Decadal Shift Research Articles

Migrating bison engineer the green wave

Newly emerging plants provide the best forage for herbivores. To exploit this fleeting resource, migrating herbivores align their movements to surf the wave of spring green-up. With new technology to track migrating animals, the Green Wave Hypothesis has steadily gained empirical support across a diversity of migratory taxa. This hypothesis assumes the green wave is controlled by variation in climate, weather, and topography, and its progression dictates the timing, pace, and extent of migrations. However, aggregate grazers that are also capable of engineering grassland ecosystems make some of the world's most impressive migrations, and it is unclear how the green wave determines their movements. Here we show that Yellowstone's bison (Bison bison) do not choreograph their migratory movements to the wave of spring green-up. Instead, bison modify the green wave as they migrate and graze. While most bison surfed during early spring, they eventually slowed and let the green wave pass them by. However, small-scale experiments indicated that feedback from grazing sustained forage quality. Most importantly, a 6-fold decadal shift in bison density revealed that intense grazing caused grasslands to green up faster, more intensely, and for a longer duration. Our finding broadens our understanding of the ways in which animal movements underpin the foraging benefit of migration. The widely accepted Green Wave Hypothesis needs to be revised to include large aggregate grazers that not only move to find forage, but also engineer plant phenology through grazing, thereby shaping their own migratory movements.

Spatiotemporal variability in daily observed precipitation and its relationship with snow cover of Hindukush, Karakoram and Himalaya region in northern Pakistan

This work quantifies recent spatiotemporal variations in daily observed precipitation in the mountains of the northern Pakistan (HKNP) for 30-year period (1986–2015) on annual as well as on seasonal basis. The probability distribution functions (PDFs) are employed to identify decadal changes in the observed precipitation of the region. The precipitation based PDFs depict a positive mean decadal shift of 0.13 mm/day from 1996–2005 to 2006–2015 in the HKNP region, on annual basis. Similar positive mean decadal shift of 0.18, 0.18, and 0.16 mm/day for precipitation in winter, summer and autumn seasons is noticed, from 1996 to 2005 to 2006–2015, respectively. It is further observed that the intensity of extreme precipitation events also increases progressively (from 213.8 to 257.0 mm/day) during 1986–1995 to 2006–2015 for summer season in the HKNP region. Along with temporal shifts, spatial positive (negative) precipitation shifts are also noticed in the western (eastern) parts of the HKNP region. Furthermore, significant increasing trends in the wet days at weather stations of the Karakoram subregion in the HKNP are directly related with increasing snow cover fraction trends for summer season in a changing climate. A progressively increasing high non-parametric correlation (0.49 to 0.76) between the precipitation and the river discharge of the region is observed in recent decade (2006–2015) which requires better water resource management in the HKNP region in coming years.

Modulation of tropical cyclones in the southeastern part of western North Pacific by tropical Pacific decadal variability

The tropical cyclone (TC) genesis number in the western North Pacific (WNP) exhibits a pronounced decadal decrease around the mid-1990s, with prominent seasonal and spatial inhomogeneity. This decadal shift of TC activity is mostly confined to the southeastern part of the WNP and occurs mainly during the second half of the calendar year. Accordingly, westward and northeastward TC recurving movements strongly decreased in recent decades after 1995 compared with TC tracks in the earlier period (1979–1994). We find that this TC activity decadal change is associated with tropical Pacific decadal variability, which is measured here by a low-pass filtered Nino3.4 index. In contrast to the earlier period, the anomalous cold mean state in the tropical Pacific during recent decades favored the enhancement of zonal vertical wind shear (UVWS) and suppressed TC activity. This tropical Pacific mean state change is possibly related to decadal changes of El Nino–Southern Oscillation (ENSO) properties (i.e., more La Nina events occurred during recent decades). This relationship between tropical Pacific mean state change and the UVWS in the southeastern WNP on decadal timescales is further validated based on longer observations (1951–2017) and control simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The statistical relationships between TC activity and the Pacific Decadal Oscillation (PDO) or Atlantic Multidecadal Oscillation (AMO) are weaker and insignificant, both for the observations and for simulations. Our results imply that decadal variations of the tropical Pacific mean state should be taken into account when predicting WNP TC activities on decadal timescales.

