Easterly Wind Regimes Research Articles

Quasi-biennial modulation of total ozone distribution from equator to polar regions based on observations

The modulation of total ozone (TOZ) by the quasi-biennial oscillation (QBO) not only at the equator, but also at subtropics, middle and high latitudes has long been known. As well, a large number of studies have shown seasonal synchronization and interhemispheric asymmetry of QBO effects in the TOZ in extratropical regions. However, the proposed mechanisms of seasonality and asymmetry as a consequence of the interaction of the quasi-biennial and annual cycles are not well understood. A great problem in comparing model simulations with observations and, hence, in verifying theoretical assumptions about the mechanism of extratropical ozone QBO, is the continually changing phase relationship between the annual and quasi-biennial cycles. This issue is exacerbated by the fact that, as is commonly believed, the wind QBO period is not constant, but varies irregularly from one cycle to another. In this study the influence of the QBO of equatorial stratospheric wind on the global distribution of the TOZ has been analyzed using the SBUV Merged Ozone Data Set (MOD). The investigation of the QBO effects in the TOZ in the subtropics, middle and high latitudes of both hemispheres is based on accounting for the seasonal regularities of the wind QBO, which leads to a limited number of only certain combinations of the quasi-biennial and annual cycles that can be realized. The smaller variability serves to clarify the nature of changes in extratropical ozone. A detailed analysis of the TOZ data for 1970–2021 showed that the interannual TOZ variability at subtropics, middle and high latitudes depends on the alternation of wind QBO cycles of different scenarios. The key moments of the TOZ variations are clearly associated with certain points in time during the downward propagation of the easterly and westerly QBO wind regimes and the seasonally dependent QBO circulation produced by them. These results contradict the general view that it is difficult to isolate the effect of QBO on the TOZ in extratropics because of the irregularly varying QBO period, and can be used in comparing simulated and observed extratropical ozone QBO, as well as in long-term forecast of variations in the TOZ distribution.

Changes in Frequency and Location of East Coast Low Pressure Systems Affecting Southeast Australia

Low pressure systems off the southeast coast of Australia can generate intense rainfall and associated flooding, destructive winds, and coastal erosion, particularly during the cool season (April–September). Impacts depend on coastal proximity, strength and latitude. Therefore, it is important to investigate changes in frequency, duration, location, and intensity of these systems. First, an existing observation-based database of these low pressure systems, for 1970–2006, is extended to 2019, focusing on April–September and using archived Australian Bureau of Meteorology MSLP charts. Second, data consistency between 1970 and 2006 and 2007 and 2019 is confirmed. Third, permutation testing is performed on differences in means and variances between the two 25-year intervals 1970–1994 and 1995–2019. Additionally, trends in positions, durations and central pressures of the systems are investigated. p-values from permutation tests reveal statistically significant increases in mean low pressure system frequencies. Specifically, a greater frequency of both total days and initial development days only, occurred in the latter period. Statistically significant lower variance for both latitude and longitude in systems that developed in both subtropical easterly and mid-latitude westerly wind regimes indicate a shift south and east in the latter period. Furthermore, statistically significant differences in variance of development location of explosive low pressure systems that develop in a low level easterly wind regime indicate a shift further south and east. These changes are consistent with fewer systems projected to impact the east coast. Finally, important changes are suggested in the large scale atmospheric dynamics of the eastern Australian/Tasman Sea region.

