Synoptic Variability Research Articles

Evaluation of the Antarctic Mesoscale Prediction System based on snow accumulation observations over the Ross Ice Shelf

Recent snow height measurements (2008–15) from nine automatic weather stations (AWSs) on the Ross Ice Shelf are used to examine the synoptic and seasonal variability in snow accumulation, and also to evaluate the performance of the Antarctic Mesoscale Prediction System (AMPS) for precipitation. The number of snow accumulation events varies from one station to another between 2008 and 2015, thus demonstrating geographic dependence. The interannual variability in snow accumulation is too high to determine its seasonality based on the current AWS observations with limited time coverage. Comparison between the AMPS and AWS snow height measurements show that approximately 28% of the AWS events are reproduced by AMPS. Furthermore, there are significant correlations between AMPS and AWS coincident event sizes at five stations (p < 0.05). This finding suggests that AMPS has a certain ability to represent actual precipitation events.

Contribution of the cold sector of extratropical cyclones to mean state features over the Gulf Stream in winter

Recent studies have shown that midlatitude air–sea interactions are strongly modulated by synoptic variability. The present study investigates how air–sea interactions over the Gulf Stream sea surface temperature (SST) front vary in different synoptic regimes. We focus more particularly on the variation of three atmospheric mean state features which are colocated with the SST front in winter: enhanced ascent and precipitation on the warm side of the SST front, and enhanced low‐level baroclinicity at the entrance of the storm track. These three fields are partitioned depending on whether they occur in the cold sector of extratropical cyclones or in any other synoptic features. The analysis is based on ERA‐Interim winter data covering the period 1979–2012. Results are twofold:(i) Cold sector precipitation is confined within a 5° latitude band south of the SST front and reaches 2 mm day−1, whereas precipitation occurring outside the cold sector forms a broader spatial pattern. The same partitioning applied to vertical wind shows that the ascent on the warm side of the Gulf Stream is not a feature of the cold sector. These results mean that a significant part of the anchoring effect of the SST front on precipitation occurs via the cold sector and that the observed colocation of ascent and precipitation is not causal, in contrast to what was suggested by previous studies, but is rather the result of two different mechanisms.(ii) The surface heat fluxes and convection occurring in the cold sector restore low‐level atmospheric temperature gradients within two days after a time maximum of meridional eddy heat flux, such that low‐level baroclinicity remains largely unchanged after the passage of an extratropical cyclone.This ‘cold path mechanism’ opens new avenues to understand how SST forces climate variability in the midlatitudes.

The variability of surface radiation fluxes over landfast sea ice near Zhongshan station, east Antarctica during austral spring

ABSTRACTSurface radiative fluxes over landfast sea ice off Zhongshan station have been measured in austral spring for five springs between 2010 and 2015. Downward and upward solar radiation vary diurnally with maximum amplitudes of 473 and 290 W m−2, respectively. The maximum and minimum long-wave radiation values of the mean diurnal cycle are 218 and 210 W m−2 for downward radiation, 277 and 259 W m−2 for upward radiation and 125 and −52 W m−2 for net radiation. The albedo has a U-shaped mean diurnal cycle with a minimum of 0.64 at noon. Sea ice thickness is in the growth phase for most spring days, but can be disturbed by synoptic processes. The surface temperature largely determines the occurrence of ice melting. Surface downward and upward long-wave radiation show synoptic oscillations with a 5–8 day period and intraseasonal variability with a 12–45 day period. The amplitudes of the diurnal, synoptic and intraseasonal variability show some differences during the five austral springs considered here. The intraseasonal and synoptic variability of downward and upward long-wave radiation are associated with the variability of cloud cover and surface temperature induced by the atmospheric circulation.

