Abstract

Sensitivity of climate drift to selected convection and cloudiness parameters is investigated with a coupled ocean–atmosphere general circulation model. The dependence of the coupled model climatology upon parameterizations of convective entrainment and stratocumulus cloud cover is studied. The methodology relies upon short uncoupled (1 yr) and coupled (3 yr) simulations. The coupled model climatology is very sensitive to both parameterizations. For instance, the air–sea interface mean state can be too warm or too cold depending on the profile of the convective entrainment rate. Enhanced entrainment at lower levels breaks the symmetry of the tropical precipitation pattern observed in both forced and coupled control simulations. Furthermore, the zonal wind stress strength and related thermocline slope around 150°W are shown to be crucial in determining the warm pool–cold tongue structure in the tropical Pacific. The model sensitivity is found to be the result of complex feedbacks between convection, cloud, and boundary layer processes, sea surface temperature (SST), and large-scale ocean–atmosphere dynamics.

Abstract

The relationship between global sea surface temperatures (SSTs) and the North Atlantic–Europe (NAE) atmospheric circulation is investigated using an ensemble of eight simulations with the ARPEGE atmospheric global circulation model forced with prescribed SSTs over the 1948–97 period. The model mean state is first validated against NCEP reanalyses. The interannual SST-forced variability is then compared to the internal one using analysis of variance (ANOVA) techniques. Both components are maximum in winter over the Northern Hemisphere and the associated potential predictability shows weak but significant values located over the Icelandic low (IL) and the Azores high (AH).

The North Atlantic oscillation (NAO) is found to be the leading internal variability mode over the NAE sector as shown by principal component analysis of a control simulation with climatological SSTs. The noise imprint dominates the forced response estimated from the ensemble mean. The latter is related first to the El Niño–Southern Oscillation (ENSO) activity. During warm (cold) events in the Pacific, the AH shows negative (positive) pressure anomalies and weakened opposition with the IL. The AH fluctuations exhibit a 3.7-yr peak and result from changes in the activity of the Atlantic Hadley cell and from the eastward extension of the Pacific North America teleconnection pattern. Eliassen–Palm diagnostics show that eddy–mean flow interaction acts to maintain the anomalous Atlantic stationary wave pattern as described by Fraedrich in a review based on observational results. The simulated ENSO–NAE connection is, however, too strong in the model and this dominance may be related to the simulated mean state biases. Second, the North Atlantic atmospheric forced signal is associated with the Atlantic SSTs. A tripole structure over the North Atlantic basin with maximum loading in its tropical branch is linked to the phase of the simulated NAO. A local Hadley cell mechanism associated with Rossby wave excitation over the Atlantic is suggested to explain tropical–midlatitude interactions in the model.

Abstract

In this study, the ENSO teleconnection with the tropical North Atlantic (TNA) sea surface temperatures (SSTs) in boreal spring is analyzed in ocean–atmosphere coupled global circulation models. To assess the role played by horizontal resolution of models on this teleconnection, we used a multimodel dataset that is the first to combine models with both low and high resolution. The TNA response to ENSO projects onto the most significant SST mode of the tropical Atlantic at interannual time scales, the Atlantic meridional mode (AMM). Its evolution is primarily driven by the wind–evaporation–SST (WES) feedback, which in turn is based on the development of an initial SST gradient. This study examines and quantifies the relative contribution of a dynamic-related (upwelling) and a thermodynamic-related (evaporation) process in triggering this gradient in the case of the ENSO–TNA teleconnection. While no major contribution is found with the evaporation, a consistent contribution from the coastal upwelling off northwest Africa is identified. This contribution is enhanced in high-resolution models and highlights the close link between the upwelling in winter and the development of the AMM in spring. It is further shown that high-resolution models present a thinner and more realistic ocean mixed layer within the upwelling area, which enhances the effect of surface winds on upwelling and SSTs. As a consequence, high-resolution models are more sensitive than low-resolution models to surface wind errors, thereby they do not ensure improved reliability or predictability of the TNA SST response to ENSO.

Abstract

This study elucidates the physical mechanisms underlying internal and forced components of winter surface air temperature (SAT) trends over North America during the past 50 years (1963–2012) using a combined observational and modeling framework. The modeling framework consists of 30 simulations with the Community Earth System Model (CESM) at 1° latitude–longitude resolution, each of which is subject to an identical scenario of historical radiative forcing but starts from a slightly different atmospheric state. Hence, any spread within the ensemble results from unpredictable internal variability superimposed upon the forced climate change signal. Constructed atmospheric circulation analogs are used to estimate the dynamical contribution to forced and internal components of SAT trends: thermodynamic contributions are obtained as a residual. Internal circulation trends are estimated to account for approximately one-third of the observed wintertime warming trend over North America and more than half locally over parts of Canada and the United States. Removing the effects of internal atmospheric circulation variability narrows the spread of SAT trends within the CESM ensemble and brings the observed trends closer to the model’s radiatively forced response. In addition, removing internal dynamics approximately doubles the signal-to-noise ratio of the simulated SAT trends and substantially advances the “time of emergence” of the forced component of SAT anomalies. The methodological framework proposed here provides a general template for improving physical understanding and interpretation of observed and simulated climate trends worldwide and may help to reconcile the diversity of SAT trends across the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5).

