Variability in the southern atmospheric circulation at mid- to high latitudes with a dominant quasi-stationary wavenumber-3 pattern has been reported in many observational studies. The variability is barotropic in nature with signals in the middle troposphere as well as at the atmosphere–ocean interface. Moreover, there are preferred fixed centers for the strongest anomalies. These features are well reproduced by the Commonwealth Scientific and Industrial Research Organisation coupled model on various timescales. On the interannual timescale, an index of the modeled wavenumber-3 pattern shows little correlation with the modeled Southern Oscillation index, suggesting that the variability associated with wavenumber-3 anomalies is separate to modeled ENSO-like events. However, the variation of the pattern index is strikingly similar to, and highly correlated with, the modeled oceanic variability. The associated oceanic anomalies move eastward and are similar to those of the observed Antarctic circumpolar wave (ACW). The modeled ACW-like anomalies exist not only at the surface but also through middle ocean depths, with a similar barotropic nature to those of the atmospheric anomalies. The oceanic anomalies also display a wavenumber-3 pattern. The essential elements of the dynamics of the modeled ACW are the advection of SST anomalies by the surface Antarctic Circumpolar Current (ACC), and the interactions between anomalies of SST and mean sea level pressure (MSLP). Associated with the standing wavenumber-3 pattern, there are fixed centers for the strongest MSLP anomalies. As a positive SST anomaly advected by surface ACC approaches a center of a positive MSLP anomaly, the MSLP decreases. The positive (negative) SST anomalies are generated by anomalous latent and heat fluxes, which are in turn induced by southward (northward) meridional wind stress anomalies resulting from geostrophic balance. These MSLP anomalies change sign when the positive (negative) SST anomalies move to a location near the centers. Once MSLP anomalies change sign, positive (negative) SST anomalies are generated again reinforcing the anomalies entering from the west. The time for the surface ACC to advect one-sixth of the circuit around the pole corresponds to the time of a half-cycle of the standing MSLP oscillations. Thus the surface ACC determines the frequency of the standing oscillation. In the present model, the speed of the surface ACC is such that the period of the standing oscillation is 4–5 yr, and it would take 12–16 yr for an anomaly to encircle the pole. These and other features of the modeled ACW, together with associated dynamic processes, are analyzed and discussed.
Abstract An asymmetry, and its multidecadal variability, in a rainfall teleconnection with the El Niño–Southern Oscillation (ENSO) are described. Further, the breakdown of this relationship since 1980 is offered as a cause for a rainfall reduction in an ENSO-affected region, southeast Queensland (SEQ). There, austral summer rainfall has been declining since around the 1980s, but the associated process is not understood. It is demonstrated that the rainfall reduction is not simulated by the majority of current climate models forced with anthropogenic forcing factors. Examination shows that ENSO is a rainfall-generating mechanism for the region because of an asymmetry in its impact: the La Niña–rainfall relationship is statistically significant, as SEQ summer rainfall increases with La Niña amplitude; by contrast, the El Niño–induced rainfall reductions do not have a statistically significant relationship with El Niño amplitude. Since 1980, this asymmetry no longer operates, and La Niña events no longer induce a rainfall increase, leading to the observed SEQ rainfall reduction. A similar asymmetric rainfall teleconnection with ENSO Modoki exists and shares the same temporal evolutions. This breakdown is caused by an eastward shift in the Walker circulation and the convection center near Australia’s east coast, in association with a post-1980 positive phase of the interdecadal Pacific oscillation (IPO). Such a breakdown occurred before 1950, indicating that multidecadal variability alone could potentially be responsible for the recent SEQ rainfall decline. An aggregation of outputs from climate models to distill the impact of climate change suggests that the asymmetry and the breakdown may not be generated by climate change, although most models do not perform well in simulating the ENSO–rainfall teleconnection over the SEQ region.
