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Brian J. Hoskins - One of the best experts on this subject based on the ideXlab platform.
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Subtropical Anticyclones and Summer Monsoons
Journal of Climate, 2001Co-Authors: Mark J. Rodwell, Brian J. HoskinsAbstract:The summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and Anticyclones over the oceans. In winter, the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography. Here, the mechanics of the summer and winter lowertropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations. By prescribing in the model the heatings associated with several of the world’s monsoons, it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the Kelvin wave response to the monsoon heating over the continent to the west. A poleward-flowing low-level jet into a monsoon (such as the Great Plains jet) is required for Sverdrup vorticity balance. This jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region. The low-level jet into North America induced by its monsoon heating is augmented by a remote response to the Asian monsoon heating. The Rossby wave response to the west of subtropical monsoon heating, interacting with the midlatitude westerlies, produces a region of adiabatic descent. It is demonstrated here that a local ‘‘diabatic enhancement’’ can lead to a strengthening of the descent. Longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region. Below the descent, Sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through Ekman transport. Feedbacks, involving, for example, sea surface temperatures, may further enhance the descent in these regions. The conclusion is that the Mediterranean-type climates of regions such as California and Chile may be induced remotely by the monsoon to the east. Hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems, each involving monsoon rains, a low-level poleward jet, a subtropical anticyclone to the east, and descent and equatorward flow to the west. In winter, it is demonstrated how the nonlinear interaction between the strong zonal-mean circulation, associated with the winter ‘‘Hadley cell,’’ and the mountains can define many of the large-scale features of the subtropical circulation. The blocking effect of the longitudinal mountain chains is shown to be very important. Subsequent diabatic effects, such as a local diabatic enhancement, would appear to be essential for producing the observed amplitude of these features.
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On the existence and strength of the summer subtropical Anticyclones : Bernhard Haurwitz memorial lecture
Bulletin of the American Meteorological Society, 1996Co-Authors: Brian J. HoskinsAbstract:The subtropical Anticyclones are usually related to radiative cooling and the descending arm of the Hadley Cell. This explanation works well for the winter anticyclone belt. However, the summer subtropical Anticyclones are in most measures stronger, as well as being more longitudinally localized, than their winter counterparts. The nature of the circulations in the summer and winter subtropics, and the Tropics are compared. It is hypothesized that the basic cause of the summer Anticyclones is the monsoonal latent heat release over the neighboring continents to the east. As this diabatic heating pushes poleward to near 25° latitude, so deep descent is induced poleward and westward of it. Orographic effects can act to localize this descent. Enhanced radiative cooling and suppressed convection are then viewed as amplifiers of the descent. Under this descent is the equatorward moving air of an anticyclone, which is centered to the west. This wind acts to drive oceanic upwelling, leading to cold sea surface temperatures and reinforcement of the suppression of convection.
Alexander Gershunov - One of the best experts on this subject based on the ideXlab platform.
