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 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.

Air–Sea Heat Exchanges Characteristic of a Prominent Midlatitude Oceanic Front in the South Indian Ocean as Simulated in a High-Resolution Coupled GCM

2009 ◽  
Vol 22 (24) ◽  
pp. 6515-6535 ◽  
Author(s):  
Masami Nonaka ◽  
Hisashi Nakamura ◽  
Bunmei Taguchi ◽  
Nobumasa Komori ◽  
Akira Kuwano-Yoshida ◽  
...  
Keyword(s):  
Heat Flux ◽  
Frontal Zone ◽  
Sensible Heat Flux ◽  
Storm Track ◽  
Sensible Heat ◽  
South Indian Ocean ◽  
Sst Front ◽  
South Indian ◽  
Atmospheric Disturbances ◽  
Heat Exchanges

Abstract An integration of a high-resolution coupled general circulation model whose ocean component is eddy permitting and thus able to reproduce a sharp gradient in sea surface temperature (SST) is analyzed to investigate air–sea heat exchanges characteristic of the midlatitude oceanic frontal zone. The focus of this paper is placed on a prominent SST front in the south Indian Ocean, which is collocated with the core of the Southern Hemisphere storm track. Time-mean distribution of sensible heat flux is characterized by a distinct cross-frontal contrast. It is upward and downward on the warmer and cooler flanks, respectively, of the SST front, acting to maintain the sharp gradient of surface air temperature (SAT) that is important for preconditioning the environment for the recurrent development of storms and thereby anchoring the storm track. Induced by cross-frontal advection of cold (warm) air associated with migratory atmospheric disturbances, the surface flux is highly variable with intermittent enhancement of the upward (downward) flux predominantly on the warmer (cooler) flank of the front. Indeed, several intermittent events of cold (warm) air advection, whose total duration accounts for only 21% (19%) of the entire analysis period, contribute to as much as 60% (44%) of the total amount of sensible heat flux during the analysis period on the warmer (cooler) flank. This antisymmetric behavior yields the sharp cross-frontal gradient in the time-mean flux. Since the flux intensity is strongly influenced by local magnitude of the SST–SAT difference that tends to increase with the SST gradient, the concentration of the flux variance to the frontal zone and cross-frontal contrasts in the mean and skewness of the flux all become stronger during the spinup of the SST front. Synoptically, the enhanced sensible heat flux near the SST front can restore SAT toward the underlying SST effectively with a time scale of a day, to maintain a frontal SAT gradient against the relaxing effect of atmospheric disturbances. The restoration effect of the differential surface heating at the SST front, augmented by the surface latent heating concentrated on the warm side of the front, represents a key process through which the atmospheric baroclinicity and ultimately the storm track are linked to the underlying ocean.

Atmospheric response to Gulf Stream SST front shifting: impact of horizontal resolution in an ensemble of global climate models

2021 ◽  
Author(s):  
Luca Famooss Paolini ◽  
Alessio Bellucci ◽  
Paolo Ruggieri ◽  
Panos Athanasiadis ◽  
Silvio Gualdi
Keyword(s):  
High Resolution ◽  
Gulf Stream ◽  
General Circulation ◽  
Diabatic Heating ◽  
Storm Track ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Atmospheric Response ◽  
Sst Front ◽  
Pressure Anomaly

