Bergen Climate Model Research Articles

Improvement the Largest Lyapunov Exponent for Measurement the Northeast Monsoon Prediction

Thailand is an agricultural country. So that, the water resources are important. The water management is very important for keep the water used in necessary time. The monsoon is causes a heavy rain. So that, the monsoon prediction by the global climate model is important. The accuracy of the forecasts by the predictability measurement method is very important. In this research, the northeast monsoon prediction in Thailand by the global climate model. The data from The Bjerknes Centre for Climate Research (BCCR), University of Bergen, Norway. The global climate model is Bergen Climate Model (BCM) Version 2.0 (BCCR-BCM2.0) of the Intergovernmental Panel on Climate Change (IPCC) and the European Centre for Medium Range Weather Forecast (ECMWF) are used. The largest Lyapunov exponent (LLE) is the predictability measurement method for verify the efficiency of the global climate model and improvement the LLE by limit theorems. The result to show that the improvement the LLE by limit theorems can be measure the accuracy of the northeast monsoon prediction in Thailand by the global climate model are suitable.

Modulation of ENSO evolution by strong tropical volcanic eruptions

The simulated responses of the El Nino–Southern Oscillation (ENSO) to volcanic forcings are controversial, and some mechanisms of these responses are not clear. We investigate the impacts of volcanic forcing on the ENSO using a long-term simulation covering 1400–1999 as simulated by the Bergen Climate Model (BCM) and a group of simulations performed with the Community Atmosphere Model version 4.0 (CAM4) and the BCM’s ocean component Miami Isopycanic Coordinated Ocean Model (MICOM). The analysis of the long-term BCM simulation indicates that ENSO has a negative-positive-negative response to strong tropical volcanic eruptions (SVEs), which corresponds to the different stages of volcanic forcing. In the initial forcing stage, a brief and weak La Nina-like response is caused by the cooling along the west coast of the South American continent and associated enhancement of the trade winds. In the peak forcing stage, westerly wind anomalies are excited by both reduced east–west sea level pressure gradients and weakened and equatorward shifted tropical convergence zones. These westerly wind anomalies extend to the equatorial eastern Pacific, leading to an El Nino-like response. At the same time, easterly wind anomalies west of 120°E and strong cooling effects can promote a discharged thermocline state and excite an upwelling Kelvin wave in the western Pacific. In the declining forcing stage, forced by the recovered trade winds, the upwelling Kelvin wave propagates eastward and reaches the equatorial eastern Pacific. Through the Bjerknes feedback, a strong and temporally extended La Nina-like response forms. Additional CAM4 simulations suggest a more important role of the surface cooling over the Maritime Continent and surrounding ocean in shaping the westerly wind anomalies over the equatorial central-eastern Pacific and the easterly wind anomalies west of 120° E, which are key to causing the El Nino-like responses and subsequent La Nina-like responses, respectively. The MICOM sensitivity simulations confirm that SVE-induced tropical atmospheric circulation anomalies play a dominant role in regulating post-eruption ENSO evolution in the observation, while the influences of anomalous buoyance forcing (heat and freshwater fluxes) are secondary. Therefore, SVEs play an important role in modulating the ENSO evolution. Compared with proxy data, the simulated El Nino-like responses and subsequent La Nina-like responses are consistent with the reconstructed ENSO responses to SVEs. However, the simulated initial brief La Nina-like response, which is reproduced by most models, is seen in only one proxy dataset and is absent in most of the reconstructed ENSOs and those observed. The reason for this model-data mismatch will require further investigation.

Sea-ice free Arctic contributes to the projected warming minimum in the North Atlantic

Projected global warming is not spatially uniform and one of the minima in warming occurs in the North Atlantic (NA). Several models from the Coupled Model Intercomparison Project Phase 5 even projected a slight NA cooling in 2081–2100 relative to 1986–2005. Here we show that, by our simulations performed with the Bergen Climate Model (BCM), an autumn (September to November) sea-ice free Arctic (SIF) contributes to the NA warming minimum by weakening the Atlantic meridional overturning circulation (AMOC). The role of the air–sea interaction in the response to the SIF, which has not been widely discussed in the literature, has been highlighted by the results presented in this study.

