Atlantic Storm Track Research Articles

The North Atlantic Oscillation and the North Atlantic Jet Variability: Precursors to NAO Regimes and Transitions

Abstract In this paper, precursors to the North Atlantic Oscillation (NAO) and its transitions are investigated to understand the dynamical cause of the interdecadal NAO variability from dominant negative (NAO−) events during 1950–77 (P1) to dominant positive (NAO+) events during 1978–2010 (P2). It is found that the phase of the NAO event depends strongly on the latitudinal position of the North Atlantic jet (NAJ) prior to the NAO onset. The NAO− (NAO+) events occur frequently when the NAJ core prior to the NAO onset is displaced southward (northward), as the situation within P1 (P2). Thus, the northward (southward) shift of the NAJ from its mean position is a precursor to the NAO+ (NAO−) event. This finding is further supported by results obtained from a weakly nonlinear model. Furthermore, the model results show that, when the Atlantic mean zonal wind exceeds a critical strength under which the dipole anomaly prior to the NAO onset is stationary, in situ NAO− (NAO+) events, which are events not preceded by opposite events, can occur frequently during P1 (P2) when the Atlantic storm track is not too strong. This mean zonal wind condition is easily satisfied during P1 and P2. However, when the Atlantic storm track (mean zonal wind) prior to the NAO onset is markedly intensified (weakened), the NAO event can undergo a transition from one phase to another, especially in a relatively strong background westerly wind, the Atlantic storm track has to be strong enough to produce a phase transition.

Weather Regime Transitions and the Interannual Variability of the North Atlantic Oscillation. Part II: Dynamical Processes

Abstract In this study, attention is focused on identifying the dynamical processes that contribute to the negative North Atlantic Oscillation (NAO)− to positive NAO (NAO+) and NAO+ to NAO− transitions that occur during 1978–90 (P1) and 1991–2008 (P2). By constructing Atlantic ridge (AR) and Scandinavian blocking (SBL) indices, the composite analysis demonstrates that in a stronger AR (SBL) winter NAO− (NAO+) event can more easily transition into an NAO+ (NAO−) event. Composites of 300-hPa geopotential height anomalies for the NAO− to NAO+ and NAO+ to NAO− transition events during P1 and P2 are calculated. It is shown for P2 (P1) that the NAO+ to SBL to NAO− (NAO− to AR to NAO+) transition results from the retrograde drift of an enhanced high-latitude, large-scale, positive (negative) anomaly over northern Europe during the decay of the previous NAO+ (NAO−) event. This finding cannot be detected for NAO events without transition. Moreover, it is found that the amplification of retrograding wavenumber 1 is more important for the NAO− to NAO+ transition during P1, but the marked reintensification and retrograde movement of both wavenumbers 1 and 2 after the NAO+ event decays is crucial for the NAO+ to NAO− transition during P2. It is further shown that destructive (constructive) interference between wavenumbers 1 and 2 over the North Atlantic during P1 (P2) is responsible for the subsequent weak NAO+ (strong NAO−) anomaly associated with the NAO− to NAO+ (NAO+ to NAO−) transition. Also, the weakening (strengthening) of the vertically integrated zonal wind (upstream Atlantic storm track) is found to play an important role in the NAO regime transition.

Response of the North Atlantic storm track to climate change shaped by ocean–atmosphere coupling

In contrast to those in other regions, the North Atlantic storm track responds to anthropogenic greenhouse-gas forcing by strengthening and extending farther east. A regression analysis of an ensemble of coupled climate model simulations shows that this response is, to a large part, shaped by the interaction between ocean and atmosphere. A poleward shift of the mid-latitude storm tracks in response to anthropogenic greenhouse-gas forcing has been diagnosed in climate model simulations1,2. Explanations of this effect have focused on atmospheric dynamics3,4,5,6,7. However, in contrast to storm tracks in other regions, the North Atlantic storm track responds by strengthening and extending farther east, in particular on its southern flank8. These adjustments are associated with an intensification and extension of the eddy-driven jet towards western Europe9 and are expected to have considerable societal impacts related to a rise in storminess in Europe10,11,12. Here, we apply a regression analysis to an ensemble of coupled climate model simulations to show that the coupling between ocean and atmosphere shapes the distinct storm-track response to greenhouse-gas forcing in the North Atlantic region. In the ensemble of simulations we analyse, at least half of the differences between the storm-track responses of different models are associated with uncertainties in ocean circulation changes. We compare the fully coupled simulations with both the associated slab model simulations and an ocean-forced experiment with one climate model to establish causality. We conclude that uncertainties in the response of the North Atlantic storm track to anthropogenic emissions could be reduced through tighter constraints on the future ocean circulation.

