Strong Annual Signal Research Articles

Seasonality of reproduction in an ever-wet lowland tropical forest in Amazonian Ecuador.

Flowering and fruiting phenology have been little studied in the ever-wet hyperdiverse lowland forests of northwestern equatorial Amazonia. These Neotropical forests are typically called aseasonal with reference to climate because they are ever-wet, and it is often assumed they are also aseasonal with respect to phenology. The physiological limits to plant reproduction imposed by water and light availability are difficult to disentangle in seasonal forests because these variables are often temporally correlated, and both are rarely studied together, challenging our understanding of their relative importance as drivers of reproduction. Here we report on the first long-term study (18 years) of flowering and fruiting phenology in a diverse equatorial forest, Yasuní in eastern Ecuador, and the first to include a full suite of on-site monthly climate data. Using twice monthly censuses of 200 traps and > 1000 species, we determined whether reproduction at Yasuní is seasonal at the community- and species-levels and analyzed the relationships of environmental variables to phenology. We also tested the hypothesis that seasonality in phenology, if present, is driven primarily by irradiance. Both the community- and species-level measures demonstrated strong reproductive seasonality at Yasuní. Flowering peaked in September-November and fruiting peaked in March-April, with a strong annual signal for both phenophases. Irradiance and rainfall were also highly seasonal, even though no month on average experienced drought (a month with < 100 mm rainfall). Flowering was positively correlated with current or near-current irradiance, supporting our hypothesis that the extra energy available during the period of peak irradiance drives seasonality of flowering at Yasuní. As Yasuní is representative of lowland ever-wet equatorial forests of northwestern Amazonia, we expect that reproductive phenology will be strongly seasonal throughout this region. This article is protected by copyright. All rights reserved.

A composite approach towards understanding the mechanisms and driving variables of river mouth variability: A case study of the Da Dien River mouth

ABSTRACT This research aims to present a composite approach to characterizing the intra-annual and interannual variability in inlet throat width and understanding its mechanism (how, when, and why it varies). The techniques used include remote sensing imagery analysis, spectral and cross-spectral analyses, correlation analyses, and random forest variable importance. The Da Dien River mouth (DDRM), which is located in southern Central Vietnam and experiences a strong annual signal of climate regimes, was selected as a case study to demonstrate the applicability of the approach. The results show that the narrowing/widening of the DDRM throat width is highly dynamic and variable under the significant dominant influence of wave height, tide and river flow, which seasonally and interannually vary according to the monsoon regime, El Niño/Southern Oscillation and sunspot number variations (better correlation with ENSO events). The overall throat width decreases during El Niño/the positive QBO phase and increases during La Niña periods/the negative QBO phase. The difference in river flow magnitudes is the main cause for the constriction/expansion of the DDRM throat width. The successful application of the study approach to the DDRM case study demonstrates its usefulness and ability to be applied to other case studies in tropical monsoon regions.

Wavelet spectrum analysis of PM10 data in Bangkok, Thailand

PM10 is one of the key factors which influences the air quality in metropolitan areas throughout the world. In this research we investigate the variation period of PM10 concentration and its temporal patterns from 2009 to 2017 in Bangkok, Thailand using wavelet spectrum analysis. We also utilize the cross wavelet transform to study the potential relation between PM10 and the temperature. From the wavelet power spectrum of PM10, we can distinguish two dominant bands, one in the period between 8-16 months and the other between 1-8 months. The first oscillation is obviously related to natural annual periodicities that the high power occurs from December 2011to February 2015. The PM10 concentrations are high in winter and low in the rainy season. The second band between 1-8 months is the transient pattern with very high concentration of PM10 occurs only in the year 2013. Wavelet spectrum of the temperature is similar in pattern for it shows the strong annual signal and the lowest temperature band between 4-8 months. The cross wavelet transform (XWT) power spectra between PM10 and the temperature show significant common power, in the 8-16 month band from November 2009 to June 2016, and in the 1-8 month band around November 2012 to February 2014. The wavelet transform coherence (WTC) spectra show that PM10 and the temperature co-varied out of phase during all observed time intervals. This indicates the high seasonal dependence between PM10 and the temperature. Knowledge of the variation period of PM10 concentration and its evolution feature in Bangkok area will help the authorities to better prepare for public health and environmental hazard from the next PM10 pollution.

