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John C. H. Chiang and Adam H. Sobel

Abstract

The warming of the entire tropical free troposphere in response to El Niño is well established, and suggests a tropical mechanism for the El Niño–Southern Oscillation (ENSO) teleconnection. The potential impact of this warming on remote tropical climates is examined through investigating the adjustment of a single-column model to imposed tropospheric temperature variations, assuming that ENSO controls interannual tropospheric temperature variations at all tropical locations. The column model predicts the impact of these variations in three typical tropical climate states (precipitation > evaporation; precipitation < evaporation; no convection) over a slab mixed layer ocean. Model precipitation and sea surface temperature (SST) respond significantly to the imposed tropospheric forcing in the first two climate states. Their amplitude and phase are sensitive to the imposed mixed layer depth, with the nature of the response depending on how fast the ocean adjusts to imposed tropospheric temperature forcing. For larger mixed layer depth, the SST lags the tropospheric temperature by a longer time, allowing greater disequilibrium between atmosphere and ocean. This causes larger surface flux variations, which drive larger precipitation variations. Moist convective processes are responsible for communicating the tropospheric temperature signal to the surface in this model.

Preliminary observational analysis suggests that the above mechanism may be applicable to interpreting interannual climate variability in the remote Tropics. In particular, it offers a simple explanation for the gross spatial structure of the observed surface temperature response to ENSO, including the response over land and the lack thereof over the southeast tropical Atlantic and southeast tropical Indian Oceans. The mechanism predicts that the air–sea humidity difference variation is a driver of ENSO-related remote tropical surface temperature variability, an addition to wind speed and cloudiness variations that previous studies have shown to be important.

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Benjamin R. Lintner and John C. H. Chiang

Abstract

The applicability of a weak temperature gradient (WTG) formulation for the reorganization of tropical climate during El Niño–Southern Oscillation (ENSO) events is investigated. This idealized dynamical framework solves for the divergent portion of the tropical circulation by assuming a spatially homogeneous perturbation temperature profile and a mass balance constraint applied over the tropical belt. An intermediate-level complexity model [the Quasi-Equilibrium Tropical Circulation Model (QTCM)] configured with the WTG assumptions is used to simulate El Niño conditions and is found to yield an appropriate level of tropospheric warming, a plausible pattern of precipitation anomalies in the tropical Pacific source region of El Niño, and a gross precipitation deficit over the Tropics outside the Pacific (hereafter the “remote Tropics”). Additional tests of the WTG framework with La Niña forcing conditions and enhanced greenhouse gas concentrations support its applicability. However, the ENSO response under the WTG framework fails in some respects when compared to the standard QTCM: in particular, some regional features of the anomalous precipitation response, especially in the remote Tropics, differ markedly between the two model versions. These discrepancies appear to originate in part from the lack of anomalous tropospheric temperature gradients (and circulations) in the framework presented here. Nevertheless, the WTG approach appears to be a useful lowest-order model for the tropical climate adjustment to ENSO. The WTG framework is also used to argue that El Niño may not represent a good proxy for tropical rainfall changes under greenhouse gas warming scenarios because the large-scale subsidence occurring with the tropospheric warming in the El Niño scenario has an effect on rainfall that is distinct from the effect of increased tropospheric temperatures common to both the greenhouse gas warming and El Niño scenarios.

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John C. H. Chiang and Daniel J. Vimont

Abstract

From observational analysis a Pacific mode of variability in the intertropical convergence zone (ITCZ)/cold tongue region is identified that possesses characteristics and interpretation similar to the dominant “meridional” mode of interannual–decadal variability in the tropical Atlantic. The Pacific and Atlantic meridional modes are characterized by an anomalous sea surface temperature (SST) gradient across the mean latitude of the ITCZ coupled to an anomalous displacement of the ITCZ toward the warmer hemisphere. Both are forced by trade wind variations in their respective northern subtropical oceans. The Pacific meridional mode exists independently of ENSO, although ENSO nonlinearity projects strongly on it during the peak anomaly season of boreal spring. It is suggested that the Pacific and Atlantic modes are analogous, governed by physics intrinsic to the ITCZ/ cold tongue complex.

