North American Precipitation Research Articles

Does El Niño affect MJO-AR connections over the North Pacific and associated North American precipitation?

Does El Niño affect MJO-AR connections over the North Pacific and associated North American precipitation?

Machine Learning Approaches to Improve North American Precipitation Forecasts

Machine Learning Approaches to Improve North American Precipitation Forecasts

Destructive Interference of ENSO on North Pacific SST and North American Precipitation Associated with Aleutian Low Variability

Abstract Identifying the origins of wintertime climate variations in the Northern Hemisphere requires careful attribution of the role of El Niño–Southern Oscillation (ENSO). For example, Aleutian low variability arises from internal atmospheric dynamics and is remotely forced mainly via ENSO. How ENSO modifies the local sea surface temperature (SST) and North American precipitation responses to Aleutian low variability remains unclear, as teasing out the ENSO signal is difficult. This study utilizes carefully designed coupled model experiments to address this issue. In the absence of ENSO, a deeper Aleutian low drives a positive Pacific decadal oscillation (PDO)-like SST response. However, unlike the observed PDO pattern, a coherent zonal band of turbulent heat flux–driven warm SST anomalies develops throughout the subtropical North Pacific. Furthermore, non-ENSO Aleutian low variability is associated with a large-scale atmospheric circulation pattern confined over the North Pacific and North America and dry precipitation anomalies across the southeastern United States. When ENSO is included in the forcing of Aleutian low variability in the experiments, the ENSO teleconnection modulates the turbulent heat fluxes and damps the subtropical SST anomalies induced by non-ENSO Aleutian low variability. Inclusion of ENSO forcing results in wet precipitation anomalies across the southeastern United States, unlike when the Aleutian low is driven by non-ENSO sources. Hence, we find that the ENSO teleconnection acts to destructively interfere with the subtropical North Pacific SST and southeastern United States precipitation signals associated with non-ENSO Aleutian low variability.

Lead Isotopes in North American Precipitation Record the Presence of Saharan Dust

Abstract Atmospheric dust is an important mass transfer and nutrient supply process in Earth surface ecosystems. For decades, Saharan dust has been hypothesized as a supplier of nutrients to the Amazon rainforest and eastern North America. However, isotope studies aimed at detecting Saharan dust in the American sedimentary record have been ambiguous. A large Saharan dust storm emerged off the coast of Africa in June 2020 and extended into the southeastern United States. This storm provided a means to evaluate the influence of Saharan dust in North America confirmed by independent satellite and ground observations. Precipitation samples from 17 sites within the National Atmospheric Deposition Program (NADP) were obtained from throughout the southeastern United States prior to, during, and after the arrival of Saharan dust. Precipitation samples were measured for their lead (Pb) isotopic composition, total Pb content, and 210Pb activity using multicollector inductively coupled plasma mass spectrometry. We measured a significant isotopic shift (approximately 0.7% in the 208Pb/206Pb relative to the 207Pb/206Pb) in precipitation that peaked in late June 2020 when the dust blanketed the southeastern United States. However, the magnitude and short time period of the isotopic shift would make it difficult to detect in sedimentary records.

A 10 km North American precipitation and land-surface reanalysis based on the GEM atmospheric model

Abstract. Environment and Climate Change Canada has initiated the production of a 1980–2018, 10 km, North American precipitation and surface reanalysis. ERA-Interim is used to initialize the Global Deterministic Reforecast System (GDRS) at a 39 km resolution. Its output is then dynamically downscaled to 10 km by the Regional Deterministic Reforecast System (RDRS). Coupled with the RDRS, the Canadian Land Data Assimilation System (CaLDAS) and Precipitation Analysis (CaPA) are used to produce surface and precipitation analyses. All systems used are close to operational model versions and configurations. In this study, a 7-year sample of the reanalysis (2011–2017) is evaluated. Verification results show that the skill of the RDRS is stable over time and equivalent to that of the current operational system. The impact of the coupling between RDRS and CaLDAS is explored using an early version of the reanalysis system which was run at 15 km resolution for the period 2010–2014, with and without the use of CaLDAS. Significant improvements are observed with CaLDAS in the lower troposphere and surface layer, especially for the 850 hPa dew point and absolute temperatures in summer. Precipitation is further improved through an offline precipitation analysis which allows the assimilation of additional observations of 24 h precipitation totals. The final dataset should be of particular interest for hydrological applications focusing on transboundary and northern watersheds, where existing products often show discontinuities at the border and assimilate very few – if any – precipitation observations.

