Large Scale Weather Patterns

This investigation performs diagnostic analyses with NCEP/NCAR reanalysis data and Israeli precipitation data to examine the dynamical processes that drive the teleconnection pattern associated with Israeli winter precipitation anomalies. A dipole teleconnection pattern with one centre of action over western Europe and another centre of action over the eastern Mediterranean (EM) is found to be associated with the winter precipitation anomalies in Israel. This pattern is referred to as the southern Levant (SL) pattern. The positive (negative) phase of the SL pattern is found to be closely associated with enhanced (reduced) precipitation in Israel. Composites of the 300 hPa stream-function field show that both phases of the SL pattern are associated with eastward propagating wave packets that traverse about three-quarters of the distance across the globe. These wave packets, which have a phase speed that is close to zero, originate over the northeast Pacific (the east coast of North America) for the positive (negative) SL phase, and propagate eastward across the North Atlantic, western Europe, the EM, and southern Asia, until they eventually decay over the northwest Pacific. As these wave packets pass over western Europe and the EM, the SL pattern is triggered and anomalies in Israeli winter precipitation occur. The SL pattern is found to have an e-folding timescale of 4 days, which is determined by the group velocity of the wave packet. The time-averaged spatial structure of the circumglobal wave packet closely resembles the first empirical orthogonal function of the hemispheric 300 hPa meridional wind field. This suggests that the circumglobal wave packet may be a fundamental pattern of variability which is relevant for the entire Northern Hemisphere.

Increased use of solar photovoltaic electricity requires a better understanding of the impact of large‐scale atmospheric teleconnections on incident short wave (SW) solar radiation. Our focus is on the relationship between winter (December to February) SW radiation in northwest Europe and the dominant Euro‐Atlantic atmospheric teleconnection patterns using multiple multi‐decadal observational and gridded reanalysis datasets, with a focus on the islands of Ireland and Britain. Our study reveals that the previously reported west–east seesaw in the correlation pattern between the winter North Atlantic Oscillation (NAO) index and winter SW radiation across the United Kingdom is complex, involving several zonal changes in the sign of the NAO–SW correlations (multiple seesaws). By comparison with the NAO, the east Atlantic pattern exerts only a weak control on winter SW radiation across the United Kingdom and Ireland, although in the western part of the Iberian Peninsula and adjacent Atla...

Samenvatting Περίληψη Acknowledgements Curriculum vitae viii Chapter 1.2.2 Rossby waves and their propagation characteristics Atmospheric Rossby waves are disturbances from the zonal symmetry in the atmospheric circulation that move westward under the inuence of the rotation of the Earth. In this thesis we study Rossby waves that are excited by tropical Rossby wave sources and travel into the extratropics, creating so-called teleconnection patterns. Rossby waves are named after Carl-Gustaf Arvid Rossby (Rossby (1939)) who identied them and explained their westward propagation in 1939. Rossby waves are dispersive, the longer the wavelength the faster the westward propagation. Their propagation is oset by the background winds such that in the region of the westerly winds, only the longest waves retrograde, move against the ow westward, while the smaller waves are carried eastward. The largest waves are called planetary waves. The planetary Rossby waves determine β * = β − ∂ 2Ū ∂y 2 (1.6) The jet stream is a fast westerly current owing around the globe in the upper troposphere with a sharp meridional prole, that is a consequence of the Earth's rotation and its dierential heating (section 1.2.1). Due to its sharp prole β * tends to have a larger relative maximum thanŪ (see Eq. 1.6). Therefore K s (Eq. 1.7) takes its maximum values along the jet core and reduces quickly toward the anks of the jet. This curvature and the attribute of the rays to bend towards larger K-values allows the changes the waveguiding properties and hence inuence the formation and propagation characteristics of Rossby waves. The pattern is most vigorous in the boreal winter when the circulation is strongest. The eect of NAO variability on the local weather conditions are distinct over Eastern North America, North Atlantic and Northern Europe and often across Southern Europe and the Middle East. Contrary to the extratropical mean ow, the NAO has a pronounced 1.4.2 The Indian Ocean Dipole Another important source of IO variability is the atmosphere-ocean coupled phenomenon widely known as the Indian Ocean Dipole (IOD) (Saji et al (1999)). On a regional scale the IOD has a signicant impact on the atmospheric circulation and precipitation anomalies and on a larger scale the phenomenon correlates with temperature and precipitation anomalies in remote regions, such as Europe, America, South Africa and Northeast Asia (Saji and Yamagata (2003a)). The IOD is characterized by an eastwest SST gradient along the central tropical IO that strongly impacts the atmospheric convection over the basin. When the Central-West IO is warmer than the Eastern IO the event is called a positive event and vice-Along the equator the strong east to southeasterly trade winds allow the cold deep currents, that ow from Antarctica to the west coast of South America, to upwell and keep the SSTs relatively cool along the equatorial cold tongue. In the Western Pacic the trade winds deepen the thermocline and the surface water is warmer. This area of warm ocean waters, the Pacic warm pool, is a major source for atmospheric convection program, which is nanced by the Netherlands Organization for Scientic Research (NWO). We also thank the two anonymous reviewers that provided valuable suggestions and remarks that improved the outcome of this work.

where G(x,y,t) = [g1(x,y,t), g2(x,y,t), g3(x,y,t)] is a generic vector function, k is the zonal wavenumber, n is the meridional mode and r=1 for Rossby, r=2 for WGW, r=3 for EGW. The MRGW is included in the r=1 and n=0 mode. The Kelvin wave is represented by n=-1 and r=3. Substitution of (2) in (1), multiplication by the conjugate of the eigenfunctions and global integration lead to: Apart from the well defined annual cycle, higher frequency variability is also detected in the convection in Amazon region up to the very prominent diurnal variation of convection. Theoretical studies have explored the role of the heat source of tropical S. America in the generation of Rossby (RW), Mixed Rossby-Gravity (MRGW), Kelvin (KW) and Gravity waves (GW) for the maintenance of the observed circulation patterns, such as the upper tropospheric anticyclonic circulation known as the Bolivian High. MRGW and KW are also generated by the stationary heat source. However, observational studies of the equa...

