No abstract

The causes of the Texas-northern Mexico drought during 2010-11 are shown, using observations, reanalyses, and model simulations, to arise from a combination of ocean forcing and internal atmospheric variability. The drought began in fall 2010 and winter 2010/11 as a La Niña event developed in the tropical Pacific Ocean. Climate models forced by observed sea surface temperatures (SSTs) produced dry conditions in fall 2010 through spring 2011 associated with transient eddy moisture flux divergence related to a northward shift of the Pacific-North American storm track, typical of La Niña events. In contrast the observed drought was not associated with such a clear shift of the transient eddy fields and instead was significantly influenced by internal atmospheric variability including the negative North Atlantic Oscillation of winter 2010/11, which created mean flow moisture divergence and drying over the southern Plains and southeast United States. The models suggest that drought continuation into summer 2011 was not strongly SST forced. Mean flow circulation and moisture divergence anomalies were responsible for the summer 2011 drought, arising from either internal atmospheric variability or a response to dry summer soils not captured by the models. The summer of 2011 was one of the two driest and hottest summers over recent decades but it does not represent a clear outlier to the strong inverse relation between summer precipitation and temperature in the region. Seasonal forecasts at 3.5-month lead time did predict onset of the drought in fall and winter 2010/11 but not intensification into summer 2011, demonstrating the current, and likely inherent, inability to predict important aspects of North American droughts.

The prevailing wet climate in the western Amazon is not favorable to the natural occurrence of fires. Nevertheless, the current process of clearing of humid forests for agriculture and cattle ranching has increased the vulnerability of the region to the spread of fires. Using meteorological stations precipitation and the Moderate Resolution Spectroradiometer (MODIS) Active‐Fires (AF) during 2000–2009, we show that fire anomalies vary closely with July‐August‐September (JAS) precipitation variability as measured by the Standardized Precipitation Index (SPI). The precipitation variability is, in turn, greatly determined by sea surface temperature (SST) anomalies in the North Tropical Atlantic (NTA). We develop a linear regression model to relate local fire activity to an index of the NTA‐SST. By using seasonal forecasts of SST from a coupled model, we are able to predict anomalous JAS fire activity as early as April. We applied the method to predict the severe 2010 JAS season, which indicated strongly positive seasonal fire anomalies within the 95% prediction confidence intervals in most western Amazon. The spatial distribution of predicted SPI was also in accordance with observed precipitation anomalies. This three months lead time precipitation and fire prediction product in the western Amazon could help local decision makers to establish an early warning systems or other appropriate course of action before the fire season begins.

Pacific decadal variability is studied in a series of coupled global ocean-atmosphere simulations aided by two ''modeling surgery'' strategies: partial coupling (PC) and partial blocking (PB). The PC experiments retain full ocean-atmosphere coupling in selected regions, but constrain ocean-atmosphere coupling elsewhere by prescribing the model climatological SST to force the atmospheric component of the coupled system. In PB experiments, sponge walls are inserted into the ocean component of the coupled model at specified latitudinal bands to block the extratropical-tropical oceanic teleconnection. Both modeling and observational studies suggest that Pacific decadal variability is composed of two distinct modes: a decadal to bidecadal tropical Pacific mode (TPM) and a multidecadal North Pacific mode (NPM). The PC and PB experiments showed that the tropical Pacific mode originates predominantly from local coupled ocean-atmosphere interaction within the tropical Pacific. Extratropical-tropical teleconnections, although not a necessary precondition for the genesis of the tropical decadal variability, can enhance SST variations in the Tropics. The decadal memory in the Tropics seems to be associated with tropical higher baroclinic modes. The North Pacific mode originates from local atmospheric stochastic processes and coupled ocean-atmosphere interaction. Atmospheric stochastic forcing can generate a weaker NPM-like pattern in both the atmosphere and ocean, but with no preferred timescales. In contrast, coupled ocean-atmosphere feedback can enhance the variability substantially and generate a basin-scale multidecadal mode in the North Pacific. The multidecadal memory in the midlatitudes seems to be associated with the delayed response of the subtropical/subpolar gyre to wind stress variation in the central North Pacific and the slow growing/decaying of SST anomalies that propagate eastward in the Kuroshio Extension region. Oceanic dynamics, particularly the advection of the mean temperature by anomalous meridional surface Ekman flow and western boundary currents, plays an important role in generating the North Pacific mode.

