NCEP NCAR Reanalysis Research Articles

Statistical Deterministic and Ensemble Seasonal Prediction of Tropical Cyclones in the Northwest Australian Region

Abstract Statistical seasonal prediction of tropical cyclones (TCs) has been ongoing for quite some time in many different ocean basins across the world. While a few basins (e.g., North Atlantic and western North Pacific) have been extensively studied and forecasted for many years, Southern Hemispheric TCs have been less frequently studied and generally grouped as a whole or into two primary basins: southern Indian Ocean and Australian. This paper investigates the predictability of TCs in the northwest Australian (NWAUS) basin of the southeast Indian Ocean (105°–135°E) and describes two statistical approaches to the seasonal prediction of TC frequency, TC days, and accumulated cyclone energy (ACE). The first approach is a traditional deterministic seasonal prediction using predictors identified from NCEP–NCAR reanalysis fields using multiple linear regression. The second is a 100-member statistical ensemble approach with the same predictors as the deterministic model but with a resampling of the dataset with replacement and smearing input values to generate slightly different coefficients in the multiple linear regression prediction equations. Both the deterministic and ensemble schemes provide valuable forecasts that are better than climatological forecasts. The ensemble approach outperforms the deterministic model as well as adding quantitative uncertainty that reflects the predictability of a given TC season.

Summer Rainfall Seesaw between Hetao and the Middle and Lower Reaches of the Yangtze River and Its Relationship with the North Atlantic Oscillation

Using the NCEP–NCAR reanalysis and other observational datasets, the authors have investigated the relationship of summer rainfall variations between the Hetao region of northern China and the middle and lower reaches of Yangtze River (MLRYR). The results have demonstrated that rainfall in Hetao varies out of phase with that in MLRYR on the interannual time scales. This phenomenon is referred to as the Hetao–Yangtze rainfall seesaw (HYRS). An HYRS index is defined to reveal both spatial and temporal features of HYRS. It is found that the North Atlantic Oscillation (NAO) affects the HYRS. In years when the NAO is in its positive phase, anomalous divergences in the lower troposphere and anomalous convergences in the upper troposphere are observed in regions of the Mediterranean and eastern Europe. The anomalous convergences in the upper troposphere occur as the positive Rossby wave source excites a circumglobal teleconnection (CGT) in the midlatitudes, exhibiting the eastward propagation of Rossby wave energy along the Asian jet. Meanwhile, the Eurasian–Pacific (EUP) teleconnection also affects the HYRS. Influenced mainly by the CGT pattern, the circulations over Hetao and MLRYR are consequently perturbed. The atmosphere over Hetao converges anomalously in the lower troposphere and diverges anomalously in the upper troposphere, facilitating more than normal rainfall there. At the same time, the atmosphere over MLRYR diverges anomalously in the lower troposphere and converges anomalously in the upper troposphere, resulting in more than normal summer rainfall in MLRYR. In this way, the north–south rainfall seesaw is formed. This NAO-induced rainfall seesaw is potentially useful for summer rainfall predictions in both MLRYR and the Hetao region of northern China.

Intraseasonal Variability of Air Temperature Over East Asia in Boreal Summer

The intraseasonal oscillation (ISO) of atmospheric temperature at 2m over East Asia (EA) in boreal summer is investigated through NCEP-NCAR reanalysis data for the period of 1979-2013. It is found that the surface air temperature in EA has a dominant period of 10-30 days and exhibits a southeastward propagation characteristic. The average zonal and meridional phase speed of the intraseasonal air temperature anomaly is about 2.6 longitudes and 1.7 latitudes per day, respectively. A diagnosis of the temperature budget suggests that the propagation of the intraseasonal temperature perturbation is primarily attributed to the advection by the mean wind and the advection of the summer mean temperature by the intraseasonal wind. An upper-tropospheric geopotential positive (negative) height anomaly is closely associated with a low-level warmer (cooler) air temperature anomaly and both anomalies propagate southeastward in a similar way. A wave activity flux analysis reveals that mid-latitude Rossby wave energy also propagates southeastward, consistent with the upper-tropospheric geopotential height propagation. This indicates that in addition to low-level advection mechanism, upper-tropospheric forcing through the hydrostatic relationship is another possible mechanism. A further statistical analysis indicates that heat waves in EA summer are modulated by the southeastward-propagating intraseasonal temperature mode.

