Wave climate and long-term changes for the Southern North Sea obtained from a high-resolution hindcast 1958–2002

An analysis of the extreme wave conditions in 1958–2002 in the North Sea as obtained from a regional model hindcast is presented. The model was driven by hourly wind fields obtained from a regional atmosphere model forced with reanalysis data from the National Center for Environmental Prediction (NCEP/NCAR). Furthermore, observed sea ice conditions from the Norwegian Meteorological Institute have been accounted for in the simulation. It is shown that the model is capable of reproducing extreme wave height statistics at a reasonable degree of approximation. The analysis of severe wave height events reveals that for much of the Southern North Sea, their number has increased since the beginning of the simulation period (1958), although the increase has attenuated later and leveled off around about 1985. On the other hand, the intensity and duration of severe wave height events decreased within the last few years of the simulation so that annual 99%-ile wave heights have also reduced since about 1990–1995. For the UK North Sea coast, a different behavior was found characterized by a reduction in severe wave conditions over much of the hindcast period.     

Performance evaluation of WAVEWATCH III model in the Persian Gulf using different wind resources

The third-generation wave model, WAVEWATCH III, was employed to simulate bulk wave parameters in the Persian Gulf using three different wind sources: ERA-Interim, CCMP, and GFS-Analysis. Different formulations for whitecapping term and the energy transfer from wind to wave were used, namely the Tolman and Chalikov (J Phys Oceanogr 26:497–518, 1996), WAM cycle 4 (BJA and WAM4), and Ardhuin et al. (J Phys Oceanogr 40(9):1917–1941, 2010) (TEST405 and TEST451 parameterizations) source term packages. The obtained results from numerical simulations were compared to altimeter-derived significant wave heights and measured wave parameters at two stations in the northern part of the Persian Gulf through statistical indicators and the Taylor diagram. Comparison of the bulk wave parameters with measured values showed underestimation of wave height using all wind sources. However, the performance of the model was best when GFS-Analysis wind data were used. In general, when wind veering from southeast to northwest occurred, and wind speed was high during the rotation, the model underestimation of wave height was severe. Except for the Tolman and Chalikov (J Phys Oceanogr 26:497–518, 1996) source term package, which severely underestimated the bulk wave parameters during stormy condition, the performances of other formulations were practically similar. However, in terms of statistics, the Ardhuin et al. (J Phys Oceanogr 40(9):1917–1941, 2010) source terms with TEST405 parameterization were the most successful formulation in the Persian Gulf when compared to in situ and altimeter-derived observations.     

A Bayesian hierarchical spatio-temporal model for significant wave height in the North Atlantic

Bad weather and rough seas continue to be a major cause for ship losses and is thus a significant contributor to the risk to maritime transportation. This stresses the importance of taking severe sea state conditions adequately into account in ship design and operation. Hence, there is a need for appropriate stochastic models describing the variability of sea states, taking into account long-term trends related to climate change. Various stochastic models of significant wave height are reported in the literature, but most are based on point measurements without considering spatial variations. As far as the authors are aware, no model of significant wave height to date exploits the flexible framework of Bayesian hierarchical space-time models. This framework allows modelling of complex dependence structures in space and time and incorporation of physical features and prior knowledge, yet at the same time remains intuitive and easily interpreted. This paper presents a Bayesian hierarchical space-time model for significant wave height. The model has been fitted by significant wave height data for an area in the North Atlantic ocean. The different components of the model will be outlined, and the results from applying the model to monthly and daily data will be discussed. Different model alternatives have been tried and long-term trends in the data have been identified for all model alternatives. Overall, these trends are in reasonable agreement and also agree fairly well with previous studies. Furthermore, a discussion of possible extensions to the model, e.g. incorporating regression terms with relevant meteorological data will be presented.     

