Meridional Wind Components Research Articles

Interannual and Decadal Variability of Sea Surface Temperature and Sea Ice Concentration in the Barents Sea

Sea ice loss and accelerated warming in the Barents Sea have recently been one of the main concerns of climate research. In this study, we investigated the trends and possible relationships between sea surface temperature (SST), sea ice concentration (SIC), and local and large-scale atmospheric parameters over the last 39 years (1982 to 2020). We examined the interannual and long-term spatiotemporal variability of SST and SIC by performing an empirical orthogonal function (EOF) analysis. The SST warming rate from 1982 through 2020 was 0.35 ± 0.04 °C/decade and 0.40 ± 0.04 °C/decade in the ice-covered and ice-free regions, respectively. This climate warming had a significant impact on sea-ice conditions in the Barents Sea, such as a strong decline in the SIC (−6.52 ± 0.78%/decade) and a shortening of the sea-ice season by about −26.1 ± 7.5 days/decade, resulting in a 3.4-month longer summer ice-free period over the last 39 years. On the interannual and longer-term scales, the Barents Sea has shown strong coherent spatiotemporal variability in both SST and SIC. The temporal evolution of SST and SIC are strongly correlated, whereas the Atlantic Multidecadal Oscillation (AMO) influences the spatiotemporal variability of SST and SIC. The highest spatial variability (i.e., the center of action of the first EOF mode) of SST was observed over the region bounded by the northern and southern polar fronts, which are influenced by both warm Atlantic and cold Arctic waters. The largest SIC variability was found over the northeastern Barents Sea and over the Storbanken and Olga Basin. The second EOF mode revealed a dipole structure with out-of-phase variability between the ice-covered and ice-free regions for the SST and between the Svalbard and Novaya Zemlya regions for SIC. In order to investigate the processes that generate these patterns, a correlation analysis was applied to a set of oceanic (SST) and atmospheric parameters (air temperature, zonal, and meridional wind components) and climate indices. This analysis showed that SST and SIC are highly correlated with air temperature and meridional winds and with two climate indices (AMO and East Atlantic Pattern (EAP)) on an interannual time scale. The North Atlantic Oscillation (NAO) only correlated with the second EOF mode of SST on a decadal time scale.

On the derivation of zonal and meridional wind components from Aeolus horizontal line-of-sight wind

Abstract. Since its launch in 2018, the European Space Agency's Earth Explorer satellite Aeolus has provided global height resolved measurements of horizontal wind in the troposphere and lower stratosphere for the first time. Novel datasets such as these provide an unprecedented opportunity for the research of atmospheric dynamics and provide new insights into the dynamics of the upper troposphere and lower stratosphere (UTLS) region. Aeolus measures the wind component along its horizontal line-of-sight, but for the analysis and interpretation of atmospheric dynamics, zonal and/or meridional wind components are most useful. In this paper, we introduce and compare three different methods to derive zonal and meridional wind components from the Aeolus wind measurements. We find that the most promising method involves combining Aeolus measurements during ascending and descending orbits. Using this method, we derive global estimates of the zonal wind in the latitude range 79.7∘ S to 84.5∘ N with errors of less than 5 m s−1 (at the 2σ level). Due to the orbit geometry of Aeolus, the estimation of meridional wind in the tropics and at midlatitudes is more challenging and the quality is less reliable. However, we find that it is possible to derive meridional winds poleward of 70∘ latitude with absolute errors typically below 5 m s−1 (at the 2σ level). This further demonstrates the value of Aeolus wind measurements for applications in weather and climate research, in addition to their important role in numerical weather prediction.

Analysis of extreme wind events in the Weddell Sea region (Antarctica) at Belgrano II Station

