Variational assimilation in combination with a regularization method for sea level pressure retrieval from QuikSCAT scatterometer data Ⅱ:simulation experiment and actual case study

The sea level pressure field can be computed from sea surface winds retrieved from satellite microwave scatterometer measurements, based on variational assimilation in combination with a regularization method given in part I of this paper. First, the validity of the new method is proved with a simulation experiment. Then, a new processing procedure for the sea level pressure retrieval is built by combining the geostrophic wind, which is computed from the scatterometer 10-meter wind using the University of Washington planetary boundary layer model using this method. Finally, the feasibility of the method is proved using an actual case study.

Variational assimilation in combination with the regularization method for sea level pressure retrieval from QuikSCAT scatterometer data I:Theoretical frame construction

A new method of constructing a sea level pressure field from satellite microwave scatterometer measurements is presented. It is based on variational assimilation in combination with a regularization method using geostrophic vorticity to construct a sea level pressure field from scatterometer data that are given in this paper, which offers a new idea for the application of scatterometer measurements. Firstly, the geostrophic vorticity from the scatterometer data is computed to construct the observation field, and the vorticity field in an area and the sea level pressure on the borders are assimilated. Secondly, the gradient of sea level pressure （semi-norm） is used as the stable functional to educe the adjoint system, the adjoint boundary condition and the gradient of the cost functional in which a weight parameter is introduced for the harmony of the system and the Tikhonov regularization techniques in inverse problem are used to overcome the ill-posedness of the assimilation. Finally, the iteration method of the sea level pressure field is developed.

Influence of Sea Level Pressure on Inter-Annual Rainfall Variability in Northern Senegal in the Context of Climate Change

This study examines the inter-annual variability of rainfall and Mean Sea Level Pressure (MSLP) over west Africa based on analysis of the Global Precipitation Climatology Project (GPCP) and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis respectively. An interconnection is found in this region, between Mean Sea Level Pressure (MSLP) anomaly (over Azores and St. Helena High) and monthly mean precipitation during summer (June to September: JJAS). We also found that over northern Senegal (15°N - 17°N;17°W - 13°W) the SLP to the north is strong;the wind converges at 200 hPa corresponding to the position of the African Easterly Jet (AEJ) the rotational wind 700 hPa (corresponding to the position of the African Easterly Jet (AEJ) coming from the north-east is negative. In this region, the precipitation is related to the SLP to the north with the opposite sign. The Empirical Orthogonal Functions (EOF) of SLP is also presented, including the mean spectrum of precipitation and pressures to the north (15°N - 40°N and 50°W - 25°W) and south (40°S - 10°S and 40°W - 0°E). The dominant EOF of Sea Level Pressures north and south of the Atlantic Ocean for GPCP represents about 62.2% and 69.4% of the variance, respectively. The second and third EOFs of the pressure to the north account for 24.0% and 6.5% respectively. The second and third EOFs of the pressure to the south represent 12.5% and 8.9% respectively. Wet years in the north of Senegal were associated with anomalous low-pressure areas over the north Atlantic Ocean as opposed to the dry years which exhibited an anomalous high-pressure area in the same region. On the other hand, over the South Atlantic, an opposition is noted. The wavelet analysis method is applied to the SLP showings to the north, south and precipitation in our study area. The indices prove to be very consistent, especially during intervals of high variance.

Role of sea level pressure in variations of the Canadian Arctic Archipelago throughflow

