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Yoshio Kawatani, Shingo Watanabe, Kaoru Sato, Timothy J. Dunkerton, Saburo Miyahara, and Masaaki Takahashi

Abstract

The roles of equatorial trapped waves (EQWs) and internal inertia–gravity waves in driving the quasi-biennial oscillation (QBO) are investigated using a high-resolution atmospheric general circulation model with T213L256 resolution (60-km horizontal and 300-m vertical resolution) integrated for three years. The model, which does not use a gravity wave drag parameterization, simulates a QBO. Although the simulated QBO has a shorter period than that of the real atmosphere, its amplitudes and structure in the lower stratosphere are fairly realistic. The zonal wavenumber/frequency spectra of simulated outgoing longwave radiation represent realistic signals of convectively coupled EQWs. Clear signals of EQWs are also seen in the stratospheric wind components. In the eastward wind shear of the QBO, eastward EQWs including Kelvin waves contribute up to ∼25%–50% to the driving of the QBO. The peaks of eastward wave forcing associated with EQWs and internal inertia–gravity waves occur at nearly the same time at the same altitude. On the other hand, westward EQWs contribute up to ∼10% to driving the QBO during the weak westward wind phase but make almost zero contribution during the relatively strong westward wind phase. Extratropical Rossby waves propagating into the equatorial region contribute ∼10%–25%, whereas internal inertia–gravity waves with zonal wavelength ≲1000 km are the main contributors to the westward wind shear phase of the simulated QBO.

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Gan Zhang, Zhuo Wang, Timothy J. Dunkerton, Melinda S. Peng, and Gudrun Magnusdottir

Abstract

With warm sea surface temperature (SST) anomalies in the tropical Atlantic and cold SST anomalies in the east Pacific, the unusually quiet hurricane season in 2013 was a surprise to the hurricane community. The authors’ analyses suggest that the substantially suppressed Atlantic tropical cyclone (TC) activity in August 2013 can be attributed to frequent breaking of midlatitude Rossby waves, which led to the equatorward intrusion of cold and dry extratropical air. The resultant mid- to upper-tropospheric dryness and strong vertical wind shear hindered TC development. Using the empirical orthogonal function analysis, the active Rossby wave breaking in August 2013 was found to be associated with a recurrent mode of the midlatitude jet stream over the North Atlantic, which represents the variability of the intensity and zonal extent of the jet. This mode is significantly correlated with Atlantic hurricane frequency. The correlation coefficient is comparable to the correlation of Atlantic hurricane frequency with the main development region (MDR) relative SST and higher than that with the Niño-3.4 index. This study highlights the extratropical impacts on Atlantic TC activity, which may have important implications for the seasonal predictability of Atlantic TCs.

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