Decadal shifts of coastal microphytoplankton communities in a semi-enclosed bay of NW Mediterranean Sea subjected to multiple stresses

Long-term evolution of microphytoplankton communities remains poorly studied in anthropized coastal zones submitted to multiple stressors. Here, we investigate decadal (2005–2017) microphytoplankton community changes, focusing on abundance and biovolume of major taxa related to both local abiotic conditions (rainfall rate, temperature and salinity) and regional convection events (wintering deep mixing) in the highly urbanized and semi-enclosed Toulon Bay (NW Mediterranean Sea). Results showed that persistent variations in local rainfall regime were followed by major changes in microphytoplankton community composition. Wet period (P2) (increase of wintering precipitations observed between late 2008 and early 2015) was associated to an increase of large heterotrophic dinoflagellates and disappearance of dominant diatom taxa, while dry periods (2005–2008 (P1) and 2015–2017 (P3)) promoted diatoms, microflagellates and small mixotrophic/heterotrophic dinoflagellates including potentially toxic species. Concomitance between intense deep mixing events, reported in open Ligurian Basin (particularly during winters 2005 and 2006) and higher values in the total microphytoplankton abundance and in spring diatom abundance regardless of rainfall conditions, presents this meso-scale process as the main fertilization mechanism in Toulon Bay. Although no change was detected in the chlorophyll a concentration during the 2006–2017 period, its trend was negatively correlated to the total microphytoplankton abundance. This negative relation as well as a change of size in dinoflagellates suggested a shift in the primary producer nature, from large autotrophic cells (diatoms and microflagellates) to smaller ones, driven by a runoff intensification. Finally, different communities composition were observed during both dry periods (i.e. diatoms-dominated and autotrophic microflagellate-dominated communities during P1 and P3, respectively), suggesting another environmental driver of change for phytoplankton communities of this coastal ecosystem.

The Zonal Oscillation and the Driving Mechanisms of the Extreme Western North Pacific Subtropical High and Its Impacts on East Asian Summer Precipitation

AbstractThis paper scrutinizes the zonal oscillation of the western North Pacific subtropical high (WNPSH) via diagnosing its two extreme phases, which are defined by the top 10% strongest (positive phase) and the weakest (negative phase) WNPSH index (WNPSHI) days during summers in 1979–2016. Key findings include the following: a tripole pattern consisting of intensified (weakened) precipitation over the Maritime Continent and the East Asian summer monsoon regions, and suppressed (strengthened) precipitation over the western North Pacific summer monsoon region during positive (negative) WNPSH phases; a westward movement of WNPSH-induced precipitation anomalies that subsequently affects eastern China, Japan, and the Korean Peninsula at different time lags; an OLR–vorticity pattern explained by atmospheric responses to thermal sources is suggested to drive the oscillation; and the competitive interaction of local air–sea feedbacks, especially during the positive phase. In addition, moderate-to-strong positive correlations between the WNPSHI and the Niño-3.4 index are found on 1–2-, 2–3-, and 3–6-yr time scales; both exhibit decadal shifts to a higher-frequency mode, suggesting the intensification of both the zonal WNPSH oscillation and the ENSO under the changing climate and their close interdecadal association. A nonlinear quasi-biennial WNPSH–ENSO relationship is identified: the positive (negative) WNPSH phase sometimes occurs during 1) a decaying El Niño (La Niña) in the preceding summer/autumn, and/or 2) a developing La Niña (El Niño) in the current summer/autumn. A full ENSO transition from moderate-to-strong El Niño to La Niña is often seen during the positive phase, offering potential in predicting ENSO events and extreme WNPSH phases and thereby the summer monsoon rainfall in East Asia.

