Rotated Empirical Orthogonal Function Research Articles

An integrated analysis of dry-wet variability in western China for the last 4–5 centuries

The dry-wet variability in western China and its spatiotemporal structure during the last 4–5 centuries was examined using 24 climate proxies from sediments, ice cores, historical documents, and tree rings. Spatial patterns and temporal evolutions of dryness and wetness were not only extracted from the proxy data using rotated empirical orthogonal function (REOF) analysis for the last 4 centuries, but also for instrumental data in the last 40 years. The leading five REOF modes indicate that 5 dry-wet variation centers exist in western China. Moreover, long-term variability in dryness and wetness is seen on long (centennial) to short (inter-decadal) timescales. An out-of-phase relationship for the inter-decadal variation was observed between the Hetao-upper Yangtze River region and north Xinjiang, indicating influences on dry-wet variations of the East Asian summer monsoon and the westerly winds over the two regions, respectively. A particularly long dry spell was found in the central Tibetan Plateau in the 19th century. A predominance of wet decades in the last 4 centuries was found in the arid and Hetao regions. Three regional dry-wet series with annual resolution in north Xinjiang, the upper Yellow River valley, and the Hetao area were constructed for analyses of the last 500 years. Dry-wet oscillations with periodicities of 16, 50, and 150 years in north Xinjiang, 50 years in the upper Yellow River valley, and 70–80 years in the Hetao region were identified by wavelet analysis. In general, these periods correspond to large-scale oscillations found in the climate system, are mainly related to ocean-atmosphere interaction.

Independent Component Analysis of Climate Data: A New Look at EOF Rotation

Abstract The complexity inherent in climate data makes it necessary to introduce more than one statistical tool to the researcher to gain insight into the climate system. Empirical orthogonal function (EOF) analysis is one of the most widely used methods to analyze weather/climate modes of variability and to reduce the dimensionality of the system. Simple structure rotation of EOFs can enhance interpretability of the obtained patterns but cannot provide anything more than temporal uncorrelatedness. In this paper, an alternative rotation method based on independent component analysis (ICA) is considered. The ICA is viewed here as a method of EOF rotation. Starting from an initial EOF solution rather than rotating the loadings toward simplicity, ICA seeks a rotation matrix that maximizes the independence between the components in the time domain. If the underlying climate signals have an independent forcing, one can expect to find loadings with interpretable patterns whose time coefficients have properties that go beyond simple noncorrelation observed in EOFs. The methodology is presented and an application to monthly means sea level pressure (SLP) field is discussed. Among the rotated (to independence) EOFs, the North Atlantic Oscillation (NAO) pattern, an Arctic Oscillation–like pattern, and a Scandinavian-like pattern have been identified. There is the suggestion that the NAO is an intrinsic mode of variability independent of the Pacific.

The linkage between the Pacific-North American teleconnection pattern and the North Atlantic Oscillation

In this study, using the ECMWF reanalysis data, the possible linkage between the Pacific-North American teleconnection pattern (PNA) and the North Atlantic Oscillation (NAO) during boreal winter (December- February) is investigated. The PNA and the NAO pattern are obtained by performing Rotated Empirical Orthogonal Function (REOF) analysis on an anomalous daily mean 300-hPa geopotential height field. The composite daily NAO indices show that the NAO indices are prone to be negative (positive) when the contemporary PNA indices are extremely positive (negative). The correlation coefficients between the daily PNA and NAO indices also confirm that, indeed, there is a significant anti-correlation between the PNA and NAO indices. The correlation peaks at a lag of 0 days (meaning contemporary correlation), and its value is 0.202. Analyses of a newly defined Rossby wave breaking index and diagnostics of the stream function tendency equation indicate that the anti-correlation between PNA and NAO may be caused by the anomalous Rossby wave breaking events associated with the PNA pattern.