The Development of Predictability Measurement Method for Winter Monsoon Forecasts in Thailand

The forecasting method is important because it can predict phenomena in the future. The accuracy of the forecast depends on the model and the initial conditions. In addition, the predictability measurement method is important can be check the accurate of forecasts. In this research is winter monsoon forecasts in Thailand by the shallow water model. The data from The Bjerknes Centre for Climate Research (BCCR), University of Bergen, Norway. The global climate model is Bergen Climate Model (BCM) Version 2.0 (BCCR-BCM2.0) of the Intergovernmental Panel on Climate Change (IPCC) is used. The Lyapunov exponent (LE) is the predictability measurement method for verify the efficiency of model and establish the new predictability measurement method by the time series analysis. The result to show that the new predictability measurement method by the time series analysis can be measure the efficiency of the winter monsoon forecasts in Thailand by the shallow water model for December 2015 to December 2056 are suitable.

The Meridional Pressure Gradient Prediction by the Global Climate Model

The pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The meridional pressure gradient can be prediction for the main forces acting on the air to make it move as wind. The randomly selected cases for this experiment are the Asian northeast monsoon downscaling for December 2049. The global climate model is the Bjerknes Centre for Climate Research (BCCR), University of Bergen, Norway. Bergen Climate Model (BCM) Version 2.0 (BCCR-BCM2.0). The data predictions are the A2 scenario and COMMIT scenario. In this research maximum Lyapunov exponent (MLE) and finite size Lyapunov exponent (FSLE) for every 24-hr interval of the meridional pressure gradient from the BCCR-BCM2.0 model are calculated. The results show that the meridional pressure gradient prediction by the BCCR-BCM2.0 is not sensitive to scenario.

A Novel Spatiotemporal Statistical Downscaling Method for Hourly Rainfall

Finer spatiotemporal resolution rainfall data is essential for assessing hydrological impacts of climate change on medium and small basins. However, existing methods pay less attention to the inter-day correlation and diurnal cycle, which can strongly influence the hydrological cycle. To address this problem, we present a spatiotemporal downscaling method that is capable of reproducing the inter-day correlation, the diurnal cycle, and rainfall statistics on daily and hourly scales. The large-scale datasets, which we obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset (NNR) and general circulation model (GCM) outputs, and local rainfall data are analyzed to assess the impacts of climate change on rainfall. Our proposed method consists of two steps: spatial downscaling and temporal downscaling. We apply spatial downscaling first to obtain the relationship between large-scale datasets and daily rainfall at a site scale using a k-nearest neighbor method (KNN). Then, we conduct an hourly downscaling of daily rainfall in the second step using a genetic algorithm-based KNN (GAKNN) with the inter-day correlation and the diurnal cycle. Furthermore, we analyzed changes in rainfall statistics for the periods 2046–2065 and 2081–2100 under the A2, A1B, and B1 scenarios of the third generation Coupled Global Climate Model (CGCM3.1) and Bergen Climate Model version 2 (BCM2.0). An application of our proposed method to the Shihmen Reservoir basin (Taiwan) has shown that it could accurately reproduce local rainfall and its statistics on daily and hourly scales. Overall, the results demonstrated that the proposed spatiotemporal method is a powerful tool for downscaling hourly rainfall data from a large-scale dataset. The understanding of future changes of rainfall characteristics through our proposed method is also expected to assist the planning and management of water resources systems.

Bjerknes compensation in the Bergen Climate Model

The meridional transport of heat is a critical component of the Earth’s climate system. If the total heat transported by the climate system is approximately constant, then the anomalies of heat transported by the atmosphere and ocean should be approximately equal and opposite, a scenario now called Bjerknes compensation. This has previously been found in two coupled climate models, with both showing multi-decadal variability in the heat transports. This work identifies Bjerknes compensation in the Bergen Climate Model, adding to the understanding of the robust features of Bjerknes compensation in coupled climate models. The atmospheric and oceanic heat transports are investigated in the 600-year control run of a fully-coupled global climate model. The presence of Bjerknes compensation is confirmed by the strong anti-correlation and equal magnitude of the anomalies of these heat transports. The heat transport anomalies contain a signal of multi-decadal variability. Since natural variability in global heat transport could mask anthropogenic climate change signals, understanding Bjerknes compensation is of socio-economic importance. Using regression analysis the atmospheric and oceanic responses to the multi-decadal variability of the Bjerknes compensation signal are investigated. This highlights the importance of the marginal ice zones of the Greenland and Barents Seas as the critical location for coupling the atmosphere and ocean. During periods of increased heat transport in the ocean, these regions show decreased sea-ice, leading to increased fluxes and local temperatures, and giving rise to a thermal low-pressure center and a non-local high-pressure centre, thus changing the atmospheric flow on multi-decadal timescales.