Sedimentary processes on the NW Iberian Continental Shelf since the Little Ice Age

The OMEX core CD110 W90, retrieved from the Douro Mud Patch (DMP) off the River Douro in the north of Portugal, records the period since the beginning of Little Ice Age (LIA). The core chronology is based upon the data attributes for 210Pb, 137Cs and a 14C dating from a level near the core base. Geochemical, granulometric, microfaunal (benthic foraminifera) and compositional data suggest the occurrence of precipitation changes which may have been, at least partially, influenced by the North Atlantic Oscillation (NAO), that contributes to the regulation of the ocean–atmosphere dynamics in the North Atlantic.Southwesterly Atlantic storm track is associated with the negative phases of the NAO, when the Azores High is anomalously weak, higher oceanographic hydrodynamism, downwelling events and increased rainfall generally occurs. Prevalence of these characteristics during the LIA left a record that corresponds to phases of major floods. During these phases the DMP received a higher contribution of relatively coarse-grained terrigenous sediments, enriched in quartz particles, which diluted the contribution of other minerals, as indicated by reduced concentrations of several lithogenic chemical elements such as: Al, As, Ba, Ce, Co, Cu, Fe, K, La, Li, Mg, Mn, Mo, Na, Ni, P, Rb, Sc, Sn, Th, V and Y. The presence of biogenic carbonate particles also underwent dilution, as revealed by the smaller abundance of foraminifera and correlative lower concentrations of Ca and Sr. During this period, the DMP also received an increased contribution of organic matter, indicated by higher values of lignin remains and a benthic foraminifera high productivity index, or BFHP, which gave rise to early diagenetic changes with pyrite formation. Since the beginning of the 20th century this contribution diminished, probably due to several drier periods and the impact of human activities in the river basins, e.g. construction of dams, or, on the littoral areas, construction of hard-engineering structures and sand extraction activities.During the first half of the 20th century mainly positive phases of the NAO prevailed, caused by the above normal strengthening of the subtropical high pressure centre of the Azores and the deepening of the low pressure centre in Iceland. These phases may have contributed to the reduction in the supply of both terrigenous sediments and organic matter from shallow water to the DMP. During the positive phases of the NAO, sedimentation became finer.The development of mining and industrial activities during the 20th century is marked, in this core, by higher concentrations of Pb. Furthermore, the erosion of heaps resulting from wolfram exploitation leaves its signature as a peak of W concentrations recorded in the sediments of the DMP deposited between the 1960s and the 1990s. Wolfram exploitation was an important activity in the middle part of the 20th century, particularly during the period of the Second World War.

Further Investigation of the Impact of Idealized Continents and SST Distributions on the Northern Hemisphere Storm Tracks

Abstract Building on previous studies of the basic ingredients of the North Atlantic storm track (examining land–sea contrast, orography, and SST), this article investigates the impact of Eurasian topography and Pacific SST anomalies on North Pacific and Atlantic storm tracks through a hierarchy of atmospheric GCM simulations using idealized boundary conditions in the Hadley Centre HadGAM1 atmospheric circulation model. The Himalaya–Tibet mountain complex is found to play a crucial role in shaping the North Pacific storm track. The northward deflection of the westerly flow around northern Tibet generates an extensive pool of very cold air in the northeastern tip of the Asian continent, which strengthens the meridional temperature gradient and favors baroclinic growth in the western Pacific. The Kuroshio SST front is also instrumental in strengthening the Pacific storm track through its impact on near-surface baroclinicity, while the warm waters around Indonesia tend to weaken it through the impact on baroclinicity of stationary Rossby waves propagating poleward from the convective heating regions. Three mechanisms by which the Atlantic storm track may be affected by changes in the boundary conditions upstream of the Rockies are discussed. In the model configuration used here, stationary Rossby waves emanating from Tibet appear to weaken the North Atlantic storm track substantially, whereas those generated over the cold waters off Peru appear to strengthen it. Changes in eddy-driven surface winds over the Pacific generally appear to modify the flow over the Rocky Mountains, leading to consistent modifications in the Atlantic storm track. The evidence for each of these mechanisms is, however, ultimately equivocal in these simulations.