Cold surges were un usu ally ac tive in sub trop i cal East Asia dur ing Jan u ary-Feb ru ary 2005. These cold surges were pre ceded by up stream wave trains, which orig i nated in the Med i ter ra nean-Sa hara re gion and prop a gated east ward along the sub trop i cal jet stream over the Eur asian con ti nent. The north erly of the up per-level cy clonic anom aly in East Asia cou pled with the low-level north erly upon the ar rival of wave ac tiv ity, and this was fol lowed by a quick south ward pen e tra tion of cold air mass and sur face an ti cy clone. Di ag nos tic and nu mer i cal re sults sug gest that the an oma lously ac tive wave ac tiv ity af fect ing the East Asian cold surges may be at trib uted to an an oma lously en hanced jet stream over the Mid dle East and an an oma lously west ward ex ten sion of the East Asian Jet Stream. The con fig u ra tion of these two sub trop i cal jet streams es tab lished a strong wave guide through which wave ac tiv ity forced in the Med i ter ra nean-Sa hara re gion could ef fi ciently prop a gate to East Asia, re sult ing in above av er age cold surge events in sub trop i cal East Asia. Wave-like per tur ba tion tended to be amplified at the entrance to the East Asian jet through barotropic energy conversion from the mean flow.

This study supports efforts directed toward research on large-scale atmospheric patterns and on the variability of tornado outbreaks. Specifically, we applied rotated principal component analysis to identify synoptic-scale patterns of 500-hPa geopotential height associated with tornado outbreaks in the United States. We created a database of historic tornado outbreaks using kernel density estimation on events composed of at least seven tornadoes of magnitude (E)F2 or higher (major tornado outbreaks) that occurred in May from 1950 to 2019 (91 events). Results of the analysis show that the first three principal components explained the majority, that is, 74% of the total variation. Based on the analysis of congruence coefficients, the Promax oblique transformation was chosen as the most representative rotation in portraying physically meaningful modes and resulted in three main atmospheric patterns. Two of these, to the best of our knowledge, have not been yet identified as the most representative of tornado outbreaks in any previous studies. Additionally, results suggest that although synoptic patterns associated with major May tornado outbreaks remain the same over time, partial variability in their geographic location exhibits some cyclical behavior, especially on decadal and multidecadal scales. Identifying these patterns can serve as a first step in determining how they may change under anthropogenic climate change in the future.

The purpose of this chapter is to discuss certain disturbances around the pole of a Venus-type planet that result as a response to barotropic instability processes in a zonal flow. We discuss a linear instability of normal modes in a zonal flow through the barotropic vorticity equations (BVEs). By using a simple idealization of a zonal flow, the instability is employed on measurements of the upper atmosphere of Venus. In 1998, the tropical cyclone Mitch gave way to the observational study of a dipole vortex. This dipole vortex might have helped to intensify the cyclone and moved it towards the SW. In order to examine this process of interaction, the nonlinear BVE was integrated in time applied to the 800-200 hPa average layer in the previous moment when it moved towards the SW. The 2-day integrations carried out with the model showed that the geometric structure of the solution can be calculated to a good approximation. The solution HLC moves very fast westwards as observed. On October 27, the HLA headed north-eastward and then became quasi-stationary. It was also observed that HLA and HLC as a coupled system rotates in the clockwise direction.

The purpose of this chapter is to discuss certain disturbances around the pole of a Venus-type planet that result as a response to barotropic instability processes in a zonal flow. We discuss a linear instability of normal modes in a zonal flow through the barotropic vorticity equations (BVEs). By using a simple idealization of a zonal flow, the instability is employed on measurements of the upper atmosphere of Venus. In 1998, the tropical cyclone Mitch gave way to the observational study of a dipole vortex. This dipole vortex might have helped to intensify the cyclone and moved it towards the SW. In order to examine this process of interaction, the nonlinear BVE was integrated in time applied to the 800-200 hPa average layer in the previous moment when it moved towards the SW. The 2-day integrations carried out with the model showed that the geometric structure of the solution can be calculated to a good approximation. The solution HLC moves very fast westwards as observed. On October 27, the HLA headed north-eastward and then became quasi-stationary. It was also observed that HLA and HLC as a coupled system rotates in the clockwise direction.

