Abstract
The coupling between the stratosphere and the troposphere following two major stratospheric sudden warmings is studied in the Canadian Middle Atmosphere Model using a nudging technique by ...which the zonal-mean evolution of the reference sudden warmings are artificially induced in an ~100-member ensemble spun off from a control simulation. Both reference warmings are taken from a freely running integration of the model. One event is a displacement, the other is a split, and both are followed by extended recoveries in the lower stratosphere. The methodology permits a statistically robust study of their influence on the troposphere below.
The nudged ensembles exhibit a tropospheric annular mode response closely analogous to that seen in observations, confirming the downward influence of sudden warmings on the troposphere in a comprehensive model. This tropospheric response coincides more closely with the lower-stratospheric annular mode anomalies than with the midstratospheric wind reversal. In addition to the expected synoptic-scale eddy feedback, the planetary-scale eddies also reinforce the tropospheric wind changes, apparently responding directly to the stratospheric anomalies.
Furthermore, despite the zonal symmetry of the stratospheric perturbation, a highly zonally asymmetric near-surface response is produced, corresponding to a strongly negative phase of the North Atlantic Oscillation with a much weaker response over the Pacific basin that matches composites of sudden warmings from the Interim ECMWF Re-Analysis (ERA-Interim). Phase 5 of the Coupled Model Intercomparison Project models exhibit a similar response, though in most models the response’s magnitude is underrepresented.
Mediterranean-type climates are defined by temperate, wet winters, and hot or warm dry summers and exist at the western edges of five continents in locations determined by the geography of winter ...storm tracks and summer subtropical anticyclones. The climatology, variability, and long-term changes in winter precipitation in Mediterranean-type climates, and the mechanisms for model-projected near-term future change, are analyzed. Despite commonalities in terms of location in the context of planetary-scale dynamics, the causes of variability are distinct across the regions. Internal atmospheric variability is the dominant source of winter precipitation variability in all Mediterranean-type climate regions, but only in the Mediterranean is this clearly related to annular mode variability. Ocean forcing of variability is a notable influence only for California and Chile. As a consequence, potential predictability of winter precipitation variability in the regions is low. In all regions, the trend in winter precipitation since 1901 is similar to that which arises as a response to changes in external forcing in the models participating in phase 5 of the Coupled Model Intercomparison Project. All Mediterranean-type climate regions, except in North America, have dried and the models project further drying over coming decades. In the Northern Hemisphere, dynamical processes are responsible: development of a winter ridge over the Mediterranean that suppresses precipitation and of a trough west of the North American west coast that shifts the Pacific storm track equatorward. In the Southern Hemisphere, mixed dynamic–thermodynamic changes are important that place a minimum in vertically integrated water vapor change at the coast and enhance zonal dry advection into Mediterranean-type climate regions inland.
Climate models exhibit a wide range in latitudinal position of the Southern Hemisphere westerly jet. Previous work has demonstrated, in the annual mean, that models with lower latitude jets, exhibit ...greater poleward jet shifts under climate forcings. It has been argued that this behavior is due to stronger eddy/mean flow feedbacks in models with lower latitude jets, as inferred from the timescale of the Southern Annular Mode (SAM). Here we revisit this question with a focus on seasonality. Using a larger set of models and forcing scenarios from the Coupled Model Intercomparison Project, phase 5, we find that the jet position/jet shift relationship is strong in winter but insignificant in summer, whereas the model spread in SAM timescales arises primarily in summer, with winter timescales similar across models. The results, therefore, question previous interpretations and motivate an improved understanding of the spread in model behavior.
Key Points
The SH jet position/jet shift correlation is highly seasonal in CMIP5 models
It is strong in JJA but weak in DJF, opposite to the spread in SAM timescales
Proposed theories for the jet position/jet shift relationship appear inadequate to explain this seasonality
Zonal-mean or basin-mean analyses often conclude that the midlatitude circulation will undergo a poleward shift with global warming. In this study, the models from phase 5 of the Coupled Model ...Intercomparison Project are used to provide a detailed examination of midlatitude circulation change as a function of longitude and season. The two-dimensional vertically integrated momentum budget is used to identify the dominant terms that maintain the anomalous surface wind stress, thereby allowing a distinction between features that are maintained by high-frequency eddies and those that involve changes in the lower-frequency or stationary flow.
The U.S. Southwest experienced a strong hydroclimate trend from the 1980s to the 2010s, from cool and wet to warm and dry conditions. Attribution of this trend is challenging due to the influence of ...internal variability but desired by water managers eager to plan for robust signals of climate change in this water‐scarce region. Here we use an empirical method based on constructed circulation analogues to assess the contribution of atmospheric circulation variability to the recent observed hydroclimate trend. Consistent with other studies, we find the observed precipitation trend from 1983 to 2012 to be mainly due to internal atmospheric circulation variability that is driven in part by decadal‐scale tropical Pacific sea surface temperature changes. Removing this internal dynamical component brings the observed precipitation trend into closer agreement with the anthropogenically forced response in climate models, demonstrating progress toward an integrated perspective on climate change attribution.
