Detection and evaluation of sudden stratospheric warming : A case study of water vapor transport

Abstract

Stratospheric Sudden Warming, (SSW), events represent the most dramatic and influential disruptions of the wintertime circulation in the Northern Hemisphere (NH). These phenomena are characterized by a rapid increase in polar temperatures and a subsequent weakening or reversal of the polar night jet, carrying profound implications for both stratospheric composition and tropospheric weather patterns. Furthermore, the modulation of these events by the Quasi-Biennial Oscillation (QBO) is evaluated, confirming that the easterly phase (QBO–) significantly enhances planetary wave activity and the probability of vortex breakdowns through the Holton-Tan effect. This thesis provides an comprehensive evaluation of SSW dynamics, their representation in modern climate models, and their critical role in the vertical transport of water vapor (WV). The primary objective is to move beyond simple statistical descriptions and characterize the evolution of these events through their feedback mechanisms within the global climate system. Given that NH winter conditions are heavily influenced by the coupling between the stratosphere and troposphere, understanding the physical triggers that lead to the breakdown of the polar vortex is essential for improving long-range seasonal predictability. The sudden surges in stratospheric temperature, often exceeding 30–40K at the 10 hPa level, are induced by the vertical propagation of large-scale Rossby waves from the troposphere, leading to intense wave-mean flow interactions and the collapse of the polar night jet. A central methodological contribution of this research is the implementation of the Threshold Exceedance Area (TEA), method for identifying and classifying SSW events. Conventional detection methods, primarily based on the reversal of zonal mean winds at 60N, often fail to capture the full thermodynamic evolution and the spatial extent of the warming. The TEA method addresses this gap by analyzing temperature anomalies at the 10 hPa level, requiring a 30 K threshold to be exceeded for at least 6 consecutive days. Using ERA5 reanalysis data for the 1980 to 2005 period, this study demonstrates that the TEA approach provides a more refined understanding of the Main Phase Strength (MPS). The MPS is calculated by multiplication the Main Phase Area (MPA) over the Main Phase Duration (MPD) of the event, allowing for a robust classification into Minor, Major, and Extreme categories. The findings indicate that during the peak of these events, the stratospheric temperature variance decreases by 44–54%, reflecting a fundamental reorganization of the polar vortex energy state. This variance reduction suggests that SSWs act as a critical stabilizing feedback mechanism, redistributing heat from the mid-latitudes to the over-cooled polar region. With the ERA5 reanalysis dataset serving as the primary reference for validation, the evaluation of five specific climate models, namely ACCESS1.3 (M1), HadGEM2-CC (M2), MPI-ESM-MR (M3), FGOALS-g2 (M4), and GFDL-ESM2G (M5), reveals significant disparities in their ability to simulate stratospheric variability. While models such as M1, M2 and M3 show a high degree of fidelity in reproducing the frequency and magnitude of NH SSWs relative to ERA5, others suffer from a persistent cold pole bias. This bias leads to a polar night jet that is too stable and strong, resulting in an underestimation of the frequency of vortex breakdowns. In particular, the vertical resolution and the height of the model top are identified as decisive factors in capturing the upward propagation of planetary waves. Low-top models like M4 and M5 fail to reproduce realistic SSW frequencies due to their limited representation of the upper stratosphere. Under the RCP 4.5 future climate scenario, the analysis suggests that while the absolute frequency of SSWs may not increase drastically, the duration and vertical depth of these events are projected to lengthen. These modifications in SSW characteristics are expected to influence the downward propagation of stratospheric anomalies, thereby affecting surface weather regimes and atmospheric oscillations in a warming climate. Furthermore, this study investigates the hydrological implications of SSWs through a detailed case study of the January 2003 major warming event. This event is notable for a dramatic vortex split, which facilitated a significant injection of water vapor from the troposphere into the lower stratosphere. Quantitative analysis reveals a substantial +5.22% increase in the stratospheric moisture index (100–10 hPa), which is physically balanced by a –4.91% moisture deficit in the middle troposphere (500–100 hPa) due to the vertical hoisting mechanism. By analyzing vertical velocity fields and moisture flux, the research demonstrates that the mechanical disruption of the vortex creates pathways for moisture transport through the Upper Troposphere Lower Stratosphere (UTLS) region. Normal atmospheric barriers at the tropopause are breached during these extreme events, allowing moist air from lower levels to be injected into the dry stratospheric reservoir. This transport is not limited to large-scale circulation but also occurs through filamentous and spiral structures formed during the vortex fragmentation. The enhanced stratospheric water vapor alters radiative cooling rates and impacts the recovery time of the polar vortex, as the additional moisture acts as a radiative forcing agent that delays the return to a stable state. A critical discovery of this research is the persistence of the hydrological response; while dynamical anomalies peak during the warming phase and the event concludes on 30 January 2003, stratospheric moisture concentrations remain elevated during the recovery phase. The role of the Brewer-Dobson Circulation (BDC) is central to explaining this transport mechanism. During an SSW, the BDC is significantly accelerated due to increased wave breaking in the stratosphere. This acceleration drives the upward motion of air at high latitudes, effectively pumping water vapor into the middle atmosphere. The case study of 2003 provides empirical evidence for this pump effect, showing that the moisture anomalies persist in the stratosphere for months after the initial warming has subsided. This suggests that SSWs are not merely dynamical events but are critical drivers of the stratospheric moisture budget and chemical composition. The persistence of these moisture anomalies provides a memory effect that can influence the radiative balance of the NH for an entire season. In conclusion, this thesis advances the understanding of SSW events by introducing a robust detection algorithm and elucidating the physical mechanisms of stratospheric-tropospheric moisture exchange. The results highlight the necessity for improved stratospheric representation in climate models, particularly regarding vertical motion and UTLS interactions. By linking the dynamical classification of events via MPS with the hydrological consequences via BDC-driven transport, this research provides a holistic view of how the middle atmosphere responds to extreme forcing. These findings are vital for refining future climate models, as the inclusion of accurate stratospheric water vapor feedback is essential for predicting the long-term stability of the global climate system. The results underscore that high-resolution vertical grids and accurate stratospheric water vapor feedback are essential prerequisites for improving long-range seasonal predictability and refining future climate projections. The mysteries of the NH stratosphere continue to be a focal point for meteorological research, and this work establishes a new framework for integrating dynamical warming events with the global hydrological cycle.