ALBERT

Description: South-Eastern Brazil experienced a devastating drought associated with significant agricultural losses in austral summer 2014. The drought was linked to the development of a quasi-stationary anticyclone in the South Atlantic in early 2014 that affected local precipitation patterns over South-East Brazil. Previous studies have suggested that the unusual blocking was triggered by tropical Pacific sea surface temperature (SST) anomalies and, more recently, by convection over the Indian Ocean related to the Madden-Julian Oscillation. Further investigation of the proposed teleconnections appears crucial for anticipating future economic impacts. In this study, we use numerical experiments with an idealized atmospheric general circulation model forced with the observed 2013/2014 SST anomalies in different ocean basins to understand the dominant mechanism that initiated the 2014 South Atlantic anticyclonic anomaly. We show that a forcing with global 2013/2014 SST anomalies enhances the chance for the occurrence of positive geopotential height anomalies in the South Atlantic. However, further sensitivity experiments with SST forcings in separate ocean basins suggest that neither the Indian Ocean nor tropical Pacific SST anomalies alone have contributed significantly to the anomalous atmospheric circulation that led to the 2014 South-East Brazil drought. The model study rather points to an important role of remote forcing from the South Pacific, local South Atlantic SSTs, and internal atmospheric variability in driving the persistent blocking over the South Atlantic.

Description: The present study describes Rossby wave packet (RWP) properties in the upper-troposphere and lower-stratosphere (UTLS) with the use of Global Navigation Satellite System radio occultation (GNSS-RO) measurements. This global study covering both hemisphere's extratropics is the first to tackle medium and synoptic-scale waves with GNSS-RO. We use one decade of GNSS-RO temperature and pressure data from the CHAMP, COSMIC, GRACE, Metop-A, Metop-B, SAC-C and TerraSAR-X missions; combining them into one gridded dataset for the years 2007–2016. Our approach to extract RWP anomalies and their envelope uses Fourier and Hilbert transforms over longitude without pre- or post-processing the data. Our study is purely based on observations, only using ERA-Interim winds to provide information about the background wind regimes. The RWP structures that we obtain in the UTLS agree well with theory and earlier studies, in terms of coherent phase/group propagation, zonal scale and distribution over latitudes. Furthermore, we show that RWP pressure anomalies maximize around the tropopause, while RWP temperature anomalies maximize right above tropopause height with a contrasting minimum right below. RWP activity follows the zonal-mean tropopause during all seasons. RWP anomalies in the lower stratosphere are dynamically coupled to the upper troposphere. They are part of the same system with a quasi-barotropic structure across the UTLS. RWP activity often reaches up to 20 km height and occasionally higher, defying the Charney–Drazin criterion. We note enhanced amplitude and upward propagation of RWP activity during sudden stratospheric warmings. We provide observational support for improvements in RWP diagnostics and wave trend analysis in models and reanalyses. Wave quantities follow the tropopause, and diagnosing them on fixed pressure levels (which the tropopause does not follow) can lead to aliasing. Our novel approach analysing GNSS-RO pressure anomalies provides wave signals with better continuity and coherence across the UTLS and the stratosphere, compared to temperature anomalies. Thus, RWP vertical propagation is much easier to analyse with pressure data.

Description: Data from 1997-2018 were logged every 8 minutes in the Inner Kiel Fjord (54°19'46.0"N; 10°08'58.3"E) in shallow waters (Hydrometeorological station: Fa. Driesen und Kern, Bad Bramstedt). Until 2013 the sensor was deployed floating at the surface, but due to settlements on the floating device, the actual depth is not perfectly certain. Therefore, in 2013 the sensor was mounted to a fixed depth of 1.8 m (below sealevel). If a value differed more than 1.0°C from the preceding and following value, the value was interpolated between the two adjacent values. If values were 2) constant for more than 4 hours, 2) missing within ± 8 min, and if 3) changes in values exceeded 0.7°C within 16 min, the values were set to NA. This holds true for the following times: 1999-05-21 21:20 - 1999-06-17 15:00, 2000-11-10 12:00 - 2000-11-18 10:00, 2001-01-06 12:20 - 2001-01-08 07:40, 2014-08-30 18:00 - 2014-09-19 16:20, 2015-01-07 14:40 - 2015-02-23 20:00, 2016-11-09 03:20 - 2016-12-04 03:20 - 2017-01-24 23:25 - 2017-01-25 12:00. Gaps larger than 3 days were filled with data (if available) obtained from sensors very close to the actual measuring site (SeapHOx, Scripps Research Institute San Diego 1m; or Hydrometeorological station, Fa. Driesen und Kern, Bad Bramstedt 1.5m): 2000-11-11 - 2000-11-17 (Hydrometeorological station), 2001-08-03 - 2001-08-28 (Hydrometeorological station), 2005-10-07 - 2005-10-17 (Hydrometeorological station), 2006-02-09 - 2006-02-22 (Hydrometeorological station), 2014-08-29 - 2014-09-18 (Hydrometeorological station), 2015-01-08 - 2015-02-24 (Hydrometeorological station), 2016-03-10 - 2016-03-13 (SeapHOx), 2016-06-03 - 2016-06-15 (SeapHOx), 2016-11-10 - 2016-12-05 (SeapHOx). Data from 1.5 m water depth were corrected by subtracting a systematic deviance of 0.3°C.