<p>Western boundary currents transport a large amount of heat from the Tropics toward higher latitudes; furthermore they are characterized by a strong sea surface temperature (SST) gradient, which anchors zones of intense upward motion extending up to the upper-troposphere and shapes zones of intense baroclinic eddy activity (storm tracks). For such reasons they have been shown to be fundamental in influencing the climate of the Northern Hemisphere and its variability, and a potentially relevant source of atmospheric predictability. </p><p> </p><p>General circulation models show deficiencies in simulating the observed atmospheric response to SST front variability. The atmospheric horizontal resolution has been recently proposed as a key element in understanding such differences. However, the number of studies on this subject is still limited. Furthermore, a multi-model analysis to systematically investigate differences between low-resolution and high-resolution atmospheric response to oceanic forcing is still lacking. </p><p> </p><p>The present work has the objective to fill this gap, analysing the atmospheric response to Gulf Stream SST front shifting using data from recent High Resolution Model Intercomparison Project (HighResMIP). This project was designed with the specific objective of investigating the impact of increased model horizontal resolution on the representation of the observed climate. Ensembles of historical simulations performed with three atmospheric general circulation models (AGCMs) have been analysed, each conducted with a low-resolution (LR, about 1°) and a high-resolution (HR, about 0.25°) configuration. AGCMs have been forced with observed SSTs (HadISST2 dataset), available at daily frequency on a 0.25° grid, during 1950–2014. </p><p><br>Results show atmospheric responses to the SST-induced diabatic heating anomalies that are strongly resolution dependent. In LR simulations a low-pressure anomaly is present downstream of the SST anomaly, while the diabatic heating anomaly is mainly balanced by meridional advection of air coming from higher latitudes, as expected for an extra-tropical shallow heat source. In contrast, HR simulations generate a high-pressure anomaly downstream of the SST anomaly, thus driving positive temperature advection from lower latitudes (not balancing diabatic heating). Along the vertical direction, both in LR and HR simulation, the diabatic heating in the interior of the atmosphere is balanced by upward motion south of GS SST front and downward motion north and further south of the Gulf Stream. Finally, LR simulations show a reduction in storm-track activity over the North Atlantic, whereas HR simulations show a meridional displacement of the storm-track considerably larger (yet in the same direction) than that of the SST front. HR simulations reproduce the atmospheric response obtained from observations, albeit weaker. This is a hint for the existence of a positive feedback between ocean and atmosphere, as proposed in previous studies. These findings are qualitatively consistent with previous results in literature and, leveraging on recent coordinated modelling efforts, shed light on the effective role of atmospheric horizontal resolution in modelling the atmospheric response to extra-tropical oceanic forcing.</p>

scholarly journals The Role of Tropical Interbasin SST Gradients in Forcing Walker Circulation Trends

2017 ◽  
Vol 30 (2) ◽  
pp. 499-508 ◽  
Author(s):  
Lei Zhang ◽  
Kristopher B. Karnauskas
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Walker Circulation ◽  
Equatorial Pacific ◽  
Trade Winds ◽  
Low Level ◽  
The Pacific ◽  
Indian Ocean Warming ◽  
Tropical Sea ◽  
Sst Gradient

The effects of externally forced tropical sea surface temperature (SST) anomalies on long-term Walker circulation changes are investigated through numerical atmospheric general circulation model (AGCM) experiments. In response to the observed tropics-wide SST trend, which exhibits a prominent interbasin warming contrast (IBWC) with smaller warming in the Pacific than the Indian and Atlantic Oceans that includes a weak La Niña–like pattern in the equatorial Pacific, pronounced low-level easterly anomalies emerge over the equatorial Pacific. Through sensitivity experiments, the intensification of the Pacific trade winds (PTWs) is attributable to the IBWC, whereas the slightly enhanced zonal SST gradient within the equatorial Pacific plays a small role relative to the observed IBWC. It is further demonstrated that the greater Indian Ocean warming forces low-level easterly anomalies over the entire equatorial Pacific, while the greater tropical Atlantic warming-driven enhancement of PTWs is located over the central equatorial Pacific. In contrast to observations, a negligible IBWC emerges in the tropical SST trends of CMIP5 historical simulations due to a strong El Niño–like warming in the tropical Pacific. Lacking the observed IBWC (and the observed enhancement of the zonal SST gradient within the equatorial Pacific), the PTWs in the CMIP5 ensemble can only weaken.

