse modeling (LIM) approach, we propose a method to partly correct the errors in MJO linear dynamic operators to improve the MJO predictions of three operational dynamic S2S models. Correcting the deficiencies of the too‐fast decay rates and the unrealistic propagating phase speeds lead to MJO prediction skills being extended by approximately 2–4 days. The improvements are more significant for the models with larger biases in MJO amplitude and propagation. This approach in principle may be extendable to predictions of other types of climate variability such as ENSO on one hand, and possible inclusions of nonlinear dynamics effects on the other hand.
Plain Language Summary
The Madden‐Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics and can exert a significant influence on global circulation and extreme weather events. Most of the current generations of state‐of‐the‐art operational dynamic models used in MJO predictions have yet to fully capture the essential features of the MJO. These deficiencies hinder the models’ MJO prediction skills. By taking advantage of mass hindcast data, a method is proposed in this study to correct the effect of errors of the linear dynamics of each model on its MJO predictions by carrying out a dynamics‐based postprocessing on the model predictions. This proposed approach is applied to three different operational dynamic models and shows improvements in MJO prediction skills. This kind of approach may be extended to improve the predictions of other types of climate variability such as ENSO and may be expanded to include nonlinear dynamics effects as well.
Key Points
The operational dynamic forecast models mostly suffer errors in capturing Madden‐Julian oscillation's (MJO) fundamental dynamic properties
A dynamics‐based postprocessing correction method may extend model's MJO forecast skill by approximately 2–4 days
The gain in skill comes from reducing errors in the decay rate and propagating phase speed in the model's MJO predictions
Two Types of El Niño Events Kug, Jong-Seong; Jin, Fei-Fei; An, Soon-Il
Journal of climate,
03/2009, Letnik:
22, Številka:
6
Journal Article
Recenzirano
Odprti dostop
In this study, two types of El Niño events are classified based on spatial patterns of the sea surface temperature (SST) anomaly. One is the cold tongue (CT) El Niño, which can be regarded as the ...conventional El Niño, and the other the warm pool (WP) El Niño. The CT El Niño is characterized by relatively large SST anomalies in the Niño-3 region (5°S–5°N, 150°–90°W), while the WP El Niño is associated with SST anomalies mostly confined to the Niño-4 region (5°S–5°N, 160°E–150°W). In addition, spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of El Niño events. Furthermore, the difference in the transition mechanism between the two types of El Niño is clearly identified. That is, the discharge process of the equatorial heat content associated with the WP El Niño is not efficient owing to the spatial structure of SST anomaly; as a result, it cannot trigger a cold event. It is also demonstrated that zonal advective feedback (i.e., zonal advection of mean SST by anomalous zonal currents) plays a crucial role in the development of a decaying SST anomaly associated with the WP El Niño, while thermocline feedback is a key process during the CT El Niño.
Global sea level rise due to the thermal expansion of the warming oceans and freshwater input from melting glaciers and ice sheets is threatening to inundate low-lying islands and coastlines ...worldwide. At present the global mean sea level rises at 3.1 ± 0.7 mm yr−1with an accelerating tendency. However, the magnitude of recent decadal sea level trends varies greatly spatially, attaining values of up to 10 mm yr−1in some areas of the western tropical Pacific. Identifying the causes of recent regional sea level trends and understanding the patterns of future projected sea level change is of crucial importance. Using a wind-forced simplified dynamical ocean model, the study shows that the regional features of recent decadal and multidecadal sea level trends in the tropical Indo-Pacific can be attributed to changes in the prevailing wind regimes. Furthermore, it is demonstrated that within an ensemble of 10 state-of-the-art coupled general circulation models, forced by increasing atmospheric CO₂ concentrations over the next century, wind-induced redistributions of upper-ocean water play a key role in establishing the spatial characteristics of projected regional sea level rise. Wind-related changes in near-surface mass and heat convergence near the Solomon Islands, Tuvalu, Kiribati, the Cook Islands, and French Polynesia oppose—but cannot cancel—the regional signal of global mean sea level rise.
During June–July 2020, the record‐breaking flooding in the recent four decades struck the plum rain belt over China, Japan, and Korea. Concurrent with this persistent heavy rainfall, pronounced ...Indian Ocean basin warming (IOBW) was observed, following the previous El Niño event in the transition to La Niña‐like mean state this summer. This tropical Indo‐Pacific large‐scale thermal condition provided favorable conditions for rainfall surpluses over the plum rain belt via the western North Pacific anticyclone. Superimposed on the tropical Indo‐Pacific large‐scale thermal condition, an extraordinary long‐lasting and quasistationary Madden‐Julian Oscillation (MJO) active phase persisted in the Indian Ocean throughout June–July 2020, lasting for 59 days. The MJO‐associated teleconnection was mainly responsible for the extreme rainfall over the plum rain belt, which was facilitated by the conducive large‐scale Indo‐Pacific oceanic‐atmospheric condition.
Plain Language Summary
The East Asian rainy season (commonly called the plum rain, also known as Meiyu in China, Baiu in Japan, and Jangma in Korea), is caused by rainfall along a persistent stationary front. The plum rain season usually starts in June and persists into the middle of July. During June–July 2020, the region of the plum rain belt over China, Japan, and Korea, was ravaged by the strongest flooding in decades with catastrophic losses. The physical mechanisms driving this extreme flooding remains unclear. Concurrent with this persistent rainfall surpluses, the Indo‐Pacific oceans featured a developing La Niña‐like state in the eastern tropical Pacific and pronounced sea surface temperature warming in the Indian Ocean basin. Although these Indo‐Pacific thermal conditions can provide a favorable large‐scale background for the rainfall surpluses over East Asia, they cannot explain the extreme rainfall amount over the plum rain belt during 2020. We show that an exceptionally long‐lasting and quasistationary Madden‐Julian Oscillation activity over the Indian Ocean was the key reason for this extreme event.
Key Points
The rainfall amount over the East Asian plum‐rain belt during summer 2020 was the largest in the recent four decades
The intense flooding coincided with an exceptionally persistent Madden‐Julian Oscillation (MJO) active phase in the Indian Ocean throughout June and July
This extraordinary MJO activity was facilitated by the conducive pantropic background oceanic‐atmospheric conditions
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