Reconstruction of the three-dimensional salinity and temperature fields of the Black Sea on the basis of satellite altimetry measurements

The methodology of joint processing of the satellite altimetry and occasional hydrological observations in the Black Sea for 1993–2012 is developed. The original technique for reconstruction of the 3D temperature and salinity fields in the deep-sea part is proposed and implemented. This technique makes it possible to identify the depths at which a contribution of adiabatic processes to the deformation of the temperature and salinity profiles of the sea is predominant. Daily-averaged 3D fields of the seawater temperature and salinity in a baroclinic layer on a regular grid are reconstructed. The evaluation of accuracy of the reconstructed temperature and salinity arrays is performed by comparing them with the data of hydrological exploration. Structures of the temperature and salinity fields are correlated naturally with topography of the altimetric level and clearly indicate the synoptic variability. Seasonal and interannual variabilities of the kinetic energy (averaged over horizons of the 63–400 m layer) of the geostrophic currents calculated using the dynamic method makes it possible to reveal a sharp increase in the kinetic energy of the currents in the winter season of 2002. A high correlation is found between the interannual variability of the ERA-Interim wind stress curl averaged over the surface of the deep sea part and the kinetic energy of the geostrophic currents in the 63–400 m layer.

Modeling the impact of river discharge and wind on the hypoxia off Yangtze Estuary

Abstract. The phenomenon of low dissolved oxygen (known as hypoxia) in a coastal ocean system is closely related to a combination of anthropogenic and natural factors. Marine hypoxia occurs in the Yangtze Estuary, China, with high frequency and long persistence. It is related primarily to organic and nutrient enrichment influenced by river discharges and physical factors, such as water mixing. In this paper, a three-dimensional hydrodynamic model was coupled to a biological model to simulate and analyze the ecological system of the East China Sea. By comparing with the observation data, the model results can reasonably capture the physical and biochemical dynamics of the Yangtze Estuary. In addition, the sensitive experiments were also used to examine the role of physical forcing (river discharge, wind speed, wind direction) in controlling hypoxia in waters adjacent to the Yangtze Estuary. The results showed that the wind field and river discharge have significant impact on the hypoxia off the Yangtze Estuary. The seasonal cycle of hypoxia was relatively insensitive to synoptic variability in the river discharge, but integrated hypoxic areas were sensitive to the whole magnitude of river discharge. Increasing the river discharge was shown to increase hypoxic areas, while decreasing the river discharge tended to decrease hypoxic areas. The variations of wind speed and direction had a great impact on the integrated hypoxic areas.

Synoptic variation of stable isotopes in precipitation and ground level vapour in a humid tropical region, Kerala, India

The controlling factors of spatio-temporal variation in δ18O and δD composition of rainwater and the rain–vapour interaction in the southern, middle and northern regions of Kerala, India, are presented. The isotopic distribution (δ18O) of rainfall ranged between 0 and −6‰ during January to September and between −6 to −12‰ during October to December. The rain during post-monsoon season has significant contribution to recycled vapour. Evaporation from falling raindrops is significant despite high relative humidity. The sudden isotopic depletion of ground level vapour from baseline isotopic composition is due to interaction and isotopic exchange with falling raindrops.

The statistical structure of synoptic variability ocean currents at the continental slope of the Laptev Sea and features of their generation by anemobaric forces

Based on vector-algebraic analysis of random processes, we study the statistical structure of the synoptic variability of currents measured by an ADCP in the upper mixed layer in the central part of the continental slope of the Laptev Sea in 2006–2007. The results of statistical analysis show that in some cases the synoptic currents in the surface layer of the sea are signs of wind drift currents. This is indicated by the high correlation between the tangential friction of wind and currents, as well as the reversal of the depth of current vectors and the major axes of the ellipses of the mean-square deviation of the Ekman spiral. Due to the large variability of wind flows and stratification of water masses, the penetration depth of these currents is small and varies from 6 to 30 m, with pronounced seasonal variation. In deeper layers, no relationship between the currents and anemobaric forces is traced. It is concluded that the fluctuations of synoptic scale currents in the area of the continental slope of the Laptev Sea represent a superposition of Ekman drift currents and movements associated with free baroclinic Kelvin waves. These currents are the dominant contributor in the upper 30-m layer of the ocean, while waves play a key role in deeper waters.

