The scaling and skewness of optimally transported meshes on the sphere

In the context of numerical solution of PDEs, dynamic mesh redistribution methods (r-adaptive methods) are an important procedure for increasing the resolution in regions of interest, without modifying the connectivity of the mesh. Key to the success of these methods is that the mesh should be sufficiently refined (locally) and flexible in order to resolve evolving solution features, but at the same time not introduce errors through skewness and lack of regularity. Some state-of-the-art methods are bottom-up in that they attempt to prescribe both the local cell size and the alignment to features of the solution. However, the resulting problem is overdetermined, necessitating a compromise between these conflicting requirements. An alternative approach, described in this paper, is to prescribe only the local cell size and augment this an optimal transport condition to provide global regularity. This leads to a robust and flexible algorithm for generating meshes fitted to an evolving solution, with minimal need for tuning parameters. Of particular interest for geophysical modelling are meshes constructed on the surface of the sphere. The purpose of this paper is to demonstrate that meshes generated on the sphere using this optimal transport approach have good a-priori regularity and that the meshes produced are naturally aligned to various simple features. It is further shown that the sphere's intrinsic curvature leads to more regular meshes than the plane. In addition to these general results, we provide a wide range of examples relevant to practical applications, to showcase the behaviour of optimally transported meshes on the sphere. These range from axisymmetric cases that can be solved analytically to more general examples that are tackled numerically. Evaluation of the singular values and singular vectors of the mesh transformation provides a quantitative measure of the mesh aniso...Comment: Updated following reviewer comment

The Madden–Julian oscillation wind-convection coupling and the role of moisture processes in the MM5 model

The Madden–Julian oscillation (MJO) produced by a mesoscale model is investigated using standardized statistical diagnostics. Results show that upper- and lower-level zonal winds display the correct MJO structure, phase speed (8 m s[superscript −1]) and space–time power spectrum. However, the simulated free atmosphere moisture, outgoing longwave radiation and precipitation do not exhibit any clear MJO signal. Yet, the boundary layer moisture, moist static energy and atmospheric instability, measured using a moist static energy instability index, have clear MJO signals. A significant finding is the ability of the model to simulate a realistic MJO phase speed in the winds without reproducing the MJO wind-convection coupling or a realistic propagation in the free atmosphere water vapor. This study suggests that the convergence of boundary layer moisture and the discharge and recharge of the moist static energy and atmospheric instability may be responsible for controlling the speed of propagation of the MJO circulation.National Science Foundation (U.S.) (Grant ATM0733698

Interannual variability of sea surface temperature in the eastern tropical Pacific Ocean and Central American rainfall

Sea surface temperature (SST) in the east Pacific warm pool (EPWP) plays a potentially important role in Central American rainfall, tropical cyclogenesis, ocean biology, large-scale tropical heating, and the El Niño-Southern Oscillation (ENSO). The first part of this dissertation is aimed at understanding what processes govern the interannual variability of SST in the EPWP. Interannual wind stress, shortwave radiation, and precipitation were used as forcing to an ocean general circulation model. Shortwave heating was identified as the primary driver of the interannual SST tendency in the EPWP. The high correlation between the EPWP and the equatorial Pacific Ocean is explained by the fact that equatorial SST anomalies modify the distribution of atmospheric vertical motions and therefore cloud cover and shortwave heating. In a parallel set of experiments, the low-frequency variability of the Tehuantepec gap winds was also shown to have a considerable effect on that of SST in the EPWP. Motivated by the results of the first part of this dissertation, the second part offers significant improvements to the mean state of the equatorial Pacific Ocean in a climatology ocean model experiment by including the Galápagos Islands in the model topography. In this context, the equatorial cold bias is reduced. Furthermore, when the ocean model is coupled to the atmosphere through zonal wind stress, the problem of an excessively regular and biennial ENSO is also reduced. The change in ENSO timescale is a result of the same dynamics operating on a different mean state. The third part of this dissertation is aimed at understanding the role of the interannual variability of SST in the EPWP in that of Central American rainfall. An anomalously warm EPWP can trigger a rapid enhancement of the east Pacific intertropical convergence zone (ITCZ) in rainy seasons following peak ENSO events, which leads to a rainfall anomaly over Central America. Moreover, the timing and amplitude of the SST-enhanced ITCZ depends on the persistence of the ENSO event. The longer the equatorial SST anomaly persists, the longer the EPWP is subject to anomalous shortwave heating and thus the greater the subsequent SST enhancement of the ITCZ

Quantifying the SST biases in data assimilative ocean simulations of the Benguela Upwelling System