Interannual variability of summertime outgoing longwave radiation over the Maritime Continent in relation to East Asian summer monsoon anomalies

The Maritime Continent (MC) is under influences of both the tropical Pacific and the Indian Ocean. Anomalous convective activities over the MC have significant impacts on the East Asian summer monsoon (EASM) and climate in China. In the present study, the variation in convective activity over the MC in boreal summer and its relationship to EASM anomalies are investigated based on regression analysis of NCEP–NCAR reanalysis and CMAP [Climate Prediction Center (CPC) Merged Analysis of Precipitation] data, with a focus on the impacts of ENSO and the Indian Ocean Dipole (IOD). The most significant interannual variability of convective activity is found over 10°S–10°N, 95°–145°E, which can be roughly defined as the key area of the MC (hereafter, KMC). Outgoing longwave radiation anomaly (OLRA) exhibits 3- to 7-yr periodicities over the KMC, and around 70% of the OLRA variance can be explained by the ENSO signal. However, distinct convection and precipitation anomalies still exist over this region after the ENSO and IOD signals are removed. Abnormally low precipitation always corresponds to positive OLRA over the KMC when negative diabatic heating anomalies and anomalous cooling of the atmospheric column lead to abnormal descending motion over this region. Correspondingly, abnormal divergence occurs in the lower troposphere while convergence occurs in the upper troposphere, triggering an East Asia–Pacific/Pacific–Japan (EAP/PJ)-like anomalous wave train that propagates northeastward and leads to a significant positive precipitation anomaly from the Yangtze River valley in China to the islands of Japan. This EAP/PJ-like wave pattern becomes even clearer after the removal of the ENSO signal and the combined effects of ENSO and IOD, suggesting that convective anomalies over the KMC have an important impact on EASM anomalies. The above results provide important clues for the prediction of EASM anomalies and associated summer precipitation anomalies in China.

The Nature of the Arctic Oscillation and Diversity of the Extreme Surface Weather Anomalies It Generates

Through a cluster analysis of daily NCEP–NCAR reanalysis data, this study demonstrates that the Arctic Oscillation (AO), defined as the leading empirical orthogonal function (EOF) of 250-hPa geopotential height anomalies, is not a unique pattern but a continuum that can be well approximated by five discrete, representative AO-like patterns. These AO-like patterns grow simultaneously from disturbances in the North Pacific, the North Atlantic, and the Arctic, and both the feedback from the high-frequency eddies in the North Pacific and North Atlantic and propagation of the low-frequency wave trains from the North Pacific across North America into the North Atlantic play important roles in the pattern formation. Furthermore, it is shown that the structures and frequencies of occurrence of the five AO-like patterns are significantly modulated by El Niño–Southern Oscillation (ENSO). Warm (cold) ENSO enhances the negative (positive) AO phase, compared with ENSO neutral winters. Finally, the surface weather effects of these AO-like patterns and their implications for the AO-related weather prediction and the AO-North Atlantic Oscillation (NAO) relationship are discussed.

Association of the Poleward Shift of East Asian Subtropical Upper-Level Jet with Frequent Tropical Cyclone Activities over the Western North Pacific in Summer

The NCEP–NCAR reanalysis dataset and the tropical cyclone (TC) best-track dataset from the Regional Specialized Meteorological Center (RSMC) Tokyo Typhoon Center were employed in the present study to investigate the possible linkage of the meridional displacement of the East Asian subtropical upper-level jet (EASJ) with the TC activity over the western North Pacific (WNP). Results indicate that summertime frequent TC activities would create the poleward shift of the EASJ through a stimulated Pacific–Japan (PJ) teleconnection pattern as well as the changed large-scale meridional temperature gradient. On the contrary, in the inactive TC years, the EASJ is often located more southward than normal with an enhanced intensity. Therefore, TC activities over the WNP are closely related to the location and intensity of the EASJ in summer at the interannual time scale.