Quantifying atmosphere and ocean origins of North American precipitation variability

How atmospheric and oceanic processes control North American precipitation variability has been extensively investigated, and yet debates remain. Here we address this question in a 50 km-resolution flux-adjusted global climate model. The high spatial resolution and flux adjustment greatly improve the model’s ability to realistically simulate North American precipitation, the relevant tropical and midlatitude variability and their teleconnections. Comparing two millennium-long simulations with and without an interactive ocean, we find that the leading modes of North American precipitation variability on seasonal and longer timescales exhibit nearly identical spatial and spectral characteristics, explained fraction of total variance and associated atmospheric circulation. This finding suggests that these leading modes arise from internal atmospheric dynamics and atmosphere-land coupling. However, in the fully coupled simulation, North American precipitation variability still correlates significantly with tropical ocean variability, consistent with observations and prior literature. We find that tropical ocean variability does not create its own type of atmospheric variability but excites internal atmospheric modes of variability in midlatitudes. This oceanic impact on North American precipitation is secondary to atmospheric impacts based on correlation. However, relative to the simulation without an interactive ocean, the fully coupled simulation amplifies precipitation variance over southwest North America (SWNA) during late spring to summer by up to 90%. The amplification is caused by a stronger variability in atmospheric moisture content that is attributed to tropical Pacific sea surface temperature variability. Enhanced atmospheric moisture variations over the tropical Pacific are transported by seasonal mean southwesterly winds into SWNA, resulting in larger precipitation variance.

The Full Extent of El Niño's Precipitation Influence on the United States and the Americas: The Suboptimality of the Niño 3.4 SST Index

AbstractKey features of El Niño's influence on North American winter precipitation are well‐known but we show that this influence has, hitherto, been suboptimally characterized in winter, and especially in fall and spring. The suboptimality has arisen from the historical over‐reliance on regressions on the Niño 3.4 SST index—a widely used marker of El Niño variability. We show that El Niño's full influence is obtained from assembling the regressions on the spatiotemporal modes constituting El Niño variability, rather than from regressions on an SST index keyed just to its mature phase (Niño 3.4 index). The notably different influence of central and eastern Pacific El Niños on Eastern Seaboard is documented. The full influence of El Niño on North American precipitation is shown to be substantially larger than previously recognized and pronounced not just in northern winter but even in fall and spring—enhancing prospects for year‐round seasonal prediction.

Impacts of the Atlantic warm pool on North American precipitation and global sea surface temperature in a coupled general circulation model

The responses of North American precipitation to the Atlantic warm pool (AWP) are investigated by using observational data and the NCAR Community Earth System Model. We show that the responses are controlled by different physical mechanisms in different seasons. In the warm season, a large AWP reduces the North Atlantic subtropical high, consistent with Gill’s physics. The corresponding influence over North America is northerly wind anomalies in the lower atmosphere, which leads to a precipitation suppression in the central United States. However, in the cold season the AWP’s impact on North American precipitation is operated via the teleconnection of cold SST anomalies in the tropical Pacific which are induced by the AWP. A large AWP enhances the local Hadley circulation that moves across the equator to the tropical southeastern Pacific in the boreal summer. This inter-hemispheric process strengthens the atmospheric sinking and then increases the South Pacific subtropical high, resulting in the enhancement of the surface easterly trade wind and thus cold SST anomalies in tropical southeastern Pacific. The wind-evaporation-SST and Bjerknes feedbacks further lead tropical central Pacific SST to a La Nina-like pattern, which is consistent with that the significant and negative correlation between ENSO and the AWP is observed after the 1990s when the AWP leads by 6–9 months. The AWP-induced La Nina-like SST anomalies in the tropical central Pacific further prompts a negative phase of the Pacific North American teleconnection, resulting in decreased precipitation over the southern United States; and vice versa. In addition to the AWP-induced central Pacific-type of ENSO events, the paper also shows the influences of the AWP on SSTs in the North Pacific and Indian Oceans.

