El Niño Dynamics and Effects Observed with Radio Occultation

El Niño Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon, which links sea surface temperature (SST) and ocean heat content to atmospheric dynamics. ENSO is the most significant mode of interannual climate variability of the tropical troposphere. Variations of convection, atmospheric temperature and circulation are strongly pronounced at tropospheric low latitudes but signatures associated with ENSO have also been observed at high latitudes as well as at stratospheric altitudes. Numerous studies over the last decades have analyzed the influence of ENSO on atmospheric temperature using observational data, e.g., from radiosondes or satellites, reanalyses, or model data. Since traditional observations have limited spatial coverage and most satellite data have a low vertical resolution in the upper troposphere and lower stratosphere (UTLS) region, substantial uncertainties remain in atmospheric dynamics in the UTLS during El Niño and La Niña conditions. In this context radio occultation (RO) offers new possibilities by providing high quality observations in the UTLS with global coverage. Atmospheric parameters like temperature, pressure, and geopotential height are retrieved with high vertical resolution and high accuracy and precision. A continuous RO record is available since 2001 but the number of RO measurements increased significantly in 2006, with the launch of the six Formosat-3/COSMIC (F3C) satellites. This constellation tracked more than 2000 RO events per day, enabling the derivation of fine- resolved atmospheric fields. The central aim of the proposed project DYNOCC is the assessment of ENSO related atmospheric dynamics and its effects on troposphere-stratosphere processes from this novel multi-year RO record. In a first step I will perform a thorough investigation of UTLS ENSO signals in temperature, pressure, geopotential height, and water vapor to analyze the detailed vertical and horizontal structure of ENSO. The consistency of results obtained from RO will be checked using data from other observations as well as from analyses and reanalyses. Furthermore, I will validate a state-of-the-art global climate model with regard to its representation of the vertical UTLS ENSO structure. In a second step I will use high resolution RO data to investigate details of atmospheric waves associated with local heating induced by El Niño. This will allow gaining deeper understanding of natural climate variability in the transition between the troposphere and stratosphere, an important coupling layer in the atmosphere which is a topic of active current research. Finally, I will use precise RO-based geopotential height profiles on constant pressure surfaces to investigate the vertical and horizontal displacement of the geostrophic zonal wind during El Niño and La Niña conditions. Addressing these key issues, DYNOCC aims at adding detailed understanding of atmospheric dynamics associated with natural climate variability. Extending the application of RO to further research fields, DYNOCC will strengthen its value also outside of the RO community.

El NiñoSouthern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon, which links sea surface temperature (SST) and ocean heat content to atmospheric dynamics. ENSO is the most significant mode of inter-annual climate variability of the tropical troposphere, the lowest layer of the Earths atmosphere. Numerous studies over the last decades have analyzed the influence of ENSO on atmospheric temperature using data from radiosondes, satellites, reanalyses, or models. Since traditional observations have limited spatial coverage and most satellite data have a low vertical resolution in the upper troposphere and lower stratosphere (UTLS) region, substantial uncertainties remain in atmospheric dynamics in the UTLS during warm and cold ENSO phases (El Niño/La Niña conditions).In this context radio occultation (RO) offers new possibilities by providing high quality observations in the UTLS with global coverage. Atmospheric parameters such as temperature, pressure, and geopotential height are retrieved with high vertical resolution and high accuracy and precision. The central aim of the project DYNOCC was the assessment of ENSO-related atmospheric dynamics and its effects on troposphere-stratosphere processes from this novel multi-year RO record. A principal result of DYNOCC was that atmospheric temperature does not only increase on very large scales during El Niño conditions, but has also pronounced spatial and vertical impacts on much smaller scales. A very strong ENSO cooling signal was found, for example, in the upper troposphere above the tropical Pacific.The influence of ENSO on the Earths atmosphere has also been investigated in terms of the three-dimensional structure of pressure and wind fields. For this study, RO data have been used for the first time to derive climatological wind fields. It has been shown that global wind fields are strongly modulated by ENSO, which also affects global precipitation patterns.The investigation of tropical high-frequency waves (i.e., Kelvin waves) between 15km and 30km did not reveal any evidence that wave activity is associated with ENSO dynamics. However, the strength of these waves in the lower stratosphere strongly depends on background wind fields. No distinct period could be identified between 17km and 20km. The cause for these irregular variations is not entirely clear yet but some relationship to short-term variability of convection near the equator and to local tropospheric wind is assumed. This is still a topic of active research.Addressing these key issues, DYNOCC added to deeper understanding of atmospheric dynamics associated with natural climate variability in the UTLS region.