In this paper, mechanisms for the formation of quasi-stationary extratropical wave trains associated with four distinct winter patterns of seasonal mean tropical Pacific convection anomalies, including the western CP (W-CP) pattern, La Niña (LN) pattern, CP El Niño (CPEN) pattern and EP El Niño (EPEN) pattern, are investigated by utilizing observational and six Atmospheric Model Intercomparison Project Phase 5 (AMIP5) high-skilled models (ACCESS1.0, CanAM4, CCSM4, CMCC-CM, HadGEM2-A, MPI-ESM-LR) datasets. The observational results show that tropical Pacific forcing induces upper-level anomalous Rossby wave source that force vorticity over the central North Pacific through modulating the local Hadley circulation, then excite eastward propagation of quasi-stationary planetary waves penetrating into the North Atlantic, and finally establish the associated teleconnections in mid-and high-latitudes by local synoptic eddy-mean flow interaction. Different structures of wave trains may be explained by evident differences in intensity and scale of the extratropical vorticity forcing originating from distinct convection anomalies over the tropical Pacific. The MME results further confirm the possible physical processes obtained from observations, but there still exist some significant differences. These cover the North Pacific and North America and can possibly be attributed to the models' simulation ability of extratropical vorticity forcing and local storm track intensity. In addition, the stratosphere-troposphere interaction plays an important role for extratropical atmospheric circulation anomalies. The stratospheric polar vortex not only modulates the underlying wave train, especially for the North Atlantic/Europe sector, but is also affected by the upper-tropospheric disturbance in high-latitudes through the upward wave activity flux. Keywords Tropical Pacific convection • Extratropical wave train • Synoptic eddy-mean flow interaction • Extratropical vorticity forcing • Storm track Hans-F. Graf: visiting fellow of the Chinese Academy of Sciences.

Midlatitude baroclinic waves drive extratropical weather and climate variations, but their predictability beyond 2 weeks has been deemed low. Here we analyze a large ensemble of climate simulations forced by observed sea surface temperatures (SSTs) and demonstrate that seasonal variations of baroclinic wave activity (BWA) are potentially predictable. This potential seasonal predictability is denoted by robust BWA responses to SST forcings. To probe regional sources of the potential predictability, a regression analysis is applied to the SST-forced large ensemble simulations. By filtering out variability internal to the atmosphere and land, this analysis identifies both well-known and unfamiliar BWA responses to SST forcings across latitudes. Finally, we confirm the model-indicated predictability by showing that an operational seasonal prediction system can leverage some of the identified SST-BWA relationships to achieve skillful predictions of BWA. Our findings help to extend long-r...

A process-oriented model study of the tropical Atlantic Ocean is presented. It is based on a hierarchy of reduced-gravity primitive equation layer models, including one, two or three active layers; the domain of integration spans the latitudes from 35°S to 35°N and the wind forcing is provided by the ECMWF Re-Analysis data, which are decomposed in EOF. The full oceanic response is first compared with that obtained with a limited number of EOF in order to identify the main patterns of the wind-driven circulation. Particular attention is devoted to the analysis of beta-refracted baroclinic Rossby waves, that shape the oceanic variability in the eastern tropical region. The wind forcing is then confined to zonal bands with different meridional widths centered at the equator, so that the thermocline variability along the African coasts outside such bands is merely due to coastal Kelvin waves originating from eastward-traveling equatorial Kelvin waves through a well known teleconnection ...

Numerical weather prediction (NWP) plays significant role on daily human life, agriculture, economy and disaster warning. The main aim of this study is to predict (24, 48, 72 and 96) hours ahead of forecast mid-latitude (500) hpa geopotential height by solution of barotropic vorticity equation. Inputted wind velocity was obtained from (ECMWF) for (500) hpa pressure level on one selected day for both winter and summer. The result shows that the model have good accuracy in predict low and high systems but there is a little shifting in determining systems location, And the model present same geopotential height gradient with pattern but it has a shifting from actual pattern. Also The model sensitivity test are carried out, which shows that efficient prediction is below horizontal wind value of (60 m/h), while vice versa for high wind lead to loss of predictability.

The impacts of El Niño and La Niña on the U.S. climate during northern summer are analyzed separately. Composite analyses reveal that a continental-scale anomalous high dominates over most of North America during La Niña events and leads to hot and dry summers over the central United States. However, the impacts of El Niño over North America are weaker and more variable. A linear barotropic model is used to explore the maintenance of the anomalous patterns. Various forcing terms derived from observations via a single-level vorticity budget analysis are used to drive the model. When the barotropic model is driven by the total forcing (Rossby wave source plus transient eddy forcing plus nonlinear interactions), the model simulations resemble the observed patterns, and a strong and extensive anticyclone is reproduced in the La Niña simulation. The model responses to the individual forcing terms suggested that the vorticity stretching term ( fD) and the transient eddy forcing contribute...

A series of experiments with rotating, electromagnetically forced, turbulent flows were carried out at the Sapienza University of Rome to investigate the eddy-wave duality in flows with a β-effect and the electromagnetic force acting in the westward direction. When the β-effect is significant, i.e., as in planetary atmospheric and oceanic circulations, nonlinear eddy/wave interactions facilitate flow self-organization into zonal patterns in which Rossby waves and westward propagating cyclonic and anticyclonic eddies coexist. Upon time averaging, eddies disappear and the flow pattern transforms into a system of alternating zonal jets. What is the relationship between eddies, jets, and Rossby waves? To address this issue, we designed a laboratory experiment in which a westward zonal flow is produced by applying an electromagnetic small-scale forcing to a thin layer of a rotating fluid. In order to investigate different levels of flow zonality and a wider range of zonal modes, we varied the forcing intensity and the area of the forced sector. The zonal flow evolves as a system of westward propagating, large scale, cyclonic, and anticyclonic eddies. The propagation speed of the traveling structures was calculated from the Hovmöller diagrams of both the streamfunction and the centroids of clusters of different types (cyclonic and anticyclonic eddy cores and saddle point neighborhoods) obtained via an Okubo-Weiss analysis. The results were compared with the theoretical phase speed of a Rossby wave. The correspondence between these two characteristics at the radius of maximum shear corresponding to the epicenter of the barotropic instability is quite good, particularly after including the radial variation of the zonal velocity in the β-term. It is concluded that the Rossby waves and eddies are inseparable as the former maintain the instability that sustains the latter. This symbiosis visually resembles the Rossby soliton.