The objective of the present work is to analyze the wind pattern (direction and speed) at Belgrano II Station, evaluating the influence of large-scale atmospheric systems on extreme wind speed events. The study area is the Weddell Sea, located at the southern end of the Atlantic Ocean, between latitudes of 83° S and 55° S and longitudes of 84° W and 20° E. Daily meteorological data were used in the period from 1979 to 2015, whose variables are: Sea Level Pres-sure (SLP), Wind Speed (WS), Wind Direction (WD) and Air Temperature at 2 Meters (T2M), collected at Belgrano II Station, under the care of the Ar- gentine Antarctic Institute (AAI). To analyze the anomalies in the fields of air temperature, atmospheric circulation and surface pressure on a large scale, we used the reanalysis data obtained from the ERA-Interim database, which are: air temperature at 2 m (oC), geopotential height (m) at the 500 hPa level, and zonal and meridional components of wind (ms−1) on the surface. The ex-treme event was defined as values above the 95% percentile of each month of the year. The results indicate that the wind direction at Belgrano II Station was predominantly southeast. However, when the wind speed was predominant from the northeast, approximately 20% of the cases presented wind speeds above 16 ms−1. Therefore, when there is a change in synoptic conditions near the Belgrano II station, the predominance of the winds becomes northeasterly, with an intensification in its speed. An extra-tropical cyclone approaching the Belgrano II region caused the wind to become of the northeastern on the days before the extreme event, so that two days beforehand there was an increase in air temperature in surface. When we analyzed the wind speed in Belgrano II, we identified that there was not a marked change and over the years, however, we identified that the seasonality presented changes, with more intense winds between the months of May and September.

Intensity Evolution of Zonal Shear Line over the Tibetan Plateau in Summer: A Perspective of Divergent and Rotational Kinetic Energies

Based on the ERA5 reanalysis datasets during 1980–2019, a total of eleven zonal shear lines (ZSLs) that caused heavy precipitation and lasted more than 60 hours over the Tibetan Plateau in summer are selected for composite analysis. By decomposing the kinetic energy (K) near the ZSL into divergent and rotational kinetic energies (KD and KR) and the kinetic energy of interaction between the divergent wind and the rotational wind (KRD), the influence of the rotational and divergent winds on the evolution of the ZSL intensity is investigated from the perspective of KD and KR. The main results are as follows. The ZSL is a comprehensive reflection of rotation and convergence. The intensity evolution of ZSL is essentially synchronized with those of K, KR, and KRD but lags behind KD by about three hours. The enhancement of K is mainly contributed by KR, which is governed by the conversion from KD to KR. Furthermore, the increase in the conversion from KD to KR is controlled by the geostrophic effect term Af, which is determined by the joint enhancement of the zonal rotational and meridional divergent wind components (uR and vD). Therefore, the joint enhancement of uR and vD controls the increase of the ZSL intensity, leading to increased precipitation.

Interannual Variability and Trends of Sea Surface Temperature Around Southern South America

The interannual variability and trends of sea surface temperature (SST) around southern South America are studied from 1982 to 2017 using monthly values of the Optimally Interpolation SST version 2 gridded database. Mid-latitude (30°–50°S) regions in the eastern South Pacific and western South Atlantic present moderate to intense warming (~0.4°C decade−1), while south of 50°S the region around southern South America presents moderate cooling (~ −0.3°C decade−1). Two areas of statistically significant trends of SST anomalies (SSTa) with opposite sign are found on the Patagonian Shelf over the southwest South Atlantic: a warming area delimited between 42 and 45°S (Northern Patagonian Shelf; NPS), and a cooling area between 49 and 52°S (Southern Patagonian Shelf; SPS). Between 1982 and 2017 the warming rate has been 0.15 ± 0.01°C decade−1 representing an increase of 0.52°C at NPS, and the cooling rate has been –0.12 ± 0.01°C decade−1 representing a decrease of 0.42°C at SPS. On both regions, the largest trends are observed during 2008–2017 (0.35 ± 0.02°C decade−1 at NPS and –0.27 ± 0.03°C decade−1 at SPS), while the trends in 1982–2007 are non-significant, indicating the record-length SSTa trends are mostly associated with the variability observed during the past 10 years of the record. The spectra of the records present significant variance at interannual time scales, centered at about 80 months (~6 years). The observed variability of SSTa is studied in connection with atmospheric forcing (zonal and meridional wind components, wind speed, wind stress curl and surface heat fluxes). During 1982–2007, the local meridional wind explains 25–30% of the total variance at NPS and SPS on interannual time scales. During 2008–2017, the SSTa at NPS is significantly anticorrelated with the local zonal wind (r = –0.85), while at SPS it is significantly anticorrelated with the meridional wind (r = –0.61). Our results show that a substantial fraction of the interannual variability of SSTa around southern South America can be described by the first three empirical orthogonal function (EOF) modes which explain 28, 16, and 12% of the variance, respectively. The variability of the three EOF principal components time series is associated with the combined variability of El Niño–Southern Oscillation, the Interdecadal Pacific Oscillation and the Southern Annular Mode.

