Intraseasonal Fluctuations Research Articles

Annual patterns of ultraviolet radiation effects on temperate marine phytoplankton off Patagonia, Argentina

We carried out experiments to evaluate the effects of solar ultraviolet radiation (UVR; 280–400 nm) upon primary production of different natural phytoplankton assemblages (i.e. characteristic of a seasonal cycle) from Patagonia (Argentina) from January 2001 to January 2002. The short-term impact of UVR (i.e. measured as radiocarbon incorporation) was assessed by exposing samples to solar radiation under six radiation treatments: uncovered quartz tubes and tubes covered with different cut-off Schott filters (WG295, WG305, WG320, WG360), and Plexiglas UF-3 (cut-off at 400 nm), so that samples received radiation at five different intervals within the UVR in addition to photosynthetically active radiation (PAR), and only PAR, respectively. Phytoplankton composition and abundance allowed us to differentiate pre-bloom, bloom and post-bloom periods, with pre- and post-bloom samples characterized by small cells (e.g. flagellates <10 μm), whereas the bloom was dominated by large diatoms (∼50 μm). Absolute values of photosynthesis inhibition were lower during the bloom, but biological weighting functions (i.e. inhibition per unit energy) indicated that this assemblage was more sensitive to UVR (especially in the UV-B region, 280–320 nm) than those of the pre- and post-bloom periods. UV-A radiation (320–400 nm) accounted for most of the reduction in carbon incorporation (>60%), especially during the pre- and post-bloom periods. Most of the observed variability was inter-seasonal, although small intra-seasonal fluctuations were also observed. Our results indicate that the taxonomic composition and cellular size are especially important when addressing UVR effects upon these assemblages. However, other factors such as mixing can also contribute to the variability in responses to UVR.

Intraseasonal variability in the southwestern Arabian Sea and its relation to the seasonal circulation

An analysis of TOPEX/POSEIDON altimeter data and in situ current and temperature data obtained between April 1995 and October 1996 from a moored array shows strong intraseasonal fluctuations in the southwestern Arabian Sea, an oceanic region where the Great Whirl (GW), a predominantly wind-generated, very energetic anticyclone, is present during the Southwest Monsoon. Fluctuation periods between 30 and 50 days, up to 100 days during some years, are observed in the 8-year altimetric dataset, mostly during late summer and fall. These fluctuations are largest in a 1000 km-wide region off the Somali, Omani and Yemeni coasts north of 5°N, suggesting a local generation mechanism. The in situ data at different moorings show strong and coherent fluctuations that are characterized by southwestward phase propagation and northward energy propagation. Their periods range from 30 to 60 days and increase steadily from July 1995 to January 1996. In the first stage, these periods are at and below the cut-off period of freely propagating, first baroclinic mode Rossby waves, but approach this theoretical limit later in the year. Instabilities of the flow in the transition region between the Southern Gyre and the GW are likely sources of these fluctuations.

Variability of daily precipitation in China (1980–1993): PCA and wavelet analysis of observation and ECMWF reanalysis data

In this study, principal component analysis (PCA) and complex Morlet wavelet transform were used with daily rainfall in China for the period 1980–1993 (1 May–31 Dec.) from observation and ECMWF reanalysis to study its variability and eaaluate the validation of reanalyzed precipitation. The results showed that northward movement of the summer rain belt was a wavelike propagation, which was always accompanied by rainfall breaks and could be treated as one event under time scale of about 1 month only. The first 4 EOFs accounted for 28% and 35% of total variance from observation and reanalysis, respectively, and were roughly consistent with each other. The first and third EOFs for observation mainly represented interweekly, interseasonal and interannual variations and contained some summer intraseasonal fluctuations also. The second and fourth ones mainly represented some rather strong summer intraseasonal fluctuations for a paticular year and contained interweekly, interseasonal and interannual variations also. Although there is still room for improvement, the ECMWF reanalysis is the best available dataset with global coverage and daily variability.

Intra-Seasonal Convective Variability over Southern Africa: Principal Component Analysis of Pentad Outgoing-longwave Radiation Departures 1976-1994

Intra-seasonal fluctuations of summer convection over southern Africa are studied through principal components (PC) analysis. Pentad (5 day) satellite outgoing long-wave radiation (OLR) departures are used to characterise the space and time scales of terrestrial cloudiness in the domain 10–35° S, 10–40° E. Areas of intra-seasonal convective influence are analysed according to spatial pattern and corresponding temporal character. Eight distinct geographic domains are identified, four tropical and four sub-tropical. The three most significant modes occur over southern Tanzania, Namibia, and Zambia, and refer to pulsing of: the Indian NE monsoon, surface heating in the western desert, and the zonal ITCZ, respectively. Temporal characteristics vary widely but an underlying near-monthly rhythm is detected. The variety of modes suggests that convective weather systems respond to external forcing (wave trains) and internal dynamics, to produce intra-seasonal fluctuations over southern Africa.

