scholarly journals Role of Cross-Equatorial Waves in Maintaining Long Periods of Low Convective Activity over Southern Africa

2015 ◽  
Vol 72 (2) ◽  
pp. 682-692 ◽  
Author(s):  
J. V. Ratnam ◽  
Swadhin K. Behera ◽  
Toshio Yamagata
Keyword(s):  
Northern Hemisphere ◽  
Southern Africa ◽  
Wave Train ◽  
Longwave Radiation ◽  
Convective Activity ◽  
Weather Forecasts ◽  
Medium Range ◽  
Tropical Temperate Trough ◽  
Upper Level

Abstract Periods of low convective activity over southern Africa during the peak rainy season from December to February are known to be due to the northeastward displacement of the tropical temperate trough (TTT) systems from the landmass. In this study, using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, the authors show that the displacement of the TTT systems during long periods of low convective activity has origins in the Northern Hemisphere. Using standardized area-averaged outgoing longwave radiation (OLR) daily anomalies over southern Africa, long periods of low convective activity are defined as periods of positive OLR anomalies lasting consecutively for 5 or more days with a standard deviation of 1 or more. An eddy streamfunction anomaly composite of the periods of low convective activity shows an upper-level anomalous wave originating in the Northern Hemisphere and extending to southern Africa from the eastern Pacific and displacing the tropical–extratropical cloud bands from the southern African landmass into the southwestern Indian Ocean. The wave train is also seen to generate an anticyclonic anomaly over southern Africa, resulting in suppressed convective activity. Understanding the causes of the long periods of low convective activity will help in improving their predictability and also the predictability of seasonal rainfall over southern Africa.

scholarly journals The Global Distribution of Atmospheric Eddy Length Scales

2012 ◽  
Vol 25 (9) ◽  
pp. 3409-3416 ◽  
Author(s):  
Elizabeth A. Barnes ◽  
Dennis L. Hartmann
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Seasonal Cycle ◽  
Winter Season ◽  
Global Distribution ◽  
Correlation Lengths ◽  
Weather Forecasts ◽  
European Centre ◽  
Medium Range ◽  
Upper Level

The correlation lengths of vorticity anomalies from temporal averages are examined in the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis dataset. It is shown that, in the annual mean, eddies in the Southern Hemisphere are significantly larger than those in the Northern Hemisphere. The eddy vorticity lengths exhibit a strong seasonal cycle, with the largest scales occurring in the winter season. The maximum zonal eddy lengths closely follow the contours of the strong upper-level winds, while the maximum meridional lengths are found in jet exit regions and in the stratosphere.

Four Years of Tropical ERA-40 Vorticity Maxima Tracks. Part I: Climatology and Vertical Vorticity Structure

2008 ◽  
Vol 136 (11) ◽  
pp. 4301-4319 ◽  
Author(s):  
Brandon Kerns ◽  
Kantave Greene ◽  
Edward Zipser
Keyword(s):  
Large Scale ◽  
Easterly Waves ◽  
Weather Forecasts ◽  
East Pacific ◽  
Cloud Clusters ◽  
Geographical Separation ◽  
Medium Range ◽  
Strong Gradient ◽  
Upper Level ◽  
Vorticity Structure

Abstract Using the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), vorticity maxima (VM) have been manually tracked and classified as developing and nondeveloping. The VM are identified on Hovmöller plots for June–October 1998–2001, within 0°–35°N, 140°–10°W. Over 600 low-level and midlevel VM are tracked. The ERA-40 VM track climatology compares favorably with previous knowledge about easterly waves. Some new results have also been found. The VM are not equivalent to easterly waves, so it is important to distinguish between the large-scale wave and the embedded VM. Unlike waves, individual VM leaving Africa generally do not survive to cross the entire Atlantic. Unlike waves, which can cross Central America, most individual east Pacific VM originate in the east Pacific. Genesis productivity is defined as the fraction of nontropical cyclone VM that eventually develop. It reaches 50% in the eastern North Pacific (EPAC) and 30% in the Atlantic, where there is geographical separation between the locations of maximum nondeveloping and pregenesis track density. There is a strong gradient in daily genesis potential (DGP) near 10°N, associated with weaker upper-level anticyclonic vorticity equatorward of 10°N. The maximum genesis productivity is obtained north of 10°N, where the upper-anticyclonic vorticity and DGP are higher. Finally, there is no obvious distinction in VM strength between developing VM prior to genesis and nondeveloping VM. A major factor is the minimum vorticity threshold for VM as opposed to cloud clusters.

Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby Wave Energy Dispersion*

2010 ◽  
Vol 138 (1) ◽  
pp. 42-54 ◽  
Author(s):  
Xuyang Ge ◽  
Tim Li ◽  
Melinda S. Peng
Keyword(s):  
Rossby Wave ◽  
Wave Energy ◽  
Large Scale ◽  
Wave Train ◽  
Energy Dispersion ◽  
Sensitivity Experiment ◽  
Low Level ◽  
Filtering Technique ◽  
Upper Level

Abstract The genesis of Typhoon Prapiroon (2000), in the western North Pacific, is simulated to understand the role of Rossby wave energy dispersion of a preexisting tropical cyclone (TC) in the subsequent genesis event. Two experiments are conducted. In the control experiment (CTL), the authors retain both the previous typhoon, Typhoon Bilis, and its wave train in the initial condition. In the sensitivity experiment (EXP), the circulation of Typhoon Bilis was removed based on a spatial filtering technique of Kurihara et al., while the wave train in the wake is kept. The comparison between these two numerical simulations demonstrates that the preexisting TC impacts the subsequent TC genesis through both a direct and an indirect process. The direct process is through the conventional barotropic Rossby wave energy dispersion, which enhances the low-level wave train, the boundary layer convergence, and the convection–circulation feedback. The indirect process is through the upper-level outflow jet. The asymmetric outflow jet induces a secondary circulation with a strong divergence tendency to the left-exit side of the outflow jet. The upper-level divergence boosts large-scale ascending motion and promotes favorable environmental conditions for a TC-scale vortex development. In addition, the outflow jet induces a well-organized cyclonic eddy angular momentum flux, which acts as a momentum forcing that enhances the upper-level outflow and low-level inflow and favors the growth of the new TC.

scholarly journals Seasonal Modulations of the Active MJO Cycle Characterized by Nonlinear Principal Component Analysis

2011 ◽  
Vol 139 (7) ◽  
pp. 2259-2275 ◽  
Author(s):  
Johannes Jenkner ◽  
William W. Hsieh ◽  
Alex J. Cannon
Keyword(s):  
Longwave Radiation ◽  
Principal Component ◽  
Boreal Winter ◽  
Convective Activity ◽  
Marginal Contributions ◽  
The Indian Ocean ◽  
The Mean ◽  
The Difference ◽  
Upper Level ◽  
Mean Cycle

Abstract A novel methodology is presented for the identification of the mean cycle of the Madden–Julian oscillation (MJO) along the equator. The methodology is based on a nonlinear principal component (NLPC) computed with a neural network model. The bandpass-filtered input data encompass 30 yr with zonal winds at 850 and 200 hPa plus outgoing longwave radiation (OLR). The NLPC is conditioned on a sufficiently strong MJO activity and is computed both for the pooled dataset and for the dataset stratified into seasons. The NLPC for all data depicts a circular mode formed by the first two linear principal components (LPCs) with marginal contributions by the higher-order LPCs. Hence, the mean MJO cycle throughout the year is effectively captured by the amplitude of the leading two LPCs varying in quadrature. The NLPC for individual seasons shows additional variability, which mainly arises from a subordinate oscillation of the second pair of LPCs superimposed on the annual MJO signal. In reference to the all-year solution, the difference in resolved variability approximately accounts for 9% in solstitial seasons and 3% in equinoctial seasons. The phasing of the third LPC is such that convective activity oscillations over the Maritime Continent as well as wind oscillations over the Indian Ocean appear enhanced (suppressed) during boreal winter (summer). Also, convective activity oscillations appear more pronounced at the date line during both winter and summer. The phasing of the fourth LPC is such that upper-level westerlies over the Atlantic region are more persistent during boreal spring than during other seasons.

scholarly journals The role of convective overshooting clouds in tropical stratosphere–troposphere dynamical coupling

2014 ◽  
Vol 14 (16) ◽  
pp. 23745-23761 ◽  
Author(s):  
K. Kodera ◽  
B. M. Funatsu ◽  
C. Claud ◽  
N. Eguchi
Keyword(s):  
Deep Convection ◽  
Correlation Coefficients ◽  
Longwave Radiation ◽  
Convective Activity ◽  
Convective Clouds ◽  
Circulation Change ◽  
Tropospheric Circulation ◽  
Dynamical Coupling ◽  
The Tropics

Abstract. This paper investigates the role of deep convection and overshooting convective clouds in stratosphere–troposphere dynamical coupling in the tropics during two large major stratospheric sudden warming events in January 2009 and January 2010. During both events, convective activity and precipitation increased in the equatorial Southern Hemisphere as a result of a strengthening of the Brewer–Dobson circulation induced by enhanced stratospheric planetary wave activity. Correlation coefficients between variables related to the convective activity and the vertical velocity were calculated to identify the processes connecting stratospheric variability to the troposphere. Convective overshooting clouds showed a direct relationship to lower stratospheric upwelling at around 70–50 hPa. As the tropospheric circulation change lags behind that of the stratosphere, outgoing longwave radiation shows almost no simultaneous correlation with the stratospheric upwelling. This result suggests that the stratospheric circulation change first penetrates into the troposphere through the modulation of deep convective activity.