Response of the East Asian Winter Monsoon to Strong Tropical Volcanic Eruptions

Abstract In this study, a 600-yr integration performed with the Bergen Climate Model (BCM), version 2.0, was used to investigate the impact of strong tropical volcanic eruptions (SVEs) on the East Asian winter monsoon (EAWM). It is found that SVEs have an important influence on the East Asian winter climate. The volcanic forcing can cause changes in surface heat fluxes and tropospheric circulation, particularly over the tropics and high-latitude regions. As a result, Arctic Oscillation enters into its positive phase in the first two winters after SVEs. The associated circulation weakens the Siberian high and reduces the cold air transport to East Asia, which is not conducive to the enhancement of the EAWM during this period. At the same time, the North Pacific Ocean gradually cools and shapes La Niña–like sea surface temperature (SST) anomalies in the third winter after SVEs. The Walker circulation is strengthened over the Pacific, and two anomalous lower-tropospheric cyclones are located over the South China Sea and southeast of Japan, respectively. Therefore, related northeasterly wind anomalies appear along the East Asian coast, indicating a strengthened EAWM during this period. Meanwhile, the enhanced Siberian high and East Asian trough further contribute to the enhancement of EAWM in the third winter. It is therefore concluded that the SVEs-induced climate changes over the tropical Pacific and north polar regions play an important role in regulating the EAWM in the posteruption winters.

On the reconstruction of ocean circulation and climate based on the “Gardar Drift”

AbstractSediment‐based reconstructions of bottom water velocity at the Gardar Drift are commonly interpreted to reflect changes in the eastern Nordic Seas overflows. Here we investigate the relationship between changes in the water that overflows through the Faroe Shetland Channel and downstream bottom velocity at the location of the Gardar Drift as represented in a 500 year long simulation with the Bergen Climate Model. We identify a region in our simulation proximal to the geographical location of the northern Gardar Drift where 76% of the variance in bottom velocity can be explained by changes in the volume transport and density of the Faroe Shetland Channel overflow. By contrast, Labrador Sea Water changes do not appear to play a significant role in the bottom flow over the Gardar Drift. Our findings support the assumption in the paleo literature that reconstructions of bottom water velocity at the Gardar Drift reflect past changes in the eastern Nordic Seas overflows. However, our results suggest that velocity changes downstream are not a simple metric for the strength of the overflow, rather overflow density plays the largest role.

Modelled and observed sea surface temperature trends for the Caribbean and Antilles

The ocean occupies 95% of the Caribbean's area and plays a leading role in the region's climate, thus making the sea surface temperature (SST) a very important regional climate index. This, in conjunction with the lack of a regionally consistent, quality-controlled surface temperature dataset increases the scientific value of using SST to characterize the regional climate and its trends. This study determines the magnitudes of the long-term SST trends in the Wider Caribbean (WC) and the Antilles. We overcome the presence of discontinuity points in the SST time series using the change point statistical technique. Annual mean SST trends combining the subperiods 1906–1969 and 1972–2005 are 1.32 ± 0.41 °C per century for the Antilles and 1.08 ± 0.32 °C per century for the WC. For the same regions during the subperiod 1972–2005, the corresponding trends are 1.41 ± 0.68 °C per century and 1.18 ± 0.49 °C per century, illustrating the warming intensification during the last four decades. A significant correlation is found between the SSTs in the Caribbean and Atlantic Multidecadal Oscillation (AMO) index, suggesting a potential link between Caribbean SSTs and the mechanisms governing the Atlantic basin-wide SSTs. Finally, the capabilities of two state-of-the-art coupled climate models, the Norwegian Earth System Model (NorESM1-M) and the Bergen Climate Model (BCM), to simulate SST in the Caribbean were tested. Both models produce an adequate simulation of the annual mean SST anomalies and SST seasonal cycle for the WC and the Antilles. The simulated annual and monthly mean SSTs are colder in the two models compared to the observations, a common feature among the majority of general circulation models participating in the Coupled Model Intercomparison Project Phase 5. However, despite these minor deficiencies both BCM and NorESM1-M are considered adequate for conducting SST simulations relevant for future climate change research in the Caribbean.

