scholarly journals Impacts of a Midlatitude Oceanic Frontal Zone for the Baroclinic Annular Mode in the Southern Hemisphere

2021 ◽  
pp. 1-63
Author(s):  
MORIO NAKAYAMA ◽  
HISASHI NAKAMURA ◽  
FUMIAKI OGAWA
Keyword(s):  
Southern Hemisphere ◽  
General Circulation ◽  
Frontal Zone ◽  
Circulation Model ◽  
Southern Annular Mode ◽  
Internal Dynamics ◽  
Near Surface ◽  
Annular Mode ◽  
Major Mode ◽  
Sst Gradient

AbstractAs a major mode of annular variability in the Southern Hemisphere, the baroclinic annular mode (BAM) represents the pulsing of extratropical eddy activity. Focusing mainly on sub-weekly disturbances, this study assesses the impacts of a midlatitude oceanic frontal zone on the BAM and its dynamics through a set of “aqua-planet” atmospheric general circulation model experiments with zonally uniform sea-surface temperature (SST) profiles prescribed. Though idealized, one experiment with realistic frontal SST gradient reasonably well reproduces observed BAM-associated anomalies as a manifestation of a typical lifecycle of migratory baroclinic disturbances. Qualitatively, these BAM features are also simulated in the other experiment where the frontal SST gradient is removed. However, the BAM-associated variability weakens markedly and shifts equatorward, in association with the corresponding modifications in the climatological-mean stormtrack activity. The midlatitude oceanic frontal zone amplifies and anchors the BAM variability by restoring near-surface baroclinicity through anomalous sensible heat supply from the ocean and moisture supply to cyclones, although the BAM is essentially a manifestation of atmospheric internal dynamics. Those experiments and observations further indicate that the BAM modulates momentum flux associated with transient disturbances to induce a modest but robust meridional shift of the polar-front jet, suggesting that the BAM can help maintain the southern annular mode. They also indicate that the quasi-periodic behavior of the BAM is likely to reflect internal dynamics in which atmospheric disturbances on both sub-weekly and longer time scales are involved.

scholarly journals Climate variability of the mid- and high-latitudes of the Southern Hemisphere in ensemble simulations from 1500 to 2000 AD

2012 ◽  
Vol 8 (1) ◽  
pp. 373-390 ◽  
Author(s):  
S. B. Wilmes ◽  
C. C. Raible ◽  
T. F. Stocker
Keyword(s):  
Climate Variability ◽  
Southern Hemisphere ◽  
General Circulation ◽  
Circulation Model ◽  
Volcanic Eruptions ◽  
Ocean General Circulation Model ◽  
Southern Annular Mode ◽  
Significant Shift ◽  
External Forcing ◽  
Proxy Records

Abstract. To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability, we assess an ensemble of 4 transient simulations over the last 500 yr performed with a state-of-the-art atmosphere ocean general circulation model. The model is forced with reconstructions of solar irradiance, greenhouse gas (GHG) and volcanic aerosol concentrations. A 1990 control simulation shows that the model is able to represent the Southern Annular Mode (SAM), and to some extent the South Pacific Dipole (SPD) and the Zonal Wave 3 (ZW3). During the past 500 yr we find that SPD and ZW3 variability remain stable, whereas SAM shows a significant shift towards its positive state during the 20th century. Regional temperatures over South America are strongly influenced by changing both GHG concentrations and volcanic eruptions, whereas precipitation shows no significant response to the varying external forcing. For temperature this stands in contrast to proxy records, suggesting that SH climate is dominated by internal variability rather than external forcing. The underlying dynamics of the temperature changes generally point to a combination of several modes, thus, hampering the possibilities of regional reconstructing the modes from proxy records. The linear imprint of the external forcing is as expected, i.e. a warming for increase in the combined solar and GHG forcing and a cooling after volcanic eruptions. Dynamically, only the increase in SAM with increased combined forcing is simulated.

scholarly journals Cross-Seasonal Influence of the December–February Southern Hemisphere Annular Mode on March–May Meridional Circulation and Precipitation

2015 ◽  
Vol 28 (17) ◽  
pp. 6859-6881 ◽  
Author(s):  
Fei Zheng ◽  
Jianping Li ◽  
Lei Wang ◽  
Fei Xie ◽  
Xiaofeng Li
Keyword(s):  
Southern Hemisphere ◽  
General Circulation ◽  
Large Scale ◽  
Meridional Circulation ◽  
Circulation Model ◽  
Annular Mode ◽  
Southern Hemisphere Annular Mode ◽  
Middle Latitudes ◽  
Mean Meridional Circulation ◽  
Sst Anomalies