Intraseasonal Variability of the Summer Monsoon over Southeast Arizona

Abstract Intraseasonal fluctuations associated with the Mexican monsoon system are examined for the semiarid Sonoran Desert region. Daily rain gauge accumulations, radiosonde reports, satellite imagery, and global analyses are all analyzed. Composite wet and dry periods during July and August of 1985–92 are compared, and the statistical significance of differences between the composite fields are assessed. Significant differences exist between many of the wet and dry fields over the Sonoran Desert. As the monsoon shifts from dry to wet conditions, the subtropical ridge moves ∼5° lat northward, and the middle- and upper-tropospheric (700–300 mb) winds back from southwesterly to southeasterly. The midtropospheric transport of water vapor from the southeast strengthens, and the precipitable water values increase by as much as ∼1.2 cm (∼0.5 in.). Middle-tropospheric air parcels ascend into the region from the southeast, while low-level air parcels continue to stream inland from the Gulf of California and up t...

Characteristics of the Indo‐Pacific Throughflow in the eastern Indian Ocean

Five hydrographic sections across the Indonesian throughflow from the World Ocean Circulation Experiment (WOCE) and the Java Australia Dynamic Experiment (JADE) are used to examine seasonal water mass and density distributions, including semiannual variation of the South Java Current. Altimetric sea surface height variations match dynamic height along each WOCE cruise track remarkably well. They also reveal energetic eddies and meanders of the current system south of Java with intraseasonal (60‐day) changes in surface dynamic height comparable in magnitude to seasonal and interannual variations. Through the formation of discrete eddies and meanders of the large‐scale zonal flows, intraseasonal fluctuations account for much of the variation in water mass structure between cruises.

多重解像度フーリエ解析と北極アラートにおけるCO&lt;sub&gt;2&lt;/sub&gt;濃度変動の解析への適用

Difficulties in using conventional Fourier spectral analysis to explore the temporal variation of CO 2 short-term fluctuations have indicated the need for some new statistical techniques. It is suggested here that the Multiresolution Fourier Transform (MFT) can be used for interpreting such fluctuations. The technique is adapted from wavelet theory in signal processing. To demonstrate the usefulness of this technique, some examples of analytic fluctuation are first examined. The examples suggest that this technique might be useful for analysis of fluctuations that exhibit locality in both frequency and time. MFT is then applied to study the CO2 fluctuations measured at Alert from 1988 to 1995. The results show that the C02 concentration over Alert has two dominant regimes of quasi-periodic short-term fluctuation : 1)20-50 day intraseasonal fluctuation and 2) 6-14 day synoptic fluctuation. Both the amplitude and frequency of these fluctuations are strongly modulated by the seasonal cycle and the largest amplitude appears during winter time. In addition, these short-term fluctuations have significant interannual variability, especially for the synoptic fluctuation.

Global Ocean Circulation from Satellite Altimetry and High-Resolution Computer Simulation

The sea surface elevation relative to the geoid, a dynamic boundary condition for the three-dimensional oceanic pressure field, is being determined over the global ocean every 10 days by a precision radar altimeter aboard the TOPEX/POSEIDON satellite. This is the most accurate altimeter data stream to date for the study of the ocean general circulation and its variability. The authors compare results from 2 years (October 1992–October 1994) of the satellite observations to computer simulations for the same period using a state-of-the-art ocean general circulation model driven by realistic winds from an atmospheric weather-prediction model. The average horizontal resolution of the model is 1/5° (varying from 30 km at the equator to 6 km at the polar latitudes), the highest for a global simulation performed to date. Comparisons of the mean circulation, the mesoscale variability, the amplitude, and phase of the annual cycle, as well as intraseasonal and interannual changes show that the simulations and observations agree fairly well over a broad range of temporal and spatial scales. However, the sea level variance produced by the model is generally less than the observation by a factor of 2, primarily in the eddy-rich regions. Comparison of wavenumber spectra indicates that even higher spatial resolution is needed to fully resolve the mesoscale eddies. The absolute dynamic topography determined from either the data or the model has an error on the order of 10 cm at wavelengths larger than 2000 km. Geoid errors are the limiting factor for the utility of the altimeter data at smaller scales. Heat flux forcing is a key factor in determining the simulated annual cycle of the ocean. Improved agreement with the observation is achieved when the model is driven by heat flux forcing instead of being nudged to sea surface temperature climatology. The temporal evolution of the intraseasonal fluctuations at mid- and high latitudes as well as the interannual variability of the tropical oceans, both are primarily wind driven, is simulated fairly well by the model.

