Sea Surface Temperature Forcing Research Articles

The Linear Sensitivity of the North Atlantic Oscillation and Eddy-Driven Jet to SSTs

Abstract The North Atlantic Oscillation (NAO) and eddy-driven jet contain a forced component arising from sea surface temperature (SST) variations. Due to large amounts of internal variability, it is not trivial to determine where and to what extent SSTs force the NAO and jet. A linear statistical–dynamic method is employed with a large climate ensemble to compute the sensitivities of the winter and summer NAO and jet speed and latitude to the SSTs. Key regions of sensitivity are identified in the Indian and Pacific basins, and the North Atlantic tripole. Using the sensitivity maps and a long observational SST dataset, skillful reconstructions of the NAO and jet time series are made. The ability to skillfully forecast both the winter and summer NAO using only SST anomalies is also demonstrated. The linear approach used here allows precise attribution of model forecast signals to SSTs in particular regions. Skill comes from the Atlantic and Pacific basins on short lead times, while the Indian Ocean SSTs may contribute to the longer-term NAO trend. However, despite the region of high sensitivity in the Indian Ocean, SSTs here do not provide significant skill on interannual time scales, which highlights the limitations of the imposed SST approach. Given the impact of the NAO and jet on Northern Hemisphere weather and climate, these results provide useful information that could be used for improved attribution and forecasting.

The Asymmetric Atmospheric Response to the Midlatitude North Pacific SST Anomalies

AbstractForced by Pacific Decadal Oscillation‐related sea surface temperature (SST) anomalies with the same pattern but opposite signs in the western‐central North Pacific, nonlinear wintertime atmospheric responses are produced by a state‐of‐the‐art atmospheric general circulation model (GFDL AM2.1); that is, an obvious equivalent barotropic geopotential low appears over the cold SST forcing (“CSST”), whereas a weak baroclinic structure shows up corresponding to the warm SST forcing (“WSST”), and both of them have similar characteristics in the lower troposphere. Specifically, because of the relatively dry environment in the central North Pacific, nonlinear responses of moisture process including latent heat flux and low‐level atmosphere moisture advection induce asymmetric diabatic heating (Qd): in WSST, Qd tends to increase in the middle‐lower troposphere but decrease in the middle‐upper level, whereas it always increases in the whole troposphere in CSST. Thus, Qd has the same low‐level positive vertical gradient in both CSST and WSST, which produces similar atmospheric circulation anomalies in the lower troposphere. In turn, the asymmetric responses of low‐level temperature advection further modify air temperature meridional gradient as well as atmospheric baroclinicity in the lower troposphere, significantly shifting the transient eddy activities southward in CSST and greatly weakening their intensity in WSST, respectively. Accordingly, the transient eddy vorticity forcing primarily determines the upper‐level atmospheric responses in CSST, but it has unsystematic effects in WSST that are overtaken by Qd. Therefore, the dominance of diabatic heating in WSST and transient eddy forcing in CSST over the central North Pacific lead to the asymmetric atmospheric responses among which the asymmetry of moisture plays an essential role.

Tropical cyclone sensitivities to CO2 doubling: roles of atmospheric resolution, synoptic variability and background climate changes

Responses of tropical cyclones (TCs) to CO2 doubling are explored using coupled global climate models (GCMs) with increasingly refined atmospheric/land horizontal grids (~ 200 km, ~ 50 km and ~ 25 km). The three models exhibit similar changes in background climate fields thought to regulate TC activity, such as relative sea surface temperature (SST), potential intensity, and wind shear. However, global TC frequency decreases substantially in the 50 km model, while the 25 km model shows no significant change. The ~ 25 km model also has a substantial and spatially-ubiquitous increase of Category 3–4–5 hurricanes. Idealized perturbation experiments are performed to understand the TC response. Each model’s transient fully-coupled 2 × CO2 TC activity response is largely recovered by “time-slice” experiments using time-invariant SST perturbations added to each model’s own SST climatology. The TC response to SST forcing depends on each model’s background climatological SST biases: removing these biases leads to a global TC intensity increase in the ~ 50 km model, and a global TC frequency increase in the ~ 25 km model, in response to CO2-induced warming patterns and CO2 doubling. Isolated CO2 doubling leads to a significant TC frequency decrease, while isolated uniform SST warming leads to a significant global TC frequency increase; the ~ 25 km model has a greater tendency for frequency increase. Global TC frequency responds to both (1) changes in TC “seeds”, which increase due to warming (more so in the ~ 25 km model) and decrease due to higher CO2 concentrations, and (2) less efficient development of these“seeds” into TCs, largely due to the nonlinear relation between temperature and saturation specific humidity.