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John C. H. Chiang and Yue Fang

Abstract

Model evidence is presented to make the case that the midlatitude North Pacific wintertime transient eddy activity may have been significantly weaker during the mid-Holocene (∼6000 yr BP). A simulation of the mid-Holocene climate in an atmospheric general circulation model coupled to a reduced gravity ocean model showed significant reduction to transient eddy activity, up to 30% in the main storm-track region. The reduced baroclinic eddy activity is associated with basinwide climate changes over the northern and tropical Pacific, including a deepening of the Aleutian low, colder SSTs in the western and central North Pacific, a strengthening and southward shift of the subtropical jet, and a strengthened South Pacific convergence zone. These associated climate changes are consistently simulated across a range of Paleoclimate Modeling Intercomparison Project Phase II (PMIP2) coupled models forced with mid-Holocene climate forcings, suggesting they are a robust response to mid-Holocene orbital forcing. The authors link the mid-Holocene climate changes to two related modern-day analogs: (i) interannual variations in wintertime North Pacific storminess and (ii) the phenomenon of midwinter suppression whereby North Pacific transient eddy activity in today’s climate is reduced in midwinter. In both instances, the associated North Pacific climate conditions resemble those seen in the mid-Holocene simulations. While it remains to be seen which analog is dynamically more appropriate, the latter link—midwinter suppression—offers the simple physical interpretation that the mid-Holocene reduction in storminess is a consequence of a “more winterlike” climate resulting from the mid-Holocene precessional forcing.

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John C. H. Chiang and Benjamin R. Lintner

Abstract

The authors demonstrate through atmospheric general circulation model (the Community Climate Model version 3.10) simulations of the 1997/98 El Niño that the observed “remote” (i.e., outside the Pacific) tropical land and ocean surface warming appearing a few months after the peak of the El Niño event is causally linked to the Tropics-wide warming of the troposphere resulting from increased atmospheric heating in the Pacific, with the latter acting as a conduit for the former. Unlike surface temperature, the surface flux behavior in the remote Tropics in response to El Niño is complex, with sizable spatial variation and compensation between individual flux components; this complexity suggests a more fundamental control (i.e., tropospheric temperature) for the remote tropical surface warming. Over the remote oceans, latent heat flux acting through boundary layer humidity variations is the important regulator linking the surface warming in the model simulations to the tropospheric warming over the remote tropical oceans. Idealized 1997/98 El Niño simulations using an intermediate tropical circulation model (the Quasi-Equilibrium Tropical Circulation Model) in which individual surface fluxes are directly manipulated confirms this result. The findings over the remote ocean are consistent with the “tropospheric temperature mechanism” previously proposed for the tropical ENSO teleconnection, with equatorial planetary waves propagating tropospheric temperature anomalies from the eastern Pacific to the remote Tropics and moist convective processes mediating the troposphere-to-remote-surface connection. The latter effectively requires the boundary layer moist static energy to vary in concert with the free tropospheric moist static energy. Over the remote land regions, idealized model simulations suggest that sensible heat flux regulates the warming response to El Niño, though the underlying mechanism has not yet been fully determined.

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Benjamin R. Lintner and John C. H. Chiang

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The adjustment of the tropical climate outside the Pacific (the “remote Tropics”) to the abrupt onset of El Niño conditions is examined in a tropical atmosphere model that assumes simplified vertical structure and quasi-equilibrium (QE) convective closure. The El Niño signal is rapidly (∼1 week) communicated to the remote Tropics via an eastward-propagating Kelvin-like wave that induces both anomalous subsidence and tropospheric warming. Widespread reductions in convective precipitation occur in conjunction with the spreading of the temperature and subsidence anomalies. The remote rainfall suppression peaks roughly 5–15 days after the initiation of El Niño conditions, after which the anomalous remote rainfall field recovers to a state characterized by a smaller remote areal mean rainfall deficit and the appearance of localized positive rainfall anomalies. The initial remote precipitation reduction after El Niño onset is tied to both tropospheric warming (i.e., stabilization of the troposphere to deep convection) and the suppression of remote humidity levels; recovery of the initial deficits occurs as feedbacks modulate the subsequent evolution of humidity anomalies in the tropospheric column. Apart from the short-term response, there is a longer-term adjustment of the remote climate related to the thermal inertia of the underlying surface: surface thermal disequilibrium, which is related to the depth of the ocean mixed layer, maintains larger precipitation deficits than would be expected for equilibrated conditions. This result supports a previous prediction by one of the authors for a significant disequilibrium mechanism in the precipitation teleconnection to El Niño resulting from the local vertical coupling of the troposphere to the surface through moist convection.

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John C. H. Chiang and Stephen E. Zebiak

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Previous diagnostic studies of surface wind momentum balances over tropical oceans showed that, under a linear friction assumption, the meridional friction coefficient is two to three times larger than the zonal friction coefficient, and that both friction coefficients exhibit a pronounced meridional dependence. The authors’ diagnosis of a global marine surface dataset confirms these results. Furthermore, it is shown that to first approximation the friction coefficients are independent of longitude and season in the tropical band between ∼20°S and ∼20°N. Poleward of 20°N and 20°S, the coefficients are no longer solely a function of latitude. To explain these empirical results, a simple analytical model of the friction coefficient is formulated based on the simplest K-theory mixed-layer parameterization, assuming constant viscosity. The model does a good job of reproducing the observed zonal friction coefficient, but does poorly for meridional friction. The poor result is thought to be from model sensitivity to the specified planetary boundary layer (PBL) thickness. By reversing the calculation, using observed meridional friction coefficients, and assuming no meridional winds at PBL top, model PBL heights were derived that compared favorably with zonally averaged inversion heights for June–August over the tropical Atlantic.