A high-resolution satellite-derived sea surface temperature climatology for the western North Atlantic Ocean

Long-term and high-resolution (∼1.2 km) satellite-derived sea surface temperature (SST) fields of a monthly mean time series for the 1985–1999 period, and a daily climatology have been calculated for the North West Atlantic Ocean. The SST fields extend from 78°W to 41°W in longitude, and 30°N to 56°N in latitude, encompassing the region off Cape Hatteras, North Carolina, to the southern Labrador Sea. The monthly mean time series, consists of 180 cloud-masked monthly mean SST fields, derived from a full-resolution NOAA/NASA Pathfinder SST data set for the 1985–1999 period. The satellite-derived monthly mean SST fields, as compared with in situ monthly mean near-surface ocean temperatures from buoys located in the western North Atlantic, yield an overall RMS difference of 1.15 °C. The daily climatology, which consists of 365 fields, was derived by applying a least-squares harmonic regression technique on the monthly mean SST time series for the full study period. The monthly mean and daily climatological SST fields will be useful for studying inter-annual variability related to climate variability of SST over the study domain.     

The variability of the Atlantic meridional circulation since 1980, as hindcast by a data-driven nonlinear systems model

The Atlantic meridional overturning circulation (AMOC), an important component of the climate system, has only been directly measured since the RAPID array’s installation across the Atlantic at 26°N in 2004. This has shown that the AMOC strength is highly variable on monthly timescales; however, after an abrupt, short-lived, halving of the strength of the AMOC early in 2010, its mean has remained?~?15% below its pre-2010 level. To attempt to understand the reasons for this variability, we use a control systems identification approach to model the AMOC, with the RAPID data of 2004–2017 providing a trial and test data set. After testing to find the environmental variables, and systems model, that allow us to best match the RAPID observations, we reconstruct AMOC variation back to 1980. Our reconstruction suggests that there is inter-decadal variability in the strength of the AMOC, with periods of both weaker flow than recently, and flow strengths similar to the late 2000s, since 1980. Recent signs of weakening may therefore not reflect the beginning of a sustained decline. It is also shown that there may be predictive power for AMOC variability of around 6 months, as ocean density contrasts between the source and sink regions for the North Atlantic Drift, with lags up to 6 months, are found to be important components of the systems model.     

Evaluation of wave model performance in the South Atlantic Ocean: a study about physical parameterization and wind forcing calibration

Ocean Dynamics - This paper evaluates the performance of the spectral wave model WAVEWATCH III for the South Atlantic Ocean forced by wind inputs from the most recent reanalyses, NCEP/CFSR and...     

Interdecadal change of the linkage between the North Atlantic Oscillation and the tropical cyclone frequency over the western North Pacific

The relationship between the North Atlantic Oscillation(NAO) and the tropical cyclone frequency over the western North Pacific(WNPTCF) in summer is investigated by use of observation data. It is found that their linkage appears to have an interdecadal change from weak connection to strong connection. During the period of 1948–1977, the NAO was insignificantly correlated to the WNPTCF. However, during the period of 1980–2009, they were significantly correlated with stronger(weaker) NAO corresponding to more(fewer) tropical cyclones in the western North Pacific. The possible reason for such a different relationship between the NAO and the WNPTCF during the former and latter periods is further analyzed from the perspective of large-scale atmospheric circulations. When the NAO was stronger than normal in the latter period, an anomalous cyclonic circulation prevailed in the lower troposphere of the western North Pacific and the monsoon trough was intensified, concurrent with the eastward-shifting western Pacific subtropical high as well as anomalous low-level convergence and high-level divergence over the western North Pacific. These conditions favor the genesis and development of tropical cyclones, and thus more tropical cyclones appeared over the western North Pacific. In contrast, in the former period, the impact of the NAO on the aforementioned atmospheric circulations became insignificant, thereby weakening its linkage to the WNPTCF. Further study shows that the change of the wave activity flux associated with the NAO during the former and latter periods may account for such an interdecadal shift of the NAO–WNPTCF relationship.     

Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12 000 cal yr: long-term North Atlantic oceanographic variability and solar forcing

Giant piston core MD99-2269 recovered 25 m of sediment in Hunáfloáall, a deep trough on the North Iceland margin fronting the Iceland Sea, and the site of a shelf sediment drift. The rate of sediment accumulation is 2 m/kyr (5 yr/cm); the core terminated in the Vedde tephra (12 cal ka). The sediment was sampled at between 5 and 50 yr/sample, including rock magnetic, grain-size, and sediment properties. Data reduction was carried out using principal component analysis. Two PC axes for the 5-yr/sample magnetic data are strongly correlated with measures of coercivity (ARM_{20 mT}/ARM) and magnetic concentrations (kARM). In turn ARM_{20 mT}/ARM is highly correlated (negatively) with grain-size and the mean size of the sortable silt fraction. Analyses of the two PC axes with MTM spectral methods indicate a series of significant (>99%) periodicities at millennial to multidecadal scales, including those at 200, 125, and 88 yr which are associated with solar variability. We also document a strong correlation between the sediment magnetic properties and the ∂¹⁸O on benthic foraminifera on the North Iceland inner shelf. We hypothesize that the links between variations in grain-size, magnetic concentrations, and solar forcing are controlled by atmospheric and oceanographic changes linked to changes in the relative advection of Atlantic and polar waters along the North Iceland margin. Today these changes are associated with variations in the deep convection in the Greenland and Iceland Seas. The precise linkages are, however, presently elusive although a combination of coarser sediments and low ∂¹⁸O values define a Holocene thermal maximum between 8 and 6 cal ka.     

Sensitivity of Sverdrup transport to surface wind products over the tropical North Pacific Ocean

Ocean Dynamics - This study investigates the sensitivity of the Sverdrup transport to the NCEP/NCAR, ERA-Interim, CCMP, and QSCAT wind products over the tropical North Pacific Ocean during the...     

Simulation of irregular waves in an offshore wind farm with a spectral wave model

A numerical study of irregular waves in the Norwegian continental shelf wind farm (HAVSUL-II) was conducted using 3rd generation spectral wave models. The study was composed of two parts: the study of the effect of a single windmill monopile in the local incoming wave field using an empirical JONSWAP spectrum, and a wave hindcast study in the wind farm area using realistic incoming wave spectra obtained from large scale simulations for the 1991-1992 winter period. In the single windmill monopile study the SWAN wave model was used, while the hindcast study was conducted by successively nesting from a coarse grid using the WAM model up to a high-resolution (56 m) grid covering 26.2 km² of the HAVSUL-II windmill farm using the SWAN model. The effect of a single monopile on incident waves with realistic spectra was also studied. In the single windmill study the monopile was represented as a closed circular obstacle and in the hindcast study it was represented as a dry grid point. The results showed that the single windmill monopile creates a shadow zone in the down wave region with lower significant wave height (H_s) values and a slight increase of H_s in the up wave region. The effects of the windmill monopile on the wave field were found to be dependent on the directional distribution of the incoming wave spectrum and also on the wave diffraction and reflection. The hindcast study showed that the group of windmill monopiles may contribute to the reduction of the wave energy inside the offshore wind farm and that once the waves enter into the offshore wind farm they experience modifications due to the presence of the windmill monopiles, which cause a blocking of the wave energy propagation resulting in an altered distribution of the H_s field.     

