Martin P. King

Atlantic forcing of Pacific decadal variability

This paper investigates the Atlantic Ocean influence on equatorial Pacific decadal variability. Using an ensemble of simulations, where the ICTPAGCM (“SPEEDY”) is coupled to the NEMO/OPA ocean model in the Indo-Pacific region and forced by observed sea surface temperatures in the Atlantic region, it is shown that the Atlantic Multidecadal Oscillation (AMO) has had a substantial influence on the equatorial Pacific decadal variability. According to AMO phases we have identified three periods with strong Atlantic forcing of equatorial Pacific changes, namely (1) 1931–1950 minus 1910–1929, (2) 1970–1989 minus 1931–1950 and (3) 1994–2013 minus 1970–1989. Both observations and the model show easterly surface wind anomalies in the central Pacific, cooling in the central-eastern Pacific and warming in the western Pacific/Indian Ocean region in events (1) and (3) and the opposite signals in event (2). The physical mechanism for these responses is related to a modification of the Walker circulation because a positive (negative) AMO leads to an overall warmer (cooler) tropical Atlantic. The warmer (cooler) tropical Atlantic modifies the Walker circulation, leading to rising (sinking) and upper-level divergence (convergence) motion in the Atlantic region and sinking (rising) motion and upper-level convergence (divergence) in the central Pacific region.

On the Need of Intermediate Complexity General Circulation Models: A “SPEEDY” Example

processes that allows realistic and fast climate simula -tions that often involve large ensembles for the purpose of reducing uncertainty and estimation of the forced and internal variability of the system. The forced signal is typically estimated by an ensemble mean of many simulations, but ensembles of state-of-the-art models are often too small to reduce the remaining internal variability. The ensemble size needed to estimate the mean accurately depends on the signal-to-noise ratio for the variable and region under consideration. For example, the ensemble size to estimate midlatitude 500-hPa height accurately is about 20, which is larger than most ensembles used in seasonal hindcast data-sets or climate projections performed by individual centers. Intermediate complexity models can also be used efficiently to investigate the sensitivity of simu-lated climate to changes in parameters in the physical parameterizations. Another application is related to climate change. For example, Forest et al. (2002) and Sokolov at al. (2009) use the MIT Integrated Global System Model (MIT IGSM) to investigate topics such as climate sensitivity, aerosol forcing, ocean heat uptake rate, and probabilistic projections of climate change. There are many intermediate complexity system models of intermediate complexity (EMICs). A number of them are participating in the IPCC Fifth Assessment Report and can be found at http://climate .uvic.ca/EMICAR5 (one of which is based on a previous version of the model introduced here). This website also provides information about experiments that are performed with these models that range from en-sembles of 1,000-year-long historical simulations to the assessment of different CO

The Roles of External Forcings and Internal Variabilities in the Northern Hemisphere Atmospheric Circulation Change from the 1960s to the 1990s

Abstract The Northern Hemisphere atmospheric circulation change from the 1960s to the 1990s shows a strong positive North Atlantic Oscillation (NAO) and a deepening of the Aleutian low. The issue regarding the contributions of external forcings and internal atmospheric variability to this circulation change has not been resolved satisfactorily. Previous studies have found the importance of tropical SST forcing. Here, this hypothesis is examined again using relatively large ensembles of atmospheric general circulation model simulations of the twentieth-century climate forced only by historically varying SST. The resulting ensemble-mean amplitude underestimates the observed change by at least 70%, although the spatial pattern is reproduced well qualitatively. Furthermore, AGCM experiments are performed to investigate other driving factors, such as the greenhouse gases, sea ice, the stratospheric ozone, as well as the contribution from atmospheric internal variability. The increase in ensemble-mean trend amp...

Observed Low-Frequency Covariabilities between the Tropical Oceans and the North Atlantic Oscillation in the Twentieth Century

Abstract The low-frequency covariabilities of tropical sea surface temperature (SST) and the North Atlantic Oscillation (NAO) during twentieth-century winters are investigated by maximum covariance analysis (MCA) using reanalysis data. It was found that the positive NAO phase is positively correlated to an increase in tropical SST, especially during the recent decades. The western tropical Pacific SST displays high correlation with the NAO throughout the whole of the twentieth century. For this ocean region, the MCA homogeneous map has a SST spatial pattern with meridional gradients. It was also found that a cooling of tropical Atlantic SST is correlated with positive NAO. The influence of the tropical Atlantic SST on the NAO is strongest during the pre-1960s period.

Numerical Simulations of Convective Heat Transfer in Rayleigh-Bénard Convection and a Rotating Annulus

ABSTRACT Numerical studies of fluid and heat flows in a sealed rotating annulus having a heated outer cylinder and a cooled inner cylinder are performed. Flow is induced by buoyancy in the centrifugal field and affected by the Coriolis force. We focus on the heat transfer efficiencies characterized by correlations of Nusselt number with Rayleigh number ( Nu ∼ Raγ); however, the flow structures are also discussed. We begin with a discussion of recent progress in Rayleigh-Bénard convection. Its connection to the rotating annulus systems used in the current study are then described. We hope to present a clearer picture of free-convective heat transfer in enclosures by comparing and interpreting results from various systems. Typical values for γ vary between 1/5 and 1/2 inclusive.

