Dynamical systems modeling of low-frequency variability in low-order atmospheric models

We investigate the Atlantic multidecadal variability (AMV) in observations and in a large CMIP6 historical climate model ensemble. We compare four different definitions of the AMV aimed at extracting the variability intrinsic to the Atlantic region. They are based on removing from the Atlantic temperature the linear or polynomial trend, the part congruent to the global average, or the part congruent to the multimodel mean of the global average. The definitions agree on the well-known low-frequency oscillatory variability in observations but show larger differences for the models. In general, large differences between ensemble members are found. We estimate the forced response in the AMV as the mean of the multimodel ensemble. This response converges fast with the ensemble size and saturates for a size of around 25. The forced response resembles the observed low-frequency oscillatory variability for the detrended AMV definitions, but these definitions are also the most ineffective in removing the forced global mean signal. The forced response is very weak for the other definitions, and few of their individual ensemble members show oscillatory variability and even fewer with the observed phase. The observed spatial temperature pattern related to the AMV is well captured for all AMV definitions but is more confined for the definitions with the global means removed. These definitions also depend the least on the considered period. The observed instantaneous connection between NAO and AMV is well represented in the models for all definitions. Only nonsignificant evidence of NAO leading the AMV on decadal time scales is found.

Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951–2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean temperature and frequency of hot summers (66 and 82 %) over the wUS can be attributed to the anthropogenic global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that AMO- and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies are not robust during the periods when PDO and AMO are in phase. The prolonged PDO–AMO antiphase period since the late twentieth century resulted in the substantial component of multidecadal anomalies in temperature and precipitation over the wUS.

ABSTRACTHuang et al. recently reported that opposite phases of the interdecadal Pacific oscillation (IPO) and the Atlantic multidecadal oscillation (AMO) can affect the shift of the East Asian polar front jet (EAPJ) and the East Asian subtropical jet (EASJ) in the boreal winter. To give a full image of the relationship among the IPO, AMO, and concurrent variation of jets throughout the whole year, this study investigates the changes in summer jets response to the combinations of the IPO and AMO, and mostly focuses on the quantitative analysis in the role of the IPO and AMO. Both of the diagnostic analysis and atmospheric model simulations confirm that combinations of the negative phase of the IPO (“−IPO”) and the positive phase of the AMO (“+AMO”) can significantly enhance the EAPJ and reduce the EASJ in the summer, via the meridional temperature gradient and the Eady growth rate, and vice versa in the “+IPO −AMO” combination. The reanalysis data show that this relationship is particularly evident between the periods of 1999–2014 and 1979–98. Based on the simulations, the multilinear regression has indicated that −IPO plays a more important role than +AMO, particularly for the reduced EASJ. We have further revealed two pathways of the stationary Rossby wave activity anomaly, eastward from the North Atlantic to East Asia along 60°N and westward from the North Pacific to East Asia along 40°N. The two activities are associated with anomalous anticyclone along the active regions between EAPJ and EASJ, and therefore affect the jet variations.

Recent concurrent shifts of the East Asian polar-front jet (EAPJ) and the East Asian subtropical jet (EASJ) in the boreal winter have raised concerns, since they could result in severe weather events over East Asia. However, the possible mechanisms are not fully understood. In this study, the roles of the interdecadal Pacific oscillation (IPO) and the Atlantic multidecadal oscillation (AMO) are investigated by analyzing reanalysis data and model simulations. Results show that combinations of opposite phases of the IPO and AMO can result in significant shifts of the two jets during 1920–2014. This relationship is particularly evident during 1999–2014 and 1979–98 in the reanalysis data. A combination of a negative phase of the IPO (−IPO) and a positive phase of the AMO (+AMO) since the late 1990s has enhanced the meridional temperature gradient and the Eady growth rate and thus westerlies over the region between the two jets, but weakened them to the south and north of the region, thereby contributing to the equatorward and poleward shifts of the EAPJ and EASJ, respectively. Atmospheric model simulations are further used to investigate the relative contribution of −IPO and +AMO to the jet shifts. The model simulations show that the combination of −IPO and +AMO favors the recent jet changes more than the individual −IPO or +AMO. Under a concurrent −IPO and +AMO, the meridional eddy transport of zonal momentum and sensitive heat strengthens, and more mean available potential energy converts to the eddy available potential energy over the region between the two jets, which enhances westerly winds there.