Responses of extreme convective storms in the southern Himalayan front to the Indian summer monsoon

South Asian monsoon convection in the southern Himalayan front (SHF) plays an important role in the entry of South Asia pollutants to the Qinghai-Tibetan Plateau and even to the stratosphere. Combining the 16-yr TRMM observations and Indian summer monsoon (ISM) index, the responses of extreme convective storms to the ISM are investigated. The results showed that the top height of convective storms increases gradually while their convective intensity weakens with the evolution from pre-monsoon to monsoon season. The establishment period of ISM (late May to early June) is found to be an important period for convective storms in the SHF. Not only does the frequency of storms occur more frequently, but also the convective intensity of storms is stronger compared with pre-monsoon and monsoon seasons. The ISM is proved to have an important impact on the formation of convective storms in northwestern inland Indian subcontinent, dominated by the water vapor supply, rather than the ‘juicy’ eastern SHF. Cloud top height of storms is more sensitive to the strength of ISM than the convective intensity. The water vapor transportation originated from the Bay of Bengal have more significant contribution in convective storm formation there than that from Arabian Sea. In stronger ISM years, the southerly meridional wind component over the Bay of Bengal is stronger, which forces more water vapor to be transported to further inland areas along the Himalayas. Together with the warm center and anticyclone in upper troposphere, resulting in convective storms in the westernmost of SHF occur more frequently and can grow up to higher top heights.

Intercomparisons of Tropospheric Wind Velocities Measured by Multi-Frequency Wind Profilers and Rawinsonde

Concurrent measurements of three-dimensional wind velocities made with three co-located wind profilers operated at frequencies of 52 MHz, 449 MHz, and 1.29 GHz for the period 12–16 September 2017 are compared for the first time in this study. The velocity–azimuth display (VAD) method is employed to estimate the wind velocities. The result shows that, in the absence of precipitation, the root mean square difference (RMSD) in the horizontal wind speed velocities U and wind directions D between different pairs of wind profilers are, respectively, in the range of 0.94–0.99 ms−1 and 7.7–8.3°, and those of zonal wind component u and meridional wind component v are in the respective ranges of 0.91–1.02 ms−1 and 1.1–1.24 ms−1. However, the RMSDs between wind profilers and rawinsonde are in the range of 2.89–3.26 ms−1 for horizontal wind speed velocity and 11.17–14.48° for the wind direction, which are around 2–3 factors greater than those between the wind profilers on average. In addition to the RMSDs, MDs between wind profilers and radiosonde are around one order of magnitude larger than those between wind profilers. These results show that the RMSDs, MDs, and Stdds between radars are highly consistent with each other, and they are much smaller than those between radar and rawinsonde. This therefore suggests that the wind profiler-measured horizontal wind velocities are much more reliable, precise, and accurate than the rawinsonde measurement.

Performance of surface winds from atmospheric reanalyses in the Southwestern South Atlantic Ocean

AbstractSea surface wind (10‐m high) is one of the most important variables for oceanic applications. The zonal and meridional surface wind components from seven global atmospheric reanalyses [National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research Reanalysis 1, NCEP/Department of Energy Reanalysis 2, NCEP/climate forecast system reanalysis (CFSR), European Centre for Medium‐Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA‐Interim), Japanese 55‐year Reanalysis (JRA‐55), the Modern‐Era Retrospective Analysis for Research and Application V2, and the ERA5 Reanalysis] were compared with surface winds retrieved from QuikScat/SeaWinds and Ascat scatterometers, and with surface ground winds measured in eight coastal meteorological stations, at the Southwestern South Atlantic Ocean, between 35° and 55°S and 55° and 70°W, approximately. Bias, root mean squared error and the Pearson linear correlation coefficient for the zonal and meridional wind components were estimated in this study. The effects of the atmospheric stability, the surface currents and the discontinuity between land and sea roughness on the satellite data are discussed in this paper. It was concluded that Modern‐Era Retrospective Analysis for Research and Application, Version 2 reanalysis is the product that better fitted with satellite data, and ERA5 follows it in performance. CFSR, ERA‐Interim and JRA‐55 reanalyses also presented very good performances. Surface winds from NR1 and NR2 reanalysis showed the largest differences with satellite data. ERA5 reanalysis was the database that better compared with the coastal observations measured at the meteorological stations. Surface winds measured at meteorological stations are, in general, worse represented by the reanalyses than satellite data. This last could be due to the discontinuity between land and sea roughness, and because the topography is not completely represented by the atmospheric global models.