Understanding the inter‐decadal variability of autumn precipitation over North Central China using model simulations

AbstractA recent study has shown a robust relationship between the Pacific Decadal Oscillation (PDO) and inter‐decadal variations in autumn precipitation over North Central China (NCC). However, the physical processes underlying these inter‐decadal precipitation variations are not fully understood. Here we analyse multi‐member ensembles of atmospheric reanalysis and model simulations to examine the atmospheric and precipitation responses to sea surface temperature (SST) forcing. Despite the large inter‐member spread resulting from atmospheric internal variability, the model simulations forced by the observed SSTs show an important role of mid‐latitude atmospheric circulation in influencing the inter‐decadal precipitation variations over NCC. We also analyse the sensitivity experiments using three atmospheric models forced by the Inter‐decadal Pacific Oscillation (IPO)/PDO‐like SST fields. The results suggest that the SST anomalies associated with a negative IPO phase can induce anomalous positive pressure anomalies over the East Asia‐Japan‐North Pacific region, which weakens the East Asian trough (EAT) and produces southerly advection of warm and moist air into NCC, leading to increased precipitation there. This study provides a depiction of how the IPO/PDO‐associated SSTs induce inter‐decadal oscillations in mid‐latitude atmospheric circulations, which in turn cause inter‐decadal precipitation variations over NCC.

Climate Change Amplification of Natural Drought Variability: The Historic Mid-Twentieth-Century North American Drought in a Warmer World

AbstractIn the mid-twentieth century (1948–57), North America experienced a severe drought forced by cold tropical Pacific sea surface temperatures (SSTs). If these SSTs recurred, it would likely cause another drought, but in a world substantially warmer than the one in which the original event took place. We use a 20-member ensemble of the GISS climate model to investigate the drought impacts of a repetition of the mid-twentieth-century SST anomalies in a significantly warmer world. Using observed SSTs and mid-twentieth-century forcings (Hist-DRGHT), the ensemble reproduces the observed precipitation deficits during the cold season (October–March) across the Southwest, southern plains, and Mexico and during the warm season (April–September) in the southern plains and the Southeast. Under analogous SST forcing and enhanced warming (Fut-DRGHT, ≈3 K above preindustrial), cold season precipitation deficits are ameliorated in the Southwest and southern plains and intensified in the Southeast, whereas during the warm season precipitation deficits are enhanced across North America. This occurs primarily from greenhouse gas–forced trends in mean precipitation, rather than changes in SST teleconnections. Cold season runoff deficits in Fut-DRGHT are significantly amplified over the Southeast, but otherwise similar to Hist-DRGHT over the Southwest and southern plains. In the warm season, however, runoff and soil moisture deficits during Fut-DRGHT are significantly amplified across the southern United States, a consequence of enhanced precipitation deficits and increased evaporative losses due to warming. Our study highlights how internal variability and greenhouse gas–forced trends in hydroclimate are likely to interact over North America, including how changes in both precipitation and evaporative demand will affect future drought.