Plain Language Summary
The U.S. Southwest has been getting drier and warmer over the last few decades. These changes fit the common narrative of what might be expected to happen in response to increasing greenhouse gas concentrations. However, natural variability of precipitation and temperature is known to be large in this region, making it difficult to clearly attribute the recent drying and warming to greenhouse gas forcing. Here we show that while the warming is largely due to greenhouse gas forcing, the drying is mostly due to internal climate variability. To date, only an insignificant drying remains after accounting for this internal climate variability. Unlike previous studies that relied exclusively on climate models, we are able to reach these conclusions based on a combination of observations, an empirical statistical method, and climate models.
Key Points
Most of the drying over the U.S. Southwest from 1983 to 2012 is driven by internal variability rather than anthropogenic climate change
Removing the effects of atmospheric circulation variability largely reconciles drying trends in observations and climate model simulations
Prospects and limitations for the detection of forced temperature and precipitation trends in observations are discussed
The variability of the summer North Pacific Subtropical High (NPSH) has substantial socioeconomic impacts. However, state‐of‐the‐art climate models significantly disagree on the response of the NPSH ...to anthropogenic warming. Inter‐model spread in NPSH projections originates from models' inconsistency in simulating tropical precipitation changes. This inconsistency in precipitation changes is partly due to inter‐model spread in tropical sea surface temperature (SST) changes, but it can also occur independently of uncertainty in SST changes. Here, we show that both types of precipitation uncertainty influence the NPSH via the Matsuno‐Gill wave response, but their relative impact varies by region. Through the modulation of low cloud fraction, inter‐model spread of the NPSH can have a further impact on extra‐tropical land surface temperature. The teleconnection between tropical precipitation and the NPSH is examined through a series of numerical experiments.
Plain Language Summary
The North Pacific Subtropical High (NPSH) is a semi‐permanent high‐pressure system located in the subtropical North Pacific. The variability in the summer NPSH has a significant impact on the monsoon and typhoons over East Asia and the hydroclimate of California. However, future projections of the NPSH using state‐of‐the‐art climate models remain highly uncertain. By evaluating how much individual models deviate from the multi‐model mean at different locations, we find four hot spots of high uncertainty in NPSH projections. Our analysis further reveals that the primary source of model variance in changes in the NPSH is tropical precipitation, which can be attributed to both inter‐model SST‐driven and non‐inter‐model SST‐driven factors. Through numerical experiments, we demonstrate that the teleconnection between tropical precipitation and the NPSH is achieved through wave propagation.
Key Points
Model spread in the response of the summer North Pacific Subtropical High (NPSH) to CO2 stems from model spread in simulating tropical processes
Model spread in tropical sea surface temperature (SST) changes modulates the NPSH by influencing tropical precipitation
Model spread in tropical precipitation changes independent of model spread in SST changes also adds to the uncertainty of the NPSH response
Abstract
In this study, observations and simulations are used to investigate the mechanisms behind the different surface responses over the North Pacific and North Atlantic basins in response to ...sudden stratospheric warmings associated with a polar-night jet oscillation event (PJO SSWs). In reanalysis and a free-running preindustrial simulation, on average, a negative North Atlantic Oscillation (NAO) response is seen, corresponding to an equatorward shift of the eddy-driven jet. This is considered as the canonical tropospheric response to PJO SSWs. In contrast, the response over the North Pacific is muted. This basin-asymmetric response is shaped by the North Pacific air–sea interactions spun up by the tropospheric precursor to PJO SSWs, which prevent the Pacific eddy-driven jet from responding to the downward influence from the stratosphere. To isolate the downward influence from the sudden warming itself from any preconditioning of the troposphere that may have occurred prior to the warming, a nudging technique is used by which a reference PJO SSW is artificially imposed in a 195-member ensemble spun off from a control simulation. The nudged ensembles show a more basin-symmetric negative Northern Annular Mode (NAM) response, in which the eddy-driven jet shifts equatorward in both the Pacific and Atlantic sectors. Monitoring the atmospheric and oceanic conditions in the North Pacific before and at the onset of PJO SSWs may be useful for forecasting whether a basin-asymmetric negative NAO or basin-symmetric negative NAM response is more likely to emerge. This can be further used to improve subseasonal-to-seasonal predictions of weather and climate.
Significance Statement
Stratospheric sudden warming events (SSWs) occur when the eastward winds usually found above the Arctic in the winter spontaneously and rapidly reverse. Following their occurrence, the Northern Hemisphere surface westerlies move southward, sometimes over both the North Atlantic and North Pacific and other times over the North Atlantic only. We therefore wanted to understand this uncertainty in the North Pacific surface westerlies response. We find that the North Pacific surface westerlies response to SSWs can be muted by air–sea interactions over the North Pacific. Our results highlight the importance of monitoring the atmospheric and oceanic conditions in the North Pacific before the occurrence of SSWs to forecast whether the Pacific westerlies are likely to respond to SSWs.
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