Dynamics, Variability, and Change in Seasonal Precipitation Reconstructions for North America

AbstractCool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

The Impact of Sea Surface Temperature Biases on North American Precipitation in a High-Resolution Climate Model

AbstractPositive precipitation biases over western North America have remained a pervasive problem in the current generation of coupled global climate models. These biases are substantially reduced, however, in a version of the Geophysical Fluid Dynamics Laboratory Forecast-Oriented Low Ocean Resolution (FLOR) coupled climate model with systematic sea surface temperature (SST) biases artificially corrected through flux adjustment. This study examines how the SST biases in the Atlantic and Pacific Oceans contribute to the North American precipitation biases. Experiments with the FLOR model in which SST biases are removed in the Atlantic and Pacific are carried out to determine the contribution of SST errors in each basin to precipitation statistics over North America. Tropical and North Pacific SST biases have a strong impact on northern North American precipitation, while tropical Atlantic SST biases have a dominant impact on precipitation biases in southern North America, including the western United States. Most notably, negative SST biases in the tropical Atlantic in boreal winter induce an anomalously strong Aleutian low and a southward bias in the North Pacific storm track. In boreal summer, the negative SST biases induce a strengthened North Atlantic subtropical high and Great Plains low-level jet. Each of these impacts contributes to positive annual mean precipitation biases over western North America. Both North Pacific and North Atlantic SST biases induce SST biases in remote basins through dynamical pathways, so a complete attribution of the effects of SST biases on precipitation must account for both the local and remote impacts.

Stable isotopes in large herbivore tooth enamel capture a mid-Miocene precipitation spike in the interior Pacific Northwest

Stable carbon and oxygen isotopes of fossil ungulate tooth enamel were used to investigate the paleoclimate of the Pacific Northwest from the mid-Oligocene through Pleistocene, particularly targeting the mid-Miocene Climatic Optimum (MMCO). We specifically test whether precipitation, as inferred from tooth enamel δ13C values relative to the δ13C values of atmospheric CO2 (Δ13C), increased with increasing partial pressure of atmospheric CO2 (pCO2). Analysis emphasized water-dependent equids, rhinos, and proboscideans, but several artiodactyls and Castor were also analyzed as available. Values of Δ13C>+19.5‰ in the lower John Day (~27 and ~18Ma) and Mascall (15–16Ma) localities imply relatively high mean annual precipitation (MAP=ca. 550–800mm/yr). Values of Δ13C<+18.5‰ at ~18Ma and at four levels between 15 and 3Ma imply low MAP (≤250mm/yr), similar to modern climate. High vs. low MAP generally correlates with high vs. low pCO2, implicating pCO2 as a probable driver of MAP in the Pacific Northwest. A climate oscillation model best explains data, such that warm-wet conditions during high pCO2 events alternated with cool-dry conditions during low pCO2 events on timescales of 100kyr. Because of poor time constraints associated with many of the specimens, changes in pCO2 are mainly inferred from marine isotopic data, rather than direct comparison between terrestrial pCO2 reconstructions and MAP. High δ18O values in specimens from the John Day (+21.8±1.0‰; VSMOW) and Mascall (+21.3±0.8‰) Formations could reflect lower elevations for the upwind Cascade Range prior to ~7Ma, or proximity to the coast compared to younger, more inland sites (δ18O=+17.7±0.9 to +19.6±1.1‰). Unusually high δ18O values of Dromomeryx sp. from Red Basin (+27.4±0.2‰) probably reflect drought tolerance. General circulation models commonly reproduce temperature but underestimate precipitation during the MMCO in the Pacific Northwest, suggesting that refinements are needed to predict more accurately how western North American precipitation will respond to increases in pCO2 during the next century.

Categorical representation of North American precipitation projections.