Seasonal precipitation change in the Western North Pacific and East Asia under global warming in two high-resolution AGCMs

To estimate seasonal precipitation change over the western North Pacific and East Asia region (WNP-EA), we use high-resolution atmospheric Model (HiRAM) (50 km resolution) to conduct a series of simulations forced by Representative Concentration Pathways 8.5 (RCP8.5) scenario with the fifth phases of the Coupled Model Intercomparison project (CMIP5) ensemble sea surface temperature (SST) changes (RCP_Ens). The sensitivity of future projections to various SST forcing ( $$ {\text{SST}}_{\text{spa}}^{{\prime }} $$ ) is also assessed. We also assess the sensitivity associated with model dependence by comparing with the results of the Meteorological Research Institute-Atmospheric General Circulation Model (MRI-AGCM3.2S) projections. The major findings are: (1) weakened atmospheric circulation in all seasons in RCP_Ens experiment; (2) more precipitation over most of the northern East Asian continent and the northern WNP oceanic region in all seasons; (3) an anomalous anticyclonic circulation (AAC) together with decreased precipitation over the oceanic WNP-EA in the typhoon season; and (4) largest sensitivity to various $$ {\text{SST}}_{\text{spa}}^{{\prime }} $$ in spring comparing with other seasons. The aforementioned changes are similarly seen in both the HiRAM and MRI-AGCM3.2S, suggesting the reliability of our findings. The moisture budget indicates the dominance of dynamic contribution to the reduced precipitation. By contrast, the increased precipitation may be associated with various processes such as enhanced upward motion, increased water vapor or surface evaporation.

Idealized Modeling of the Atmospheric Boundary Layer Response to SST Forcing in the Western Indian Ocean

Abstract The atmospheric response to sea surface temperature (SST) variations forced by oceanic downwelling equatorial Rossby waves is investigated using an idealized convection-resolving model. Downwelling equatorial Rossby waves sharpen SST gradients in the western Indian Ocean. Changes in SST cause the atmosphere to hydrostatically adjust, subsequently modulating the low-level wind field. In an idealized cloud model, surface wind speeds, surface moisture fluxes, and low-level precipitable water maximize near regions of strongest SST gradients, not necessarily in regions of warmest SST. Simulations utilizing the steepened SST gradient representative of periods with oceanic downwelling equatorial Rossby waves show enhanced patterns of surface convergence and precipitation that are linked to a strengthened zonally overturning circulation. During these conditions, convection is highly organized, clustering near the maximum SST gradient and ascending branch of the SST-induced overturning circulation. When the SST gradient is reduced, as occurs during periods of weak or absent oceanic equatorial Rossby waves, convection is much less organized and total rainfall is decreased. This demonstrates the previously observed upscale organization of convection and rainfall associated with oceanic downwelling equatorial Rossby waves in the western Indian Ocean. These results suggest that the enhancement of surface fluxes that results from a steepening of the SST gradient is the leading mechanism by which oceanic equatorial Rossby waves prime the atmospheric boundary layer for rapid convective development.

Relative Contributions of North and South Pacific Sea Surface Temperature Anomalies to ENSO

AbstractVariations in the sea surface temperature (SST) field in both the North Pacific (represented by the Victoria mode [VM]) and the South Pacific (represented by the South Pacific quadrapole [SPQ] mode) are related to the state of the El Niño–Southern Oscillation (ENSO) three seasons later. Here with the aid of observational data and numerical experiments, we demonstrate that both VM and SPQ SST forcing can influence the development of ENSO events through a similar air‐sea coupling mechanism. By comparing ENSO amplitudes induced by the VM and SPQ, as well as the percentages of strong ENSO events followed by the VM and SPQ events, we find that the VM and SPQ make comparable contributions and therefore have similar levels of importance to ENSO. Additional analysis indicates that although VM or SPQ SST forcing alone may serve as a good predictor for ENSO events, it is more effective to consider their combined influence. A prediction model based on both VM and SPQ indices is developed, which is capable of yielding skillful forecasts for ENSO at lead times of three seasons.

