The role of the Maritime Continent SST anomalies in maintaining the Pacific–Japan pattern on decadal time scales

The decadal Pacific–Japan (PJ) pattern, the dominant decadal mode of summer vorticity anomaly over East Asia, is characterized as a meridionally arranged wave pattern with one anomalous cyclone located over Taiwan, and two anomalous anticyclones around the South China Sea (SCS) and the Bohai Sea. This pattern can cause wetter and colder conditions in Southeast China and dryer and warmer conditions in North China. Local SST–rainfall relationship reveals that the Maritime Continent (MC) SST can act as an engine to regulate and maintain the decadal PJ pattern. Driven by enhanced convection over the MC, anomalous divergent flows in the upper troposphere move northward, cross the equator and then converge and subside over the SCS. The SCS low-level divergence, maintained by this meridional overturning circulation under the Sverdrup vorticity balance, further works as a Rossby wave source and excites the decadal PJ pattern pointing straight northward. The transhemispheric impacts of the MC SST are well reproduced by both the atmospheric general circulation model and the dry linear baroclinic model, with the former emphasizing the MC’s original forcing role and the latter highlighting the SCS anticyclone’s role in relaying and amplifying those climatic impacts. Thus, our results indicate that SST variations over the MC region can be viewed as a potential source of East Asian decadal climate predictability.

Dipole Pattern of Summer Ozone Pollution in the east of China and Its Connection with Climate Variability

Surface O3 pollution has become one of the most severe air pollution problems in China, which makes it of practical importance to understand O3 variability. A south-north dipole pattern of summer-mean O3 concentration in the east of China (DP-O3), which were centered at North China (NC) and the Pearl River Delta (PRD) respectively, has been identified from the simulation of a global 3-D chemical transport model for the period 1980–2019. Large-scale anticyclonic (cyclonic) and cyclonic (anticyclonic) anomalies over NC and the PRD resulted in a sharp contrast of meteorological conditions between the above two regions. The enhanced (restrained) photochemistry and natural emissions of O3 precursors in NC and restrained (enhanced) O3 production in the PRD contributed to the DP-O3. Decreased sea ice anomalies near the Franz Josef Land and associated warm sea surface in May enhanced the Rossby-wave source over northern Europe and West Siberia, which eventually induced an anomalous Eurasia-like pattern to influence the formation of the DP-O3. The thermodynamic signals of the southern Indian Ocean dipole were stored in the subsurface and influenced spatial pattern of O3 pollution in the east of China mainly through the Hadley circulation. The physical mechanisms behind the modulation of the atmospheric circulations and related DP-O3 by these two climate anomalies at different latitudes were evidently verified by large-scale ensemble simulations of the earth system model.

Tropical and subtropical forcing of future southern hemisphere stationary wave changes

Stationary wave changes play a significant role in the regional climate change response in Southern Hemisphere (SH) winter. In particular, almost all CMIP5 models feature a substantial strengthening of the westerlies to the south of Australia and enhancement of the subtropical jet over the eastern Pacific in winter. In this study we investigate the mechanisms behind these changes, finding that the stationary wave response can largely be explained via reductions in the magnitude of the upper level Rossby wave source over the tropical / subtropical East Pacific. The Rossby wave source changes in this region are robust across the model ensemble and are strongly correlated with changes to low latitude circulation patterns, in particular, the projected southward migration of the Hadley cell and weakening of the Walker circulation. To confirm our mechanism of future changes, we employ a series of barotropic model experiments in which the barotropic model is given a background state identical to a particular CMIP5 model and an anomalous Rossby wave source is imposed. This simple approach is able to capture the primary features of the ensemble mean change, including the cyclonic anomaly south of Australia, and is also able to capture many of the inter-model differences. These findings will help to advance our understanding of the mechanisms underpinning SH extratropical circulation changes under climate change.

