A global empirical system for probabilistic seasonal climate prediction

Preparing for episodes with risks of anomalous weather a month to a year ahead is an important challenge for governments, non-governmental organisations, and private companies and is dependent on the availability of reliable forecasts. The majority of operational seasonal forecasts are made using process-based dynamical models, which are complex, computationally challenging and prone to biases. Empirical forecast approaches built on statistical models to represent physical processes offer an alternative to dynamical systems and can provide either a benchmark for comparison or independent supplementary forecasts. Here, we present a simple empirical system based on multiple linear regression for producing probabilistic forecasts of seasonal surface air temperature and precipitation across the globe. The global CO2-equivalent concentration is taken as the primary predictor; subsequent predictors, including large-scale modes of variability in the climate system and local-scale information, are selected on the basis of their physical relationship with the predictand. The focus given to the climate change signal as a source of skill and the probabilistic nature of the forecasts produced constitute a novel approach to global empirical prediction. Hindcasts for the period 1961–2013 are validated against observations using deterministic (correlation of seasonal means) and probabilistic (continuous rank probability skill scores) metrics. Good skill is found in many regions, particularly for surface air temperature and most notably in much of Europe during the spring and summer seasons. For precipitation, skill is generally limited to regions with known El Niño–Southern Oscillation (ENSO) teleconnections. The system is used in a quasi-operational framework to generate empirical seasonal forecasts on a monthly basis.

An assessment of Indian monsoon seasonal forecasts and mechanisms underlying monsoon interannual variability in the Met Office GloSea5-GC2 system

We assess Indian summer monsoon seasonal forecasts in GloSea5-GC2, the Met Office fully coupled subseasonal to seasonal ensemble forecasting system. Using several metrics, GloSea5-GC2 shows similar skill to other state-of-the-art forecast systems. The prediction skill of the large-scale South Asian monsoon circulation is higher than that of Indian monsoon rainfall. Using multiple linear regression analysis we evaluate relationships between Indian monsoon rainfall and five possible drivers of monsoon interannual variability. Over the time period studied (1992-2011), the El Nino-Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) are the most important of these drivers in both observations and GloSea5-GC2. Our analysis indicates that ENSO and its teleconnection with the Indian rainfall are well represented in GloSea5-GC2. However, the relationship between the IOD and Indian rainfall anomalies is too weak in GloSea5-GC2, which may be limiting the prediction skill of the local monsoon circulation and Indian rainfall. We show that this weak relationship likely results from a coupled mean state bias that limits the impact of anomalous wind forcing on SST variability, resulting in erroneous IOD SST anomalies. Known difficulties in representing convective precipitation over India may also play a role. Since Indian rainfall responds weakly to the IOD, it responds more consistently to ENSO than in observations. Our assessment identifies specific coupled biases that are likely limiting GloSea5-GC2 prediction skill, providing targets for model improvement.

The combined influences of westerly phase of the quasi biennial oscillation and 11-year solar maximum conditions on the Northern Hemisphere extratropical winter circulation

The combined influences of the westerly phase of the quasi-biennial oscillation (QBO-W) and solar maximum (Smax) conditions on the Northern Hemisphere extratropical winter circulation are investigated using reanalysis data and Center for Climate System Research/National Institute for Environmental Studies chemistry climate model (CCM) simulations. The composite analysis for the reanalysis data indicates strengthened polar vortex in December followed by weakened polar vortex in February–March for QBO-W during Smax (QBO-W/Smax) conditions. This relationship need not be specific to QBO-W/Smax conditions but may just require strengthened vortex in December, which is more likely under QBO-W/Smax. Both the reanalysis data and CCM simulations suggest that dynamical processes of planetary wave propagation and meridional circulation related to QBO-W around polar vortex in December are similar in character to those related to Smax; furthermore, both processes may work in concert to maintain stronger vortex during QBO-W/Smax. In the reanalysis data, the strengthened polar vortex in December is associated with the development of north–south dipole tropospheric anomaly in the Atlantic sector similar to the North Atlantic oscillation (NAO) during December–January. The structure of the north–south dipole anomaly has zonal wavenumber 1 (WN1) component, where the longitude of anomalous ridge overlaps with that of climatological ridge in the North Atlantic in January. This implies amplification of the WN1 wave and results in the enhancement of the upward WN1 propagation from troposphere into stratosphere in January, leading to the weakened polar vortex in February–March. Although WN2 waves do not play a direct role in forcing the stratospheric vortex evolution, their tropospheric response to QBO-W/Smax conditions appears to be related to the maintenance of the NAO-like anomaly in the high-latitude troposphere in January. These results may provide a possible explanation for the mechanisms underlying the seasonal evolution of wintertime polar vortex anomalies during QBO-W/Smax conditions and the role of troposphere in this evolution.