Cross‐shelf water exchange in the East China Sea as estimated by satellite altimetry and in situ hydrographic measurement

AbstractCombining satellite altimetry and in situ hydrographic measurement, we estimated the cross‐shelf transport (CST) and its spatial and temporal variations across 200 m isobath in the East China Sea (ECS) from 1993 to 2014. The vertically integrated CST can be dynamically divided into three parts: surface Ekman transport, geostrophic transport, and bottom Ekman transport. The results show that the 22 year‐mean, sectionally integrated CST to be 1.7 ± 2.0 Sv (positive in the on‐shelf direction), comprised of bottom and surface Ekman transports of 2.7 ± 1.0 Sv and 0.6 ± 0.6 Sv, respectively, that are partially offset by a geostrophic transport of −1.5 ± 1.7 Sv. The sectionally integrated CST shows significantly high power at roughly annual period from 1999 to 2001, with lower power at intra‐annual period. The vertically integrated CST to the northeast of Taiwan is the main source of sectionally integrated CST. The vertically integrated CST also shows significant variations in the 6–15 month period band to the northeast of Taiwan as well. The temporal variations of the sectionally integrated and vertically integrated CST are both controlled primarily by geostrophic transport and modulated by bottom Ekman transport. In the upper 50 m, the geostrophic current to the northeast of Taiwan exhibits large mean and significant variability. The empirical orthogonal function analysis of vertical structure of geostrophic current shows two significant modes with strong annual signal. The first mode is associated with the migration of Kuroshio axis near Taiwan, while the second mode is associated with the variation of the meander of the Kuroshio to the northeast of Taiwan.

Population Genetic Structure and the Migration of Puccinia striiformis f. sp. tritici Between the Gansu and Sichuan Basin Populations of China.

Puccinia striiformis f. sp. tritici is the causal pathogen of interregional epidemics of wheat stripe rust in China via long-distance migration. Gansu Province serves as putative inoculum center providing oversummering inoculum, while Sichuan Basin area serves as a region providing huge amounts of overwintering inoculum. Thus, the relationship between these two regions in population exchange and migration become important in prediction of interregional epidemics. In this study, we compared the population genetic structure and race composition between Gansu and Sichuan Basin populations to infer their migration relationships. A total of 526 isolates, spanning 3 years, were genotyped using eight pairs of amplified fragment length polymorphism markers, and a subset of 98 isolates were inoculated onto 19 Chinese differentials to perform the race analysis. Twenty-three common races and 26 shared genotypes supplied molecular evidence for migration between Gansu and Sichuan Basin populations. Bayesian assignment and principal component analysis revealed that the genetic group assignment of the Sichuan Basin populations (10SB and 11SB) changed in the spring to align with the fall Gansu populations in the prior seasons (09GS and 10GS), which indicated an asymmetric migration from Gansu Province to the Sichuan Basin area. The linkage disequilibrium and the parsimony tree length permutation test revealed a strong annual recombination signal in the Gansu populations and an inconsistent signal in the Sichuan Basin populations.

Temporal dynamics of monthly rainfall time scales in semi-arid area, Morocco

Historically, Morocco has been subject to severe climate conditions in which intense rainfall and floods may occur after long periods of drought. This paper addresses the temporal dynamics of rainfall at monthly time scales in a semi-arid climate by using the Fourier and wavelet transforms. The analysis shows a concentration of power within the 8–16-month band, meaning that the rainfall time series have a strong annual signal. However, this band is not continuous, which means that the rainfall time series is affected by dry and wet periods. Other bands were observed centered mostly in the 2–4-month band, but sometimes covered a 2–8-month period. Concerning long-term bands, we identified the transition from a dry period to a wet period as lasting between 2 and 8 years, which witnessed a recent temporal evolution of rainfall supported by the evidence that the area is witnessing the effects of global warming.