Quasi-Biennial Oscillations in Atmospheric Ozone from the Chemistry-Climate Model and Ozone Reanalysis

The quasi-biennial oscillation is the primary mode of variability of the equatorial mean zonal wind in the lower stratosphere, which is characterized by downward propagating easterly and westerly wind regimes from 10 hPa level with a period approximately 28 months. The effects of the stratospheric quasi-biennial oscillation in zonal winds (SQBO) are not only confined to atmospheric dynamics but also seen in the chemical constituent (trace gases) anomalies such as ozone, water vapor, carbon monoxide and methane in the lower stratosphere. In this study, we examined the SQBO and associated ozone quasi-biennial oscillation (OQBO) using the chemistry-climate model CHASER (MIROC-ESM) simulations and ECMWF ERA-Interim ozone reanalysis for the period 2000-2015. We used lower stratospheric zonal wind from the radiosonde observations and total column ozone (TCO) from Aura Satellite (OMI Instruments) over Singapore to compare the SQBO and OQBO phases with model and reanalysis. The SQBO shows large variations in magnitude and periodicity during the period of study and the amplitude of OQBO also changes in accordance with the westerly (+ve ozone anomaly) and easterly (-ve ozone anomaly) phases of SQBO. During the Westerly phase of Ozone QBO (WQBO) corresponds to average positive total ozone anomaly of ~10 DU and in the Easterly phase of Ozone QBO (EQBO) corresponds to an average negative total ozone anomaly ~−10 DU in the tropical lower stratosphere. Since the SQBO phases were explained by the vertical propagations of Mixed-Ross by Gravity (MRG) waves and Kelvin waves, the correlation of ozone volume mixing ratio with zonal and vertical velocities gives quasi-biennial signals, which indicate the OQBO mechanism more related to dynamical transport than the stratospheric photochemical variations. Since the average amplitude of OQBO phases gives ~+/− 10 DU from the observations during easterly and westerly phases SQBO, we need more research focused on the dynamical transport than the photochemical changes to understand the tropical ozone variability due to the ozone quasi-biennial oscillations.

Seasonal dependence of the quasi-biennial oscillation (QBO): New evidence from IGRA data

The seasonal features of the quasi-biennial oscillation (QBO) of the zonal wind in the equatorial stratosphere were examined using daily data from the Integrated Global Radiosonde Archive (IGRA). Analysis of forty years of daily IGRA data (1977–2016) shows the characteristics of a descent rate of easterly and westerly wind regimes that cannot be found from the mean monthly data. This study clearly demonstrates the stepwise descent of the easterly regime in the whole height range from ∼10 to ∼70 hPa, with the alternation of intervals with faster and slower descent. Furthermore, it seems that the easterly descent always sharply accelerates (decelerates) near equinoxes (solstices). Thus, the easterly regime descends faster in intervals from equinox to solstice, while from solstice to equinox the descent rate is lower and often is near zero or negative. These findings cannot be explained by the influence of annually varying tropical upwelling with a stronger and weaker strength in Northern Hemisphere winter and summer, respectively. Also it seems that the westerly wind regime, irrespective of the time of its appearance at ∼10 hPa, always reaches the level ∼70 hPa simultaneously with the appearance of the next easterly wind regime at ∼10 hPa. Consequently, seasonal alignment of the QBO seems to be primarily associated with the regularities of the easterly descent. The sharp changes of the easterly descent rate near equinoxes and solstices are especially evident in the layer 20–50 hPa, and the QBO period can be determined using these seasonal features of the easterly. The advantage of this method is the possibility of the long-term forecasting of the QBO due to the discretely changing QBO period, which turns out to be equal to 24, 30 or 36 months.

How Momentum Coupling Affects SST Variance and Large-Scale Pacific Climate Variability in CESM