The role of local atmospheric forcing on the modulation of the ocean mixed layer depth in reanalysis and a coupled single column ocean model

The role of the local atmospheric forcing on the ocean mixed layer depth (MLD) over the global oceans is studied using ocean reanalysis data products and a single-column ocean model coupled to an atmospheric general circulation model. The focus of this study is on how the annual mean and the seasonal cycle of the MLD relate to various forcing characteristics in different parts of the world's ocean, and how anomalous variations in the monthly mean MLD relate to anomalous atmospheric forcings. By analysing both ocean reanalysis data and the single-column ocean model, regions with different dominant forcings and different mean and variability characteristics of the MLD can be identified. Many of the global oceans' MLD characteristics appear to be directly linked to different atmospheric forcing characteristics at different locations. Here, heating and wind-stress are identified as the main drivers; in some, mostly coastal, regions the atmospheric salinity forcing also contributes. The annual mean MLD is more closely related to the annual mean wind-stress and the MLD seasonality is more closely to the seasonality in heating. The single-column ocean model, however, also points out that the MLD characteristics over most global ocean regions, and in particular the tropics and subtropics, cannot be maintained by local atmospheric forcings only, but are also a result of ocean dynamics that are not simulated in a single-column ocean model. Thus, lateral ocean dynamics are essentially in correctly simulating observed MLD.

Examining the predictability of the Stratospheric Sudden Warming of January 2013 using multiple NWP systems

The first multi-model study to estimate the predictability of a boreal Sudden Stratospheric Warming (SSW) is performed using five NWP systems. During the 2012-2013 boreal winter, anomalous upward propagating planetary wave activity was observed towards the end of December, which followed by a rapid deceleration of the westerly circulation around 2 January 2013, and on 7 January 2013 the zonal mean zonal wind at 60°N and 10 hPa reversed to easterly. This stratospheric dynamical activity was followed by an equatorward shift of the tropospheric jet stream and by a high pressure anomaly over the North Atlantic, which resulted in severe cold conditions in the UK and Northern Europe. In most of the five models, the SSW event was predicted 10 days in advance. However, only some ensemble members in most of the models predicted weakening of westerly wind when the models were initialized 15 days in advance of the SSW. Further dynamical analysis of the SSW shows that this event was characterized by the anomalous planetary wave-1 amplification followed by the anomalous wave-2 amplification in the stratosphere, which resulted in a split vortex occurring between 6 January 2013 and 8 January 2013. The models have some success in reproducing wave-1 activity when initialized 15 days in advance, they but generally failed to produce the wave-2 activity during the final days of the event. Detailed analysis shows that models have reasonably good skill in forecasting tropospheric blocking features that stimulate wave-2 amplification in the troposphere, but they have limited skill in reproducing wave-2 amplification in the stratosphere.

A common framework for approaches to extreme event attribution

The extent to which a given extreme weather or climate event is attributable to anthropogenic climate change is a question of considerable public interest. From a scientific perspective, the question can be framed in various ways, and the answer depends very much on the framing. One such framing is a risk-based approach, which answers the question probabilistically, in terms of a change in likelihood of a class of event similar to the one in question, and natural variability is treated as noise. A rather different framing is a storyline approach, which examines the role of the various factors contributing to the event as it unfolded, including the anomalous aspects of natural variability, and answers the question deterministically. It is argued that these two apparently irreconcilable approaches can be viewed within a common framework, where the most useful level of conditioning will depend on the question being asked and the uncertainties involved.