A consistent combination of GNSS and SLR with minimum constraints

In this article, the realization of a global terrestrial reference system (TRS) based on a consistent combination of Global Navigation Satellite System (GNSS) and Satellite Laser Ranging (SLR) is studied. Our input data consists of normal equation systems from 17 years (1994–2010) of homogeneously reprocessed GPS, GLONASS and SLR data. This effort used common state of the art reduction models and the same processing software (Bernese GNSS Software) to ensure the highest consistency when combining GNSS and SLR. Residual surface load deformations are modeled with a spherical harmonic approach. The estimated degree-1 surface load coefficients have a strong annual signal for which the GNSS- and SLR-only solutions show very similar results. A combination including these coefficients reduces systematic uncertainties in comparison to the single-technique solution. In particular, uncertainties due to solar radiation pressure modeling in the coefficient time series can be reduced up to 50 % in the GNSS+SLR solution compared to the GNSS-only solution. In contrast to the ITRF2008 realization, no local ties are used to combine the different geodetic techniques. We combine the pole coordinates as global ties and apply minimum constraints to define the geodetic datum. We show that a common origin, scale and orientation can be reliably realized from our combination strategy in comparison to the ITRF2008.

Quantifying tropical dry forest succession in the Americas using CHRIS/PROBA

This study explores the possibility of estimating different stages of ecological succession in tropical dry forests using CHRIS/PROBA hyperspectral and multi-angular satellite images. Tropical dry forests (TDFs) in the Americas are considered one of the most threatened ecosystems in region, experiencing also high rates of secondary growth. In this study, TDFs are studied along three successional stages (early, intermediate and late) in sites across a latitudinal gradient in the Americas. Sites are located in Mexico, Nicaragua and Brazil. Due to their strong annual phenological signal, reflectance values of CHRIS/PROBA Mode-4 were extracted for the dry and wet season, and afterwards analyzed using scatterplots, n-D visualization tools, and Transformed Divergence separability tests. Spectral separation of three successional stages within TDFs was observed in images corresponding to the dry season, at all study sites. In this season, it is possible to discriminate the different successional stages in all viewing angles. For the spectral separation of the three successional stages in images of the wet season, it is required the use of the 18 bands of the sensor. Our analysis also indicates that better spectral separation is achieved at ±55°, in both seasons. Our results confirm the utility of multi-angular and hyperspectral sensors as a critical tool for environmental planning in TDFs environments.

Evidences of Intermittent Wind-Induced Flow in the Yellow Sea obtained from AVHRR SST Data

Ten-year AVHRR sea surface temperature data obtained in the Yellow Sea are put into EOF analyses. Temperature variation is predominated by the first mode which is associated with the seasonal fluctuation of temperature with annual range decreasing with the bottom depth. Since such a strong annual signal may mask the upwind or downwind flows occurring intermittently during the winter, only the data obtained during this season are put into EOF analyses. Every winter shows similar results. The first mode, explaining more than 90% of total variance, appears to be a part of the seasonal variation of temperature mentioned above. In the second mode, the time coefficient is well correlated with northerly winds to which the responses of the trough and shallow coastal areas are opposite to each other. A simple theoretical consideration suggests the following physical explanation: The northerly wind stress anomaly creates an upwind (downwind) flow over the trough (coastal) areas, which then induces a temperature increase (decrease) by advection of heat, and vice versa for the southerly wind stress anomaly. Hence, this paper provides further evidence of the intermittent upwind or downwind flows occurring in the Yellow Sea every winter.