The response of the East Asian summer monsoon to strong tropical volcanic eruptions

A 600-year integration performed with the Bergen Climate Model and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data were used to investigate the impact of strong tropical volcanic eruptions on the East Asian summer monsoon (EASM) and EASM rainfall. Both the simulation and NCEP/NCAR reanalysis data show a weakening of the EASM in strong eruption years. The model simulation suggests that North and South China experience droughts and the Yangtze-Huaihe River Valley experiences floods during eruption years. In response to strong tropical volcanic eruptions, the meridional air temperature gradient in the upper troposphere is enhanced, which leads to a southward shift and an increase of the East Asian subtropical westerly jet stream (EASWJ). At the same time, the land-sea thermal contrast between the Asian land mass and Northwest Pacific Ocean is weakened. The southward shift and increase of the EASWJ and reduction of the land-sea thermal contrast all contribute to a weakening of the EASM and EASM rainfall anomaly.

Modulation of the Pacific Decadal Oscillation on the summer precipitation over East China: a comparison of observations to 600-years control run of Bergen Climate Model

Observations show that the summer precipitation over East China often goes through decadal variations of opposite sign over North China and the Yangtze River valley (YRV), such as the “southern flood and northern drought” pattern that occurred during the late 1970s–1990s. In this study it is shown that a modulation of the Pacific Decadal Oscillation (PDO) on the summer precipitation pattern over East China during the last century is partly responsible for this characteristic precipitation pattern. During positive PDO phases, the warm winter sea surface temperatures (SSTs) in the eastern subtropical Pacific along the western coast of North American propagate to the tropics in the following summer due to weakened oceanic meridional circulation and the existence of a coupled wind–evaporation–SST feedback mechanism, resulting in a warming in the eastern tropical Pacific Ocean (5°N–20°N, 160°W–120°W) in summer. This in turn causes a zonal anomalous circulation over the subtropical–tropical Pacific Ocean that induces a strengthened western Pacific subtropical high (WPSH) and thus more moisture over the YRV region. The end result of these events is that the summer precipitation is increased over the YRV region while it is decreased over North China. The suggested mechanism is found both in the observations and in a 600-years fully coupled pre-industrial multi-century control simulations with Bergen Climate Model. The intensification of the WPSH due to the warming in the eastern tropical Pacific Ocean was also examined in idealized SSTA-forced AGCM experiments.

Role of natural external forcing factors in modulating the Indian summer monsoon rainfall, the winter North Atlantic Oscillation and their relationship on inter-decadal timescale

We use reconstructed data and multi-centennial integrations performed with the Bergen Climate Model Version 2 to investigate the impact of natural external forcing factors on the Indian summer monsoon (ISM) rainfall, the winter North Atlantic Oscillation (NAO), and the potential relationship between the ISM rainfall and the winter NAO on decadal to inter-decadal timescales. The model simulations include a 600-year control integration (CTL600) and a 600-year integration with time-varied natural external forcing factors from 1400 to 1999 (EXT600). Both reconstructed data and the simulation showed increased ISM rainfall 2–3 years after strong volcanic eruptions. Strong volcanic eruptions decrease the Indian Ocean sea surface temperature (SST), which increases the strength of the southwesterly winds over the Arabian Sea. With negative externally-forced radiative anomaly, the lower stratospheric pole-to-equator winter temperature gradient is enhanced, leading to a positive winter NAO anomaly with a time lag of 1 year. There is no significant correlation between the winter NAO and ISM rainfall in CTL600. However, the ISM rainfall is significantly positively correlated with the winter NAO in EXT600, with the NAO leading by 2–4 years, which is consistent with the NAO–ISM rainfall relationship in the reconstructed data. We suggest that natural external forcing factors regulate the inter-decadal variability of both the winter NAO and the ISM rainfall and thus likely lead to an increased statistical but not causal relationship between them on the inter-decadal timescale over the past centuries.