The role of equatorial oceanic waves on the evolution of the 2007 positive Indian Ocean Dipole (pIOD) event was evaluated using available observations and output from a quasi-analytical linear wave model. It was found that the 2007 pIOD event was a weak and short-lived event: developed in the midsummer (July), matured in the early-fall (September), and terminated in the mid-fall (October). The evolution of the 2007 pIOD event was linked to the equatorial wave dynamics. The event development was associated with the generation of upwelling equatorial Kelvin waves (westward current anomalies) generated by easterly wind anomalies. The event termination was associated with the occurrence of eastward zonal current anomalies resulting from a complex interplay between the wind-forced down welling Kelvin waves and the eastern-boundary-reflected Rossby waves. Results from a quasi-analytical linear wave model show that during the event development and maturation, the wind-forced Kelvin waves played a dominant role in generating zonal current anomalies along the equatorial Indian Ocean, while the easternboundary-reflected Rossby waves tended to weaken the wind-forced Kelvin wave signals. During the event termination our model shows that the initiation of anomalous eastward current resulted from the reflected Rossby waves at the eastern boundary. The wind-forced Kelvin waves associated with the seasonal reversal of the monsoon further strengthened the eastward zonal currents generated by the boundary-generated Rossby waves in late-October/early-November. This highlights the importance of the eastern-boundary-reflected Rossby waves on the IOD event termination.

We review teleconnections within the atmosphere and ocean, their dynamics and their role in coupled climate variability. We concentrate on teleconnections in the latitudinal direction, notably tropical-extratropical and interhemispheric interactions, and discuss the timescales of several teleconnection processes. The tropical impact on extratropical climate is accomplished mainly through the atmosphere. In particular, tropical Pacific sea surface temperature anomalies impact extratropical climate variability through stationary atmospheric waves and their interactions with midlatitude storm tracks. Changes in the extratropics can also impact the tropical climate through upper ocean subtropical cells at decadal and longer timescales. On the global scale the tropics and subtropics interact through the atmospheric Hadley circulation and the oceanic subtropical cell. The thermohaline circulation can provide an effective oceanic teleconnection for interhemispheric climate interactions.

This study describes the climate conditions and primary climate influences in the greater Uruguay region and shows results of a climate change prediction for that region performed using a coupled ocean-atmosphere model. The study demonstrates the strong influences of the El Niño phenomenon on the region's precipitation variability, with flood conditions during El Niño and drought conditions during La Niña events. Future climate model simulations indicate temperature increases that reach about two degrees Celsius by the end of the 21 st century. Precipitation in the greater Uruguay region increases by about 0.5 mm/day and most of the increase happens in the summer months. One worrisome feature of the model future climate simulation is an increase in the time scales of precipitation variability. This points to longer periods of drought and flood in a warmer climate. If such a prediction materializes it would worsen the already negative influences of the prolonged precipitation extremes that are already experienced in the region.

The Silk Road pattern (SRP) is a leading mode of Eurasian atmospheric variability in boreal summer. It remains challenging for many models to predict the temporal phase of the SRP. This study investigates whether the forcing mechanism of the SRP can explain SRP prediction difficulty. Rossby-wave sources (RWS) associated with the SRP are first identified in a reanalysis. An idealised barotropic model is then used to test wave propagation from all identified RWS, to isolate the relevant forcing locations of the SRP, namely the RWS hotspots. In addition to previous findings, a new hotspot in the central North Pacific is located, which can force the SRP by westward dispersion of a zonally elongated Rossby wave. Furthermore, a new mechanism of the Indian summer monsoon in forcing the SRP is discovered. Results also suggest that Rossby-wave propagation by itself only explains a small fraction of the SRP amplitude, consistent with baroclinic energy conversion from the mean state, which has previously been shown to be vital for SRP growth-not represented in the idealised model. Seasonal hindcasts from the Pusan National University (PNU) coupled general circulation model are also analysed. The hindcasts can predict the SRP spatially but not temporally. Within a seasonal timescale, the hindcast develops an upper-level mean-state wind bias compared with the reanalysis, which is shown by the barotropic model to affect RWS hotspot locations of the SRP. While the reanalysis SRP is associated with North Atlantic sea-surface temperature (SST) anomalies, the hindcast SRP is associated with tropical Pacific SST. Wind biases in the North Atlantic jet exit, subtropical western Pacific, and tropical Pacific are found to alter wave propagation from the North Atlantic and the North and tropical Pacific, respectively, and can explain the association of the SRP with different SSTs in the reanalysis and hindcast. This sensitivity of hotspots to mean-state winds is proposed to reduce SRP prediction skill.

Abstract: Dynamic education must be able to follow the progress of the times that continue to develop. The demands of 21st century skills have an impact on improving the quality of vocational education through innovation of educational facilities and infrastructure that are in line with sustainable development goals in realizing quality education. To achieve this goal, the school seeks to improve the quality of learning through structuring the interior elements of the learning space. The purpose of this study was to describe the interior elements of Animaxx studio in terms of lighting, color, shape, and material aspects and to determine the effect of interior elements on the learning process in terms of motifs, affective, and learning achievement. Combined research methods (mix methods) were used in this study, quantitative methods were used to collect data about user perceptions of interior elements and responses to motives, affective, and learning achievement through questionnaire...