Automated ice-core layer-counting with strong univariate signals

Abstract. We present an automated process for determining the annual layer chronology of an ice-core with a strong annual signal, utilising the hydrogen peroxide record from an Antarctic Peninsula ice-core as a test signal on which to count annual cycles and explain the methods. The signal is de-trended and normalised before being split into sections with a deterministic cycle count and those that need more attention. Possible reconstructions for the uncertain sections are determined which could be used as a visual aid for manual counting, and a simple method for assigning probability measures to each reconstruction is discussed. The robustness of this process is explored by applying it to versions of two different chemistry signals from the same stretch of the NGRIP (North Greenland Ice Core Project) ice-core, which shows more variation in annual layer thickness, with and without thinning to mimic poorer quality data. An adapted version of these methods is applied to the more challenging non-sea-salt sulphur signal from the same Antarctic Peninsula core from which the hydrogen peroxide signal was taken. These methods could readily be adapted for use on much longer datasets, thereby reducing manual effort and providing a robust automated layer-counting methodology.

Hydrographic variability on a coastal shelf directly influenced by estuarine outflow

Hydrographic variability on the Alabama shelf just outside of Mobile Bay, a major source of river discharge into the Gulf of Mexico, is examined using time series of water column temperature and surface and bottom salinity from a mooring site with a depth of 20 m in conjunction with a series of across-shelf CTD surveys. The time series data show variability in a range of time scales. The density variation is affected by both salinity and temperature, with its relatively strong annual signal mostly determined by temperature and its year to year variability mostly determined by salinity. Seasonal mean structures of temperature, salinity, and density show a transition from estuarine to shelf conditions in which three regions with distinct seasonal characteristics in their horizontal and vertical gradient structures are identified. Correlation analysis with the available forcing functions demonstrates the influence of Mobile Bay on the variability at the mooring site. At low frequencies, river discharge from Mobile Bay has a varying influence on salinity, which is absent during the periods with unusually low discharge. At shorter synoptic time scales, both the estuarine response to the across-shelf wind stress and the shelf response to the along-shelf wind stress are significantly correlated with temperature/salinity variability: the former becoming important for the surface layer during winter whereas the latter for the bottom layer during both winter and summer. These forcing functions are important players in determining the estuarine–shelf exchange, which in turn is found to contribute to the shelf hydrographic structure.

Investigation of the North Brazil Current retroflection and North Equatorial Countercurrent variability

Eight years of TOPEX/Poseidon altimeter‐derived sea height anomaly and climatological hydrographic data are used to investigate the temporal and spatial variability of the North Brazil Current (NBC) retroflection and the North Equatorial Countercurrent (NECC). The NBC retroflection exhibits a mean position of 6.6°N ± 2.0°, with a strong annual signal and year‐to‐year variability. The annual cycle of its location follows the meridional migration of the ITCZ and the strength of the wind stress curl over the equatorial Atlantic. A marked shift in the annual mean location of the NBC retroflection is observed in 1998–1999 that appears to be linked to an increase in wind stress curl during this period. The mean latitude of the NECC is estimated to be 6.0°N ± 1.3°, with a semiannual cycle showing maximum northerly locations in February and August. The baroclinic transport of the NECC varies predominantly on an annual cycle with maximum transport of 12 Sv occurring in November.

Quantification of nearshore morphology based on video imaging

The Argus network is a series of video cameras with aerial views of beaches around the world. Intensity contrasts in time exposure images reveal areas of preferential breaking, which are closely tied to underlying bed morphology. This relationship was further investigated, including the effect of tidal elevation and wave height on the presence of wave breaking and its cross-shore position over sand bars. Computerized methods of objectively extracting shoreline and sand bar locations were developed, allowing the vast quantity of data generated by Argus to be more effectively examined. Once features were identified in the images, daily alongshore mean values were taken to create time series of shoreline and sand bar location, which were analyzed for annual cycles and cross-correlated with wave data to investigate environmental forcing and response. These data extraction techniques were applied to images from four of the Argus camera sites. A relationship between wave height and shoreline location was found in which increased wave heights resulted in more landward shoreline positions; given the short lag times over which this correlation was significant, and that the strong annual signal in wave height was not replicated in the shoreline time series, it is likely that this relationship is a result of set-up during periods of large waves. Wave height was also found to have an effect on sand bar location, whereby an increase in wave height resulted in offshore bar migration. This correlation was significant over much longer time lags than the relationship between wave height and shoreline location, and a strong annual signal was found in the location of almost all observed bars, indicating that the sand bars are migrating with changes in wave height. In the case of the site with multiple sand bars, the offshore bars responded more significantly to changes in wave height, whereas the innermost bar seemed to be shielded from incident wave energy by breaking over the other bars. A relationship was also found between a site's mean wave height and inner sand bar location; sites with the highest wave heights tended to have sand bars farther from shore than those with relatively low wave heights.