The decadal shift of the long persistent rainfall over the northern part of China and the associated ocean conditions

The long persistent rainfall (LPR) over the northern part of China has a great impact on its society and economics. This study investigates the decadal shift of the regional‐mean LPR frequency over the northern part of China (LPR frequency) from 1971 to 2014 and the associated ocean conditions. The LPR frequency has experienced a significant decadal shift around 1996, with an enhanced (reduced) anomaly before (after) 1996, which is closely associated with a meridional tri‐pole sea surface temperature (SST) anomaly over the central Pacific, a La Niña‐like anomaly for the pre‐1996 epoch (1971–1996) and an obvious warming over the south Indian Ocean and the western part of south Pacific Ocean in the post‐1996 epoch (1997–2014). In the pre‐1996 epoch, associated with the typical SSTs, the ascent motion with increasing humidity anomalies are over the northern part of China. Meanwhile, high (low) pressure anomalies are over western Asia and East Asia (Europe and central Asia), which resembles as the positive phase of the Silk Road pattern (SRP). It would be associated with the polewards shift of the East Asian jet stream (EAJS) and therefore result in the northwards shift of the rain belt and benefit the occurrence of the LPR. In the post‐1996 epoch, associated with the warming over the specific south oceans, a descend anomaly dominates over the northern part of China, as well as the resembled negative phase of the SRP over mid‐high latitudes and equatorwards shift of the EAJS. These would suppress the rainfall over the northern part of China and reduce the LPR frequency over there after 1996.

Decadal Transition of the Leading Mode of Interannual Moisture Circulation over East Asia–Western North Pacific: Bonding to Different Evolution of ENSO

AbstractThe El Niño–Southern Oscillation (ENSO) cycle has a great impact on the summer moisture circulation over East Asia (EA) and the western North Pacific [WNP (EA-WNP)] on an interannual time scale, and its modulation is mainly embedded in the leading mode. In contrast to the stable influence of the mature phase of ENSO, the impact of synchronous eastern Pacific sea surface temperature anomalies (SSTAs) on summer moisture circulation is negligible during the 1970s–80s, while it intensifies after 1991. In response, the interannual variation of moisture circulation exhibits a much more widespread anticyclonic/cyclonic pattern over the subtropical WNP and a weaker counterpart to the north after 1991. Abnormal moisture moves farther northward with the enhanced moisture convergence, and thus precipitation shifts from the Yangtze River to the Huai River valley. The decadal shift in the modulation of ENSO on moisture circulation arises from a more rapid evolution of the bonding ENSO cycle and its stronger coupling with circulation over the Indian Ocean after 1991. The rapid development of cooling SSTAs over the central-eastern Pacific, and warming SSTAs to the west over the eastern Indian Ocean–Maritime Continent (EIO-MC) in summer, stimulates abnormal descending motion over the western-central Pacific and ascending motion over the EIO-MC. The former excites an anticyclone over the WNP as a Rossby wave response, sustaining and intensifying the WNP anticyclone; the latter helps anchor the anticyclone over the tropical–subtropical WNP via an abnormal southwest–northeast vertical circulation between EIO-MC and WNP.

Recent decadal variability of daily observed temperatures in Hindukush, Karakoram and Himalaya region in northern Pakistan

Spatiotemporal variability in the observed daily temperatures for a recent 30-year period (1986–2015) is examined from a total of 18 different weather stations in the Hindukush, Karakoram and Himalaya region in northern Pakistan (HKNP) by employing probability distribution functions (PDFs) on annual and seasonal basis. The region is a hub of glaciers and perennial snow cover which fulfills a large fraction of Pakistan’s water demand for irrigation, power generation and for drinking purposes. The temperature-based PDFs show a significant mean decadal warming of 0.45 °C, 0.03 °C, and 0.25 °C, in the maximum (Tmax), minimum (Tmin) and mean temperature (Tmean) of the region, on annual basis, respectively. However, the observed river discharges-based PDFs of the region show a mean negative decadal shift of − 40.15 m3/s on annual basis. The negative decadal shift in river discharge in warm climate is discussed in terms of percentile-based analysis which quantifies temperature changes for each percentile. The results revealed that the decadal changes in Tmin percentiles are more correlated with river discharge than decadal changes in Tmax and Tmean percentiles, on annual basis. The seasonal analysis showed a significant positive decadal shift of 1.93 °C for Tmax in spring season, whereas winter season showed a significant negative decadal shift of − 0.56 °C in Tmin of the HKNP region, from first decade (1986–1995) to third decade (2006–2015), respectively. The rest of seasons (i.e., summer and autumn) displayed high variability in the Tmax, Tmin and Tmean in the HKNP region. A high observed (non-parametric) correlation between the observed cloud cover and temperatures of the region indicates that changes in regional cloud cover may influence the regional temperatures. This work highlights the importance of recent temperature variations in the HKNP and its connection with the downstream river discharge of the region in changing climate of northern Pakistan.

