Short-Term Variability of Atmospheric Tides in Earth’s Mesosphere and Lower Thermosphere Region

In the early 2000’s, advances in satellite observations and the development of whole atmosphere models led to a paradigm shift in our understanding of what drives space climate/weather in near-Earth space. In addition to solar and magnetospheric driving influences from above, it was realized that meteorological events (such as changes in convection, tropical cyclones, sudden stratospheric warmings, El Niño, to name just a few) are important drivers of space climate/weather due to the generation and upward propagation of atmospheric waves (tides, planetary waves (PW), and gravity waves (GW)) from lower atmospheric sources. Atmospheric tides are key to understanding the global-scale connection between tropospheric/stratospheric weather/climate and space weather/climate in the mesosphere and lower thermosphere (MLT) region and further above in the ionosphere-thermosphere (IT) region, including dynamo processes. Much progress has been made in delineating and understanding the “tidal climate” of the MLT region, i.e., tidal variability on seasonal or longer timescales. Tidal variability on shorter timescales, however, is much less understood, mainly due to the observational constraints imposed by satellite local solar time sampling.This thesis presents a study of the causes of the “tidal weather” or short-term (day-to-day to intraseasonal, i.e., \u3c90-day) tidal variability from satellite observations in the MLT region in connection to lower atmospheric driving. The tidal baseline data used is based on the “tidal deconvolution” approach performed on 18 years of daily tidal temperature tides observed by the SABER instrument onboard the TIMED satellite. In addition, SD-WACCMX tidal simulations are used to get further insights into the results obtained from the SABER observations. This allows one to resolve non-linear tidal-PW interactions that cause tidal variability on a \u3c30-day timescale, and variability on a 30-90-day timescale that occurs as a response to the recurring Madden-Julian Oscillation (MJO) in tropical convection. This research mainly focuses on two prominent diurnal (D, ~24 hours period) tides which are the westward-propagating (W) zonal wave number 1 (DW1) and the eastward-propagating (E) nonmigrating diurnal tide zonal wave number 3 (DE3) tides, originating from tropospheric radiative and latent heating distributions. The results in this thesis contribute toward a better understanding of the physical causes of day-to-day to intraseasonal (\u3c90-day) variability in the DW1 and DE3 tides and shed new light on how various propagation and forcing conditions– such as the stratospheric Quasi-Biennial Oscillation (QBO), El Niño and La Niña, MJO and the solar cycle – impact short-term tidal variability. The thesis first discusses the use of an information-theoretic approach from climate science for the statistical characterization of the \u3c30-day short-term tidal variability and proceeds to the regression analysis of multi-year variations in the Sun-Earth system to delineate causes of such characteristics. A key result is that the teleconnection effects due to the QBO in the tropical stratosphere coupled with the solar cycle through the polar vortex disturbances change the \u3c30-day short-term tidal variability. This was not previously known. In the second segment of the thesis, the analysis focuses on the intraseasonal timescale of the tidal variability, where a statistical analysis of SABER observations and SD-WACCMX simulations reveals how the MLT tides respond to the various locations of active-MJO events over the Indian and Pacific Oceans. This confirmed previously unverified model predictions of a 10-25% tidal modulation by the MJO as a function of MJO-locations up to the MLT region. The tides largely respond to the MJO in the tropospheric tidal forcing, and the tidal advection and GW drag forcing in the MLT region. Filtering by tropospheric/stratospheric background winds is comparatively less important. These findings have broader implications as tides can also couple variability on PW and MJO timescales from the MLT region to the IT through dynamo processes, which is important for a better understanding and prediction of space weather