Cold Anomalies Research Articles

Submesoscale eddies in Peter the Great Bay of the Japan/East Sea in winter

Cold-core (T 27.24 kg/m3 were observed in the near-bottom layer at the shelf break in Peter the Great Bay (PGB) of the Japan/East Sea in March 2010. The anomalously cold water was 0.4 ml/l richer in oxygen than was the surrounding water, and it contained more suspended particles. The observations were carried out by using a moored automatic mobile Aqualog profiler. Profiling occurred as frequent as every hour, allowing us to obtain data with high temporal resolution. The Aqualog profiler delivered vertical profiles of the ocean current velocity, acoustic backscatter at 2 MHz, temperature, and salinity between the depths of 20 and 105 m. Other oceanographic instruments were mounted at fixed depths on the mooring line to measure current velocity, temperature, conductivity, dissolved oxygen, chlorophyll-a fluorescence, and turbidity. Complementary data included ship-borne CTD casts and satellite-borne imagery and scatterometry as well as coastal weather station records. The Regional Ocean Modeling System (ROMS) was employed to study the origin and evolution of the cold water. The model simulated the ocean dynamics at a 600-m horizontal resolution in PGB from 2009 to 2010. The model was forced by the surface momentum, heat and fresh water fluxes of the NCEP-DOE Reanalysis 2. The lateral boundary condition of the model was obtained from nesting into a Japan Coastal Ocean Predictability Experiment model data set. According to the ROMS simulation, the circulation in Ussuri Bay in the northeastern bay of PGB was anticyclonic in February–March 2010. The submesoscale cyclonic vortices generated around the anticyclonic gyre. The submesoscale cyclones tended to move southward out of the bay, and they transported the anomalously cold water towards the outer shelf. As a result, the cold water anomalies were often observed to persist for as long as 2 days near 42.5°N, 132°E. Lagrangian analysis confirmed that this cold water observed by the Aqualog profiler originated in Ussuri Bay. The model simulation showed that the submesoscale cyclonic eddies played a specific role in supplying the densest water from the northern part of PGB to the outer shelf, where the dense water was then entrained by the mesoscale eddies in the Primorye Current zone and could cascade down the continental slope into the deep northern basin of the sea. This transport of the densest water by the submesoscale eddies was estimated to be 5–10·10−3 Sv in February–March 2010.

Defining the Northeast Monsoon of India

AbstractThis study introduces an objective definition for onset and demise of the northeast Indian monsoon (NEM). The definition is based on the land surface temperature analysis over the Indian subcontinent. It is diagnosed from the inflection points in the daily anomaly cumulative curve of the area-averaged surface temperature over the provinces of Andhra Pradesh, Rayalseema, and Tamil Nadu located in the southeastern part of India. Per this definition, the climatological onset and demise dates of the NEM season are 6 November and 13 March, respectively. The composite evolution of the seasonal cycle of 850-hPa winds, surface wind stress, surface ocean currents, and upper-ocean heat content suggest a seasonal shift around the time of the diagnosed onset and demise dates of the NEM season. The interannual variations indicate onset date variations have a larger impact than demise date variations on the seasonal length, seasonal anomalies of rainfall, and surface temperature of the NEM. Furthermore, it is shown that warm El Niño–Southern Oscillation (ENSO) episodes are associated with excess seasonal rainfall, warm seasonal land surface temperature anomalies, and reduced lengths of the NEM season. Likewise, cold ENSO episodes are likely to be related to seasonal deficit rainfall anomalies, cold land surface temperature anomalies, and increased lengths of the NEM season.