We explore use of the familiar tercile framework of seasonal forecasting for the characterization of 21st-century precipitation projections over North America. Consistent with direct analyses of modeled precipitation change, in a superensemble of CMIP5 simulations an unambiguous pattern of shifted tercile population statistics develops as the globe warms. Expressed categorically, frequencies for the low (i.e., dry) tercile increase in the southwestern United States and southward into Mexico and decrease across the northern tier of North America, while counts for the high tercile shift in the opposite sense. We show that as the 21st-century proceeds, changes become statistically significant over wide regions in the pointwise sense, and also when considered as projections on model-specific climate change “fingerprints”. Background noise in the superensemble, against which significance is established, comprises both structural model uncertainty and natural climate variability. The robustness of these findings makes a compelling case for long-range planning for a dryer future in the American Southwest and southward, and wetter one to the north and especially northeast, while communication is facilitated by widespread user familiarity with the tercile format.

Atmospheric Responses to North Atlantic SST Anomalies in Idealized Experiments. Part II: North American Precipitation

Abstract This is the second part of a two-part paper that addresses deterministic roles of the sea surface temperature (SST) anomalies associated with the Atlantic multidecadal oscillation (AMO) in variations of atmospheric circulation and precipitation in the Northern Hemisphere, using a sequence of idealized model runs at the spring equinox conditions. This part focuses on the effect of the SST anomalies on North American precipitation. Major results show that, in the model setting closest to the real-world situation, a warm SST anomaly in the North Atlantic Ocean causes suppressed precipitation in central, western, and northern North America but more precipitation in the southeast. A nearly reversed pattern of precipitation anomalies develops in response to the cold SST anomaly. Further examinations of these solutions reveal that the response to the cold SST anomaly is less stable than that to the warm SST anomaly. The former is “dynamically charged” in the sense that positive eddy kinetic energy (EKE) exists over the continent. The lack of precipitation in its southeast is because of an insufficient moisture supply. In addition, the results show that the EKE of the short- (2–6 day) and medium-range (7–10 day) weather-producing processes in North America have nearly opposite signs in response to the same cold SST anomaly. These competing effects of eddies in the dynamically charged environment (elevated sensitivity to moisture) complicate the circulation and precipitation responses to the cold SST anomaly in the North Atlantic and may explain why the model results show more varying precipitation anomalies (also confirmed by statistical test results) during the cold than the warm SST anomaly, as also shown in simulations with more realistic models. Results of this study indicate a need to include the AMO in the right context with other forcings in an effort to improve understanding of interannual-to-multidecadal variations in warm season precipitation in North America.

Boreal peatland water table depth and carbon accumulation during the Holocene thermal maximum, Roman Warm Period, and Medieval Climate Anomaly

The Boreal Shield and James Bay Lowland regions of Ontario have few Holocene reconstructions specific to surface moisture despite their potential importance to documenting the geographic patterns and teleconnections of Holocene pluvials and droughts, as well as their role in the global carbon cycle. We reconstructed water table depth using preserved testate amoebae in four stratigraphic peatland cores, supplemented with a literature review to investigate the effects of the Holocene Thermal Maximum (HTM) and Medieval Climate Anomaly (MCA) on surface moisture and long-term apparent carbon accumulation (LARCA) in the regions. A 7320 calendar years before AD 1950 (yrBP) length record registered a wet shift at 4600 yrBP in concert with the HTM and a later post-2550 yrBP shift towards wetter and more stable conditions, possibly related to the Roman Warm Period. During the late-Holocene the three most southern boreal records indicated a lack of droughts during the MCA. Medieval pluvials in boreal Ontario would be consistent with North American precipitation patterns under La Niña conditions. Of the three wet-warm periods identified (HTM, 2550 yrBP, MCA), the 2550 yrBP shift had the most consistent positive influence on LARCA regionally. Although past precipitation was positively correlated with temperature in the region, recent drying may be indicative of a fundamental change in evaporation-precipitation balance.

North American rainfall and temperature prediction response to the diversity of ENSO

