
Hans-F. GrafUniversity of Cambridge | Cam · Centre for Atmospheric Science
Hans-F. Graf
Dr. Sc.
About
251
Publications
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Introduction
Hans-F. Graf worked at the Centre for Atmospheric Science, University of Cambridge. Hans-F. does research in Meteorology and Volcanology. Their most recent publication is 'ENSO forecasts near the spring predictability barrier and possible reasons for the recently reduced predictability'.
He recently joined Hommel&Graf Environmental, see www.env.earth for more information.
Additional affiliations
September 1980 - December 1990
January 1991 - October 2003
October 2003 - present
Publications
Publications (251)
The North Pacific Oscillation (NPO), an important mode of atmospheric variability, is a crucial trigger for the development of El Niño-Southern Oscillation (ENSO) via the seasonal footprinting mechanism. How the NPO effect on ENSO changes in response to greenhouse warming remains unclear, however. Here, using climate model simulations under high-em...
Plain Language Summary
Arctic sea ice change not only regulates the local ecosystem but extends its influences into mid‐even low‐latitudes through several complicated physical processes. Sea ice variation in EsCB Seas exhibits an increased amplitude and more crucial role in climate change under global warming. The new findings hinted that autumn Es...
In this study, we reveal a marked enhanced impact of the early-spring Aleutian Low (AL) on the following winter El Niño and Southern Oscillation (ENSO) after the late-1990s. This enhanced impact of the early-spring AL may have an important contribution to the increased emergence of the central Pacific ENSO during recent decades. After the late-1990...
Based on reanalysis datasets and sea-ice sensitivity experiments, this study has pointed out that the autumn sea ice loss in East Siberian-Chukchi-Beaufort (EsCB) Seas significantly increases the frequency of winter extreme low temperature over western-central China. Autumn sea ice loss warms the troposphere and generates anticyclonic anomaly over...
Understanding the influence of volcanism on ENSO and associated climatic impacts is of great scientific and social importance. Although many studies on the volcano–ENSO nexus are available, a thorough review of ENSO sensitivity to explosive eruptions is still missing. Therefore, this study aims to provide an in-depth assessment of the ENSO response...
In this study, we provide a new perspective on the recent increased emergence of the central Pacific type of El Niño and Southern Oscillation (ENSO). Our results indicate that early-spring Aleutian Low (AL) intensity has a remarkable impact on the following winter ENSO especially after the late-1990s. Decrease (increase) in the early-spring AL stre...
The present study investigates different impacts of the East Asian winter monsoon (EAWM) on surface air temperature (Ts) in North America (NA) during ENSO and neutral ENSO episodes. In neutral ENSO years, the EAWM shows a direct impact on the Ts anomalies in NA on an interannual time scale. Two Rossby wave packets appear over the Eurasian–western P...
The present study reveals a close relation between the interannual variation of Aleutian Low intensity (ALI) in March and the subsequent winter El Niño-Southern Oscillation (ENSO). When March ALI is weaker (stronger) than normal, an El Niño (a La Nina)-like sea surface temperature (SST) warming (cooling) tends to appear in the equatorial central-ea...
In this paper, mechanisms for the formation of quasi-stationary extratropical wave trains associated with four distinct winter patterns of seasonal mean tropical Pacific convection anomalies, including the western CP (W-CP) pattern, La Niña (LN) pattern, CP El Niño (CPEN) pattern and EP El Niño (EPEN) pattern, are investigated by utilizing observat...
Kobresia pastures in the eastern Tibetan highlands occupy 450000 km ² and form the world’s largest pastoral alpine ecosystem. The main constituent is an endemic dwarf sedge, Kobresia pygmaea , which forms a lawn with a durable turf cover anchored by a felty root mat, and occurs from 3000 m to nearly 6000 m a.s.l. The existence and functioning of th...
In this paper, the performance of 18 Coupled Model Intercomparison Project Phase 5 (CMIP5) models forced by observational SSTs in simulating the tropical Pacific convective variation and the atmospheric responses in the extratropics are assessed. The multi-model ensemble mean results of 18 CMIP5 models show that five major patterns of tropical Paci...
In this paper, distinct patterns of boreal winter convection anomalies over the tropical Pacific and associated wave trains in the extratropics are addressed. The first leading mode (EOF1) of convection anomalies as measured by outgoing longwave radiation demonstrates an east–west oscillation of deep convection with centers over the equatorial cent...