The contribution of buoyancy (thermal + freshwater fluxes) versus momentum (wind driven) coupling to SST variance in climate models is a longstanding question. Addressing this question has proven difficult because a gap in the model hierarchy exists between the fully coupled (momentum + buoyancy + ocean dynamics) and slab–mixed layer ocean coupled (thermal with no ocean dynamics) versions. The missing piece is a thermally coupled configuration that permits anomalous ocean heat transport convergence decoupled from the anomalous wind stress. A mechanically decoupled model configuration is provided to fill this gap and diagnose the impact of momentum coupling on SST variance in NCAR CESM. A major finding is that subtropical SST variance increases when momentum coupling is disengaged. An “opposing flux hypothesis” may explain why the subtropics (midlatitudes) experience increased (reduced) variance without momentum coupling. In a subtropical easterly wind regime, Ekman fluxes [Formula: see text] oppose thermal fluxes [Formula: see text], such that when the air and sea are mechanically decoupled [Formula: see text], [Formula: see text] variance increases. As a result, SST variance increases. In a midlatitude westerly regime where [Formula: see text] and [Formula: see text] typically reinforce each other, SST variance is reduced. Changes in mean surface winds with climate change could impact the [Formula: see text] and [Formula: see text] covariance relationships. A by-product of mechanically decoupling the model is the absence of ENSO variability. The Pacific decadal oscillation operates without momentum coupling or tropical forcing, although the pattern is modified with enhanced (reduced) variability in the subtropics (midlatitudes). Results show that Ekman fluxes are an important component to tropical, subtropical, and midlatitude SST variance.

Winter Wind Regimes over Israel Using Self-Organizing Maps

AbstractThe aim of this study is to objectively define and automatically reconstruct surface wind regimes over Israel. Unlike other previous studies that subjectively examined case studies or applied the semiobjective synoptic classification (SOC), this study shows the ability of the method of self-organizing maps (SOM) to directly define well-known wind regimes at the synoptic hours (0000, 0600, 1200, and 1800 UTC) during the winter. This ability sets the groundwork for future automatic climatological analysis and applications. The investigation is performed by analyzing surface wind measurements from 53 Israel Meteorological Service stations. The relation between the synoptic variables and the wind regimes is revealed from the averages of ECMWF interim reanalysis (ERA-Interim) variables at each SOM wind regime. The inspection of wind regimes and their average pressure anomalies has shown that wind regimes relate to the gradient of the pressure anomalies rather than to the specific isobaric pattern. Two main wind regimes—strong western and strong eastern—are well known over this region. During daytime, SOM classification identifies these two regimes while SOC reveals only strong western regimes since SOC considers depth of the pressure gradients only in the case of low pressure centers. In accordance with previous studies, two main groups—winter low and high pressure centers and/or Red Sea troughs—are related to the strong westerly or easterly wind regimes with low diurnal variability and high daily persistence. Regimes under weak pressure gradients have higher diurnal variability, relatively lower steadiness, and weaker speed. Their daily persistence is not necessarily low.

The tropical tropopause inversion layer: variability and modulation by equatorial waves

Abstract. The tropical tropopause layer (TTL) acts as a transition layer between the troposphere and the stratosphere over several kilometers, where air has both tropospheric and stratospheric properties. Within this region, a fine-scale feature is located: the tropopause inversion layer (TIL), which consists of a sharp temperature inversion at the tropopause and the corresponding high static stability values right above, which theoretically affect the dispersion relations of atmospheric waves like Rossby or inertia–gravity waves and hamper stratosphere–troposphere exchange (STE). Therefore, the TIL receives increasing attention from the scientific community, mainly in the extratropics so far. Our goal is to give a detailed picture of the properties, variability and forcings of the tropical TIL, with special emphasis on small-scale equatorial waves and the quasi-biennial oscillation (QBO).We use high-resolution temperature profiles from the COSMIC satellite mission, i.e., ∼ 2000 measurements per day globally, between 2007 and 2013, to derive TIL properties and to study the fine-scale structures of static stability in the tropics. The situation at near tropopause level is described by the 100 hPa horizontal wind divergence fields, and the vertical structure of the QBO is provided by the equatorial winds at all levels, both from the ERA-Interim reanalysis.We describe a new feature of the equatorial static stability profile: a secondary stability maximum below the zero wind line within the easterly QBO wind regime at about 20–25 km altitude, which is forced by the descending westerly QBO phase and gives a double-TIL-like structure. In the lowermost stratosphere, the TIL is stronger with westerly winds. We provide the first evidence of a relationship between the tropical TIL strength and near-tropopause divergence, with stronger (weaker) TIL with near-tropopause divergent (convergent) flow, a relationship analogous to that of TIL strength with relative vorticity in the extratropics.To elucidate possible enhancing mechanisms of the tropical TIL, we quantify the signature of the different equatorial waves on the vertical structure of static stability in the tropics. All waves show, on average, maximum cold anomalies at the thermal tropopause, warm anomalies above and a net TIL enhancement close to the tropopause. The main drivers are Kelvin, inertia–gravity and Rossby waves. We suggest that a similar wave modulation will exist at mid- and polar latitudes from the extratropical wave modes.