Winds aloft over three water bodies influence spring stopover distributions of migrating birds along the Gulf of Mexico coast

Abstract Migrating birds contend with dynamic wind conditions that ultimately influence most aspects of their migration, from broad-scale movements to individual decisions about where to rest and refuel. We used weather surveillance radar data to measure spring stopover distributions of northward migrating birds along the northern Gulf of Mexico coast and found a strong influence of winds over nonadjacent water bodies, the Caribbean Sea and Atlantic Ocean, along with the contiguous Gulf of Mexico. Specifically, we quantified the relative influence of meridional (north–south) and zonal (west–east) wind components over the 3 water bodies on weekly spring stopover densities along western, central, and eastern regions of the northern Gulf of Mexico coast. Winds over the Caribbean Sea and Atlantic Ocean were just as, or more, influential than winds over the Gulf of Mexico, with the highest stopover densities in the central and eastern regions of the coast following the fastest winds from the east over the Caribbean Sea. In contrast, stopover density along the western region of the coast was most influenced by winds over the Gulf of Mexico, with the highest densities following winds from the south. Our results elucidate the important role of wind conditions over multiple water bodies on region-wide stopover distributions and complement tracking data showing Nearctic–Neotropical birds flying nonstop from South America to the northern Gulf of Mexico coast. Smaller-bodied birds may be particularly sensitive to prevailing wind conditions during nonstop flights over water, with probable orientation and energetic consequences that shape subsequent terrestrial stopover distributions. In the future, the changing climate is likely to alter wind conditions associated with migration, so birds that employ nonstop over-water flight strategies may face growing challenges.

Assessment of Extreme Precipitation Indices over Indochina and South China in CMIP6 Models

AbstractPrecipitation extremes over the Indochina and South China (INCSC) region simulated by 40 global climate models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) were quantitatively assessed based on the skill score metrics of four extreme precipitation indices when compared with observational results from a high-resolution daily precipitation dataset for 1958–2014. The results show that it is difficult for most of the CMIP6 models to reproduce the observed spatial pattern of extreme precipitation indices in the INCSC region. The interannual variability of the extreme precipitation indices is relatively better simulated for South China than for Indochina. In general, most of the CMIP6 models perform better in South China compared with Indochina when taking both the simulations of spatial pattern and interannual variability into consideration. Only three models (EC-EARTH3, EC-EARTH3-Veg, and NorESM2-MM) can successfully reproduce both the spatial pattern and the interannual variability for the INCSC region. Through model ranking, the multimodel ensemble generated by a selection of the most skillful models leads to a more realistic simulation of the extreme precipitation indices both in South China and Indochina. Better simulation of the meridional wind component over South China and the water vapor convergence over Indochina can partly reduce the wet biases, resulting in a more realistic simulation of extreme precipitation indices over the INCSC region.

The Effect of Midnight Temperature Maximum Winds on Post‐Midnight Equatorial Spread F

AbstractThe SAMI3/equatorial spread F (Sami3 is also a model of the ionosphere/ESF) code is used to simulate the growth of equatorial plasma bubbles in the presence of a background wind field based on measured winds. The measured winds exhibit the well‐known “midnight temperature maximum” (MTM) pattern, in which an equatorward wind occurs simultaneously with a cessation in the zonal wind. The MTM is often preceded by strong equatorward winds (about 100 m/s). The circumstance where the MTM winds are symmetric across the equator is considered; here the meridional wind component in the southern hemisphere is the reverse of the northern meridional wind. The timing of the wind pattern relative to the imposition of a seed for the ESF instability is explored. We find that the simultaneous occurrence of a seed wave and a strong converging meridional wind pattern can produce post‐midnight ESF. We further find that the seed wave and the sudden cessation of the zonal winds can also produce post‐midnight ESF. The Magnetic mEridional NeuTrAl Thermospheric code verifies the occurrence of converging meridional wind patterns such as those simulated here, based on ionosonde data. Results suggest that regional‐scale wind measurements would aid in the prediction signal‐disrupting ionospheric bubbles.