The Benguela Upwelling System (BUS) on the west coast of southern Africa is one of the global ocean’s most productive upwelling systems supporting a large fishing industry, a fledgling aquaculture sector and offshore mining interests. Despite intensive monitoring and modelling studies, there is no regionally tailored ocean forecasting system that is explicitly developed to deal with the unique ocean dynamics of the Benguela. In this study, the Hybrid Coordinate Ocean Model (HYCOM) is used in conjunction with the Ensemble Optimal Interpolation (EnOI) assimilation scheme to study the impact of assimilating sea surface temperature (SST) and along-track sea level anomalies (SLA) observations on predicted upwelling dynamics in the Benguela. In order to evaluate the predictive skill and impact of data assimilation, three experiments with HYCOMEnOI are evaluated: (1) with no assimilation (HYCOMFREE), (2) only assimilating along-track SLA (HYCOMSLA) and (3) assimilating both SLA and SST (HYCOMSLA+SST). Using MODIS Terra SST as reference, the model SST outputs are evaluated. HYCOMFREE is found to exhibit a warm bias along the coast, HYCOMSLA shows an even greater warm bias while HYCOMSLA+SST conversely shows a much improved SST forecast skill. It is hypothesised that the warm biases could be due to errors in boundary conditions and/or the ERA-interim wind product used to force the model. Furthermore, a comparison of the assimilated SST product (the Operational Sea Surface Temperature and Sea Ice Analysis; OSTIA) with MODIS SST reveals biases in OSTIA up to ±1 ◦C, raising questions over its suitability for assimilation in upwelling regions. Studying the effect of assimilation on SSH, SST and surface currents before and after the assimilation suggests that an increase in SSH from assimilated SLA leads to increased warm SST biases in HYCOMSLA. This is due to an incorrect relationship between SSH and SST in the free-running HYCOM, from which the static ensemble is derived for the EnOI. HYCOMSLA+SST exhibits slightly enhanced SSH increments but the associated increase in SST is significantly reduced by the assimilated SST, resulting in a reduction of the bias with very little impact on the current dynamics. This is reflected in the surface velocitiy increments, which are similar to or worse than that of HYCOMSLA. Investigating the potential of HYCOM-EnOI as an operational forecasting system has revealed that the assimilation of SST and along-track SLA vastly improves modelled SST for the BUS upwelling. Errors in the free-running model, which constitutes the static ensemble, need addressing and comparisons between MODIS and OSTIA SSTs suggests that OSTIA may not be ideally suited for assimilation in the case of coastal upwelling, due to limitations in capturing the dynamics correctly