Abstract

Diagnostics combining atmospheric reanalysis and station-based temperature data for 1950–2003 indicate that European heat waves can be associated with the occurrence of two specific summertime atmospheric circulation regimes. Evidence is presented that during the record warm summer of 2003, the excitation of these two regimes was significantly favored by the anomalous tropical Atlantic heating related to wetter-than-average conditions in both the Caribbean basin and the Sahel. Given the persistence of tropical Atlantic climate anomalies, their seasonality, and their associated predictability, the suggested tropical–extratropical Atlantic connection is encouraging for the prospects of long-range forecasting of extreme weather in Europe.

Abstract

Time of emergence of anthropogenic climate change is a crucial metric in risk assessments surrounding future climate predictions. However, internal climate variability impairs the ability to make accurate statements about when climate change emerges from a background reference state. None of the existing efforts to explore uncertainties in time of emergence has explicitly explored the role of internal atmospheric circulation variability. Here a dynamical adjustment method based on constructed circulation analogs is used to provide new estimates of time of emergence of anthropogenic warming over North America and Europe from both a local and spatially aggregated perspective. After removing the effects of internal atmospheric circulation variability, the emergence of anthropogenic warming occurs on average two decades earlier in winter and one decade earlier in summer over North America and Europe. Dynamical adjustment increases the percentage of land area over which warming has emerged by about 30% and 15% in winter (10% and 5% in summer) over North America and Europe, respectively. Using a large ensemble of simulations with a climate model, evidence is provided that thermodynamic factors related to variations in snow cover, sea ice, and soil moisture are important drivers of the remaining uncertainty in time of emergence. Model biases in variability lead to an underestimation (13%–22% over North America and <5% over Europe) of the land fraction emerged by 2010 in summer, indicating that the forced warming signal emerges earlier in observations than suggested by models. The results herein illustrate opportunities for future detection and attribution studies to improve physical understanding by explicitly accounting for internal atmospheric circulation variability.

Abstract

The observed low-frequency winter atmospheric variability of the North Atlantic–European region and its relationship with global surface oceanic conditions is investigated based on the climate and weather regimes paradigm.

Asymmetries between the two phases of the North Atlantic Oscillation (NAO) are found in the position of the Azores high and, to a weaker extent, the Icelandic low. There is a significant eastward displacement or expansion toward Europe for the NAO+ climate regime compared to the NAO− regime. This barotropic signal is found in different datasets and for two quasi-independent periods of record (1900–60 and 1950–2001); hence, it appears to be intrinsic to the NAO+ phase. Strong spatial similarities between weather and climate regimes suggest that the latter, representing long time scale variability, can be interpreted as the time-averaging signature of much shorter time scale processes. Model results from the ARPEGE atmospheric general circulation model are used to validate observed findings. They confirm in particular the eastward shift of the Atlantic centers of action for the NAO+ phase and strongly suggest a synoptic origin as it can be extracted from daily analyses. These results bring together present-day climate variability and scenario studies where such an NAO shift was suggested, as it is shown that the last three decades are clearly dominated by the occurrence of NAO+ regimes when concentrations of greenhouse gases are rapidly increasing. These findings highlight that the displacement of the North Atlantic centers of action should be treated as a dynamical property of the North Atlantic atmosphere and not as a mean longitudinal shift of climatological entities in response to anthropogenic forcings.

The nonstationarity with time of the atmospheric variability is documented. Late-century decades differ from early ones by the predominance of NAO climate regimes versus others. In such a context, comments on the relevance of the station-based NAO index is provided. Both tropical and extratropical sea surface temperature (SST) anomalies alter the frequency distribution of the North Atlantic regimes. Evidence is presented that the so-called ridge regime is preferably excited during La Niña events, while the NAO regimes are associated with the North Atlantic SST tripole. The ARPEGE model results indicate that the tropical branch of the SST tripole affects the NAO regimes occurrence. Warm tropical SST anomalies are more efficient at exciting NAO− regimes than cold anomalies are at forcing NAO+ regimes. The extratropical portion of the North Atlantic SST tripole also seems to play a significant role in the model, tending to counteract the dominant influence of the tropical Atlantic basin on NAO regimes.

