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Modeling of the Antarctic stratospheric polar vortex from the Coupled Model Intercomparison Project phase 3 (CMIP3) to phase 6 (CMIP6) is evaluated in this study. On average, a wide coverage of warm biases appears in the Antarctic stratosphere, which is greatest in the early CMIP and is gradually diminished in the two later CMIPs with the number of models reproducing QBO increasing (0, 5, and 19). Four metrics of the Antarctic stratospheric polar vortex are assessed for three generations of CMIPs. Biases such as the overly weak strength, the overly large aspect ratio and the westward drifted vortex centroid are commonly shared across the CMIPs. While with improvements of the model resolution, model top, interactive chemistry and physical process, the intermodel spread narrows generation by generation, especially for high-top models than low-top models in the simulation of vortex area. Further, intermodel spread of Antarctic stratospheric vortex is obviously associated with the bias of austral winter sea surface temperature (SST). Specifically, as cold SST bias in the southern tropical Pacific Ocean and warm SST bias in the Southern Hemisphere Niño 1 + 2 region (Nino) improve, biases in the modelled vortex metrics also decrease, which can be partly attributed to the modifying of the upward propagations of planetary waves over tropical and extratropical oceans. The strengthened relationships in the focused regions further confirms the importance of the SST simulation for the stratosphere vortex simulation. In general, despite biases of the polar vortex existing across CMIPs, marked progresses have been achieved for most models. [ABSTRACT FROM AUTHOR]

The large-scale extratropical circulation in the Southern Hemisphere is much more zonally symmetric than that of the Northern Hemisphere, but its zonal departures, albeit highly relevant for regional impacts, have been less studied. In this study we analyse the joint variability of the zonally asymmetric springtime stratospheric and tropospheric circulation using Complex Empirical Orthogonal Functions (cEOF) to characterise planetary waves of varying amplitude and phase. The leading cEOF represents variability of a zonal wave 1 in the stratosphere that correlates slightly with the Symmetric Southern Annular Mode (S-SAM). The second cEOF (cEOF2) is an alternative representation of the Pacific-South American modes. One phase of this cEOF is also very highly correlated with the Asymmetric SAM (A-SAM) in the troposphere. Springs with an active ENSO tend to lock the cEOF2 to a specific phase, but have no consistent impact on its magnitude. Furthermore, we find indications that the location of Pacific Sea Surface Temperature anomalies affect the phase of the cEOF2. As a result, the methodology proposed in this study provides a deeper understanding of the zonally asymmetric springtime extratropical SH circulation. [ABSTRACT FROM AUTHOR]

Using reanalysis datasets and a coupled general circulation model, the relationship between springtime Arctic total column ozone (TCO) and surface (5 m) ocean currents in the North Pacific is investigated. We found that as March Arctic TCO decreases, a statistically significant northwestward ocean current anomaly occurs in the northern North Pacific in surface layer, but an anomalous southward ocean current appears in the central North Pacific in April, and vice versa. The decreased Arctic TCO favors an enhanced Arctic stratospheric circulation, which tends to induce the tropospheric positive Arctic Oscillation anomaly with easterly anomalies over the midlatitude eastern Asia in late March through stratosphere-troposphere dynamical coupling. The easterly anomaly over eastern Asia in late March further extends eastward and induces an easterly anomaly over the midlatitude North Pacific, which favors negative North Pacific Oscillation (–NPO)-like circulation anomaly via anomalous zonal wind shear and the interactions between synoptic scale eddies and the mean flow in early-middle April. The –NPO anomaly forces anomalous northwestward/southward surface ocean currents in the northern/central North Pacific through direct friction of wind and the Coriolis force. Our coupled numerical simulations with high- and low-ozone scenarios also support that the Arctic stratospheric ozone affects the North Pacific surface ocean currents by NPO anomalies. Moreover, the ozone-related ocean current anomalies contribute to Victoria mode-like sea surface temperature anomalies in the North Pacific by horizontal heat advection. These results imply that Arctic ozone signal could be a predictor for variations of the North Pacific surface ocean currents. [ABSTRACT FROM AUTHOR]

We assess the interactions between stratospheric water vapor (SWV) and surface temperature during the past two decades using satellite observations and the Community Earth System Model (CESM). From 1992 to 2013, to first order, the observed SWV exhibited three distinct piece-wise trends: a steady increase from 1992 to 2000, an abrupt drop from 2000 to 2004, and a gradual recovery after 2004, while the global-mean surface temperature experienced a strong increase until 2000 and a warming hiatus after 2000. The atmosphere-only CESM shows that the seasonal variation of tropical-mean (30°S-30°N) SWV is anticorrelated with that of the tropical-mean sea surface temperature (SST), while the correlation between the tropical SWV and SST anomalies on the interannual time scale is rather weak. By nudging the modeled SWV to prescribed profiles in coupled atmosphere-slab ocean experiments, we investigate the impact of SWV variations on surface temperature change. We find that a uniform 1 ppmv (0.5 ppmv) SWV increase (decrease) leads to an equilibrium global mean surface warming (cooling) of 0.12 ± 0.05 °C (−0.07 ± 0.05 °C). Sensitivity experiments show that the equilibrium response of global mean surface temperature to SWV perturbations over the extratropics is larger than that over the tropics. The observed sudden drop of SWV from 2000 to 2004 produces a global mean surface cooling of about −0.048 ± 0.041 °C, which suggests that a persistent change in SWV would make an imprint on long-term variations of global-mean surface temperature. A constant linear increase in SWV based on the satellite-observed rate of SWV change yields a global mean surface warming of 0.03 ± 0.01 °C/decade over a 50-year period, which accounts for about 19 % of the observed surface temperature increase prior to the warming hiatus. In the same experiment, trend analyses during different periods reveal a multi-year adjustment of surface temperature before the response to SWV forcing becomes strong relative to the internal variability in the model. [ABSTRACT FROM AUTHOR]

The stratospheric quasi-biennial oscillation (QBO) and its association with the interannual variability in the stratosphere and troposphere, as well as in tropical sea surface temperature anomalies (SSTA), are examined in the context of a QBO life cycle. The analysis is based on the ERA40 and NCEP/NCAR reanalyses, radiosonde observations at Singapore, and other observation-based datasets. Both reanalyses reproduce the QBO life cycle and its associated variability in the stratosphere reasonably well, except that some long-term changes are detected only in the NCEP/NCAR reanalysis. In order to separate QBO from variability on other time scales and to eliminate the long-term changes, a scale separation technique [Ensemble Empirical Mode Decomposition (EEMD)] is applied to the raw data. The QBO component of zonal wind anomalies at 30 hPa, extracted using the EEMD method, is defined as a QBO index. Using this index, the QBO life cycle composites of stratosphere and troposphere variables, as well as SSTA, are constructed and examined. The composite features in the stratosphere are generally consistent with previous investigations. The correlations between the QBO and tropical Pacific SSTA depend on the phase in a QBO life cycle. On average, cold (warm) SSTA peaks about half a year after the maximum westerlies (easterlies) at 30 hPa. The connection of the QBO with the troposphere seems to be associated with the differences of temperature anomalies between the stratosphere and troposphere. While the anomalies in the stratosphere propagate downward systematically, some anomalies in the troposphere develop and expand vertically. Therefore, it is possible that the temperature difference between the troposphere and stratosphere may alter the atmospheric stability and tropical deep convection, which modulates the Walker circulation and SSTA in the equatorial Pacific Ocean. [ABSTRACT FROM AUTHOR]