North Pacific temperature and precipitation response to El Niño-like equatorial heating: sensitivity to forcing location

This study investigates the sensitivity of oceanic and atmospheric response in the extra-tropics, especially in the North Pacific, to the position of equatorial El Nino-like heating within a slab-ocean climate model. In a suite of numerical experiments, we impose an idealized equatorial sea surface temperature (SST) anomaly in the Pacific and systematically vary its longitudinal position along the equator to mimic different “flavors” of El Nino. We find that regardless of the forcing location, the induced SST pattern closely resembles a positive phase of the Pacific Decadal Oscillation with a characteristic warming along the North American coast as part of an arc-shaped pattern, accompanied by wind anomalies around the Aleutian low. However, the extent and magnitude of the coastal warming vary nonmonotonically when the forcing shifts westward along the equator. The strongest response is found when the equatorial forcing is located in the central Pacific close to the Dateline. In contrast, precipitation response over Southern California is strongest for an eastern Pacific warming centered at 150°W, even though its magnitude is highly uncertain since the boundary between dry and wet precipitation anomalies passes through this region. We repeat the experiments for cold (i.e. La Nina-like) anomalies and observe a significant asymmetry in the SST and atmospheric response between the warm and cold cases. Finally, our experiments suggest that tropical heating (or cooling) over the Western Pacific warm pool generates the largest tropical rainfall response and hence the largest global-mean SST anomaly.

Summer Arctic Cold Anomaly Dynamically Linked to East Asian Heat Waves

During recent years, the rapidly warming Arctic and its impact on winter weather and climate variability in the mid- and low latitudes have been the focus of many research efforts. In contrast, anomalous cool Arctic summers and their impacts on the large-scale circulation have received little attention. In this study, we use atmospheric reanalysis data to reveal a dominant pattern of summer 1000–500-hPa thickness variability north of 30°N and its association with East Asian heat waves. It is found that the second thickness pattern exhibits strong interannual variability but does not exhibit any trend. Spatially, the positive phase of the second thickness pattern corresponds with significant Arctic cold anomalies in the mid- and low troposphere, which are surrounded by warm anomalies outside the Arctic. This pattern is the thermodynamic expression of the leading pattern of upper-tropospheric westerly variability and significantly correlated with the frequency of East Asian heat waves. The Arctic has experienced frequent summer cold anomalies since 2005, accompanied by strengthened tropospheric westerly winds over most of the Arctic and weakened westerlies over the mid- and low latitudes of Asia. The former significantly enhances baroclinicity over the Arctic, which dynamically contributes to increased frequency of anomalous low surface pressure during summer along with decreased frequency over high latitudes of Eurasia and North America. The latter is exhibited by sustained high pressure anomalies in the mid- and low troposphere that dynamically facilitate the occurrence of East Asian heat waves. A systematic northward shift of Asian zonal winds dynamically links Arctic cold anomalies with East Asian heat waves and produces a seesaw structure in zonal wind anomalies over the Arctic and the Tibetan Plateau (the third pole). Evidence suggests that enhanced Arctic westerlies may provide a precursor to improve predictions of the East Asian winter monsoon, though the mechanism for this lag association is unclear.

On the Role of the Eastern Pacific Teleconnection in ENSO Impacts on Wintertime Weather over East Asia and North America

Previous studies have mainly focused on the influence of El Niño–Southern Oscillation (ENSO) on seasonal-mean conditions over East Asia and North America. This study, instead, proposes an ENSO pathway that influences the weather events over East Asia and North America, in which the eastern Pacific teleconnection pattern (EP) plays an important role. On the one hand, the EP pattern can induce significant surface temperature anomalies over East Asia during its development and mature stages, with the positive (negative) EPs causing colder (warmer) than normal weather events. Besides, the frequency of occurrence of EPs is significantly modulated by ENSO, with 50% of the positive EPs occurring in La Niña winters, and 47% of the negative EPs occurring in El Niño winters. As a result, in El Niño winters, more negative and fewer positive EPs tend to occur, and thus more warm and fewer cold weather events are expected in East Asia. For La Niña winters, the reverse is true. On the other hand, for the EP pattern without its canonical convection pattern (referred to as the nonconvective EP), extremely cold anomalies over the northern United States and western Canada are induced in its negative phase. Moreover, when there are positive sea surface temperature anomalies in the central equatorial Pacific, the frequency of occurrence of negative nonconvective EPs is 2.0 times greater than the climatological value, and thus an enhanced likelihood of extremely cold spells over North America may be expected.

