Summertime moisture transport over Southeastern South America and extratropical cyclones behavior during inter-El Niño events

The impact of the inter-El Niño (EN) variability on the moisture availability over Southeastern South America (SESA) is investigated. Also, an automatic tracking scheme was used to analyze the extratropical cyclones properties (system density - SD and central pressure - CP) in this region. During the austral summer period from 1977–2000, the differences for the upper-level wave train anomaly composites seem to determine the rainfall composite differences. In fact, the positive rainfall anomalies over most of the SESA domain during the strong EN events are explained by an upper-level cyclonic center over the tropics and an anticyclonic center over the eastern subtropical area. This pattern seems to contribute to upward vertical motion at 500 hPa and reinforcement of the meridional moisture transport from the equatorial Atlantic Ocean and western Amazon basin to the SESA region. These features may contribute to the positive SD and negative CP anomalies explaining part of the positive rainfall anomalies found there. On the other hand, negative rainfall anomalies are located in the northern part of SESA for the weak EN years when compared to those for the strong events. Also, positive anomalies are found in the southern part, albeit less intense. It was associated with the weakening of the meridional moisture transport from the tropics to the SESA that seems have to contributed with smaller SD and CP anomalies over the most part of subtropics, when compared to the strong EN years.     

An intimate coupling of ocean–atmospheric interaction over the extratropical North Atlantic and Pacific

The inter-basin teleconnection between the North Atlantic and the North Pacific ocean–atmosphere interaction is studied using a coupled ocean–atmosphere general circulation model. In the model, an idealized oceanic temperature anomaly is initiated over the Kuroshio and the Gulf Stream extension region to track the coupled evolution of ocean and atmosphere interaction, respectively. The experiments explicitly demonstrate that both the North Pacific and the North Atlantic ocean–atmosphere interactions are intimately coupled through an inter-basin atmospheric teleconnection. This fast inter-basin communication can transmit oceanic variability between the North Atlantic and the North Pacific through local ocean-to-atmosphere feedbacks. The leading mode of the extratropical atmospheric internal variability plays a dominant role in shaping the hemispheric-scale response forced by oceanic variability over the North Atlantic and Pacific. Modeling results also suggest that a century (two centuries) long observations are necessary for the detection of Pacific response to Atlantic forcings (Atlantic response to Pacific forcing).     

A Climatology of Extratropical Cyclones over East Asia During 1958–2001

A climatology of extratropical cyclones (ECs) over East Asia (20 -75 N, 60 -160 E) is analyzed by applying an improved objective detection and tracking algorithm to the 4-time daily sea level pressure fields from the European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis data. A total of 12914 EC processes for the period of 1958-2001 are identified, with an EC database integrated and EC activities reanalyzed using the objective algorithm. The results reveal that there are three major cyclogenesis regions: West Siberian Plain, Mongolia (to the south of Lake Baikal), and the coastal region of East China; whereas significant cyclolysis regions are observed in Siberia north of 60 N, Northeast China, and Okhotsk Sea-Northwest Pacific. It is found that the EC lifetime is largely 1-7 days while winter ECs have the shortest lifespan. The ECs are the weakest in summer among the four seasons. Strong ECs often appear in West Siberia, Northeast China, and Okhotsk Sea-Northwest Pacific. Statistical analysis based on k-means clustering has identified 6 dominating trajectories in the area south of 55 N and east of 80 E, among which 4 tracks have important impacts on weather/climate in China. ECs occurring in spring (summer) tend to travel the longest (shortest). They move the fastest in winter, and the slowest in summer. In winter, cyclones move fast in Northeast China, some areas of the Yangtze-Huaihe River region, and the south of Japan, with speed greater than 15 m s 1 . Explosively-deepening cyclones are found to occur frequently along the east coast of China, Japan, and Northwest Pacific, but very few storms occur over the inland area. Bombs prefer to occur in winter, spring, and autumn. Their annual number and intensity in 1990 and 1992 in East Asia (EA) are smaller and weaker than their counterparts in North America.     

