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  • 11
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    How Does Downward Planetary Wave Coupling Affect Polar Stratospheric Ozone in the Arctic Winter Stratosphere? (2017)
    Copernicus Publications (EGU)
    In:  Atmospheric Chemistry and Physics, 17 . pp. 2437-2458.
    Details
    Publication Date: 2020-02-06
    Description: It is well established that variable wintertime planetary wave forcing in the stratosphere controls the variability of Arctic stratospheric ozone through changes in the strength of the polar vortex and the residual circulation. While previous studies focused on the variations in upward wave flux entering the lower stratosphere, here the impact of downward planetary wave reflection on ozone is investigated for the first time. Utilizing the MERRA2 reanalysis and a fully coupled chemistry–climate simulation with the Community Earth System Model (CESM1(WACCM)) of the National Center for Atmospheric Research (NCAR), we find two downward wave reflection effects on ozone: (1) the direct effect in which the residual circulation is weakened during winter, reducing the typical increase of ozone due to upward planetary wave events and (2) the indirect effect in which the modification of polar temperature during winter affects the amount of ozone destruction in spring. Winter seasons dominated by downward wave reflection events (i.e., reflective winters) are characterized by lower Arctic ozone concentration, while seasons dominated by increased upward wave events (i.e., absorptive winters) are characterized by relatively higher ozone concentration. This behavior is consistent with the cumulative effects of downward and upward planetary wave events on polar stratospheric ozone via the residual circulation and the polar temperature in winter. The results establish a new perspective on dynamical processes controlling stratospheric ozone variability in the Arctic by highlighting the key role of wave reflection.
    Type: Article , PeerReviewed
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  • 12
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    The modulating influence of convectively coupled equatorial waves (CCEWs) on the variability of tropical precipitation (2015)
    Royal Meteorological Society
    In:  International Journal of Climatology, 35 (7). pp. 1465-1483.
    Details
    Publication Date: 2017-07-01
    Description: A detailed examination of the Tropical Rainfall Measuring Mission (TRMM) daily estimates, merging high-quality (HQ)/infrared (IR) precipitation from 1998 to 2009, revealed the modulating influence of convectively coupled equatorial waves (CCEWs), including Kelvin, n = 1 equatorial Rossby (ER), mixed Rossby-gravity (MRG), and tropical depression (TD)-type waves, on the variability of tropical precipitation. Consistent with inviscid β-plane shallow water theory, the wave-induced convergence zones are found to be an active location for precipitation. Modulated precipitation by ER waves exhibits slow westward phase progression of approximately −4.8 m s−1, and it is distributed as a symmetric pair of off-equatorial maxima and a weak equatorial peak. MRG waves show an asymmetrically modulated precipitation distribution and a faster phase progression of about −16.1 m s−1 followed by enhanced symmetrical gyres. Peak precipitation within TD-type waves originates in the trough axis of off-equatorial vortex trains and propagates to the west at approximately −9.5 m s−1. Regarding Kelvin waves, typical positive precipitation anomalies travelling in the east–west direction occur in the maxima of low-level wind convergences, with an approximate phase progression of about 15.4 m s−1. Variability of tropical precipitation due to CCEWs behaves relatively varied over different seasons and locations. TD-type waves exhibit more predominant impacts than other waves, with maximum impacts of up to 9.75 ± 4.54% of the total precipitation variance during boreal summer, which is three times higher than the MRG peak (2.90 ± 0.82%). On the other hand, Kelvin and n = 1 ER waves each have more prominent effects during boreal winter; up to 6.99 ± 3.30 % and 3.77 ± 1.79 %, respectively. On average, our results suggest that, although being less dominant than other tropical oscillations (e.g. ENSO, MJO), these four types of CCEWs can considerably affect precipitation by contributing up to 16–20% of the total intraseasonal (2.5–72 days) precipitation variance in the tropics.
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  • 13
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    Analysis of the Equatorial Lower Stratosphere Quasi-Biennial Oscillation (QBO) Using ECMWF-Interim Reanalysis Data Set (2016)
    IOPscience
    In:  IOP Conference Series: Earth and Environmental Science, 31 (1). 012032.
    Details
    Publication Date: 2019-02-01
    Description: The ERA-Interim data set from Europe Center for Medium Range Weather Forecasting (ECMWF) was used to quantitatively analyze the characteristic of equatorial quasi-biennial oscillation (QBO). Analysis of spatial and temporal of the data showed that the zonally symmetric easterly and westerly phase of QBO regimes alternate with period of ~27.7 months. Based on Equivalent QBO Amplitude (EQA) method, the maximum amplitudes in zonal mean zonal wind (u), temperature (T), vertical shear (du/dz) and quadratic vertical shear (d2u/dz2) are ~28.3 m/s, ~3.4 K, ~4.8 m/s/km, and ~1.0 m/s/km2 respectively. The amplitudes decay exponentially with a Gaussian distribution in latitude. The twofold-structure of QBO descends downward at rate of ~1 km/month. The temperature anomaly can be used to analyze the characteristic of QBO which satisfies the thermal wind balance relation in the lower- stratosphere due to very small contribution of the mean meridional and vertical motion. Moreover, the concentration of the total column ozone (TCO) in the tropics is significantly influenced by QBO. During the westerly phase of QBO, the TCO is relatively increased in the lower-stratosphere, but decreased during the opposite phase.
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  • 14
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    Key Role of the Ocean Western Boundary currents in shaping the Northern Hemisphere climate (2019)
    Nature Research
    In:  Scientific Reports, 9 (1). Art.Nr. 3014.
    Details
    Publication Date: 2022-01-31
    Description: The individual impact of North Atlantic and Pacific Ocean Western Boundary Currents (OWBCs) on the tropospheric circulation has recently been studied in depth. However, their simultaneous role in shaping the hemisphere-scale wintertime troposphere/stratosphere-coupled circulation and its variability have not been considered. Through semi-idealized Atmospheric General-Circulation-Model experiments, we show that the North Atlantic and Pacific OWBCs jointly maintain and shape the wintertime hemispheric circulation and its leading mode of variability Northern Annular Mode (NAM). The OWBCs energize baroclinic waves that reinforce quasi-annular hemispheric structure in the tropospheric eddy-driven jetstreams and NAM variability. Without the OWBCs, the wintertime NAM variability is much weaker and its impact on the continental and maritime surface climate is largely insignificant. Atmospheric energy redistribution caused by the OWBCs acts to damp the near-surface atmospheric baroclinicity and compensates the associated oceanic meridional energy transport. Furthermore, the OWBCs substantially weaken the wintertime stratospheric polar vortex by enhancing the upward planetary wave propagation, and thereby affecting both stratospheric and tropospheric NAM-annularity. Whereas the overall impact of the extra-tropical OWBCs on the stratosphere results mainly from the Pacific, the impact on the troposphere results from both the Pacific and Atlantic OWBCs.
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