Diminishing clear winter skies in Beijing towards a possible future

Severe haze during the winter season has been troubling the citizens of Beijing over the past few decades, and the trend seems to be continuing. However, occasionally such as the winter of 2017 (2017/12–2018/2), one would be amazed to see unusually few hazy days throughout the winter that brings memories of a long past. It is controversial to say whether such a nowadays-rare event is a result of policy-driven emission cuts or an opportunity brought about by natural climate variability. This paper investigates the probability of such anomalous atmospheric circulation events in winter from a climate perspective. Based on updated observations, only three winters during the past 38 years are found to be similar to that of 2017. These events were accompanied by a strong Siberian High to the north and cold anomalies in the mid-lower troposphere in association with a strong East Asian Trough, which favored the strengthening of northwesterly winds and effective ventilation. The occurrences of such favorable winter circulation anomalies are found to have decreased by about 50% from the 1st to the 2nd half of the 20th century. A further 60% [11.4%, 89.3%] reduction between 1951–2000 and 2050–2099 is projected by climate models under the IPCC’s Representative Concentration Pathway 8.5 scenario. Without serious cuts in pollution emissions, winters are projected to be dismal for the 20 million people of Beijing to a possible future under global warming.

ENSO's Shrinking Twentieth‐Century Footprint Revealed in a Half‐Millennium Coral Core From the South Pacific Convergence Zone

AbstractA 492‐year‐long, continuous δ18O time series from a massive Porites coral colony in Ta'u, American Samoa, records contrasting responses to different types of El Niño‐Southern Oscillation (ENSO) through a mixed sea surface temperature and salinity signal. Currently, conventional El Niño (La Niña) events generate cold and salty (warm and fresh) anomalies at Ta'u, while Modoki El Niño (La Niña) events warm (cool) the waters at Ta'u. Over the course of the twentieth century, the Ta'u δ18O record underwent a polarity shift in its response to conventional ENSO: A warm and fresh (cool and salty) response to El Niño (La Niña) was replaced by the opposite pattern. We interpret this as evidence for the movement of the Eastern Pacific ENSO null zone, the narrow band of the surface ocean where sea surface temperature variability is not on average correlated with ENSO. This movement appears to be related to overall shrinking of the ENSO footprint over the twentieth century. We infer no such trend in the Modoki footprint. The five‐century‐long Ta'u record shows dramatic, century‐scale changes in ENSO‐band variability. Comparisons with other ENSO reconstructions lead to conflicting interpretations: The Ta'u coral may have recorded changes in the strength of ENSO or in its spatial footprint. Changes in the spatial footprint manifest as a changing sensitivity to ENSO at any given location, presenting challenges to established methods of ENSO reconstruction.

Impact of the Atlantic Meridional Mode on Gulf Stream North Wall Position

The path of the Gulf Stream as it leaves the continental shelf near Cape Hatteras is marked by a sharp gradient in ocean temperature known as the North Wall. Previous work in the literature has considered processes related to the North Atlantic Oscillation (NAO) in triggering latitudinal displacements of the North Wall position. This paper presents evidence that the Atlantic meridional mode (AMM) also impacts interannual variations of the North Wall position. The AMM signal from the tropics propagates to the Gulf Stream near the 200-m depth, and there are two time scales for this interaction. Anomalous Ekman suction induced by AMM cools the tropical Atlantic. The cold water in the Caribbean Sea is entrained into the currents feeding the Gulf Stream, and this cooling signal reaches the North Wall within a year. A second mechanism involves cold anomalies in the western tropical Atlantic, which initially propagate westward as baroclinic planetary waves, reaching the Gulf Stream and resulting in a southward shift in the North Wall position after a delay of about one year. In an analysis for the period 1961–2015, AMM’s signal dominates North Wall fluctuations in the upper 300 m, while NAO is the major influence below ~500 m; the influence of both the teleconnections is seen between 300 and 500 m. The relationship between the Atlantic meridional overturning circulation (AMOC) and the North Wall is investigated for the 2005–15 period and found to be statistically significant only at the sea surface in one of the three North Wall indices used.