Indo-western Pacific ocean capacitor and coherent climate anomalies in post-ENSO summer: A review

Abstract ENSO induces coherent climate anomalies over the Indo-western Pacific, but these anomalies outlast SST anomalies of the equatorial Pacific by a season, with major effects on the Asian summer monsoon. This review provides historical accounts of major milestones and synthesizes recent advances in the endeavor to understand summer variability over the Indo-Northwest Pacific region. Specifically, a large-scale anomalous anticyclone (AAC) is a recurrent pattern in post-El Ni˜no summers, spanning the tropical Northwest Pacific and North Indian oceans. Regarding the ocean memory that anchors the summer AAC, competing hypotheses emphasize either SST cooling in the easterly trade wind regime of the Northwest Pacific or SST warming in the westerly monsoon regime of the North Indian Ocean. Our synthesis reveals a coupled ocean–atmosphere mode that builds on both mechanisms in a two-stage evolution. In spring, when the northeast trades prevail, the AAC and Northwest Pacific cooling are coupled via wind–evaporation–SST feedback. The Northwest Pacific cooling persists to trigger a summer feedback that arises from the interaction of the AAC and North Indian Ocean warming, enabled by the westerly monsoon wind regime. This Indo-western Pacific ocean capacitor (IPOC) effect explains why El Ni˜no stages its last act over the monsoonal Indo-Northwest Pacific and casts the Indian Ocean warming and AAC in leading roles. The IPOC displays interdecadal modulations by the ENSO variance cycle, significantly correlated with ENSO at the turn of the 20th century and after the 1970s, but not in between. Outstanding issues, including future climate projections, are also discussed.

Attribution of variations in the quasi-biennial oscillation period from the duration of easterly and westerly phases

This study reports the main features of quasi-biennial oscillation (QBO) period variability at stratospheric levels from 70 to 10 hPa and its attribution from the duration variability of westerly and easterly phases using monthly mean zonal wind data from August 1956 to July 2013, archived by Free University of Berlin. A total of 24 QBO events have been distinguished based on the zonal wind field and wavelet analysis for it. The QBO period varies in phase at various stratospheric levels and shows no significant long-term trend but decadal to multi-decadal variability. The noted case-to-case variations in QBO period are due to variations in durations of the westerly and easterly phases at the same level. The highly coupled variability of the easterly duration in the upper levels above 30 hPa and westerly durations in the lower levels below, which manifests the stalling or accelerating of the descent rate of easterly wind regimes around 30 hPa, is found to be the dominant variability of the easterly and westerly durations at various stratospheric levels. Accordingly, the period of QBO in the lower levels below 40 hPa/upper levels above 20 hPa is determined by the westerly/easterly durations there in about 75 % of the 24 QBO events; and at 30 hPa, variations in the durations of both easterly and westerly phases contribute to the QBO period variability. On the contrary, in only 4 out of 24 QBO events, the variations of the westerly/easterly durations in the upper/lower levels are greater than the variations of the easterly/westerly durations in the upper/lower levels, making deterministic contributions to the QBO period variability.

