Search Results

You are looking at 11 - 20 of 21 items for

  • Author or Editor: Noel Keenlyside x
  • Refine by Access: All Content x
Clear All Modify Search
Shunya Koseki
,
Benjamin Pohl
,
Bhuwan Chandra Bhatt
,
Noel Keenlyside
, and
Arielle Stela Nkwinkwa Njouodo

Abstract

Adopting a state-of-the-art numerical model system, we investigate how the diurnal variations in precipitation and local breeze systems are characterized by lower-boundary conditions related to the Drakensberg highland and warm SST associated with the Agulhas Current. A control simulation can simulate the hydrometeorological climates in the region realistically, but the terrestrial rainfall is overestimated. During daytime, the precipitation is confined to the Drakensberg highland, and there is an onshore local breeze, while during midnight to morning, the rainfall is confined to the Agulhas Current, and the breeze is offshore. These variations are captured by the numerical simulation, although the timing of maximum rainfall is early over the land and delayed over the ocean. The sensitivity experiment in which the Drakensberg is absent shows a drastic modification in the diurnal variations over land and ocean. The terrestrial precipitation is largely decreased around the Drakensberg and is largest along the coast during daytime. The nocturnal marine precipitation along the Agulhas Current is also reduced. Although the daily residual breeze is still pronounced even without the Drakensberg, wind speed is weakened. We attribute this to the reduction of precipitation. In another sensitivity experiment with smoothened warm SST due to the Agulhas Current, the amplitudes of diurnal variations are not modified remarkably, but the coastal rainfall is diminished to some extent due to less evaporation along the Agulhas Current. This study concludes that the Drakensberg plays a crucial role for the diurnal cycle, and the impact of the Agulhas Current is limited on the diurnal cycle of the coastal precipitation in this region.

Full access
Martin P. King
,
Ivana Herceg-Bulić
,
Ileana Bladé
,
Javier García-Serrano
,
Noel Keenlyside
,
Fred Kucharski
,
Camille Li
, and
Stefan Sobolowski

Abstract

Recent studies have indicated the importance of fall climate forcings and teleconnections in influencing the climate of the northern mid- to high latitudes. Here, we present some exploratory analyses using observational data and seasonal hindcasts, with the aim of highlighting the potential of the El Niño–Southern Oscillation (ENSO) as a driver of climate variability during boreal late fall and early winter (November and December) in the North Atlantic–European sector, and motivating further research on this relatively unexplored topic. The atmospheric ENSO teleconnection in November and December is reminiscent of the east Atlantic pattern and distinct from the well-known arching extratropical Rossby wave train found from January to March. Temperature and precipitation over Europe in November are positively correlated with the Niño-3.4 index, which suggests a potentially important ENSO climate impact during late fall. In particular, the ENSO-related temperature anomaly extends over a much larger area than during the subsequent winter months. We discuss the implications of these results and pose some research questions.

Full access
Vladimir A. Semenov
,
Mojib Latif
,
Dietmar Dommenget
,
Noel S. Keenlyside
,
Alexander Strehz
,
Thomas Martin
, and
Wonsun Park

Abstract

The twentieth-century Northern Hemisphere surface climate exhibits a long-term warming trend largely caused by anthropogenic forcing, with natural decadal climate variability superimposed on it. This study addresses the possible origin and strength of internal decadal climate variability in the Northern Hemisphere during the recent decades. The authors present results from a set of climate model simulations that suggest natural internal multidecadal climate variability in the North Atlantic–Arctic sector could have considerably contributed to the Northern Hemisphere surface warming since 1980. Although covering only a few percent of the earth’s surface, the Arctic may have provided the largest share in this. It is hypothesized that a stronger meridional overturning circulation in the Atlantic and the associated increase in northward heat transport enhanced the heat loss from the ocean to the atmosphere in the North Atlantic region and especially in the North Atlantic portion of the Arctic because of anomalously strong sea ice melt. The model results stress the potential importance of natural internal multidecadal variability originating in the North Atlantic–Arctic sector in generating interdecadal climate changes, not only on a regional scale, but also possibly on a hemispheric and even a global scale.