The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part II: Vortex Tilt

AbstractWe investigate a class of tropical cyclones (TCs) that undergo rapid intensification (RI) in moderate vertical wind shear through analysis of a series of idealized model simulations. Two key findings derived from observational analysis are that the average 200–850-hPa shear value is 7.5 m s−1 and that the TCs displayed coherent cloud structures, deemed tilt-modulated convective asymmetries (TCA), which feature pulses of deep convection with periods of between 4 and 8 h. Additionally, all of the TCs are embedded in an environment that is characterized by shear associated with anticyclones, a factor that limits depth of the strongest environmental winds in the vertical. The idealized TC develops in the presence of relatively shallow environmental wind shear of an anticyclone. An analysis of the TC tilt in the vertical demonstrates that the source of the observed 4–8-h periodicity of the TCAs can be explained by smaller-scale nutations of the tilt on the longer, slower upshear precession. When the environmental wind shear occurs over a deeper layer similar to that of a trough, the TC does not develop. The TCAs are characterized as collections of updrafts that are buoyant throughout the depth of the TC since they rise into a cold anomaly caused by the tilting vortex. At 90 h into the simulation, RI occurs, and the tilt nutations (and hence the TCAs) cease to occur.

A Statistical Analysis of Tropical Upper-Tropospheric Trough Cells over the Western North Pacific during 2006–15

AbstractA statistical analysis of tropical upper-tropospheric trough (TUTT) cells over the western North Pacific Ocean (WNP) during 2006 to 2015 is performed using the NCEP Final reanalysis. A total of 369 TUTT-cell events or 6836 TUTT cells are identified, with a peak frequency in July. Most TUTT cells form to the east of 150°E and then move southwestward with a mean speed of 6.6 m s−1 and a mean life span of 4.4 days. About 75% of the TUTT cells have radii of <500 km with 200-hPa central heights of <1239.4 dam. In general, TUTT cells exhibit negative height anomalies above 450 hPa, with their peak amplitudes at 200 hPa, pronounced cold anomalies in the 650–200-hPa layer, and significant cyclonic vorticity in the 550–125-hPa layer. A comparison of the composite TUTT cells among the eastern, central, and western WNP areas shows the generation of an intense cold-cored vortex as a result of the southward penetration of a midlatitude trough into a climatological TUTT over the eastern WNP region. The TUTT cell with pronounced rotation is cut off from the midlatitude westerlies after moving to the central WNP region, where it enters its mature phase, under the influence of northeasterly flow. The TUTT cell weakens in rotation and shrinks in size, diminishing within the TUTT after arriving at the western WNP region. Results suggest that, although most TUTT cells may diminish before reaching the western WNP, their vertical influences may extend to the surface layer and last longer than their signals at 200 hPa.

Atmospheric energetics over the tropical Indian Ocean during Indian Ocean dipole events

The evolution of atmospheric perturbation potential energy (PPE) over the tropical Indian Ocean is analyzed during a composite positive IOD event using reanalysis datasets for the period 1948–2015. The IOD modulates the variation in PPE, which then affects perturbation kinetic energy (PKE) through energy conversion. The PPE anomalies in the lower troposphere (1000–850 hPa) as the dominant layer of the PPE in the whole troposphere (1000–150 hPa) present a dipole pattern corresponding to the anomalous variation in SST during the IOD event. When cold SST anomalies (SSTAs) first appear in the eastern Indian Ocean (IOD-E), they reduce the atmospheric PPE in the lower troposphere rapidly. The negative PPE anomalies lead to less energy conversion to PKE, restraining the surface wind convergence over the IOD-E and weakening the climatological Walker circulation. Meanwhile, the surface easterly wind anomalies strengthen, which depresses the thermocline to the west and gives warmer SSTAs in the western Indian Ocean (IOD-W). The PPE anomalies and energy conversion (CK) over the IOD-W are opposite to those over the IOD-E, the anomalous easterly wind continues to develop, and the positive SSTAs in the IOD-W reach a peak. Thus, the response of the Walker circulation over the Indian Ocean provides a positive feedback during the IOD event and explains the delayed effect of IOD-E SSTAs on IOD-W SSTAs.