This model suggests that both coefficients increase away from the equator because of the decrease in PBL thickness. Furthermore, the zonal friction coefficient is smaller than the meridional coefficient because strong zonal winds at the top of the boundary layer mixes down, reducing the retarding influence of surface zonal momentum fluxes. The results also suggest that the boundary layer top winds and height are important components in modeling surface winds over the tropical oceans.

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Wenwen Kong and John C. H. Chiang

Abstract

This study explores how the termination of the mei-yu is dynamically linked to the westerlies impinging on the Tibetan Plateau. It is found that the mei-yu stage terminates when the maximum upper-tropospheric westerlies shift beyond the northern edge of the plateau, around 40°N. This termination is accompanied by the disappearance of tropospheric northerlies over northeastern China. The link between the transit of the jet axis across the northern edge of the plateau, the disappearance of northerlies, and termination of the mei-yu holds on a range of time scales from interannual through seasonal and pentad. Diagnostic analysis indicates that the weakening of the meridional moisture contrast and meridional wind convergence, mainly resulting from the disappearance of northerlies, causes the demise of the mei-yu front. The authors propose that the westerlies migrating north of the plateau and consequent weakening of the extratropical northerlies triggers the mei-yu termination. Model simulations are employed to test the causality between the jet and the orographic downstream northerlies by repositioning the northern edge of the plateau. As the plateau edge extends northward, orographic forcing on the westerlies strengthens, leading to persistent strong downstream northerlies and a prolonged mei-yu. Idealized simulations with a dry dynamical core further demonstrate the dynamical link between the weakening of orographically forced downstream northerlies with the positioning of the jet from south to north of the plateau. Changes in the magnitude of orographically forced stationary waves are proposed to explain why the downstream northerlies disappear when the jet axis migrates beyond the northern edge of the plateau.

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John C. H. Chiang, C.-Y. Chang, and M. F. Wehner

Abstract

Multidecadal and longer changes to the Atlantic interhemispheric sea surface temperature gradient (AITG) in phase 5 of the Coupled Model Intercomparison Project (CMIP5) historical simulations are investigated. Observations show a secular trend to this gradient over most of the twentieth century, with the southern lobe warming faster relative to its northern counterpart. A previous study of phase 3 of the CMIP (CMIP3) suggests that this trend is partially forced by anthropogenic sulfate aerosols. This analysis collectively confirms the partially forced trend for the CMIP5 and by anthropogenic aerosols. Like the CMIP3, the CMIP5 also simulates a reversal in the AITG trend in the late 1970s, which was attributed to a leveling off of the anthropogenic aerosol influence and increased influence of greenhouse gases in the late twentieth century. Two (of 25) CMIP5 models, however, systematically simulate a twentieth-century trend opposite to observed, leading to some uncertainty regarding the forced nature of the AITG trend. The observed AITG also exhibits a pronounced multidecadal modulation on top of the trend, associated with the Atlantic multidecadal oscillation (AMO). Motivated by a recent suggestion that the AMO is a forced response to aerosols, the causes of this multidecadal behavior were also examined. A few of the CMIP5 models analyzed do produce multidecadal AITG variations that are correlated to the observed AMO-like variation, but only one, the Hadley Centre Global Environmental Model, version 2 (HadGEM2), systematically simulates AMO-like behavior with both the requisite amplitude and phase. The CMIP5 simulations thus point to a robust aerosol influence on the historical AITG trend but not to the AMO-like multidecadal behavior.

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Hyo-Seok Park, John C. H. Chiang, and Seok-Woo Son

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The role of the central Asian mountains on North Pacific storminess is examined using an atmospheric general circulation model by varying the height and the areas of the mountains. A series of model integrations show that the presence of the central Asian mountains suppresses the North Pacific storminess by 20%–30% during boreal winter. Their impact on storminess is found to be small during other seasons. The mountains amplify stationary waves and effectively weaken the high-frequency transient eddy kinetic energy in boreal winter. Two main causes of the reduced storminess are diagnosed. First, the decrease in storminess appears to be associated with a weakening of downstream eddy development. The mountains disorganize the zonal coherency of wave packets and refract them more equatorward. As the zonal traveling distance of wave packets gets substantially shorter, downstream eddy development gets weaker. Second, the central Asian mountains suppress the global baroclinic energy conversion. The decreased baroclinic energy conversion, particularly over the eastern Eurasian continent, decreases the number of eddy disturbances entering into the western North Pacific. The “barotropic governor” does not appear to explain the reduced storminess in our model simulations.

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