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This study examines coupled relationships among clouds, atmospheric circulation, and sea ice in Antarctic winter. We find that the wave-3 pattern dominates the leading covariability mode among cloud, atmospheric circulation, and sea ice. Both horizontal transport and vertical motion contribute to cloud formation, resulting in maximum cloud anomalies spatially between maximum meridional wind and pressure anomalies in the coupled system. The radiative effect of the clouds related to the wave-3 pattern can generate sea ice anomalies up to 12 cm thick in one month in the Amundsen Sea. It also strengthens the sea ice anomalies that are directly induced by low-level atmospheric circulation anomalies. In addition, the radiative forcing of the leading cloud mode in the lower troposphere is suppressed by the dynamic and thermodynamic effects of the circulation anomalies. These discoveries provide a better understanding of Antarctica’s interactive processes, and also offer physical evidence for climate model validations.

The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of the Coupled Model Intercomparison Project do not support such a case either in terms of a shift in the mean circulation or in variance that would favor increased intensity or frequency of ridges. The models also do not support shifts toward a drier mean climate or more frequent or intense dry winters or to tropical SST states that would favor west coast ridges. However, reanalyses do show that over the last century there has been a trend toward circulation anomalies over the Pacific–North American domain akin to those during the height of the California drought. The trend has been associated with a trend toward preferential warming of the Indo–west Pacific, an arrangement of tropical oceans and Pacific–North American circulation similar to that during winter 2013/14, the driest winter of the California drought. These height trends, however, are not reproduced in SST-forced atmosphere model ensembles. In contrast, idealized atmosphere modeling suggests that increased tropical Indo-Pacific zonal SST gradients are optimal for forcing height trends that favor a west coast ridge. These results allow a tenuous case for human-driven climate change driving increased gradients and favoring the west coast ridge, but observational data are not sufficiently accurate to confirm or reject this case.

The article “Climate change and the Syrian civil war revisited” by Selby et al., 2017 (henceforth S2017) challenges research exploring the links between anthropogenic climate change, water scarcity and drought, impacts on agricultural production and economic stability, and the initial 2011 unrest in Syria. More broadly, the authors contest any causal link between climate and conflict. This is an important, evolving area of study and we encourage improved, expanded, and updated analysis of these connections. In their criticism, the authors of S2017 single out three papers (Werrell, Femia, & Sternberg, 2015; Gleick, 2014; and Kelley, Mohtadi, Cane, Seager, & Kushnir, 2015; here after K2015). Here we comment directly on our own work in K2015, though the others are equally unscathed by S2017's criticisms. We do so by refuting S2017's claims regarding the role of climate change, summarizing the sizeable evidence of the long-term drying trend in the region, and bolstering the link between the drought, migration and subsequent unrest by providing further supporting evidence. Our response contends overall that S2017 fail to make their case. K2015 (as well as Gleick, 2014; Werrell et al., 2015) claim climate as one of many contributing factors to the unrest. Nothing in S2017 refutes this, and none of their supportable arguments even offer reason for doubting this view. While K2015 can and did make a quantitative estimate of the impact of anthropogenic effects on the drought, we cannot quantify the impact of climate or any other factor, separately, on the conflict.

East Africa has two rainy seasons: the long rains [March–May (MAM)] and the short rains [October–December (OND)]. Most CMIP3/5 coupled models overestimate the short rains while underestimating the long rains. In this study, the East African rainfall bias is investigated by comparing the coupled historical simulations from CMIP5 to the corresponding SST-forced AMIP simulations. Much of the investigation is focused on the MRI-CGCM3 model, which successfully reproduces the observed rainfall annual cycle in East Africa in the AMIP experiment but its coupled historical simulation has a similar but stronger bias as the coupled multimodel mean. The historical–AMIP monthly climatology rainfall bias in East Africa can be explained by the bias in the convective instability (CI), which is dominated by the near-surface moisture static energy (MSE) and ultimately by the MSE’s moisture component. The near-surface MSE bias is modulated by the sea surface temperature (SST) over the western Indian Ocean. The warm SST bias in OND can be explained by both insufficient ocean dynamical cooling and latent flux, while the insufficient shortwave radiation and excess latent heat flux mainly contribute to the cool SST bias in MAM.

Decadal variability of the East African precipitation during the season of March–May (long rains) is examined and the performance of a series of models in simulating the observed features is assessed. Observational results show that the drying trend of the long rains is associated with decadal natural variability associated with sea surface temperature (SST) variations over the Pacific Ocean. Empirical orthogonal function (EOF), linear regression, and composite analyses all show the spatial pattern of the associated SST field to be La Niña like. The SST-forced International Research Institute for Climate and Society (IRI) forecast models are able to capture the East African precipitation climatology, the decadal variability of the long rains, and the associated SST anomaly pattern but are not consistent with observations from the 1970s. The multimodel mean of the SST-forced models from the Coupled Model Intercomparison Project phase 5 (CMIP5) Atmospheric Model Intercomparison Project (AMIP) experiment captures the climatology and the drying trend in recent decades. The fully coupled models from the CMIP5 historical experiment, however, have systematic errors in simulating the East African precipitation climatology by underestimating the long rains while overestimating the short rains. The multimodel mean of the historical simulations of the long rains anomalies, which is the best estimate of the radiatively forced change, shows a weak wetting trend associated with anthropogenic forcing. The SST anomaly pattern associated with the long rains has large discrepancies with the observations. The results herein suggest caution in projections of East African precipitation from CMIP5 or the relationship between the East African precipitation and the SST spatial pattern found in paleoclimate studies with coupled climate models.