External forcing of the early 20th century Arctic warming

The observed Arctic warming during the early 20th century was comparable to present-day warming in terms of magnitude. The causes and mechanisms for the early 20th century Arctic warming are less clear and need to be better understood when considering projections of future climate change in the Arctic. The simulations using the Bergen Climate Model (BCM) can reproduce the surface air temperature (SAT) fluctuations in the Arctic during the 20th century reasonably well. The results presented here, based on the model simulations and observations, indicate that intensified solar radiation and a lull in volcanic activity during the 1920s–1950s can explain much of the early 20th century Arctic warming. The anthropogenic forcing could play a role in getting the timing of the peak warming correct. According to the model the local solar irradiation changes play a crucial role in driving the Arctic early 20th century warming. The SAT co-varied closely with local solar irradiation changes when natural external forcings are included in the model either alone or in combination with anthropogenic external forcings. The increased Barents Sea warm inflow and the anomalous atmosphere circulation patterns in the northern Europe and north Atlantic can also contribute to the warming. In summary, the early 20th century warming was largely externally forced.

Weakening AMOC connects Equatorial Atlantic and Pacific interannual variability

Observations indicate that since the 1970s Equatorial Atlantic sea surface temperature (SST) variations in boreal summer tend to modulate El Niño in the following seasons, indicating that the Atlantic Ocean can have importance for predicting the El Niño–Southern Oscillation (ENSO). The cause of the change in the recent decades remains unknown. Here we show that in the Bergen Climate Model (BCM), a freshwater forced weakening of the Atlantic meridional overturning circulation (AMOC) results in a strengthening of the relation between the Atlantic and the Pacific similar to that observed since the 1970s. During the weakening AMOC phase, SST and precipitation increase in the central Equatorial Atlantic, while the mean state of the Pacific does not change significantly. In the Equatorial Atlantic the SST variability has also increased, with a peak in variability in boreal summer. In addition, the characteristic timescales of ENSO variability is shifted towards higher frequencies. The BCM version used here is flux-adjusted, and hence Atlantic variability is realistic in contrast to in many other models. These results indicate that in the BCM a weakening AMOC can change the mean background state of the Tropical Atlantic surface conditions, enhancing Equatorial Atlantic variability, and resulting in a stronger relationship between the Tropical Atlantic and Pacific Oceans. This in turn alters the variability in the Pacific.

Heat-related mortality risk model for climate change impact projection

We previously developed a model for projection of heat-related mortality attributable to climate change. The objective of this paper is to improve the fit and precision of and examine the robustness of the model. We obtained daily data for number of deaths and maximum temperature from respective governmental organizations of Japan, Korea, Taiwan, the USA, and European countries. For future projection, we used the Bergen climate model2 (BCM2) general circulation model, the Special Report on Emissions Scenarios (SRES) A1B socioeconomic scenario, and the mortality projection for the 65+-year-old age group developed by the World Health Organization (WHO). The heat-related excess mortality was defined as follows: The temperature-mortality relation forms a V-shaped curve, and the temperature at which mortality becomes lowest is called the optimum temperature (OT). The difference in mortality between the OT and a temperature beyond the OT is the excess mortality. To develop the model for projection, we used Japanese 47-prefecture data from 1972 to 2008. Using a distributed lag nonlinear model (two-dimensional nonparametric regression of temperature and its lag effect), we included the lag effect of temperature up to 15days, and created a risk function curve on which the projection is based. As an example, we perform a future projection using the above-mentioned risk function. In the projection, we used 1961-1990 temperature as the baseline, and temperatures in the 2030s and 2050s were projected using the BCM2 global circulation model, SRES A1B scenario, and WHO-provided annual mortality. Here, we used the "counterfactual method" to evaluate the climate change impact; For example, baseline temperature and 2030 mortality were used to determine the baseline excess, and compared with the 2030 excess, for which we used 2030 temperature and 2030 mortality. In terms of adaptation to warmer climate, we assumed 0% adaptation when the OT as of the current climate is used and 100% adaptation when the OT as of the future climate is used. The midpoint of the OTs of the two types of adaptation was set to be the OT for 50% adaptation. We calculated heat-related excess mortality for 2030 and 2050. Our new model is considered to be better fit, and more precise and robust compared with the previous model.

