The climate of the Western Antarctic Peninsula (WAP) is the most rapidly changing in the Southern Hemisphere, with a rise in atmospheric temperature of nearly 3°C since 1951 and associated cryospheric impacts. We demonstrate here, for the first time, that the adjacent ocean showed profound coincident changes, with surface summer temperatures rising more than 1°C and a strong upper‐layer salinification. Initially driven by atmospheric warming and reduced rates of sea ice production, these changes constitute positive feedbacks that will contribute significantly to the continued climate change. Marine species in this region have extreme sensitivities to their environment, with population and species removal predicted in response to very small increases in ocean temperature. The WAP region is an important breeding and nursery ground for Antarctic krill, a key species in the Southern Ocean foodweb with a known dependence on the physical environment. The changes observed thus have significant ecological implications.

While global mean surface air temperature (SAT) has increased over recent decades, the rate 28 of regional warming has varied markedly 10 , with some of the most rapid SAT increases 29 recorded in the polar regions [11][12][13] . In Antarctica, the largest SAT increases have been 30 observed in the Antarctic Peninsula (AP) and especially on its west coast 1 : in particular, 31Vernadsky (formerly Faraday) station (Fig. 1) experienced an increase in annual mean SAT 32 of 2. 8° C between 1951 and 2000. 33 The AP is a challenging area for the attribution of the causes of climate change 34 because of the shortness of the in-situ records, the large inter-annual circulation variability 14 35 and the sensitivity to local interactions between the atmosphere, ocean and ice. In addition, 36 the atmospheric circulation of the AP and South Pacific are quite different between summer 37 (December -February) and the remainder of the year. 38Since the late 1970s the springtime loss of stratospheric ozone has contributed to the 39 warming of the AP, particularly during summer 7 . However, during the extended winter 40 period of March -September, when teleconnections between the tropics and high southern 41 latitudes are strongest 15 , tropical sea surface temperature (SST) anomalies in the Pacific and 42Atlantic Oceans 16 can strongly modulate the climate of the AP. The teleconnections are 43 further affected by the mid-latitude jet, which influences regional cyclonic activity and AP 44SATs. While the jet is strong for most of the year, during the summer it is weaker, there are 45 fewer cyclones, and tropical forcing plays little part in AP climate variability. 46The annual mean SAT records from six coastal stations located in the northern AP 47 (Fig. 1) show a warming through the second half of the Twentieth Century, followed by little 48 change or a decrease during the first part of the Twenty First Century 17 . We investigate the 49 3 differences in high and low latitude forcing on the climate of the AP during what we 50 henceforth term the 'warming' and 'cooling' periods, focussing particularly on the period 51 since 1979, since this marks the start of the availability of reliable, gridded atmospheric 52 analyses and fields of sea ice concentration (SIC). We use a stacked and normalized SAT 53 anomaly record (Fig. 2a) response to stratospheric ozone depletion and increasing greenhouse gas concentrations 5,18 . 68The trend in the SAM led to a greater flow of mild, north-westerly air onto the AP (Extended 69 Data Fig. 2a), with SAT on the northeastern side increasing most because of amplification 70 through the foehn effect 7 . This atmospheric circulation trend contributed to the large decrease 71 in SIC in summer (Extended Data Fig. 3a) and for the year as a whole (Fig. 3a). However, 72there was no significant trend in annual mean sea level pressure (SLP) across the AP during 73 4 the warming period (Fig. 3b). During the summer, tropical climate variability had little 74 influence on the AP SATs 15 and the trend in the...

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 (ϳ62°S), both at the surface and throughout the troposphere. This is in contrast to a station farther south (ϳ65°S) where the SAM exerts little influence.

Surface air temperature records from stations on the west coast of the Antarctic Peninsula show a higher degree of interannual variability and stronger long-term warming trends than recorded elsewhere in Antarctica. Possible mechanisms for driving these fluctuations are investigated. The extreme climatic sensitivity of this region may be linked to a stronger coupling between temperatures and regional sea-ice extent than is seen elsewhere in Antarctica. Significant intcrannual persistence of air temperature anomalies suggests a link with ocean temperatures or circulation.

No abstract

This paper describes the development and evaluation of the UK's new high resolution global coupled model, HiGEM, which is based on the latest climate configuration of the Met Office Unified Model, HadGEM1. In HiGEM, the horizontal resolution has been increased to 1.25 • x 0.83 • in longitude and latitude for the atmosphere, and 1/3 • x 1/3 • globally for the ocean. Multi-decadal integrations of HiGEM, and the lower resolution HadGEM, are used to explore the impact of resolution on the fidelity of climate simulations.Generally SST errors are reduced in HiGEM. Cold SST errors associated with the path of the North Atlantic drift improve, and warm SST errors are reduced in upwelling stratocumulus regions where the simulation of low level cloud is better at higher resolution. The ocean model in HiGEM allows ocean eddies to be partially resolved, which dramatically improves the representation of sea surface height variability. In the Southern Ocean, most of the heat transports in HiGEM is achieved by resolved eddy motions which replaces the parametrised eddy heat transport in the lower resolution model. HiGEM is also able to more realistically simulate small-scale features in the windstress curl around islands and oceanic SST fronts, which may have implications for oceanic upwelling and ocean biology.Higher resolution in both the atmosphere and the ocean allows coupling to occur on small spatial scales. In particular the small scale interaction recently seen in satellite imagery between the atmosphere and Tropical instability waves in the Tropical Pacific ocean is realistically captured in HiGEM. Tropical instability waves play a role in improving the simulation of the mean state of the Tropical Pacific which has important implications for climate variability.In particular all aspects of the simulation of ENSO (spatial patterns, the timescales at which ENSO occurs, and global teleconnections) are much improved in HiGEM.2

The foehn jets -apparent in aircraft observations where available and MetUM simulations of all three cases -are mesoscale features (up to 60 km in width) originating from the mouths of leeside inlets. Through back trajectory analysis they are identified as a type of gap flow. In cases A and B the jets are distinct, being strongly accelerated relative to the background flow, and confined to low levels above the Larsen C Ice Shelf. They resemble the 'shallow foehn' of the Alps. Case C resembles a case of 'deep foehn', with the jets less distinct. The foehn jets are considerably cooler and moister relative to adjacent regions of calmer foehn air. This is due to a dampened foehn effect in the jet regions: in case A the jets have lower upwind source regions, and in the more linear case C there is less diabatic warming and precipitation along jet trajectories due to the reduced orographic uplift across the mountain passes.

[1] We demonstrate that recent observed trends in the annual and austral summer Southern Hemisphere Annular Mode (SAM) are unlikely to be due to internal climate variability, since they exceed any equivalent-length trends in a millennial General Circulation Model (GCM) control run with constant forcings. In contrast we show that observed trends in the SAM are consistent with the combined effects of anthropogenic and natural forcings in GCM simulations. As these trends begin prior to stratospheric ozone depletion we challenge the assertion that this process is primarily responsible for changes in the SAM. Moreover, anthropogenic forcings have a larger effect on the austral summer SAM in combination with natural forcings than when acting in isolation.