Full access
Hyacinth C. Nnamchi
,
Jianping Li
,
Fred Kucharski
,
In-Sik Kang
,
Noel S. Keenlyside
,
Ping Chang
, and
Riccardo Farneti

Abstract

Equatorial Atlantic variability is dominated by the Atlantic Niño peaking during the boreal summer. Studies have shown robust links of the Atlantic Niño to fluctuations of the St. Helena subtropical anticyclone and Benguela Niño events. Furthermore, the occurrence of opposite sea surface temperature (SST) anomalies in the eastern equatorial and southwestern extratropical South Atlantic Ocean (SAO), also peaking in boreal summer, has recently been identified and termed the SAO dipole (SAOD). However, the extent to which and how the Atlantic Niño and SAOD are related remain unclear. Here, an analysis of historical observations reveals the Atlantic Niño as a possible intrinsic equatorial arm of the SAOD. Specifically, the observed sporadic equatorial warming characteristic of the Atlantic Niño (~0.4 K) is consistently linked to southwestern cooling (~−0.4 K) of the Atlantic Ocean during the boreal summer. Heat budget calculations show that the SAOD is largely driven by the surface net heat flux anomalies while ocean dynamics may be of secondary importance. Perturbations of the St. Helena anticyclone appear to be the dominant mechanism triggering the surface heat flux anomalies. A weakening of the anticyclone will tend to weaken the prevailing northeasterlies and enhance evaporative cooling over the southwestern Atlantic Ocean. In the equatorial region, the southeast trade winds weaken, thereby suppressing evaporation and leading to net surface warming. Thus, it is hypothesized that the wind–evaporation–SST feedback may be responsible for the growth of the SAOD events linking southern extratropics and equatorial Atlantic variability via surface net heat flux anomalies.

Full access
Wan-Ling Tseng
,
Huang-Hsiung Hsu
,
Noel Keenlyside
,
Chiung-Wen June Chang
,
Ben-Jei Tsuang
,
Chia-Ying Tu
, and
Li-Chiang Jiang

Abstract

This study uses the atmospheric general circulation model (AGCM) ECHAM5 coupled with the newly developed Snow–Ice–Thermocline model (ECHAM5-SIT) to examine the effects of orography and land–sea contrast on the Madden–Julian oscillation (MJO) in the Maritime Continent (MC) during boreal winter. The ECHAM5-SIT is one of the few AGCMs that realistically simulate the major characteristics of the MJO. Three experiments are conducted with realistic topography, without orography, and with oceans only in the MC region to evaluate the relative effects of orography and land–sea contrast. Orography and land–sea contrast have the following effects on the MJO in the MC: 1) a larger amplitude, 2) a smaller zonal scale, 3) more realistic periodicity and stronger eastward-propagating signals, 4) a stronger southward detour during the eastward propagation, 5) a distorted coupled Kelvin–Rossby wave structure, and 6) larger low-level moisture convergence. The existence of mountainous islands also enhances the mean westerly in the eastern Indian Ocean and the western MC, as well as the moisture content over the MC. This enhancement of mean states contributes to the stronger eastward-propagating MJO. The findings herein suggest that theoretical and empirical studies, which are largely derived from an aquaplanet framework, have likely provided an oversimplified view of the MJO. The effects of mountainous islands should be considered for better understanding and more accurate forecast of the MJO.