The throughflow in the Canadian Arctic Archipelago(CAA)had a significant impact on the North Atlantic Ocean with the Arctic climate change.The findings of physical mechanisms driving the throughflow in the CAA differed and few studies about the impact of sea level pressure(SLP)on the CAA throughflow were made.A high-resolution ice-ocean coupled Arctic Ocean Finite-Volume Community Ocean Model(AO-FVCOM)was used over the period 1978-2016 to examine the interannual and seasonal variability of the outflows in the CAA and the mechanism of SLP in driving the variation of the CAA throughflow quantitively.The simulated volume transport through Davis Strait,Nares Strait,Lancaster Sound and Jones Sound showed consistent increasing trends over 1978-2016 and the larger flux in winter and spring than in summer and fall.The variation of volume transport through Nares Strait contributed more than Lancaster and Jones Sound to the variation through Davis Strait.Five process-oriented experiments were made to further explore the role of SLP in setting up and controlling the sea surface height(SSH)difference and thus the throughflow transport in the CAA.The SLP was a primary forcing to control the SSH difference and the outflow transport compared with the wind forcing.The memory of the SSH to the SLP was short and an equilibrium state could be reached if the SLP varied with time.The upstream and downstream SLP difference,however,made a slight direct contribution to driving the volume transport of the CAA throughflow.In addition to the external forcing of SLP and wind,the variability of the CAA outflow was also influenced by the variability of the inflow/outflow and SSH on boundaries connected to the Pacific and Atlantic Oceans.The choice of SLP datasets from CORE-v2,ECMWF and NCEP was sensitive to the simulated uncertainty of volume transport.

VARIATIONS OF SEA SURFACE TEMPERATURE AND SEA LEVEL PRESSURE OVER PACIFIC AND INDIAN OCEANS AND THEIR INFLUENCE ON COLD SUMMER IN EAST ASIA

An EOF analysis was performed to investigate the variations of sea surface temperature (SST) of Pacific and Indian Oceans.Result shows that the distribution of SST anomaly exhibits a distinct anticorrelation pattern between Northwest and Southeast Pacific,as well as between Northwest Pacific and the Arabian Sea.It also shows that the sea level pressure (SLP) anomaly between North Pacific and North Indian Oceans is of a seesaw pattern,which we named the North Paci- fic and North Indian Ocean Oscillation (PIO).Such a phenomenon is closely correlated with the cold summer in East Asia.

Characteristics of extratropical cyclone variability in the Northern Hemisphere and their response to rapid changes in Arctic sea ice

Extratropical cyclones are critical weather systems that affect large-scale weather and climate changes at mid-high latitudes.However,prior research shows that there are still great difficulties in predicting extratropical cyclones for occurrence,frequency,and position.In this study,mean sea level pressure(MSLP)data from the European Centre for Medium-Range Weather Forecasts(ECMWF)reanalysis(ERA5)are used to calculate the variance statistics of the MSLP to reveal extratropical cyclone activity(ECA).Based on the analysis of the change characteristics of ECA in the Northern Hemisphere,the intrinsic link between ECA in the Northern Hemisphere and Arctic sea ice is explored.The results show that the maximum ECA mainly occurs in winter over the mid-high latitudes in the Northern Hemisphere.The maximum ECA changes in the North Pacific and the North Atlantic,which are the largest variations in the Northern Hemisphere,are independent of each other,and their mechanisms may be different.Furthermore,MSLP is a significant physical variable that affects ECA.The North Atlantic Oscillation(NAO)and North Pacific Index(NPI)are significant indices that impact ECA in the North Atlantic and North Pacific,respectively.The innovation of this paper is to explore the relationship between the activity of extratropical cyclones in the Northern Hemisphere and the abnormal changes in Arctic sea ice for the first time.The mechanism is that the abnormal changes in summer-autumn and winter Arctic sea ice lead to the phase transition of the NPI and NAO,respectively,and then cause the occurrence of ECA in the North Pacific and North Atlantic,respectively.Arctic sea ice plays a crucial role in the ECA in the Northern Hemisphere by influencing the polar vortex and westerly jets.This is the first exploration of ECAs in the Northern Hemisphere using Arctic sea ice,which can provide some references for the in-depth study and prediction of ECAs in the Northern Hemisphere.

