The relationship between the El Niño Southern Oscillation and Australian monsoon rainfall on decadal time-scales

<p>This research analyses the observed relationship between eastern and central Pacific El Niño Southern Oscillation (ENSO) events and Australian monsoon rainfall (AUMR) on a decadal timescale during the December to March monsoon months. To assess the decadal influence of the different flavours of ENSO on the AUMR, we focus on the phases of the Interdecadal Pacific Oscillation (IPO) over the period 1920 to 2020.  The AUMR is characterized by substantial decadal variability, which appears to be linked to the positive and negative phases of the IPO. During the past two historical negative IPO phases, significant correlations have been observed between central Pacific sea surface temperature (SST) anomalies and AUMR over both the northeast and northwest of Australia. This central Pacific SST-AUMR relationship has strengthened from the first negative IPO phase (mid-1940s to the mid-1970s) to the second (late 1990s to mid-2010s), while the eastern Pacific SST-AUMR influence has weakened. Composite rainfall anomalies over Australia reveal a different response of AUMR to central Pacific El Niño/La Niña and eastern Pacific La Niña events during positive IPO and negative IPO phases. This research clearly shows that ENSO's influence on AUMR is modulated by Pacific decadal variability, however this teleconnection, in itself, can change between similar decadal Pacific states.  Going forward, as decadal prediction systems improve and become more mainstream, the IPO phase could be used as a potential source for decadal predictability of the tendency of AUMR.  </p>

Interactions between Decadal to Multidecadal Ocean Variability and Stratospheric Sudden Warmings

<p>Major sudden stratospheric warmings (SSWs) are largest instances of the boreal polar stratospheric variability. Their effects extend farther from the polar stratosphere, affecting for example near-surface circulation. According to observations, SSWs are not equally distributed along time, with decades with almost no events and decades with SSWs happening almost every winter. This suggests the existence of multidecadal variability of SSWs. Some previous studies have pointed to phenomena in the ocean surface as the main precursors of this low-frequency variability. However, the relatively short observational record and the need of long model simulations with daily output have not enabled an analysis of the influences of these oceanic phenomena on SSWs</p><p>The goal of this study is to investigate the effects of Atlantic Multidecadal Variability (AMV) and Pacific Decadal Variability (PDV) on SSWs. To do so, we use for the first time a large ensemble of historical experiments (Max Planck Grand Ensemble) that allows us to examine the modulation of the frequency, precursors and surface impact of SSWs by both types of oceanic variability. Our results reveal that PDV has an impact on the frequency of SSWs, with a significant higher rate of SSWs for its positive than the negative phase. As for AMV, the main effect of AMV is centered on the tropospheric response to SSWs, with almost no modulation in the occurrence of the event. This last finding would be useful in order to predict the tropospheric fingerprint of SSWs.</p>

Global modulation of ENSO teleconnections by Pacific Decadal Variability

<p>The El Niño Southern Oscillation (ENSO) is the leading mode of global climate variability on interannual timescales. On longer time-scales, decadal variability in the Pacific is responsible for modulating the rate of global warming (Meehl et al 2013, Maher et al 2014 Henley & King 2017). Whether Pacific Decadal Variability (PDV) modulates ENSO teleconnections is an important research question that has largely been investigated using the observational record. PDV is shown to modulate Australian rainfall (Power et al 1999, Arblaster et al 2002, Verdon et al, 2004, King et al 2013), which has impacts for flood frequency (Franks and Kuczera, 2002, Kiem et al, 2003, Pui et al 2011). PDV has also been shown to modulate ENSO precipitation teleconnections over Africa (Dong & Dai 2015), Texas (Khedun et al 2014), and Europe (Zanchettin et al 2008) as well as ENSO temperature teleconnections over New Zealand (Salinger et al 2001). While these observationally based studies suggest connections between ENSO teleconnections and PDV, the short observational record contains only two PDV phase changes. In addition, calculating PDV using a lowpass filter on the region that contains ENSO could also cause statistical artefacts in the results (Power at al, 2006, Westra et al 2015). These limitations can be addressed using climate models. Arblaster et al (2002) use atmosphere only simulations and find similar results to observational studies over Australia. Dong and Dai (2015) further investigate global modulation using 4 ensemble members of a single model. These modelling studies are limited in their use of single models and while they include a larger dataset than the observational record, previous work has only used small ensemble sizes. In this study, we address both the issue of small datasets and the dependence on results on the model used by utilising four single model initial-condition large ensembles.  Each model ensemble has a minimum of 20 members enabling investigation of multiple realizations of PDV and ENSO covariability. Over the historical period, using one ensemble member results in a record that is indeed too short to accurately quantify the influence of PDV on ENSO teleconnections. We then composite events for different phases of the PDV and ENSO using all ensemble members. Initial results show that PDV strongly influences ENSO temperature teleconnections over North America.  We find that stronger teleconnections occur when an El Niño occurs during a positive phase of PDV or a La Niña occurs in a negative phase of the PDV. Similarly, PDV phase affects precipitation over Australia, where co-occurring El Niño and positive PDV phases and La Niña and negative PDV phases have larger precipitation anomalies. Finally we investigate whether this modulation of ENSO teleconnections by the PDV is projected to change under strong anthropogenic forcing. We find greater inter-model agreement for precipitation teleconnections than for temperature teleconnections.  Ongoing work will assess the underlying physical mechanisms behind these results. </p>