Research has shown that there is significant diversity in the location of the maximum sea surface temperature anomaly (SSTA) associated with the El Nino Southern Oscillation (ENSO). In one extreme, warm SSTA peak near the South American coast (often referred to as Eastern Pacific of EP El Nino), and at the other extreme, warm SSTA peak in the central Pacific (Central Pacific or CP El Nino). Due to the differing tropical Pacific SSTA and precipitation structure, there are differing extratropical responses, particularly over North America. Recent work involving the North American Multi-Model Ensemble (NMME) system for intra-seasonal to inter-annual prediction on prediction of the differences between El Nino events found excess warming in the eastern Pacific during CP El Nino events. This manuscript investigates the ensemble and observational agreement of the NMME system when forecasting the North American response to the diversity of ENSO, focusing on regional land-based 2-meter temperature and precipitation. NMME forecasts of North American precipitation and T2m agree with observations more often during EP events. Ensemble agreement of NMME forecasts is regional. For instance, ensemble agreement in Southeast North America demonstrates a strong connection to NINO3 precipitation and SSTA amplitude during warm ENSO events. Ensemble agreement in Northwest North America demonstrates a weak connection to NINO4 precipitation and SSTA amplitude during warm ENSO events. Still other regions do not show a strong connection between ensemble agreement and strength of warm ENSO events.

Stochastically Generated North American Megadroughts

Abstract The importance of interannual-to-decadal sea surface temperature (SST) influences on drought in the United States is examined using a suite of simulations conducted with the T31×3 resolution version of the NCAR Community Earth System Model (CESM1.0.3). The model captures tropical Pacific teleconnections to North American precipitation reasonably well, although orographic features are somewhat enhanced at higher resolution. The contribution of SST anomalies is isolated by comparing two idealized, 1000-yr CESM1.0.3 experiments: a fully coupled control and an atmosphere-only (CAM4) run forced with the SST climatology from the control. Droughts are identified using the Palmer Drought Severity Index (PDSI), which is computed over four U.S. regions from the CESM1.0.3 experiments and compared with the North American Drought Atlas (NADA). The CESM1.0.3 reproduces the persistence of NADA droughts quite well, although the model underestimates drought severity. Within the CESM1.0.3 framework, SST forcing does not significantly affect drought intensity or frequency of occurrence, even for very persistent “megadroughts” of 15 yr or more in length. In both the CESM1.0.3 and NADA, with the exception of the Southeast United States, droughts in all regions have intensities, persistence lengths, and occurrence frequencies statistically consistent with a red noise null hypothesis. This implies that SST forcing is not the dominant factor in generating drought and therefore that many decadal megadroughts are caused by a combination of internal atmospheric variability and coupling with the land surface, with SST anomalies playing only a secondary role.

Decadal predictability of soil water, vegetation, and wildfire frequency over North America

The potential decadal predictability of land hydrological and biogeochemical variables in North America is examined using a 900-year-long pre-industrial control simulation, conducted with the NCAR Community Earth System Model (CESM) version 1.0.3. The leading modes of simulated North American precipitation and soil water storage are characterized essentially by qualitatively similar meridional seesaw patterns associated with the activity of the westerly jet. Whereas the corresponding precipitation variability can be described as a white noise stochastic process, power spectra of vertically integrated soil water exhibit significant redness on timescales of years to decades, since the predictability of soil water storage arises mostly from the integration of precipitation variability. As a result, damped persistence hindcasts following a 1st order Markov process are skillful with lead times of up to several years. This potential multi-year skill estimate is consistent with ensemble hindcasts conducted with the CESM for various initial conditions. Our control simulation further suggests that decadal variations in soil water storage also affect vegetation and wildfire occurrences. The long-term potential predictability of soil water variations in combination with the slow regrowth of vegetation after major disruptions leads to enhanced predictability on decadal timescales for vegetation, terrestrial carbon stock, and fire frequency, in particular in the Southern United States (US)/Mexico region. By contrast, the prediction skill of fire frequency in the Northern US is limited to 1 year. Our results demonstrate that skillful decadal predictions of soil water storage, carbon stock, and fire frequency are feasible with proper initialization of soil conditions. Although the potential predictability in our idealized modeling framework would overestimate the real predictability of the coupled climate-land-vegetation system, the decadal climate prediction may become beneficial for water resource management, forestry, and agriculture.