Precursory signals of East Asian winter cold anomalies in stratospheric planetary wave pattern

The impacts of the stratospheric polar vortex changes on the tropospheric climate variability have been emphasized in previous studies. The present study presents evidence for precursory signals of cold anomalies over East Asia in the stratospheric wave pattern during November through March. The leading two empirical orthogonal function (EOF) modes of daily 50-hPa geopotential height anomalies along 65°N correspond to changes in the amplitude and phase of the stratospheric planetary wavenumber 1, respectively. Composite analysis is performed to unravel the temporal evolution of anomalies during 8 phases of 34 events identified based on the second EOF mode with a complete life cycle for the period 1979–2016. The surface cold anomalies over eastern China emerge in phase 3 (corresponding to negative and positive 50-hPa height anomalies on the North Atlantic and Russian Far East sector, respectively) and peak in phase 5 (corresponding to positive 50-hPa height anomalies over Eurasia) when the Siberian high extends southeastward and the East Asian trough deepens with anomalous surface northerly winds. Downward propagation of stratospheric signal is detected over western Europe and western Siberia. The former is a source region of the tropospheric Rossby wave trains propagating along the polar front jet and subtropical jet. The downward propagation of stratospheric signal is related to the reflection of stratospheric planetary wavenumber 1 pattern and contributes to the development of the tropospheric Rossby wave train that, in turn, leads to the strengthening of the Siberian high and the deepening of the East Asian trough.

Winter matters: Sensitivity to winter climate and cold events increases towards the cold distribution margin of European beech (Fagus sylvatica L.)

AbstractAimThe dominant forest tree in Europe, European beech (Fagus sylvatica L.), covers large areas of continental Europe and thus experiences diverse climatic conditions. In the face of predicted climate change and shifts of distribution ranges, it is important to understand the diverse climate–growth relationships towards distribution margins. Beech is generally reported to be sensitive to summer drought towards dry and continental regions; yet, few studies have investigated climate sensitivity towards the cold distribution margin of beech. We hypothesized that at colder sites (a) growth of beech is more sensitive to winter cold, (b) growth is less influenced by summer drought, and (c) stand‐wide growth reductions (negative pointer years) are related to extreme winter cold events.TaxonEuropean beech (Fagus sylvatica L.).LocationA large gradient of decreasing winter temperature (ΔT >4 K along 500 km) from Rostock (Germany) to Gdańsk (Poland).MethodsWe analysed climate–growth relationships and the nature of growth reductions of 11 beech stands from more central to cold marginal beech populations.ResultsTowards the cold marginal populations, growth became increasingly sensitive to winter cold (February temperature) and less sensitive to summer water availability (June precipitation). Likewise, negative pointer years coincided with winter cold anomalies at the colder sites and with summer drought anomalies at the warmer sites. Thus, over the studied gradient, the general sensitivity of beech to summer drought transitions into sensitivity to winter cold.Main conclusionsA range shift of beech across the current cold distribution margin is often assumed to compensate for habitat and productivity losses of drought‐prone southern and central populations. With respect to the winter cold sensitivity found in our study, such assumptions should be taken with caution. Since winter cold events are predicted to persist with similar frequency and magnitude even during predicted climate warming, beech populations in the newly colonized habitat might be significantly sensitive to winter cold.

Atlantic Ocean Heat Transport Influences Interannual-to-Decadal Surface Temperature Predictability in the North Atlantic Region

An analysis of a three-member ensemble of initialized coupled simulations with the MPI-ESM-LR covering the period 1901–2010 shows that Atlantic northward ocean heat transport (OHT) at 50°N influences surface temperature variability in the North Atlantic region for several years. Three to ten years after strong OHT phases at 50°N, a characteristic pattern of sea surface temperature (SST) anomalies emerges: warm anomalies are found in the North Atlantic and cold anomalies emerge in the Gulf Stream region. This pattern originates from persistent upper-ocean heat content anomalies that originate from southward-propagating OHT anomalies in the North Atlantic. Interannual-to-decadal SST predictability of yearly initialized hindcasts is linked to this SST pattern: when ocean heat transport at 50°N is strong at the initialization of a hindcast, SST anomaly correlation coefficients in the northeast Atlantic at lead years 2–9 are significantly higher than when the ocean heat transport at 50°N is weak at initialization. Surface heat fluxes that mask the predictable low-frequency oceanic variability that influences SSTs in the northwest Atlantic after strong OHT phases, and in the northwest and northeast Atlantic after weak OHT phases at 50°N lead to zonally asymmetrically predictable SSTs 7–9 years ahead. This study shows that the interannual-to-decadal predictability of North Atlantic SSTs depends strongly on the strength of subpolar ocean heat transport at the start of a prediction, indicating that physical mechanisms need to be taken into account for actual temperature predictions.