Impact on oceanic dynamics from assimilation of satellite surface height anomaly data into the Hybrid Coordinate Ocean Model Ocean Model (HYCOM) over the Atlantic Ocean

Observed along-track data of sea surface height anomalies (SSHAs) over the Atlantic Ocean from the Jason-1 and Jason-2 satellites were assimilated into the Hybrid-Coordinate Ocean Model (HYCOM) with the Ensemble Optimal Interpolation scheme (EnOI). The impact of assimilation of SSHA with focus on oceanic dynamics was investigated. Time series of analyzed and forecasted values were compared with a model free run with the same forcing but without assimilation. In addition, the results were compared with an independent run, the so-called HYCOM + NCODA analysis from the US Navy. The study shows that the assimilation technique with some modifications allowed substantial improvement in the 24 h ocean prediction by reducing the forecast errors in comparison with the free run. It is also shown that the analyzed sea surface fields contain mesoscale and synoptic variability, which are poorly seen in the free run.

Multidisciplinary experiment on studying short-period variability of the sedimentary process in the northeastern part of the Black Sea

The principal aim of this work is to reveal the regularities of short-period synoptic variability of vertical flows and the composition of settling sedimentary material, to obtain information on the quantitative characteristics of the processes that influence sound-scattering layers in the water layer above the continental slope behind the shelf edge in the northeastern part of the Black Sea. The results were obtained due to improvement of the equipment and the procedures for performing sea experiments on studying physicogeological, biological, and hydrophysical processes in the upper illuminated layer of phytoplankton development.

Atmospheric forcing of the upper ocean transport in the Gulf of Mexico: From seasonal to diurnal scales

AbstractThe 2010 Deepwater Horizon oil spill in the Gulf of Mexico (GoM) was an environmental disaster, which highlighted the urgent need to predict the transport and dispersion of hydrocarbon. Although the variability of the atmospheric forcing plays a major role in the upper ocean circulation and transport of the pollutants, the air‐sea interaction on various time scales is not well understood. This study provides a comprehensive overview of the atmospheric forcing and upper ocean response in the GoM from seasonal to diurnal time scales, using climatologies derived from long‐term observations, in situ observations from two field campaigns, and a coupled model. The atmospheric forcing in the GoM is characterized by striking seasonality. In the summer, the time‐average large‐scale forcing is weak, despite occasional extreme winds associated with hurricanes. In the winter, the atmospheric forcing is much stronger, and dominated by synoptic variability on time scales of 3–7 days associated with winter storms and cold air outbreaks. The diurnal cycle is more pronounced during the summer, when sea breeze circulations affect the coastal regions and nighttime wind maxima occur over the offshore waters. Realtime predictions from a high‐resolution atmosphere‐wave‐ocean coupled model were evaluated for both summer and winter conditions during the Grand LAgrangian Deployment (GLAD) in July–August 2012 and the Surfzone Coastal Oil Pathways Experiment (SCOPE) in November–December 2013. The model generally captured the variability of atmospheric forcing on all scales, but suffered from some systematic errors.

Time-Sampling Errors of Earth Radiation From Satellites: Theory for Monthly Mean Albedo

The Earth Radiation Budget Experiment wide-field-of-view (WFOV) radiometers aboard the Earth Radiation Budget Satellite (ERBS) provided a 15-year record of high-quality measurements for research into the radiant energy balance of the Earth. Monthly mean maps of RSR and outgoing longwave radiation (OLR) are primary data products from these measurements. The ERBS orbit had an inclination of 57° so as to precess through all local times every 72 days. Because of limited temporal sampling, some regions were not measured sufficiently often by the WFOV radiometers to produce accurate radiation flux values for these maps. The temporal sampling of any one region is very irregular; therefore, it is necessary to consider each region in detail for each month. An analysis of the errors, which result from computing the average value of the albedo of a region over a day or month based on limited sampling, is presented. It is necessary to take into account synoptic variations and their time correlations and differences of the regions' diurnal cycle from that assumed by the time-averaging algorithms. An expression is derived for the variance of the error of the computed daily and monthly mean albedo. Temporal correlation and variability of the albedo field are specified a priori . This analysis has been used for quality assurance to evaluate the temporal sampling errors of monthly mean RSR maps computed from the measurements by the WFOV radiometers aboard the ERBS and to delete those values for which the error variance is excessive.