We explore the path between Indian Ocean observations and monsoon dynamics, the societal impacts of interannual climate variations and applications of resource predictions in southeastern Africa, the Mascarene Islands, India, southeast Asia and Australia. Recent progress in understanding ocean dynamics associated with SST variation is reviewed. The global El Niño-Southern Oscillation (ENSO) affects monsoon winds and ocean temperatures in a manner consistent with, but lagging, the Pacific. The ENSO influence often propagates across the tropical Indian Ocean from Africa to Indonesia, modulating the tropospheric moisture flux over the Indian Ocean and rainfall in surrounding continents. An east-west dipole in SST anomalies and monsoon rainfall is identified and related to the atmospheric Walker Cell. It appears partially in response to global ENSO conditions during build-up phase (July-Nov.). The eastern ‘node’ is confined near Sumatra, whilst the western centre of action extends from ...

Characteristics of atmospherically forced mid-latitude, subinertial barotropic motions in the oceans are studied with measurements of bottom pressure, barotropic currents, surface winds and air pressure from the central North Pacific. The bottom pressure is found to be dominated by atmospherically forced variability within the periods of about 1.5 to 300 days. The bottom pressure exhibits characteristics of evanescent, nonfree-wave motions at periods shorter than the shortest allowable free Rossby wave (from linear theory this cutoff period is about 2.6 days for the study area). At longer periods, the bottom pressure variability is consistent with the propagation of topographic Rossby waves. The main features of the bottom pressure energy densities and the patterns of coherence of bottom pressure with local and non-local atmospheric variables are predicted reasonably well by simple analytical models, both above and below the Rossby wave cutoff period. The spatial patterns of coheren...

We investigate the opposing effects of direct radiative forcing and sea surface warming on the atmospheric circulation using a hierarchy of models. In large ensembles of three general circulation models, direct CO 2 forcing produces a wavenumber 5 stationary wave over the Northern Hemisphere in summer. Sea surface warming produces a similar wave, but with the opposite sign. The waves are also present in the Coupled Model Intercomparison Project phase 5 ensemble with opposite signs due to direct CO 2 and sea surface warming. Analyses of tropical precipitation changes and equivalent potential temperature changes and the results from a simple barotropic model show that the wave is forced from the tropics. Key forcing locations are the Western Atlantic, Eastern Atlantic and in the Indian Ocean just off the east coast of Africa. The stationary wave has a significant impact on regional temperature anomalies in the Northern Hemisphere summer, explaining some of the direct effect that CO 2 concentration has on temperature extremes. Ultimately, the climate sensitivity and future changes in the land-sea temperature contrast will dictate the balance between the opposing effects on regional changes in mean and extreme temperature and precipitation under climate change.

The response of the tropical atmosphere to steady forcing for all four seasons is computed using a simple two-layer linear primitive-equation model allowing an arbitrary basic flow with two-dimensional shear. The character of each seasonal response is studied, and two distinct forms of behavior are found. Near the Equator, the forcing excites a slowly varying Kelvin wave of the time scale of months while, in midlatitudes, the atmosphere responds via the Rossby mode. Both regimes are separated by the critical latitudes existing on the poleward limits of the easterly channel of the basic flow. The long-wave scale response of the equatorial motions noted by Krishnamurti are shown to be the results of the natural filtering of the slowly varying Kelvin wave. The temporal variation of this zonal circulation is discussed.

The present study examines the mean features of Indian summer monsoon and the boreal summer intra-seasonal oscillations (BSISO) as simulated by the Weather Research and Forecasting (WRF) regional atmospheric model. For this the WRF model at 30-km horizontal resolution is forced with NCEP-II reanalysis as the initial and boundary condition and integrated for a period of 8 years. The simulated results are compared with TRMM rainfall as well as with the reanalysis datasets of NCEP-II and ERA-Interim. Analysis reveals that the WRF model is able to reproduce the observed characteristics of large scale mean features and spatial variations of rainfall and winds at 850hPa and 200hPa. The northward propagating intra-seasonal oscillation (ISO) components over the monsoon trough and the equatorial Indian Ocean is reasonably simulated by the WRF model and shows some promise in understanding the regional characteristics of Indian summer monsoon and its ISO components. WRF model can act as suitab...

Samenvatting Περίληψη Acknowledgements Curriculum vitae viii Chapter 1.2.2 Rossby waves and their propagation characteristics Atmospheric Rossby waves are disturbances from the zonal symmetry in the atmospheric circulation that move westward under the inuence of the rotation of the Earth. In this thesis we study Rossby waves that are excited by tropical Rossby wave sources and travel into the extratropics, creating so-called teleconnection patterns. Rossby waves are named after Carl-Gustaf Arvid Rossby (Rossby (1939)) who identied them and explained their westward propagation in 1939. Rossby waves are dispersive, the longer the wavelength the faster the westward propagation. Their propagation is oset by the background winds such that in the region of the westerly winds, only the longest waves retrograde, move against the ow westward, while the smaller waves are carried eastward. The largest waves are called planetary waves. The planetary Rossby waves determine β * = β − ∂ 2Ū ∂y 2 (1.6) The jet stream is a fast westerly current owing around the globe in the upper troposphere with a sharp meridional prole, that is a consequence of the Earth's rotation and its dierential heating (section 1.2.1). Due to its sharp prole β * tends to have a larger relative maximum thanŪ (see Eq. 1.6). Therefore K s (Eq. 1.7) takes its maximum values along the jet core and reduces quickly toward the anks of the jet. This curvature and the attribute of the rays to bend towards larger K-values allows the changes the waveguiding properties and hence inuence the formation and propagation characteristics of Rossby waves. The pattern is most vigorous in the boreal winter when the circulation is strongest. The eect of NAO variability on the local weather conditions are distinct over Eastern North America, North Atlantic and Northern Europe and often across Southern Europe and the Middle East. Contrary to the extratropical mean ow, the NAO has a pronounced 1.4.2 The Indian Ocean Dipole Another important source of IO variability is the atmosphere-ocean coupled phenomenon widely known as the Indian Ocean Dipole (IOD) (Saji et al (1999)). On a regional scale the IOD has a signicant impact on the atmospheric circulation and precipitation anomalies and on a larger scale the phenomenon correlates with temperature and precipitation anomalies in remote regions, such as Europe, America, South Africa and Northeast Asia (Saji and Yamagata (2003a)). The IOD is characterized by an eastwest SST gradient along the central tropical IO that strongly impacts the atmospheric convection over the basin. When the Central-West IO is warmer than the Eastern IO the event is called a positive event and vice-Along the equator the strong east to southeasterly trade winds allow the cold deep currents, that ow from Antarctica to the west coast of South America, to upwell and keep the SSTs relatively cool along the equatorial cold tongue. In the Western Pacic the trade winds deepen the thermocline and the surface water is warmer. This area of warm ocean waters, the Pacic warm pool, is a major source for atmospheric convection program, which is nanced by the Netherlands Organization for Scientic Research (NWO). We also thank the two anonymous reviewers that provided valuable suggestions and remarks that improved the outcome of this work.