Causes of seasonal sea level anomalies in the coastal region of the East China Sea

Based on the analysis of sea level, air temperature, sea surface temperature（SST）, air pressure and wind data during 1980-2013, the causes of seasonal sea level anomalies in the coastal region of the East China Sea（ECS） are investigated. The research results show：（1） sea level along the coastal region of the ECS takes on strong seasonal variation. The annual range is 30-45 cm, larger in the north than in the south. From north to south, the phase of sea level changes from 140° to 231°, with a difference of nearly 3 months.（2） Monthly mean sea level（MSL）anomalies often occur from August to next February along the coast region of the ECS. The number of sea level anomalies is at most from January to February and from August to October, showing a growing trend in recent years.（3） Anomalous wind field is an important factor to affect the sea level variation in the coastal region of the ECS. Monthly MSL anomaly is closely related to wind field anomaly and air pressure field anomaly. Wind-driven current is essentially consistent with sea surface height. In August 2012, the sea surface heights at the coastal stations driven by wind field have contributed 50%-80% of MSL anomalies.（4） The annual variations for sea level,SST and air temperature along the coastal region of the ECS are mainly caused by solar radiation with a period of12 months. But the correlation coefficients of sea level anomalies with SST anomalies and air temperature anomalies are all less than 0.1.（5） Seasonal sea level variations contain the long-term trends and all kinds of periodic changes. Sea level oscillations vary in different seasons in the coastal region of the ECS. In winter and spring, the oscillation of 4-7 a related to El Ni？o is stronger and its amplitude exceeds 2 cm. In summer and autumn, the oscillations of 2-3 a and quasi 9 a are most significant, and their amplitudes also exceed 2 cm. The height of sea level is lifted up when the different oscillations superposed. On the other hand, the height of sea level is fallen down.

Characteristics and possible causes of the seasonal sea level anomaly along the South China Sea coast

Based on sea level, air temperature, sea surface temperature（SST）, air pressure and wind data during 1980–2014,this paper uses Morlet wavelet transform, Estuarine Coastal Ocean Model（ECOM） and so on to investigate the characteristics and possible causes of seasonal sea level anomalies along the South China Sea（SCS） coast. The research results show that：（1） Seasonal sea level anomalies often occur from January to February and from June to October. The frequency of sea level anomalies is the most in August, showing a growing trend in recent years. In addition, the occurring frequency of negative sea level anomaly accounts for 50% of the total abnormal number.（2） The seasonal sea level anomalies are closely related to ENSO events. The negative anomalies always occurred during the El Ni？o events, while the positive anomalies occurred during the La Ni？a（late El Ni？o） events. In addition, the seasonal sea level oscillation periods of 4–7 a associated with ENSO are the strongest in winter, with the amplitude over 2 cm.（3） Abnormal wind is an important factor to affect the seasonal sea level anomalies in the coastal region of the SCS. Wind-driven sea level height（SSH） is basically consistent with the seasonal sea level anomalies. Moreover, the influence of the tropical cyclone in the coastal region of the SCS is concentrated in summer and autumn, contributing to the seasonal sea level anomalies.（4） Seasonal variations of sea level, SST and air temperature are basically consistent along the coast of the SCS, but the seasonal sea level anomalies have no much correlation with the SST and air temperature.

SPATIAL VARIATION OF WINTER SEA SURFACE TEMPERATURE IN NORTH PACIFIC AND ITS RELATION TO ATMOSPHERIC OSCILLATION MODES

The spatial variation of sea surface temperature anomalies(SSTA) in the North Pacific Ocean during winter is investigated using the EOF decomposition method.The first two main modes of SSTA are associated with Pacific Decadal Oscillation(PDO) mode and North Pacific Gyre Oscillation(NPGO) mode,respectively.Moreover,the first mode(PDO) is switched to the second mode(NPGO),a dominant mode after mid-1980.The mechanism of the modes' transition is analyzed.As the two oceanic modes are forced by the Aleutian Low(AL) and North Pacific Oscillation(NPO) modes,the AR-1 model is further used to examine the possible effect and mechanism of AL and NPO in generating the PDO and NPGO.The results show that compared to the NPO,the AL plays a more important role in generating the NPGO mode since the 1970s.Likewise,both the AL and NPO affect the PDO mode since the 1980s.

