Newswise — HOUSTON – (June 6, 2023) –Climate scientists at Rice University have unearthed an inherent "cyclical nature" - a recurring pattern transpiring every 150 days - within the north-south undulation of atmospheric pressure formations that propel the motion of the predominant westward winds in the Southern Hemisphere and the jet stream in Antarctica.
"This phenomenon originates from the inherent workings of the atmosphere," stated Pedram Hassanzadeh, a study co-author, discussing the finding published in the open-access journal AGU Advances. "While experimenting with novel equations derived for the turbulent circulation of the atmosphere, we unexpectedly came across projections indicating the existence of a natural cycle within the Southern Annular Mode (SAM). Initially skeptical, we proceeded to analyze observational data and confirmed its actual presence."
Co-author Sandro Lubis said, “It was really a surprise, because it goes against the conventional wisdom that the atmosphere is all chaos and disorganization.”
The Southern Annular Mode (SAM), also recognized as the Antarctic Oscillation, holds significant influence as a climatic catalyst for Australia, New Zealand, and Antarctica. This phenomenon has undergone extensive scrutiny and research for several decades.
Hassanzadeh, an assistant professor of mechanical engineering and Earth, environmental, and planetary sciences at Rice University, emphasized the significance of the SAM to the climate community. "The SAM has held immense importance as it exerts its influence over various aspects of the Antarctic environment, including ice, oceans, and the ozone layer," he stated. "However, the oscillations, observable through the north-south shifts of the jet stream winds, have hitherto been regarded as random events occurring on timescales of 10-20 days."
Lubis, a research scientist at the Pacific Northwest National Laboratory and former postdoctoral research fellow in Hassanzadeh's lab at Rice University, pointed out that the 150-day oscillation significantly affects the fluctuations in hemispheric-scale precipitation and ocean surface wind stress. This observation implies that the oscillation could potentially have wider repercussions on the weather patterns and overall climate of the Southern Hemisphere, as well as its oceans and cryosphere.
Hassanzadeh and Lubis each said the paper’s biggest impact will likely be in the arena of climate modeling.
Lubis highlighted a noteworthy aspect, stating, "Remarkably, we observed that numerous cutting-edge climate models fail to replicate this periodicity. This discovery sheds light on the previously identified limitations of these models in simulating SAM variability." Building upon these findings, Lubis further added, "This realization enabled us to propose novel metrics and concepts for assessing the accuracy of climate models in simulating the SAM, comprehending their deficiencies, and potentially enhancing their performance."
The formation of the jet stream arises from two significant components of Earth's atmospheric circulation. Firstly, there is a propensity for air to descend in the subtropics, approximately 30 degrees latitude north or south of the equator. Secondly, as the air approaches the pole, around 60 degrees latitude, it has a tendency to ascend. Consequently, where air descends, pressure intensifies, leading to the emergence of high-pressure regions. Conversely, where air ascends, pressure decreases, leading to the formation of low-pressure areas.
Within both hemispheres, the region between the 60th and 30th parallels experiences distinctive characteristics. It is enclosed by expansive belts of low pressure on the side towards the poles and high pressure on the side closer to the subtropics. These low-pressure areas align with vigorous upper-level winds referred to as the polar jet stream. The polar jet stream follows nearly circular, or annular, trajectories encircling the poles.
The polar jet stream encircling Antarctica exhibits a recurrent pattern of migration between southerly tracks that closely follow the icy continent and northerly tracks that traverse or approach regions such as Australia, South Africa, and South America. These oscillations between north and south trajectories generally endure for approximately two weeks. However, the timing and duration of these oscillations are characterized by randomness, lacking a predictable pattern.
The oscillations observed in the polar jet stream coincide with balanced anomalies in air pressure, where one sign of anomalies is present near the 60th parallel and the opposite sign near the 30th parallel. The Southern Annular Mode (SAM) serves as a statistical index representing these anomalies, which exhibit a seesaw-like pattern. As the westerly winds shift northward or southward, the SAM index fluctuates, rising and falling along the contrasting boundaries.
A positive SAM index indicates an intensification of the jet stream, causing cold air to be confined closer to the pole. Conversely, a negative SAM index signifies increased occurrences of atmospheric lows, leading to enhanced precipitation, rain, and storms in the mid-latitudes.
Hassanzadeh emphasized that the identification of the 150-day periodicity within the SAM emerged through a reconsideration of the conventional mathematical and statistical methods employed in comprehending atmospheric circulation. This fresh perspective enabled the researchers to uncover a previously unnoticed aspect of the SAM's behavior.
Hassanzadeh explained that various atmospheric variables, such as wind or temperature, can be simplified and represented by distinct patterns known as the leading, second leading, third leading patterns, and so on. Conventionally, the statistical analysis technique known as principal component analysis has been employed, assuming that these patterns are independent of each other.
Based on previous studies by other groups, Hassanzadeh and Lubis thought, some of the patterns might be dependent at some lag times.
"We loosened certain assumptions, formulated a fresh mathematical framework, and subsequently produced an intricate manuscript demonstrating its superior performance," expressed Lubis. "Eventually, as we scrutinized the model, we exclaimed, 'This suggests periodicity. Surely, that must be erroneous! Nevertheless, let us delve into the data for verification.'"
Hassanzadeh stated that the 150-day cyclic nature arises due to the interdependence of the SAM's primary north-south movement patterns. These patterns do not act independently but rather engage in interactions with and are influenced by other prominent wind patterns.
"The primary pattern, known as the SAM, involves the consistent shifting of the jet stream towards the north or south," explained Hassanzadeh. "The secondary pattern entails variations in the speed of the jet stream, either accelerating or decelerating. The mechanism behind this periodicity is as follows: the SAM, the first pattern, reinforces itself, thereby amplifying its own strength. Simultaneously, the second pattern also contributes to the reinforcement of the SAM. However, when the SAM becomes exceedingly robust, it begins to diminish the impact of the second pattern, consequently reducing its own reinforcement."
Hassanzadeh emphasized that the subsequent phase of the research entails delving into the reasons behind the failure of certain state-of-the-art climate models to accurately capture both the interactions between patterns and the 150-day cyclic nature of the SAM.
“In the long run, our hope is that this new knowledge will help improve model accuracy for climate change projections,” he said.
The study received support from various sources, including the Office of Naval Research (N00014-20-1-2722), the National Science Foundation (2046309, 1921413), and the Department of Energy (DE-AC05-76RL01830). Additionally, computational resources were made available through the Extreme Science and Engineering Discovery Environment (XSEDE), the National Center for Atmospheric Research (NCAR), and the Rice University Center for Research Computing.
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