El Niño-Southern Oscillation (ENSO) is a key feature for seasonal weather and climate prediction in the extra-tropics since related sea surface temperature anomalies induce precipitation anomalies that generate poleward propagating Rossby waves and teleconnections. The East Asian winter monsoon (EAWM) is driven by processes originating over the Asi...
A cross-validated statistical model has been developed to produce hindcasts for the 1980-2016 Nov-Dec-Jan (NDJ, assumed El Niño peak) mean Niño3.4 sea surface temperature anomalies (SSTA). A linear combination of two parameters is sufficient to successfully predict the peak SSTA: (1) the 130oE-180o, 5oN-5oS, 5m-250m oceanic potential temperature an...
A cross-validated statistical model has been developed to produce hindcasts for the 1980-2016 November-December-January (NDJ; assumed El Niño peak) mean Niño-3.4 sea surface temperature anomalies (SSTA). A linear combination of two parameters is sufficient to successfully predict the peak SSTA: 1) the 5°N-5°S, 130°E-180°, 5-250-m oceanic potential...
Upper tropospheric equatorial westerly ducts over the Pacific Ocean are the preferred location for Rossby wave breaking events during boreal winter and spring. These subtropical wave breaking events lead to the intrusion of high PV (potential vorticity) air along the extra-tropical tropopause and transport ozone rich dry stratospheric air into the...
Significance
Cold and dry glacial-state climate conditions persisted in the Southern Hemisphere until approximately 17.7 ka, when paleoclimate records show a largely unexplained sharp, nearly synchronous acceleration in deglaciation. Detailed measurements in Antarctic ice cores document exactly at that time a unique, ∼192-y series of massive haloge...
The Sichuan basin is one of the most densely populated regions of China, making the area particularly vulnerable to the adverse impacts associated with future climate change. As such, climate models are important for understanding regional and local impacts of climate change and variability, like heat stress and drought. In this study, climate mode...
The Sichuan Basin is one of the most densely populated regions of China, making the area particularly vulnerable to the adverse impacts associated with future climate change. As such, climate models are important for understanding regional and local impacts of climate change and variability, like heat stress and drought. In this study, the Coupled...
A decadal change in summer rainfall in the Asian inland plateau (AIP) region is identified around 1999. This decadal change is characterized by an abrupt decrease in summer rainfall of about 15.7% of the climatological average amount, leading to prolonged drought in the Asian inland plateau region. Both the surface air temperature and potential eva...
Since 1960's, India experiences a series of extreme drought and flood events during the summer months. The Humid Subtropical Climatic Zone (HSTC), which comprises the Indo-Gangetic Plain (IGP) region of India, is highly vulnerable to the climatic extremities. This region is the home for ~40% of total population and yields ~50% of total agricultural...
Combined impacts of the Pacific decadal oscillation (PDO) and two types of La Niña on climate anomalies in Europe are studied. Particularly, the conjunction of the negative PDO phase and two different types of La Niña events favors strong and significant North Atlantic Oscillation (NAO) pattern anomalies with opposite polarity. For the central Paci...
The eruption of Tambora (Indonesia) in April 1815 had substantial effects on global climate and led to the ‘Year Without a Summer’ of 1816 in Europe and North America. Although a tragic event—tens of thousands of people lost their lives—the eruption also was an ‘experiment of nature’ from which science has learned until today. The aim of this study...
Drawn from multiple reanalysis datasets, an increasing trend and westward shift in the number of Potential Vorticity intrusion events over the Pacific are evident. The increased frequency can be linked to a long-term trend in upper tropospheric equatorial westerly wind and subtropical jets during boreal winter to spring. These may be resulting from...
North Atlantic sea-surface temperature anomalies are known to affect tropical Pacific climate variability and El Niño-Southern Oscillation (ENSO) through thermocline adjustment in the equatorial Pacific Ocean. Here, coupled climate simulations featuring repeated idealized cycles of the Atlantic Multidecadal Oscillation (AMO) generated by nudging it...
The Tibetan Plateau plays a significant role in at-
mospheric circulation and the Asian monsoon system. Tur-
bulent surface fluxes and the evolution of boundary-layer
clouds to deep and moist convection provide a feedback sys-
tem that modifies the plateau’s surface energy balance on
scales that are currently unresolved in mesoscale models.