Eddies and variability in the Mozambique Channel

Between 1995 and 2000, on average 4 eddies per year are observed from satellite altimetry to propagate southward through the Mozambique Channel, into the upstream Agulhas region. Further south, these eddies have been found to control the timing and frequenc yof Agulhas ring shedding. Within the Mozambique Channel, anomalous SSH amplitudes rise to 30 cm ; in agreement with in situ measured velocities. Comparison of an observed velocit ysection with GCM model results shows that the Mozambique Channel eddies in these models are too surface intensified. Also, the number of eddies formed in the models is in disagreement with our observational analysis. Moored current meter measurements observing the passage of three eddies in 2000 are extended to a 5-year time series b yreferencing the anomalous surface currents estimated from altimeter data to a s ynoptic LADCP velocit y measurement. The results show intermittent edd ypassage at the mooring location. A statistical analysis of SSH observations in different parts of the Mozambique Channel shows a southward decrease of the dominant frequency of the variability, going from 7 per year in the extension of the South Equatorial Current north of Madagascar to 4 per year south of Madagascar. The observations suggest that frequency reduction is related to the Rossb ywaves coming in from the east

Abrupt transitions in the NAO control of explosive North Atlantic cyclone development

Explosive cyclones are intense extra-tropical low pressure systems featuring large deepening rates. In the Euro-Atlantic sector, they are a major source of life-threatening weather impacts due to their associated strong wind gusts, heavy precipitation and storm surges. The wintertime variability of the North Atlantic cyclonic activity is primarily modulated by the North Atlantic Oscillation (NAO). In this study, we investigate the interannual and multi-decadal variability of explosive North Atlantic cyclones using track density data from two reanalysis datasets (NCEP and ERA-40) and a control simulation of an atmosphere/ocean coupled General Circulation Model (GCM—ECHAM5/MPIOM1). The leading interannual and multi-decadal modes of variability of explosive cyclone track density are characterized by a strengthening/weakening pattern between Newfoundland and Iceland, which is mainly modulated by the NAO at both timescales. However, the NAO control of interannual cyclone variability is not stationary in time and abruptly fluctuates during periods of 20–25 years long both in NCEP and ECHAM5/MPIOM1. These transitions are accompanied by structural changes in the leading mode of explosive cyclone variability, and by decreased/enhanced baroclinicity over the sub-polar/sub-tropical North Atlantic. The influence of the ocean is apparently important for both the occurrence and persistence of such anomalous periods. In the GCM, the Atlantic Meridional Overturning Circulation appears to influence the large-scale baroclinicity and explosive cyclone development over the North Atlantic. These results permit a better understanding of explosive cyclogenesis variability at different climatic timescales and might help to improve predictions of these hazardous events.

Annular modes and apparent eddy feedbacks in the Southern Hemisphere

Lagged correlation analysis is often used to infer intraseasonal dynamical effects but is known to be affected by non-stationarity. We highlight a pronounced quasi-two-year peak in the anomalous zonal wind and eddy momentum flux convergence power spectra in the Southern Hemisphere, which is prima facie evidence for non-stationarity. We then investigate the consequences of this non-stationarity for the Southern Annular Mode and for eddy momentum flux convergence. We argue that positive lagged correlations previously attributed to the existence of an eddy feedback are more plausibly attributed to non-stationary interannual variability external to any potential feedback process in the mid-latitude troposphere. The findings have implications for the diagnosis of feedbacks in both models and re-analysis data as well as for understanding the mechanisms underlying variations in the zonal wind.