Benchmarking Northern Hemisphere midlatitude atmospheric synoptic variability in centennial reanalysis and numerical simulations

AbstractThe representation of midlatitude winter atmospheric synoptic variability in centennial reanalysis products, which assimilate surface observations only, and atmospheric model simulations constrained by observation‐based data sets is assessed. Midlatitude waves activity in twentieth century reanalyses (20CR, ERA‐20C) and atmospheric model simulations are compared with those estimated from observationally complete reanalysis products. All reanalyses are in good agreement regarding the representation of the synoptic variability during the last decades of the twentieth century. This suggests that the assimilation of surface observations can generate high‐quality extratropical upper air fields. In the first decades of the twentieth century a suppression of high‐frequency variability is apparent in the centennial reanalysis products. This behavior does not have a counterpart in the atmospheric model integrations. Since the latter differ from one of the reanalysis products considered here (ERA‐20C) only in the assimilation of surface observations, it seems reasonable to attribute the high‐frequency variability suppression to the poor coverage of the observations assimilated.

Multiscale Temporal Mean Features of Perturbation Kinetic Energy and Its Budget in the Tropics: Review and Computation

Abstract The authors examined the maintenance mechanisms of perturbation kinetic energy (PKE) in the tropical regions for multiple time scales by computing and analyzing its budget equation. The emphasis has been placed on the mean features of synoptic and subseasonal variabilities using a 33-yr dataset. From analysis of the contributions from u-wind and υ-wind components, the PKE maximum in the Indian Ocean is attributed less to synoptic variability and more to intraseasonal variability in which the Madden–Julian oscillation (MJO) dominates; however, there is strong evidence of seasonal variability affiliated with the Asian monsoon systems. The ones in the eastern Pacific and Atlantic Oceans are closely related to both intraseasonal and synoptic variability that result from the strong MJO and the relatively large amplitude of equatorial waves. The maintenance of the PKE budget mainly depends on the structure of time mean horizontal flows, the location of convection, and the transport of PKE from the extratropics. In the regions with strong convective activities, such as the eastern Indian Ocean to the western Pacific, the production of PKE occurs between 700 and 200 hPa at the expense of perturbation available potential energy (PAPE), which is generated by convective heating. This gain in PKE is largely offset by divergence of the geopotential component of vertical energy flux; that is, it is redistributed to the upper- and lower-atmospheric layers by the pressure field. Strong PKE generation through the horizontal convergence of the extratropical energy flux takes place in the upper troposphere over the eastern Pacific and Atlantic Ocean, and is largely balanced by a PKE loss due to barotropic conversion, which is determined solely by the sign of longitudinal stretching deformation. However, over the Indian Ocean, there is a net PKE loss due to divergence of energy flux, which is compensated by PKE gain through the shear generation.

Natural hazards in Australia: heatwaves

As part of a special issue on natural hazards, this paper reviews the current state of scientific knowledge of Australian heatwaves. Over recent years, progress has been made in understanding both the causes of and changes to heatwaves. Relationships between atmospheric heatwaves and large-scale and synoptic variability have been identified, with increasing trends in heatwave intensity, frequency and duration projected to continue throughout the 21st century. However, more research is required to further our understanding of the dynamical interactions of atmospheric heatwaves, particularly with the land surface. Research into marine heatwaves is still in its infancy, with little known about driving mechanisms, and observed and future changes. In order to address these knowledge gaps, recommendations include: focusing on a comprehensive assessment of atmospheric heatwave dynamics; understanding links with droughts; working towards a unified measurement framework; and investigating observed and future trends in marine heatwaves. Such work requires comprehensive and long-term collaboration activities. However, benefits will extend to the international community, thus addressing global grand challenges surrounding these extreme events.