Decadal Shifts in Wind Patterns Reduced Continental Outflow and Suppressed Ozone Trend in the 2010s in the Lower Troposphere Over Japan

AbstractThe large increase in the springtime free tropospheric ozone over the western and eastern North Pacific over the last decades has been linked to the increase in anthropogenic emissions in China. However, the increasing trend at Mt. Happo, Japan, has been unexpectedly suppressed since 2008, by 5–10 ppbv. In this paper, we analyzed the tropospheric ozone records at Mt. Happo, along with the changes in climate and anthropogenic emissions. We revealed, based only on observational data, that the changes in climate pattern play an important role in controlling the decadal ozone trend. We found that the persistent La Niña‐like wind pattern during 2008–2013 reduced the export of polluted air masses from East Asia to the western Pacific, even though Chinese emissions continued to increase. On the other hand, an El Niño‐like wind pattern during 1992–1996 and enhanced storm track activity during 2000–2006 enhanced the export of polluted air masses from East Asia, contributing to the accelerated ozone trends. The climate modulations were identified in the O3 trends at the surface sites in western and central Japan and at a remote marine boundary layer site, Minamitorishima. Without this climatic effect driven by the persistent La Niña‐like wind pattern, the ozone trend would have been further upward over the North Pacific in the late 2000s to the early 2010s.

Malaria incidences in South Africa linked to a climate mode in southwestern Indian Ocean

Millions of individuals are at risk of malaria infection in sub-Saharan Africa. Compared to other highly affected countries on the continent, South Africa has an excellent record of malaria control. Nevertheless, the northeastern districts of the country, neighboring some of the worst malaria affected regions in southern Africa, still experience seasonal malaria outbreaks particularly during the summer months of September-February. The year to year variations of the malaria outbreaks in southern Africa, as in many other parts of the world, are often linked to interannual variations in rainfall and temperature. These meteorological factors in turn are seen to be associated with large-scale climate phenomena such as El Niño/La Niña. Here, we present evidence of a new mode of climate variation in the Indian Ocean that could explain the interannual variation of malaria incidences in South Africa. This phenomenon appeared as a dipolar association in the sea surface temperature (SST) anomalies of southwestern Indian Ocean. Both poles of the dipole strongly correlated with the malaria incidence index of Vhembe district, one of South Africa's highest malaria-affected districts. The identified correlations were stronger than those found with other climate phenomena such as El Niño/La Niña and Indian Ocean Dipole. A decadal shift in the SST dipole pattern was also observed, and the associated decrease in seasonal rainfall could partly explain the recent reduction in malaria cases.

Decadal Shift of NAO-Linked Interannual Sea Level Variability along the U.S. Northeast Coast

Recent studies have linked interannual sea level variability and extreme events along the U.S. northeast coast (NEC) to the North Atlantic Oscillation (NAO), a natural internal climate mode that prevails in the North Atlantic Ocean. The correlation between the NAO index and coastal sea level north of Cape Hatteras was weak from the 1960s to the mid-1980s, but it has markedly increased since around 1987. The causes for the decadal shift remain unknown. Yet understanding the abrupt change is vital for decadal sea level prediction and is essential for risk management. Here we use a robust method, the Bayesian dynamic linear model (DLM), to explore the nonstationary NAO impact on NEC sea level. The results show that a spatial pattern change of NAO-related winds near the NEC is a major cause of the NAO–sea level relationship shift. A new index using regional sea level pressure is developed that is a significantly better predictor of NEC sea level than is the NAO and is strongly linked to the intensity of westerly winds near the NEC. These results point to the vital importance of monitoring regional changes of wind and sea level pressure patterns, rather than the NAO index alone, to achieve more accurate predictions of sea level change along the NEC.