Abstract New evidence suggests that interannual variability in zonal-mean meridional circulation and precipitation can be partially attributed to the Southern Hemisphere annular mode (SAM), the dominant mode of climate variability in the Southern Hemisphere (SH) extratropics. A cross-seasonal correlation exists between the December–February (DJF) SAM and March–May (MAM) zonal-mean meridional circulation and precipitation. This correlation is not confined to the SH: it also extends to the Northern Hemisphere (NH) subtropics. When the preceding DJF SAM is positive, counterclockwise, and clockwise meridional cells, accompanied by less and more precipitation, occur alternately between the SH middle latitudes and NH subtropics in MAM. In particular, less precipitation occurs in the SH middle latitudes, the SH tropics, and the NH subtropics, but more precipitation occurs in the SH subtropics and the NH tropics. A framework is built to explain the cross-seasonal impact of SAM-related SST anomalies. Evidence indicates that the DJF SAM tends to lead to dipolelike SST anomalies in the SH extratropics, which are referred to in this study as the SH ocean dipole (SOD). The DJF SOD can persist until the following MAM when it begins to modulate MAM meridional circulation and large-scale precipitation. Atmospheric general circulation model simulations further verify that MAM meridional circulation between the SH middle latitudes and the northern subtropics responds to the MAM SOD.

Global Thermohaline Circulation. Part II: Sensitivity with Interactive Atmospheric Transports

1999 ◽  
Vol 12 (1) ◽  
pp. 83-91 ◽  
Author(s):  
Xiaoli Wang ◽  
Peter H. Stone ◽  
Jochem Marotzke
Keyword(s):  
Southern Hemisphere ◽  
General Circulation ◽  
Large Scale ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Ocean Atmosphere ◽  
Heat And Moisture ◽  
Sst Gradient ◽  
Moisture Transports ◽  
Flux Adjustment

Abstract A hybrid coupled ocean–atmosphere model is used to investigate the stability of the thermohaline circulation (THC) to an increase in the surface freshwater forcing in the presence of interactive meridional transports in the atmosphere. The ocean component is the idealized global general circulation model used in Part I. The atmospheric model assumes fixed latitudinal structure of the heat and moisture transports, and the amplitudes are calculated separately for each hemisphere from the large-scale sea surface temperature (SST) and SST gradient, using parameterizations based on baroclinic stability theory. The ocean–atmosphere heat and freshwater exchanges are calculated as residuals of the steady-state atmospheric budgets. Owing to the ocean component’s weak heat transport, the model has too strong a meridional SST gradient when driven with observed atmospheric meridional transports. When the latter are made interactive, the conveyor belt circulation collapses. A flux adjustment is introduced in which the efficiency of the atmospheric transports is lowered to match the too low efficiency of the ocean component. The feedbacks between the THC and both the atmospheric heat and moisture transports are positive, whether atmospheric transports are interactive in the Northern Hemisphere, the Southern Hemisphere, or both. However, the feedbacks operate differently in the Northern and Southern Hemispheres, because the Pacific THC dominates in the Southern Hemisphere, and deep water formation in the two hemispheres is negatively correlated. The feedbacks in the two hemispheres do not necessarily reinforce each other because they have opposite effects on low-latitude temperatures. The model is qualitatively similar in stability to one with conventional “additive” flux adjustment, but quantitatively more stable.

Importance of Midlatitude Oceanic Frontal Zones for the Annular Mode Variability: Interbasin Differences in the Southern Annular Mode Signature

2016 ◽  
Vol 29 (17) ◽  
pp. 6179-6199 ◽  
Author(s):  
Fumiaki Ogawa ◽  
Hisashi Nakamura ◽  
Kazuaki Nishii ◽  
Takafumi Miyasaka ◽  
Akira Kuwano-Yoshida
Keyword(s):  
General Circulation ◽  
Lower Boundary ◽  
Surface Air Temperature ◽  
Circulation Model ◽  
Southern Annular Mode ◽  
Lower Boundary Condition ◽  
Annular Mode ◽  
Frontal Zones ◽  
Dynamical Regimes ◽  
Strong Control