Extracting the Dominant SST Modes Impacting North America's Observed Climate*

Abstract The seasonal impacts of the dominant sea surface temperature (SST) modes to North American climate are assessed comprehensively in observations using the multivariate generalized equilibrium feedback assessment (GEFA) method. The GEFA method is first validated before applying it to observations. Impacts of each individual SST mode are quantified and the associated mechanisms are discussed. Four critical SST modes for North American climate are found: the ENSO mode, Indian Ocean Basin (IOB) mode, North Pacific first empirical orthogonal function (EOF) mode, and tropical Atlantic second EOF mode. The impacts of the ENSO mode are consistent with previous studies qualitatively, while the impact strength is further quantified here. The IOB mode has a strong influence on surface air temperature across North America, and it is demonstrated for the first time that its impact strength might even exceed that of ENSO during both winter and summer. The IOB mode also affects the year-round precipitation. A deeper understanding of the impact of North Pacific SSTs on wintertime surface air temperature is achieved: namely, positive SST anomalies in the Kuroshio Extension region correspond to colder (warmer) air in western (eastern) North America. The tropical Atlantic has a more significant influence on North American precipitation than does the extratropical Atlantic, with colder than normal tropical North Atlantic SSTs supporting wetter conditions across much of the United States, especially during autumn. Because of the linearity of GEFA, the total impacts of multiple SST modes can be obtained by the linear combination of each individual mode's impact. The GEFA method is a potentially powerful tool for seasonal climate prediction.

AMO-Forced Regional Processes Affecting Summertime Precipitation Variations in the Central United States

Abstract Numerous previous studies have provided insight into the influence of the Atlantic multidecadal oscillation (AMO) on North American precipitation. However, these studies focused on large-scale processes, and additional studies are needed to gain understanding of local and regional processes that develop in different phases of the AMO and substantiate its influences on precipitation. In this study, the Weather Research and Forecasting (WRF) regional model is used to examine AMO-forced local and regional processes and how they have affected summertime precipitation variation in the central United States. While moisture transport and convergence by the Great Plains low-level jet have been recognized as necessary conditions for summer precipitation, model simulations show similar low-level moisture flux convergence in the central United States between the cold and warm phases of the AMO. However, there was a strong moistening in the lower troposphere during the AMO cold phase, making the atmosphere more unstable for convection and precipitation. The source of the moisture was found to be a strong positive surface evaporation–precipitation feedback initiated and sustained by increased relative vorticity along a frontal zone. Along the frontal zone, isentropic stretching of the upper-level atmosphere and cyclonic circulation anomalies increased the relative vorticity during the AMO cold phase, providing the dynamic support needed to release the low-level moist instability and produce the increased precipitation. These results indicate that the dynamics of the circulation in the AMO cold phase played key roles to organize regional vorticity processes that further sustained a coupling of precipitation and the surface evaporation and perpetuated the precipitation.

Mechanisms of Tropical Atlantic SST Influence on North American Precipitation Variability*

Abstract The dynamical mechanisms associated with the impact of year-to-year variability in tropical North Atlantic (TNA) sea surface temperatures (SSTs) on North American precipitation, during the cold and warm halves of the hydrological year (October–September) are examined. Observations indicate that during both seasons warmer-than-normal TNA SSTs are associated with a reduction of precipitation over North America, mainly west of ∼90°W, and that the effect can be up to 30% of the year-to-year seasonal precipitation RMS variability. This finding confirms earlier studies with observations and models. During the cold season (October–March) the North American precipitation variability associated with TNA fluctuations is considerably weaker than its association with ENSO. During the warm season (April–September), however, the Atlantic influence, per one standard deviation of SST anomalies, is larger than that of ENSO. The observed association between TNA SST anomalies and global and North American precipitation and sea level pressure variability is compared with that found in the output of an atmospheric general circulation model (AGCM) forced with observed SST variability, both globally and in the tropical Atlantic alone. The similarity between model output and observations suggests that TNA SST variability is causal. The mechanisms of the “upstream” influence of the Atlantic on North American precipitation are seasonally dependent. In the warm season, warmer-than-normal TNA SSTs induce a local increase in atmospheric convection. This leads to a weakening of the North Atlantic subtropical anticyclone and a reduction in precipitation over the United States and northern Mexico, associated with the anomalous southward flow there. In the cold season, a response similar to the warm season over the subtropical Atlantic is identified, but there is also a concomitant suppression of convection over the equatorial Pacific, which leads to a weakening of the Aleutian low and subsidence over western North America, similar to the impact of La Niña although weaker in amplitude. The impact of TNA SST on tropical convection and the extratropical circulation is examined by a set of idealized experiments with a linear general circulation model forced with the tropical heating field derived from the full AGCM.