Anthropogenic aerosols are a major factor contributing to human‐induced climate change, particularly over the densely populated Asian monsoon region. Understanding the physical processes controlling the aerosol‐induced changes in monsoon rainfall is essential for reducing the uncertainties in the future projections of the hydrological cycle. Here we use multiple coupled and atmospheric general circulation models to explore the physical mechanisms for the aerosol‐driven monsoon changes on different time scales. We show that anthropogenic aerosols induce an overall reduction in monsoon rainfall and circulation, which can be largely explained by the fast adjustments over land north of 20 ∘ N. This fast response occurs before changes in sea surface temperature (SST), largely driven by aerosol‐cloud interactions. However, aerosol‐induced SST feedbacks (slow response) cause substantial changes in the monsoon meridional circulation over the oceanic regions. Both the land‐ocean asymmetry and meridional temperature gradient are key factors in determining the overall monsoon circulation response.

In this study, a new modeling approach is used to look for potential causes of the North Pacific decadal climate regime shift. This new modeling approach is specifically designed to assess not only how changes of the wind-driven ocean circulation induce SST variability, but also the subsequent feedback to climate. Observations appear to indicate that the 1970s North Pacific climate regime shift may be attributed to the coupled ocean–atmosphere interaction over the North Pacific in response to persistent wind stress anomalies in the previous decade. This tends to be supported by modeling results, which suggest that the delayed adjustment of the subtropical ocean circulation may generate sea surface temperature (SST) anomalies in the western subtropical Pacific that may potentially induce a shift of atmospheric circulation, leading to a change of SST in the central and midlatitude North Pacific. This study appears to unify the recent contradictory views of the roles of ocean circulation in the North Pacific decadal climate variability.

This study analyzes the association between wintertime high-wind events (HWEs) in the northeastern United States and extratropical cyclones. Sustained wind maxima in the daily summary data from the National Climatic Data Center’s integrated surface database are analyzed for 1979–2012. For each station, a generalized Pareto distribution is fit to the upper tail of the daily maximum wind speed data, and probabilistic return levels at 1, 3, and 5 yr are derived. Wind events meeting the return-level criteria are termed HWEs. The HWEs occurring on the same day are grouped into simultaneous wind exceedance dates, termed multistation events. In a separate analysis, extratropical cyclones are tracked using ERA-Interim. The multistation events are associated with the extratropical cyclone tracks on the basis of cyclone proximity on the day of the event. The multistation wind events are found to be most often associated with cyclones traveling from southwest to northeast, originating west of the Appalachian Mountains. To quantify the relative frequency of the strong-wind-associated cyclones, the full set of northeastern cyclone tracks is separated on the basis of path, using a crosshairs algorithm designed for this region. The tracks separate into an evenly distributed set of four pathways approaching the northeastern United States: from due west, from the southwest, and from the southeast and storms starting off the coast north of the Carolinas. Using the frequency of the tracks in each of the pathways, it is shown that the storms associated with multistation wind events are most likely to approach the northeastern United States from the southwest.

During the strong 2015/16 El Niño, only normal to below-average precipitation fell across California in the late winter. This disagrees with both predictions by the ensemble mean of forecast models and expectations for strong El Niños. The authors examine one of the possible reasons why this event did not bring expected precipitation to California in the late winter. The maximum equatorial Pacific sea surface temperature anomalies (SSTAs) were located, compared to the 1982/83 and 1997/98 strong El Niños, farther to the west in the 2015/16 winter, which possibly caused less convection in the eastern tropical Pacific and shifted the teleconnection patterns westward in the North Pacific, thus weakening the influences on California. The SSTA and precipitation forecast for February–April 2016, based on the North American Multimodel Ensemble, showed large discrepancies from observations, with the ensemble mean of most of the models overestimating SSTAs in the eastern tropical Pacific and California precipitation. Atmospheric general circulation model (AGCM) experiments were conducted to test the hypothesis that the warmer eastern tropical Pacific SSTA forecast may have caused the wetter forecast in California in 2015/16 compared to observations. The AGCM experiments suggest it is difficult to assert that the eastern tropical Pacific SSTAs caused the too-wet California precipitation forecast, especially in Southern California, given that the models disagree. Results indicate forecast error can be influenced by atmosphere-model sensitivity to forecast SSTs, but they also indicate atmospheric internal variability may have been responsible for the combination of a strong El Niño and near-normal California precipitation.