A high-resolution modelling study of the circulation along the Campania coastal system, with a special focus on the Gulf of Naples

A high-resolution modelling study of the circulation along the Campania coastal system (CCS) in the southern Tyrrhenian Sea is presented. The area includes three adjacent, semi-enclosed shallow basins (the gulfs of Gaeta, Naples and Salerno) facing the open deep sea, and represents an ideal site for investigating relevant dynamical processes of general interest in coastal oceanography. A Campania Regional sigma-coordinate Ocean Model (CROM) has been implemented in this region, with a 1/144° resolution; nesting with an operational circulation model covering the whole Tyrrhenian Sea with a 1/48° resolution is performed. Forcing is provided by surface momentum, heat, and freshwater fluxes computed from the non-hydrostatic SKIRON/Eta atmospheric modelling system outputs. A winter and a summer period of the reference year 2009 are analyzed in detail. The relative importance of the flow induced by remote large-scale currents through topographic interactions and of that induced locally by the wind are found to vary, even over a weekly time scale, in all of the three gulfs; the Gulf of Salerno appears to be the location where remote and local forcing are more often competing. An analysis of the high frequency variability shows that, besides the current changes induced by the typical mid-latitude atmospheric synoptic variability, inertial currents in the open sea and sea breeze-induced currents in the gulfs are present. Model-data comparison is performed in the CCS with altimeter data and satellite-derived turbidity distributions, and in the Gulf of Naples with the latter and with measurements performed with a high-frequency radar system. Significant model-data agreement is generally found.

Interannual Variability in Baseline Ozone and Its Relationship to Surface Ozone in the Western U.S.

Baseline ozone refers to observed concentrations of tropospheric ozone at sites that have a negligible influence from local emissions. The Mount Bachelor Observatory (MBO) was established in 2004 to examine baseline air masses as they arrive to North America from the west. In May 2012, we observed an O3 increase of 2.0-8.5 ppbv in monthly average maximum daily 8-hour average O3 mixing ratio (MDA8 O3) at MBO and numerous other sites in the western U.S. compared to previous years. This shift in the O3 distribution had an impact on the number of exceedance days. We also observed a good correlation between daily MDA8 variations at MBO and at downwind sites. This suggests that under specific meteorological conditions, synoptic variation in O3 at MBO can be observed at other surface sites in the western U.S. At MBO, the elevated O3 concentrations in May 2012 are associated with low CO values and low water vapor values, consistent with transport from the upper troposphere/lower stratosphere (UT/LS). Furthermore, the Real-time Air Quality Modeling System (RAQMS) analyses indicate that a large flux of O3 from the UT/LS in May 2012 contributed to the observed enhanced O3 across the western U.S. Our results suggest that a network of mountaintop observations, LiDAR and satellite observations of O3 could provide key data on daily and interannual variations in baseline O3.

On mechanisms that determine synoptic time scale convection over East Africa

ABSTRACTThis study explores potential mechanisms to explain the marked synoptic scale variability in convection seen over East Africa during the July–September season. Regression composites based on analysis of 22 years of brightness temperature and reanalysis datasets highlights a complicated evolution for this synoptic convection. Active convection in the region is typically preceded by a coherent eastward moving convective signal that is followed by a period with a prominent westward moving convective signal away from the region. Results suggest that this sequence is associated with eastward propagation of convectively coupled Kelvin waves (CCKWs) that subsequently trigger westward‐moving African easterly waves (EWs). Results also indicate that synoptic convection in the East African region is impacted by westward moving upper tropospheric EWs originating on the tropical easterly jet in the South Asian region. These upper EWs propagate through East Africa with about a 4‐day period, consistent with the synoptic convection variability observed over East Africa. Consistent with local forecaster experience, analysis of the regional spatial patterns of convection and associated moisture flux convergence show that a coherent signal propagates northeastwards from the equatorial Congo region to East Africa on synoptic timescales. The evolution resembles the CCKW‐associated signal suggesting that CCKWs contribute to this synoptic variability.