The Atlantic Multidecadal Variability (AMV) is a prominent mode of low-frequency climate variability, characterized by basin-scale sea surface temperature (SST) variations in the North Atlantic and strong global impacts. AMV can be forced externally by surface radiative fluxes or internally generated. The latter generation mechanism is commonly attributed to variations in the Atlantic Meridional Overturning Circulation (AMOC). Here we show that a robust AMV can arise and be sustained by large-scale atmosphere–ocean interactions, even in the absence of a dominant role for AMOC variations, in a fully coupled model—the Department of Energy’s Energy Exascale Earth System Model version 2 (E3SMv2). The simulated AMV is driven primarily by surface shortwave and turbulent heat fluxes across the North Atlantic. Essentially, an initial warming over the Gulf Stream region strengthens and spreads by reducing low cloud cover and enhancing surface shortwave radiation. This mechanism is enabled by the relatively narrow width of the North Atlantic, compared to the North Pacific. Our results broaden the conceptual understanding of AMV physics and underscore the importance of atmosphere–ocean interactions in sustaining it.

Interannual to Decadal Rainfall Variability over Greater Horn of Africa: Non-Stationary Teleconnections, Physical Mechanisms, and Predictability

The Atlantic Multidecadal Oscillation (AMO) significantly influences the climate of the surrounding continents and has previously been attributed to variations in the Atlantic Meridional Overturning Circulation. Recently, however, similar multidecadal variability was reported in climate models without ocean circulation variability. We analyze the relationship between turbulent heat fluxes and sea surface temperatures (SSTs) over the midlatitude North Atlantic in observations and coupled climate model simulations, both with and without ocean circulation variability. SST anomalies associated with the AMO are positively correlated with heat fluxes on decadal time scales in both observations and models with varying ocean circulation, whereas in models without ocean circulation variability the anomalies are negatively correlated when heat flux anomalies lead. These relationships are captured in a simple stochastic model and rely crucially on low‐frequency forcing of SST. The fully coupled models that better capture this signature more effectively reproduce the observed impact of the AMO on European summertime temperatures.

Against the backdrop of warming of the Northern Hemisphere it has recently been acknowledged that North Atlantic temperature changes undergo considerable variability over multidecadal periods. The leading component of natural low-frequency temperature variability has been termed the Atlantic Multidecadal Oscillation (AMO). Presently, correlative studies on the biological impact of the AMO on marine ecosystems over the duration of a whole AMO cycle (∼60 years) is largely unknown due to the rarity of continuously sustained biological observations at the same time period. To test whether there is multidecadal cyclic behaviour in biological time-series in the North Atlantic we used one of the world's longest continuously sustained marine biological time-series in oceanic waters, long-term fisheries data and historical records over the last century and beyond. Our findings suggest that the AMO is far from a trivial presence against the backdrop of continued temperature warming in the North Atlantic and accounts for the second most important macro-trend in North Atlantic plankton records; responsible for habitat switching (abrupt ecosystem/regime shifts) over multidecadal scales and influences the fortunes of various fisheries over many centuries.

Arabian Sea (AS) warming has been significantly accelerated since the 1990s, in particular in the spring season. Here we link the AS warming changes to the Atlantic multidecadal oscillation (AMO). A set of Atlantic pacemaker experiments with a slab mixed‐layer ocean model successfully reproduces the AS spring multidecadal variability and its connection with the AMO. An atmospheric teleconnection from the Atlantic to the AS in the preceding winter and associated thermodynamic air‐sea feedback is found to be important. The teleconnection can be reestablished by the atmospheric model when the AMO sea surface temperature (SST) and its trans‐basin footprint over the western Pacific are prescribed simultaneously. The western Pacific SST warming associated with the AMO positive phase induces a Gill‐type Rossby wave over the AS, showing anomalously low pressures and converging southerlies that weaken winter northerlies. Thus, the wind‐evaporation‐SST feedback results in and maintains the AS warm SST anomalies to the subsequent spring.