The validity of long-term prediction of quasi-biennial oscillation (QBO) as a proof of the exact seasonal synchronization of the equatorial stratospheric QBO cycle

Sixty years of radiosonde data of the equatorial stratospheric zonal wind have been used to develop the method of long-term wind variations forecast based on the seasonal regularities of quasi-biennial oscillation (QBO) evolution. The wind changes during all easterly wind regime descents in 1953–2014 in the middle stratosphere (from 10 to 70hPa) have been analyzed elaborately by the experimental data. Arguments are presented which show that the delay of easterly downward propagation is observed during each QBO cycle, not just in some cycles, as is commonly believed. Moreover it is shown that the start and the end of all easterly delays (the stages of stagnation) are strongly linked to seasons of a year. It causes the discretely varying length of the stagnation stages and the clearly defined quantized QBO cycle period which can only be equal to 24, 30 or 36 months. These regularities allow developing the method of long-term QBO-cycle prediction. The verification of previous forecasts by comparing with actually observed zonal wind changes in 2005–2014 is presented. Also the QBO development for the following period as far as December 2018 is predicted. We show that the predictability of the wind QBO in the equatorial stratosphere for a long time interval (up to five years) is the result of the existence of a strong seasonal dependence of the QBO evolution. On the other hand, the correctness of these QBO predictions is a strong proof of the exact seasonal synchronization of the QBO cycle in the equatorial stratosphere.

On the simulation of the quasi-biennial oscillation in the Community Atmosphere Model, version 5

The quasi-biennial oscillation (QBO) of the tropical zonal mean wind is a prominent feature of the tropical stratosphere. The easterly and westerly wind regimes alternate with a period of about 28 months. The QBO is believed to be forced by a combination of equatorial waves, in particular, Kelvin and mixed Rossby-gravity waves, as well as smaller-scale gravity waves. Although the QBO is well observed and basic forcing mechanism well understood, it has been a challenge to simulate in General Circulation Models (GCMs). In this paper we examine the role of vertical resolution and gravity wave parameterization on the simulation of the QBO in the Community Atmosphere Model, version 5. We show that in this model vertical resolution of 500 m and adequate gravity wave drag are needed to obtain a realistic QBO. At 500 m vertical resolution, CAM5 generates significantly more mixed Rossby-gravity and Kelvin waves as compared to CAM5 with 700 m or 1200 m vertical resolution. These waves then contribute to the forcing of the easterly and westerly phases of the QBO, respectively. In this work, we also briefly explore the effects of horizontal resolution on the QBO and conclude that the QBO can be adequately represented with horizontal resolution of ∼200 km as long as vertical resolution of the model is fine enough.

Forecast of development of quasi-biennial oscillation in the equatorial stratospheric wind until April 2014

As is well known, the quasi-biennial oscillation (QBO) of wind in the equatorial stratosphere has a significant influence on the processes not only at tropical but also at middle and high latitudes in both stratosphere and troposphere. Consequently, a forecasting the dates of QBO-phase (W or E) transitions is required for the solution of many problems (for instance, for investigation of possible climate changes, for studying of solar activity effects in atmospheric phenomena, for predicting the trends of various atmospheric parameters, etc.). The type of QBO-phase is commonly determined by the zonal wind direction at fixed altitude. The quasi-biennial alternations of wind direction are related with a successive descent of easterly and westerly wind regimes. The paper describes the peculiarities of the wind evolution during QBO-cycle and reveals that the easterly regime propagates downward always with stop at some altitude in the range 20–40hPa. This time period (no less than three months) of the easterly descent interruption is termed as the stagnation stage. Seasonal regularities of stagnation stage cause the discreteness of QBO-cycle period. If the interval between beginnings of successive stagnation stages is identified as a QBO-cycle length, it appears that no QBO-period other than 24, 30, or 36 months can be realized. The QBO-cycle period is unambiguously related with the stagnation duration that allows making an early forecast of the QBO-phase transition dates. The paper discusses the stagnation stage features occurring during QBO-cycles of different period, and describes the technique of forecasting the QBO-cycle evolution by analyzing the vertical profiles of wind speed. The study is based on experimental observation of zonal wind in the equatorial stratosphere during 1953–2011. It is shown that the previous quasi-biennial cycle has begun in January 2009, lasted for 30 months and completed in June 2011. The current cycle has begun in July 2011, will last for 30 months and will end in December 2013. Subsequent cycle will begin in January 2014. The QBO-phase transition dates for different height levels are determined for all months up to April 2014.

