Contribution of the cold sector of extratropical cyclones to mean state features in winter

Recent studies have shown that midlatitude air-sea interactions are strongly modulated by synoptic variability. The present study investigates how air-sea interactions over the Gulf-Stream sea surface temperature (SST) front vary in different synoptic regimes. We focus more particularly on the variation of three atmospheric mean state features which are colocated with the SST front in winter : enhanced ascent and precipitation on the warm side of the SST front and enhanced low-level baroclinicity at the entrance of the storm track. These three fields are partitioned depending on whether they occur in the cold sector of extratropical cyclones or in any other synoptic features. The analysis is based on ERA Interim winter data covering the period 1979–2012. Results are twofold. (i) Cold sector precipitation is confined within a 5° of latitude band south of the SST front and reaches 2 mm day-1, whereas precipitation occurring outside the cold sector forms a broader spatial pattern. The same partitioning applied to vertical wind shows that the ascent on the warm side of the Gulf Stream is not a feature of the cold sector. These results mean that a significant part of the anchoring effect of the SST front on precipitation occurs via the cold sector and that the observed colocation of ascent and precipitation is not causal, in contrast to what was suggested by previous studies, but is rather the result of two different mechanisms. (ii) The surface heat fluxes and convection occurring in the cold sector restore low-level atmospheric temperature gradients within 2 days after a time maximum of meridional eddy heat flux, such that low-level baroclinicity remains largely unchanged after the passage of an extratropical cyclone. This "cold path mechanism" opens new avenues to understand how SST forces climate variability in the midlatitudes

Mechanisms controlling the SST air-sea heat flux feedback and its dependence on spatial scale

Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Climate Dynamics 48 (2017): 1297–1307, doi:10.1007/s00382-016-3142-3.The turbulent air-sea heat flux feedback (α, in W m-2 K-1) is a major contributor to setting the damping timescale of sea surface temperature (SST) anomalies. In this study we compare the spatial distribution and magnitude of α in the North Atlantic and the Southern Ocean, as estimated from the ERA-Interim reanalysis dataset. The comparison is rationalized in terms of an upper bound on the heat flux feedback, associated with \fast" atmospheric export of temperature and moisture anomalies away from the marine boundary layer, and a lower bound associated with "slow" export. It is found that regions of cold surface waters (≤10°C) are best described as approaching the slow export limit. This conclusion is not only valid at the synoptic scale resolved by the reanalysis data, but also on basin scales. In particular, it applies to the heat flux feedback acting as circumpolar SST anomaly scales are approached in the Southern Ocean, with feedbacks of ≤10 W m-2 K-1. In contrast, the magnitude of the heat flux feed-back is close to that expected from the fast export limit over the Gulf Stream and its recirculation with values on the order of ≈40 W m-2 K-1. Further analysis suggests that this high value reflects a compensation between a moderate thermo-dynamic adjustment of the boundary layer, which tends to weaken the heat flux feedback, and an enhancement of the surface winds over warm SST anomalies, which tend to enhance the feedback.Ute Hausmann and John Marshall acknowledge support by the FESD program of NSF.2017-05-0

Ocean heat storage rate unaffected by MOC weakening in an idealized climate model

To study the role of the Atlantic meridional overturning circulation (AMOC) in climate change, we perform an abrupt CO2-doubling experiment using a coupled atmosphere-ocean-ice model with a simple geometry that separates the ocean into small and large basins. As in observations and high-end climate models, the small basin exhibits a MOC and warms at a faster rate than the large basin. In our set-up, this contrast in heat storage rates is 0.6 ± 0.1 W m2, and we argue that this is due to the small basin MOC. However, the MOC weakens significantly, yet this has little impact on the small basin’s heat storage rate. We find this is due to the effects of both compensating warming pat- 22 terns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant

On the contribution of synoptic transients to the mean atmospheric state in the Gulf Stream region

Abstract A new decomposition of the time mean sea level pressure, precipitation, meridional velocity and pressure vertical velocity is applied to ERAInterim reanalysis data over the North Atlantic ocean for the December