성층권 준2년주기진동이 매든-줄리안 진동에 미치는 영향

학위논문(박사)--서울대학교 대학원 :자연과학대학 지구환경과학부,2020. 2. 손석우.최근 성층권 준2년주기진동이 북반구 겨울철에 매든-줄리안 진동에 영향을 미칠 수 있다는 가능성이 제시된 바 있다. 성층권 준2년주기진동이 동풍일 때, 겨울철 매든-줄리안 진동의 대류 활동이 서풍일 때에 비해 활발해지는 경향이 있는데, 이는 최근에 발표된 연구로써 구체적인 현상과 그 원인에 대한 이해가 부족할 뿐만 아니라, 현업 예측 모형의 계절내 시간 규모의 예측성 향상에도 직접적으로 연관되어 있다는 점에서 이 현상을 이해하는 것은 매우 중요하다. 이에 따라, 본 학위논문은 성층권 준2년주기진동이 매든-줄리안 진동에 미치는 영향과 그 원인을 다양한 방법을 바탕으로 이해해보고자 하였다. 관측 자료를 바탕으로, 성층권 준2년주기진동의 위상에 따라 매든-줄리안 진동의 활동 특성뿐만 아니라 매든-줄리안 진동의 원격 상관성까지 변화하는 것으로 나타났다. 성층권 준2년주기진동이 동풍일 때, 단순히 매든-줄리안 진동의 대류 활동 강도뿐만 아니라 대류 활동의 동진 속도가 느려지고 지속기간이 길어지는 경향이 있으며, 더 나아가, 매든-줄리안 진동의 중위도 원격상관성 강도까지 강화시키는 것으로 나타났다. 이러한 성층권 준2년주기진동에 따른 매든-줄리안 진동의 체계적인 변화는 현상에 대한 당위성을 높임과 동시에 이에 대한 검증 및 이해의 필요성을 더욱 높였다. 따라서, 다양한 예측 모형을 활용하여 다음과 같이 현상을 이해하고자 하였다. 첫 번째로 성층권 준2년주기진동과 매든-줄리안 진동을 직접 모의하는 기후 모형을 이용하여 현상이 존재하는 지 검증해보고자 하였고, 대부분의 모형에서 모의하지 못하였으나, 하나의 기후모형에서 유일하게 관측에 비해서는 약하지만 현상을 모의하였다. 성층권 준2년주기진동이 매든-줄리안 진동에 영향을 미칠 수 있는 원인으로 크게 성층권 준2년주기진동에 따른 대류권 상부와 성층권 하부 간 동서방향평균 정적 안정도의 변화, 그리고 매든-줄리안 진동의 연직 구조 변화로 인한 정적안정도의 지역적 변화가 주요 원인으로 알려져 있는데, 기후 모형에서 이를 약하지만 모의하는 것으로 나타났고 현상을 모의하는데 기여했을 것으로 추정되었다. 두 번째로는 역학코어모형을 바탕으로 성층권 준2년주기진동이 매든-줄리안 진동의 연직 구조 변화에 역학적으로 영향을 미칠 수 있는지 검증해보았고, 정적 안정도 이외에도 다른 역학적 영향 가능성이 있음을 확인하였다. 마지막으로 10개의 현업 기관 예측 모형을 바탕으로 현상에 대해 검증해본 결과, 현상이 모의될 뿐 아니라 매든-줄리안 진동의 예측성에도 영향을 미치는 것으로 나타났다. 모형에 따라 차이가 있으나, 성층권 준2년주기진동이 동풍일 때, 매든-줄리안 진동의 예측성이 1-10일 더 높은 것으로 확인되었다. 현재까지 여러 모형에서 두 현상의 상관성을 약하게 나타내긴 하였으나, 그럼에도 불구하고 모형에서 성층권 준2년주기진동과 매든-줄리안 진동 현상 각각을 모의하는 데 한계가 있기 때문에, 관측에서 나타난 만큼 두 현상의 높은 상관성을 모의하기에는 어려움이 있었다. 이를 향후 개선하기 위한 방안 중 하나로써, 매든-줄리안 진동의 주요 물리 과정을 바탕으로 구름-복사 피드백 작용과 수분의 평균적 공간 분포 모의 능력이 매든-줄리안 진동의 모의에 중요한 것을 확인하였다. 이러한 물리 과정의 모의 능력이 향상된다면, 향후 성층권 준2년주기진동과 매든-줄리안 진동의 상관관계를 이해하고 예측성 향상에도 크게 기여할 것으로 기대된다.Recent studies have shown that the Quasi-Biennial Oscillation (QBO) affects the boreal winter Madden-Julian Oscillation (MJO). During the easterly phase of QBO (EQBO) winters, the MJO activity is amplified, and the opposite is shown during the westerly phase of QBO (WQBO) winters. Since this relationship is very recently reported with simple correlation analysis, it should be confirmed and understood in detail. This thesis is to investigate the QBO-MJO connection using a variety of datasets, such as the observations, dynamical core model, climate models, and subseasonal-to-seasonal (S2S) prediction models. Their possible mechanism(s) and the impacts on the MJO prediction are also evaluated and discussed. In the observational study, it is shown that the overall MJO characteristics are closely linked with the stratospheric QBO. The MJO activity around the Maritime Continent becomes stronger and more organized during EQBO than during WQBO winters. The QBO-related MJO change explains up to 40% of the interannual variation of the boreal winter MJO amplitude. During EQBO winters, the MJO convections propagate further eastward with a slower propagation, and more enhanced MJO teleconnection is also presented. These systematic changes in MJO activity confirm the QBO-MJO connection, emphasizing the stratospheric impact on the MJO. Due to the short analysis period of the observational data, the model outputs are helpful for a better understanding of this phenomenon. In the climate models, however, a weak hint of the QBO-MJO link is found only in the medium-resolution Max Planck Institute Earth System Model (MPI-ESM-MR) among four CMIP5 models that internally generate the QBO. In this model, the MJO anomalies become slightly stronger and more organized during EQBO than during WQBO winters. Overall differences, however, are still much weaker and less organized than the observation. When daily MJO-index amplitude is compared, their differences are not robust. The reasons for weak QBO-MJO connection might result from the weak QBO and MJO amplitudes, and weak static stability change in response to the QBO in the model. To better simulate the QBO structure and to examine the dynamical process, the QBO-MJO connection is tested in