Dynamics on Seasonal Variability of EKE Associated with TIWs in the Eastern Equatorial Pacific Ocean

AbstractEnergetic mesoscale eddies (vortices) associated with tropical instability waves (TIWs) exist in the eastern equatorial Pacific Ocean between 0° and 8°N. This study examines the seasonal variations in eddy kinetic energy (EKE) of TIWs using in situ and satellite observations and elucidates the underlying dynamical mechanisms. The results reveal that the cross-equatorial southerly winds are key to sustaining the high-level EKE (up to ~600 cm2 s−2) from boreal summer to winter in 0°–6°N and 155°–110°W. Because of the β effect and the surface wind divergence, the southerly winds generate anticyclonic wind curls north of the equator that intensify the sea surface temperature (SST) fronts and force the downwelling annual Rossby waves. The resultant sea surface height ridge induces strong horizontal current shears between 0° and 5°N. The intensified current shears and SST fronts generate EKE via barotropic and baroclinic instabilities, respectively. To the extent that the seasonal migration of a northward-displaced intertropical convergence zone intensifies the southerly winds north of, but not south of, the equator, our study suggests that the climatic asymmetry is important for the oceanic eddy generations in the eastern equatorial Pacific Ocean—a result with important implications for coupled climate simulation/prediction.

Dominant Interannual Covariations of the East Asian–Australian Land Precipitation during Boreal Winter

AbstractThe present study applies the empirical orthogonal function (EOF) method to investigate the interannual covariations of East Asian–Australian land precipitation (EAALP) during boreal winter based on observational and reanalysis datasets. The first mode of EAALP variations is characterized by opposite-sign anomalies between East Asia (EA) and Australia (AUS). The second mode features an anomaly pattern over EA similar to the first mode, but with a southwest–northeast dipole structure over AUS. El Niño–Southern Oscillation (ENSO) is found to be a primary factor in modulating the interannual variations of land precipitation over EA and western AUS. By comparison, the Indian Ocean subtropical dipole mode (IOSD) plays an important role in the formation of precipitation anomalies over northeastern AUS, mainly through a zonal vertical circulation spanning from the southern Indian Ocean (SIO) to northern AUS. In addition, the ENSO-independent cold sea surface temperature (SST) anomalies in the western North Pacific (WNP) impact the formation of the second mode. Using the atmospheric general circulation model ECHAM5, three 40-yr numerical simulation experiments differing in specified SST forcings verify the impacts of the IOSD and WNP SST anomalies. Further composite analyses indicate that the dominant patterns of EAALP variability are largely determined by the out-of-phase and in-phase combinations of ENSO and IOSD. These results suggest that in addition to ENSO, IOSD should be considered as another crucial factor influencing the EAALP variability during the boreal winter, which has large implications for improved prediction of EAALP land precipitation on the interannual time scale.

Pacific Ocean Forcing and Atmospheric Variability Are the Dominant Causes of Spatially Widespread Droughts in the Contiguous United States

AbstractThe contributions of oceanic and atmospheric variability to spatially widespread summer droughts in the contiguous United States (hereafter, pan‐CONUS droughts) are investigated using 16‐member ensembles of the Community Climate Model version 3 (CCM3) forced with observed sea surface temperatures (SSTs) from 1856–2012. The employed SST forcing fields are either (i) global or restricted to the (ii) tropical Pacific or (iii) tropical Atlantic to isolate the impacts of these two ocean regions on pan‐CONUS droughts. Model results show that SST forcing of pan‐CONUS droughts originates almost entirely from the tropical Pacific because of atmospheric highs from the northern Pacific to eastern North America established by La Niña conditions, with little contribution from the tropical Atlantic. Notably, in all three model configurations, internal atmospheric variability influences pan‐CONUS drought occurrence by as much or more than the ocean forcing and can alone cause pan‐CONUS droughts by establishing a dominant high centered over the U.S. montane west. Similar results are found for the Community Atmosphere Model version 5 (CAM5). Model results are compared to the observational record, which supports model‐inferred contributions to pan‐CONUS droughts from La Niñas and internal atmospheric variability. While there may be an additional association with warm Atlantic SSTs in the observational record, this association is ambiguous due to the limited number of observed pan‐CONUS droughts. The ambiguity thus opens the possibility that the observational results are limited by sampling over the twentieth century and not at odds with the suggested dominance of Pacific Ocean forcing in the model ensembles.