Teleconnections in the Atmosphere and Oceans

AFFILIATIONS: KucharsKi—international centre for Theoretical physics, earth system physics section, Trieste, italy; Kang—seoul national University, seoul, Korea; straus—center for oceanland-Atmosphere studies, George mason University, Fairfax, Virginia; King—monash University, Jalan lagoon selatan, Bandar sunway, malaysia CORRESPONDING AuThOR: Fred Kucharski, The Abdus salam international centre for Theoretical physics, earth system physics section, Trieste, italy e-mail: kucharsk@ictp.it

Multi-model assessment of linkages between eastern Arctic sea-ice variability and the Euro-Atlantic atmospheric circulation in current climate

A set of ensemble integrations from the Coupled Model Intercomparison Project phase 5, with historical forcing plus RCP4.5 scenario, are used to explore if state-of-the-art climate models are able to simulate previously reported linkages between sea-ice concentration (SIC) anomalies over the eastern Arctic, namely in the Greenland–Barents–Kara Seas, and lagged atmospheric circulation that projects on the North Atlantic Oscillation (NAO)/Arctic Oscillation (AO). The study is focused on variability around the long-term trends, so that all anomalies are detrended prior to analysis; the period of study is 1979–2013. The model linkages are detected by applying maximum covariance analysis. As also found in observational data, all the models considered here show a statistically significant link with sea-ice reduction over the eastern Arctic followed by a negative NAO/AO-like pattern. If the simulated relationship is found at a lag of one month, the results suggest that a stratospheric pathway could be at play as the driving mechanism; in observations this is preferentially shown for SIC in November. The interference of a wave-like anomaly over Eurasia, accompanying SIC changes, with the climatological wave pattern appears to be key in setting the mediating role of the stratosphere. On the other hand, if the simulated relationship is found at a lag of two months, the results suggest that tropospheric dynamics are dominant, presumably due to transient eddy feedback; in observations this is preferentially shown for SIC in December. The results shown here and previous evidence from atmosphere-only experiments emphasize that there could be a detectable influence of eastern Arctic SIC variability on mid-latitude atmospheric circulation anomalies. Even if the mechanisms are robust among the models, the timing of the simulated linkages strongly depends on the model and does not generally mimic the observational ones. This implies that the atmospheric sensitivity to sea-ice changes largely depends on the mean-flow and parameterizations, which could lead to misleading conclusions elsewhere if a multi-model ensemble-mean approach is adopted. It might also represent an important source of uncertainty in climate prediction and projection. Modelling efforts are hence further required to improve representation of the background atmospheric circulation and reduce biases, in order to attain more accurate covariability.

Assessment of downscaled current and future projections of diurnal rainfall patterns for the Himalaya

In this study we investigate the diurnal precipitation cycle in high-resolution regional climate simulations for present (2000–2010) and future time periods (2030–2040 and 2040–2050) over subregions of the Himalayas. The future periods are simulated under a high-emission scenario, Representative Concentration Pathway 8.5 (RCP8.5) in order to maximize any projected externally driven precipitation signal. For present climate, 4-hourly simulated precipitation is first evaluated against observations to establish model credibility. The diurnal cycle, which is typically characterized by a bimodal structure with primary (secondary) maxima in nighttime (afternoon), is reasonably well represented in the present, giving us confidence when assessing potential future changes. The timing of the precipitation maxima and minima is found to match the observed timings well in the diurnal cycle. In general, the Weather Research and Forecasting model captures the principal shape of the diurnal cycle observed over all the subregions. Under projected future conditions, no significant changes in the diurnal cycle occur. The results suggest modest changes in diurnal precipitation under RCP8.5 emission scenario, as evidenced by an increase in afternoon precipitation around the Himalayas. Although the projected future changes of precipitation presented in this article are within of the expected range of precipitation changes caution must be exercised when interpreting single-model experiments.

Interannual tropical Pacific sea surface temperature anomalies teleconnection to Northern Hemisphere atmosphere in November

We investigate the Northern Hemisphere atmospheric circulation anomalies associated to the sea surface temperature (SST) anomalies that are related to the eastern-Pacific and central-Pacific El Nino-Southern Oscillations in the late autumn (November). This research is motivated by the need for improving understanding of the autumn climate conditions which can impact on winter climate, as well as the relative lack of study on the boreal autumn climate processes compared to winter. Using reanalysis and SST datasets available from the late nineteenth century through the recent years, we found that there are two major atmospheric responses; one is a hemispheric-wide wave number-4 pattern, another has a more annular pattern. Both of these project on the East Atlantic pattern (southward-shifted North Atlantic Oscillation) in the Atlantic sector. Which of the patterns is active is suggested to depend on the background mean flow, with the annular anomaly active in the most recent decades, while the wave-4 pattern in the decades before. This switch is associated with a change of correlation sign in the North Pacific. We discuss the robustness of this finding. The ability of two atmospheric general circulation models (ICTP-AGCM and ECHAM-AGCM) to reproduce the teleconnections is also examined. Evidence provided shows that the wave-4 pattern and the East Atlantic pattern signals can be reproduced by the models, while the shift from this to an annular response for the recent years is not found conclusively.

Potential ocean atmosphere preconditioning of late autumn Barents-Kara sea ice concentration anomaly

Many recent studies have revealed the importance of the climatic state in November on the seasonal climate of the subsequent winter. In particular, it has been shown that interannual variability of sea ice concentration (SIC) over the Barents-Kara (BK) seas in November is linked to winter atmospheric circulation anomaly that projects on the North Atlantic Oscillation. Understanding the lead–lag processes involving the different components of the climate system from autumn to winter is therefore important. This note presents dynamical interpretation for the ice-ocean–atmosphere relationships that can affect the BK SIC anomaly in late autumn. It is found that cyclonic (anticyclonic) wind anomaly over the Arctic in October, by Ekman drift, can be responsible for positive (negative) SIC in the BK seas in November. The results also suggest that ocean heat transport via the Barents Sea Opening in September and October can contribute to BK SIC anomaly in November.