Impact of biomass burning aerosol on the monsoon circulation transition over Amazonia

Ensemble simulations of a regional climate model (RegCM3) forced by aerosol radiative forcing suggest that biomass burning aerosols can work against the seasonal monsoon circulation transition, thus re-enforce the dry season rainfall pattern for Southern Amazonia. Strongly absorbing smoke aerosols warm and stabilize the lower troposphere within the smoke center in southern Amazonia (where aerosol optical depth >0.3). These changes increase the surface pressure in the smoke center, weaken the southward surface pressure gradient between northern and southern Amazonia, and consequently induce an anomalous moisture divergence in the smoke center and an anomalous convergence in northwestern Amazonia (5 degrees S-5 degrees N, 60 degrees W-70 degrees W). The increased atmospheric thermodynamic stability, surface pressure, and divergent flow in Southern Amazonia may inhibit synoptic cyclonic activities propagated from extratropical South America, and re-enforce winter-like synoptic cyclonic activities and rainfall in southeastern Brazil, Paraguay and northeastern Argentina. Citation: Zhang, Y., R. Fu, H. Yu, Y. Qian, R. Dickinson, M. A. F. Silva Dias, P. L. da Silva Dias, and K. Fernandes (2009), Impact of biomass burning aerosol on the monsoon circulation transition over Amazonia, Geophys. Res. Lett., 36, L10814, doi: 10.1029/2009GL037180.

An event of stratospheric air intrusion and its associated secondary surface cyclogenesis over the South Atlantic Ocean

This work presents an analysis of a lowermost stratospheric air intrusion event over the coast of Brazil, which may have been responsible for a secondary surface cyclogenesis over the southwestern Atlantic Ocean. The surface cyclone initiated at 0600 UTC 17 April 1999 in a cold air mass in the rear of a cold front after a primary cyclone developed over the same region. The analysis of the secondary cyclone revealed the presence of lowermost stratospheric air intrusion characterized by anomalous potential vorticity (PV), dry air, and high concentration of ozone in atmospheric column. The system developed on the eastern side of an upper level core of PV anomaly, which induced a cyclonic wind circulation at lower levels and favored the onset of the secondary cyclone. In midlevels (500 hPa), the cutoff low development contributed to reduce the propagation speed of the wave pattern. This feature seemed to (1) allow the low-level cold/dry air to heat/moisten associated with sensible and latent fluxes transferred from the ocean to the atmosphere, which intensified a baroclinic zone parallel to the coast, and (2) contribute to the long duration of the system. The present analysis indicates that this secondary cyclone development could be the result of the coupling between the PV anomaly in the upper levels and low-level air-sea interaction.

The role of the South Indian and Pacific oceans in South American monsoon variability

This work has investigated the impact of three different low-frequency sea surface temperature (SST) variability modes located in the Indian and the Pacific Oceans on the interannual variability of the South American Monsoon System (SAMS) using observed and numerical data. Rotated Empirical Orthogonal Function (REOF) analysis and numerical simulations with a General Circulation Model (GCM) were used. One of the three SST variability modes is located close to southeastern Africa. According to the composites, warmer waters over this region are associated with enhanced austral summer precipitation over the sub-tropics. The GCM is able to reproduce this anomalous precipitation pattern, simulating a wave train emanating from the Indian Ocean towards South America (SA). A second SST variability mode was located in the western Pacific Ocean. REOF analysis indicates that warmer waters are associated with drought conditions over the South Atlantic Convergence Zone (SACZ) and enhanced precipitation over the sub-tropics. The GCM indicates that the warmer waters over Indonesia generate drought conditions over tropical SA through a Pacific South America-like (PSA) wave pattern emanating from the western Pacific. Finally, the third SST variability mode is located over the southwestern South Pacific. The composites indicate that warmer waters are associated with enhanced precipitation over the SACZ and drought conditions over the sub-tropics. There is a PSA-like wave train emanating from Indonesia towards SA, and another crossing the Southern Hemisphere in the extra-tropics, probably associated with transient activity. The GCM is able to reproduce the anomalous precipitation pattern, although it is weaker than observed. The PSA-like pattern is simulated, but the model fails in reproducing the extra-tropical wave activity.