Abstract A midlatitude oceanic frontal zone is a confluent region of warm and cool ocean currents, characterized by a strong meridional gradient in both sea surface temperature (SST) and surface air temperature (SAT). While recent observational and modeling studies indicate potential impacts of midlatitude oceanic fronts on the extratropical climatological circulation, including storm tracks and eddy-driven westerlies, their impacts on the atmospheric-dominant low-frequency variability (i.e., the annular mode) still remain to be understood. This study explores possible impacts of midlatitude oceanic frontal zones on annular mode signatures in the wintertime Southern Hemisphere (SH). To mimic the SH, sets of idealized aquaplanet experiments are conducted with zonally symmetric distributions of SST prescribed globally at the lower boundary of an atmospheric general circulation model. By systematically changing the latitude of frontal gradient in the SST profile, the experiments reveal that the characteristics of the wintertime annular mode exhibit strong sensitivity to the position of the SST front if situated at midlatitude or subpolar latitude. The annular mode may be interpreted as a manifestation of wobble of the extratropical tropospheric circulation between two “dynamical regimes”—one under the strong influence of SST gradient and the other under the strong control of atmospheric internal dynamics unrelated to the lower-boundary condition. In fact, this interpretation offers insight into the observed interbasin differences in the wintertime signature of the southern annular mode (SAM) that are embedded in the zonally symmetric anomalies. The findings suggest a possible reinterpretation of the climatological-mean state observed in the wintertime SH as the superposition of those two dynamical regimes.

scholarly journals Climate variability of the mid- and high-latitudes of the Southern Hemisphere in ensemble simulations from 1500 to 2000 AD

2011 ◽  
Vol 7 (5) ◽  
pp. 3091-3129 ◽  
Author(s):  
S. B. Wilmes ◽  
C. C. Raible ◽  
T. F. Stocker
Keyword(s):  
Climate Variability ◽  
Southern Hemisphere ◽  
General Circulation ◽  
Circulation Model ◽  
Volcanic Eruptions ◽  
Ocean General Circulation Model ◽  
Southern Annular Mode ◽  
Significant Shift ◽  
External Forcing ◽  
Proxy Records

Abstract. To increase the sparse knowledge of long-term Southern Hemisphere (SH) climate variability we assess an ensemble of 4 transient simulations over the last 500 yr performed with a state-of-the-art atmosphere ocean general circulation model. The model is forced with reconstructions of solar irradiance, greenhouse gas (GHG) and volcanic aerosol concentrations. A 1990 control simulation shows that the model is able to represent the Southern Annular Mode (SAM), and to some extent the South Pacific Dipole (SPD) and the Zonal Wave 3 (ZW3). During the past 500 yr we find that SPD and ZW3 variability remain stable, whereas SAM shows a significant shift towards its positive state during the 20th century. Regional temperatures over South America are strongly influenced by changing both GHG concentrations and volcanic eruptions whereas precipitation shows no significant response to the varying external forcing. For temperature this stands in contrast to proxy records suggesting that SH climate is dominated by internal variability rather than external forcing. The underlying dynamics of the temperature changes generally point to a combination of several modes, thus, hampering the possibilities regional reconstructions of the modes from proxy records. The linear imprint of the external forcing is as expected, i.e. a warming for increase in the combined solar and GHG forcing and a cooling after volcanic eruptions. Dynamically only the increase in SAM with increased combined forcing is simulated.

scholarly journals Significance of a Midlatitude SST Frontal Zone in the Formation of a Storm Track and an Eddy-Driven Westerly Jet*

2010 ◽  
Vol 23 (7) ◽  
pp. 1793-1814 ◽  
Author(s):  
Takeaki Sampe ◽  
Hisashi Nakamura ◽  
Atsushi Goto ◽  
Wataru Ohfuchi
Keyword(s):  
General Circulation ◽  
Frontal Zone ◽  
Sensible Heat Flux ◽  
Lower Boundary ◽  
Storm Track ◽  
Lower Boundary Condition ◽  
Near Surface ◽  
Low Level ◽  
Sst Front ◽  
Sst Gradient

Abstract In a set of idealized “aquaplanet” experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature (SST) is prescribed globally as the lower boundary condition, an assessment is made of the potential influence of the frontal SST gradient upon the formation of a storm track and an eddy-driven midlatitude polar front jet (PFJ), and on its robustness against changes in the intensity of a subtropical jet (STJ). In experiments with the frontal midlatitude SST gradient as that observed in the southwestern Indian Ocean, transient eddy activity in each of the winter and summer hemispheres is organized into a deep storm track along the SST front with an enhanced low-level baroclinic growth of eddies. In the winter hemisphere, another storm track forms just below the intense STJ core, but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath. The near-surface westerlies are strongest near the midlatitude SST front as observed, consistent with westerly momentum transport associated with baroclinic eddy growth. The sharp poleward decline in the surface sensible heat flux across the SST frontal zone sustains strong near-surface baroclinicity against the relaxing effect by vigorous poleward eddy heat transport. Elimination of the midlatitude frontal SST gradient yields marked decreases in the activity of eddies and their transport of angular momentum into midlatitudes, in association with equatorward shifts of the PFJ-associated low-level westerlies and a subtropical high pressure belt, especially in the summer hemisphere. These impacts of the midlatitude frontal SST gradient are found to be robust against modest changes in the STJ intensity as observed in its interannual variability, suggesting the potential importance of midlatitude atmosphere–ocean interaction in shaping the tropospheric general circulation.