In this paper, zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ECMWF Interim Re-Analysis (ERA-Interim). It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis force, and friction. The nonlinear advection term plays a significant role only in the Asian summer monsoon regions within the ABL. In the upper troposphere, the pressure gradient force, the Coriolis force, and the nonlinear advection are the dominant terms. The transient eddy force and the residual force (which can be explained as convective momentum transfer over open oceans) are secondary, yet cannot be neglected near the equator. Zonal-mean equatorial upper-troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross-equatorial Hadley circulation. Equatorial upper-troposphere easterlies over the Asian monsoon regions are also controlled by the absolute angular momentum advection but are mainly maintained by the pressure gradient force in January. The equivalent linear Rayleigh friction, which is widely applied in simple tropical models, is calculated and the corresponding spatial distribution of the local coefficient and damping time scale are estimated from the linear regression. It is found that the linear momentum model is in general capable of crudely describing the tropical atmospheric circulation dynamics, yet the caveat should be kept in mind that the friction coefficient is not uniformly distributed and is even negative in some regions.

The midlatitude response to tropical Pacific SST anomalies involves changes in transient eddy propagation, but the processes leading to the transient eddy changes are still not clear. In a recent study, we used a series of controlled general circulation model (GCM) experiments in which an imposed tropical Pacific sea-surface temperature (SST) anomaly is turned on abruptly and the response is analyzed in terms of its high- and low-frequency parts, to show that the changes in transient eddies induced by El Nino Southern Oscillation (ENSO) arise from changes in wave refraction on the altered mean flow. In this work, we use a quasi-geostrophic linear model and a linear stationary wave model, to interpret the GCM experiments and obtain the sequence of events that lead from a tropical SST anomaly to the quasi-equilibrium change in the mean and transient atmospheric circulation. The initial direct response of the mean flow is confined to the tropical and subtropical Pacific, similar to what is obtained from a stationary wave model. This tropical–subtropical mean flow change initiates a transient eddy response, which induces a midlatitude mean flow anomaly. The wave–mean flow system evolves towards a state in which the eddy anomalies maintain the mean flow anomalies, allowing them to persist. It is further shown that, while eddy momentum fluxes persistently accelerate and decelerate the subtropical and midlatitude mean flow, the eddy heat flux effect on the zonal mean flow is much more variable, and only marginally significant. The linear quasi-geostrophic model calculations capture the evolution of eddy momentum flux anomalies equatorwards of 60°N quite well, suggesting linear wave refraction can explain the midlatitude ENSO anomalies. However, other processes, like stationary waves or changes in the nonlinear stage of eddy life cycles, are needed to explain the ENSO-related anomalies at high latitudes, polewards of around 60°N. Copyright © 2010 Royal Meteorological Society

Is the transport of heat northward by the Gulf Stream and North Atlantic Drift, and its subsequent release into the midlatitude westerlies, the reason why Europe's winters are so much milder than those of eastern North America and other places at the same latitude? Here, it is shown that the principal cause of this temperature difference is advection by the mean winds. South-westerlies bring warm maritime air into Europe and north-westerlies bring frigid continental air into north-eastern North America. Further, analysis of the ocean surface heat budget shows that the majority of the heat released during winter from the ocean to the atmosphere is accounted for by the seasonal release of heat previously absorbed and not by ocean heat-flux convergence. Therefore, the existence of the winter temperature contrast between western Europe and eastern North America does not require a dynamical ocean. Two experiments with an atmospheric general-circulation model coupled to an ocean mixed layer confirm this conclusion. The difference in winter temperatures across the North Atlantic, and the difference between western Europe and western North America, is essentially the same in these models whether or not the movement of heat by the ocean is accounted for. In an additional experiment with no mountains, the flow across the ocean is more zonal, western Europe is cooled, the trough east of the Rockies is weakened and the cold of north-eastern North America is ameliorated. In all experiments the west coast of Europe is warmer than the west coast of North America at the same latitude whether or not ocean heat transport is accounted for. In summary the deviations from zonal symmetry of winter temperatures in the northern hemisphere are fundamentally caused by the atmospheric circulation interacting with the oceanic mixed layer. Copyright © 2002 Royal Meteorological Society.