Response of London’s Urban Heat Island to a Marine Air Intrusion in an Easterly Wind Regime

Numerical simulations are conducted using the Weather Research and Forecast numerical model to examine the effects of a marine air intrusion (including a sea-breeze front), in an easterly wind regime on 7 May 2008, on the structure of London’s urban heat island (UHI). A sensitivity study is undertaken to assess how the representation of the urban area of London in the model, with a horizontal grid resolution of 1 km, affects its performance characteristics for the near-surface air temperature, dewpoint depression, and wind fields. No single simulation is found to provide the overall best or worst performance for all the near-surface fields considered. Using a multilayer (rather than single layer or bulk) urban canopy model does not clearly improve the prediction of the intensity of the UHI but it does improve the prediction of its spatial pattern. Providing surface-cover fractions leads to improved predictions of the UHI intensity. The advection of cooler air from the North Sea reduces the intensity of the UHI in the windward suburbs and displaces it several kilometres to the west, in good agreement with observations. Frontal advection across London effectively replaces the air in the urban area. Results indicate that there is a delicate balance between the effects of thermal advection and urbanization on near-surface fields, which depend, inter alia, on the parametrization of the urban canopy and the urban land-cover distribution.

Beaufort shelf break eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean detected by satellite and modeling analyses

[1] Mesoscale eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean are examined using satellite data sets and an eddy-resolving coupled sea ice–ocean model. Several surface eddy-like features along the Beaufort shelf break are detected by the Moderate-Resolution Imaging Spectroradiometer (MODIS) sea surface temperature and the Global Imager (GLI) radiance image. The QuikSCAT sea wind vectors indicate that summertime shelf-wide wind is an important factor for interannual variations in the eddy properties. A realistic numerical experiment reveals that the origin of shelf break warm eddies and timing of their generation can be classified into three types. Type I eddies are frequently produced in the vicinity of the Barrow Canyon throughout the summer season by a combination of outflow of Pacific summer water with a low potential vorticity from the Barrow Canyon and lateral velocity shear of the canyon jet. Surface wind variability modulates the timing of generation. Type II and Type III eddies originate from an eastward current along the Beaufort shelf break, although origin depth and background hydrographic structure differ between them. Type II eddies are spawned from the shelf slope at middepth during early summer. Type III eddies are generated from the surface isopycnal front over the shelf break during late summer. The shift from Type II eddies to Type III eddies is caused by surface buoyancy input in the upstream region. A sensitivity experiment using different atmospheric forcing data suggest that the mechanism controlling Pacific water transport from the Chukchi shelf to the Canada Basin differs depending on surface wind fields in the shelf region. The mesoscale eddies primarily induce the shelf-to-basin transport under weak or westerly wind conditions during summer, while wind-driven Ekman transport is a major driver in the easterly wind regime. Year 2003 (2007) corresponds to the former (latter) case.

The quasi-biennial oscillation in the equatorial stratosphere: Seasonal regularity in zonal wind changes, discrete QBO-cycle period and prediction of QBO-cycle duration

The changes of vertical wind structure in equatorial stratosphere in course of Quasi-Biennial Oscillation (QBO) demonstrates the evident seasonal dependence. The easterly wind regime descending from the middle to lower stratosphere always includes the stationary period—the stagnation stage. At stagnation stage the bottom boundary of the easterly wind is located in different QBO-cycles at different altitude in the range from ∼22 to ∼26 km, but in each QBO-cycle this altitude is actually constant during the whole stagnation period. Stagnation always begins in solstice (in June–July or December–January). Descent of the easterly wind after stagnation is always resumed near equinox—in March–April or September–October. Consequently, the duration of stagnation stage varies discretely and can be equal to one, three, or five seasons (three, nine, or fifteen months, respectively) in different QBO-cycles. The QBO-cycle period determined as an interval between the beginnings of two successive stagnation stages turns out to be of discrete duration also, and can be equal to 24, 30, or 36 months. The dependence of many atmosphere phenomena determining the Earth’s climate on QBO-phase suggests the need to forecast the QBO-cycle evolution. The established seasonal regularity and discrete QBO-cycle period make it possible the forecasting the QBO-cycle evolution, its duration, and the dates of the QBO-phase changing.

