scholarly journals Southern Hemisphere Annular Mode Variability and the Role of Optimal Nonmodal Growth

2005 ◽  
Vol 62 (6) ◽  
pp. 1947-1961 ◽  
Author(s):  
Harun A. Rashid ◽  
Ian Simmonds
Keyword(s):  
Southern Hemisphere ◽  
Zonal Wind ◽  
Positive Feedback ◽  
General Model ◽  
Initial Perturbation ◽  
Low Frequency ◽  
Southern Annular Mode ◽  
Feedback Mechanism ◽  
Annular Mode ◽  
Positive Feedback Mechanism

Abstract The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large amplitudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.

Reconstructions of the southern annular mode (SAM) during the last millennium

2017 ◽  
Vol 41 (6) ◽  
pp. 834-849 ◽  
Author(s):  
Amy Hessl ◽  
Kathryn J Allen ◽  
Tessa Vance ◽  
Nerilie J Abram ◽  
Krystyna M Saunders
Keyword(s):  
Southern Hemisphere ◽  
Low Frequency ◽  
Southern Annular Mode ◽  
Climate Forcing ◽  
Atmospheric Variability ◽  
Annular Mode ◽  
Indian Ocean Sector ◽  
Ocean Sector ◽  
Proxy Reconstructions ◽  
Frequency Behavior

The leading mode of atmospheric variability in the Southern Hemisphere is the Southern Annular Mode (SAM), which affects the atmosphere and ocean from the mid-latitudes to the Antarctic. However, the short instrumental record of the SAM does not adequately represent its multi-decadal to centennial-scale variability. Long palaeoclimatic reconstructions of the SAM would improve our understanding of its low frequency behavior and its effects on regional temperature, rainfall, sea ice, and ecosystem processes. In this progress report, we review three published palaeoclimatic reconstructions available for understanding multi-decadal to centennial-scale variability of the SAM. Reconstructions reviewed here show similar patterns of decadal SAM variability during the last two centuries, but earlier centuries are less coherent. Reconstructions clearly maintain similar trends towards more positive SAM states since the onset of significant anthropogenic climate forcing from rising greenhouse gas (GHG) concentrations and ozone depletion and these excursions appear unprecedented over at least the last 500 years. We describe how new multi-proxy reconstructions of the SAM could further improve our understanding of its long-term variability and effects across all geographic sectors of the Southern Hemisphere. Here, we recommend careful selection and development of proxies in SAM-sensitive regions and seasons. In particular, proxies related to cool-season conditions and from the poorly-sampled Indian Ocean sector would allow for a true circumpolar and year-round reconstruction of past SAM variability.

scholarly journals Possible Dynamical Mechanisms for Southern Hemisphere Climate Change due to the Ozone Hole

2012 ◽  
Vol 69 (10) ◽  
pp. 2917-2932 ◽  
Author(s):  
Andrew Orr ◽  
Thomas J. Bracegirdle ◽  
J. Scott Hosking ◽  
Thomas Jung ◽  
Joanna D. Haigh ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Positive Feedback ◽  
Ozone Depletion ◽  
General Circulation ◽  
Stratospheric Ozone ◽  
Circulation Model ◽  
Feedback Mechanism ◽  
Planetary Waves ◽  
Ozone Hole ◽  
Positive Feedback Mechanism

Abstract The authors report a hypothesis for the dynamical mechanisms responsible for the strengthening of the Southern Hemisphere circumpolar winds from the lower stratosphere to the surface due to the ozone hole. A general circulation model forced by stratospheric ozone depletion representative of the ozone hole period successfully reproduced these observed changes. Investigation of the dynamical characteristics of the model therefore provides some insight into the actual mechanisms. From this the authors suggest the following: 1) An initial (radiative) strengthening of the lower-stratospheric winds as a result of ozone depletion conditions the polar vortex so that fewer planetary waves propagate up from the troposphere, resulting in weaker planetary wave driving. 2) This causes further strengthening of the vortex, which results in an additional reduction in upward-propagating planetary waves and initiates a positive feedback mechanism in which the weaker wave driving and the associated strengthened winds are drawn downward to the tropopause. 3) In the troposphere the midlatitude jet shifts poleward in association with increases in the synoptic wave fluxes of heat and momentum, which are the result of a positive feedback mechanism consisting of two components: 4) increases in low-level baroclinicity, and the subsequent generation of baroclinic activity (associated with a poleward heat flux), are collocated with the jet latitudinal position, and 5) strengthening anticyclonic shear increases the refraction of wave activity equatorward (associated with a poleward momentum flux). Finally, 6) confinement of planetary waves in the high-latitude troposphere is an important step to couple the stratospheric changes to the tropospheric response.

scholarly journals A positive feedback mechanism ensures proper assembly of the functional inner centromere during mitosis in human cells

2018 ◽  
Vol 294 (5) ◽  
pp. 1437-1450 ◽  
Author(s):  
Cai Liang ◽  
Zhenlei Zhang ◽  
Qinfu Chen ◽  
Haiyan Yan ◽  
Miao Zhang ◽  
...  
Keyword(s):  
Positive Feedback ◽  
Histone H3 ◽  
Causal Link ◽  
Feedback Mechanism ◽  
Histone H2a ◽  
Defective Mutant ◽  
Chromosomal Passenger ◽  
Phosphatase 2A ◽  
Elevated Rate ◽  
Positive Feedback Mechanism