Paracas dust storms: Sources, trajectories and associated meteorological conditions

Dust storms that develop along the Pisco-Ica desert in Southern Peru, locally known as “Paracas” winds have ecological, health and economic repercussions. Here we identify dust sources through MODIS (Moderate Resolution Imaging Spectroradiometer) imagery and analyze HYSPLIT (Hybrid Single Particles Lagrangian Integrated Trajectory) model trajectories and dispersion patterns, along with concomitant synoptic-scale meteorological conditions from National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis (NCEP/NCAR). Additionally, surface pressure data from the hourly METeorological Aerodrome Report (METAR) at Arica (18.5°S, 70.3°W) and Pisco (13.7°S, 76.2°W) were used to calculate Alongshore (sea-level) Pressure Gradient (APG) anomalies during Paracas dust storms, their duration and associated wind-speeds and wind directions. This study provides a review on the occurrence and strength of the Paracas dust storms as reported in the Pisco airfield for five-year period and their correspondence with MODIS true-color imagery in terms of dust-emission source areas. Our results show that most of the particle fluxes moving into the Ica-Pisco desert area during Paracas wind events originate over the coastal zone, where strong winds forced by steep APGs develop as the axis of a deep mid-troposphere trough sets in along north-central Chile. Direct relationships between Paracas wind intensity, number of active dust-emission sources and APGs are also documented, although the scarcity of simultaneous METAR/MODIS data for clearly observed MODIS dust plumes prevents any significant statistical inference. Synoptic-scale meteorological composites from NCEP/NCAR reanalysis data show that Paracas wind events (steep APGs) are mostly associated with the strengthening of anticyclonic conditions in northern Chile, that can be attributed to cold air advection associated with the incoming trough. Compared to the MODIS images, HYSPLIT outputs were able to spatially reproduce trajectories and dust dispersion plumes during the Paracas wind storms. HYSPLIT trajectories revealed that part of the wind-eroded lithological material can be transported downwind several kilometers along the Peruvian coast and also deposited over the nearby coastal ocean, giving support to the presence of an aeolian signal in continental shelf sediments, of great importance for paleoenvironmental studies.

Characteristics and possible formation mechanisms of severe storms in the outer rainbands of Typhoon Mujiga (1522)

To better understand how severe storms form and evolve in the outer rainbands of typhoons, in this study, we investigate the evolutionary characteristics and possible formation mechanisms for severe storms in the rainbands of Typhoon Mujigae, which occurred during 2–5 October 2015, based on the NCEP–NCAR reanalysis data, conventional observations, and Doppler radar data. For the rainbands far from the inner core (eye and eyewall) of Mujigae (distance of approximately 70–800 km), wind speed first increased with the radius expanding from the inner core, and then decreased as the radius continued to expand. The Rankine Vortex Model was used to explore such variations in wind speed. The areas of strong stormy rainbands were mainly located in the northeast quadrant of Mujigae, and overlapped with the areas of high winds within approximately 300–550 km away from the inner core, where the strong winds were conducive to the development of strong storms. A severe convective cell in the rainbands developed into waterspout at approximately 500 km to the northeast of the inner core, when Mujigae was strengthening before it made landfall. Two severe convective cells in the rainbands developed into two tornadoes at approximately 350 km to the northeast of the inner core after Mujigae made landfall. The radar echo bands enhanced to 60 dBZ when mesocyclones occurred in the rainbands and induced tornadoes. The radar echoes gradually weakened after the mesocyclones weakened. The tops of parent clouds of the mesocyclones elevated at first, and then suddenly dropped about 20 min before the tornadoes appeared. Thereby, the cloud top variation has the potential to be used as an early warning of tornado occurrence.

Relationship between atmospheric heat source over the Tibetan Plateau and precipitation in the Sichuan–Chongqing region during summer