Coupled Ocean–Atmosphere Interaction and Variability in the Tropical Atlantic Ocean with and without an Annual Cycle

Abstract Many previous studies point to a connection between the annual cycle and interannual variability in the tropical Atlantic Ocean. To investigate the importance of the annual cycle in the generation of tropical Atlantic variability (TAV) as well as its associated coupled feedback mechanisms, a set of controlled experiments is conducted using a global coupled ocean–atmosphere general circulation model (GCM) in which the climatological annual cycle is modified. An anomaly coupling strategy was developed to improve the model-simulated annual cycle and mean sea surface temperature (SST), which is critical to the experiments. Experiments include a control simulation in which the annual cycle is present and a fixed annual cycle simulation in which the coupled model is forced to remain in a perpetual annual mean state. Results reveal that the patterns of TAV, defined as the leading three rotated EOFs, and their relationship to coupled feedback mechanisms are present even in the absence of the annual cycle, suggesting that the generation of TAV is not dependent on the annual cycle. Each pattern of variability arises from an alteration of the easterly trade winds. Results suggest that it is the presence of these winds in the mean state that is the determining factor for the structure of the coupled ocean–atmosphere variability. Additionally, the patterns of variability persist longer in the simulation with no annual cycle. Most remarkable is the doubling of the decay phase related to the north tropical Atlantic variability, which is attributed to the persistence of the local wind–evaporation–sea surface temperature (WES) feedback mechanism. The author concludes that the annual cycle acts to cut off or interrupt conditions favorable for feedback mechanisms to operate, therefore putting a limit on the length of the event life cycle.

Moisture variability across China and Mongolia: 1951–2005

Moisture variability across China and Mongolia (hereafter, CM) during 1951–2005 was investigated using the recently developed monthly Palmer Drought Severity Index (PDSI) dataset. In total there are 206 PDSI grid points across CM, based on a 2.5° × 2.5° gridding system. For CM as a whole a significant decreasing trend in mean moisture availability was observed during 1951–2005, but with strong decadal (17.1-year) and interannual (5.0-year, 3.2-year, 2.4–2.8 year) variations. The areas affected by moderate and severe moisture deficit over CM have increased significantly since the mid-1950s. In contrast, there is a significant decreasing trend for areas affected by moderate wetness since the mid-1950s, and no significant trend was found for the areas affected by severe wetness. Ten moisture-related spatial patterns were objectively defined for CM using rotated Empirical Orthogonal Function (REOF) analysis. These patterns are related to distinct geographical areas and are associated with distinct temporal variations. Four of these patterns, in Northeast China (NE), North China (NC), Central China (CC), and East China (EC), generally demonstrate a significant decreasing trend in moisture availability. Two patterns located in western areas of Northwest China (NW) and the Tibetan Plateau (TP) show a significant moisture increase, while four patterns in Mongolia (MN), far western China (FW), South China (SC), and Southwest China (SW) do not have significant moisture trends during 1951–2005. Based on REOF results we propose that CM should be divided into ten coherent moisture divisions. Moisture variations within each division are generally coherent, but may show either similar or contrasting covariability with adjacent divisions.

Spatial and temporal variability of precipitation over China, 1951–2005

Annual, winter and summer precipitation records for the period 1951–2005 from 160 stations in China were analysed using the rotated empirical orthogonal function (REOF), the Mann–Kendall trend test and the Continuous Wavelet Transform (CTW) method. The REOF method was used to analyse the annual and seasonal variability of precipitation patterns over China, the Mann–Kendall method was used to detect the temporal trend of the rotated principal components time series, and the continuous wavelet method was used to explore the periodicity of precipitation changes. In general, six coherent regions across China are identified using the REOF method: north-east China; the middle and lower Yangtze River basin; the Haihe River and the Liaohe River; north-west China; the middle Yellow River and the South-east Rivers (rivers in south-east China). Continuous wavelet transform results indicate that the significant 2–4 year and 6–9 year bands are the major period components. Precipitation in China is uneven in space and time, and its complex temporal structure and spatial variations are different in each season. The Mann–Kendall test results show that, in general, the middle and the lower sections of the Yangtze River are dominated by increasing annual, summer and winter precipitation. Increasing annual precipitation can be observed in north-west China. Increasing summer precipitation is found in north-east China and the Pearl River basin, and the South-east Rivers are dominated by increasing winter precipitation. The availability of water as a resource is in close association with precipitation changes; therefore, this research will be helpful to watershed-based water resource managers in China.