A Climatology of the Southwest Vortex during 1979–2008

Using a new vortex detection and tracing method, a dataset of the Southwest Vortex（SWV） is established based on Japanese 25-year Reanalysis（JRA-25） reanalysis data during 1979–2008. The spatiotemporal features of the SWV are derived from the dataset. In comparison to other seasons, summer yields the least SWVs, but with the highest probability that they will migrate from their region of origin. SWVs mostly emerge in the southwest of the Sichuan Basin and the southeast of the Tibetan Plateau. Migratory SWVs mainly move along either an eastward or southeastward path. Detailed composite analysis of warm-season SWVs shows that the subtropical high is a key factor in determining the direction of migratory SWVs. Furthermore, the steering wind at 700 hPa dominates the moving direction of migratory SWVs. Potential stability diagnosed by pseudo-equivalent potential temperature ？ se is of certain significance for the evolution and movement of SWVs. On the other hand, migratory SWVs possess relatively greater strength than stationary SWVs, due to a stronger low-level jet with enhanced baroclinicity and moisture transport providing more energy to support the growth of SWVs along their paths of movement.     

Role of the global oceans and land–atmosphere interaction on summertime interdecadal variability over northern Argentina

This study uses experiments with an atmospheric general circulation model (AGCM) to address the role of the oceans and the effect of land–atmosphere coupling on the predictability of summertime rainfall over northern Argentina focusing on interdecadal time scales during 1901–2006. Ensembles of experiments where the AGCM is forced with historical sea surface temperature (SST) in the global, Pacific and tropical-North Atlantic domains are used. The role of land–atmosphere interaction is assessed comparing the output of simulations with active and climatological soil moisture. A maximum covariance analysis between precipitation and SST reveals the impact of the Pacific Decadal Oscillation, the Atlantic Multidecadal Oscillation and the equatorial–tropical South Atlantic on rainfall over northern Argentina. Model simulations further show that while the dominant influence comes from the Pacific basin, the Atlantic influence can explain a large transition from dry to wet decades over northern Argentina during the beginning of the 1970s. Analysis of anomalies before and after the transition reveals an upper level anticyclonic circulation off the Patagonian coast with barotropic structure. This circulation enhances the moisture transport and convergence in northern Argentina and, together with enhanced evaporation, increased the rainfall after 1970. The SST pattern is dominated by cold conditions in the equatorial Atlantic and warm eastern Pacific and South Atlantic. We also found that land–atmosphere interaction leads to a representation of the long term rainfall evolution over northern Argentina that is closer to the observed one. Moreover, it leads to a smaller dispersion among ensemble members, thus resulting in a larger signal-to-noise ratio.     

Climatology of thunderstorms in the Baltic countries, 1951–2000

This paper provides an overview of the thunderstorm climate of the Baltic countries during the period of 1951–2000. Our study area is in northeastern Europe and encompasses Estonia, Latvia, and Lithuania. Visual thunderstorm observations at 59 weather stations were used as a data source. The mean annual number of thunderstorm days was 12–29.5. The seasonal cycle of thunderstorm hours with a daily step unexpectedly showed two maxima, whereas the monthly numbers of thunderstorm days had one clear mid-latitude specific peak between June and August. The diurnal cycle of thunderstorm hours showed a peak between 1400 and 1800 local time and a minimum between 0400 and 1000 hours local time. The average annual duration of thunderstorm events was 112 min. The average number of thunderstorm events per thunderstorm day was around 1.1–1.2. Our results showed that the thunderstorm climate of the Baltic countries generally resembles that of other mid-latitude study sites.     

Tropical cyclones, 6–25 day oscillations, and tropical-extratropical interaction over the northwestern Pacific