MAGNETOHYDRODYNAMIC SIMULATIONS OF RECONNECTION AND PARTICLE ACCELERATION: THREE-DIMENSIONAL EFFECTS

Magnetic fields can change their topology through a process known as magnetic reconnection. This process in not only important for understanding the origin and evolution of the large-scale magnetic field, but is seen as a possibly efficient particle accelerator producing cosmic rays mainly through the first-order Fermi process. In this work we study the properties of particle acceleration inserted in reconnection zones and show that the velocity component parallel to the magnetic field of test particles inserted in magnetohydrodynamic (MHD) domains of reconnection without including kinetic effects, such as pressure anisotropy, the Hall term, or anomalous effects, increases exponentially. Also, the acceleration of the perpendicular component is always possible in such models. We find that within contracting magnetic islands or current sheets the particles accelerate predominantly through the first-order Fermi process, as previously described, while outside the current sheets and islands the particles experience mostly drift acceleration due to magnetic field gradients. Considering two-dimensional MHD models without a guide field, we find that the parallel acceleration stops at some level. This saturation effect is, however, removed in the presence of an out-of-plane guide field or in three-dimensional models. Therefore, we stress the importance of the guide field and fully three-dimensional studies for a complete understanding of the process of particle acceleration in astrophysical reconnection environments.

Fast magnetic reconnection and energetic particle acceleration

Our numerical simulations show that the reconnection of magnetic field becomes fast in the presence of weak turbulence in the way consistent with the Lazarian and Vishniac (1999) model of fast reconnection. We trace particles within our numerical simulations and show that the particles can be efficiently accelerated via the first order Fermi acceleration. We discuss the acceleration arising from reconnection as a possible origin of the anomalous cosmic rays measured by Voyagers. (C) 2010 Elsevier Ltd. All rights reserved.

Magnetotelluric deep soundings, gravity and geoid in the south Sao Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

In the south Sao Francisco craton a circular and 8-m amplitude geoid anomaly coincides with the outcropping terrain of an Archean-Paleoproterozoic basement. Broadband magnetotelluric (MT) data inversions of two radial profiles within the positive geoid and Bouguer gravity anomaly yield geo-electrical crustal sections, whereby the lower crust is locally more conductive (10 to 100 Omega m) in spatial coincidence with a denser lower crust modeled by the gravity data. This anomalous lower crust may have resulted from magmatic underplating, associated with Mesoarchean and Proterozoic episodes of tholeiitic dike intrusion. Long-period MT soundings reveal a low electrical resistivity mantle (20 to 200 Omega m) from depths beyond 120 km. Forward geoid modeling, using the scope of the low electrical resistivity region within the mantle as a constraint, entails a density increase (40 to 50 kg/m(3)) possibly due to Fe enrichment of mantle minerals. However, this factor alone does not explain the observed resistivity. A supplemented presence of small amounts of percolated carbonatite melting (similar to 0.005 vol.%), dissolved water and enhanced oxygen fugacity within the peridotitic mantle are viable agents that could explain the less resistive upper mantle. We propose that metasomatic processes confined in the sub-continental lithospheric mantle foster the conditions for a low degree melting with variable CO(2), H(2)O and Fe content. Even though the precise age of this metasomatism is unknown it might be older than the Early Cretaceous based on the evidence that a high-degree of melting in a lithospheric mantle impregnated with carbonatites originated the tholeiitic dike intrusions dispersed from the southeastern border of the Sao Francisco craton, during the onset of the lithosphere extension and break-up of the western Gondwana. The proxies are the NE Parana and Espinhaco (130 Ma, Ar/Ar ages) tholeiitic dikes, which contain (similar to 3%) carbonatites in their composition. The occurrence of a positive geoid anomaly (+ 10 m) and pre-tholeiites (age > 138 Ma), carbonatites and kimberlites along the west African continental margin (Angola and Namibia) reinforces the presumed age of the Sao Francisco-Congo craton rejuvenation to be prior to its fragmentation in the Lower Cretaceous. (C) 2010 Elsevier B.V. All rights reserved.