Precipitation Intensity Changes in the Tropics from Observations and Models

Tropical (30°N–30°S) interdecadal precipitation changes and trends are explored for the satellite era using GPCP monthly analyses and CMIP5 outputs and focusing on precipitation intensity distributions represented by percentiles (Pct) and other parameters. Positive trends occur for the upper percentiles (Pct ≥ 70th), and become statistically significant for Pct ≥ 80th. Negative trends appear for the middle one-half percentiles (~20th–65th) and are statistically significant for the 20th–40th percentiles. As part of these trends there is a decadal shift around 1998, indicating the presence of an interdecadal [Pacific decadal oscillation (PDO)] signal. For the lower percentiles (Pct ≤ 10th), positive trends occur, although weakly. The AMIP-type simulations generally show similar trend results for their respective time periods. Precipitation intensity changes are further examined using four precipitation categories based on the climatological percentiles: Wet (Pct ≥ 70th), Intermediate (70th > Pct ≥ 30th), Dry (30th > Pct ≥ 5th), and No Rain (5th > Pct ≥ 0th). Epoch differences of occurrence frequency between 1988–97 and 1998–2015 have spatial features generally reflecting the combined effect of the PDO and external forcings, specifically the anthropogenic greenhouse gas (GHG)-related warming based on comparisons with both AMIP and CMIP results. Furthermore, precipitation intensity over Wet zones shows much stronger changes than mean precipitation including a more prominent change around 1998 associated with the PDO phase shift. Trends also appear in the sizes of Intermediate and Dry zones, especially over ocean. However, changes in the sizes of Wet and No Rain zones are generally weak. AMIP simulations reproduce these changes relatively well. Comparisons with the CMIP5 historical experiments further confirm that the observed changes and trends are a combination of the effect of the PDO phase shift and the impact of anthropogenic GHG-related warming.

An extreme climate transition in the Caribbean's Virgin Islands. I. Evidence of teleconnection with the 1976/1977 Pacific climate shift

We present the Caribbean's Virgin Islands’ climate variability over the last 60 years from indices of extreme temperature and precipitation, as well as their quantitative support for a recent climatic shift (1983.2 ± 5.5 years). The region's climate indices (defined via time series averages) and trends (defined via linear least squares regression fitting) of extreme temperature and precipitation were cross‐examined from 1952, 1961, 1978, and 1983–2014 to diagnose: the area's climatic extremes; a climatic transition's role in assessing decadal climate anomaly rates; and to provide an analytically independent test of the transition's existence. Analyses relied on the use of varying physical constraints to include weighting from statistical and non‐statistical uncertainties in our climatic outcomes. We report that diurnal warming and enhanced precipitation accompanied each interval, however, non‐negligible deviations existed in comparisons between the same anomalies in any 2 intervals. Our Virgin Islands’ long (1952, 1961–2014) versus short (1978, 1983–2014) climate variation comparisons provide evidence for the manifestation of decadal (or longer) shifts, and thus, biases, in recent Caribbean reports. Our climatic transition's overlap with the renowned 1976/1977 Pacific event insinuates a local Caribbean teleconnection, and contributes to the evidence growing globally for 1980s’ shifts in an abundance of feedback measures of Earth's energy budget.

Decadal Shift in West China Autumn Precipitation and its Association With Sea Surface Temperature

AbstractWest China autumn precipitation (WCAP) is the final stage of the rainy season in mainland China and is characterized as the secondary peak in annual cycle of precipitation in West China. This study reveals that WCAP experienced a significant interdecadal shift around the mid‐1980s, with greatly reduced precipitation after this shift. Features related to the decrease in WCAP include the weakening of warm, wet southerlies prevailing from the oceans to inland China, the weakened Eurasian pattern, and the southward displacement of the East Asian jet stream (EAJS). Further analysis indicates that the interdecadal changes in WCAP may be attributed to the interdecadal increasing of sea surface temperature (SST) in the Indo‐Pacific warm pool (SSTIOP), North Pacific (SSTNP), and central equatorial Pacific (SSTCEP) after the 1980s. The warmer SSTIOP contributes to a weaker meridional land‐sea thermal contrast, which inducts an anomalous local meridional circulation and northerly. The warmer SSTNP stimulates a Rossby wave train that leads to weakened West Pacific subtropical high and accompanying cyclonic circulation anomaly, hindering the transport of water vapor inland from the oceans. The increased SSTCEP leads to the southward displacement of the EAJS and its secondary circulation, generating an anomalous descending branch and reduced WCAP. Numerical simulations further support the conclusions derived from the diagnostic analysis that the decadal warming of the SSTIOP, SSTNP, and SSTCEP synergistically contributes to the reduction of WCAP after the 1980s.