Summary This paper investigates tropical-extratropical interactions over the northwestern Pacific Ocean that involve tropical cyclones and subtropical jet streaks. Another aspect of this study is to examine the relation between 6–25 day convective variability and tropical cyclones. This investigation is conducted for the fall and early winter season, with a focus on the months, October through December (OND). In addition to outgoing longwave radiation (OLR) data, we use 10 years (1985–1994) of WCRP/TOGA archive II analyses produced by ECMWF to compute equivalent temperature, _e , precipitable water, W, and kinematic and kinetic energy transfer variables. These variables are composited for two classes of tropical cyclones, recurving cyclones (RCs) and non-recurving cyclones (NCRs), in order to examine the influence of tropical cyclones and baroclinic processes on changes in the jet streak intensity.We found that RCs interacted with extratropical regions during all composite days. A strong baroclinic zone developed throughout the troposphere on the north side of the composite cyclone as it propagated poleward. Between the day of recurvature, DR, and the day after recurvature, DR+1, the main band of convection shifted from the RC to a frontal band within the baroclinic zone indicating a transformation of the tropical cyclone into an extratropical one. An eastward propagating jet streak at 200 hPa, located north of the RC and in the vicinity of the baroclinic zone, increased its speed from 57 ms^–1 to 79 ms^–1 on DR+1. Although we could not measure the role of baroclinic processes in this regard, we were able to infer that upper-level outflow from the RC did supply momentum and energy to the jet streak.Whereas we expected tropical-extratropical interactions for the RCs, we also found evidence that NRCs that stay south of 20° N throughout their lifetime and that dissipate over Indo-China have an influence on the subtropical jet by their upper-level outflow, especially in the late OND season. The tropical (i.e., momentum) forcing did appear to cause increases in the speed of the jet after the composited storm crossed the Phillippines on the fourth day of its life cycle, D4. Concurrently, a baroclinic zone developed along the coast of southern China by about D4, but it was confined to the lower troposphere.Finally, our spectral analysis investigations for the northwestern Pacific showed significant peaks at 6–10 days and 15–20 days from late September to early December. The first peak is well known and is associated with typhoon activity. In several of the investigated autumn seasons (1987, 1989, 1992, and 1993), the second peak was clearly related to the recurrence interval of northwestern Pacific tropical cyclones. This result is in accordance with the findings of Hartmann et al. (1992). For some years of the investigation period (1985, 1986, and 1988), however, our results showed that westward propagating convective disturbances that fail to reach tropical depression strength also contribute to the power in the 15–25 day band, whereas in a few years (1990 and 1991), no OLR peak between 15 and 20 days could be found at all. Therefore, it appears that further work needs to be done with regard to the relationship between convective systems and their accompanying relationships on time scales ranging between 10 and 25 days.With 15 Figures     

Characteristics of the internal and external sources of the Mediterranean synoptic cyclones for the period 1956–2013

This paper investigates the main sources and features of the Mediterranean synoptic cyclones affecting the basin, using the cyclone tracks. The cyclones’ tracks are identified using sea level pressure (SLP) from the NCEP/NCAR reanalysis data for the period 1956–2013. The identified cyclones are classified into two categories: basin affected and basin non-affected. Most of the basin-affected (non-affected) cyclones are internal (external), i.e., generated inside (outside) the Mediterranean basin. This study reveals four (five) main sources of internal (external) cyclones. These four (five) main sources generated about 63.76% (57.25%) of the internal (external) cyclones. Seasonal analysis shows that most of the basin-affected internal (external) cyclones were generated in the winter (spring) season. The lowest number of cyclones were found in the summer. Moreover, the synoptic study of the atmospheric systems accompanied the highest- and lowest-generated years demonstrates that the deepening of the north Europe cyclones and the relative positions of Azores- and Siberian-high systems represent the important factors that influence the number of internal cyclones. Essential factors influencing the external cyclones are the strength of the maximum upper wind, Azores high, Siberian high, and orientations of their ridges.     

Role of orography in modulating space–time scales of convection over South Asia

Propagation of convective systems in the meridional direction during boreal summer is responsible for active and break phases of monsoon over south Asia. This region is unique in the world in its characteristics of monsoon variability and is in close proximity of mountains like the Himalayas. Here, using an atmospheric general circulation model, we try to understand the role of orography in determining spatial and temporal scales of these convective systems. Absence of orography (noGlOrog) decreased the simulated seasonal mean precipitation over India by 23 % due to delay in onset by about a month vis-a-vis the full-mountain case. In noGlOrog, poleward propagations were absent during the delayed period prior to onset. Post-onset, both simulations had similar patterns of poleward propagations. The spatial and temporal scales of propagating clouds bands were determined using wavelet analysis. These scales were found to be different in full-mountain and no-mountain experiments in June–July. However, after the onset of monsoon in noGlOrog, these scales become similar to that with orography. Simulations with two different sets of convection schemes confirmed this result. Further analysis shows that the absence (presence) of meridional propagations during early (late) phase of summer monsoon in noGlOrog was associated with weaker (stronger) vertical shear of zonal wind over south Asia. Our study shows that orography plays a major role in determining the time of onset over the Indian region. However, after onset, basic characteristics of propagating convective systems and therefore the monthly precipitation over India, are less sensitive to the presence of orography and are modulated by moist convective processes.     