Abstract

The Northern Hemisphere transient atmospheric response to Arctic sea decline is investigated in autumn and winter, using sensitivity experiments performed with the CNRM-CM6-1 high-top climate model. Arctic sea ice albedo is reduced to the ocean value, yielding ice-free conditions during summer and a more moderate sea ice reduction during the following months. A strong amplification of temperatures over the Arctic is induced by sea ice loss, with values reaching up to 25°C near the surface in autumn. Significant surface temperature anomalies are also found over the midlatitudes, with a warming reaching 1°C over North America and Europe, and a cooling reaching 1°C over central Asia. Using a dynamical adjustment method based on a regional reconstruction of circulation analogs, we show that the warming over North America and Europe can be explained both by changes in the atmospheric circulation and by the advection of warmer oceanic air by the climatological flow. In contrast, we demonstrate that the sea ice–induced cooling over central Asia is solely due to dynamical changes, involving an intensification of the Siberian high and a cyclonic anomaly over the Sea of Okhotsk. In the troposphere, the abrupt Arctic sea ice decline favors a narrowing of the subtropical jet stream and a slight weakening of the lower part of the polar vortex that is explained by a weak enhancement of upward wave activity toward the stratosphere. We further show that reduced Arctic sea ice in our experiments is mainly associated with less severe cold extremes in the midlatitudes.

Abstract

This study investigates the origin and features of interannual–decadal Atlantic meridional overturning circulation (AMOC) variability from several ocean simulations, including a large (50 member) ensemble of global, eddy-permitting (1/4°) ocean–sea ice hindcasts. After an initial stochastic perturbation, each member is driven by the same realistic atmospheric forcing over 1960–2015. The magnitude, spatiotemporal scales, and patterns of both the atmospherically forced and intrinsic–chaotic interannual AMOC variability are then characterized from the ensemble mean and ensemble spread, respectively. The analysis of the ensemble-mean variability shows that the AMOC fluctuations north of 40°N are largely driven by the atmospheric variability, which forces meridionally coherent fluctuations reaching decadal time scales. The amplitude of the intrinsic interannual AMOC variability never exceeds the atmospherically forced contribution in the Atlantic basin, but it reaches up to 100% of the latter around 35°S and 60% in the Northern Hemisphere midlatitudes. The intrinsic AMOC variability exhibits a large-scale meridional coherence, especially south of 25°N. An EOF analysis over the basin shows two large-scale leading modes that together explain 60% of the interannual intrinsic variability. The first mode is likely excited by intrinsic oceanic processes at the southern end of the basin and affects latitudes up to 40°N; the second mode is mostly restricted to, and excited within, the Northern Hemisphere midlatitudes. These features of the intrinsic, chaotic variability (intensity, patterns, and random phase) are barely sensitive to the atmospheric evolution, and they strongly resemble the “pure intrinsic” interannual AMOC variability that emerges in climatological simulations under repeated seasonal-cycle forcing. These results raise questions about the attribution of observed and simulated AMOC signals and about the possible impact of intrinsic signals on the atmosphere.

Abstract

The link between the changes in equatorial background stratification and El Niño–Southern Oscillation (ENSO) modulation is investigated using a simulation from a 260-yr-long coupled general circulation model (CGCM). The work focuses on the role of nonlinearities associated with equatorial wave dynamics. As a first step, the low-frequency change in mean stratification is diagnosed and documented from the shallow-water parameters derived from a vertical mode decomposition of the CGCM. The parameters vary differently according to the baroclinic mode order, which may explain why a flattening thermocline does not necessarily lead to reduced ENSO activity. Estimations of baroclinic mode contributions to zonal current anomalies indicate that the decadal variability projects differently for the baroclinic modes as compared to the interannual variability. In particular, the high-order modes associated with decadal variability have a more pronounced signature in the western Pacific, whereas that associated with interannual variability (i.e., ENSO) shows more energy in the eastern Pacific.

In the light of the results of the CGCM vertical mode decomposition, an intermediate coupled model (ICM) is used to test whether the nonlinearities associated with the changes in the baroclinic mode energy distribution can lead to coherent ENSO modulation. The results indicate that rectification of the interannual variability (ENSO time scales) by the interdecadal variability associated with changes in the oceanic mean states takes place in the ICM. The rectified effect results mostly in an increased variability and skewness of the zonal advection, which tends to produce a zonal seesaw of the sea surface temperature anomaly. A tropical mechanism for producing ENSO modulation is then proposed that reconciles both the rectified effect resulting from nonlinearities associated with equatorial wave dynamics and the tropical decadal mode of thermocline depth arising from Ekman-pumping anomalies located in the central South Pacific.