Low-dimensional models can give insight into the climate system, in particular its response to externally imposed forcing such as the anthropogenic emission of green-house gases. Here, we use the Lorenz system, a chaotic dynamical system characterized by two “regimes”, to examine the effect of a weak imposed forcing. We show that the probability density functions (PDF's) of time-spent in the two regimes are exponential, and that the most dramatic response to forcing is a change in the frequency of occurrence of extremely persistent events, rather than the weaker change in the mean persistence time. This enhanced sensitivity of the “tails” of the PDF's to forcing is quantitatively explained by changes in the stability of the regimes. We demonstrate similar behavior in a stochastically forced double well system. Our results suggest that the most significant effect of anthropogenic forcing may be to change the frequency of occurrence of persistent climate events, such as droughts, rather than the mean.

We test the hypothesis that sea level variations associated with EL Niño events are a response to wind changes in the central Pacific and that the signal is transmitted to the coast of South America by packets of equatorial Kelvin waves. A linear model is forced by wind stress anomalies composited from six EL Niños occurring between 1951 and 1972 (Rasmusson and Carpenter). Model sea level is compared with similar composites of tidegage measurements from selected equatorial Pacific stations. Although several vertical modes are included in the calculation, only the two gravest baroclinic modes make significant contributions to the sea level signal. Model results duplicate the pattern and timing of the observed sea level anomalies but amplitudes are systematically low. This is especially true at central and western Pacific stations, and it is suggested that the poor quality of the forcing data may be at fault there. Results at the east provide better support for the theory: in particular, the twin-peaked signal characteristic of El Niño sea-level anomalies is reproduced. The second peak is shown to be a response to the massive collapse of the trades occurring in the middle of the El Niño year and its amplitude is correctly hindcast by the model. The first peak is a response to the weaker wind changes occurring in the boreal fall preceding El Niño; its calculated amplitude is too small. The implication of this discrepancy is that the linear Kelvin wave theory will have to be modified if it is to account for the initial El Niño warning.

A new low-frequency standing equatorial wave mode is described. It is composed solely of long Kelvin and Rossby waves, whereas previously described low-frequency modes involved short, eastward propagating Rossby waves. It is argued that these short waves travel too slowly to allow such modes to set up. A simple closed form expression is given for the new basin mode; this is also a new form for the sum of a Kelvin wave and its eastern boundary reflection.

Different wind field analyses are used to force a one layer adiabatic model of the near equ~torial surface circulation in the Indian Ocean in 1979. The model simulates .the major features. of the obse~atlOns: ea:>tward jets were present in April-May and in October-November In the' central Indian Ocea.n In phase With the local winds; the seasonal changes of thermocline depth in the western part of the basIn are related to the near equatorial currents. .... Significant discrepancies are also found. Some are due to the uncertaInty In. the WInd fiel.ds. Correlation between different wind fields are only of the order of 0.75 for the low frequencies and mag~ltud~s can vary by a factor of 1.5. Others are attributed fo model inadequacies, especially the ne~ect of nonhnean~y and the oversimplification of the vertical structure. Ther~ is an unrealistic energy focus In the central Indian Ocean though, in general, seasonal changes are underestimated by at least 30%. . '" Simpler dynamics failed to produce a reasonable agreement over the whole basIn. A. Yoshida Jet did well for the currents in the central part of the basin, but did not reproduce the mass changes In the west. Sverdrup equilibrium reproduces the model zonal slope of the thermocline, but not the currents.

El Nino events, the most spectacular instances of interannual variability in the ocean, have profound consequences for climate and the ocean ecosystem. The 1982-1983 El Nino is perhaps the strongest in this century. El Nino events usually have followed a predictable pattern, but the recent event differs markedly. The physical oceanography of this El Nino is described and compared with that of earlier events.

The sea level signature for the onset of the 1982–83 El Nino is hindcasted using a linear wind-driven model that consists of four vertical modes. The hindcast solution is compared with sea level time series from 18 island and coastal stations throughout the tropical Pacific. The response of the first and second baroclinic modes, when summed, account for a significant portion of the phase and amplitude of the observed sea level. The comparisons with observed sea level are best in the eastern tropical Pacific along the equator and South American coast where the solutions are a function of integrals of the zonal wind field to the west. The skill of the hindcast is notably less away from the equator.

Luyten and Roemmich have shown a strong semiannual signal in zonal velocity in the upper, western part of the equatorial Indian Ocean. Their observations are modeled by assuming that they are directly forced by the observed semiannual component of zonal wind stress, which is relatively large in the equatorial Indian Ocean. The model is linear, periodic, has linear damping, uses the long-wave approximation, and can be solved analytically. A good comparison with the observations is obtained for the phase of the oscillation across the array. The predicted magnitude is somewhat smaller than in the observations. The model sensitivity to friction and the spatial distribution of the wind stress is explored. Some additional model simplifications are discussed, but it is concluded that they all detract substantially from the comparison. The main conclusion is that the observations can be accounted for as a directly forced response to the semiannual component of the near-equatorial zonal winds.

Three different classes of numerical models successfully predicted the occurrence of the El Niño of 1986-87 at lead times of 3 to 9 months. Although the magnitude and timing of predicted ocean surface temperatures were not perfect, these results suggest that routine prediction of moderate to large El Niño events is feasible. The key to the success of the models lies in recognizing or simulating the low-frequency, large-scale changes in the tropical ocean-atmosphere system that give rise to El Niño events.