The Role of Strong Meridional Neutral Winds in the Formation of Deep Equatorial Ionization Trough in CHAMP Observations

AbstractIonospheric plasma density data from the Planar Langmuir Probe onboard the CHAllenging Minisatellite Payload (CHAMP) are used to investigate the latitudinal profile of the ionospheric plasma density. Along with the well‐known profiles with features of the equatorial ionization anomaly and equatorial plasma bubbles, a third type is observed as smooth profiles with large‐scale deep equatorial trough (DET) where the plasma density is extremely low. A global survey focusing on magnetically quiet conditions reveals that the DET profiles are generally a post‐sunset phenomenon, and their appearance depends on longitude and season. During equinoxes in active solar years, the strong pre‐reversal enhancement electric field is believed to uplift the ionosphere and produce the DET profiles. While around June solstice, the DET profiles are clustered at W– E longitude, where the zonal electric field is weak but the magnetic meridional component of the neutral wind is most significant. The SAMI2 model is then employed to simulate the ionospheric plasma distribution in the magnetic meridional plane with enhanced neutral wind, which successfully reproduced the main features of the observed DET profile. The results indicate that the neutral wind can obviously affect the equatorial ionosphere by transporting plasma in the magnetic flux tube through neutral‐ion interaction. The neutral wind is previously known to work with the inclined magnetic field at mid‐latitude, while this study emphasizes its effect around the magnetic equator especially when the electric field forcing () is weak or absent.

A High-Resolution Modeling Study of the Circulation Patterns at a Coastal Embayment: Ría de Pontevedra (NW Spain) Under Upwelling and Downwelling Conditions

Estuarine exchange flow plays an important role in the biogeochemical properties exchange between the coastal ocean and the estuaries. Circulation patterns inside the Rias Baixas (NW Iberian Peninsula) have been widely studied, while the Ría de Pontevedra has received less attention. A high-resolution ROMS-AGRIF system configuration with three online two-way nesting domains was used and validated to enhance the knowledge about the circulation patterns of this Ría. Two representative estuarine circulation patterns were studied: a coastal upwelling event and a downwelling event associated with a maximum river runoff of 84 m3/s. An analysis using Empirical Orthogonal Functions (EOF) was applied to the horizontal subtidal velocity fields at the surface layer and at 20 m depth and to the subtidal normal velocity across vertical cross-sections. Upwelling events are characterized by a reinforcement of the positive estuarine circulation, in which the surface waters outflow and deep waters inflow at the main sill of the Ría, whilst full water column inflow (outflow) is observed at the northern (southern) openings. Conversely, downwelling events induce an opposite behavior, consisting of a negative estuarine circulation with the surface (deep) inflow (outflow) at the main sill of the Ría and outflow (inflow) at the northern (southern) openings. Evidences of a front splitting two vertical clockwise circulation cells were found. These cells are associated with the ocean water subduction that enters through the surface layer under downwelling favorable winds and this surface inflow opposes to the direction of the inner estuarine circulation. The EOF study confirms that shelf wind forcing has an important contribution to the variability of the velocity field, inducing positive or negative estuarine like circulation. The temporal principal components (PC) of EOF analysis present correlations of 0.92 between the meridional wind component and the first mode, and 0.77 between the zonal wind component and the second mode. Third EOF mode represents the circulation during transitional winds when some vorticity structures are visible. The realistic high-resolution simulation of the physical conditions at the Ría de Pontevedra provides new insights into the ocean-estuarine water exchanges and allow the visualization of recirculation processes and characteristic structures that have consequences in the marine ecosystem and in their productivity.

Assimilation of individual components of radiosonde winds: An investigation to assess the impact of single-component winds from space-borne measurements on NWP