Impact of ENSO on the Atmospheric Variability over the North Atlantic in Late Winter—Role of Transient Eddies

Abstract The dynamical mechanism for the late-winter teleconnection between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) is examined using the output from a 2000-yr integration of a coupled general circulation model (GCM). The coupled model captures many salient features of the observed behavior of both ENSO and NAO, as well as their impact on the surface climate in late winter. Both the observational and model data indicate more occurrences of negative phase of NAO in late winter during El Niño events, and positive NAO in La Niña episodes. The potential role of high-frequency transient eddies in the above teleconnection is diagnosed. During El Niño winters, the intensified transient disturbances along the equatorward-shifted North Pacific storm track extend their influences farther downstream. The eddy-induced negative height tendencies are found to be more coherent and stronger over North Atlantic than that over North Pacific. These negative height tendencies over the North Atlantic are coincident with the southern lobe of NAO, and thus favor more occurrences of negative NAO events. During those El Niño winters with relatively strong SST warming in eastern equatorial Pacific, the eastward extension of eddy activity is reinforced by the enhanced near-surface baroclinicity over the subtropical eastern Pacific. This flow environment supports a stronger linkage between the Pacific and Atlantic storm tracks, and is more conducive to a negative NAO phase. These model results are supported by a parallel analysis of various observational datasets. It is further demonstrated that these transient eddy effects can be reproduced in atmospheric GCM integrations subjected to ENSO-related SST forcing in the tropical Pacific.

Seasonal and interannual variations of community metabolism rates of a Posidonia oceanica seagrass meadow

We report gross primary production (GPP), community respiration (CR), and net community production (NCP) over a Posidonia oceanica meadow at 10 m in Corsica (Bay of Revellata) based on the open water O2 mass balance from a data set of hourly measurements with an array of three O2 optodes deployed from August 2006 to October 2009. The method was checked by comparison with discrete measurements of metabolic rates derived from benthic chamber incubations also based on the diel change of O2. This comparison was satisfactory and actually highlights the potential caveats of benthic incubation measurements related to O2 accumulation in small chambers leading to photorespiration and an underestimation of GPP. Our data confirmed previous P. oceanica meadows GPP and CR values, strong seasonal variations, and net autotrophy. High‐resolution data revealed strong interannual variability, with a decrease of GPP by 35% and NCP by 87% during 2006–2007 characterized by a mild and less stormy winter compared with 2007–2008 and 2008–2009. P. oceanica meadows are then expected to decrease export of organic carbon to adjacent communities (decrease of NCP), since a decrease in frequency and intensity of marine storms is expected in the future in the Mediterranean Sea as a result of a northward shift of the Atlantic storm track.

Future Change in Extratropical Cyclones Associated with Change in the Upper Troposphere

Abstract Future changes in Northern Hemisphere wintertime storm activity as a consequence of global warming are investigated using the AGCM of Meteorological Research Institute (MRI-AGCM) with horizontal grid sizes of 60 and 20 km. A future (2075–99) climate experiment, in which the change in sea surface temperature (SST) derived from the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel ensemble mean is added to observed SST, is compared with a present-day (1979–2003) climate experiment. Results of three-member simulations using the 60-km model are presented. A single simulation using the 20-km model is also presented, showing that similar results are obtained. In the future climate experiment, the number of intense cyclones (sea level pressure below 980 hPa) shows a significant increase whereas the number of total cyclones shows a significant decrease, similar to the results obtained from the CMIP3 models themselves. The increase in intense cyclones is seen on the polar side and downstream side of Atlantic and Pacific storm tracks. At the same time, the growth rate of the cyclones increases in areas upstream of these regions. For the regions with the increasing growth rate, a high correlation is seen between the growth rate of the surface cyclones and upper-tropospheric zonal wind at a monthly-mean time scale. Months of high cyclone growth rate with strong zonal wind in these regions become more frequent, and months of low cyclone growth rate with weak zonal wind become less frequent. One of the possibilities that can explain this relationship is changes in the wave-breaking pattern, that is, a decrease in wave breakings in areas of cyclonic shear and an increase in wave breakings in areas of anticyclonic shear. Associated with these changes, rapid cyclone developments are more commonly seen, and weak, long-lived cyclones become less frequent.