scholarly journals The Role of Warm Conveyor Belts for the Intensification of Extratropical Cyclones in Northern Hemisphere Winter

2016 ◽  
Vol 73 (10) ◽  
pp. 3997-4020 ◽  
Author(s):  
Hanin Binder ◽  
Maxi Boettcher ◽  
Hanna Joos ◽  
Heini Wernli
Keyword(s):  
Northern Hemisphere ◽  
Extratropical Cyclones ◽  
Cyclone Center ◽  
Explosive Cyclones ◽  
Conveyor Belts ◽  
Upper Level ◽  
Hemisphere Winter ◽  
Warm Conveyor Belts ◽  
Northern Hemisphere Winter

Abstract The role of warm conveyor belts (WCBs) and their associated positive low-level potential vorticity (PV) anomalies are investigated for extratropical cyclones in Northern Hemisphere winter, using ERA-Interim and composite techniques. The Spearman correlation coefficient of 0.68 implies a moderate to strong correlation between cyclone intensification and WCB strength. Hereby, cyclone intensification is quantified by the normalized maximum 24-h central sea level pressure deepening and WCB strength by the WCB air mass associated with the cyclone’s 24-h period of strongest deepening. Explosively intensifying cyclones typically have strong WCBs and pronounced WCB-related PV production in the cyclone center; they are associated with a WCB of type W2, which ascends close to the cyclone center. Cyclones with similar WCB strength but weak intensification are either diabatic Rossby waves, which do not interact with an upper-level disturbance, or cyclones where much of the WCB-related PV production occurs far from the cyclone center and thereby does not contribute strongly to cyclone deepening (WCB of type W1, which ascends mainly along the cold front). The category of explosively intensifying cyclones with weak WCBs is inhomogeneous but often characterized by a very low tropopause or latent heating independent of WCBs. These findings reveal that (i) diabatic PV production in WCBs is essential for the intensification of many explosive cyclones, (ii) the importance of WCBs for cyclone development strongly depends on the location of the PV production relative to the cyclone center, and (iii) a minority of explosive cyclones is not associated with WCBs.

scholarly journals The role of convective overshooting clouds in tropical stratosphere–troposphere dynamical coupling

2015 ◽  
Vol 15 (12) ◽  
pp. 6767-6774 ◽  
Author(s):  
K. Kodera ◽  
B. M. Funatsu ◽  
C. Claud ◽  
N. Eguchi
Keyword(s):  
Deep Convection ◽  
Correlation Coefficients ◽  
Longwave Radiation ◽  
Convective Activity ◽  
Convective Clouds ◽  
Circulation Change ◽  
Tropospheric Circulation ◽  
Dynamical Coupling ◽  
The Tropics

Abstract. This paper investigates the role of deep convection and overshooting convective clouds in stratosphere–troposphere dynamical coupling in the tropics during two large major stratospheric sudden warming events in January 2009 and January 2010. During both events, convective activity and precipitation increased in the equatorial Southern Hemisphere as a result of a strengthening of the Brewer–Dobson circulation induced by enhanced stratospheric planetary wave activity. Correlation coefficients between variables related to the convective activity and the vertical velocity were calculated to identify the processes connecting stratospheric variability to the troposphere. Convective overshooting clouds showed a direct relationship to lower stratospheric upwelling at around 70–50 hPa. As the tropospheric circulation change lags behind that of the stratosphere, outgoing longwave radiation shows almost no simultaneous correlation with the stratospheric upwelling. This result suggests that the stratospheric circulation change first penetrates into the troposphere through the modulation of deep convective activity.

Skill of the MJO and Northern Hemisphere Blocking in GEFS Medium-Range Reforecasts

2014 ◽  
Vol 142 (2) ◽  
pp. 868-885 ◽  
Author(s):  
Thomas M. Hamill ◽  
George N. Kiladis
Keyword(s):  
Northern Hemisphere ◽  
Lead Time ◽  
Decadal Variability ◽  
Longwave Radiation ◽  
Lead Times ◽  
Atlantic Sector ◽  
Tropical Precipitation ◽  
Predictive Skill ◽  
Perfect Model ◽  
Medium Range

