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Andrew D. King
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Andrew D. King, David J. Karoly, and Geert Jan van Oldenborgh
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Sophie C. Lewis, Andrew D. King, and Sarah E. Perkins-Kirkpatrick

Abstract

The term “new normal” has been used in scientific literature and public commentary to contextualize contemporary climate events as an indicator of a changing climate due to enhanced greenhouse warming. A new normal has been used broadly but tends to be descriptive and ambiguously defined. Here we review previous studies conceptualizing this idea of a new climatological normal and argue that this term should be used cautiously and with explicit definition in order to avoid confusion. We provide a formal definition of a new climate normal relative to present based around record-breaking contemporary events and explore the timing of when such extremes become statistically normal in the future model simulations. Applying this method to the record-breaking global-average 2015 temperatures as a reference event and a suite of model climate models, we determine that 2015 global annual-average temperatures will be the new normal by 2040 in all emissions scenarios. At the regional level, a new normal can be delayed through aggressive greenhouse gas emissions reductions. Using this specific case study to investigate a climatological new normal, our approach demonstrates the greater value of the concept of a climatological new normal for understanding and communicating climate change when the term is explicitly defined. This approach moves us one step closer to understanding how current extremes will change in the future in a warming world.

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Mitchell T. Black, David J. Karoly, and Andrew D. King
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Andrew D. King, Mitchell T. Black, David J. Karoly, and Markus G. Donat
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Gareth J. Marshall, Andrew Orr, Nicole P. M. van Lipzig, and John C. King

Abstract

Since the mid-1960s, rapid regional summer warming has occurred on the east coast of the northern Antarctic Peninsula, with near-surface temperatures increasing by more than 2°C. This warming has contributed significantly to the collapse of the northern sections of the Larsen Ice Shelf. Coincident with this warming, the summer Southern Hemisphere Annular Mode (SAM) has exhibited a marked trend, suggested by modeling studies to be predominantly a response to anthropogenic forcing, resulting in increased westerlies across the northern peninsula.

Observations and reanalysis data are utilized to demonstrate that the changing SAM has played a key role in driving this local summer warming. It is proposed that the stronger summer westerly winds reduce the blocking effect of the Antarctic Peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern Antarctic Peninsula. When this occurs, a combination of a climatological temperature gradient across the barrier and the formation of a föhn wind on the lee side typically results in a summer near-surface temperature sensitivity to the SAM that is 3 times greater on the eastern side of the peninsula than on the west. SAM variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula (∼62°S), both at the surface and throughout the troposphere. This is in contrast to a station farther south (∼65°S) where the SAM exerts little influence.

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Kimberley J. Reid, Ian Simmonds, Claire L. Vincent, and Andrew D. King

Abstract

Australian northwest cloudbands (NWCBs) are continental-scale bands of continuous cloud that stretch from northwest to southeast Australia. In earlier studies, where the characteristics of NWCBs and their relationship with precipitation were identified from satellite imagery, there was considerable uncertainty in the results due to limited quality and availability of data. The present study identifies NWCBs from 31 years of satellite data using a pattern-matching algorithm. This new climatology is the longest record based entirely on observations. Our findings include a strong annual cycle in NWCB frequency, with a summer maximum and winter minimum, and a statistically significant increase in annual NWCB days over the period 1984–2014. Physical mechanisms responsible for NWCB occurrences are explored to determine whether there is a fundamental difference between summer and winter NWCBs as hypothesized in earlier studies. Composite analyses are used to reveal that a key difference between these is their genesis mechanisms. Whereas summer NWCBs are triggered by cyclonic disturbances, winter NWCBs tend to form when meridional sea surface temperature gradients trigger baroclinic instability. It was also found that while precipitation is enhanced over parts of Australia during a cloudband day, it is reduced in other regions. During a cloudband day, precipitation extremes are more likely over northwest, central, and southeast Australia, while the probability of extreme precipitation decreases in northeast and southwest Australia. Additionally, cold fronts and NWCBs can interact, leading to enhanced rainfall over Australia.

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David J. Karoly, Mitchell T. Black, Andrew D. King, and Michael R. Grose
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Nicole P. M. van Lipzig, Gareth J. Marshall, Andrew Orr, and John C. King

Abstract

The large regional summer warming on the east coast of the northern Antarctic Peninsula (AP), which has taken place since the mid-1960s, has previously been proposed to be caused by a trend in the Southern Hemisphere Annular Mode (SAM). The authors utilize a high-resolution regional atmospheric model climatology (14-km grid spacing) to study the mechanisms that determine the response of the near-surface temperature to an increase in the SAM (ΔT/ΔSAM). Month-to-month variations in near-surface temperature and surface pressure are well represented by the model. It is found that north of ∼68°S, ΔT/ΔSAM is much larger on the eastern (lee) side than on the western (windward) side of the barrier. This is because of the enhanced westerly flow of relatively warm air over the barrier, which warms (and dries) further as it descends down the lee slope. The downward motion on the eastern side of the barrier causes a decrease in surface-mass balance and cloud cover. South of ∼68°S, vertical deflection across the barrier is greatly reduced and the contrast in ΔT/ΔSAM between the east and west sides of the barrier vanishes. In the northeastern part of the AP, the modeled ΔT/ΔSAM distribution is similar to the distribution derived from satellite infrared radiometer data. The region of strongest modeled temperature sensitivity to the SAM is where ice shelf collapse has recently taken place and does not extend farther south over the Larsen-C Ice Shelf.

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Andrew D. King, Reto Knutti, Peter Uhe, Daniel M. Mitchell, Sophie C. Lewis, Julie M. Arblaster, and Nicolas Freychet

Abstract

Given the Paris Agreement it is imperative there is greater understanding of the consequences of limiting global warming to the target 1.5° and 2°C levels above preindustrial conditions. It is challenging to quantify changes across a small increment of global warming, so a pattern-scaling approach may be considered. Here we investigate the validity of such an approach by comprehensively examining how well local temperatures and warming trends in a 1.5°C world predict local temperatures at global warming of 2°C. Ensembles of transient coupled climate simulations from multiple models under different scenarios were compared and individual model responses were analyzed. For many places, the multimodel forced response of seasonal-average temperatures is approximately linear with global warming between 1.5° and 2°C. However, individual model results vary and large contributions from nonlinear changes in unforced variability or the forced response cannot be ruled out. In some regions, such as East Asia, models simulate substantially greater warming than is expected from linear scaling. Examining East Asia during boreal summer, we find that increased warming in the simulated 2°C world relative to scaling up from 1.5°C is related to reduced anthropogenic aerosol emissions. Our findings suggest that, where forcings other than those due to greenhouse gas emissions change, the warming experienced in a 1.5°C world is a poor predictor for local climate at 2°C of global warming. In addition to the analysis of the linearity in the forced climate change signal, we find that natural variability remains a substantial contribution to uncertainty at these low-warming targets.

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