Downslope convection north of Elephant Island, Antarctica: Influence on deep waters and dependence on ENSO

We present a long time series (1992–2001) of bottom temperature from 1040m on the continental slope north of Elephant Island, Antarctic Peninsula. A strong annual signal is apparent, with minimum temperatures during or following the austral winter, and a range similar to that found in much shallower waters on the adjacent shelf. Individual convective events are observable as large‐amplitude cold spikes that are confined to the period of seasonally coldest temperatures. These plumes are dense enough to seasonally displace the Circumpolar Deep Water, and directly inject cold water into the deep open ocean. The coldest water occurred when anomalous sea ice conditions were prevalent, caused by the strong 1997/98 ENSO event. We thus expect repeats of such strong convective events on ENSO timescales. The observed long‐term warming and reduction in sea ice along the Peninsula may, however, act to suppress this mode of ventilating the deep Southern Ocean.

Scatterometer winds explain damped Rossby waves

Westward propagating waves in the North Pacific Ocean from 10–16°N are overwhelmed by a zonally coherent response at the annual period, as observed in sea surface height (SSH) anomalies from the TOPEX/POSEIDON altimeter. SSH from a simple model of wind‐forced Rossby waves and from seasonal heating are compared with observed SSH to understand the processes responsible for the observed signal. The seasonal heating cycle is out‐of‐phase and too weak to explain the SSH. The oceanic response to wind stress curl forcing more closely resembles the observations, but the response to NCEP Reanalysis winds does not show a strong annual cycle. Wind stress curl from the QuikSCAT/SeaWinds scatterometer has a strong and zonally coherent annual cycle that produces a corresonding strong annual signal in SSH. The model forced by scatterometer winds demonstrates that the response to Ekman pumping is the source of the strong annual cycle in the SSH.

Crustal displacements due to continental water loading

The effects of long‐wavelength (&gt;100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displacements (ΔrM) have root‐mean‐square (RMS) values for 1994–1998 as large as 8 mm, with ranges up to 30 mm, and are predominantly annual in character. Regional strains are on the order of 20 nanostrain for tilt and 5 nanostrain for horizontal deformation. We compare ΔrM with observed Global Positioning System (GPS) heights (ΔrO) (which include adjustments to remove estimated effects of atmospheric pressure and annual tidal and non‐tidal ocean loading) for 147 globally distributed sites. When the ΔrO time series are adjusted by ΔrM, their variances are reduced, on average, by an amount equal to the variance of the ΔrM. Of the ΔrO time series exhibiting a strong annual signal, more than half are found to have an annual harmonic that is in phase and of comparable amplitude with the annual harmonic in the ΔrM. The ΔrM time series exhibit long‐period variations that could be mistaken for secular tectonic trends or postglacial rebound when observed over a time span of a few years.