The role of the South Indian and Pacific oceans in South American monsoon variability

This work has investigated the impact of three different low-frequency sea surface temperature (SST) variability modes located in the Indian and the Pacific Oceans on the interannual variability of the South American Monsoon System (SAMS) using observed and numerical data. Rotated Empirical Orthogonal Function (REOF) analysis and numerical simulations with a General Circulation Model (GCM) were used. One of the three SST variability modes is located close to southeastern Africa. According to the composites, warmer waters over this region are associated with enhanced austral summer precipitation over the sub-tropics. The GCM is able to reproduce this anomalous precipitation pattern, simulating a wave train emanating from the Indian Ocean towards South America (SA). A second SST variability mode was located in the western Pacific Ocean. REOF analysis indicates that warmer waters are associated with drought conditions over the South Atlantic Convergence Zone (SACZ) and enhanced precipitation over the sub-tropics. The GCM indicates that the warmer waters over Indonesia generate drought conditions over tropical SA through a Pacific South America-like (PSA) wave pattern emanating from the western Pacific. Finally, the third SST variability mode is located over the southwestern South Pacific. The composites indicate that warmer waters are associated with enhanced precipitation over the SACZ and drought conditions over the sub-tropics. There is a PSA-like wave train emanating from Indonesia towards SA, and another crossing the Southern Hemisphere in the extra-tropics, probably associated with transient activity. The GCM is able to reproduce the anomalous precipitation pattern, although it is weaker than observed. The PSA-like pattern is simulated, but the model fails in reproducing the extra-tropical wave activity.

Spatial and temporal variation of extreme precipitation over the Yangtze River Basin

The present study examines the spatial and temporal variation of extreme precipitation over the Yangtze River Basin based on meteorological data for 1960–2004. From 147 stations, regional characteristics of extreme precipitation are analyzed using a rotated empirical orthogonal function (REOF). Non-parametric trend analysis and wavelet transformation analysis are applied to analyze the temporal variation of extreme precipitation. The results show that (1) 6 leading modes of extreme precipitation exist in the following area: east/north-eastern Yangtze, north/central Yangtze, south-eastern Yangtze, north-western Yangtze, western Yangtze and south/south-western Yangtze; (2) significant positive changes can be found for extreme precipitation averaged for the east/north-eastern Yangtze and the western Yangtze basin, and statistically significant abruption is detected in year 1986 for both regions; (3) significant negative trends can be detected over the north-western Yangtze basin with abruption in the years 1968 and 1990; and (4) long-term trends and inter-decadal oscillations of extreme precipitation can be found for 6 mode domains, but not constant periodicity. The increase in heavy precipitation in the middle-lower reaches can partly explain the frequent occurrence of floods in the 1990s. It is difficult to predict the trends of heavy precipitation for future years, but possible alternating of high-frequency extreme precipitation in the middle and lower reaches indicates flood disaster might be aggravated in the incoming decade of the 21st century in the Yangtze River Basin.