The Combined Influence of the Stratospheric Polar Vortex and ENSO on Zonal Asymmetries in the Southern Hemisphere Upper Tropospheric Circulation during Austral Spring and Summer

<p>The influence of El Niño Southern Oscillation (ENSO) and the Stratospheric Polar Vortex (SPV) on the zonal asymmetries in the Southern Hemisphere atmospheric circulation during spring and summer is examined. The main objective is to explore if the SPV can modulate the ENSO teleconnections in the extratropics. We use a large ensemble of seasonal hindcasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecast System to provide a much larger sample size than is possible from the observations alone.</p><p>We find a small but statistically significant relationship between ENSO and the SPV, with El Niño events occurring with weak SPV and La Niña events occurring with strong SPV more often than expected by chance, in agreement with previous works. We show that the zonally asymmetric response to ENSO and SPV can be mainly explained by a linear combination of the response to both forcings, and that they can combine constructively or destructively. From this perspective, we find that the tropospheric asymmetries in response to ENSO are more intense when El Niño events occur with weak SPV and La Niña events occur with strong SPV, at least from September through December. In the stratosphere, the ENSO teleconnections are mostly confounded by the SPV signal. The analysis of Rossby Wave Source and of wave activity shows that both are stronger when El Niño events occur together with weak SPV, and when La Niña events occur together with strong SPV.</p>

A more complete Rossby wave source

<p>Tropical convection drives extratropical variability on subseasonal to interannual time-scales by exciting Rossby wave trains in the upper troposphere. Traditionally the relevant Rossby wave source is considered to be the sum of vortex stretching and vorticity advection by the divergent horizontal flow ( - ∇·<strong>u</strong><sub>χ</sub> (ζ+f) - <strong>u</strong><sub>χ</sub>·∇ (ζ+f)). Since absolute vorticity is very small at the equator, the equatorward flanks of the upper tropospheric jets have been regarded the source region of Rossby wave trains. In these considerations vertical momentum advection is neglected, although, it is an important source for westerly momentum at the equator. The curl of vertical momentum advection is the sum of vertical vorticity advection and vortex tilting ( -  ω ζ<sub>p</sub> - ω<sub>x</sub> v<sub>p</sub> + ω<sub>y</sub> u<sub>p</sub>). These contributions are smaller than the traditional Rossby wave source in midlatidues by about one order of magnitude but they are of similar size in the tropics.</p>

On the Linearity of the Stratospheric and Euro-Atlantic Sector Response to ENSO

The dependence of the winter stratospheric and Euro-Atlantic climate response on ENSO amplitude is investigated using the HadGEM3 model. Experiments are performed with imposed east Pacific sea surface temperature perturbations corresponding to Niño-3.4 anomalies of ±0.75, 1.5, 2.25, and 3.0 K. In the North Pacific, El Niño (EN) deepens and shifts the Aleutian low eastward, while the equivalent magnitude La Niña (LN) perturbations drive anomalies of opposite sign that are around 4 times weaker. The muted North Pacific response to LN can be traced back to the weaker response of tropical convection and the associated anomalous Rossby wave source. The EN perturbations weaken the Arctic polar vortex, with the winter mean zonal mean zonal wind at 60°N and 10 hPa decreasing approximately linearly with Niño-3.4 anomaly by around −3.6 m s−1 K−1. For the strongest EN case (+3 K), the frequency of sudden stratospheric warmings (SSWs) increases by ~60% compared to the control experiment. Hence the results do not support a saturation of the stratospheric pathway for strong EN as suggested in previous literature. The equivalent amplitude LN perturbations cause a weak strengthening of the polar vortex and no substantial change in SSW frequency, in contrast to some reanalysis-based studies. EN induces a negative North Atlantic Oscillation (NAO) index throughout boreal winter, which increases approximately linearly with the Niño-3.4 anomaly by around −0.6 standard deviations K−1. Only the response to the strongest LN perturbations projects onto a weak positive NAO in November, suggesting that the mechanism for the Euro-Atlantic response to LN may be distinct from EN.