Abstract Forecast characteristics of Northern Hemisphere atmospheric blocking and the Madden–Julian oscillation (MJO) were diagnosed using an extensive time series (December–February 1985–2012) of daily medium-range ensemble reforecasts based on a version of the NCEP Global Ensemble Forecast System (GEFS). For blocking, (i) interannual variability of analyzed blocking frequency was quite large, (ii) the GEFS slightly underforecasted blocking frequency at longer leads in the Euro-Atlantic sector, (iii) predictive skill of actual blocking was substantially smaller than its perfect-model skill, (iv) block onset and cessation were forecast less well than overall blocking frequency, (v) there was substantial variability of blocking skill between half-decadal periods, and (vi) the reliability of probabilistic blocking forecasts degraded with increasing lead time. For the MJO, (i) forecasts of strong Indian Ocean MJOs propagated too slowly, especially the component associated with outgoing longwave radiation (OLR), that is, convection; (ii) tropical precipitation was greatly overforecast at early lead times; (iii) the ensemble predictions were biased and/or underdispersive, manifested in U-shaped rank histograms of MJO indices (magnitude forecasts were especially U shaped); (iv) MJO correlation skill was larger for its wind than for its OLR component, and was larger for the higher-amplitude MJO events; (v) there was some half-decadal variability in skill; and (vi) probabilistic skill of the MJO forecast was modest, and skill was larger when measured relative to climatology than when measured relative to a lagged persistence forecast. For longer-lead forecasts, the GEFS demonstrated little ability to replicate the changes in blocking frequency due to a strong MJO that were noted in analyzed data.

scholarly journals Potential Vorticity Anomalies of the Lowermost Stratosphere: A 10-Yr Winter Climatology

2010 ◽  
Vol 138 (4) ◽  
pp. 1234-1249 ◽  
Author(s):  
Sarah F. Kew ◽  
Michael Sprenger ◽  
Huw C. Davies
Keyword(s):  
Life Cycle ◽  
Spatial Structure ◽  
Northern Hemisphere ◽  
Potential Vorticity ◽  
Weather Forecasts ◽  
European Centre ◽  
Medium Range ◽  
High Topography ◽  
Ubiquitous Presence ◽  
Shed Light

Abstract Inspection of the potential vorticity (PV) distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive PV anomalies. To examine the space–time characteristics of these anomalies, a combined “identification and tracking” tool is developed that can catalog each individual anomaly’s effective amplitude, location, overall spatial structure, and movement from genesis to lysis. A 10-yr winter climatology of such anomalies in the Northern Hemisphere is derived for the period 1991–2001 based upon the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The climatology indicates that the anomalies are frequently evident above high topography and in a quasi-annular band at about 70°N, are long lived (days to weeks), and that their effective amplitude is typically 2 PV units (PVU) higher than that of the ambient environment. In addition, the derived climatologies and associated composites pose questions regarding the origin of the anomalies, detail their life cycle, and shed light on their dynamics and role as long-lived precursors of surface cyclogenesis.

Mechanisms of Global Teleconnections Associated with the Asian Summer Monsoon: An Intermediate Model Analysis*

2013 ◽  
Vol 26 (5) ◽  
pp. 1791-1806 ◽  
Author(s):  
Fei Liu ◽  
Bin Wang
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Summer Monsoon ◽  
Rossby Wave ◽  
North Pacific ◽  
Asian Summer Monsoon ◽  
Wave Train ◽  
Westerly Jet ◽  
Asian Summer ◽  
Upper Level

Abstract The Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) are two subsystems of the Asian summer monsoon, and they exhibit different global teleconnection patterns. The enhanced ISM strengthens the South Asian high and Mascarene high, and the WNPSM excites a meridional tripolar wave train in the Northern Hemisphere and affects the Australian high in the Southern Hemisphere. To understand the dynamics behind these global teleconnections, especially the processes responsible for the cross-equatorial teleconnection, an intermediate model, describing a two-level troposphere and a steady planetary boundary layer (PBL), is linearized from the background horizontal wind field. The model results indicate that the ISM heating, located under the strong easterly vertical shear (VS) and close to the westerly jet in the Northern Hemisphere, can excite a barotropic Rossby wave that emanates northwestward and then propagates downstream along the westerly jet. Since the WNPSM heating is far away from the westerly jet over the North Pacific, it only excites a weak Rossby wave train, which cannot explain the meridional tripolar teleconnection associated with the WNPSM. It is found that both the ISM and WNPSM heating excite strong teleconnections in the Southern Hemisphere via an advection mechanism; that is, the background upper-level northerly winds can transport energy across the equator from the Northern Hemisphere summer monsoon to the Southern Hemisphere westerly jet. In addition, the PBL enhances monsoon teleconnections by trapping more energy in the upper troposphere. The elevated maximum monsoon heating also reinforces upper-level perturbations and enhances the teleconnection pattern.

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