Evaluating models' response of tropical low clouds to SST forcings using CALIPSO observations

Abstract. Recent studies have shown that, in response to a surface warming, the marine tropical low-cloud cover (LCC) as observed by passive-sensor satellites substantially decreases, therefore generating a smaller negative value of the top-of-the-atmosphere (TOA) cloud radiative effect (CRE). Here we study the LCC and CRE interannual changes in response to sea surface temperature (SST) forcings in the GISS model E2 climate model, a developmental version of the GISS model E3 climate model, and in 12 other climate models, as a function of their ability to represent the vertical structure of the cloud response to SST change against 10 years of CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) observations. The more realistic models (those that satisfy the observational constraint) capture the observed interannual LCC change quite well (ΔLCC/ΔSST=-3.49±1.01 % K−1 vs. ΔLCC/ΔSSTobs=-3.59±0.28 % K−1) while the others largely underestimate it (ΔLCC/ΔSST=-1.32±1.28 % K−1). Consequently, the more realistic models simulate more positive shortwave (SW) feedback (ΔCRE/ΔSST=2.60±1.13 W m−2 K−1) than the less realistic models (ΔCRE/ΔSST=0.87±2.63 W m−2 K−1), in better agreement with the observations (ΔCRE/ΔSSTobs=3±0.26 W m−2 K−1), although slightly underestimated. The ability of the models to represent moist processes within the planetary boundary layer (PBL) and produce persistent stratocumulus (Sc) decks appears crucial to replicating the observed relationship between clouds, radiation and surface temperature. This relationship is different depending on the type of low clouds in the observations. Over stratocumulus regions, cloud-top height increases slightly with SST, accompanied by a large decrease in cloud fraction, whereas over trade cumulus (Cu) regions, cloud fraction decreases everywhere, to a smaller extent.

Reexamination of the Late Pliocene Climate over China Using a 25-km Resolution General Circulation Model

Simulations of past warm climate provide an opportunity to better understand how the climate system may respond to increased greenhouse gas emissions. Using the ~25-km-resolution Community Atmosphere Model, version 4 (CAM4), we examine climate change over China in the Late Pliocene warm period (3.264–3.025 Ma) and further explore the influences of different sea surface temperature (SST) forcings and model horizontal resolutions. Initial evaluation shows that the high-resolution CAM4 performs well in capturing the climatological distribution of present-day temperature, precipitation, and low-level monsoon circulations over China. Based on the standard Pliocene Research, Interpretation and Synoptic Mapping (version 4; PRISM4) boundary conditions, CAM4 predicts an increase of annual mean temperature by ~0.5°C over China in the Late Pliocene relative to the preindustrial era, with the greatest warming in northwest China but cooling in southwest China. Enhanced annual mean precipitation is observed in the Late Pliocene over most of China except for northwest China where precipitation is decreased. The East Asian summer (winter) monsoon is intensified (weakened) in the Late Pliocene as suggested by geological evidence, which is attributed to the enhanced (reduced) land–sea thermal contrast. The East Asian monsoon domain exhibits a northwestward expansion in the Late Pliocene, especially over the Tibetan Plateau. Additionally, our results indicate that the modeled climate change is sensitive to the Late Pliocene SST forcings and model resolution. Particularly, different SST forcings [PRISM4-based vs Pliocene Model Intercomparison Project (PlioMIP)-based SSTs] affect the modeled phase change of summer monsoon and the associated precipitation change, while model resolution (~25 vs 400 km) mainly impacts precipitation change.

Combined Effects of El Niño and the Pacific Decadal Oscillation on Summertime Circulation over East Asia

The El Nino-Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO) are the two major modes of sea surface temperature (SST) variability over equatorial and North Pacific regions, respectively. In this study, the combined effects of the ENSO and PDO on summertime circulation and precipitation fields over East Asia are investigated. Results show that SST forcing associated with a positive ENSO phase intensifies the anticyclonic circulation anomaly over the western North Pacific (WNP) region through a meridionally propagating Rossby wave train or via downward motion due to an overturning circulation stemming from equatorial central Pacific warming. A strong negative SST anomaly over the North Pacific during positive PDO phases increases local meridional temperature gradient and thus induces anomalous westerly winds along 35°N. This wind anomaly forms the northern margin of the anticyclonic circulation anomaly over the WNP. The combined effect of positive ENSO and positive PDO phases strengthens the anticyclonic circulation anomaly more than when these forcings are considered separately. Therefore, to the north of the anomaly, precipitation is enhanced due to the increased moisture flux transport and convergence along the rim of the WNP subtropical high.