NCEP–NCAR reanalysis data and a 47-yr daily precipitation dataset from a network of 42 rain gauges are used to analyze the atmospheric heat source ( ) anomaly over the Tibetan Plateau (TP) and its influence on the summer precipitation anomaly in the Sichuan–Chongqing region. Results show that the vertical advection of over the central TP is a major factor affecting summer precipitation in the Sichuan–Chongqing region. When the vertical advection of over the central TP is strengthened, the South Asian high shifts further than normal to the south and east, the western Pacific subtropical high shifts further than normal to the south and west, and the Indian low weakens. This benefits the transport of warm moist air from the low latitude oceans to the Sichuan–Chongqing region. Correspondingly, in the high latitudes, two ridges and one trough form, which lead to cool air moving southward. These two air masses converge over the Sichuan–Chongqing region, leading to significant precipitation. In contrast, when the vertical advection of over the central TP is weakened, the South Asian high moves to the north and west, the subtropical high moves eastward and northward, and the Indian low strengthens. This circulation pattern is unfavorable for warm air advection from the south to the Sichuan–Chongqing region, and the cool air further north cannot move southward because of the presence of two troughs and one ridge at high latitude. Thus, ascent over the Sichuan–Chongqing region is weakened, resulting in less precipitation.

Historical statistics and future changes in long-duration blocking highs in key regions of Eurasia

Using NCEP/NCAR reanalysis (NNR) data and 13 models from phase 5 of Coupled Model Intercomparison Project (CMIP5), this study focuses on historical statistics and future change in blocking over key regions (Urals, Baikal, and Okhotsk regions) in Eurasia. The statistical characteristic using NNR data shows that short-duration and long-duration blocking highs are 75 and 25% of the total, respectively. It is also shown that frequency of blocking highs presents a decreased trend in Eurasia during 1956–2006, except blocking highs in summer in Baikal region, long-duration blocking highs in summer, and short-duration blocking highs in winter in Okhotsk region. Compared with NNR, the frequency of blocking highs is underestimated in Eurasia by CMIP5 models, except these with long-duration in the Baikal and Okhotsk regions. Most of the CMIP5 models can reproduce the historical trend of blocking highs over Eurasia during 1956–2005. In addition, projections show that the future change of long-duration blocking highs in Eurasia is not always consistent with that in the whole Northern Hemisphere. The results of the multiple models ensemble (MME) suggest that long-duration blocking highs in the Urals will significantly increase by 0.3 times/10 years (decrease by 0.22 times/10 years) under RCP4.5 (RCP8.5) in the wintertime. In the summertime, the frequency fluctuates with little change. In Okhotsk, long-duration blocking highs will increase by 0.23 times/10 years (decrease by 0.22 times/10 years) in the wintertime (summertime) under RCP4.5. Under RCP8.5, long-duration blocking high frequency will remain the same, and the decreasing trend in the wintertime and the increasing trend (0.32 times/10 years) in the summertime will even accelerate over the trends in the twentieth century.

Evolution of the eastward shift in the quasi-stationary minimum of the Antarctic total ozone column

Abstract. The quasi-stationary pattern of the Antarctic total ozone has changed during the last 4 decades, showing an eastward shift in the zonal ozone minimum. In this work, the association between the longitudinal shift of the zonal ozone minimum and changes in meteorological fields in austral spring (September–November) for 1979–2014 is analyzed using ERA-Interim and NCEP–NCAR reanalyses. Regressive, correlative and anomaly composite analyses are applied to reanalysis data. Patterns of the Southern Annular Mode and quasi-stationary zonal waves 1 and 3 in the meteorological fields show relationships with interannual variability in the longitude of the zonal ozone minimum. On decadal timescales, consistent longitudinal shifts of the zonal ozone minimum and zonal wave 3 pattern in the middle-troposphere temperature at the southern midlatitudes are shown. Attribution runs of the chemistry–climate version of the Australian Community Climate and Earth System Simulator (ACCESS-CCM) model suggest that long-term shifts of the zonal ozone minimum are separately contributed by changes in ozone-depleting substances and greenhouse gases. As is known, Antarctic ozone depletion in spring is strongly projected on the Southern Annular Mode in summer and impacts summertime surface climate across the Southern Hemisphere. The results of this study suggest that changes in zonal ozone asymmetry accompanying ozone depletion could be associated with regional climate changes in the Southern Hemisphere in spring.