Detectability of Decadal Anthropogenic Hydroclimate Changes over North America

Regional hydroclimate changes on decadal time scales contain substantial natural variability. This presents a challenge for the detection of anthropogenically forced hydroclimate changes on these spatiotemporal scales because the signal of anthropogenic changes is modest, compared to the noise of natural variability. However, previous studies have shown that this signal-to-noise ratio can be greatly improved in a large model ensemble where each member contains the same signal but different noise. Here, using multiple state-of-the-art large ensembles from two climate models, the authors quantitatively assess the detectability of anthropogenically caused decadal shifts in precipitation-minus-evaporation (PmE) mean state against natural variability, focusing on North America during 2000–50. Anthropogenic forcing is projected to cause detectable (signal larger than noise) shifts in PmE mean state relative to the 1950–99 climatology over 50%–70% of North America by 2050. The earliest detectable signals include, during November–April, a moistening over northeastern North America and a drying over southwestern North America and, during May–October, a drying over central North America. Different processes are responsible for these signals. Changes in submonthly transient eddy moisture fluxes account for the northeastern moistening and central drying, while monthly atmospheric circulation changes explain the southwestern drying. These model findings suggest that despite the dominant role of natural internal variability on decadal time scales, anthropogenic shifts in PmE mean state can be detected over most of North America before the middle of the current century.

A potential predictor of multi-season droughts in Southwest China: soil moisture and its memory

During the last decade, several high intensity and long duration droughts happened in Southwest China (SWC) and resulted in tremendous socioeconomic losses. Meanwhile, it is well known that soil moisture (SM) plays a key role in land–atmosphere interaction and weather/climate prediction and is a direct drought index. Thus, a general analysis of SM is beneficial to drought research and prediction over this region. Based on the SM data of Global Land Data Assimilation System V2.0, we examined the temporal variations in SM in SWC during 1961–2012. Results show that significant soil drying trend happened in autumn accompanied by an evident abrupt change in 1991. Moreover, SM exhibits a strong and season-dependent persistence. Particularly, the autumn SM anomaly shows the strongest memory that can be sustained to the next spring. Along with the decadal shift of SM, the memory time of autumn SM can extend from 3 months before 1991 to 6 months in recent years. We further used the Standardized Precipitation Evapotranspiration Index (SPEI) at multiple time scales to identify the droughts in different seasons over SWC, and the inter-annual change patterns of autumn SM and SPEIs are generally in agreement with each other, which confirms that SM is suitable for indicating the droughts. Our results suggest that the autumn SM can be a potential predictor of persistent droughts over SWC, especially for those multi-season persistent drought events started in autumn.

Decadal shifts in macroalgae assemblages in impacted urban lagoons in Brazil

Unplanned coastal urbanization affects marine environments through losses of biodiversity and potential reductions in trophic interactions. Neotropical coastal lagoons are anthropogenically dominated ecosystems and some of the world's most threatened environments. The unplanned urbanization and long-term shifts in macroalgae assemblages were examined in Brazilian urban lagoons, evaluating their salinity and structural-functional metrics. Over 25 years, 46% of the species were lost from the Tijuca Lagoon (TL) and 26% from the Araruama Lagoon (AL), the latter a hypersaline lagoon; within only seven years, 14% of the algae taxa disappeared from the Rodrigo de Freitas Lagoon (RFL). Overall, the biomass in TL was halved, and RFL showed a reduction of 72%. These lagoons are currently dominated by opportunistic species, being classified as Ecological Status Groups (ESG) IIA and IIB. The saline lagoons (AL, TL) are now dominated by green-genera (Ulva, Cladophora and Rhizoclonium spp.), while the low-salinity lagoon (RFL) is red-algae dominated (mainly Polysiphonia subtilissima). Multivariate analyses confirmed decreases in total biomass at both TL and RFL, and identified changes in their community physiognomies over time. The Ecological Evaluation Index demonstrated that the macroalgae assemblages from these lagoons were moderately to impacted, largely due to increasing and poorly planned urban development.

