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Matthew Newman
,
Gilbert P. Compo
, and
Michael A. Alexander

Abstract

Variability of the Pacific decadal oscillation (PDO), on both interannual and decadal timescales, is well modeled as the sum of direct forcing by El Niño–Southern Oscillation (ENSO), the “reemergence” of North Pacific sea surface temperature anomalies in subsequent winters, and white noise atmospheric forcing. This simple model may be taken as a null hypothesis for the PDO, and may also be relevant for other climate integrators that have been previously related to the PDO.

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John C. Fyfe

Abstract

In concert with a poleward shift in baroclinicity, the synoptic environment south of 40°S appears to have changed significantly over recent decades. South of 40°S and north of the Antarctic Ocean the number of cyclones has dramatically decreased, while over the Antarctic Ocean a modest increase has occurred. A global climate model with anthropogenic forcing produces similar historical changes, and under a “business-as-usual” emissions scenario predicts that the number of sub-Antarctic Ocean cyclones will drop by over 30% between now and century's end.

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Oleg A. Saenko
,
Andrew J. Weaver
, and
Jonathan M. Gregory

Abstract

A close link between the formation of global-scale water masses, such as North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW), and two stable modes of the thermohaline circulation (THC) is investigated in a coupled model. In the upper 2–3 km of the Atlantic, the THC modes are characterized by meridional overturning circulations of opposite sign, with either a dominance of the AAIW cell over the NADW cell (“off” THC mode) or vice versa (“on” THC mode). A transition between these THC modes is controlled by the relationship between the densities in the source regions of formation of AAIW and NADW water masses. This is shown by applying a freshwater perturbation in the region of enhanced AAIW formation in the Southern Ocean to obtain a hysteresis loop of the NADW circulation. Transitions between the two modes of the THC occur when the densities in the source regions of AAIW and NADW become comparable to each other.

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Francis W. Zwiers
and
Xuebin Zhang

Abstract

Using an optimal detection technique, the extent to which the combined effect of changes in greenhouse gases and sulfate aerosols (GS) may be detected in observed surface temperatures is assessed in six spatial domains decreasing in size from the globe to Eurasia and North America, separately. The GS signal is detected in the annual mean near-surface temperatures of the past 50 yr in all domains. It is also detected in some seasonal mean temperatures of the past 50 yr, with detection in more seasons in larger domains.

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J. M. Gregory
,
R. J. Stouffer
,
S. C. B. Raper
,
P. A. Stott
, and
N. A. Rayner

Abstract

A probability distribution for values of the effective climate sensitivity, with a lower bound of 1.6 K (5th percentile), is obtained on the basis of the increase in ocean heat content in recent decades from analyses of observed interior-ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthropogenic and natural radiative forcing of the climate system. Radiative forcing is the greatest source of uncertainty in the calculation; the result also depends somewhat on the rate of ocean heat uptake in the late nineteenth century, for which an assumption is needed as there is no observational estimate. Because the method does not use the climate sensitivity simulated by a general circulation model, it provides an independent observationally based constraint on this important parameter of the climate system.

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W. J. Ingram

Abstract

Expectations of the size of climate change are generally based on models in which relative humidity (RH) changes little under climate change. A wide variety of general circulation models (GCMs) show such a response, but it has been suggested that this may be an artifact of their having inadequate vertical resolution, and that in reality the climate is much less sensitive to external forcing.

This paper compares GCM simulations with a wide range of vertical resolutions, as well as completely different schemes for convection or advection. All these cases show similar water vapor feedbacks, suggesting that the consensus of GCMs on this feedback may well be correct.

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Mathew Barlow
,
Heidi Cullen
, and
Bradfield Lyon

Abstract

Severe drought over the past three years (1998–2001), in combination with the effects of protracted sociopolitical disruption, has led to widespread famine affecting over 60 million people in central and southwest (CSW) Asia. Here both a regional and a large-scale mode of climate variability are documented that, together, suggest a possible forcing mechanism for the drought. During the boreal cold season, an inverse relationship exists between precipitation anomalies in the eastern Indian Ocean and CSW Asia. Suppression of precipitation over CSW Asia is consistent with interaction between local synoptic storms and wave energy generated by enhanced tropical rainfall in the eastern Indian Ocean. This regional out-of-phase precipitation relationship is related to large-scale climate variability through a subset of El Niño–Southern Oscillation (ENSO) events characterized by an enhanced signal in the warm pool region of the western Pacific Ocean. Both the prolonged duration of the 1998–2001 cold phase ENSO (La Niña) event and unusually warm ocean waters in the western Pacific appear to contribute to the severity of the drought.

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Bing Lin
,
Bruce A. Wielicki
,
Lin H. Chambers
,
Yongxiang Hu
, and
Kuan-Man Xu

Abstract

Using the Tropical Rainfall Measuring Mission (TRMM) satellite measurements over tropical oceans, this study evaluates the iris hypothesis recently proposed by Lindzen et al. that tropical upper-tropospheric anvils act as a strong negative feedback in the global climate system. The modeled radiative fluxes of Lindzen et al. are replaced by the Clouds and the Earth's Radiant Energy System (CERES) directly observed broadband radiation fields. The observations show that the clouds have much higher albedos and moderately larger longwave fluxes than those assumed by Lindzen et al. As a result, decreases in these clouds would cause a significant but weak positive feedback to the climate system, instead of providing a strong negative feedback.

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Fanglin Yang
,
Arun Kumar
,
Wanqiu Wang
,
Hann-Ming Henry Juang
, and
Masao Kanamitsu

Abstract

Interannual variations in the tropical Pacific sea surface temperatures related to El Niño–Southern Oscillation (ENSO) are known to influence wintertime surface climate anomalies over North America. However, the role of local land surface processes in this phenomenon is not well understood. Here, using a suite of atmospheric general circulation model simulations, it is demonstrated that the North American surface climate anomalies related to ENSO are greatly enhanced by a local snow–albedo feedback. Implications of this feedback mechanism on seasonal climate predictions and greenhouse gas–induced climate changes are discussed.

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Niklas Schneider
and
Arthur J. Miller

Abstract

It is shown that wintertime sea surface temperature anomalies in the confluence region of the Kuroshio–Oyashio Currents in the western North Pacific can be skillfully predicted at lead times of up to 3 yr. The predictions are based on the history of the wind stress over the North Pacific and oceanic Rossby wave dynamics. The predictions may be exploitable in fisheries research and other ecological applications.

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