Changes in the Extratropical Storm Tracks in Response to Changes in SST in an AGCM

2012 ◽  
Vol 25 (6) ◽  
pp. 1854-1870 ◽  
Author(s):  
Lise Seland Graff ◽  
J. H. LaCasce
Keyword(s):  
Boundary Conditions ◽  
General Circulation ◽  
Lower Boundary ◽  
Storm Track ◽  
Circulation Model ◽  
Hadley Circulation ◽  
Meridional Overturning Circulation ◽  
Storm Tracks ◽  
Poleward Shift ◽  
The Mean

Abstract A poleward shift in the extratropical storm tracks has been identified in observational and climate simulations. The authors examine the role of altered sea surface temperatures (SSTs) on the storm-track position and intensity in an atmospheric general circulation model (AGCM) using realistic lower boundary conditions. A set of experiments was conducted in which the SSTs where changed by 2 K in specified latitude bands. The primary profile was inspired by the observed trend in ocean temperatures, with the largest warming occurring at low latitudes. The response to several other heating patterns was also investigated, to examine the effect of imposed gradients and low- versus high-latitude heating. The focus is on the Northern Hemisphere (NH) winter, averaged over a 20-yr period. Results show that the storm tracks respond to changes in both the mean SST and SST gradients, consistent with previous studies employing aquaplanet (water only) boundary conditions. Increasing the mean SST strengthens the Hadley circulation and the subtropical jets, causing the storm tracks to intensify and shift poleward. Increasing the SST gradient at midlatitudes similarly causes an intensification and a poleward shift of the storm tracks. Increasing the gradient in the tropics, on the other hand, causes the Hadley cells to contract and the storm tracks to shift equatorward. Consistent shifts are seen in the mean zonal velocity, the atmospheric baroclinicity, the eddy heat and momentum fluxes, and the atmospheric meridional overturning circulation. The results support the idea that oceanic heating could be a contributing factor to the observed shift in the storm tracks.

The Influence of an SST Front on a Heavy Rainfall Event over Coastal Taiwan during TiMREX

2014 ◽  
Vol 71 (9) ◽  
pp. 3223-3249 ◽  
Author(s):  
Michael D. Toy ◽  
Richard H. Johnson
Keyword(s):  
Northern South China Sea ◽  
Wrf Model ◽  
Early Summer ◽  
Rainfall Event ◽  
Precipitation Event ◽  
Heavy Rainfall Event ◽  
Low Level ◽  
Sst Front ◽  
Southwest Coast ◽  
Sst Gradient

Abstract A long-lived heavy precipitation area was observed along the southwest coast of Taiwan from 13 to 18 June 2008 during the Terrain-Influenced Monsoon Rainfall Experiment (TiMREX). Rainfall amounts exceeded 500 mm along portions of the coast, and the coastal plains experienced severe flooding. The precipitation systems were influenced by blocking effects, as the southerly moist monsoon flow impinged on the island. A relatively strong gradient in the sea surface temperature (SST) off the southwest coast of Taiwan existed during the rainfall event. Mesoscale SST fronts are known to influence the planetary boundary layer (PBL) such that low-level convergence and precipitation are enhanced under certain circumstances. In this study, the authors investigate the role of the SST front in enhancing the 13–18 June 2008 precipitation event over Taiwan using the Weather Research and Forecasting (WRF) Model. In control simulations with the observed SST, there is a transition from a well-mixed to a stable PBL across the front, causing the low-level flow to decelerate, resulting in an enhancement of horizontal convergence. Such a transition in the PBL and the associated convergence is greatly reduced in smoothed SST gradient model simulations, which produce over 20% less precipitation over southwest Taiwan. Sensitivity tests show that, qualitatively, the results are independent of the existence of the island of Taiwan. These findings indicate that the SST gradient over the northern South China Sea during the early summer monsoon can have a significant impact on the intensity of rainfall over Taiwan.

scholarly journals On the Appropriate Definition of Soil Profile Configuration and Initial Conditions for Land Surface-Hydrology Models in Cold Regions

2017 ◽  
Author(s):  
Gonzalo Sapriza-Azuri ◽  
Pablo Gamazo ◽  
Saman Razavi ◽  
Howard S. Wheater
Keyword(s):  
Climate Change ◽  
Soil Profile ◽  
Land Surface ◽  
General Circulation ◽  
Field Experiments ◽  
Lower Boundary ◽  
Model Parameters ◽  
Lower Boundary Condition ◽  
The Impact ◽  
Permafrost Regions