Vertical Velocity Statistics in Fair-Weather Cumuli at the ARM TWP Nauru Climate Research Facility

Abstract Fair-weather cumuli are fundamental in regulating the vertical structure of water vapor and entropy in the lowest 2–3 km of the earth’s atmosphere over vast areas of the oceans. In this study, a long record of profiling cloud radar observations at the Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) at Nauru Island is used to investigate cloud vertical air motion statistics over an 8-yr observing period. Appropriate processing of the observed low radar reflectivities provides radar volume samples that contain only small cloud droplets; thus, the Doppler velocities are used as air motion tracers. The technique is applied to shallow boundary layer clouds (less than 1000 m thick) during the 1999–2007 period when radar data are available. Using the boundary layer winds from the soundings obtained at the Nauru ACRF, the fair-weather cumuli fields are classified in easterly and westerly boundary layer wind regimes. This distinction is necessary to separate marine-forced (westerlies) from land-forced (easterlies) shallow clouds because of a well-studied island effect at the Nauru ACRF. The two regimes exhibit large diurnal differences in cloud fraction and cloud dynamics as manifested by the analysis of the hourly averaged vertical air motion statistics. The fair-weather cumuli fields associated with easterlies exhibit a strong diurnal cycle in cloud fraction and updraft strength and fraction, indicating a strong influence of land-forced clouds. In contrast over the fair-weather cumuli with oceanic origin, land-forced clouds are characterized by uniform diurnal cloudiness and persistent updrafts at the cloud-base level. This study provides a unique observational dataset appropriate for testing fair-weather cumulus mass flux and turbulence parameterizations in numerical models.

Severe marine storms in the Northern Adriatic: Characteristics and trends

This paper discusses present characteristics and trends of severe marine storminess in the Northern Adriatic. It merges oceanographic and meteorological aspects by considering storm surges, wind waves and the atmospheric cyclones that cause them. The paper introduces these three processes and the different role of south-easterly (Sirocco) and easterly (Bora) wind regimes. The specific characteristics of cyclones producing severe marine storms in terms of location where cyclogenesis occurs, trajectories and intensity, are compared with those of generic cyclones crossing northern Italy. It is shown that cyclones producing high waves and surges in most cases have different characteristics and their lists overlap only partially. However, both high wave and surge events have a similar annual cycle, with maximum activity in November and hardly any event in summer (June–July–August). The trends of severe high wave and surge events are discussed (various thresholds are considered) and they are shown to be broadly consistent. Timeseries, which show large inter-annual variability and very little overall tendencies on multi-decadal time scale, suggest progressively milder storms during the second half of the 20th century.

On The Dominance of 28-Month Harmonic in the Equatorial- Stratospheric-Wind Quasi Biennial Oscillation

The Quasi Biennial Oscillation, the downward propagating easterly and westerly wind regimes in the equatorial stratosphere, has been investigated according to robust statistics that utilizes the amplitude/vector probable error ratio to provide the confidence level of the investigated harmonic. The amplitude-to-noise ratio is found to be the highest in correspondence of the 28- month harmonic for each examined height; the relative amplitude and phase values are found, respectively, to decrease and increase with the height and to take about a year to descend from 15 to 70 hPa with a progressive lag of about 1 month/km. At the top of the stratosphere, easterlies dominate, while at the bottom, westerlies are more likely to be found.

