North Atlantic Decadal Variability Research Articles

The Key Role of Cloud–Climate Coupling in Extratropical Sea Surface Temperature Variability

Abstract Cloud radiative effects have long been known to play a key role in governing the mean climate. In recent years, it has become clear that they also contribute to climate variability in the tropics. Here we build on recent work and probe the role of cloud radiative effects in extratropical sea surface temperature (SST) variability. The impact of cloud radiative effects on climate variability is explored in “cloud-locking” simulations run on an Earth System Model. The method involves comparing the output from two climate simulations: one in which clouds are coupled to atmospheric dynamic and thermodynamic processes, and another in which clouds are prescribed and thus decoupled from them. The results reveal that cloud–climate coupling leads to widespread increases in the amplitudes of extratropical SST variability from monthly to decadal time scales. Notably, it leads to ∼40%–100% increases in the amplitude of monthly to decadal variability over both the North Atlantic and North Pacific Oceans. These increases are consistent with the “reddening” of cloud shortwave radiative effects that arises when clouds respond to the dynamic and thermodynamic state of the atmosphere. The results suggest that a notable fraction of observed Northern Hemisphere SST variability—including that associated with North Pacific and North Atlantic decadal variability—is due to cloud–climate coupling.

The Role of Eddies in the North Atlantic Decadal Variability

The role of eddies in the North Atlantic decadal variability is investigated in this study by using two ocean reanalyses, including an eddy permitting (or eddy poor) reanalysis with horizontal resolution of 0.25 degree and 75 vertical levels and an eddy resolving (or eddy rich) reanalysis with horizontal resolution of 1/12 degree and 50 vertical levels. The prominent mid-1990s warming and post-2005 cooling trend as part of the North Atlantic decadal variability is well displayed in both reanalyses with no significant difference between them. The main driver of the mid-1990s warming and post-2005 cooling trend is the increase and reduction of the meridional ocean heat transport showing similar patterns in both reanalyses. The relative contribution of the heat transport anomalies from eddies to the total heat transport anomalies is slightly larger in eddy resolving than in eddy permitting ocean reanalysis. However, the total mean ocean meridional heat transport increases by 10% in eddy resolving reanalysis with respect to eddy permitting reanalysis and is mainly due to the associated increase of the mean states (temperature and velocity). Therefore, the increase of eddy population due to the increase of horizontal resolution, found by comparing the two datasets, does not affect the MHT anomalies significantly and, consequently, the North Atlantic decadal variability. It is found that the importance of the model horizontal resolution for the North Atlantic decadal variability depends on the interaction between the eddies (small scale) and the mean state (large scale) at decadal time scales. Although the fast increase of computational power will allow soon for eddy-resolving predictions, the need to use high resolution modeling tools for decadal predictions depends on the importance of initialization methods and the interaction between small scale and large-scale variabilities. This study has pivotal implications for the development of North Atlantic decadal prediction systems.

Response of North Pacific and North Atlantic decadal variability to weak global warming

The Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Variability (AMV) are the two dominant low-frequency modes in the climate system. This research focused on the response of these two modes under weak global warming. Observational data were derived from the Hadley Center Sea Ice and Sea Surface Temperature dataset (HadISST) and coupled model outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Changes in PDO and AMV were examined using four models (bcc-csm1-1, CCSM4, IPSL-CM5A-LR, and MPI-ESM-LR) with long weak global warming scenarios (RCP2.6). These models captured the two low-frequency modes in both pre-industrial run and RCP2.6 run. Under weak global warming, the time scales of PDO and AMV significantly decreased while the amplitude only slightly decreased. Interestingly, the standard deviation of the North Pacific sea surface temperature anomaly (SSTA) decreased only in decadal time scale, and that of the North Atlantic SSTA decreased both in interannual and decadal time scales. The coupled system consists of a slow ocean component, which has a decadal time scale, and a fast atmospheric component, which is calculated by subtracting the decadal from the total. Results suggest that under global warming, PDO change is dominated by ocean dynamics, and AMV change is dominated by ocean dynamics and stochastic atmosphere forcing.

Exploring the impact of CMIP5 model biases on the simulation of North Atlantic decadal variability

AbstractInstrumental observations, paleoproxies, and climate models suggest significant decadal variability within the North Atlantic subpolar gyre (NASPG). However, a poorly sampled observational record and a diversity of model behaviors mean that the precise nature and mechanisms of this variability are unclear. Here we analyze an exceptionally large multimodel ensemble of 42 present‐generation climate models to test whether NASPG mean state biases systematically affect the representation of decadal variability. Temperature and salinity biases in the Labrador Sea covary and influence whether density variability is controlled by temperature or salinity variations. Ocean horizontal resolution is a good predictor of the biases and the location of the dominant dynamical feedbacks within the NASPG. However, we find no link to the spectral characteristics of the variability. Our results suggest that the mean state and mechanisms of variability within the NASPG are not independent. This represents an important caveat for decadal predictions using anomaly assimilation methods.

Decadal climate predictions for the period 1901-2010 with a coupled climate model

An ensemble of yearly initialized decadal predictions is performed with the Max Planck Institute Earth System Model to examine the forecast skill for the period from 1901 to 2010. Compared to the more recent period (1960 to present day), the extended period leads to an enlargement of regions with significant anomaly correlation coefficients (ACC) for predicted surface temperatures. This arises from an increased contribution of the trend, which is also found in the uninitialized runs. Additionally, in the North Atlantic decadal variability plays a larger role over the extended period, with detrended time series showing higher ACC for the extended compared to the short period. Furthermore, in contrast to the uninitialized simulations, the initialized predictions capture the North Atlantic warming events during the 1920s and 1990s, together with some of the surface climate impacts including warm European summer temperatures and a northward shift of Atlantic tropical rainfall.