Assessment of Atmospheric Reanalysis Data Based on Buoy Observations over the Tropical Western Indian Ocean in 2019

Atmospheric reanalysis data are an important data source for studying weather and climate systems.The sea surface wind and sea level pressure observations measured from a real-time buoy system deployed in Kenya’s offshore area in 2019 conducted jointly by Chinese and Kenyan scientists were used to evaluate the performance of the major high-frequency atmospheric reanalysis products in the western Indian Ocean region.Compared with observations,the sea level pressure field could be accurately simulated using the atmospheric reanalysis data.However,significant discrepancies existed between the surface wind reanalysis data,especially between meridional wind and the observational data.Most of the data provide a complete understanding of sea level pressure,except for the Japanese 55-year Reanalysis data,which hold a significant system bias.The Modern-Era Reanalysis for Research and Applications,Version-2,provides an improved description of all datasets.All the reanalysis datasets for zonal wind underestimate the strength during the study period.Among reanalysis data,NCEP-DOE Atmospheric Model Intercomparison Project reanalysis data presents an inaccurate description due to the worst correlation with the observations.For meridional wind,most reanalysis datasets underestimate the variance,while the European Centre for Medium-Range Weather Forecasts Atmospheric Composition Reanalysis 4 has a larger variance than the observations.In addition to the original data comparison,the diurnal variability of sea level pressure and surface wind are also assessed,and the result indicates that the diurnal variations have a significant gap between observation and reanalysis data.This study indicates that the current high-frequency reanalysis data still have disadvantages when describing the atmospheric parameters in the Western Indian Ocean region.

Case Study of Coastal Fog Events in Senegal Using LIDAR Ceilometer

This study aims to examine the atmospheric conditions characterising fog phenomena on the Senegalese coast focusing on two specific instances that occurred on April 3 and April 30,2023.These events were detected by the LIDAR Ceilometer installed at LPAOSF/ESP/UCAD and confirmed on the METARs of the meteorological stations at Dakar and Diass airports.The LIDAR’s backscatter signal showed that the fog of April 3 started around midnight with a vertical extension at 100 m altitude and dissipated around 10 a.m.The April 30 event characterized by a good vertical extension from the surface up to 300 m above sea level,was triggered just after 2 a.m.and lasted around 3 hours.The results showed that a decrease in temperature,accompanied by an increase in humidity and light wind,is favorable for the triggering and persistence of fog.Sea Level Pressure(SLP)anomaly fields show two distinct configurations.The April 3 event was characterized by a zonal dipole of SLP anomalies between the Sahara and the northern Senegalese coast,while the April 30 event was characterized by a meridional dipole between the Sahara and the Gulf of Guinea area as far as the equatorial Atlantic.A weakening of the pressure around the study area was observed in both cases,allowing moisture advection to favor the onset of fog.The hovmoller diagrams of relative humidity and wind show that a good vertical extension of humidity associated with a westerly wind in the lower layers plays an important role in the formation and persistence of fog.The presence of dry air associated with a weak easterly wind in the middle layers could explain the low vertical extension of the fog on April 3.A strong wind in the lower layers would be responsible for the premature dissipation of the April 30 fog.

Changes in Wind Speed and Extremes in Beijing during 1960–2008 Based on Homogenized Observations

Daily observations of wind speed at 12 stations in the Greater Beijing Area during 1960–2008 were homogenized using the Multiple Analysis of Series for Homogenization method. The linear trends in the regional mean annual and seasonal （winter, spring, summer and autumn） wind speed series were-0.26,-0.39,-0.30,-0.12 and-0.22 m s-1 （10 yr）-1 , respectively. Winter showed the greatest magnitude in declining wind speed, followed by spring, autumn and summer. The annual and seasonal frequencies of wind speed extremes （days） also decreased, more prominently for winter than for the other seasons. The declining trends in wind speed and extremes were formed mainly by some rapid declines during the 1970s and 1980s. The maximum declining trend in wind speed occurred at Chaoyang （CY）, a station within the central business district （CBD） of Beijing with the highest level of urbanization. The declining trends were in general smaller in magnitude away from the city center, except for the winter case in which the maximum declining trend shifted northeastward to rural Miyun （MY）. The influence of urbanization on the annual wind speed was estimated to be about-0.05 m s-1 （10 yr）-1 during 1960–2008, accounting for around one fifth of the regional mean declining trend. The annual and seasonal geostrophic wind speeds around Beijing, based on daily mean sea level pressure （MSLP） from the ERA-40 reanalysis dataset, also exhibited decreasing trends, coincident with the results from site observations. A comparative analysis of the MSLP fields between 1966–1975 and 1992–2001 suggested that the influences of both the winter and summer monsoons on Beijing were weaker in the more recent of the two decades. It is suggested that the bulk of wind in Beijing is influenced considerably by urbanization, while changes in strong winds or wind speed extremes are prone to large-scale climate change in the region.