Wind observations are critical for the better atmospheric analysis for Numerical Weather Prediction (NWP), particularly over the tropics. High-resolution direct wind observations are essential for defining smaller scales and deeper atmospheric structures. Recently launched Aeolus satellite delivers wind profiles that mostly satisfy these requirements, suitable for NWP assimilation. The main product from Aeolus is the horizontally projected Line of Sight wind component, a single component of wind, approximately zonal in nature over the tropics and more meridional over the Polar region, and the main limitation of this observation. Observing system experiments are conducted with the assimilation of individual components of radiosonde and pilot balloon winds to assess the impact of a single component of wind compared to the assimilation of full wind vector in the NCMRWF global assimilation and forecast system. Denial of the zonal component of wind in the assimilation system produced a larger observation increment (observation – model background) in the meridional wind than the full vector assimilation. In contrast, the observation increment of the zonal wind remains nearly the same, even after removing the meridional wind component from the assimilation system. Assimilation of both zonal and meridional components produced changes in the analysis fields of various meteorological variables; however, the zonal component plays a significant role in the tropics. Both wind components play an important role in controlling the humidity field, whereas only zonal components of wind impact the temperature field in the upper troposphere and lower stratosphere. Though the full vector wind assimilation produces a larger impact in the forecast fields of various meteorological variables, the zonal component has more impact than the meridional component. Verification of analysis and forecast wind against the satellite-derived atmospheric motion vectors clearly show the importance of both the horizontal components of winds in the lower troposphere. In contrast, the zonal component of wind alone has a high impact on the upper troposphere and lower stratosphere.

A New Technique for Century-Scale Wind Component Indices

Advancing the understanding of how variations in the climate over the ocean influences the weather over the United States can be aided by developing marine climatic indices. Herein, wind component indices are developed using nearly 125 years of wind observations from ships. A new technique using probability density functions for the values of meridional and zonal wind components is developed to create indices for a user-selected region and accumulation interval (e.g., annual or seasonal) over a climatological period. The index is a measure of the shift in the likelihood of values above or below a threshold for a given season or year as compared to the long-term (e.g., 125 year) probability distribution. The new index method is demonstrated using ship-based wind observations for select regions of the Atlantic Ocean. Ship observations are extracted from release 3.0.0 of the International Comprehensive Ocean-Atmosphere Data Set. Prior to index creation, an assessment of wind data quality is completed, and suspect observations are removed. The method to create a probabilistic wind component index is described along with a metric of the uncertainty in the calculated index. Two wind component indices, for regions in the north Atlantic and eastern Gulf of Mexico, are presented to demonstrate the technique. Using the Gulf of Mexico index as a case study, we compare the wind component indices to precipitation measured over the Gulf coastal states and identify several relationships between multi-year changes in winds in the eastern Gulf of Mexico and precipitation on a seasonal basis. Exploring the spatiotemporal patterns of the onshore/offshore component wind indices derived from seasonal wind forecasts could provide a metric for future prediction of seasonal or annual precipitation to support the agricultural sector. The index method demonstrated can be applied to other spatiotemporal regions for different parameters and using other source datasets.

Examination of the Daily Cycle Wind Vector Modes of Variability from the Constellation of Microwave Scatterometers and Radiometers

Offshore of many coastal regions, the ocean surface wind varies in speed and direction throughout the day, owing to forcing from land/sea temperature differences and orographic effects. Far offshore, both diurnal and semidiurnal wind vector variability has been noted in the Tropical Atmosphere Ocean-TRIangle Trans-Ocean buoy Network (TAO-TRITON) mooring data in the tropical Pacific Ocean. In this manuscript, the tropical diurnal wind variability is examined with microwave radiometer-derived winds from the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement (GPM), merged with RapidScat and other scatterometer data. Since the relationship between wind speed and its zonal and meridional components is nonlinear, this manuscript describes an observationally based methodology to merge the radiometer and scatterometer-based wind estimates as a function of observation time, to generate a multi-year dataset of diurnal wind variability. Compared to TAO-TRITON mooring array data, the merged satellite-derived wind components fairly well replicate the semidiurnal zonal wind variability over the tropical Pacific but generally show more variability in the meridional wind components. The meridional component agrees with the associated mooring location data in some locations better than others, or it shows no clear dominant diurnal or semidiurnal mode. Similar discrepancies are noted between two forecast model reanalysis products. It is hypothesized that the discrepancies amongst the meridional winds are due to interactions between surface convergence and convective precipitation over tropical ocean basins.