Recent global trends in atmospheric fronts

[1] An automated, objective method is used to identify atmospheric fronts in four independent reanalysis data sets for the period 1989–2009 and to calculate changes in their frequency. The analysis highlights several coherent regions of statistically significant change in the frequency of fronts. The front frequency in the North Atlantic storm track has decreased by about 10–20%, whereas changes observed over the Southern Ocean are relatively small. In the subtropical Pacific the front frequency has increased significantly, which is consistent with an expansion of the dry subtropics. The sensitivity of these trends to the detection method is tested and the results are found to be robust. The results provide a concise summary of the recent changes in a major component of synoptic weather conditions, providing a benchmark for climate models as well as an additional tool for interpreting climate change predictions.

Intermittent African Easterly Wave Activity in a Dry Atmospheric Model: Influence of the Extratropics

Abstract A dynamical model is constructed of the northern summertime global circulation, maintained by empirically derived forcing, based on the same dynamical code that has recently been used to study African easterly waves (AEWs) as convectively triggered perturbations (Thorncroft et al.; Leroux and Hall). In the configuration used here, the model faithfully simulates the observed mean distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to investigate the origin and intraseasonal intermittency of AEWs in an artificially dry (no convection) context. A long integration of the model produces a summertime climatology that includes a realistic African easterly jet and westward-propagating 3–5-day disturbances over West Africa. These simulated waves display intraseasonal intermittency as the observed AEWs also do. Further experiments designed to discern the source of this intermittency in the model show that the simulated waves are mainly triggered by dynamical precursors coming from the North Atlantic storm track. The model is at least as sensitive to this remote influence as it is to local triggering by convective heating.

Interdecadal changes in the storm track activity over the North Pacific and North Atlantic

Analysis of NCEP-NCAR I reanalysis data of 1948–2009 and ECMWF ERA-40 reanalysis data of 1958–2001 reveals several significant interdecadal changes in the storm track activity and mean flow-transient eddy interaction in the extratropics of Northern Hemisphere. First, the most remarkable transition in the North Pacific storm track (PST) and the North Atlantic storm track (AST) activities during the boreal cold season (from November to March) occurred around early-to-mid 1970s with the characteristics of global intensification that has been noticed in previous studies. Second, the PST activity in midwinter underwent decadal change from a weak regime in the early 1980s to a strong regime in the late 1980s. Third, during recent decade, the PST intensity has been enhanced in early spring whereas the AST intensity has been weakened in midwinter. Finally, interdecadal change has been also noted in the relationship between the PST and AST activities and between the storm track activity and climate indices. The variability of storm track activity is well correlated with the Pacific Decadal Oscillation and North Atlantic Oscillation prior to the early 1980s, but this relationship has disappeared afterward and a significant linkage between the PST and AST activity has also been decoupled. For a better understanding of the mid-1970s’ shift in storm track activity and mean flow-transient eddy interaction, further investigation is made by analyzing local barotropic and baroclinic energetics. The intensification of global storm track activity after the mid-1970s is mainly associated with the enhancement of mean meridional temperature gradient resulting in favorable condition for baroclinic eddy growth. Consistent with the change in storm track activity, the baroclinic energy conversion is significantly increased in the North Pacific and North Atlantic. The intensification of the PST and AST activity, in turn, helps to reinforce the changes in the middle-to-upper tropospheric circulation but acts to interfere with the changes in the low-tropospheric temperature field.