Submonthly Variability in the South American Monsoon System

In this study, we examine the South American Monsoon System (SAMS) variability with an emphasis on three-day periods or longer during which time the low-level wind in central Amazonia is consistently from the west or the east (westerly and easterly wind regimes, WWRs and EWRs, respectively). We use observational and Reanalysis datasets for the southern summer, as in previous studies on SAMS variability. Our research methodology involves the addition of model data that consists of the Coordinated Enhanced Observing Period (CEOP) Model Output Location Time Series (MOLTS) at locations in the SAMS region, and ensemble atmospheric general circulation model (AGCM) simulations of the southern summer season. For both wind regimes, the observed and simulated composites of anomalous precipitation over South America show a dipole pattern with poles in northwestern and central-southeastern Brazil. During WWRs (EWRs), precipitation anomalies at the central-southeastern pole are positive (negative), indicating a stronger (weaker) SACZ. In addition, during WWRs the upper-level monsoon high is stronger and the subtropical jet in the South American sector is stronger and closer to the equator. We also examine the diurnal cycle of rainfall from three CEOP MOLTS for Rondonia and Brasilia, which are situated at different poles of the rainfall anomalies dipole. The diurnal cycle in precipitation during both wind regimes shows an early-afternoon maximum at the selected locations. At Rondonia (Brasilia), there is a second early-morning (nocturnal) maximum on EWR (WWR) days. The diurnal mean precipitation in the regime that shows this maximum value, which is primarily due to enhanced cumulus convection, is higher than that in the other regime. In the simulation, WWRs and EWRs occur with approximately equal frequency. There is also a dipole pattern in anomalous precipitation during the days of each regime, although the poles are displaced relative to the observations. The simulated diurnal cycles of rainfall for the different wind regimes are nearly identical. Possible explanations for these AGCM difficulties are briefly discussed. Lag composites of geopotential height anomalies at 700 mb prior to the establishment of the wind regimes in Amazonia show the development of structures in the South Pacific that resemble the principal modes of variability in that region. This indicates the existence of links between the wind regimes and the modes of intraseasonal variability over South America. It is proposed that the wind regimes are associated with low-frequency variability in the extratropical circulation.

Spatial distribution of phytoplankton <13μm in the Gulf of Cádiz in relation to water masses and circulation pattern under westerly and easterly wind regimes

Spatial distribution of both abundance and biomass of phytoplankton o13mm were analysed by flow cytometry, distinguishing eukaryotic picoplankton, nanoplankton, cryptomonads, Synechococcus and Prochlorococcus. These groups were related to different circulation patterns observed in two successive mesoscale samplings in the Gulf of Cadiz. The first sampling was characterized by westerly-driven upwelling processes around Cape Santa Maria and a surface signal of the Huelva front. During the second sampling easterlies blew and warm waters covered the shelf between Cadiz and Cape Santa Maria although a reminiscent upwelling was still observed west of the cape. Under the first wind regime, lowest phytoplankton abundance was measured in the upwelling area probably due to time lag of phytoplankton response, whereas the highest abundance occurred associated with the Huelva front (T� 161C). During the second cruise and under easterly wind regime, higher phytoplankton abundance and biomass of all groups were found as compared to the first period. Phytoplankton accumulation was mainly associated with the 161C isotherm in upwelled waters and in warm shelf waters. In the mixing area between warm shelf waters and cold upwelled waters, phytoplankton size spectra were found to be less negative in the warm stratified side than in the cold mixed side. This unusual feature was attributed to the warming and nutrient enrichment of shelf water during each tidal cycle in river delta and marshes located around the Gulf of Cadiz, leading to elevated biomass around these structures. On TS diagrams, eukaryotic nanoplankton was associated with warm shelf water, eukaryotic picoplankton and Synechococcus with upwelled water, while Prochlorococcus reached highest abundances in deeper water (50-75 m) associated with the mixing line between North Atlantic Central Water and Surface Atlantic Water. r 2006 Elsevier Ltd. All rights reserved.