Intraseasonal Kelvin waves represent an important aspect of tropical ocean dynamics. There has been considerable speculation about whether intraseasonal Kelvin waves generated by intraseasonal wind events can trigger El Niño Southern Oscillation (ENSO) events, and therefore play a potentially important role in the prediction of El Niño. This study considers the different but related issue of how ENSO conditions in uence intraseasonal Kelvin waves. Understanding this issue is an important part of clarifying the total role of scale interactions in the ENSO cycle.

Cool and wet weather conditions hit northern Central Asia, East Asia and central North America during the 2009 summer in concert with a strong jet stream and a prominent meandering upper-level circulation in the Northern Hemisphere mid-latitudes despite the fact that the year 2009 is the fifth warmest year globally in the modern record. It is found that the conspicuous atmospheric variability in the entire Northern Hemisphere mid-latitudes during the summer of 2009 was caused by a combination of teleconnections associated with significant tropical thermal forcings, strong polar forcing, and interaction between high-frequency weather events and climate anomalies. The strong negative circumglobal teleconnection pattern associated with the deficient Indian summer monsoon rainfall and developing El Niño condition was the major contributor to the cool and wet summer in June. On the other hand, the July weather conditions were attributable to the high-latitude impact of the unprecedented negative Arctic Oscillation, together with the Rossby wave response to the subtropical heating generated by convective activities over the Western North Pacific summer monsoon region. It is also noted that enhanced storm track activity and frequent cold surges from high-latitudes may have played a role in the cool and wet summer over the regions of interest.

Increased use of solar photovoltaic electricity requires a better understanding of the impact of large-scale atmospheric teleconnections on incident short wave (SW) solar radiation. Our focus is on the relationship between winter (December to February) SW radiation in northwest Europe and the dominant Euro-Atlantic atmospheric teleconnection patterns using multiple multi-decadal observational and gridded reanalysis datasets, with a focus on the islands of Ireland and Britain. Our study reveals that the previously reported west-east seesaw in the correlation pattern between the winter North Atlantic Oscillation (NAO) index and winter SW radiation across the United Kingdom is complex, involving several zonal changes in the sign of the NAO-SW correlations (multiple seesaws). By comparison with the NAO, the east Atlantic pattern exerts only a weak control on winter SW radiation across the United Kingdom and Ireland, although in the western part of the Iberian Peninsula and adjacent Atlantic Ocean significant positive correlations occur. High values of the Scandinavian pattern index result in higher than average winter SW radiation in much of northern Europe, although it is evident that some regions (e.g. northeast England, east Scotland and the adjacent North Sea area) exhibit the opposite behaviour. Inter-seasonal variations in the dominant atmospheric flow and moisture transport directions, steered by large-scale atmospheric pressure patterns, combined with orographic uplift and rainout effects on the windward side of hills and mountains are interpreted to be the physical drivers of the observed zonal variations and correlation sign reversals between winter SW anomalies and the NAO index. K E Y W O R D S east Atlantic (EA)

Welcome to the first issue of the Meteorological Service Singapore (MSS) Research Letters. This publication aims at sharing information on research done within MSS, in particular research from the Centre for Climate Research Singapore (CCRS) which forms part of MSS. We hope that the MSS Research Letters (MRL) will act as a conduit for sharing information, knowledge, and experience, as well as foster future collaborations. In doing so, the Research Letters will also act as a repository for MSS research, projects, and in-house development.

Satellite data of Sea Surface Temperature (SST) and Precipitation from the Tropical Rainfall Measuring Mission (TRMM) were analyzed to investigate the interactions between them in Indian Ocean Dipole (IOD) years in the Bay of Bengal (BoB), located at 5° N -25° N and 80° E-100° E region. In an agricultural country like Bangladesh, the success or failure of sufficient rainfall and its effects on agricultural production and water scarcity on regional basis are always of great concern. Even, a small fluctuation in the seasonal rainfall can have devastating impacts on agricultural sector. Understanding of these variability's especially the extreme droughts and floods are of great importance because the country's economy has bounded with the availability of monsoon rainfall. The SST has shown a significant relation with precipitation but there was some time lag. However, IOD did not show much regular influences on precipitation pattern. The investigation also reveals that SST was not the main parameter to occur the precipitation variation in the Bay. There may exist some other factors such as oceanic heat content, incoming solar radiation, wind etc, which might influence the seasonal/annual precipitaion. Future study may reveal the other factors that control the precipitation pattern in the Bay including the whole Bangladesh land.