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Pacific cyclonic and anticyclonic transients in a global warming context: possible consequences for Western North American daily precipitation and temperature extremes
Climate Dynamics, 2008Co-Authors: Alice Favre, Alexander GershunovAbstract:Trajectories of surface cyclones and Anticyclones were constructed using an automated scheme by tracking local minima and maxima of mean daily sea level pressure data in the NCEP-NCAR reanalysis and the Centre National de Recherches Météorologiques coupled global climate Model (CNRM-CM3) SRES A2 integration. Mid-latitude lows and highs traveling in the North Pacific were tracked and daily frequencies were gridded. Transient activity in the CNRM-CM3 historical simulation (1950–1999) was validated against reanalysis. The GCM correctly reproduces winter trajectories as well as mean geographical distributions of cyclones and Anticyclones over the North Pacific in spite of a general under-estimation of cyclones' frequency. On inter-annual time scales, frequencies of cyclones and Anticyclones vary in accordance with the Aleutian Low (AL) strength. When the AL is stronger (weaker), cyclones are more (less) numerous over the central and eastern North Pacific, while Anticyclones are significantly less (more) numerous over this region. The action of transient cyclones and Anticyclones over the central and eastern North Pacific determines seasonal climate over the West Coast of North America, and specifically, winter weather over California. Relationships between winter cyclone/anticyclone behavior and daily precipitation/cold temperature extremes over Western North America (the West) were examined and yielded two simple indices summarizing North Pacific transient activity relevant to regional climates. These indices are strongly related to the observed inter-annual variability of daily precipitation and cold temperature extremes over the West as well as to large scale seasonally averaged near surface climate conditions (e.g., air temperature at 2 m and wind at 10 m). In fact, they represent the synoptic links that accomplish the teleconnections. Comparison of patterns derived from NCEP-NCAR and CNRM-CM3 revealed that the model reproduces links between cyclone/anticyclone frequencies over the Northeastern Pacific and extra-tropical climate conditions but is deficient in relation to tropical climate variability. The connections between these synoptic indices and Western weather are well reproduced by the model. Under advanced global warming conditions, that is, the last half of the century, the model predicts a significant reduction of cyclonic transients throughout the mid-latitude North Pacific with the exception of the far northern and northeastern domains. Anticyclonic transients respond somewhat more regionally but consistently to strong greenhouse forcing, with notably fewer Anticyclones over the Okhotsk/Kamchatka sector and generally more Anticyclones in the Northeastern Pacific. These modifications of synoptic weather result in regional feedbacks, that is, regional synoptic alterations of the anthropogenic warming signal around the North Pacific. In the eastern Pacific, for example, synoptic feedbacks, having to do especially with the northward shift of the eastern Pacific storm-track (responding, in turn, to a weaker equator-to-pole temperature gradient), are favorable to more anticyclonic conditions off the American mid-latitude west coast and more cyclonic conditions at higher latitudes. These circulation feedbacks further reduce the equator-to-pole temperature gradient by favoring high-latitude mean winter warming especially over a broad wedge of the Arctic north of the Bering Sea and moderating the warming along the mid-latitude west coast of north America while also reducing precipitation frequencies from California to Northern Mexico.
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Extra-Tropical cyclonic/anticyclonic activity in North-Eastern Pacific and air temperature extremes in Western North America
Climate Dynamics, 2006Co-Authors: Alice Favre, Alexander GershunovAbstract:Synoptic extra-tropical cyclone and anticyclone trajectories have been constructed from mean daily sea level pressure (SLP) data using a new automated scheme. Frequency, intensity and trajectory characteristics of these transients have been summarized to form indices describing wintertime cyclonic and anticyclonic activity over the North-Eastern Pacific (east of 170°W) during 1950 2001. During this period, the strength of Anticyclones gradually diminished and their frequency became more variable, while cyclones intensified in a discrete shift with deeper lows and further southerly trajectories occurring since the mid-1970s. These changes in synoptic transients translate into anomalously low seasonal mean SLP in the Aleutian Low, a low-level circulation anomaly consistent with the positive phase of the North Pacific Decadal Oscillation, with positive sea surface temperature (SST) anomalies along the west coast of North America and negative in the central North Pacific Ocean. A link between cyclonic/anticyclonic activity and tropical SST anomalies also exists, but this link only becomes significant after the mid-1970s, a period that coincides with more southerly cyclone trajectories. Southward excursions of mid-latitude cyclones during El Niño/positive NPO winters accomplish the northward advection of tropical air and discourage the southward penetration of polar air masses associated with transient Anticyclones. Naturally, these changes in cyclonic/anticyclonic activity directly impact surface air temperatures, especially at night. We document these profound impacts on observed wintertime minimum temperatures over Western North America.