Evolution of freshwater plumes and salinity fronts in the northern Bay of Bengal

The evolution of freshwater plumes and the associated salinity fronts in the northern Bay of Bengal (henceforth the bay) is studied using rotated empirical orthogonal function (REOF) analysis and extended associate pattern analysis (EAPA). The results show that sea surface salinity distribution is featured by eastern‐bay and western‐bay plumes in the northern bay during different seasons. The western‐bay plume begins in early July, peaks in late August, and then turns into a bay‐shaped plume with the two plumes in either side of the bay, which peaks in late October. The southward extension of the western‐bay plume can be explained by the southwestward geostrophic flow associated with the cyclonic gyre in the northern bay, which counters the northeastward Ekman drift driven by wind stress. The offshore expansion of the western‐bay plume is induced by the offshore Ekman drift which also produces a salinity front near the east coast of India. The bay‐shaped plume appears when the cyclonic gyre shifts westward and a weak anticyclonic gyre occupies the northeastern bay. As the season advances, the western part of the bay‐shaped plume decays while the eastern part persists until the following June, which is believed to be associated with the anticyclonic gyre in the northern bay. The evolution of the plumes except the eastern part of the bay‐shaped plume in fall can be partly explained by the seasonal variation of mass transport associated with the Sverdrup balance. The fact that the western‐bay (eastern‐bay) plume appears when surface freshwater flux in the northeastern bay increases (decreases) dramatically suggests that the plumes are not produced directly by surface freshwater flux. River discharge seems to be the freshwater source for the plumes and has little to do with the evolution of the plumes.

Response of vegetation in the Qinghai-Tibet Plateau to global warming

Using satellite-observed Normalized Difference Vegetation Index (NDVI) dada and station-observed surface air temperature anomalies for the Northern Hemisphere (NH), we analyze the spatio-temporal characteristics of vegetation variations in the Qinghai-Tibet Plateau and their correlations with global warming from 1982 to 2002. It is found that the late spring and early summer (May–June) are the months with the strongest responses of vegetation to global warming. Based on the Rotated Empirical Orthogonal Function (REOF) method, the study shows that the first REOF spatial pattern of average NDVI for May–June reveals the northern and southern zones with great inter-annual variations of vegetation, the northern zone from the eastern Kunlun Mountains to the southwestern Qilian Mountain and southern zone from the northern edge of the Himalayas eastward to the Hengduan Mountains. The vegetation, especially grassland, in the two zones increases significantly with global warming, with a correlation coefficient of 0.71 between the first REOF of May–June vegetation and the April–May surface air temperature anomaly in the NH during 1982–2002. A long-term increasing trend in May–June vegetation for the plateau region as a whole is also attributed mainly to global warming although there are considerable regional differences. The areas with low NDVI (grassland and shrubland) usually respond more evidently to global warming, especially since the 1990s, than those with moderate or high NDVI values.

Empirical orthogonal functions and related techniques in atmospheric science: A review

AbstractClimate and weather constitute a typical example where high dimensional and complex phenomena meet. The atmospheric system is the result of highly complex interactions between many degrees of freedom or modes. In order to gain insight in understanding the dynamical/physical behaviour involved it is useful to attempt to understand their interactions in terms of a much smaller number of prominent modes of variability. This has led to the development by atmospheric researchers of methods that give a space display and a time display of large space‐time atmospheric data.Empirical orthogonal functions (EOFs) were first used in meteorology in the late 1940s. The method, which decomposes a space‐time field into spatial patterns and associated time indices, contributed much in advancing our knowledge of the atmosphere. However, since the atmosphere contains all sorts of features, e.g. stationary and propagating, EOFs are unable to provide a full picture. For example, EOFs tend, in general, to be difficult to interpret because of their geometric properties, such as their global feature, and their orthogonality in space and time. To obtain more localised features, modifications, e.g. rotated EOFs (REOFs), have been introduced. At the same time, because these methods cannot deal with propagating features, since they only use spatial correlation of the field, it was necessary to use both spatial and time information in order to identify such features. Extended and complex EOFs were introduced to serve that purpose.Because of the importance of EOFs and closely related methods in atmospheric science, and because the existing reviews of the subject are slightly out of date, there seems to be a need to update our knowledge by including new developments that could not be presented in previous reviews. This review proposes to achieve precisely this goal. The basic theory of the main types of EOFs is reviewed, and a wide range of applications using various data sets are also provided. Copyright © 2007 Royal Meteorological Society

Ocean Model Diagnosis of Low-Frequency Climate Variability in the South Atlantic Region