Spatio-temporal variations of Arctic amplification and their linkage with the Arctic oscillation

The Arctic near-surface air temperatures are increasing more than twice as fast as the global average–a feature known as Arctic amplification (AA). A modified AA index is constructed in this paper to emphasize the contrast of warming rate between polar and mid-latitude regions, as well as the spatial and temporal characteristics of AA and their influence on atmospheric circulation over the Northern Hemisphere. Results show that AA has a pronounced annual cycle. The positive or negative phase activities are the strongest in autumn and winter, the weakest in summer. After experiencing a remarkable decadal shift from negative to positive phase in the early global warming hiatus period, the AA has entered into a state of being enlarged continuously, and the decadal regime shift of AA in about 2002 is affected mainly by decadal shift in autumn. In terms of spatial distribution, AA has maximum warming near the surface in almost all seasons except in summer. Poleward of 20°N, AA in autumn has a significant influence on the atmospheric circulation in the following winter. The reason may be that the autumn AA increases the amplitude of planetary waves, slows the wave speeds and weakens upper-level zonal winds through the thermal wind relation, thus influencing surface air temperature in the following winter. The AA correlates to negative phase of the Arctic oscillation (AO) and leads AO by 0–3 months within the period 1979–2002. However, weaker relationship between them is indistinctive after the decadal shift of AA.

Diet and trophic ecology of the tiger shark (Galeocerdo cuvier) from South African waters.

Knowledge of the diet and trophic ecology of apex predators is key for the implementation of effective ecosystem as well as species-based management initiatives. Using a combination of stomach content data and stable isotope analysis (δ15N and δ13C) the current study provides information on size-based and sex-specific variations in diet, trophic position (TP) and foraging habitat of tiger sharks (Galeocerdo cuvier) caught in the KwaZulu-Natal Sharks Board bather protection program. This study presents the longest time-series and most detailed analysis of stomach content data for G. cuvier worldwide. Prey identified from 628 non-empty stomachs revealed a size-based shift in diet. Reptiles, birds, mysticetes, and large shark species increased in dietary importance with G. cuvier size, concomitant with a decrease in smaller prey such as batoids and teleosts. Seasonal and decadal shifts in diet driven primarily by changes in the importance of elasmobranchs and mammal (cetacean) prey were recorded for medium sized (150–220 cm) G. cuvier. Both stomach content and stable isotope analysis indicated that G. cuvier is a generalist feeder at the population level. Size-based δ13C profiles indicated a movement to offshore foraging habitats by larger G. cuvier. Calculated TP varied by method ranging from 4.0 to 5.0 (TPSCA for stomach contents) and from 3.6 to 4.5 (TPscaled and TPadditive for δ15N). Large (> 220 cm) G. cuvier did not feed at discrete trophic levels, but rather throughout the food web. These data provide key information on the ecological role of G. cuvier to improve the accuracy of regional food web modelling. This will enable a better understanding of the ecological impacts related to changes in the abundance of this predator.

Modulation by the Atlantic Multidecadal Oscillation of the intensity of the interannual seesaw between the Somali and Australian cross-equatorial flows

AbstractThe decadal variation of the intensity of the interannual opposite connection (seesaw) between the Somali and Australian cross-equatorial flows (CEFs) is investigated. During the period prior to the mid-1960s, and after the early-2000s, the intensity of the interannual seesaw connection is significantly weaker relative to the period between. Such interdecadal shifts in the interannual seesaw intensity bear a resemblance to the decadal shift in ENSO’s strength, and can be further attributed to the phase transition of the Atlantic Multidecadal Oscillation (AMO). When the AMO is in a positive phase, the ENSO amplitude reduces and the seesaw strength becomes weakened, and vice versa. The historical simulation outputs of the CMIP5 models are used to verify the connection, and a similar result was obtained. Thus, the notion that the intensity of the interannual opposite connection (seesaw) between the CEFs is modulated by the AMO is robust.