On the Seasonality of the El Niño Teleconnection to the Amundsen Sea Region

The Amundsen Sea low (ASL) is a quasi-stationary low pressure system that affects climate in West Antarctica. Previous studies have shown that El Niño–Southern Oscillation (ENSO) modulates the position and strength of the ASL with the strongest teleconnection found in austral winter despite the amplitude of ENSO events generally being largest in austral autumn/summer. This study investigates the mechanisms behind the seasonality of the El Niño teleconnection to the Amundsen Sea region (ASR) using experiments with the HadGEM3 climate model forced with an idealized fixed El Niño sea surface temperature anomaly present throughout the year. The seasonality of the El Niño–ASR teleconnection is found to originate from seasonal differences in the large-scale zonal winds in the South Pacific sector. In austral winter, the region of strong absolute vorticity near ~30°S associated with the subtropical jet, in combination with the changes to upper-tropospheric divergence due to the El Niño perturbation, acts as an anomalous Rossby wave source that is largely absent in austral summer. Furthermore, in austral summer the poleward propagation of tropically sourced Rossby waves into the ASR is inhibited by the strong polar front jet in the South Pacific sector, which leads to Rossby wave reflection away from the ASR. In austral winter, Rossby waves are able to propagate into the ASR, forming part of the Pacific South America pattern. The lack of the Rossby wave source in the tropical Pacific and the absence of favorable conditions for wave propagation explains the weaker El Niño–ASR teleconnection in austral summer compared to austral winter.

The Consistency of MJO Teleconnection Patterns: An Explanation Using Linear Rossby Wave Theory

The Madden–Julian oscillation (MJO) excites strong variations in extratropical atmospheric circulations that have important implications for subseasonal-to-seasonal (S2S) prediction. A previous study showed that particular MJO phases are characterized by a consistent modulation of geopotential heights in the North Pacific and adjacent regions across different MJO events, and demonstrated that this consistency is beneficial for extended numerical weather forecasts (i.e., lead times of two weeks to one month). In this study, we examine the physical mechanisms that lead some MJO phases to have more consistent teleconnections than others using a linear baroclinic model. The results show that MJO phases 2, 3, 6, and 7 consistently generate Pacific–North American (PNA)-like patterns on S2S time scales while other phases do not. A Rossby wave source analysis is applied and shows that a dipole-like pattern of Rossby wave source on each side of the subtropical jet can increase the pattern consistency of teleconnections due to the constructive interference of similar teleconnection signals. On the other hand, symmetric patterns of Rossby wave source can dramatically reduce the pattern consistency due to destructive interference. A dipole-like Rossby wave source pattern is present most frequently when tropical heating is found in the Indian Ocean or the Pacific warm pool, and a symmetric Rossby wave source is present most frequently when tropical heating is located over the Maritime Continent. Thus, the MJO phase-dependent pattern consistency of teleconnections is a special case of this mechanism.

Baroclinic-to-Barotropic Pathway in El Niño–Southern Oscillation Teleconnections from the Viewpoint of a Barotropic Rossby Wave Source

The baroclinic-to-barotropic pathway in ENSO teleconnections is examined from the viewpoint of a barotropic Rossby wave source that results from decomposition into barotropic and baroclinic components. Diagnoses using the NCEP–NCAR reanalysis are supplemented by analysis of the response of a tropical atmospheric model of intermediate complexity to the NCEP–NCAR barotropic Rossby wave source. Among the three barotropic Rossby wave source contributions (shear advection, vertical advection, and surface drag), the leading contribution is from shear advection and, more specifically, the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind. Vertical advection is the smallest term, while surface drag tends to cancel and reinforce the shear advection in different regions through damping on the baroclinic mode, which spins up a barotropic response. There are also nontrivial impacts of transients in the barotropic wind response to ENSO. Both tropical and subtropical baroclinic vorticity advection contribute to the barotropic component of the Pacific subtropical jet near the coast of North America, where the resulting barotropic wind contribution approximately doubles the zonal jet anomaly at upper levels, relative to the baroclinic anomalies alone. In this view, the barotropic Rossby wave source in the subtropics simply arises from the basic-state baroclinic flow acting on the well-known baroclinic ENSO flow pattern that spreads from the deep tropics into the subtropics over a scale of equatorial radius of deformation. This is inseparably connected to the leading deep tropical Rossby wave source that arises from eastern Pacific climatological baroclinic winds advecting the tropical portion of the same ENSO flow pattern.