Abstract South Atlantic Ocean variability is investigated by means of an ocean general circulation model (ORCA2), forced with the NCEP–NCAR reanalyses for the 1948–99 period. A rotated EOF analysis of the mixed layer temperature suggests a breakdown of the South Atlantic into the following four subdomains, with characteristic spatial and temporal scales: (a) the tropical Atlantic, with mainly interannual fluctuations; (b) the northeastern subtropics, with variability on an interannual to decadal scale; (c) the midlatitudes, with interannual and multidecadal variability; and (d) the southwestern subtropics/midlatitudes with a mixture of interannual and decadal variability. These modes are closely connected to anomalous atmospheric circulation patterns, which induce typical forcing mechanisms for each region. Temperature changes in the western to central Tropics are found to be driven by changes in surface heat fluxes and the horizontal advection of heat, while in the central to eastern Tropics and the northern Benguela region temperature changes are connected to reduced vertical entrainment, altering the depth of the mixed layer and leading to reduced upwelling. In the western and eastern subtropics, changes in the net surface fluxes drive the upper-ocean temperature anomalies, and wind-induced vertical mixing dissipates them, inducing changes in the depth of the mixed layer. Anomalous heat and volume transports are found to be related to anomalous Ekman and geostrophic currents in the eastern subtropics. A wind-driven mechanism is suggested, whereby changes in Ekman-related heat and volume transport lead to modulations of the subtropical gyre and thus to changes in the geostrophic-related heat and volume transport. Temporal variability in the midlatitudes is mainly due to horizontal advection and wind-induced vertical mixing, whereby geostrophic advection of heat dominates in the western to central area, and Ekman-induced heat transports are confined to the eastern midlatitudes.

Diagnosing the Annual Cycle Modes in the Tropical Atlantic Ocean Using a Directly Coupled Atmosphere–Ocean GCM

Abstract The annual cycle of sea surface temperature (SST) in the tropical Atlantic of a directly coupled atmosphere–ocean general circulation model (CGCM) is decomposed into the parts forced by different surface fluxes (denoted as modes) for the two extreme months of March and August using forced ocean experiments. Almost all previous diagnostic work of the forcing of the SST annual cycle in the Atlantic has concentrated on the near-equatorial region. Here, the annual cycle is examined within the latitude range of 25°S–25°N to facilitate comparison with the interannual variability. The structure of the response to the different surface flux forcings bears some resemblance to the interannual SST modes in the tropical Atlantic, which are diagnosed using rotated empirical orthogonal function (REOF) analysis. Diagnosis of the forcing of the annual cycle modes and the interannual modes shows that they do not always have a common cause. Hence, the simple interpretation that the leading interannual modes are perturbations to the annual cycle is not always valid. In particular, the equatorial SST annual cycle mode is primarily driven by variations in vertical velocity while the equatorial interannual mode is associated with eastward-propagating thermocline anomalies and is forced by both thermocline anomalies and vertical velocity anomalies. As for the interannual modes, there exist off-equatorial annual cycle modes in both the Northern and Southern Hemispheres. The annual cycle off-equatorial mode in both hemispheres is shown to be primarily driven by heat flux variations. The Southern Hemisphere interannual mode is primarily driven by heat flux variations while the Northern Hemisphere interannual mode shows a strong influence of thermocline depth anomalies. In addition, the Southern Hemisphere interannual mode is centered about 10° south of the annual cycle mode. An interannual mode that has maximum variability along the South American coast south of the equator is shown to be associated with thermocline depth anomalies. This interannual mode has no analog in the annual cycle modes. The coupled model simulation of the annual cycle is found to be fairly realistic so that the results presented here could have applicability to the observed Atlantic.