Effects of Substorms on High‐Latitude Upper Thermospheric Winds

AbstractDuring magnetospheric substorms, high‐latitude ionospheric plasma convection is known to change dramatically. How upper thermospheric winds change, however, has not been well understood, and conflicting conclusions have been reported. Here, we study the effect of substorms on high‐latitude upper thermospheric winds by taking advantage of a chain of scanning Doppler imagers (SDIs), THEMIS all‐sky imagers (ASIs), and the Poker Flat incoherent scatter radar (PFISR). SDIs provide mosaics of wind dynamics in response to substorms in two dimensions in space and as a function of time, while ASIs and PFISR concurrently monitor auroral emissions and ionospheric parameters. During the substorm growth phase, the classical two‐cell global circulation of neutral winds intensifies. After substorm onset, the zonal component of these winds is strongly suppressed in the midnight sector, whereas away from the midnight sector two‐cell circulation of winds is enhanced. Both pre and postonset enhancements are ≥100 m/s above the quiet‐time value, and postonset enhancement occurs over a broader latitude and local‐time area than preonset enhancement. The meridional wind component in the midnight and postmidnight sectors is accelerated southward to subauroral latitudes. Our findings suggest that substorms significantly modify the upper‐thermospheric wind circulation by changing the wind direction and speed and therefore are important for the entire magnetosphere‐ionosphere‐thermosphere system.

Analysis of and Solution to the Polar Numerical Noise Within the Shallow‐Water Model on the Latitude‐Longitude Grid

AbstractThis study conducts an analysis of the polar numerical noise in the barotropic shallow‐water version of the Grid‐point Atmospheric Model of IAP LASG (GAMIL‐SW) and provides a good solution to the problem. GAMIL‐SW suffers from numerical noise in the polar region in some ideal test cases, which is likely to be detrimental to the full physical model. The noise is suspected to be related to the nonlinear advection term in the momentum equation. Thus, a new shallow‐water model with a vector‐invariant form of the momentum equation is developed on the latitude‐longitude grid to analyze the polar noise. It is found that the version with meridional wind component staggered on the pole is free from noise, while the version with zonal wind component staggered on the pole is still contaminated. By redefining the polar relative vorticity, the polar noise is eliminated in the latter version, and the global conservation properties are maintained. In addition, the test cases demonstrate that the new shallow‐water model maintains the properties of the original GAMIL‐SW with respect to numerical accuracy and computational stability. This study helps to identify appropriate governing equations to further develop the next generation of GAMIL dynamical core.

Dynamic Bayesian temporal modeling and forecasting of short-term wind measurements

Abstract We present a new Bayesian modeling approach for joint analysis of wind components and short-term wind prediction. This approach considers a truncated bivariate matrix Bayesian dynamic linear model (TMDLM) that jointly models the u (zonal) and v (meridional) wind components of observed hourly wind speed and direction data. The TMDLM takes into account calm wind observations and provides joint forecasts of hourly wind speed and direction at a given location. The proposed model is compared to alternative empirically-based time series approaches that are often used for short-term wind prediction, including the persistence method (naive predictor), as well as univariate and bivariate ARIMA models. Model performance is measured predictively in terms of mean squared errors associated to 1-h and 24-h ahead forecasts. We show that our approach generally leads to more accurate short term predictions than these alternative approaches in the context of analysis and forecasting of hourly wind measurements in 3 locations in Northern California for winter and summer months.

Tidal characteristics in the Gulf of Khambhat, northern Arabian Sea – based on observation and global tidal model data

Summary Tidal characteristics of the Gulf of Khambhat are described based on measured and modelled sea-level data. Data were recorded at three locations inside and two locations outside the Gulf with record lengths of 6–12 months to study the tidal propagation. A northward increase in tidal amplitude is noticed from Daman (eastern side) and Diu (western side) and attains maxima at Bhavnagar. A similar trend is followed by the amplitude of the major tidal constituents, although there are discrepancies for that of the minor constituents. The non-tidal factor which influences the sea-level is the local wind, especially the alongshore component of wind. A positive correlation is obtained between the sea-level and the meridional component of wind at each location. Harmonic analysis of sea-level data shows that M2 is the major tidal constituent which propagates in a non-linear fashion inside the Gulf. Tides from two global tide models (MIKE21 and FES2014) have been compared with the measured data, which could be used for further prediction of the tides and sediment transport in the Gulf. The tide elevation derived from the MIKE21 model has further been used for the harmonic analysis of tide. The tides predicted using one-month data are up to 10% smaller than those predicted using the one-year data. The global tide model FES2014 data performs well with measured data for offshore locations, whereas it fails to predict the same for the inner Gulf locations. The study manifests the fact that to understand the dynamics of complex tidal areas, regional models should better be used than global tidal models.