scholarly journals A new mechanism for atmospheric mercury redox chemistry: Implications for the global mercury budget

2017 ◽  
Author(s):  
Hannah M. Horowitz ◽  
Daniel J. Jacob ◽  
Yanxu Zhang ◽  
Theodore S. Dibble ◽  
Franz Slemr ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Wet Deposition ◽  
General Circulation ◽  
Gulf Coast ◽  
Circulation Model ◽  
Redox Chemistry ◽  
Ocean General Circulation Model ◽  
Atmospheric Mercury ◽  
Water Soluble ◽  
Second Stage

Abstract. Mercury (Hg) is emitted to the atmosphere mainly as volatile elemental Hg0. Oxidation to water-soluble HgII controls Hg deposition to ecosystems. Here we implement a new mechanism for atmospheric Hg0 / HgII redox chemistry in the GEOS-Chem global model and examine the implications for the global atmospheric Hg budget and deposition patterns. Our simulation includes a new coupling of GEOS-Chem to an ocean general circulation model (MITgcm), enabling a global 3-D representation of atmosphere-ocean Hg0 / HgII cycling. We find that atomic bromine (Br) of marine organobromine origin is the main atmospheric Hg0 oxidant, and that second-stage HgBr oxidation is mainly by the NO2 and HO2 radicals. The resulting lifetime of tropospheric Hg0 against oxidation is 2.7 months, shorter than in previous models. Fast HgII atmospheric reduction must occur in order to match the ~ 6-month lifetime of Hg against deposition implied by the observed atmospheric variability of total gaseous mercury (TGM ≡ Hg0 + HgII(g)). We implement this reduction in GEOS-Chem as photolysis of aqueous-phase HgII-organic complexes in aerosols and clouds, resulting in a TGM lifetime of 5.2 months against deposition and matching both mean observed TGM and its variability. Model sensitivity analysis shows that the interhemispheric gradient of TGM, previously used to infer a longer Hg lifetime against deposition, is misleading because southern hemisphere Hg mainly originates from oceanic emissions rather than transport from the northern hemisphere. The model reproduces the observed seasonal TGM variation at northern mid-latitudes (maximum in February, minimum in September) driven by chemistry and oceanic evasion, but does not reproduce the lack of seasonality observed at southern hemisphere marine sites. Aircraft observations in the lowermost stratosphere show a strong TGM-ozone relationship indicative of fast Hg0 oxidation, but we show that this relationship provides only a weak test of Hg chemistry because it is also influenced by mixing. The model reproduces observed Hg wet deposition fluxes over North America, Europe, and China, including the maximum over the US Gulf Coast driven by HgBr oxidation by NO2 and HO2. Low Hg wet deposition observed over rural China is attributed to fast HgII reduction in the presence of high organic aerosol concentrations. We find that 80 % of global HgII deposition takes place over the oceans, reflecting the marine origin of Br and low concentrations of marine organics for HgII reduction, and most of HO2 and NO2 for second-stage HgBr oxidation.

Zonal Wave 3 Pattern in the Southern Hemisphere generated by tropical convection

2021 ◽  
Author(s):  
Rishav Goyal ◽  
Martin Jucker ◽  
Alex Sen Gupta ◽  
Harry Hendon ◽  
Matthew England
Keyword(s):  
Sea Ice ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
General Circulation ◽  
Deep Convection ◽  
Circulation Model ◽  
Hadley Cell ◽  
Wave Source ◽  
The Tropics

Abstract A distinctive feature of the Southern Hemisphere (SH) extratropical atmospheric circulation is the quasi-stationary zonal wave 3 (ZW3) pattern, characterized by three high and three low-pressure centers around the SH extratropics. This feature is present in both the mean atmospheric circulation and its variability on daily, seasonal and interannual timescales. While the ZW3 pattern has significant impacts on meridional heat transport and Antarctic sea ice extent, the reason for its existence remains uncertain, although it has long been assumed to be linked to the existence of three major land masses in the SH extratropics. Here we use an atmospheric general circulation model to show that the stationery ZW3 pattern is instead driven by zonal asymmetric deep atmospheric convection in the tropics, with little to no role played by the orography or land masses in the extratropics. Localized regions of deep convection in the tropics form a local Hadley cell which in turn creates a wave source in the subtropics that excites a poleward and eastward propagating wave train which forms stationary waves in the SH high latitudes. Our findings suggest that changes in tropical deep convection, either due to natural variability or climate change, will impact the zonal wave 3 pattern, with implications for Southern Hemisphere climate, ocean circulation, and sea-ice.

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