This wo...
This study describes how aerosol in an aerosol-coupled climate model of the
middle atmosphere is influenced by the quasi-biennial oscillation (QBO)
during times when the stratosphere is largely unperturbed by volcanic
material. In accordance with satellite observations, the vertical extent of
the stratospheric aerosol layer in the tropics is modula...
This article investigates the year-to-year variability of the onset of the South China Sea summer monsoon (SCSSM) and the possible influences exerted by the surface temperature anomalies over land and sea. Early and late monsoon onsets are related to the temperature anomalies in different regions. It is found that an early onset follows negative se...
The Tibetan Plateau plays a significant role in the atmospheric
circulation and the Asian monsoon system. Turbulent surface fluxes
and the evolution of boundary layer clouds to deep and moist
convection provide a feedback system that modifies the Plateau's
surface energy balance on scales that are currently unresolved in
mesoscale models. This work...
Different types of El Niño (EN) events have recently been discussed. Based on NCEP–NOAA reanalysis data this analysis explores a number of key parameters that cause a range of EN types over the period 1980–2013. EN events are divided into three types depending on the spatial and temporal evolution of the sea surface temperature anomalies (SSTA): Ce...
The long-term atmospheric cycling and fate of persistent organic pollutants under the influence of a changing climate is a concern. A GCM’s realization of present-day (1970–1999) and future (2070–2099) climate, the latter under a medium scenario of greenhouse gas emissions, is used to study meridional transports and their correlations with the Arct...
The long-term atmospheric cycling and fate of persistent organic pollutants under the influence of a changing climate is a concern. A GCM's realization of present-day (1970 1999) and future (2070-2099) climate, the latter under a medium scenario of greenhouse gas emissions, is used to study meridional transports and their correlations with the Arct...
The Tibetan Plateau has a significant role with regard to atmospheric circulation and the monsoon in particular. Changes between a closed plant cover and open bare soil are one of the striking effects of land use degradation observed with unsustainable range management or climate change, but experiments investigating changes of surface properties a...
The decadal evolution of Arctic and Antarctic sea ice following strong volcanic eruptions is investigated in four climate simulation ensembles performed with the COSMOS-Mill version of the Max Planck Institute-Earth System Model. The ensembles differ in the magnitude of the imposed volcanic perturbations, with sizes representative of historical tro...
This study describes how aerosol in an aerosol-coupled climate model of the middle at-mosphere is influenced by the quasi-biennial oscillation (QBO) during times when the stratosphere is largely unperturbed from volcanic material. In accordance with satel-lite observations, the tropical stratospheric aerosol load is predominately influenced by QBO...
The Tibetan Plateau has a significant role with regard to atmospheric circulation and the monsoon in particular. Changes between a closed plant cover and open bare soil are one of the striking effects of land use degradation observed with unsustainable range management or climate change, but experiments coupling changes of surface properties and pr...
The decadal evolution of Arctic and Antarctic sea ice following strong
volcanic eruptions is investigated in four climate simulation ensembles
performed with the COSMOS-Mill version of the Max Planck Institute Earth System Model. The ensembles differ in the magnitude of the imposed volcanic
perturbations, with sizes representative of historical tro...
A composite analysis of Northern Hemisphere's mid-winter tropospheric anomalies under the conditions of strong and weak stratospheric polar vortex was performed on NCEP/NCAR reanalysis data from 1948 to 2013 considering, as additional grouping criteria, the coincidental states of major seasonally relevant climate phenomena, such as El Niño-Southern...
We investigate the various methods currently available for distinguishing between the Central Pacific (CP) El Niño (or "El Niño Modoki") and the canonical El Niño by considering nine different methods and five sea surface temperature (SST) datasets from 1880 to 2010. This is aimed to demonstrate the variety which exists between different classifica...
[1] Previous studies suggest that wintertime North Pacific Oscillation (NPO) is able to force an El Niño event during the following winter via the seasonal footprinting mechanism (SFM). In this study, we present evidence that springtime Arctic Oscillation (AO) has a significant modulation effect on the connection between the NPO and El Niño. When t...
This work investigates the influence of atmospheric temperature and rela-tive humidity profiles obtained from radiosoundings, NCEP-I and ERA-INT reanalysis and GFS-FNL analysis data on the simulated evolution of clouds and convection at Nam Co Lake on the Tibetan Plateau. In addition to differences in moisture, the initial at-mospheric profiles exh...