Observational Evidence for Poleward Expansion of the Hadley Circulation

How the Hadley circulation changes in response to global climate change and how its change impacts upon regional and global climates has generated a lot of interest in the literature in the past few years. In this paper, consistent and statistically significant poleward expansion of the Hadley circulation in the past few decades is demonstrated, using independent observational datasets as proxy measures of the Hadley circulation. Both observational outgoing longwave radiation and precipitation datasets show an annual average total poleward expansion of the Hadley cells of about 3.6° latitude. Sea level pressure from observational and reanalysis datasets show smaller magnitudes of poleward expansion, of about 1.2° latitude. Ensemble general circulation model simulations forced by observed time-varying sea surface temperatures were found to generate a total poleward expansion of about 1.23°latitude. Possible mechanisms behind the changes in the horizontal extent of the Hadley circulation are discussed.

Two Types of El Nio-related Southern Oscillation and Their Diferent Impacts on Global Land Precipitation

The contrast between the eastern and central responses of zonal and vertical circulation in the Pacific （EP- and CP-） E1 Nino is observed in the different tropics. To measure the different responses of the atmo- spheric circulation to the two types of E1 Nino, an eastern and a central Pacific southern oscillation index （EP- and CP-SOI） are defined based on the air-sea coupled relationship between eddy sea level pressure and sea surface temperature. Analyses suggest that while the EP-SOI exhibits variability on an interannual （2- 7-yr） time scale, decadal （10-15-yr） variations in the CP-SOI are more dominant; both are strongly coupled with their respective EP- and CP-E1 Nino patterns. Composite analysis suggests that, during EP-ENSO, the Walker circulation exhibits a dipole structure in the lower-level （850 hPa） and upper-level （200 hPa） velocity potential anomalies and exhibits a signal cell over the Pacific. In the case of CP-ENSO, however, the Walker circulation shows a tripole structure and exhibits double cells over the Pacific. In addition, the two types of ENSO events show opposite impacts on global land precipitation in the boreal winter and spring seasons. For example, seasonal precipitation across China's Mainland exhibits an opposite relationship with the EP- and CP-ENSO during winter and spring, but the rainfall over the lower reaches of the Yangtze River and South China shows an opposite relationship during the rest of the seasons. Therefore, the different relationships between rainfall and EP- and CP-ENSO should be carefully considered when predicting seasonal rainfall in the East Asian monsoon regions.

A Preliminary Study on the Relationship Between Arctic Oscillation and Daily SLP Variance in the Northern Hemisphere During Wintertime