The influence of Black Sea Water inflow and its synoptic time-scale variability in the North Aegean Sea hydrodynamics

The exchange water fluxes between the Black Sea and the North Aegean Sea through the Dardanelles Strait constitute an essential factor for the general circulation of the region. The Black Sea Water (BSW) inflow to the Aegean plays an important role in the hydrography and circulation of the basin and can affect the North Aegean deep water formation processes. Numerical experiments evaluating the influence of the time-scale variability (synoptic and seasonal) and the seasonality (period of maximum/minimum) of the Black Sea Water inflow on the dynamics of the North Aegean basin were performed. The experiments were carried out for the period from August 2008 to October 2009, using observed upper and lower-layer fluxes from the Dardanelles Strait, high-resolution atmospheric forcing, and boundary conditions derived from an operational system (ALERMO). The large-scale spatial patterns of the circulation and the seasonal variability of the North Aegean circulation show that dynamics of the basin can effectively absorb most of the Black Sea Water inflow variability. The overall cyclonic circulation of the North Aegean Sea and the predominant cyclonic and anti-cyclonic features are robust and are little affected by the different lateral fluxes. However, differences in the seasonality of the BSW inflow have an important impact in the North Aegean water column structure, while the synoptic variability observed in the Black Sea Water inflow affects the kinetic energy of the basin and the pathway of the Black Sea Water plume.

Spatiotemporal variability of the flow in the deep part of the Central Caspian Sea

The paper presents the analysis of hydrophysical observations executed along the Transcaspian section in 2004-2014 during the expeditions by Shirshov Institute of Oceanology of Russian Academy of Sciences. The results used in the paper are the generalization of data obtained in the last decade. Investigations of the current system of the Central Caspian Sea demonstrated that at the northeastern part of the cyclonic circulation there are considerable deviations. The deposits that energy of current disturbances of various spatiotemporal scales makes to the general energy of currents, are calculated. Preliminary results are received of the assessment of water transfer through the Apsheron threshold from the Central to the Southern Caspian Sea and vice versa. It is shown that annual recurrence of concentration of hydrogen sulfide in the benthonic hollows of the Caspian Sea is directly connected with seasonal and synoptic variability of currents at the bottom.

Increased Risk of the 2014 Australian May Heatwave Due to Anthropogenic Activity

An evaluation on the ability of the Community Earth System Model (CESM) model to simulate heatwave-relevant synoptic patterns over Australia was performed. The method of self-organizing maps was used (Kohonen 2001), which has been shown to be a powerful tool for evaluating model simulated synoptic variability in other studies (Cassano et al. 2006; Uotila et al. 2007; Gibson et al. 2015, manuscript submitted to Climate Dyn.). Each historical realization of the CESM ensemble was compared to ERAinterim over 1979–2004 in terms of the frequency of each synoptic pattern (node). A total of 20 nodes were computed, as shown in Fig. S31.1. Many of the nodes resemble the development of the observed synoptic system associated with the 19-day 2014 May heatwave (see Fig. S31.2 for snapshots of the observed system). In particular, node c4 was most relevant to the event occurring for 8 of the heatwave days, including 6 consecutive days (Fig. S31.2). Other similar nodes, also characterized by anticyclonic conditions over the Tasman Sea, occurred during the heat wave including nodes b4, c3, d3, d4. Overall, all realizations of CESM produced similar frequencies of each self-organizing map (SOM) node. Encouragingly, the frequency of node c4 (that most relevant to the heatwave in this study) in CESM was very similar to that of ERA-interim, while some of the other less relevant nodes were either slightly underestimated (nodes c3, d3, d4) or overestimated (b4) by the model (Fig. S31.3). The biggest deficiency in the model is its tendency to overstimulate the frequency of zonally elongated patterns (i.e., nodes a2 and b2), however, this is less of a concern as these patterns are not relevant to the heat event.