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake 1–4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns 5–7 , but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake 1-4 . The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns 5-7 , but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

There now exists a number of computer models that show some success in predicting the meteorological conditions in the ensuing winter in the northern hemisphere. These examine the Arctic Oscillation (AO) and are successful up to two months in advance of the beginning of meteorological winter. Other models, which are based upon the North Atlantic Oscillation (NAO), also show some predictive success depending on the period over which they are tested. Here I show that the pattern of winter circulation across the North Atlantic Basin, Arctic Basin and Western Europe can be elucidated from the patterns present as early as the preceding summer and early autumn. These are visible in 500 mb charts that are publically available. This empirical model has particular success in years where there is neither an El Niño or La Niña event. However, in some recent El Niño or La Niña years, the pattern remains rather predictable making subsequent long term forecasting possible.

A spectral coupled ocean-atmosphere model, suitable for studying the El-Nino Southern Oscillation, has been formulated. The atmospheric component is global in its extent, while the oceanic part is a hemispheric basin representing the Pacific Ocean. Multi-decadal integrations of the model have been performed with realistic topography and seasonal variations in the atmospheric radiative forcing, and for resolutions ranging from low (rhomboidal wavenumber 15 (R15)) to

We formulate a numerically efficient coupled ocean-atmosphere model. It consists of a global atmosphere and a Pacific basin ocean, with two dynamical levels in each component. The model has a realistic climatology and displays El Niño-Southern oscillation variability at the observed frequencies. In hindcasts over the period 1981 to 2000, the model displays good skill out to seven months in forecasting the tropical upper ocean temperatures and zonal current anomalies. The most skilful forecasts occur for those initialised during June to November. Skilful predictions for the atmospheric fields generally only extend to one month, but can be as much as three months during major El Niño-Southern oscillation events.

Analysis of vertical profiles of absolute horizontal velocity collected in January 1981, February 1982 and April 1982 in the central equatorial Pacific as part of the Pacific Equatorial Ocean Dynamics (PEQUOD) program, revealed two significant narrow band spectral peaks in the zonal velocity records, centered at vertical wavelengths of 560 and 350 stretched meters (sm). Both signals were present in all three cruises, but the 350 sm peak showed a more steady character in amplitude and a higher signal-to-noise ratio. In addition, its vertical scales corresponded to the scales of the conspicuous alternating flows generically called the equatorial deep jets in the past (the same terminology will be used here). Meridional velocity and vertical displacement spectra did not show any such energetic features.

Extratropical impacts on the tropical El Nin˜o-Southern Oscillation (ENSO) are studied in a coupled climate model. Idealized experiments show that the remote impact of the extratropics on the equatorial thermocline through oceanic tunnel can substantially modulate the ENSO in both magnitude and frequency. First of all, an extratropical warming can be conveyed to the equator by the mean subduction current, resulting in a warming of the equatorial thermocline. Second, the extratropical warming can weaken the Hadley cells, which in turn slow down the mean shallow meridional overturning circulations in the upper Pacific, reducing the equatorward cold water supply and the equatorial upwelling. These oceanic dynamic processes would weaken the stratification of the equatorial thermocline and retard a buildup (purge) of excess heat content along the equator, and finally result in a weaker and longer ENSO cycle. This study highlights a nonlocal mechanism in which ENSO behavior is related to the extratropical climate conditions.

The present study examines the mean features of Indian summer monsoon and the boreal summer intra-seasonal oscillations (BSISO) as simulated by the Weather Research and Forecasting (WRF) regional atmospheric model. For this the WRF model at 30-km horizontal resolution is forced with NCEP-II reanalysis as the initial and boundary condition and integrated for a period of 8 years. The simulated results are compared with TRMM rainfall as well as with the reanalysis datasets of NCEP-II and ERA-Interim. Analysis reveals that the WRF model is able to reproduce the observed characteristics of large scale mean features and spatial variations of rainfall and winds at 850hPa and 200hPa. The northward propagating intra-seasonal oscillation (ISO) components over the monsoon trough and the equatorial Indian Ocean is reasonably simulated by the WRF model and shows some promise in understanding the regional characteristics of Indian summer monsoon and its ISO components. WRF model can act as suitable tool for predicting the ISO's over the Indian summer monsoon region. Further, the results presented here suggests that reanalysis with a realistic basic state and proper representation of northward moving ISO's is required to force a regional model. This study elucidate the WRF regional model has shown a reasonable representation of BSISO.