Effects of air-sea interaction on extended-range prediction of geopotential height at 500 hPa over the northern extratropical region

The contribution of air-sea interaction on the extended-range prediction of geopotential height at 500 hPa in the northern extratropical region has been analyzed with a coupled model form Beijing Climate Center and its atmospheric components. Under the assumption of the perfect model, the extended-range prediction skill was evaluated by anomaly correlation coefficient (ACC), root mean square error (RMSE), and signal-to-noise ratio (SNR). The coupled model has a better prediction skill than its atmospheric model, especially, the air-sea interaction in July made a greater contribution for the improvement of prediction skill than other months. The prediction skill of the extratropical region in the coupled model reaches 16–18 days in all months, while the atmospheric model reaches 10–11 days in January, April, and July and only 7–8 days in October, indicating that the air-sea interaction can extend the prediction skill of the atmospheric model by about 1 week. The errors of both the coupled model and the atmospheric model reach saturation in about 20 days, suggesting that the predictable range is less than 3 weeks.     

Climate downscaling over South America for 1961–1970 using the Eta Model

This study shows the results from a regional climate simulation of the present-day climate, corresponding to the period 1961–1970 over South America, using the regional Eta Model nested within the HadAM3P model from the UK Hadley Centre. The simulation analysis is focused on assessing the capability of the nested regional model in representing spatial patterns of seasonal mean climate and the annual cycle of precipitation and temperature. The goals of this 10-year run for South America are to verify if the Eta Model can be used in climate-change scenarios and to verify if this model has the ability to generate added value for the South American continent. The Eta Model was chosen because there are few investigations using the Eta Model for long integrations over South America and because the vertical coordinate system used in this model is recommended for use over South America due to the presence of the Andes range. In the present 10-year simulation, the regional model reproduced many of the South American mesoscale climate features and together added new value to the driver model. Value was also added to the driver model by reducing seasonal biases in austral winter relative to austral summer. The regional model also exhibits better performance in the representation of low-level circulation, such as the topographically induced northwesterly flux.     

Spatial-temporal characteristics of the “cumulative effect” of torrential rain over South China

To expand torrential rain, which is a meso- and microscale weather process, to a meso- and long-scale weather process, in this paper, we choose South China as a sample region and propose the conception of the “Cumulative Effect” of torrential rain (CETR) by using daily precipitation observational data from 740 stations. Through a statistical analysis of the observations, three indexes—continuous time (L _d), control area (A _r), and precipitation contribution rate (Q _s)—are used to define the CETR and indicate the torrential rain processes. The relationships between the CETR and simultaneous total precipitation over South China are analyzed in the pre-flooding and latter flooding seasons. This analysis shows that on both interannual and interdecadal scales, the three indexes are highly correlated with simultaneous total precipitation over South China in the pre-flooding season and latter flooding season. Moreover, an empirical orthogonal function (EOF) analysis is performed to classify the spatial distribution of the CETR. In both the pre-flooding season and the latter flooding season, the four major spatial models of torrential rain are similar to those of total precipitation over South China. With regard to the amount of precipitation caused by the CETR, the latter flooding season is affected more significantly than the pre-flooding season. Regarding the geographical distribution of precipitation, the opposite result occurs. In conclusion, in both the pre-flooding season and the latter flooding season, the CETR influences and even determines the amount and distribution of precipitation over South China.