基于WRF模式的青藏高原斜坡和平台加热影响亚洲夏季风的模拟研究

青藏高原大地形的热力强迫作用对亚洲夏季风的形成和发展具有重要的影响.本文利用较高分辨率的WRF区域模式,探讨了高原不同区域（斜坡和平台）的地形加热分别对南亚夏季风和东亚夏季风的影响.结果表明：高原南部喜马拉雅山脉的斜坡地形加热对其周围局地的环流形势和降水影响十分明显,是南亚夏季风北支分量形成和维持的主导因子,也是斜坡上气流爬坡和降水发生的必要条件.斜坡加热对东亚夏季风也有明显的增强作用,它不仅加强了中国东部低空西南季风环流,还会造成北部南下的异常干冷空气的响应.斜坡上的地形加热作用也是对流层高层暖中心位置维持在斜坡上空的一个重要原因.而高原平台加热对季风环流和降水的影响虽然没有喜马拉雅山脉斜坡加热那么显著,但是对南亚夏季风的影响范围更广,对经向哈得来环流影响更明显,能够调控高原以外更远处热带洋面上的西南季风环流.通过比较高原不同区域地形加热条件下的多种季风指数,进一步表明了高原地形加热对南亚和东亚夏季风均有增强作用,但是高原不同区域的地形加热对两类夏季风子系统又会产生不一样的影响.     

Development of a stochastic weather generator for the sub-polar North Atlantic

The article presents an approach for creating a computationally efficient stochastic weather generator. In this work the method is tested by the stochastic simulation of sea level pressure over the sub-polar North Atlantic. The weather generator includes a hidden Markov model, which propagates regional circulation patterns identified by a self organising map analysis, conditioned on the state of large-scale interannual weather regimes. The remaining residual effects are propagated by a regression model with added noise components. The regression step is performed by one of two methods, a linear model or artificial neural networks and the performance of these two methods is assessed and compared. The resulting simulations express the range of the major regional patterns of atmospheric variability and typical time scales. The long term aims of this work are to provide ensembles of atmospheric data for applied regional studies and to develop tools applicable in down-scaling large-scale ocean and atmospheric simulations.     

New predictors and a new prediction model for the typhoon frequency over western North Pacific

In this paper, the impacts of the atmospheric circulation during boreal winter-spring on the western North Pacific (WNP) typhoon frequency (WNPTF) are studied. Several new factors in winter-spring in- fluencing the typhoon frequency were identified, including the sea ice cover in the North Pacific and the North Pacific oscillation. Based on these results, the multi-linear regression was applied to establishing a new forecast model for the typhoon frequency by using the datasets of 1965―1999. The forecast model shows a high correlation coefficient (0.79) between the model simulated and the actual typhoon frequencies in the period of 1965―1999. The forecast model also exhibits reasonable hindcasts for the typhoon frequencies for the years 2000―2006. Therefore, this work demonstrates that the new pre- dictors are significant for the prediction of the interannual variability of the WNPTF, which could be potentially used in the operational seasonal forecast of the typhoon frequency in the WNP to get a more physically based operational prediction model and higher forecast skill.     

区域气候模式RegCM3对华北地区未来气候变化的数值模拟

采用高水平分辨率区域气候模式进行区域未来气候变化预估,对理解全球增暖对区域气候的潜在影响和科学评估区域气候变化有很好的参考价值.这里对国家气候中心使用25 km高水平分辨率区域气候模式RegCM3单向嵌套全球模式MIROC3.2_hires在观测温室气体(1951—2000)和IPCC A1B温室气体排放情景下(2001—2100)进行的共计150年长时间模拟结果,进行华北地区未来气温、降水和极端气候事件变化的分析.模式检验结果表明:模式对当代(1981—2000)气温以及和气温有关的极端气候事件(霜冻日数、生长季长度)的空间分布和数值模拟较好;对降水及和降水有关的极端气候事件(强降水日期、降水强度、五日最大降水量)能够模拟出它们各自的主要空间分布特征,但在模拟数值上存在偏大、偏强的误差.和全球模式驱动场相比,区域模式模拟的气温、降水和极端气候事件有明显的改进.2010—2100年华北地区随时间区域平均气温升高幅度逐渐增大,随之霜冻日数逐渐减少,生长季长度逐渐增多;同时随温室效应的不断加剧,未来降水呈增加的趋势,强降水日期和五日最大降水量逐渐增多、降水强度逐渐增大.从空间分布看,21世纪末期(2081—2100)气温、降水以及有关的极端气候事件变化比21世纪中期(2041—2060)更加明显.