Sea Surface Temperature (SST) anomalies that develop in spring in the central Pacific are crucial to the El Niño Southern Oscillation (ENSO) development. Here we use a linear, continuously stratified, ocean model, and its impulse response to a typical ENSO wind pattern, to derive a simple equation that relates those SST anomalies to the low frequency evolution of zonal wind stress anomalies τx over the preceding months. We show that SST anomalies can be approximated as a “causal”  filter of τx, τx(t−t1)−cτx(t−t2), where t1 is~1–2months,t2 −t1 is ~6 months and c ranges between 0 and 1 depending on τx location (i.e. SST anomalies are approximately proportional to the wind stress anomalies 1–2 months earlier minus a fraction of the wind stress anomalies 7–8 months earlier). The  first term represents the fast oceanic response, while the second one represents the delayed negative feedback associated with wave reflection at both boundaries. This simple approach is then applied to assess the relative influence of the Indian Ocean Dipole (IOD) and of the Indian Ocean Basin-wide warming/cooling (IOB) in favouring the phase transition of ENSO. In agreement with previous studies, Atmospheric General Circulation Model experiments indicate that the equatorial Pacific wind responses to the IOD eastern and (IOB-related) western poles tend to cancel out during autumn. The abrupt demise of the IOD eastern pole thus favours an abrupt development of the IOB-cooling-forced westerly wind anomalies in the western Pacific in winter–spring (vice versa for an IOB warming). As expected from the simple SST equation above, the faster wind change fostered by the IOD enhances the central Pacific SST response as compared to the sole IOB influence. The IOD thereby enhances the IOB tendency to favour ENSO phase transition. As the IOD is more independent of ENSO than the IOB, this external influence could contribute to enhanced ENSO predictability.

Dubbed Ice Storm '98, an extreme weather event characterized by two synoptic systems in succession dropped about 70±100 mm (in terms of water equivalent) of freezing precipitation over southeastern Ontario, southwestern Quebec and northeastern New York during a 6-day period from January 5 to 10 in 1998. Individually, the two synoptic systems were not dramatically more extreme in freezing precipitation than other major freezing rain events (4 since 1961) which occurred in the past over the affected area. Some regions in the target area, however, were impacted more by the second system. Based on an analysis of the 500 hPa vorticity ®eld during the '98 event, we suggest that the 1997/98 El Nin Äo had a role in creating ā ow environment conducive to the rapid formation of the second synoptic system. In contrast, other major freezing rain events in the last 30 years involved only one synoptic system per event lasting no more than 3 days, and producing 20±50 mm of precipitation.

Field experiments show that the poleward velocity of high altitude weather balloons may, on rare occasions, be much higher than the observed poleward winds, while their eastward velocity is much slower than the observed eastward winds. Considering a simple physical model of horizontal particle's motion in the atmosphere, which includes a realistic model of the pressure eld, it is shown here that the existence of a nearly at parabolic resonance in the model gives rise to such ights of balloons on the observed time scales even though the associated atmospheric pressure eld does not support large poleward velocities.

This study describes the climate conditions and primary climate influences in the greater Uruguay region and shows results of a climate change prediction for that region performed using a coupled ocean-atmosphere model. The study demonstrates the strong influences of the El Niño phenomenon on the region's precipitation variability, with flood conditions during El Niño and drought conditions during La Niña events. Future climate model simulations indicate temperature increases that reach about two degrees Celsius by the end of the 21 st century. Precipitation in the greater Uruguay region increases by about 0.5 mm/day and most of the increase happens in the summer months. One worrisome feature of the model future climate simulation is an increase in the time scales of precipitation variability. This points to longer periods of drought and flood in a warmer climate. If such a prediction materializes it would worsen the already negative influences of the prolonged precipitation extremes that are already experienced in the region.

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake 1-4 . The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns 5-7 , but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

The impacts of wind and mass observations on analyses and forecasts are studied. Idealized perfect and complete observations are examined. A realistic three-dimensional data assimilation technique is considered. Spatial structures of analysis errors resulting from assimilation of wind or mass observations are theoretically investigated. Forecast impacts are assessed using the geostrophic adjustment theory and spatial probabilistic error models. Numerical predictability experiments are conducted to verify the theoretic conclusions. The main result is that, on average, wind observation are more important than mass observations, especially in the tropics and in the upper troposphere and stratosphere. In the mid- and high-latitude lower troposphere, the wind and mass observational data appear to be of comparable utility.

Estimating the vertical velocity (w) in the oceanic upper-layers is a key issue for understanding the cold tongue development in the Eastern Equatorial Atlantic. In this methodological paper, we develop an expanded and general formulation of the vertical velocity equation based on the primitive equation (PE) system, in order to gain new insight into the physical processes responsible for the Equatorial and Angola upwellings. This approach is more accurate for describing the real ocean than simpler considerations based on just the wind-driven patterns of surface layer divergence. The w-sources/forcings are derived from the PE w-equation and diagnosed from a realistic ocean simulation of the Equatorial Atlantic. Sources of w are numerous and express the high complexity of terms related to the turbulent momentum flux, to the circulation and to the mass fields, some of them depending explicitly on w and others not. The equatorial upwelling is found to be mainly induced by the (i) the zonal turbulent momentum flux, (ii) the curl of turbulent momentum flux and (iii) the imbalance between the circulation and the pressure fields. The Angola upwelling in the eastern part of the basin is controlled by strong curl of turbulent momentum flux. A strong cross-regulation is evidenced between the w-forcings independent of w and dependent on w, which suggests an equatorial balanced-dynamics. The w-forcing depending on w represents the negative feedback of the ocean to the wforcing independent of w: in the equatorial band, this adjustment is led by non-linear processes and by vortex stretching outside.

Identifying and removing the in¯uence of atmospheric circulation variability on central England temperature increases the statistical signi®cance of warming trends in spring, autumn and the annual mean over the last 50 years. The trends are more detectable because the circulation changes contribute greatly to the`noise' of interannual to interdecadal variability, but induce only small multi-decadal trends (the`signal'). Factoring out the circulation can thus enhance the signal-tonoise ratio. For precipitation, the recent enhancement in the difference between summer rainfall in south-east England and winter precipitation in northern Scotland can partly be explained by atmospheric circulation variability over the past 40 years ( particularly the increase in the North Atlantic Oscillation index from the 1960s to the early 1990s).