On the Structure and Teleconnections of North Atlantic Decadal Variability

Abstract Decadal variability in the North Atlantic has been associated in the literature with a tripolar pattern of sea surface temperature (SST) anomalies that show one sign in the western midlatitudinal North Atlantic and the opposite in the subpolar and tropical North Atlantic. The present analysis of observed SST from 1870 to 2009 leads to the dissection of the SST tripole into two components, each with a different time scale in the decadal band and different teleconnections in the Atlantic basin; while the subpolar and tropical poles present quasi-decadal variations with a period of about 9 years, essentially uncorrelated with other parts of the basin, the center of action in the western midlatitudes is characterized by a longer time scale of about 14 years and significant correlations with the tropical South Atlantic and the Norwegian and North Sea(s). The 9-yr period variations are associated with an atmospheric configuration resembling the east Atlantic pattern, whereas the 14-yr period fluctuations seem to be related to the North Atlantic Oscillation pattern. Each component also bears a different relationship with the decadal variability in the Pacific Ocean.

North Atlantic Decadal Variability: Air–Sea Coupling, Oceanic Memory, and Potential Northern Hemisphere Resonance*

Abstract In this paper, the causes and mechanisms of North Atlantic decadal variability are explored in a series of coupled ocean–atmosphere simulations. The model captures the major features of the observed North Atlantic decadal variability. The North Atlantic SST anomalies in the model control simulation exhibit a prominent decadal cycle of 12–16 yr, and a coherent propagation from the western subtropical Atlantic to the subpolar region. A series of additional modeling experiments are conducted in which the air–sea coupling is systematically modified in order to evaluate the importance of air–sea coupling for the North Atlantic decadal variability being studied. This shall be referred to as “modeling surgery.” The results suggest the critical role of ocean–atmosphere coupling in sustaining the North Atlantic decadal oscillation at selected time scales. The coupling in the North Atlantic is characterized by a robust North Atlantic Oscillation (NAO)-like atmospheric response to the SST tripole anomaly, which tends to intensify the SST anomaly and, meanwhile, also provide a delayed negative feedback. This delayed negative feedback is predominantly associated with the adjustment of the subtropical gyre in response to the anomalous wind stress curl in the subtropical Atlantic. Atmospheric stochastic forcing can drive SST patterns similar to those in the fully coupled ocean–atmosphere system, but fails to generate any preferred decadal time scales. The simulated North Atlantic decadal variability, therefore, can be viewed as a coupled ocean–atmosphere mode under the influence of stochastic forcing. This modeling study also suggests some potential resonance between the Pacific and the North Atlantic decadal fluctuations mediated by the atmosphere. The modeling surgery indicates that the Pacific climate, although not a necessary precondition, can impact the North Atlantic climate variability substantially.

North Atlantic decadal variability and the formation of tropical storms and hurricanes

Both the annual number of Atlantic tropical storms forming south of 23.5°N and of Atlantic major hurricanes increased between the 1970's/1980's and 1995–2000. These increases are coincident with a multi‐decadal warming in North Atlantic SST suggesting that the high activity of 1995–2000 may persist for the next ∼10 to 40 years. However, during 1950–2000 strong decadal oscillations are superimposed on the multi‐decadal changes in both SST and tropical storms (positive SST anomalies, increased storm activity). We appear to be entering a negative phase of the decadal SST signal implying that tropical storm, and most likely major hurricane, activity may be reduced in the next several years rather than remain at the very high 1995–2000 level when both signals were in their positive phase. Tropical storm activity during 2001 and 2002 is less than the expected only from the multi‐decadal signal but for 2002 the main cause may be El Niño.

On the Mechanism of North Atlantic Decadal Variability

Abstract North Atlantic decadal climate variability is studied with a coupled atmosphere–ocean–sea ice model (ECBILT). After having reached an approximate statistical equilibrium in coupled mode without applying flux corrections, a subsequent 1000-yr integration is performed and analyzed. Compared to the current climate, the surface temperatures are 2°C warmer in the Tropics to almost 8°C warmer in the polar regions. The covariability between the atmosphere and ocean is explored by performing a singular value decomposition (SVD) of boreal winter SST anomalies and 800-hPa geopotential height anomalies. The first SVD pair shows a red variance spectrum in SST and a white spectrum in 800-hPa height. The second mode shows a peak in both spectra at a timescale of about 16–18 yr. The geopotential height pattern is the model’s equivalent of the North Atlantic oscillation (NAO) pattern; the SST anomaly pattern is a north–south oriented dipole. Additional experiments have revealed that the decadal oscillation in EC...

Modes of North Atlantic Decadal Variability in the ECHAM1/LSG Coupled Ocean–Atmosphere General Circulation Model

Abstract The climate variability in the North Atlantic sector is investigated in a 325-yr integration of the ECHAM1/ LSG coupled ocean–atmosphere general circulation model. At the interannual timescale, the coupled model behaves realistically and sea surface temperature (SST) anomalies arise as a response of the oceanic surface layer to the stochastic forcing by the atmosphere, with the heat exchanges both generating and damping the SST anomalies. In the ocean interior, the temperature spectra are red up to a period of about 20 years, and substantial decadal fluctuations are found in the upper kilometer or so of the water column. Using extended empirical orthogonal function analysis, two distinct quasi-oscillatory modes of ocean–atmosphere variability are identified, with dominant periods of about 20 and 10 years, respectively. The oceanic changes in both modes reflect the direct forcing by the atmosphere through anomalous air–sea fluxes and Ekman pumping, which after some delay affects the intensity of t...