Effect of species composition and sward structure on dietary quality in cattle and sheep grazing South African sourveld

SUMMARYThe effect of sward structure and species composition on dietary quality in cattle and sheep grazing a mesic, low quality grassland (sourveld) in South Africa was investigated over the 1990/91 and 1991/92 grazing seasons. Species composition had no measurable effect on dietary quality in either cattle or sheep but this variable could affect quality over grazing periods longer than those used in the study. In contrast, sward structure, and in particular sward greenness, had a major effect on dietary quality in both animals. Dietary in vitro digestibility (IVD) was positively correlated (P &lt; 0·001) with sward greenness but negatively correlated (P &lt; 0·05) with herbage mass and sward height. Dietary IVD in sheep was consistently higher (P &lt; 0·001) then in cattle but both species reacted in a similar manner to changes in sward structure and season. Dietary quality declined over the grazing season, with IVD being highest in spring (&gt; 60%) and lowest in winter (&lt; 45%). Marked intra-seasonal fluctuations in quality also occurred in response to sward structure and apparent variation in soil moisture. Fire also appeared to have a significant effect on quality, with IVD being higher (P &lt; 0·001) in the first year following the burn than in the subsequent year. It is suggested that on sourveld, dietary quality in the growing season is determined by a combination of environmental and management factors. Conversely, in the dormant season, dietary quality is largely determined by environmental factors beyond the control of the grazier.

Atmospheric Intraseasonal Variability at the Surface in the Tropical Western Pacific Ocean

Abstract Observations from the Tropical Atmosphere Ocean (TAO) moored buoy array provide an unprecedented opportunity to explore the atmospheric variability at the tropical sea surface. Three years of data from the TAO buoys in the western Pacific are analyzed to document surface intraseasonal fluctuations and their relationships to the tropical interseasonal oscillation (ISO) in the troposphere above the boundary layer. The most robust intraseasonal variations at the surface are observed in zonal wind, wind speed, relative humidity, sea surface temperature (SST), surface turbulent sensible and latent heat fluxes, and air–sea temperature and humidity differences. Less robust intraseasonal signals are found in surface meridional wind, specific humidity, and equivalent potential temperature. Observed signals in surface air temperature and wind divergence are the weakest. Positive anomalies of atmospheric deep convection associated with the ISO tend to occur in regimes of surface westerly perturbations, wher...

Year-to-Year and interdecadal variability in the activity of intraseasonal fluctuations in the Northern Hemisphere wintertime circulation

Interannual variability in the activity of fluctuations with subseasonal time scales is investigated based upon observed data of the extratropical Northern Hemisphere circulation over the recent 38 winters. Their activity is represented in the root mean square (RMS) field of filtered geopotential height in which the fluctuations with time scales between 10 days and a season are retained. The singular value decomposition (SVD) was applied to the covariance matrix between the seasonal mean and RMS fields for the 500-hPa height.

A comparison between the TOPEX/POSEIDON data and a global ocean general circulation model during 1992–1993

The TOPEX/POSEIDON altimetric sea level observation during 1992–1993 was used to validate the simulation made by a global ocean general circulation model (OGCM) forced by the daily wind stress and heat flux derived from the National Meteorological Center operational analysis. The OGCM is a version of the modular ocean model with a horizontal resolution of 2° longitude and 1° latitude and 22 levels in the vertical. The model simulation is compared to the observation at spatial scales of the order of 500 km and larger. Only the temporal variations are examined. The variability is composed primarily of the annual cycle and intraseasonal fluctuations (periods shorter than 100 days). The basic features of the annual cycle are simulated well by the model. Major discrepancies are found in the eastern tropical Pacific, as well as the eastern North Pacific and most of the interior of the North Atlantic. The culprit is suspected to be the inadequate heat forcing and mixing parameterizations of the model. Significant intraseasonal variability is found in the central North Pacific and the Southern Ocean. The simulation is highly correlated with the observation at periods from 20 to 100 days. The spatial scales are larger than 1000 km in many places. These variabilities are apparently the barotropic response of the ocean to wind forcing. The results of the study provide a basis for future assimilation of the data into the OGCM for improved description of the large‐scale ocean variabilities.