an idealized experiment using a dynamical core model. It is found that the QBO can directly change the MJO-related vertical structure. The MJO-induced cold anomaly near the tropopause becomes colder, especially over the western Pacific in the EQBO-like experiment, which promotes the MJO activity. This result seems to be related to the Doppler shift effect by the QBO-related zonal wind, suggesting the potential impact of the dynamical process on the QBO-MJO connection. Considering both of dynamical and physical processes with a better QBO simulation, the capability of the QBO-MJO connection is evaluated in the S2S prediction models. Their relationship is also applied in the MJO prediction skill. Ten operational models participated in the S2S prediction project show a higher MJO prediction skill during EQBO winters than during WQBO winters, based on the QBO-MJO link. For the bivariate anomaly correlation coefficient of 0.5, the MJO prediction skill during EQBO winters is enhanced by up to 10 days. This enhancement is insensitive to the initial MJO amplitude, indicating that the improved MJO prediction skill is not simply the result of a stronger MJO. Instead, a longer persistence of the MJO during EQBO winters likely induces a higher prediction skill by having a higher prediction limit. Even though the QBO modulates the MJO prediction skill, the QBO-MJO connection is not fully captured even in the S2S prediction models. To improve the simulation of the QBO-MJO connection in these models, the relationship of MJO prediction skill with model biases in the mean moisture fields and the longwave cloud–radiation feedbacks are investigated, based on understanding the MJO processes. In most models, a notable dry bias develops within a few days of forecast lead time in the deep tropics, especially across the Maritime Continent. The dry bias weakens the horizontal moisture gradient over the Indian Ocean and western Pacific, likely dampening the organization and propagation of the MJO. Most S2S models also underestimate the longwave cloud–radiation feedbacks in the tropics, which may affect the maintenance of the MJO convective envelope. In the S2S prediction project, the operational models with smaller bias in the mean horizontal moisture gradient and the longwave cloud–radiation feedbacks show higher MJO prediction skills, suggesting that improving those biases would enhance MJO prediction skill and the simulation of the QBO-MJO connection.1. Introduction 1 2. QBO-MJO connection: observational features 7 2.1. Data and methods 7 2.2. Interannual variation of seasonal-mean tropical convection by the ENSO 11 2.3. Interannual modulation of subseasonal tropical convective activity by the QBO 15 2.3.1. MJO characteristics with the QBO 19 2.3.2. MJO teleconnection with the QBO 24 2.3.3. Lead-lag relationship 26 2.3.4. Seasonality 28 2.3.5. Possible mechanism(s) of the QBO-MJO connection 29 3. QBO-MJO connection in climate models 34 3.1. Data and methods 34 3.2. QBO and MJO simulations in CMIP5 models 38 3.3. QBO-MJO connection in MPI-ESM-MR simulations 44 4. A possible mechanism of the QBO-MJO connection 59 4.1. Model description and experimental design 59 4.2. Model results 63 5. QBO-MJO connection in the S2S prediction models 71 5.1. Data and methods 71 5.1.1. Data 71 5.1.2. Evaluation metrics 76 5.2. QBO prediction skill in S2S prediction models 77 5.3. MJO prediction skill with QBO 82 5.3.1. Sensitivity to initial MJO amplitude 91 5.3.2. Sensitivity to initial MJO phase 94 5.3.3. Limiting factors of MJO prediction skill 96 6. MJO prediction skill in the S2S prediction models: for improving the simulation of the QBO-MJO connection 102 6.1. Data and methods 102 6.1.1. Data 102 6.1.2. Evaluation metrics 103 6.2. MJO prediction skill 108 6.3. Mean-state biases and their impact on MJO prediction skill 125 6.3.1. Mean moisture field 126 6.3.2. Cloud-longwave radiation feedback 132 7. Summary and discussions 136 References 142 Abstract (Korean) 153Docto