J. David Neelin - One of the best experts on this subject based on the ideXlab platform.
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The Northern Monsoons and the Southern Subtropical Anticyclones
2014Co-Authors: C. Roberto Mechoso, Sang-ki Lee, Chunzai Wang, J. David NeelinAbstract:The reasons for the different seasonal evolution of subtropical highs in the southern and northern hemisphere differs are demonstrated. In the northern hemisphere, the subtropical Anticyclones are stronger and better defined in the boreal summer than in winter. By contrast, in the southern hemisphere the subtropical Anticyclones are notably stronger in the austral winter than in summer, particularly over the Atlantic and Indian Oceans. It is shown that this seasonal asymmetry of the climate system is due to the seasonal asymmetry in the interhemispheric effects of the summer monsoons. The intensification of the northern Anticyclones in the boreal summer is consistent with the “monsoon heating” paradigm, according to which heating over continents generates a Rossby wave response and adiabatic descent over the oceanic regions to the west. The summer heating over northern continents also produces subsidence over the tropical southern hemisphere via an interhemispheric meridional overturning circulation. Suppressed convection and diabatic cooling over these regions of subsidence generate stationary barotropic Rossby waves that propagate beyond the tropics enhancing the subtropical highs. The interhemispheric effects of summer southern monsoons are much weaker. Our methodology for research is based on performing AGCM runs in which a control simulation is compared to an idealized experiment with artificially weakened summer monsoons in the northern hemisphere. This weakening is achieved by synchronizing the seasonal cycles in the model’s external and boundary conditions across hemispheres, i.e. by shifting in the northern hemisphere only both the insolation at the top of the atmosphere and the sea surface temperatures and sea ice cover by one-half the seasonal cycle (6 months). We use the US National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 4 (CAM4). According to the results, the interhemispheric response to the northern summer monsoons is most dramatic in the South Pacific, where the subtropical anticyclone nearly disappears in the austral winter without the influence from the northern summer hemisphere.
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Interhemispheric Influence of the Northern Summer Monsoons on Southern Subtropical Anticyclones
Journal of Climate, 2013Co-Authors: Sang-ki Lee, C. Roberto Mechoso, Chunzai Wang, J. David NeelinAbstract:AbstractThe southern subtropical Anticyclones are notably stronger in austral winter than summer, particularly over the Atlantic and Indian Ocean basins. This is in contrast with the Northern Hemisphere (NH), in which subtropical Anticyclones are more intense in summer according to the “monsoon heating” paradigm. To better understand the winter intensification of southern subtropical Anticyclones, the present study explores the interhemispheric response to monsoon heating in the NH during austral winter. A specially designed suite of numerical model experiments is performed in which summer monsoons in the NH are artificially weakened. These experiments are performed with both an atmospheric general circulation model and a simple two-layer model. The highlight of the findings presented here is that during the boreal summer enhanced tropical convection activity in the NH plays important roles in either maintaining or strengthening the southern subtropical Anticyclones. Enhanced NH convection largely associa...
Alice Favre - One of the best experts on this subject based on the ideXlab platform.
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Pacific cyclonic and anticyclonic transients in a global warming context: possible consequences for Western North American daily precipitation and temperature extremes
Climate Dynamics, 2008Co-Authors: Alice Favre, Alexander GershunovAbstract:Trajectories of surface cyclones and Anticyclones were constructed using an automated scheme by tracking local minima and maxima of mean daily sea level pressure data in the NCEP-NCAR reanalysis and the Centre National de Recherches Météorologiques coupled global climate Model (CNRM-CM3) SRES A2 integration. Mid-latitude lows and highs traveling in the North Pacific were tracked and daily frequencies were gridded. Transient activity in the CNRM-CM3 historical simulation (1950–1999) was validated against reanalysis. The GCM correctly reproduces winter trajectories as well as mean geographical distributions of cyclones and Anticyclones over the North Pacific in spite of a general under-estimation of cyclones' frequency. On inter-annual time