Interannual linkage between Arctic/North Atlantic Oscillation and tropical Indian Ocean precipitation during boreal winter

In the study authors analyzed the interannual relationship between the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) and the tropical Indian Ocean (TIO) precipitation in boreal winter for the period 1979–2009. A significant simultaneous teleconnection between them is found. After removing the El Nino/Southern Oscillation and Indian Ocean dipole signals, the AO/NAO and the TIO precipitation (0°–10°S, 60°–80°E) yield a correlation of +0.56, which is also consistent with the AO/NAO-outgoing longwave radiation correlation of −0.61. The atmospheric and oceanic features in association with the AO/NAO-precipitation links are investigated. During positive AO/NAO winter, the Rossby wave guided by westerlies tends to trigger persistent positive geopotential heights in upper troposphere over about 20°–30°N and 55°–70°E, which is accompanied by a stronger Middle East jet stream. Meanwhile, there are anomalous downward air motions, strengthening the air pressure in mid-lower troposphere. The enhanced Arabian High brings anomalous northern winds over the northern Indian Ocean. As a result the anomalous crossing-equator air-flow enhances the intertropical convergence zone (ITCZ). On the other hand, the anomalous Ekman transport convergence by the wind stress curl over the central TIO deepens the thermocline. Both the enhanced ITCZ and the anomalous upper ocean heat content favor in situ precipitation in the central TIO. The AO/NAO-TIO precipitation co-variations in the IPCC AR4 historical climate simulation (1850–1999) of Bergen Climate Model version 2 were investigated. The Indian Ocean precipitation anomalies (particularly the convective precipitation along the ITCZ), in conjunction with the corresponding surface winds and 200 hPa anticyclonic atmospheric circulation and upper ocean heat contents were well reproduced in simulation. The similarity between the observation and simulation support the physical robustness of the AO/NAO-TIO precipitation links.

Effect of Climate Change on Runoff Generation: Application to Rift Valley Lakes Basin of Ethiopia

AbstractIn this paper, an attempt has been made to investigate the potential impact of climate change on runoff generation at two agricultural watersheds. Climate change and key future signals of its variability were assessed using general circulation models (GCMs). Given that GCMs are operating at coarser resolution, the statistical downscaling model was applied to reduce large-scale atmospheric variables into localized weather variables from the Bjerknes Center for Climate Research–Bergen Climate Model 2.0 and Commonwealth Scientific and Industrial Research Organization (CSIRO) Mark (MK) 3.0 GCM outputs. As precipitation variables are composed of biases, both linear and power transformation bias correction methods were applied to obtain bias-corrected daily precipitation. Bias-corrected daily precipitation and temperature variables were used to simulate runoff for current and future climate scenarios using the Soil and Water Assessment Tool (SWAT) model. The statistical downscaling model, followed by bi...

The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions

In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600 years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation. At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.

Arctic/Atlantic Exchanges via the Subpolar Gyre*

Abstract In the present study the decadal variability in the strength and shape of the subpolar gyre (SPG) in a 600-yr preindustrial simulation using the Bergen Climate Model is investigated. The atmospheric influence on the SPG strength is reflected in the variability of Labrador Sea Water (LSW), which is largely controlled by the North Atlantic Oscillation, the first mode of the North Atlantic atmospheric variability. A combination of the amount of LSW, the overflows from the Nordic seas, and the second mode of atmospheric variability, the East Atlantic Pattern, explains 44% of the modeled decadal variability in the SPG strength. A prior increase in these components leads to an intensified SPG in the western subpolar region. Typically, an increase of one standard deviation (std dev) of the total overflow (1 std dev = 0.2 Sv; 1 Sv ≡ 106 m3 s−1) corresponds to an intensification of about one-half std dev of the SPG strength (1 std dev = 2 Sv). A similar response is found for an increase of one std dev in the amount of LSW, and simultaneously the strength of the North Atlantic Current increases by one-half std dev (1 std dev = 0.9 Sv).