The inner centromere region of a mitotic chromosome critically regulates sister chromatid cohesion and kinetochore–microtubule attachments. However, the molecular mechanism underlying inner centromere assembly remains elusive. Here, using CRISPR/Cas9-based gene editing in HeLa cells, we disrupted the interaction of Shugoshin 1 (Sgo1) with histone H2A phosphorylated on Thr-120 (H2ApT120) to selectively release Sgo1 from mitotic centromeres. Interestingly, cells expressing the H2ApT120-binding defective mutant of Sgo1 have an elevated rate of chromosome missegregation accompanied by weakened centromeric cohesion and decreased centromere accumulation of the chromosomal passenger complex (CPC), an integral part of the inner centromere and a key player in the correction of erroneous kinetochore–microtubule attachments. When artificially tethered to centromeres, a Sgo1 mutant defective in binding protein phosphatase 2A (PP2A) is not able to support proper centromeric cohesion and CPC accumulation, indicating that the Sgo1–PP2A interaction is essential for the integrity of mitotic centromeres. We further provide evidence indicating that Sgo1 protects centromeric cohesin to create a binding site for the histone H3–associated protein kinase Haspin, which not only inhibits the cohesin release factor Wapl and thereby strengthens centromeric cohesion but also phosphorylates histone H3 at Thr-3 to position CPC at inner centromeres. Taken together, our findings reveal a positive feedback–based mechanism that ensures proper assembly of the functional inner centromere during mitosis. They further suggest a causal link between centromeric cohesion defects and chromosomal instability in cancer cells.

scholarly journals Hedgehog signaling activates a positive feedback mechanism involving insulin-like growth factors to induce osteoblast differentiation

2015 ◽  
Vol 112 (15) ◽  
pp. 4678-4683 ◽  
Author(s):  
Yu Shi ◽  
Jianquan Chen ◽  
Courtney M. Karner ◽  
Fanxin Long
Keyword(s):  
Transcriptional Activation ◽  
Positive Feedback ◽  
Hedgehog Signaling ◽  
Osteoblast Differentiation ◽  
Akt Signaling ◽  
Feedback Mechanism ◽  
Genetic Deletion ◽  
Positive Feedback Mechanism ◽  
Igf Signaling

Hedgehog (Hh) signaling is essential for osteoblast differentiation in the endochondral skeleton during embryogenesis. However, the molecular mechanism underlying the osteoblastogenic role of Hh is not completely understood. Here, we report that Hh markedly induces the expression of insulin-like growth factor 2 (Igf2) that activates the mTORC2-Akt signaling cascade during osteoblast differentiation. Igf2-Akt signaling, in turn, stabilizes full-length Gli2 through Serine 230, thus enhancing the output of transcriptional activation by Hh. Importantly, genetic deletion of the Igf signaling receptor Igf1r specifically in Hh-responding cells diminishes bone formation in the mouse embryo. Thus, Hh engages Igf signaling in a positive feedback mechanism to activate the osteogenic program.

scholarly journals Survival and Proliferation of Neural Progenitor–Derived Glioblastomas Under Hypoxic Stress is Controlled by a CXCL12/CXCR4 Autocrine-Positive Feedback Mechanism

2016 ◽  
Vol 23 (5) ◽  
pp. 1250-1262 ◽  
Author(s):  
Anda-Alexandra Calinescu ◽  
Viveka Nand Yadav ◽  
Erica Carballo ◽  
Padma Kadiyala ◽  
Dustin Tran ◽  
...  
Keyword(s):  
Positive Feedback ◽  
Neural Progenitor ◽  
Feedback Mechanism ◽  
Hypoxic Stress ◽  
Positive Feedback Mechanism

Inverter design with positive feedback field-effect transistors

2021 ◽  
Author(s):  
Changhoon Lee ◽  
Changwoo Han ◽  
Changhwan Shin
Keyword(s):  
Positive Feedback ◽  
Computer Aided Design ◽  
Field Effect ◽  
Field Effect Transistors ◽  
Feedback Mechanism ◽  
Silicon On Insulator ◽  
Logic Device ◽  
Device Applications ◽  
Positive Feedback Mechanism ◽  
Switching Devices

Abstract As the physical size of semiconductor devices continues to be aggressively scaled down, feedback field-effect transistors (FBFET) with a positive feedback mechanism among a few promising steep switching devices have received attention as next-generation switching devices. Conventional FBFETs have been studied to explore their device performance. However, this has been restricted to the case of single FBFET; basic circuit designs with FBFETs have not been investigated extensively. In this work, we propose an inverter circuit design with silicon-on-insulator (SOI) FBFETs; we verified this inverter design with mixed-mode technology computer-aided design simulation. The basic principles and mechanisms for designing FBFET inverter circuits are explained because their configuration is different from conventional inverters. In addition, the device parameters necessary to optimize circuit construction are introduced for logic device applications.

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