scales, frequencies of cyclones and Anticyclones vary in accordance with the Aleutian Low (AL) strength. When the AL is stronger (weaker), cyclones are more (less) numerous over the central and eastern North Pacific, while Anticyclones are significantly less (more) numerous over this region. The action of transient cyclones and Anticyclones over the central and eastern North Pacific determines seasonal climate over the West Coast of North America, and specifically, winter weather over California. Relationships between winter cyclone/anticyclone behavior and daily precipitation/cold temperature extremes over Western North America (the West) were examined and yielded two simple indices summarizing North Pacific transient activity relevant to regional climates. These indices are strongly related to the observed inter-annual variability of daily precipitation and cold temperature extremes over the West as well as to large scale seasonally averaged near surface climate conditions (e.g., air temperature at 2 m and wind at 10 m). In fact, they represent the synoptic links that accomplish the teleconnections. Comparison of patterns derived from NCEP-NCAR and CNRM-CM3 revealed that the model reproduces links between cyclone/anticyclone frequencies over the Northeastern Pacific and extra-tropical climate conditions but is deficient in relation to tropical climate variability. The connections between these synoptic indices and Western weather are well reproduced by the model. Under advanced global warming conditions, that is, the last half of the century, the model predicts a significant reduction of cyclonic transients throughout the mid-latitude North Pacific with the exception of the far northern and northeastern domains. Anticyclonic transients respond somewhat more regionally but consistently to strong greenhouse forcing, with notably fewer Anticyclones over the Okhotsk/Kamchatka sector and generally more Anticyclones in the Northeastern Pacific. These modifications of synoptic weather result in regional feedbacks, that is, regional synoptic alterations of the anthropogenic warming signal around the North Pacific. In the eastern Pacific, for example, synoptic feedbacks, having to do especially with the northward shift of the eastern Pacific storm-track (responding, in turn, to a weaker equator-to-pole temperature gradient), are favorable to more anticyclonic conditions off the American mid-latitude west coast and more cyclonic conditions at higher latitudes. These circulation feedbacks further reduce the equator-to-pole temperature gradient by favoring high-latitude mean winter warming especially over a broad wedge of the Arctic north of the Bering Sea and moderating the warming along the mid-latitude west coast of north America while also reducing precipitation frequencies from California to Northern Mexico.
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Extra-Tropical cyclonic/anticyclonic activity in North-Eastern Pacific and air temperature extremes in Western North America
Climate Dynamics, 2006Co-Authors: Alice Favre, Alexander GershunovAbstract:Synoptic extra-tropical cyclone and anticyclone trajectories have been constructed from mean daily sea level pressure (SLP) data using a new automated scheme. Frequency, intensity and trajectory characteristics of these transients have been summarized to form indices describing wintertime cyclonic and anticyclonic activity over the North-Eastern Pacific (east of 170°W) during 1950 2001. During this period, the strength of Anticyclones gradually diminished and their frequency became more variable, while cyclones intensified in a discrete shift with deeper lows and further southerly trajectories occurring since the mid-1970s. These changes in synoptic transients translate into anomalously low seasonal mean SLP in the Aleutian Low, a low-level circulation anomaly consistent with the positive phase of the North Pacific Decadal Oscillation, with positive sea surface temperature (SST) anomalies along the west coast of North America and negative in the central North Pacific Ocean. A link between cyclonic/anticyclonic activity and tropical SST anomalies also exists, but this link only becomes significant after the mid-1970s, a period that coincides with more southerly cyclone trajectories. Southward excursions of mid-latitude cyclones during El Niño/positive NPO winters accomplish the northward advection of tropical air and discourage the southward penetration of polar air masses associated with transient Anticyclones. Naturally, these changes in cyclonic/anticyclonic activity directly impact surface air temperatures, especially at night. We document these profound impacts on observed wintertime minimum temperatures over Western North America.
Alexandre Bernardes Pezza - One of the best experts on this subject based on the ideXlab platform.