Ozone dynamics over Greece as derived from satellite and! in situ measurements

Abstract A comprehensive analysis of the records of surface ozone available for Athens, Greece ( 38° N, 24° E) for the periods 1901–1940 and 1987–1990 is presented. Both records are analysed to explore the intraseasonal fluctuations and the harmonic components of surface ozone and also compared to other historical surface ozone records. The variation in surface ozone concentration during rainfall is also investigated, using the hourly measurements of the surface ozone concentration obtained by a network of four stations within the Greater Athens area. The results indicate that, during rainfall events which are associated with the passing of a cold front, an important decrease of the surface ozone concentration is observed. Daily measurements of surface ozone and NOx, from five stations in the Greater Athens Basin overthe period 1986–1990 are also used in order to examine the main features of basin-wide 03-HC-NOx relations. A simple regression model between the surface ozone concentration and the temperatu...

On the thermal interaction between the synoptic‐scale eddies and the intraseasonal fluctuations in the atmosphere

Abstract The characteristics of the Northern Hemisphere low‐frequency temperature variability (periods 10 to 120 d) are examined using 700‐hPa analyses for 5 winters. Of particular interest is the relationship between the evolution of the low‐frequency temperature field and that of the higher‐frequency synoptic‐scale flow. The latter produces a low‐frequency heat flux divergence that can interact with the low‐frequency temperature field. The nature of this interaction is investigated by examining the relative position, both in space and in time, of the low‐frequency temperature and the heat flux divergence. The maximum temporal variance of the low‐frequency temperature is found to occur over Canada and Siberia. Although not negligible in these regions, the low‐frequency component of the heat flux divergence by the synoptic‐scale flow reaches a maximum somewhat farther to the east, near the storm track regions. The low‐frequency temperature variations are positively correlated in time with the in situ synoptic‐scale heat flux divergence, implying that the synoptic‐scale eddies act to dissipate the low‐frequency temperature variability. The dominant patterns of the low‐frequency temperature are determined through an Empirical Orthogonal Function (EOF) analysis. The first modes over East Asia and North America take the form of monopoles, one over each of these areas. The second and the third eigenvectors are more closely connected with the fluctuations of temperature near the east coast areas of Asia and North America. The pattern of the synoptic‐scale eddy heat flux convergence associated with each of the leading EOF temperature modes is also identified. The importance of the synoptic‐eddy heat flux convergence in determining the low‐frequency temperature variability is compared with that of other processes through a thermal variance budget analysis. The main source of the low‐frequency thermal variance over the eastern portions of the Asian and North American continents and the adjacent oceanic areas is the low‐frequency heat flux down the time‐mean temperature gradient. Both the heat flux divergence by the synoptic‐scale eddies and the low‐frequency vertical motion tend to reduce the low‐frequency thermal variance.

The influence of Southern Hemispheric Equatorial Trough on rainfall during southwest monsoon

Monsoon is a complex dynamical system. Interannual and intraseasonal fluctuations have been studied in the regional and global perspective. Southern Hemispheric Equatorial Trough and its activity during the northern summer appears inversely related to the rainfall activity over the Indian sub-continent in general and Central India in particular. During typical active SHET epochs the rainfall activity becomes deficient in the Central parts of the country. The activity of the SHET is fluctuating in nature. Its duration is prolonged and intensity is increased during the years of major failure.

The Atmospheric Angular Momentum Cycle Associated with a Madden–Julian Oscillation

Abstract The period 1 December 1984 to 3 February 1985 was associated with strong intraseasonal fluctuations in both the global atmospheric angular momentum (AAM) and tropical convection. Consistent changes were observed in the length of day. The AAM budget for the 65-day period is examined here using circulation data from the National Meteorological Center. Surprisingly well-balanced global and zonal budgets are obtained for the vertically integrated AAM. This enables a closer examination of regional changes, to assess how they might be responsible for the changes in the global AAM. Both friction and mountain torques are important in the global AAM budget. The increase of AAM is associated first with a positive friction torque, then with a positive mountain torque. The subsequent decrease of AAM results from a negative friction torque. The accompanying regional changes are mostly confined to the Northern Hemisphere, with high global AAM associated with a stronger and southward-displaced subtropical jet. ...