Comparison of Intraseasonal East Asian Winter Cold Temperature Anomalies in Positive and Negative Phases of the Arctic Oscillation

AbstractThis study investigates the relationship between intraseasonal variations of the Arctic Oscillation (AO) and East Asian winter temperature. Based on the lead‐lag correlation between 30‐ and 60‐day band‐pass filtered daily AO index and regional mean surface temperature anomalies over eastern China, it is found that the cold temperature anomalies occur following both positive and negative AO phases with nearly the same probability. Composite analysis reveals prominent differences in surface temperature and tropospheric circulation anomalies between the two types of years. During the years when negative AO leads cold anomalies over East Asia, the AO‐related pressure anomalies extend to Eurasia, inducing an enhanced Siberian high with a large spatial coverage in midlatitude and high latitude; the midtropospheric wave train along the polar front jet takes an eastward path and induces the deepening of the East Asian trough; the resultant cold surface temperature anomalies affect a large region over East Asia. During the years when positive AO leads cold anomalies, the AO‐related pressure anomalies extend to North America and the high pressure anomalies over East Asia are located at midlatitudes with a relatively small spatial coverage; the midtropospheric wave train along the polar front jet takes a southward path over Asia, inducing the development of a trough over subtropical Asia with a supplementary contribution from a subtropical wave train; the resultant cold surface temperature anomalies influence inland East Asia with a relatively small extent. There appears to be a notable difference in the relationship between the stratospheric and tropospheric change between the two types of years.

Impact of Arctic sea ice variations on winter temperature anomalies in northern hemispheric land areas

Coordinated numerical ensemble experiments with six different state-of-the-art atmosphere models have been used in order to evaluate the respective impact of the observed Arctic sea ice and sea surface temperature (SST) variations on air temperature variations in mid and high latitude land areas. Two sets of experiments have been designed; in the first set (EXP1), observed daily sea ice concentration and SST variations are used as lower boundary forcing over 1982–2014 while in the second set (EXP2) the SST variations are replaced by the daily SST climatology. The observed winter 2 m air temperature (T2m) variations are relatively well reproduced in a number of mid and high latitude land areas in EXP1, with best agreement in southwestern North America and northern Europe. Sea ice variations are important for the interannual T2m variations in northern Europe but have limited impact on all other mid and high latitude land regions. In particular, sea ice variations do not contribute to the observed opposite variations in the Arctic and mid latitude in our model experiments. The spread across ensemble members is large and many ensemble members are required to reproduce the observed T2m variations over northern Europe in our models. The amplitude of T2m anomalies in the coldest observed winters over northern Europe is not reproduced by our multi-model ensemble means. However, the sea ice conditions in these respective winters and mainly the thermodynamic response to the ice anomalies lead to an enhanced likelihood for occurrence of colder than normal winters and extremely cold winters. Still, the main reason for the observed extreme cold winters is internal atmospheric dynamics. The coldest simulated northern European winters in EXP1 and EXP2 between 1982 and 2014 show the same large scale T2m and atmospheric circulation anomaly patterns as the observed coldest winters, indicating that the models are well able to reproduce the processes, which cause these cold anomalies. The results are robust across all six models used in this study.

Interdecadal variations in persistent anomalous cold events over Asian mid-latitudes

Based on the Japanese 55-year reanalysis daily data, this study finds that the frequency of persistent cold events from Lake Balkhash to Lake Baikal exhibits a clear interdecadal variation. Circulation characteristics, such as interdecadal variations in regional persistent cold event frequencies, are the focus of this study. During the 57-year period, a subperiod from the mid-1980s to the late 1990s exhibits inactive characteristics compared with those before and after during the study period. On a subseasonal timescale, the typical circulation patterns of persistent cold events are characterized by wave-train anomalies across the Atlantic Ocean and Eurasian continent, with significantly positive height anomalies that persist over the Ural Mountains in the upper troposphere, positive SLP anomalies over Eurasian mid- to high latitudes and cold SAT anomalies from Lake Balkhash to Lake Baikal. The changes in background circulation at the interdecadal timescale correspond well with the typical circulation patterns at the subseasonal timescale, indicating their important role in modulating the frequency of persistent cold events. Interdecadal changes in the background circulation can be partially explained by transient eddy feedback forcing.