In this study we investigate statistical link between external climate forcings and modes of ocean variability on inter-annual (3-year) to centennial (100-year) timescales using de-trended semi-partial-cross-correlation analysis technique. To investigate this link we employ observations (AD 1854–1999), climate proxies (AD 1600–1999), and coupled Atmosphere-Ocean-Chemistry Climate Model simulations with SOCOL-MPIOM (AD 1600–1999). We find robust statistical evidence that Atlantic multi-decadal oscillation (AMO) has intrinsic positive correlation with solar activity in all datasets employed. The strength of the relationship between AMO and solar activity is modulated by volcanic eruptions and complex interaction among modes of ocean variability. The observational dataset reveals that El Nino southern oscillation (ENSO) has statistically significant negative intrinsic correlation with solar activity on decadal to multi-decadal timescales (16–27-year) whereas there is no evidence of a link on a typical ENSO timescale (2–7-year). In the observational dataset, the volcanic eruptions do not have a link with AMO on a typical AMO timescale (55–80-year) however the long-term datasets (proxies and SOCOL-MPIOM output) show that volcanic eruptions have intrinsic negative correlation with AMO on inter-annual to multi-decadal timescales. The Pacific decadal oscillation has no link with solar activity, however, it has positive intrinsic correlation with volcanic eruptions on multi-decadal timescales (47–54-year) in reconstruction and decadal to multi-decadal timescales (16–32-year) in climate model simulations. We also find evidence of a link between volcanic eruptions and ENSO, however, the sign of relationship is not consistent between observations/proxies and climate model simulations.

In this chapter, an introduction will be given on how to use the theory and methods of stochastic dynamical systems theory to understand phenomena of climate variability. We will restrict the application of this theory to very basic aspects of the El Nino/Southern Oscillation and the Atlantic Multidecadal Oscillation.

Associations of multi-decadal sea-surface temperature variability with US drought

Tropical cyclone (TC) translation speed (TCS) over the western North Pacific (WNP) has experienced a long-term decreasing trend. To date, however, little is known about the multidecadal variability of TCS and its possible influence on this trend. This study investigated the multidecadal variability of the WNP TCS and the underlying physical mechanisms. Results show that the WNP TCS presents robust multidecadal variability during the past seven decades, which is dominated by the TCS over the extratropics. Further analysis shows that the Atlantic multidecadal oscillation (AMO) is responsible for the TCS multidecadal variability. AMO positive (negative) phases lead to favorable (unfavorable) large-scale environmental conditions for maintaining TCs over the extratropics, which results in longer (shorter) residence time for TCs having been accelerated by the midlatitude westerlies, thus, leading to higher (lower) TCS. The TCS phase shift strongly offsets its slowdown trend, leading to the inconsistent trends during past decades. This inconsistency may also relate to the influence of extratropical transitioned cyclones without being totally excluded. These cyclones may be inhomogeneously recorded due to the absence of satellite observation before the 1980s. Our results indicate that internal variation such as AMO may dominate TCS low-frequency variations over the past several decades. Previous studies have attributed the inconsistent trends of TCS during different subperiods to data inhomogeneity. This study shows that AMO can modulate the TCS trends in different subperiods with phase shift, thus providing new evidence for the recent controversial TCS slowdown.

North Central China (NCC) (34°–42°N, 95°–107°E), a typical transitional climate zone between westerlies and monsoon, shows multiple time‐scale variations in precipitation under the background of global warming. Thus, NCC moisture transport (NMT) was analysed based on reanalysis data from 1979–2015. By using the spatially unbounded dynamic recycling model, main NMT pathways and moisture sources were identified for the summer rainfall of NCC. The trend pattern of NMT manifests as a seesaw pattern with a weakening northwesterly transport but an enhancing southwesterly transport, suggesting an interaction between the mid‐latitude westerlies and the Indian monsoon. The temporal evolution of the NMT trend pattern exhibits interannual and multidecadal variability superimposed on long‐term trends, which correspond to the atmospheric and oceanic physical processes, respectively. The partial least squares regression analysis demonstrated that the temporal evolution of NMT trend patterns can be well explained by atmospheric and oceanic internal climate variability (ICV). At interannual time scales, the atmospheric ICV, composed of the circumglobal teleconnection (CGT) and East Atlantic (EA) and East Atlantic/Western Russia (EAWR) teleconnections, forms a Eurasian wave train that weakens westerly transport via the Europe blocking flow and enhances southwesterly transport via local circulation anomalies with a zonal dipole structure. However, oceanic ICV, composed of the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), exerts influence on a multidecadal time scale to decelerate the mid‐latitude westerly jet over the North Atlantic, providing favourable upstream background conditions for the formation of the Europe blocking and further maintaining the atmospheric ICV‐induced Eurasian wave train. Thus, the internal oceanic and atmospheric processes at different time scales couple to contribute to long‐term changes in precipitation over NCC under the background of global warming.

Long-term trends and wave climate variability in the South Atlantic Ocean: The influence of climate indices