Tropical Pacific Decadal Variability and ENSO Precursor in CMIP5 Models

Observational analyses suggest that a significant fraction of the tropical Pacific decadal variability (TPDV) (~60%–70%) is energized by the combined action of extratropical precursors of El Niño–Southern Oscillation (ENSO) originating from the North and South Pacific. Specifically, the growth and decay of the basin-scale TPDV pattern (time scale = ~1.5–2 years) is linked to the following sequence: ENSO precursors (extratropics, growth phase) → ENSO (tropics, peak phase) → ENSO successors (extratropics, decay phase) resulting from ENSO teleconnections. This sequence of teleconnections is an important physical basis for Pacific climate predictability. Here we examine the TPDV and its connection to extratropical dynamics in 20 models from phase 5 of the Coupled Model Intercomparison Project (CMIP). We find that most models (~80%) can simulate the observed spatial pattern (R > 0.6) and frequency characteristics of the TPDV. In 12 models, more than 65% of the basinwide Pacific decadal variability (PDV) originates from TPDV, which is comparable with observations (~70%). However, despite reproducing the basic spatial and temporal statistics, models underestimate the influence of the North and South Pacific ENSO precursors to the TPDV, and most of the models’ TPDV originates in the tropics. Only 35%–40% of the models reproduce the observed extratropical ENSO precursor patterns (R > 0.5). Models with a better representation of the ENSO precursors show 1) better basin-scale signatures of TPDV and 2) stronger ENSO teleconnections from/to the tropics that are consistent with observations. These results suggest that better representation of ENSO precursor dynamics in CMIP may lead to improved Pacific decadal variability dynamics and predictability.

Tropical Pacific Decadal Variability Induced by Nonlinear Rectification of El Niño–Southern Oscillation

On the basis of 32 long-term simulations with state-of-the-art coupled GCMs, we investigate the relationship between tropical Pacific decadal variability (TPDV) and El Niño–Southern Oscillation (ENSO). The first empirical orthogonal function (EOF) mode for the 11-yr moving sea surface temperatures (SSTs) in the coupled models is commonly characterized by El Niño–like decadal variability with Bjerknes air–sea interaction. However, the second EOF mode can be separated into two groups, such that 1) some models have a zonal dipole SST pattern and 2) other models are characterized by a meridional dipole pattern. We found that models with the zonal dipole pattern in the second mode tend to simulate strong ENSO amplitude and asymmetry in comparison with those of the other models. Also, the residual patterns, which are defined as the summation of El Niño and La Niña SST composite anomalies, are very similar to the decadal dipole pattern, which suggests that ENSO residuals can cause the dipole decadal variability. It is found that decadal modulation of ENSO variability in these models strongly depends on the phase of the dipole decadal variability. The decadal changes in ENSO residual correspond well with the decadal changes in the dipole pattern, and the nonlinear dynamic heating terms by ENSO anomalies are well matched with the decadal dipole pattern.