Quasi-stationary waves and their impact on European weather and extreme events

Large-scale, quasi-stationary atmospheric waves (QSWs) have long been known to be associated with weather extremes such as the European heatwave in 2003. There is much debate in the scientific literature as to whether QSW activity may increase under a changing climate, providing a strong motivation for developing a better understanding of the behaviour and drivers of QSWs. This paper presents the first steps in this regard: the development of a robust objective method for a simple identification and characterisation of these waves. A clear connection between QSWs and European weather and extreme events is confirmed for all seasons, indicating that blocking anticyclones are often part of a broader scale wave pattern. Investigation of the QSW climatology in the Northern Hemisphere reveals that wave activity is typically strongest in midlatitudes, particularly at the exit of the Atlantic and Pacific storm track with weaker intensities in summer. In general, the structure of individual QSW events tends to follow the climatological pattern, except in winter where the strongest and most persistent QSWs are typically shifted polewards, indicating a distinct evolution of the ’strongest’ QSW events. Modes of inter-annual variability are calculated to better understand their importance and connection to European temperatures and to identify relevant QSW patterns. This analysis highlights that European winter temperatures are strongly associated with the meridional location of QSW activity whereas warm European summer temperatures are associated with increases in the overall intensity of midlatitude QSW activity. QSWs are shown to be strongly connected to commonly used indices to describe the large scale atmospheric circulation (NAO, AO, Ni˜no 3.4, PNA) but offer a more direct link to understanding their impact on regional weather events. It is therefore hoped that objective identification of QSWs will provide a useful new viewpoint for interpreting large-scale weather alongside more traditional measures and metrics

The role of Atlantic Ocean-atmosphere coupling in affecting North Atlantic oscillation variability

We review the role of ocean-atmosphere interactions over the Atlantic sector in North Atlantic Oscillation (NAO) variability. The emphasis is on physical mechanisms, which are illustrated in simple models and analyzed in observations and numerical models. Some directions of research are proposed to better assess the relevance of Atlantic air-sea interactions to observed and simulated NAO variability

A ‘warm path’ for Gulf Stream–troposphere interactions

Warm advection by the Gulf Stream creates a characteristic ‘tongue’ of warm water leaving a strong imprint on the sea surface temperature (SST) distribution in the western North Atlantic. This study aims at quantifying the climatological impact of this feature on cyclones travelling across this region in winter using a combination of reanalysis data and numerical experiments. It is suggested that the Gulf Stream ‘warm tongue’ is conducive to enhanced upward motion in cyclones because (i) it helps maintain a high equivalent potential temperature of air parcels at low levels which favors deep ascent in the warm conveyor belt of cyclones and (ii) because the large SST gradients to the north of the warm tongue drive a thermally direct circulation reinforcing and, possibly, destabilizing, the transverse circulation embedded in cyclones. This hypothesis is confirmed by comparing simulations at 12 km resolution from the Met Office Unified Model forced with realistic SST distribution to simulations with an SST distribution from which the Gulf Stream warm tongue was artificially removed or made colder by . It is also supported by a dynamical diagnostic applied to the ERA interim data-set over the wintertime period (1979–2012). The mechanism of oceanic forcing highlighted in this study is associated with near thermal equilibration of low level air masses with SST in the warm sector of cyclones passing over the Gulf Stream warm tongue, which is in sharp contrast to what occurs in their cold sector. It is suggested that this ‘warm path’ for the climatic impact of the Gulf Stream on the North Atlantic storm-track is not currently represented in climate models because of their coarse horizontal resolution

Will high-resolution global ocean models benefit coupled predictions on short-range to climate timescales?

As the importance of the ocean in the weather and climate system is increasingly recognised, operational systems are now moving towards coupled prediction not only for seasonal to climate timescales but also for short-range forecasts. A three-way tension exists between the allocation of computing resources to refine model resolution, the expansion of model complexity/capability, and the increase of ensemble size. Here we review evidence for the benefits of increased ocean resolution in global coupled models, where the ocean component explicitly represents transient mesoscale eddies and narrow boundary currents. We consider lessons learned from forced ocean/sea-ice simulations; from studies concerning the SST resolution required to impact atmospheric simulations; and from coupled predictions. Impacts of the mesoscale ocean in western boundary current regions on the large-scale atmospheric state have been identified. Understanding of air-sea feedback in western boundary currents is modifying our view of the dynamics in these key regions. It remains unclear whether variability associated with open ocean mesoscale eddies is equally important to the large-scale atmospheric state. We include a discussion of what processes can presently be parameterised in coupled models with coarse resolution non-eddying ocean models, and where parameterizations may fall short. We discuss the benefits of resolution and identify gaps in the current literature that leave important questions unanswered

Poleward energy transport: is the standard definition physically relevant at all time scales?

Poleward energy transport in the atmosphere and oceans constitutes an important branch of the global energy budget, and its role in the climate system has been the subject of many studies. In the atmosphere, the transport is affected by “eddies” and large scale meridional cells, both with zero net mass transport across latitude circles, but also partly by processes associated with a net transport of mass across latitude circles. The latter must cease to operate in steady state, but they may be significant when time variability of the heat budget is considered. Indeed, examination of reanalysis data on short (daily to monthly) timescales shows that mass variations on these timescales result in surprisingly large fluctuations (in excess of 10^15 W = 1 PW) in the poleward heat transport. These fluctuations are referred to as “extensive”, for they primarily alter the mass integrated energy of the region considered, but not its averaged value. It is suggested that extensive fluctuations mask more meaningful climate signals present in the heat transport variability on monthly and interannual timescales, and a new formulation is proposed to isolate the latter. This new formulation is applied successfully to reanalysis data and climate model simulations