In the present study, the authors investigated the relationship between the Arctic Oscillation (AO) and the high-frequency variability of daily sea level pressures in the Northern Hemisphere in winter (November through March), using NCEP/NCAR reanalysis datasets for the time period of 1948/49-2000/01. High-frequency signals are defined as those with timescales shorter than three weeks and measured in terms of variance, for each winter for each grid. The correlations between monthly mean AO index and high-frequency variance are conducted. A predominant feature is that several regional centers with high correlation show up in the middle to high latitudes. Significant areas include mid- to high-latitude Asia centered at Siberia, northern Europe and the middle-latitude North Atlantic east of northern Africa. Their strong correlations can also be confirmed by the singular value decomposition analysis of covariance between mean SLP and high-frequency variance. This indicates that the relationship of AO with daily Sea Level Pressure (SLP) is confined to some specific regions in association with the inherent atmospheric dynamics. In middle-latitude Asia, there is a significant (at the 95% level) trend of variance of-2.26% (10 yr)-1. Another region that displays a strong trend is the northwestern Pacific with a significant rate of change of 0.80% (10 yr)-1. If the winter of 1948/49, an apparent outlier, is excluded, a steady linear trend of +1.51% (10 yr)-1 shows up in northern Europe. The variance probability density functions (PDFs) are found to change in association with different AO phases. The changes corresponding to high and low AO phases, however, are asymmetric in these regions. Some regions such as northern Europe display much stronger changes in high AO years, whereas some other regions such as Siberia show a stronger connection to low AO conditions. These features are supported by ECMWF reanalysis data. However, the dynamical mechanisms involved in the AO-high frequency SLP variance connection have not been well understood, and this needs further study.

A Scheme for Estimating Tropical Cyclone Intensity Using AMSU-A Data

Brightness temperature anomalies measured by the Advanced Microwave Sounding Unit （AMSU） on the National Oceanic and Atmospheric Administration （NOAA） polar-orbiting series are suited to estimate tropical cyclone （TC） intensity by virtue of their ability to assess changes in tropospheric warm core struc-ture in the presence of clouds. Analysis of the measurements from different satellites shows that the variable horizontal resolution of the instrument has significant effects on the observed brightness temperature anoma-lies. With the aim to decrease these effects on TC intensity estimation more easily and effectively, a new simple correction algorithm, which is related to the product of the brightness temperature gradient near the TC center and the size of the field-of-view （FOV） observing the TC center, is proposed to modify the observed anomalies. Without other measurements, the comparison shows that the performance of the new algorithm is better than that of the traditional, physically-based algorithm. Furthermore, based on the correction algorithm, a new scheme, in which the brightness temperature anomalies at 31.4 GHz and 89 GHz accounting for precipitation effects are directly used as the predictors with those at 54.94 GHz and 55.5 GHz, is developed to estimate TC intensity in the western North Pacific basin. The collocated AMSU-A observations from NOAA-16 with the best track （BT） intensity data from the Japan Meteorological Agency （JMA） in 2002-2003 and in 2004 are used respectively to develop and validate regression coefficients. For the independent validation dataset, the scheme yields 8.4 hPa of the root mean square error and 6.6 hPa of the mean absolute error. For the 81 collocated cases in the western North Pacific basin and for the 24 collocated cases in the Atlantic basin, compared to the BT data, the standard deviations of the estimation differences of the results are 15% and 11% less than those of the CIMSS （Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison） TC intensity AMSU estimation products.

近百年冬季东亚大气活动中心年际一年代变率及其对中国气温的影响(英文)

The interannual and interdecadal varinbility of the Siberian High (SH) and the Aleutian Low (AL) from aspects of strength and location for the past one hundred years as well as their possible relations with temperature changes over China's Mainland are investigated. The data sets used are the historical sea level pressure for 1871-1995 and surface air temperature (SAT) over China in the last 100 years. The results show that the SAT in different regions over China, central strength of the SH and the AL, the south-reaching latitude of the 1030 hPa contour of the SH and the pressure gradient between the SH and the AL experienced two obvious changes during this period. One occurred in the 1920s, with a more prominent one in the 1980s. These variations are closely linked with the change of winter temperature over China in the interdecadal timescale. In the last 50 years, there is a remarkable interannual correlation between the strength of Active Centers of Atmosphere (Acas) and the winter temperature of northern and eastern regions in China. The abrupt change of Acas in the 1980s is consistent with the rising of the SAT in China. Since the late 1980s, the atmospheric circulation is experiencing a remarkable modulation, which may cause the interdecadal transition of warming trend.