Autumn atmospheric response to the 2007 low Arctic sea ice extent in coupled ocean–atmosphere hindcasts

The autumn and early winter atmospheric response to the record-low Arctic sea ice extent at the end of summer 2007 is examined in ensemble hindcasts with prescribed sea ice extent, made with the European Centre for Medium-Range Weather Forecasts state-of-the-art coupled ocean–atmosphere seasonal forecast model. Robust, warm anomalies over the Pacific and Siberian sectors of the Arctic, as high as 10°C at the surface, are found in October and November. A regime change occurs by December, characterized by weaker temperatures anomalies extending through the troposphere. Geopotential anomalies extend from the surface up to the stratosphere, associated to deeper Aleutian and Icelandic Lows. While the upper-level jet is weakened and shifted southward over the continents, it is intensified over both oceanic sectors, especially over the Pacific Ocean. On the American and Eurasian continents, intensified surface Highs are associated with anomalous advection of cold (warm) polar air on their eastern (western) sides, bringing cooler temperatures along the Pacific coast of Asia and Northeastern North America. Transient eddy activity is reduced over Eurasia, intensified over the entrance and exit regions of the Pacific and Atlantic storm tracks, in broad qualitative agreement with the upper-level wind anomalies. Potential predictability calculations indicate a strong influence of sea ice upon surface temperatures over the Arctic in autumn, but also along the Pacific coast of Asia in December. When the observed sea ice extent from 2007 is prescribed throughout the autumn, a higher correlation of surface temperatures with meteorological re-analyses is found at high latitudes from October until mid-November. This further emphasises the relevance of sea ice for seasonal forecasting in the Arctic region, in the autumn.

An examination of storm activity in the northeast Atlantic region over the 1851–2003 period using the EMULATE gridded MSLP data series

[1] The gridded mean sea level pressure (MSLP) series developed by the European and North Atlantic Daily to Multidecadal Climate Variability (EMULATE) project is used in this paper to analyze changes in storm activity in the northeast Atlantic region over the period 1851–2003. Zonal and meridional geostrophic wind speed components were calculated for each grid square over the domain 65°W–45°E, 30°N–65°N, and seasonal percentiles from these data were used as a measure of storm activity in the region. Despite the relatively coarse temporal (daily) and spatial (5° × 5°) resolution of the data, the results indicate that the series are able to provide useful information about storm activity across the northeast Atlantic domain back to 1881 and in the data-rich area of the North Sea back to 1851. The variability of winter storm activity observed across the North Sea over the last 153 years, and particularly the increase to high values in the 1990s, is closely associated with the variability of the North Atlantic storm track. In contrast, high summer values in the late nineteenth century appear to have been related to a Greenland-blocking system, leading to high meridional flow in the region. This latter conclusion is limited, however, by poor data coverage in the northwest Atlantic region.

The Basic Ingredients of the North Atlantic Storm Track. Part II: Sea Surface Temperatures

Abstract The impact of North Atlantic SST patterns on the storm track is investigated using a hierarchy of GCM simulations using idealized (aquaplanet) and “semirealistic” boundary conditions in the atmospheric component (HadAM3) of the third climate configuration of the Met Office Unified Model (HadCM3). This framework enables the mechanisms determining the tropospheric response to North Atlantic SST patterns to be examined, both in isolation and in combination with continental-scale landmasses and orography. In isolation, a “Gulf Stream” SST pattern acts to strengthen the downstream storm track while a “North Atlantic Drift” SST pattern weakens it. These changes are consistent with changes in the extratropical SST gradient and near-surface baroclinicity, and each storm-track response is associated with a consistent change in the tropospheric jet structure. Locally enhanced near-surface horizontal wind convergence is found over the warm side of strengthened SST gradients associated with ascending air and increased precipitation, consistent with previous studies. When the combined SST pattern is introduced into the semirealistic framework (including the “North American” continent and the “Rocky Mountains”), the results suggest that the topographically generated southwest–northeast tilt in the North Atlantic storm track is enhanced. In particular, the Gulf Stream shifts the storm track south in the western Atlantic whereas the strong high-latitude SST gradient in the northeastern Atlantic enhances the storm track there.