An analytic solution of the stochastic storage problem applicable to soil water

The accumulation of soil water during rainfall events and the subsequent depletion of soil water by evaporation between storms can be described, to first order, by simple accounting models. When the alternating supplies (precipitation) and demands (potential evaporation) are viewed as random variables, it follows that soil‐water storage, evaporation, and runoff are also random variables. If the forcing (supply and demand) processes are stationary for a sufficiently long period of time, an asymptotic regime should eventually be reached where the probability distribution functions of storage, evaporation, and runoff are stationary and uniquely determined by the distribution functions of the forcing. Under the assumptions that the potential evaporation rate is constant, storm arrivals are Poisson‐distributed, rainfall is instantaneous, and storm depth follows an exponential distribution, it is possible to derive the asymptotic distributions of storage, evaporation, and runoff analytically for a simple balance model. A particular result is that the fraction of rainfall converted to runoff is given by (1 ‐ R−1)/(eα(1−R−1) − R−1), in which R is the ratio of mean potential evaporation to mean rainfall and a is the ratio of soil water‐holding capacity to mean storm depth. The problem considered here is analogous to the well‐known problem of storage in a reservoir behind a dam, for which the present work offers a new solution for reservoirs of finite capacity. A simple application of the results of this analysis suggests that random, intraseasonal fluctuations of precipitation cannot by themselves explain the observed dependence of the annual water balance on annual totals of precipitation and potential evaporation.

Determining torques over the ocean and their role in the planetary momentum budget

Knowledge of wind torques at the atmosphere‐ocean interface is important in understanding the global angular momentum balance. A surface wind data set that combines special sensor microwave/imager (SSM/I) and European Centre for Medium Range Weather Forecasts (ECMWF) wind products and covers the period July 1987 to June 1989 is used to estimate zonal torques over the global ocean. Comparison of these fields and fields computed from ECMWF wind analyses alone shows substantial differences in the statistics of both locally and globally integrated torques; the impact of SSM/I data on the torque analysis is largest over the Indian Ocean and eastern Pacific regions. The characteristics of the torques from the combined SSM/I‐ECMWF data set are evaluated. In general, local torques show the largest means and variances over mid‐latitudes (especially in the southern ocean), with fluctuations at periods shorter than 1 month dominating the records. In contrast, variability in the globally integrated ocean torque is largest at seasonal time scales (3 months to 1 year); midsouthern and tropical northern latitudes contribute substantially to this variability. Comparison between the global ocean torque and atmospheric angular momentum M indicates that indirect exchanges of momentum between the atmosphere and solid Earth via oceanic stresses cannot be neglected at seasonal time scales. Intraseasonal fluctuations (1–3 months) in both tropical and global ocean torques are coherent with M, but too weak to account alone for the variability in M. At submonthly periods the ocean torque is apparently not important in the atmospheric momentum balance.

Seasonal Variations of the 40–50-Day Oscillation in Atmospheric Angular Momentum

Abstract Seasonally varying spectral and cross-spectral calculations are carried out on multiyear time series of vertically and zonally averaged daily zonal wind fields to describe the seasonal cycle of the 40–50-day oscillation of atmospheric angular momentum. Intraseasonal variability (including 40–50-day fluctuations) of global momentum is largest in late boreal winter and smallest in boreal autumn; however, the 40–50-day spectral peak is most pronounced in boreal summer when lower-frequency intraseasonal variance is depressed. The 40–50-day spectral peak in global momentum is much less pronounced and apparently is restricted to a narrower frequency band, than corresponding peaks in zonal wind spectra from individual tropical rawinsonde stations. Contributions to global momentum fluctuations from three near-equal-area latitude bands (tropics, Northern Hemisphere, and Southern Hemisphere) are compared, confirming that intraseasonal momentum fluctuations are tropical in origin. The variance of extratropi...

Relationships between circulation and rainfall over India during the southwest monsoon season Part I ? surface pressure

Rainfall over India during the southwest monsoon season exhibits large intraseasonal fluctuations. The surface pressure fields illustrate the important circulation changes and the general conditions of active and break monsoon situations. We have studied the relationship between these two successive fields at daily to monthly time scales using montly data, from July through September for an 11 year period (1966–1976). Lag relationships were also investigated to ascertain the nature of evolutionary patterns through which pressure affects rainfall and so assess the potential for predicting rainfall with the use of pressure fields. Finally, the relationship between pressure and rainfall (linear or non-linear) was examined with the use of quartile plots.