Inter ENSO variability and its influence over the South American monsoon system

Abstract. Previous studies have discussed the interannual variability of a meridional seesaw of dry and wet conditions over South America (SA) associated to the modulation of the South Atlantic Convergence Zone (SACZ). However, they did not explore if the variability inter ENSO (El Niño Southern Oscillation) can be related to the phase changes of this dipole. To answer this question, an observational work was carried out to explore the atmospheric and Sea Surface Temperature (SST) conditions related to the same ENSO signal and to opposite dipole phases. Rotated Empirical Orthogonal Function (REOF) analysis was applied over normalized Chen precipitation seasonal anomalies in order to find the dipole mode in the Austral Summer (December to February). The fourth rotated mode, explaining 6.6% of the total variance, consists of positive loading over the SACZ region and negative loading over northern Argentina. Extreme events were selected and enhanced activity of SACZ during the Summer season (SACZ+) was identified in nine years: five during La Niña events (LN) and two in El Niño episodes (EN). On the other hand, inhibited manifestations of this system (SACZ-) were identified in seven years: four in EN and two during LN. Power spectrum analysis indicated that the interannual variability of the precipitation dipole seems to be related to the low frequency and to the quasi-biennial part of ENSO variability. The ENSO events with the same signal can present opposite phases for the dipole. The results suggest that the displacement of the convection over Indonesia and western Pacific can play an important role to modulate the seesaw pattern.

In search of simple structures in climate: simplifying EOFs

Empirical orthogonal functions (EOFs) are widely used in climate research to identify dominant patterns of variability and to reduce the dimensionality of climate data. EOFs, however, can be difficult to interpret. Rotated empirical orthogonal functions (REOFs) have been proposed as more physical entities with simpler patterns than EOFs. This study presents a new approach for finding climate patterns with simple structures that overcomes the problems encountered with rotation. The method achieves simplicity of the patterns by using the main properties of EOFs and REOFs simultaneously. Orthogonal patterns that maximise variance subject to a constraint that induces a form of simplicity are found. The simplified empirical orthogonal function (SEOF) patterns, being more ‘local’, are constrained to have zero loadings outside the main centre of action. The method is applied to winter Northern Hemisphere (NH) monthly mean sea level pressure (SLP) reanalyses over the period 1948–2000. The ‘simplified’ leading patterns of variability are identified and compared to the leading patterns obtained from EOFs and REOFs. Copyright © 2005 Royal Meteorological Society.

Validation of simulated precipitation patterns over Ireland for the period 1961–2000

The Rotated Empirical Orthogonal Function (REOF) method was used to analyse the annual and interannual variability of the precipitation patterns over Ireland as simulated by the Rossby Centre Regional Climate Model Version 2 (RCA2) for the period 1961–2000. The annual and monthly precipitation results show that the model captures the major spatial pattern of each month, but the simulation shows a larger percentage of the variance explained by the leading spatial pattern and tends to have a less pronounced seasonal cycle compared to the observations. The simulation for the wet winter season is in slightly better agreement with the observations compared with the drier summer season. Spatial analysis of the monthly precipitation over a 40-year period shows two major patterns. The first, showing a precipitation maximum in the southwest, is well simulated; the second, showing a maximum in the west and northwest, shows a strongly increasing trend, which is slightly underestimated in the model simulation. Both patterns show seasonal and interannual variability. The wavelet energy density analysis shows that the interannual variability is more important than the seasonal one. In general, the model captures the major spatial and seasonal patterns observed between 1961 and 2000 but slightly overestimates monthly average precipitation levels. Copyright © 2005 Royal Meteorological Society.

Statistical analysis of surface hydrography and circulation variations in northern South China Sea

To study the variations in surface hydrography and circulation in northern South China Sea (NSCS), rotated empirical orthogonal function (REOF) and extended associate pattern analysis (EAPA) are used with daily sea surface salinity (SSS), sea surface temperature (SST) and sea surface height (SSH) datasets covering 1 126 days from American Navy Experimental Real-Time East Asian Seas Ocean Nowcast System in this paper. Results show that in summer, the SCS Diluted Water Expansion (SDWE) is the most dominant factor controlling SSS variations in the NSCS. The remarkable SDWE usually begins in early July, reaches its maximum in middle August and weakens in late September. In summer fluorishing period, its low saline core is just limited between 21°N and 22°N because of strong surface anomalous anticyclonic circulation in the NSCS. In early or late stage, the anomalous anticyclonic circulation becomes weak or turns into cyclonic one, thus the weak SCS diluted water can disperse. And its influence on the SSS variations has obviously decreased. The Kuroshio intrusion is the second controlling factor, and it has the almost opposite seasonal or intraseasonal oscillations and spatial characteristics to the SDWE. Winter Kuroshio Intrusion (WKI) begins in early November and lasts about three months. Intraseasonal Kuroshio Intrusion (IKI) takes place at any seasons. The westward Ekman transport produced by the north anomaly of East Asia Monsoon (EAM) pushes warmer and more saline seawater into the NSCS through the Bashi Strait and seems to decide the intensity of seasonal and intraseasonal Kuroshio intrusions.