Abstract Drought affects virtually every region of the world, and potential shifts in its character in a changing climate are a major concern. This article presents a synthesis of current understanding of meteorological drought, with a focus on the large-scale controls on precipitation afforded by sea surface temperature (SST) anomalies, land surface feedbacks, and radiative forcings. The synthesis is primarily based on regionally focused articles submitted to the Global Drought Information System (GDIS) collection together with new results from a suite of atmospheric general circulation model experiments intended to integrate those studies into a coherent view of drought worldwide. On interannual time scales, the preeminence of ENSO as a driver of meteorological drought throughout much of the Americas, eastern Asia, Australia, and the Maritime Continent is now well established, whereas in other regions (e.g., Europe, Africa, and India), the response to ENSO is more ephemeral or nonexistent. Northern Eurasia, central Europe, and central and eastern Canada stand out as regions with few SST-forced impacts on precipitation on interannual time scales. Decadal changes in SST appear to be a major factor in the occurrence of long-term drought, as highlighted by apparent impacts on precipitation of the late 1990s “climate shifts” in the Pacific and Atlantic SST. Key remaining research challenges include (i) better quantification of unforced and forced atmospheric variability as well as land–atmosphere feedbacks, (ii) better understanding of the physical basis for the leading modes of climate variability and their predictability, and (iii) quantification of the relative contributions of internal decadal SST variability and forced climate change to long-term drought.
A positive Indian Ocean Dipole (IOD) tends to have stronger cold sea surface temperature anomalies (SSTAs) over the eastern Indian Ocean with greater impacts than warm SSTAs that occur during its negative phase. Two feedbacks have been suggested as the cause of positive IOD skewness, a positive Bjerknes feedback and a negative SST-cloud-radiation (SCR) feedback, but their relative importance is debated. Using inter-model statistics, we show that the most important process for IOD skewness is an asymmetry in the thermocline feedback, whereby SSTAs respond to thermocline depth anomalies more strongly during the positive phase than negative phase. This asymmetric thermocline feedback drives IOD skewness despite positive IODs receiving greater damping from the SCR feedback. In response to global warming, although the thermocline feedback strengthens, its asymmetry between positive and negative IODs weakens. This behaviour change explains the reduction in IOD skewness that many models display under global warming.
Abstract Ningaloo Niño/Niña is a mode of climate variability in the southeastern Indian Ocean with huge impacts on Australian climate. El Niño‐Southern Oscillation (ENSO), as the dominant remote forcing, triggers Ningaloo Niño/Niña. However, how this teleconnection will respond to greenhouse warming is unclear. Using Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel simulations, we find a weakened ENSO‐Ningaloo Niño/Niña teleconnection under greenhouse warming, which manifests as weakened atmospheric teleconnection from La Niña to Ningaloo Niño. Such weakened teleconnection can be linked to the tropical Pacific mean state changes including an El Niño‐like warming pattern and more stable atmosphere in the future climate, both suppressing the atmospheric convection in the western tropical Pacific, leading to a weaker Matsuno‐Gill response in the southeastern Indian Ocean. Our results suggest that Ningaloo Niño/Niña becomes more challenging to predict as greenhouse warming continues.
Abstract Future changes in both the mean climate of the tropical Pacific and characteristics of the El Niño–Southern Oscillation (ENSO) are now established as being likely. Determining the time of emergence (ToE) of detectable climate change signals against background noise of natural variability is critical to mitigation strategies and adaptation planning. Here, we find that the annual-mean SST signal, mainly reflecting the tropical-mean warming signal, has already emerged in the historical period across much of the tropical Pacific, with the latest ToE in the east. The annual-mean rainfall signal is expected to emerge by around mid-century based on the multi-model ensemble mean (MEM) result, with some sensitivity to emission scenarios. By contrast, the signal of ENSO-related rainfall variability is projected to emerge by around 2040 ± 3 based on the MEM regardless of emission scenarios, ~ 30 years sooner than that of the ENSO-related SST variability. Our results are instructive for detection of climate change signals in the tropical Pacific and reinforce the severe risks of ENSO-induced climate extremes that are fast emerging regardless of any mitigation actions.
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