Characterizing the winter concurrent variation patterns of the subtropical and polar-front jets over East Asia

In this study, the concurrent variation relationships between the East Asian subtropical jet (EASJ) and polar-front jet (PFJ) over the East Asian land mass in the winter season on different timescales are identified and the impacts of the jet concurrent variation patterns on the atmospheric circulation in mid–high latitude regions and climate anomalies in China are examined, using NCEP–NCAR reanalysis data and observational data. The major variability modes of the winter upper-level wind field on interannual timescales are characterized by the meridional shift of the PFJ and out-of-phase variation in the intensity of the subtropical jet and PFJ. On subseasonal and synoptic timescales, the concurrent variation relationships can be categorized into four configuration patterns: a strong (weak) subtropical jet accompanied by a weak (strong) PFJ, or a strong (weak) subtropical jet with a strong (weak) PFJ. The out-ofphase variation [i.e., a stronger (weaker) EASJ and weaker (stronger) PFJ] is found to be more common than the inphase variation [i.e., a stronger (weaker) EASJ and stronger (weaker) PFJ]. These concurrent variation relationships represent the integral structure and variation features of the atmospheric general circulation over East Asia, and have significant impacts on the weather and climate. The strong subtropical jet/weak PFJ (weak subtropical jet/strong PFJ) pattern leads to anomalous negative (positive) geopotential height in midlatitude regions and favors cold (warm) conditions, and positive (negative) rainfall anomalies in southern China. For both strong jet configurations, the geopotential height anomaly in the mid–high latitudes shows a northwest–southeast tilted dipole pattern, resulting in northern warm–southern cold temperature anomalies, and positive rainfall anomalies in southern China. For both weak jet situations, positive geopotential height anomalies dominate the East Asian area, and warm conditions occur over most areas in China, corresponding to less negative rainfall anomalies in southern China. The complicated rainfall and temperature anomaly patterns in China can be explained by the concurrent variation relationships between the two jets. A close relationship may exist between the synoptic-scale transient eddy activity (STEA) and the intensity of jet streams, especially for the PFJ. Significantly reduced (strengthened) STEA over the polar-front area is intimately associated with a decreased (increased) intensity of the PFJ.

Comparison of the seasonal evolution of the South Asian high associated with two types of El Niño event

AbstractEl Nino (EN) episodes can be classified based on their time of onset as spring onset EN (SPEN) events and summer onset EN (SUEN) events. To evaluate the different influences of SPEN and SUEN events on the South Asian high (SAH), this study compared the seasonal evolution of the SAH (SESAH) associated with SPEN and SUEN events through analysis of geopotential height and zonal wind data derived from NCEP–NCAR Reanalysis-1 and sea surface temperature data obtained from the Hadley Center. The main features of the SESAH during an EN event are similar to its climatological characteristics. Climatologically, the SAH forms in May, strengthens, and moves northwestward in June and July. It does not change much in August, but then it returns south and weakens during September and October. However, its lifespan is shorter and its intensity weaker during EN periods. Furthermore, there are significant differences between the SESAH during SPEN and SUEN events. During a SPEN episode, the movement of the SAH to th...

Simulating ENSO SSTAs from TAO/TRITON Winds: The Impacts of 20 Years of Buoy Observations in the Pacific Waveguide and Comparison with Reanalysis Products

Abstract The fundamental importance of near-equatorial zonal wind stress in the evolution of the tropical Pacific Ocean’s seasonal cycle and El Niño–Southern Oscillation (ENSO) events is well known. It has been two decades since the TAO/TRITON buoy array was deployed, in part to provide accurate surface wind observations across the Pacific waveguide. It is timely to revisit the impact of TAO/TRITON winds on our ability to simulate and thereby understand the evolution of sea surface temperature (SST) in this region. This work shows that forced ocean model simulations of SST anomalies (SSTAs) during the periods with a reasonably high buoy data return rate can reproduce the major elements of SSTA variability during ENSO events using a wind stress field computed from TAO/TRITON observations only. This demonstrates that the buoy array usefully fulfills its waveguide-wind-measurement purpose. Comparison of several reanalysis wind fields commonly used in recent ENSO studies with the TAO/TRITON observations reveals substantial biases in the reanalyses that cause substantial errors in the variability and trends of the reanalysis-forced SST simulations. In particular, the negative trend in ERA-Interim is much larger and the NCEP–NCAR Reanalysis-1 and NCEP–DOE Reanalysis-2 variability much less than seen in the TAO/TRITON wind observations. There are also mean biases. Thus, even with the TAO/TRITON observations available for assimilation into these wind products, there remain oceanically important differences. The reanalyses would be much more useful for ENSO and tropical Pacific climate change study if they would more effectively assimilate the TAO/TRITON observations.