     

Variations of Surface Heat Fluxes over the Tibetan Plateau before and after the Onset of the South Asian Summer Monsoon during 1979–2016

As the "Third Pole of the World," the Tibetan Plateau (TP) is an important thermal forcing to the South Asian summer monsoon (ASM) and even the global atmospheric circulation. In this paper, surface heat fluxes from the ERA-Interim reanalysis data during March-October of 1979-2016 in the TP and its surrounding areas are examined and analyzed. The results are as follows.(1) From March to May (before the ASM onset), the main body of the TP is dominated by sensible heat flux, which increases rapidly with high (low) values in the west (east), while the change of latent heat flux is small but it increases with time.(2) From June to August (after the ASM onset), sensible heat flux over the TP decreases, while latent heat flux increases rapidly with high (low) values in the east (west).(3) From September to October (after the ASM withdrawal), sensible and latent heat fluxes are comparable to each other in strength, again with high (low) sensible heat flux in the west (east).(4) During 1979-2016, surface sensible heat flux in the whole TP shows a slightly downward trend, while latent heat flux shows an increasing trend. Specifically, in the western TP, sensible (latent) heat flux shows a weak decreasing (an increasing) trend;while in the eastern TP, sensible (latent) heat flux decreases (increases obviously). These variations are consistent with the observed warming and moistening in the TP region. The above results are useful for further analysis of the change of atmospheric heat sources and surface heat fluxes over the TP based on the data from the Third Tibetan Plateau Atmospheric Science Experiment (TIPEX-Ⅲ).     

Influences of tropical–extratropical interaction on the multidecadal AMOC variability in the NCEP climate forecast system

We have examined the mechanisms of a multidecadal oscillation of the Atlantic Meridional Overturning Circulation (AMOC) in a 335-year simulation of the Climate Forecast System (CFS), the climate prediction model developed at the National Centers for Environmental Prediction (NCEP). Both the mean and seasonal cycle of the AMOC in the CFS are generally consistent with observation-based estimates with a maximum northward volume transport of 16?Sv (10⁶?m³/s) near 35°N at 1.2?km. The annual mean AMOC shows an intermittent quasi 30-year oscillation. Its dominant structure includes a deep anomalous overturning cell (referred to as the anomalous AMOC) with amplitude of 0.6?Sv near 35°N and an anomalous subtropical cell (STC) of shallow overturning spanning across the equator. The mechanism for the oscillation includes a positive feedback between the anomalous AMOC and surface wind stress anomalies in mid-latitudes and a negative feedback between the anomalous STC and AMOC. A strong AMOC is associated with warm sea surface temperature anomaly (SSTA) centered near 45°N, which generates an anticyclonic easterly surface wind anomaly. This anticyclonic wind anomaly enhances the regional downwelling and reinforces the anomalous AMOC. In the mean time, a wind-evaporation-SST (WES) feedback extends the warm SSTA to the tropics and induces a cyclonic wind stress anomaly there, which drives a tropical upwelling and weakens the STC north of the equator. The STC anomaly, in turn, drives a cold upper ocean heat content anomaly (HCA) in the northern tropical Atlantic and weakens the meridional heat transport from the tropics to the mid-latitude through an anomalous southward western boundary current. The anomalous STC transports cold HCA from the subtropics to the mid-latitudes, weakening the mid-latitude deep overturning.     

The influence of Madden–Julian Oscillation in the genesis of North Indian Ocean tropical cyclones

Cyclonic storms having maximum winds of 34 knots and above that had genesis in north Indian Ocean have been studied with respect to the eastward passage of Madden–Julian Oscillation (MJO). In the three decades (1979–2008), there were a total of 118 cyclones reported in which 96 formed in the region chosen (0–15^oN, 60^oE–100^oE) for the study. Although the percentage of MJO days inducing cyclogenesis is small, it is found that tropical cyclone genesis preferentially occurred during the convective phase of MJO. This accounted for 44 cyclones of the total 54 cyclones (i.e., 81.5%) formed under MJO amplitude 1 and above. The study has shown that, when the enhanced convection of MJO is over the maritime continent and the adjoining eastern Indian Ocean, it creates the highest favorable environment for cyclogenesis in the Bay of Bengal. During this phase, westerlies at 850 hPa are strong in the equatorial region south of Bay of Bengal creating strong cyclonic vorticity in the lower troposphere along with the low vertical wind shear.