Interannual teleconnections between the summer North Atlantic Oscillation and the East Asian summer monsoon

[1] Here we present a study of the relationship between July–August (JA) mean climate over China, which is strongly linked to the East Asian summer monsoon (EASM), and the summer (JA) North Atlantic Oscillation (SNAO). The variations of temperature, precipitation, and cloud cover related to the SNAO were analyzed for the period 1951–2002 using gridded data sets as well as instrumental data from 160 stations in China. It was shown that the major patterns of summer climate over China are highly connected with the interannual variation of the SNAO, supporting a teleconnection between the North Atlantic region and East Asia. Based on the analyses of the daily and monthly reanalysis data sets, we propose possible mechanisms of this teleconnection. Changes in the position of the North Atlantic storm tracks and transient eddy activity associated with the positive (negative) SNAO phase contribute downstream to negative (positive) sea level pressure anomalies in northeastern East Asia. In negative SNAO years, a stationary wave pattern is excited from the southern SNAO center over northwestern Europe to northeastern East Asia. However, during positive SNAO years, a stationary wave pattern is excited extending from the SNAO center across the central Eurasian continent at around 40°N and downstream to the southeast. This may explain a connection between the positive SNAO and atmospheric circulation in middle and southeastern China.

Cold-season atmospheric response to the natural variability of the Atlantic meridional overturning circulation

The influence of the natural variability of the Atlantic meridional overturning circulation (AMOC) on the atmosphere is studied in multi-centennial simulations of six global climate models, using Maximum Covariance Analysis (MCA). In all models, a significant but weak influence of the AMOC changes is found during the Northern Hemisphere cold-season, when the ocean leads the atmosphere by a few years. Although the oceanic pattern slightly varies, an intensification of the AMOC is followed in all models by a weak sea level pressure response that resembles a negative phase of the North Atlantic Oscillation (NAO). The signal amplitude is typically 0.5 hPa and explains about 10% of the yearly variability of the NAO in all models. The atmospheric response seems to be due primarily due to an increase of the heat loss along the North Atlantic Current and the subpolar gyre, associated with an AMOC-driven warming. Sea-ice changes appear to be less important. The stronger heating is associated to a southward shift of the lower-tropospheric baroclinicity and a decrease of the eddy activity in the North Atlantic storm track, which is consistent with the equivalent barotropic perturbation resembling the negative phase of the NAO. This study thus provides some evidence of an atmospheric signature of the AMOC in the cold-season, which may have some implications for the decadal predictability of climate in the North Atlantic region.

Climate change projections and stratosphere–troposphere interaction

Climate change is expected to increase winter rainfall and flooding in many extratropical regions as evaporation and precipitation rates increase, storms become more intense and storm tracks move polewards. Here, we show how changes in stratospheric circulation could play a significant role in future climate change in the extratropics through an additional shift in the tropospheric circulation. This shift in the circulation alters climate change in regional winter rainfall by an amount large enough to significantly alter regional climate change projections. The changes are consistent with changes in stratospheric winds inducing a change in the baroclinic eddy growth rate across the depth of the troposphere. A change in mean wind structure and an equatorward shift of the tropospheric storm tracks relative to models with poor stratospheric resolution allows coupling with surface climate. Using the Atlantic storm track as an example, we show how this can double the predicted increase in extreme winter rainfall over Western and Central Europe compared to other current climate projections.