Full access
Luca Famooss Paolini
,
Nour-Eddine Omrani
,
Alessio Bellucci
,
Panos J. Athanasiadis
,
Paolo Ruggieri
,
Casey R. Patrizio
, and
Noel Keenlyside

Abstract

The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of the Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of nonstationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO. This study assesses the possible nonstationarity in the NAO–GSF interaction and the mechanisms underlying this interaction during 1950–2020, using reanalysis data. Results show that the NAO and GSF indices covary on the decadal time scale but only during 1972–2018. A secondary peak in the NAO–GSF covariability emerges on multiannual time scales but only during 2005–15. The nonstationarity in the decadal NAO–GSF covariability is also manifested in variations in their lead–lag relationship. Indeed, the NAO tends to lead the GSF shifts by 3 years during 1972–90 and by 2 years during 1990–2018. The response of the GSF to the NAO at the decadal time scale can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only during 1972–90. Here it is suggested that the nonstationarity of Rossby wave propagation caused the time lag between the NAO and the GSF shifts on the decadal time scale to differ between the two time periods.

Open access
Tianjun Zhou
,
Rucong Yu
,
Jie Zhang
,
Helge Drange
,
Christophe Cassou
,
Clara Deser
,
Daniel L. R. Hodson
,
Emilia Sanchez-Gomez
,
Jian Li
,
Noel Keenlyside
,
Xiaoge Xin
, and
Yuko Okumura

Abstract

The western Pacific subtropical high (WPSH) is closely related to Asian climate. Previous examination of changes in the WPSH found a westward extension since the late 1970s, which has contributed to the interdecadal transition of East Asian climate. The reason for the westward extension is unknown, however. The present study suggests that this significant change of WPSH is partly due to the atmosphere’s response to the observed Indian Ocean–western Pacific (IWP) warming. Coordinated by a European Union’s Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE), five AGCMs were forced by identical idealized sea surface temperature patterns representative of the IWP warming and cooling. The results of these numerical experiments suggest that the negative heating in the central and eastern tropical Pacific and increased convective heating in the equatorial Indian Ocean/Maritime Continent associated with IWP warming are in favor of the westward extension of WPSH. The SST changes in IWP influences the Walker circulation, with a subsequent reduction of convections in the tropical central and eastern Pacific, which then forces an ENSO/Gill-type response that modulates the WPSH. The monsoon diabatic heating mechanism proposed by Rodwell and Hoskins plays a secondary reinforcing role in the westward extension of WPSH. The low-level equatorial flank of WPSH is interpreted as a Kelvin response to monsoon condensational heating, while the intensified poleward flow along the western flank of WPSH is in accord with Sverdrup vorticity balance. The IWP warming has led to an expansion of the South Asian high in the upper troposphere, as seen in the reanalysis.

Full access
Francine Schevenhoven
,
Noel Keenlyside
,
François Counillon
,
Alberto Carrassi
,
William E. Chapman
,
Marion Devilliers
,
Alok Gupta
,
Shunya Koseki
,
Frank Selten
,
Mao-Lin Shen
,
Shuo Wang
,
Jeffrey B. Weiss
,
Wim Wiegerinck
, and
Gregory S. Duane

Abstract

The modeling of weather and climate has been a success story. The skill of forecasts continues to improve and model biases continue to decrease. Combining the output of multiple models has further improved forecast skill and reduced biases. But are we exploiting the full capacity of state-of-the-art models in making forecasts and projections? Supermodeling is a recent step forward in the multimodel ensemble approach. Instead of combining model output after the simulations are completed, in a supermodel individual models exchange state information as they run, influencing each other’s behavior. By learning the optimal parameters that determine how models influence each other based on past observations, model errors are reduced at an early stage before they propagate into larger scales and affect other regions and variables. The models synchronize on a common solution that through learning remains closer to the observed evolution. Effectively a new dynamical system has been created, a supermodel, that optimally combines the strengths of the constituent models. The supermodel approach has the potential to rapidly improve current state-of-the-art weather forecasts and climate predictions. In this paper we introduce supermodeling, demonstrate its potential in examples of various complexity, and discuss learning strategies. We conclude with a discussion of remaining challenges for a successful application of supermodeling in the context of state-of-the-art models. The supermodeling approach is not limited to the modeling of weather and climate, but can be applied to improve the prediction capabilities of any complex system, for which a set of different models exists.