Arctic and East Asia Winter Climate Variations Associated with the Eastern Atlantic Pattern

The wintertime Arctic temperature (T; surface–400 hPa) decreased from 1979 to 1997 and increased rapidly from 1998 to 2012, in contrast to the global mean surface air temperature. Here aspects of circulation variability that are associated with these temperature changes are examined using the NCEP–NCAR reanalysis and ERA-Interim products. It is found that the Nordic–Siberia seesaw of meridional winds near 70°N is associated with two-thirds of the variance of the Arctic winter mean T, possibly contributing to the cooling and warming trends. It is suggested here that the seesaw accounts for much of the difference in Arctic amplification between observations and climate models. Growth of sea ice in winter is hindered by southerly winds over the Nordic region (0°–60°E). Through modulation of the wind seesaw, the eastern Atlantic (EA) pattern is found to be significantly associated with Arctic and East Asia winter climate variations. In one phase of the EA pattern, a midlatitude North Atlantic ridge anomaly is associated with a poleward shift of the mean storm track, a weakened eddy-driven jet over Eurasia, and above-normal sea level pressure (SLP) over Siberia, most significantly in the region to the northwest of Lake Baikal. The EA pattern is associated with two-thirds of the variance of winter-average SLP over Siberia.

Possible combined influences of absorbing aerosols and anomalous atmospheric circulation on summertime diurnal temperature range variation over the middle and lower reaches of the Yangtze River

Based on the temperature data from the China Meteorological Administration, NCEP–NCAR reanalysis data, and the TOMS Aerosol Index (AI), we analyze the variations in the summertime diurnal temperature range (DTR) and temperature maxima in the middle and lower reaches of the Yangtze River (MLRYR) in China. The possible relationships between the direct warming effect of the absorbing aerosol and temperature variations are further investigated, although with some uncertainties. It is found that the summertime DTR exhibits a decreasing trend over the most recent 50 years, along with a slight increasing tendency since the 1980s. The trend of the maximum temperature is in agreement with those of the DTR and the absorbing aerosols. To investigate the causes of the large anomalies in the temperature maxima, composite analyses of the circulation anomalies are performed. When anomalous AI and anomalous maximum temperature over the MLRYR have the same sign, an anomalous circulation with a quasi-barotropic structure occurs there. This anomalous circulation is modulated by the Rossby wave energy propagations from the regions northwest of the MLRYR and influences the northwestern Pacific subtropical high over the MLRYR. In combination with aerosols, the anomalous circulation may increase the maximum temperature in this region. Conversely, when the anomalous AI and anomalous maximum temperature in the MLRYR have opposite signs, the anomalous circulation is not equivalently barotropic, which possibly offsets the warming effect of aerosols on the maximum temperature changes in this region. These results are helpful for a better understanding of the DTR changes and the occurrences of temperature extremes in the MLRYR region during boreal summer.

Baroclinic-to-Barotropic Pathway in El Niño–Southern Oscillation Teleconnections from the Viewpoint of a Barotropic Rossby Wave Source

Abstract The baroclinic-to-barotropic pathway in ENSO teleconnections is examined from the viewpoint of a barotropic Rossby wave source that results from decomposition into barotropic and baroclinic components. Diagnoses using the NCEP–NCAR reanalysis are supplemented by analysis of the response of a tropical atmospheric model of intermediate complexity to the NCEP–NCAR barotropic Rossby wave source. Among the three barotropic Rossby wave source contributions (shear advection, vertical advection, and surface drag), the leading contribution is from shear advection and, more specifically, the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind. Vertical advection is the smallest term, while surface drag tends to cancel and reinforce the shear advection in different regions through damping on the baroclinic mode, which spins up a barotropic response. There are also nontrivial impacts of transients in the barotropic wind response to ENSO. Both tropical and subtropical baroclinic vorticity advection contribute to the barotropic component of the Pacific subtropical jet near the coast of North America, where the resulting barotropic wind contribution approximately doubles the zonal jet anomaly at upper levels, relative to the baroclinic anomalies alone. In this view, the barotropic Rossby wave source in the subtropics simply arises from the basic-state baroclinic flow acting on the well-known baroclinic ENSO flow pattern that spreads from the deep tropics into the subtropics over a scale of equatorial radius of deformation. This is inseparably connected to the leading deep tropical Rossby wave source that arises from eastern Pacific climatological baroclinic winds advecting the tropical portion of the same ENSO flow pattern.