Seasonal heat balance in the Gulf of California

Most of the available hydrographic data (1939–1988) from the Gulf of California are used to describe the seasonal cycle of heat storage in the upper 400 m of water. The seasonal cycle of surface heat flux is obtained by using the sea surface temperature from the hydrographic data bank and meteorological data from coastal stations along the gulf. Monthly values of surface heat flux and heat storage are fitted to a mean value plus an annual harmonic. The longitudinal (i.e., along‐gulf) heat flux is then obtained by integrating the difference between the rate of change of heat storage and the surface heat flux. It is found that the surface heat flux has a positive (into the sea) annual mean all along the gulf; the average annual net surface heat flux for the whole gulf being ∼118 W m−2. The southern part of the gulf gains more heat (∼130 W m−2) than the northern part (∼100 W m−2), owing to the larger loss by evaporation in the latter (∼0.6 m/yr versus ∼1 m/yr) caused by lower humidity and stronger winds. These values are in agreement with previous estimates for the northern gulf. The amplitude of the seasonal signal shows a maximum of ∼230 W m−2 in the central part. Winter heat losses occur only from the middle of the gulf, the Guaymas Basin, to its head. There is a strong annual signal superimposed on the average longitudinal heat flux, whose amplitude is larger than the annual harmonic of the surface heat flux. Input of heat from the Pacific Ocean occurs from mid‐March to mid‐July, with a maximum at the mouth in May (21×1012 W). Attempts to explain this flux by diffusive processes were unsuccessful. The proposition that advection is the main longitudinal heat carrier is congruent with the generally accepted circulation patterns.

Observation of mesoscale ocean features in the northeast Pacific using Geosat radar altimetry data

In this paper, Geosat altimetry data from the northeast Pacific are examined. The data are (1) binned data on 1° latitude by 2° longitude blocks and (2) collinear track data. The binned data cover the region from 20°N to 60°N from the North American coast westward to 150°W and span from December 15, 1986, to June 1, 1989. To examine the data for evidence of westward propagation of sea level anomalies, three zonal transects (45°N, 49°N, and 57°N) are made. Complex empirical orthogonal function analysis of the data transects at these latitudes reveals phase speed and amplitude information. The first eigenmode at each latitude has a very strong annual signal. Two‐dimensional spectral analysis of the first eigenmode at 45°N and 57°N shows westward propagation of sea level anomalies at 45°N only. The propagating features are annual in period and of order 1000 km in length. At 57°N, westward propagation is evident (as at 45°N), but there is also an eastward component. To examine the variability of the coastal sea level signal, collinear track data from three ascending tracks crossing the Gulf of Alaska are analyzed revealing mesoscale features of the order of 100 km that parallel the Canadian and Alaskan coast. The most notable feature is the Sitka eddy. Westward propagation of several of these features is observed. The results from the analysis of both types of Geosat data are compared with other observational data.

The isotopic record of Miocene-Recent environmental changes in shallow marine habitats on either side of the Panamanian isthmus

The Pliocene uplift of the Panamanian isthmus isolated previously contiguous Caribbean and eastern Pacific marine faunas, and initiated the development of different selective regimes on opposing sides of the barrier. Contrasting environmental conditions provided the context for different rates and kinds of evolutionary change on either side of the isthmus. The goal of our study is to characterize the rates and types of these paleoenvironmental changes; we employ stable isotopic profiles of venerid bivalve shells from the Miocene-Recent deposits of Panama and Costa Rica.We analyzed geographic and depth variation in Recent shells as a means of calibrating the fossil analyses. Results from Recent venerids highlight the differences between the Caribbean and eastern Pacific shallow marine environments. Pacific shells typically exhibit strong annual fluctuations in δ18O, associated with seasonal upwelling and its associated cool waters. In shallow, near-shore habitats, freshwater runoff during the rainy season also contributes significantly to annual δ18O cycles. Caribbean shells exhibit negligible annual cyclicity.We analyzed fossil shells from the Miocene Gatun Formation, and the Pliocene Cayo Agua, Escudo de Veraguas, and Rio Banano Formations from the Caribbean coast, and the Plio-Pleistocene Charco Azul Group from the Pacific coast. Results from fossil shells indicate that annual cycles are well-preserved, and not obscured by diagenesis or other alteration. A strong annual signal in δ18O (amplitude &gt;1.5‰) is found back at least to the Early Pleistocene along the Pacific coast. Some post-uplift Caribbean samples exhibit stronger seasonality than that observed in similar habitats today.