Drought Variability and Trends over the Central United States in the Instrumental Record

Abstract We examined drought variability and trends over the last century (1916–2013) over the conterminous United States (CONUS) using observed precipitation P, temperature T, and reconstructed total moisture percentiles (TMP) and runoff from four land surface models. We used an integrated drought index (IDI), which we defined as the equally weighted mean of the 3-month standardized runoff index (SRI3) and TMP from four land surface models mapped onto a uniform probability distribution. Using a definition of drought as IDI less than 0.3 for 6 months or longer, we identified 16 drought events, which we termed great droughts that covered more than 50% of the CONUS during our study period. We examined the properties of great droughts and compared these with the 2012 event. The great droughts were located at least partially over the central United States (30°–42°N, 85°–110°W). We found that 12 of these great droughts occurred when cold sea surface temperature anomalies (SSTAs) were located in the tropical Pacific with warm SSTAs in the North Atlantic. We also found a predominance of decreasing trends in IDI; droughts occurred less often and events were less severe as time progressed. In particular, only 2 of the 16 great droughts (2012 and 1988) occurred in the second half of the record.

Mantle Transition Zone Thickness Beneath the Middle East: Evidence for Segmented Tethyan Slabs, Delaminated Lithosphere, and Lower Mantle Upwelling

AbstractWe use a very large seismic data set to provide a comprehensive image of the mantle transition zone (MTZ) beneath the Middle East. We utilized the technique of Common Conversion Point stacking of P wave receiver functions to investigate the topography on the 410‐ and 660‐km discontinuities defining the upper and lower boundaries of the MTZ. Our results show significant topography on the 410‐ and 660‐km discontinuities and corresponding variations in the MTZ thickness. The MTZ topography is broadly consistent with the results of seismic tomography studies, implying the presence of both cold thermal anomalies imparted by detached Tethyan slabs and lithospheric segments and hot thermal anomalies induced by upwelling of lower mantle material. The MTZ topography in the northern Middle East is dominated by the presence of patches of cold material that are intermittently separated by regions of hot to normal MTZ. Our results suggest that instead of a continuous slab, the Tethyan slab in the Middle East is strongly segmented along the strike of the subduction boundary. Furthermore, we find evidence for a significant gap in subduction extending from the eastern edge of the Cyprean arc to NW Iran. The southern Middle East is dominated by the processes related to the mantle upwelling beneath the Afar depression. Our results imply that buoyant lower mantle material enters the MTZ beneath the Afar depression and then spreads laterally to the northeast beneath the western Arabia, flowing within the MTZ and in the upper mantle.

A possible mechanism for the occurrence of wintertime extreme precipitation events over South China

This study examines the formation of circulation patterns favorable to wintertime extreme precipitation events over South China between 1979 and 2013. During these extreme precipitation events, a barotropic wave train having seven centers of action was observed to extend from the Arabian Sea to the west coast of the North American continent with a maximum amplitude at 300 hPa. A center of action located over South China, comprised of cyclonic anomalies, favored powerful updrafts and large-scale moisture convergence over South China. About 77% of wintertime extreme precipitation events in South China were preceded by European blocking events. The formation mechanism for the 141 precipitation events with pre-existing European blocking highs is presented. Rossby wave energy propagation associated with the positive phase of circumglobal teleconnection enabled the European blocking event with a lead time of $$\sim$$ 10 days to generate cyclonic anomalies over South China. Moreover, significant warm anomalies were present over South China before the onset of these blocking-related extreme precipitation events. Increases in atmospheric moisture holding capacity associated with these warm anomalies enabled a buildup of precipitable water via moisture fluxes into South China through the western and southern boundaries. Onset of the extreme precipitation events was then triggered by the intrusion of cold temperature anomalies from the north, which lifted warm moist air upward from the surface and lowered the moisture holding capacity, producing large amounts of precipitation.