Open access
Panos J. Athanasiadis
,
Fumiaki Ogawa
,
Nour-Eddine Omrani
,
Noel Keenlyside
,
Reinhard Schiemann
,
Alexander J. Baker
,
Pier Luigi Vidale
,
Alessio Bellucci
,
Paolo Ruggieri
,
Rein Haarsma
,
Malcolm Roberts
,
Chris Roberts
,
Lenka Novak
, and
Silvio Gualdi

Abstract

Starting to resolve the oceanic mesoscale in climate models is a step change in model fidelity. This study examines how certain obstinate biases in the midlatitude North Atlantic respond to increasing resolution (from 1° to 0.25° in the ocean) and how such biases in sea surface temperature (SST) affect the atmosphere. Using a multimodel ensemble of historical climate simulations run at different horizontal resolutions, it is shown that a severe cold SST bias in the central North Atlantic, common to many ocean models, is significantly reduced with increasing resolution. The associated bias in the time-mean meridional SST gradient is shown to relate to a positive bias in low-level baroclinicity, while the cold SST bias causes biases also in static stability and diabatic heating in the interior of the atmosphere. The changes in baroclinicity and diabatic heating brought by increasing resolution lead to improvements in European blocking and eddy-driven jet variability. Across the multimodel ensemble a clear relationship is found between the climatological meridional SST gradients in the broader Gulf Stream Extension area and two aspects of the atmospheric circulation: the frequency of high-latitude blocking and the southern-jet regime. This relationship is thought to reflect the two-way interaction (with a positive feedback) between the respective oceanic and atmospheric anomalies. These North Atlantic SST anomalies are shown to be important in forcing significant responses in the midlatitude atmospheric circulation, including jet variability and the storm track. Further increases in oceanic and atmospheric resolution are expected to lead to additional improvements in the representation of Euro-Atlantic climate.

Open access

Decadal Prediction

Can It Be Skillful?

Gerald A. Meehl
,
Lisa Goddard
,
James Murphy
,
Ronald J. Stouffer
,
George Boer
,
Gokhan Danabasoglu
,
Keith Dixon
,
Marco A. Giorgetta
,
Arthur M. Greene
,
Ed Hawkins
,
Gabriele Hegerl
,
David Karoly
,
Noel Keenlyside
,
Masahide Kimoto
,
Ben Kirtman
,
Antonio Navarra
,
Roger Pulwarty
,
Doug Smith
,
Detlef Stammer
, and
Timothy Stockdale

A new field of study, “decadal prediction,” is emerging in climate science. Decadal prediction lies between seasonal/interannual forecasting and longer-term climate change projections, and focuses on time-evolving regional climate conditions over the next 10–30 yr. Numerous assessments of climate information user needs have identified this time scale as being important to infrastructure planners, water resource managers, and many others. It is central to the information portfolio required to adapt effectively to and through climatic changes. At least three factors influence time-evolving regional climate at the decadal time scale: 1) climate change commitment (further warming as the coupled climate system comes into adjustment with increases of greenhouse gases that have already occurred), 2) external forcing, particularly from future increases of greenhouse gases and recovery of the ozone hole, and 3) internally generated variability. Some decadal prediction skill has been demonstrated to arise from the first two of these factors, and there is evidence that initialized coupled climate models can capture mechanisms of internally generated decadal climate variations, thus increasing predictive skill globally and particularly regionally. Several methods have been proposed for initializing global coupled climate models for decadal predictions, all of which involve global time-evolving three-dimensional ocean data, including temperature and salinity. An experimental framework to address decadal predictability/prediction is described in this paper and has been incorporated into the coordinated Coupled Model Intercomparison Model, phase 5 (CMIP5) experiments, some of which will be assessed for the IPCC Fifth Assessment Report (AR5). These experiments will likely guide work in this emerging field over the next 5 yr.

Full access