Abstract. Arctic and sub-arctic regions are amongst the most susceptible regions on Earth to global warming and climate change. Understanding and predicting the impact of climate change in these regions require a proper process representation of the interactions between climate, the carbon cycle, and hydrology in Earth system models. This study focuses on Land Surface Models (LSMs) that represent the lower boundary condition of General Circulation Models (GCMs) and Regional Climate Models (RCMs), which simulate climate change evolution at the global and regional scales, respectively. LSMs typically utilize a standard soil configuration with a depth of no more than 4 meters, whereas for cold, permafrost regions, field experiments show that attention to deep soil profiles is needed to understand and close the water and energy balances, which are tightly coupled through the phase change. To address this, we design and run a series of model experiments with a one-dimensional LSM, called CLASS (Canadian Land Surface Scheme), as embedded in the MESH (Modélisation Environmentale Communautaire – Surface and Hydrology) modelling system, to (1) characterize the effect of soil profile depth under different climate conditions and in the presence of parameter uncertainty, and (2) develop a methodology for temperature profile initialization in permafrost regions, where the system has an extended memory, by the use of paleo-records and bootstrapping. Our study area is in Norman Wells, Northwest Territories of Canada, where measurements of soil temperature profiles and historical reconstructed climate data are available. Our results demonstrate that the adequate depth of soil profile in an LSM varies for warmer and colder conditions and is sensitive to model parameters and the uncertainty around them. In general, however, we show that a minimum of 20 meters of soil profile is essential to adequately represent the temperature dynamics. Our results also indicate the significance of model initialization in permafrost regions and our proposed spin-up method requires running the LSM over more than 300 years of reconstructed climate time series.

scholarly journals Heuristic estimation of low-level cloud fraction over the globe based on a decoupling parameterization

2019 ◽  
Vol 19 (8) ◽  
pp. 5635-5660 ◽  
Author(s):  
Sungsu Park ◽  
Jihoon Shin
Keyword(s):  
General Circulation ◽  
Cloud Amount ◽  
Cloud Fraction ◽  
Spatiotemporal Variation ◽  
Interannual Variations ◽  
Marine Stratocumulus ◽  
Near Surface ◽  
Low Level ◽  
Inversion Strength ◽  
Lifting Condensation Level

Abstract. Based on the decoupling parameterization of the cloud-topped planetary boundary layer, a simple equation is derived to compute the inversion height. In combination with the lifting condensation level and the amount of water vapor in near-surface air, we propose a low-level cloud suppression parameter (LCS) and estimated low-level cloud fraction (ELF), as new proxies for the analysis of the spatiotemporal variation of the global low-level cloud amount (LCA). Individual surface and upper-air observations are used to compute LCS and ELF as well as lower-tropospheric stability (LTS), estimated inversion strength (EIS), and estimated cloud-top entrainment index (ECTEI), three proxies for LCA that have been widely used in previous studies. The spatiotemporal correlations between these proxies and surface-observed LCA were analyzed. Over the subtropical marine stratocumulus deck, both LTS and EIS diagnose seasonal–interannual variations of LCA well. However, their use as a global proxy for LCA is limited due to their weaker and inconsistent relationship with LCA over land. EIS is anti-correlated with the decoupling strength more strongly than it is correlated with the inversion strength. Compared with LTS and EIS, ELF and LCS better diagnose temporal variations of LCA, not only over the marine stratocumulus deck but also in other regions. However, all proxies have a weakness in diagnosing interannual variations of LCA in several subtropical stratocumulus decks. In the analysis using all data, ELF achieves the best performance in diagnosing spatiotemporal variation of LCA, explaining about 60 % of the spatial–seasonal–interannual variance of the seasonal LCA over the globe, which is a much larger percentage than those explained by LTS (2 %) and EIS (4 %). Our study implies that accurate prediction of inversion base height and lifting condensation level is a key factor necessary for successful simulation of global low-level clouds in general circulation models (GCMs). Strong spatiotemporal correlation between ELF (or LCS) and LCA identified in our study can be used to evaluate the performance of GCMs, identify the source of inaccurate simulation of LCA, and better understand climate sensitivity.