Similarities between the tropical Atlantic seasonal cycle and ENSO: An energetics perspective

The tropical Pacific and Atlantic Oceans have similar mean states - easterly winds and a zonally sloping thermocline which shoals in the east - but strikingly different sea surface temperature..

Influence of high latitude anomalies on tropical climate phenomena and global climate

The tropical ocean and atmosphere are a highly active and very important region of the globe. Climate phenomena such as El Ni˜no (Philander, 1990), the Tropical Atlantic Dipole, and the Indian Ocean Dipole, play an important role in global climate variability. The tropical atmospheric boundary layer is very sensitive to even small changes in the sea surface temperature (SST). Small SST anomalies in the tropics can lead to shifts in the large scale convection cells and result in atmospheric heating. There is potential for positive feedback between the tropical ocean and atmosphere.Ocean waves are capable of propagating long distances very fast. Barotropic waves (adjustments in free surface height) can propagate round the globe within days. Baroclinic waves, propagating along the thermocline are able to cross the equatorial Atlantic in 2 – 3 months. This work shows the potential for ocean wave propagation to influence global climate, by linking high latitude anomalies to tropical climate phenomena.The first part of this thesis is a detailed examination of the “Tropical Atlantic Dipole” (TAD). Analysis of model data shows a dipole pattern in the SST, with strong cross-equatorial asymmetry in the surface mixed layer. Below the mixed layer the pattern becomes symmetric, and Kelvin and Rossby wave like adjustment can be seen to occur. However, the timeseries is not sufficiently long to provide confidence in resolving the power spectrum, and as such the results are inconclusive. The complexity of the model makes it difficult to identify the mechanism(s) which are responsible for driving the dipole. An idealised basin model is used to examine high latitude anomalies which create equatorward propagating coastal Kelvin waves as a possible driving mechanism for the TAD. The results show that coastal Kelvin wave propagation can quickly transmit a signal from the high latitude anomaly to the equator, and equatorial Kelvin and Rossby wave propagation can quickly influence the entire tropical ocean. This suggests that forcing of the TAD may come from higher latitudes, although it is still not fully understood how a symmetric sub-surface signal can become asymmetric at the surface. Restoring surface boundary conditions limit the response of the model, restricting the formation of a TAD. A similar experiment, using an idealised coupled model configuration is suggested, but not possible in the time available.The second part of this thesis looks in detail at the role of the ocean in rapidly transmitting a high latitude response to the equator, using an existing coupled climate model configured with realistic land geometry and bottom topography. Simulations of a salinity anomaly in the Southern Ocean show that it is possible to create an equatorial response in SST within a month, with SST anomalies of 2.5± after 6 months. Barotropic Kelvin and Rossby wave propagation is shown to be important in creating such a rapid equatorial response. Two points that are identified from this experiment are examined in further detail using an idealised basin model. Firstly, a mechanism for energy exchange within the equatorial waveguide is tested. Results suggest that it is not the mechanism responsible for the signals seen in the coupled climate model. Secondly, idealised model integrations confirm that transmission of signals along topographic ridges is possible. Signals strong enough to excite equatorward coastal Kelvin wave propagation are able to use topography to cross the Southern Ocean and reach the coast of Australia

Intraseasonal variability in the diurnal cycle of precipitation in the Philippines

2019 Spring.Includes bibliographical references.Precipitation in the region surrounding the South China Sea (SCS) over land and coastal waters exhibits a strong diurnal cycle associated with a land-sea temperature contrast that drives a sea-breeze circulation. The boreal summer intraseasonal oscillation (BSISO) is an important modulator of the daily mean precipitation rate and the amplitude of the diurnal cycle. Using 19 years of the CMORPH precipitation product for the Philippines, it is shown that in aggregate the diurnal cycle amplitude is maximized before the arrival of the broader oceanic convective envelope associated with the BSISO. Over Luzon Island in the northern Philippines, the diurnal cycle amplitude is not in phase with daily mean precipitation, which peaks with the large-scale BSISO convection. An increase in nocturnal and morning precipitation more than compensates for the reduced precipitation rates during the afternoon peak amidst the BSISO active period. This pattern is not seen over Mindanao Island in the southern Philippines, where diurnal cycle amplitude tends to determine daily mean precipitation. A strong diurnal cycle in coastal waters west of the Philippines is evident in the transition from the inactive to active phase, due to offshore propagation of convection generated over land. This behavior is dramatically different on small spatial scales within the Philippine archipelago, depending strongly on topography. For example, the BSISO influence on the diurnal cycle on the eastern side of the high mountains of Luzon is nearly opposite to the western side. It is proposed, using wind, moisture, and radiation budget products from the ERA-Interim reanalysis, that the enhanced diurnal cycle over land and coastal waters west of the mountains during BSISO suppressed phases is a consequence of increased insolation and weaker prevailing onshore winds. Offshore propagation, and thus the diurnal cycle over the coastal waters of the SCS, is suppressed until ambient mid-level moisture increases during the transition to the active BSISO phase. In the BSISO enhanced phases, strong low level winds out of the southwest combine with increased cloudiness to suppress the sea-breeze circulation and thus the diurnal cycle of precipitation in the SCS region. Strong frictional moisture convergence leading the BSISO is not found to be concurrent with the peak in the diurnal cycle. Results are consistent when examined in other precipitation products or BSISO indices, and support conclusions derived from studies focusing on intraseasonal modulation of precipitation in other regions of the Maritime Continent, with some important local distinctions owed to geography