A study on the Antarctic circumpolar wave mode-A coexistence system of standing and traveling wave

The Antarctic circumpolar wave （ACW） has become a focus of the air-sea coupled Southern Ocean study since 1996, when it was discovered as an air-sea coupled interannual signal propagating eastward in the region of the Antarctic Circumpolar Current （ACC）. In order to analyze the mechanism of discontinuity along the latitudinal propagation, a new idea that ACW is a system with a traveling wave in the Southern Pacific and Atlantic Ocean and with a concurrent standing wave in the southern Indian Ocean is proposed in this paper. Based on the ideal wave principle, the average wave parameters of ACW is achieved using a non-linear approximation method, by which we find that the standing part and the traveling part possess similar radius frequency, proving their belonging to an integral system. We also give the latitudinal distribution of wave speed with which we could tell the reason for steady propagation during the same period. The spatial distribution of the propagation reveals complex process with variant spatial and temporal scales--The ENSO scale oscillation greatly impacts on the traveling process, while the result at the south of Australia indicates little connection between the Indian Ocean and the Pacific, which may be blocked by the vibration at the west of the Pacific. The advective effect of ACC on the propagation process should be examined clearly through dynamical method.

A STUDY ON VARIABILITY OF SEA SURFACE TEMPERATURE IN TROPICAL PACIFIC, INDIAN OCEAN AND RELATED AIR CIRCULATION

Canonical Correlation Analysis (CCA) was a-dopted in the present paper tostudy the of Sea Surface Temperature (SST) in the tropical Pacific, Indian Ocean and related aircirculation. The results show that on the seasonal time scale, El Nino events can be divided intotwo types: the east one and the middle one. For the middle type the SST variations appear contrarilyin the tropical Pacific and Indian O-cean, and the anomalous SST decreases in the east butincreases in the northwest and south-middle of the tropical Indian O-cean, specially in the east ofMadagascar Island. And vice versa. On annual time scale, when the Asian continent high gets strongerand the deepened Aleutian low shifts southeastward, both of them trigger an onset of the El Ninoevents. Contrarily, the La Nina events take place. On decadal time scale, there are two basic modesof air-sea system over the tropical Pacific and Indian Ocean. Firstly, when the Asian continent highgets stronger and deepened Aleutian low shifts southeastward, the anomalous SST increases in themiddle and east of the proical Pacific, extending to the subtropical regions, and so in most of thetropical Indian Ocean, specially in the northeast of Madagascar Island and nearby. And vice versa.Secondly, when the Asian continent high gets stronger in the north and the Aleutian low decreasesfixedly or even disappears, the anomalous SST decreases slightly in middle of the tropical Pacificand the temperate northern Pacific but increases weakly in other regions, the anomalous SSTincreases in the south but decreases in the north of the tropical Indian O-cean, and the SSTincreases more obviously in southeast of Madagascar Island. And vice versa. The linear trends ofglob- al warming seems to play a certain role for the El Nino onsets.

Genetic climatic regionalization of the Balkan Peninsula using cluster analysis

Sea Level Pressure（SLP） data for the period 1950–2012 at 61 stations located in or around the Balkan Peninsula was used. The main concept is that intra-annual course of SLP represents the best different air masses that are situated over the Balkan Peninsula during the year. The method for differentiation of climatic zones is cluster analysis. A hierarchical clustering technique–average linkage between groups with Pearson correlation for measurement of intervals was employed in the research. The climate of the Balkan Peninsula is transitional between oceanic and continental and also between subtropical and temperate climates. Several major changes in atmospheric circulation over the Balkan Peninsula have happened over the period 1950–2012. There is a serious increase of the influence of the Azores High in the period January–Marchwhich leads to an increase of SLP and enhances oceanic influence. There is an increase of the influence of the north-west extension of the monsoonal low in the period June–September. This leads to more continental climatebut also to more tropical air masses over the Balkan Peninsula. Accordinglythe extent of subtropical climate widens in northern direction. There is an increase of the influence of the Siberian High in the period October–December. This influence covers central and eastern part of the peninsula in October and Novemberand it reaches western parts in December. Thusthe climate becomes more continental.