The potential of satellite passive microwave sensors to provide quantitative information about near-source volcanic ash cloud parameters is assessed. To this aim, ground-based microwave weather radar and spaceborne microwave radiometer observations are used together with forward-model simulations. The latter are based on 2-D simulations with the nu...
This paper uses the cloud resolving model ATHAM coupled to the interactive surface model Hybrid in order to investigate the diurnal
development of a lake-breeze system at Nam Co Lake on the Tibetan Plateau. Simulations with several background wind speeds are
conducted and the interaction of the lake-breeze with topography and background wind in tri...
The goal of this work is to show potentials and drawbacks of Dual Polarization measurements of volcanic plume from microwave ground-based X-band radar (DPX). Measurements of brightness temperature (BT) from the space-orbiting microwave radiometer are used as well and compared with DPX retrievals of total columnar content (TCC). The latter is estima...
Background conditions have the potential to influence the climate
response to strong tropical volcanic eruptions. In this contribution, we
systematically assess the decadal climate response to the April 1815
Tambora eruption in a set of full-complexity Earth-system-model
simulations. Three 10-member simulation ensembles are evaluated which
describe...
While a large body of studies has indicated a significant statistical
relationship between episodes of strong (weak) polar stratospheric
vortex in winter with positive (negative) phases of the North Atlantic
Oscillation (NAO) or Arctic Oscillation and, consequently, weather and
climate over the North Atlantic, Europe and large parts of Siberia,
asp...
The sub-glacial Plinian explosive eruption of the Grímsvötn volcano, which occurred on May 2011, is for the first time analyzed and quantitatively interpreted by using ground-based weather radar data and the Volcanic Ash Radar Retrieval (VARR) physically-based technique. The prevailing southerly winds stretched the erupted plume toward the Artic po...
Climate simulations suggest that strong tropical volcanic eruptions
(SVEs) induce decadal dynamical responses in the coupled
ocean-atmosphere system, which protract the climate recovery beyond the
short-lived radiative forcing. Here, for the first time, we diagnose the
signature of such responses in European seasonal climate reconstructions
over th...
This study examines whether there exist significant differences in tropical cyclone (TC) landfall between central Pacific (CP) El Niño, eastern Pacific (EP) El Niño, and La Niña during the peak TC season (June- October) and how and towhat extent CP ElNiño influences TC landfall over EastAsia for the period 1961-2009. The peak TC season is subdivide...
We investigate the various methods currently available for
distinguishing between the Central Pacific (CP) El Niño (or "El
Niño Modoki") and the canonical El Niño by considering 10
different methods and 5 sea surface temperature (SST) datasets from 1880
to 2010. Years which are classified as CP El Niños with the
greatest convergence between method...
The regional atmospheric chemistry and climate model REMOTE has been used to conduct numerical simulations of the atmosphere during the catastrophic Indonesian fires of 1997. These simulations represent one possible scenario of the event, utilizing the RETRO wildland fire emission database. Emissions from the fires dominate the atmospheric concentr...
Decadal and bi-decadal climate responses to tropical strong volcanic eruptions (SVEs) are inspected in an ensemble simulation covering the last millennium based on the Max Planck Institute-Earth system model. An unprecedentedly large collection of pre-industrial SVEs (up to 45) producing a peak annual-average top-of-atmosphere radiative perturbatio...
Based on the NCEP/ NCAR reanalysis data the interannual variability of the East Asian winter mon-soon (EAWM) is studied with
a newly defined EAWM intensity index. The marked features for a strong (weak) winter monsoon include strong (weak) northerly
winds along coastal East Asia, cold (warm) East Asian continent and surrounding sea and warm (cold)...
This paper introduces a surface model with two soil-layers for use in a high-resolution circulation model that has been modified with an extrapolated surface temperature, to be used for the calculation of turbulent fluxes. A quadratic temperature profile based on the layer mean and base tem-perature is assumed in each layer and extended to the sur-...
This forum article is the product of interdisciplinary discussion at a conference on climate histories held in Cambridge, United Kingdom, in early 2011, with the specific aim of building a network around the issue of communicating cultural knowledge of environmental change. The lead articles, by Kirsten Hastrup as an anthropologist and Simon Schaff...