scholarly journals Structure and Mechanisms of the Southern Hemisphere Summertime Subtropical Anticyclones

2010 ◽  
Vol 23 (8) ◽  
pp. 2115-2130 ◽  
Author(s):  
Takafumi Miyasaka ◽  
Hisashi Nakamura
Keyword(s):  
Southern Hemisphere ◽  
Rossby Wave ◽  
Sensible Heat Flux ◽  
Wave Activity ◽  
Convective Heating ◽  
Surface Cooling ◽  
Near Surface ◽  
Low Level ◽  
Zonal Wavenumber ◽  
Upper Level

Abstract The three-dimensional structure and dynamics of the climatological-mean summertime subtropical anticyclones in the Southern Hemisphere (SH) are investigated. As in the Northern Hemisphere (NH), each of the surface subtropical anticyclones over the South Pacific, South Atlantic, and South Indian Oceans is accompanied by a meridional vorticity dipole aloft, exhibiting barotropic and baroclinic structures in its poleward and equatorward portions, respectively, in a manner that is dynamically consistent with the observed midtropospheric subsidence. Their dynamics are also similar to their NH counterpart. It is demonstrated through the numerical experiments presented here that each of the SH surface anticyclones observed over the relatively cool eastern oceans can be reproduced as a response to a local near-surface cooling–heating couplet. The cooling is mainly due to radiative cooling associated with low-level maritime clouds, and the heating to the east is due to sensible heat flux over the dry, heated continental surface. The low-level clouds act to maintain the coolness of the underlying ocean surface, which is also maintained by the alongshore surface southerlies. As in the NH, the presence of a local atmosphere–ocean–land feedback loop is thus suggested, in which the summertime subtropical anticyclones and continental cyclones to their east are involved. Both the model experiments conducted here and the diagnosed upward flux of Rossby wave activity suggest that, in addition to continental deep convective heating, the land–sea heating–cooling contrasts across the west coasts of the three continents can contribute to the formation of the summertime upper-level planetary wave pattern observed in the entire subtropical SH, characterized by the zonal wavenumber-3 component. Though rather subtle, there are some interhemispheric differences in the summertime subtropical anticyclones, including their smaller magnitudes in the SH and the stronger equatorward propagation of upper-level Rossby wave activity emanating from the SH surface anticyclones.

The Non-Gaussianity and Spatial Asymmetry of Temperature Extremes Relative to the Storm Track: The Role of Horizontal Advection

2017 ◽  
Vol 30 (2) ◽  
pp. 445-464 ◽  
Author(s):  
Chaim I. Garfinkel ◽  
Nili Harnik
Keyword(s):  
General Circulation ◽  
Extreme Temperature ◽  
Storm Track ◽  
Reanalysis Data ◽  
Temperature Extremes ◽  
Tropospheric Temperature ◽  
Positive Temperature ◽  
The West ◽  
Near Surface ◽  
Temperature Advection

The distribution of near-surface and tropospheric temperature variability in midlatitudes is distinguishable from a Gaussian in meteorological reanalysis data; consistent with this, warm extremes occur preferentially poleward of the location of cold extremes. To understand the factors that drive this non-Gaussianity, a dry general circulation model and a simple model of Lagrangian temperature advection are used to investigate the connections between dynamical processes and the occurrence of extreme temperature events near the surface. The non-Gaussianity evident in reanalysis data is evident in the dry model experiments, and the location of extremes is influenced by the location of the jet stream and storm track. The cause of this in the model can be traced back to the synoptic evolution within the storm track leading up to cold and warm extreme events: negative temperature extremes occur when an equatorward propagating high–low couplet (high to the west) strongly advects isotherms equatorward over a large meridional fetch over more than two days. Positive temperature anomalies occur when a poleward propagating low–high couplet (low to the west) advects isotherms poleward over a large meridional fetch over more than two days. The magnitude of the extremes is enhanced by the meridional movement of the systems. Overall, horizontal temperature advection by storm track systems can account for the warm/cold asymmetry in the latitudinal distribution of the temperature extremes.

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