Precipitation concentration changes in Spain 1946–2005

Abstract. An analysis was made of the Precipitation Concentration Index using the new MOPREDAS database of monthly precipitation in Spain (Monthly Precipitation Data base of Spain). The database was compiled after exhaustive quality control of the complete digitalized Spanish Meterological Agency (AEMet) archives and contains a total set of 2670 complete and homogeneous monthly precipitation series from 1946 to 2005. Thus, MOPREDAS currently holds the densest information available for the 1946–2005 period for Spain and ensures a high resolution of results. The Precipitation Concentration Index (PCI) is a powerful indicator of the temporal distribution of precipitation, traditionally applied at annual scales; as the value increases, the more concentrated the precipitation. Furthermore PCI is a part of the well-known Fournier index, with a long tradition on natural system analyses, as for example soil erosion. In this paper, the mean values of annual, seasonal and wet and dry periods of PCI in the conterminous Spain and for two normal periods (1946–1975 and 1976–2005) were studied. Precipitation in Spain follows a general NW-SE spatial pattern during the wet (months) period due to the Atlantic storm track, while during the dry (months) period, it follows a predominantly N-S spatial pattern. As a result, the annual values of PCI combine the two patterns and show a SW-NE PCI gradient. The analyses of the two sub-periods show significant changes in the precipitation occurred in conterminous Spain from 1946 to 2005, and precipitation concentration increased across most of the IP. At an annual scale, PCI increases mostly due to an increase in precipitation concentration during the wet season. At a seasonal scale significant changes were detected between 1945–1975 and 1976–2005, particularly in autumn (increase of PCI values), while changes in winter, spring and summer were mostly localized and not generalized (both increase and decrease). Changes in PCI seem to be complex and appear to be related to global atmospheric features and synoptic and local factors affecting precipitation trends. We discuss the possible explanation linked to the atmospheric pattern and monthly trends and their implications.

Large‐scale circulations and Tibetan Plateau summer drought and wetness in a high‐resolution climate model

AbstractExtratropical and tropical influences on Tibetan Plateau severe and extreme dry and wet summer months are investigated focussing on the large‐scale circulation and using results of the coupled climate model ECHAM5/MPI‐OM. A pre‐industrial control run and scenario runs for the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) are considered. Tibetan Plateau precipitation in months of wetness and drought is related to atmospheric circulation anomalies in the North‐Atlantic/European sector and to sea surface temperature anomalies in the Tropics. Drought on the Tibetan Plateau is associated with a pronounced wave train bridging Eurasia from the North Atlantic to Asia. Increased transient eddy activity in the North Atlantic storm track has a more south‐west to north‐east orientation. This supports a high pressure anomaly over the eastern North Atlantic and Scandinavia which excites a cross Eurasian wave train reducing the moisture inflow to the Tibetan Plateau from the Arabian Sea. A concurrent warming in the tropical Indian Ocean increases the low level monsoonal westerlies deviating the moisture transport from the Bay of Bengal towards the Indochinese Peninsula and the Philippines. Wetness on the Tibetan Plateau is dominated by a cooling in the tropical oceans, whereas atmospheric flow is predominantly zonal in the extratropics of North America and Europe. Thus, moisture inflow can reach the Tibetan Plateau via the Arabian Sea, the Bay of Bengal and the mid‐latitude westerlies. Future scenarios show little change of atmospheric flow composites for wetness and dryness; the Tibetan Plateau droughts increase by 10% for an A1B‐scenario, while extreme wet summer months are reduced by approximately 1%. Copyright © 2010 Royal Meteorological Society

An energetic view of the relation between the Mediterranean storm track and the North Atlantic Oscillation

AbstractThe time tendency equation of eddy kinetic energy (EKE) includes some important forcing terms such as baroclinic conversion (BCC), barotropic conversion (BTC) and energy flux. This article compares the ensemble mean distribution of these forcing terms, as well as the baroclinic generation (BCG) term, in critical positive and critical negative months of the North Atlantic Oscillation (NAO), focusing on the Mediterranean and Middle East regions. Results show that all the computed terms attain larger values and all the extrema are stronger in the positive phase. The energy‐flux vectors point to energy transfer from the west of the Atlantic and north of Europe to the Mediterranean region, with the transfer being weaker in the positive phase. There is energy‐flux divergence over the central region of the Mediterranean Sea, which is more extensive in the positive phase relative to the negative phase. The energy‐flux divergence leads to a radiation source in this region. The direction of the energy‐flux vectors then indicates that the energy radiated from the central Mediterranean region is transferred southeastward, leading to an energy‐flux convergence over the Red Sea and the northeast of Africa. This convergence is stronger and more extensive in the positive phase, which may be responsible for the observed larger value of the upper‐level eddy kinetic energy in the positive phase. The patterns of the BCG and BCC, together with the energy‐flux convergence, suggest that while the Mediterranean storm track is affected by the North Atlantic storm track more strongly in the negative phase it acts as an independent centre of action by radiating energy southeastward in the positive phase. Copyright © 2011 Royal Meteorological Society