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severe heat waves in southern australia synoptic climatology and large scale connections
Climate Dynamics, 2012Co-Authors: Alexandre Bernardes Pezza, Peter Van Rensch, Wenju CaiAbstract:This paper brings a new perspective on the large scale dynamics of severe heat wave (HW) events that commonly affect southern Australia. Through an automatic tracking scheme, the cyclones and Anticyclones associated with HWs affecting Melbourne, Adelaide and Perth are tracked at both the surface and upper levels, producing for the first time a synoptic climatology that reveals the broader connections associated with these extreme phenomena. The results show that a couplet (or pressure dipole) formed by transient cyclones and Anticyclones can reinforce the HW similarly to what is observed in cold surges (CS), with an obvious opposite polarity. Our results show that there is a large degree of mobility in the synoptic signature associated with the passage of the upper level ridges before they reach Australia and the blocking is established, with HW-associated surface Anticyclones often initiating over the west Indian Ocean and decaying in the eastern Pacific. In contrast to this result the 500 hPa anticyclone tracks show a very small degree of mobility, responding to the dominance of the upper level blocking ridge. An important feature of HWs is that most of the cyclones are formed inland in association with heat troughs, while in CS the cyclones are typically maritime (often explosive), associated with a strong cold front. Hence the influence of the cyclone is indirect, contributing to reinforce the blocking ridge through hot and dry advection on the ridge’s western flank. Additional insights are drawn for the record Adelaide case of March 2008 with fifteen consecutive days above 35°C breaking the previous record by 7 days. Sea surface temperatures suggest a significant air-sea interaction mechanism, with a broad increase in the meridional temperature gradient over the Indian Ocean amplifying the upstream Rossby waves that can trigger HW events. A robust cooling of the waters close to the Australian coast also contributes to the maintenance of the blocking highs locally, which is a fundamental ingredient to sustain the HWs.
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southern hemisphere cyclones and Anticyclones recent trends and links with decadal variability in the pacific ocean
International Journal of Climatology, 2007Co-Authors: Alexandre Bernardes Pezza, Ian Simmonds, James A RenwickAbstract:The aim of this paper is to study the association between the extratropical Southern Hemisphere and the decadal variability in the Pacific Ocean (PO). We discuss a pattern of coherent large-scale anomalies and trends in cyclone and anticyclone behaviour in light of the climate variability in the PO over the ERA40 reanalysis period (1957–2002). The two representative PO indices are the Pacific Decadal and Interdecadal Oscillations (PDO and IPO), and here the PDO is chosen owing to it being less associated with the southern oscillation index (SOI). Composites of the indicators of the density and intensity of cyclones/Anticyclones given by an automatic tracking scheme were calculated for the years when the PDOI was more than one standard deviation above or below its mean. Although the ERA40 is not free from noise and assimilation changes, the results show a large-scale feature, which seems to be robust and agrees with earlier studies using different data sets. The sea-level pressure shows a strong annular structure related to the PDO, which is not seen for the SOI, with lower pressure around Antarctica during the positive phase and vice versa. More intense (and fewer) cyclones and Anticyclones were observed during the positive PDO. This is less consistent for the SOI, particularly during the summer when a different PDO/SOI pattern arises at high latitudes. The trends project a pattern coincident with the positive PDO phase and seem to be linked with the main climate shift in the late seventies. Trends observed over the Tasman Sea are consistent with declining winter rainfall over southeastern Australia. Most patterns are statistically significant and seem robust, but random changes in ENSO may play a part, to a certain degree, in modulating the results, and a physical mechanism of causality has not been demonstrated. Although global warming and related changes in the Southern Annular Mode (SAM) may also help explain the observed behaviour, the large-scale response presented here provides a new insight and would be of considerable interest for further modelling studies. Copyright © 2007 Royal Meteorological Society
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Variability of Southern Hemisphere cyclone and anticyclone behavior: Further analysis
Journal of Climate, 2003Co-Authors: Alexandre Bernardes Pezza, Tércio AmbrizziAbstract:Abstract This paper presents some additional results on the use of an automatic scheme for tracking surface cyclones and Anticyclones. The Southern Hemisphere (SH) total amount of synoptic tracks (every 12 h) was analyzed for the 1973–96 period using sea level pressure from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis. Composites for seven El Nino (EN) and La Nina (LN) years were constructed in order to analyze the association between the hemispheric cyclone and anticyclone propagation and the phase of the El Nino–Southern Oscillation (ENSO) phenomenon. A climatological view of cyclone and anticyclone tracks and orphan centers superposed on the same map is presented and analyzed. A large area with overlapped cyclone and anticyclone tracks is seen between 30° and 60°S, which is approximately the climatological position of the SH transient activity. To the north of 30°S, the subtropical South Atlantic high is embedded in a region with just...