Singular value decomposition analysis for examining the impact of Siberian High on winter precipitation variability over South Asia

The relationship and linkage of Siberian High (SH) with winter climate over central Asia, East Asia, and Eurasia have been focused in the available literature. The effect of Siberian High over the South Asian region has received little attention. Thus, this article examines the possible teleconnection of SH pressure on winter precipitation variability over the region of South Asia. Moreover, the recent research studies focused on large-scale circulation phenomena: El Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO), and regarded them as responsible for variability of winter rainfall. This paper gives the evidence that the winter rainfall variability is strongly influenced by Siberian High pressure. The effect of Siberian High pressure on winter precipitation over the South Asian region (65–76° E, 33–40° N) has been investigated using singular value decomposition analysis (SVDA) that is performed on gridded datasets of NCEP-NCAR Reanalysis and Climate Research Unit (CRU) of University of East Anglia, UK. SVDA isolates the leading pairs of the coupled signals that mainly explain the relationship between the coupled fields. By employing SVDA, it is found that the Siberian High has dominant impact on precipitation variability during winter season over Northern areas of South Asia.

New Tools for Comparing Beliefs about the Timing of Recurrent Events with Climate Time Series Datasets

Abstract For climate services to be relevant and informative for users, scientific data definitions need to match users’ perceptions or beliefs. This study proposes and tests novel yet simple methods to compare beliefs of timing of recurrent climatic events with empirical evidence from multiple historical time series. The methods are tested by applying them to the onset date of the monsoon in Bangladesh, where several scientific monsoon definitions can be applied, yielding different results for monsoon onset dates. It is a challenge to know which monsoon definition compares best with people’s beliefs. Time series from eight different scientific monsoon definitions in six regions are compared with respondent beliefs from a previously completed survey concerning the monsoon onset. Beliefs about the timing of the monsoon onset are represented probabilistically for each respondent by constructing a probability mass function (PMF) from elicited responses about the earliest, normal, and latest dates for the event. A three-parameter circular modified triangular distribution (CMTD) is used to allow for the possibility (albeit small) of the onset at any time of the year. These distributions are then compared to the historical time series using two approaches: likelihood scores, and the mean and standard deviation of time series of dates simulated from each belief distribution. The methods proposed give the basis for further iterative discussion with decision-makers in the development of eventual climate services. This study uses Jessore, Bangladesh, as an example and finds that a rainfall definition, applying a 10 mm day−1 threshold to NCEP–NCAR reanalysis (Reanalyis-1) data, best matches the survey respondents’ beliefs about monsoon onset.

Annual variations of the tropopause height over the Tibetan Plateau compared with those over other regions

By using radiosonde data, the annual variation of the tropopause height over the Tibetan Plateau (TP) compared with that over the same latitude plain area and tropics is first analyzed, and then the tropopause heights over the TP obtained from radiosondes, ERA-40, NCEP-NCAR analysis, and the HALOE occultation measurement are compared. It is found that the second (first) tropopause is easier to be observed over the TP than that over the plain area in June–September (October–May). The first tropopause height (TH1) over the TP is higher than that over the plain area in boreal winter (November–March), while the second tropopause height (TH2) over the TP is higher in each month, and from April to September it is even higher than that over the tropics. The maximum differences of both TH1 and TH2 between the TP and tropics appear in boreal middle summer. The annual variation of TH2 over the TP exhibits a single-peak structure, while it shows a bimodal structure over the tropics. It is revealed that all the tropopause heights derived from ERA-40, NCEP-NCAR analysis, and HALOE measurement are between TH1 and TH2. During October–May, all of these heights are close to the average of TH1 and TH2. During June–September, all of them are close to TH2, and the HALOE-based height is most close to TH2 and their difference is less than 0.5km. Therefore, extreme caution should be exercised if ERA-40, NCEP-NCAR reanalysis, and the HALOE occultation measurement are selected to analyze the tropopause over the TP.