Dominant patterns of AVHRR NDVI interannual variability over the Sahel and linkages with key climate signals (1982–2003)

The spatio‐temporal evolution of Sahelian vegetation is analyzed using the Normalized Difference Vegetation Index (NDVI) obtained from the NOAA/AVHRR sensor (1982–2003). Dominant patterns are identified using rotated EOFs. While the first four modes are associated with specific bio‐geo‐climatic conditions in space, significant time scales are detected using a multi‐tapers method. Three interannual time scales (∼6.2‐, 4.5‐ and 3.6‐year) are present in the first and third NDVI modes over the western Sahel. A quasi‐biennial time scale (∼2.6‐year) is present in second and fourth NDVI modes over the northeast Sahel. During summer, significant lagged correlations are found between the NDVI second (9‐month lag) and third (10‐month lag) modes, the meridional Atlantic Sea Surface Temperature (SST) gradient, and the zonal SST gradient in the Indian Ocean. Potential physical linkages and dynamics with known climate fluctuations are discussed.

Characteristics of zonal anomaly of annual precipitation in the Northeastern China

The characteristics of zonal anomaly and change rule of temporal distribution of annual precipitation in the northeastern China are revealed in this paper with EOF (Empirical Orthogonal Function) and REOF (Rotated Empirical Orthogonal Function) methods and results are drawn in the standard relief maps with GIS technology for practical application. Data used in the study were obtained from 208 meteorological stations over the northeastern China from 1961 to 2001. EOF results show that the first 3 loading vectors could give entire spatial anomaly structure of annual precipitation. In the Northeast Plain including the Songneng Plain and the Liaohe Plain, there is a regional compatibility (whether wet or dry) of annual precipitation change and this precipitation pattern has occurred since the late 1980s to the present. There also exist annual precipitation patterns of wet (or dry) in south and dry (or wet) in north and wet (or dry) in east and dry (or wet) in west. REOF results display 8 principal precipitation anomaly areas by the first 8 rotated loading vectors: the west plain, the Liaodong hills, the Sanjiang Plain, the Liaoxi hills, the Changbai Mountains, the Hulun Buir Plateau, the southwest plateau and the Liaodong Peninsula.

Seasonal and intraseasonal variations of the surface Taiwan Warm Current

To study seasonal and intraseasonal variations of the Taiwan Warm Current (TWC) in detail, Rotated Empirical Orthogonal Function (REOF) and Extended Associate Pattern Analysis (EAPA) are jointly adopted with daily sea surface salinity (SSS), sea surface temperature (SST) and sea surface height (SSH) datasets covering 1126 days from American Navy Experimental Real-Time East Asian Seas Ocean Nowcast System in the present paper. Results show that the first and second REOFs of SST in the southern East China Sea (SECS) account for 50.8% and 39.8% of the total variance. The surface TWC contains persistent (multi-year mean), seasonal and intraseasonal components. The persistent one mainly inosculates with the Kuroshio but the seasonal and intraseasonal ones are usually active only on the continental shelf. Its persistent component is produced by inertial flow of the Kuroshio, however its seasonal and intraseasonal ones seems coming from seasonal and intraseasonal oscillations of monsoon force. The seasonal one reaches its maximum in late summer, lasting about four months and the intraseasonal one takes place at any seasons, lasting more than 40 days.