On the role of the Atlantic Ocean in forcing tropic cyclones in the Arabian Sea

This study examines the impacts of tropical South Atlantic sea surface temperature (SST) anomalies on the frequency of the Arabian Sea tropical cyclones (TCs) during the pre-monsoon season (May–June) using observations and climate model experiments. There is a statistically significant association between the Atlantic SST anomalies and the frequency of Arabian Sea TCs during May–June of 1979–2016 based on the observations. These results can be explained from a physical perspective through the classic Matsuno-Gill responses to the Atlantic SST forcing, leading to changes in vertical wind shear in the Arabian Sea. The reduced vertical wind shear related to the Atlantic SST warming is associated with anomalous upper-level westerlies and lower-level easterlies. The physical mechanisms identified in the observations are strongly supported by a suite of experiments with an atmospheric general circulation model. Overall, the experiments reproduce the observed Matsuno-Gill responses, which are responsible for changes in vertical wind shear, lending confidence to the strong impacts of the Atlantic SST anomalies on the frequency of pre-monsoon Arabian Sea TCs.

Sea surface temperature, ocean color and wind forcing patterns in the Bay of La Paz, Gulf of California: Seasonal variability

Monthly climatologies are used to estimate the mean seasonal cycle of MODIS-Aqua satellite-derived sea surface temperature (SST), surface chlorophyll (Chl- a ) and their relationship with surface wind stress curl (from the Cross-Calibrate Multi-Platform, CCMP) in the Bay of La Paz located in the southwestern Gulf of California, a region identified as having high biological productivity and significant seasonal variability. Harmonic fit applied to the monthly climatologies of these variables indicate a best fit with annual and semi-annual constituents. The annual amplitude was the dominant signal in both SST and Chl- a , corresponding to the bay-wide seasonal cycle of warming and cooling associated with the influx of upwelled nutrient-rich coastal waters jointly with the water exchange between the bay and the Gulf of California. Empirical orthogonal function (EOF) decomposition yielded the annual and semi-annual patterns of the seasonal cycle that respond to wind forcing. The EOF 1 explained 93% of the SST variance and 48% of the Chl- a variance, showing a homogeneous variability of both variables across the bay and a strong thermal and biological gradient in the vicinity of the Gulf of California with their corresponding amplitude time series representing out-of-phase annual cycles, which corresponded to the regional dynamics of wind forcing pattern. The EOF 2 accounted for 6% of the SST variance and 31% of the Chl- a variance, which reached its peak during winter associated with the high Chl- a anomaly values (>1.5 mg m –3 ) observed along the coast. The intense biological productivity covaried strongly with the wind stress curl annual cycle ( R = 0.5, P < 0.05 ), suggesting that these Chl- a blooms could be associated with coastal currents and upwelling events regulated by the seasonal wind forcing pattern playing an essential role in the modulation of this quasi-permanent biophysical coupling observed in the Bay of La Paz during an annual cycle.

Impacts of Pacific SSTs on Atmospheric Circulations Leading to California Winter Precipitation Variability: A Diagnostic Modeling

One of the primary meteorological causes of the winter precipitation deficits and droughts in California (CA) is anomalous developments and maintenance of upper-tropospheric ridges over the northeastern Pacific. In order to understand and find the key factors controlling the winter precipitation variability in CA, the present study examines two dominant atmospheric modes of the 500 hPa geopotential height in the Northern Hemisphere using an Empirical Orthogonal Function (EOF) and their associated large-scale circulation patterns for the last 41 winters (1974/75–2014/15). Explaining 17.5% of variability, the second mode (EOF2) shows strong anti-cyclonic circulations in the North Pacific and cyclonic circulations in the eastern USA and mid-latitude North Atlantic, similar to the atmospheric circulation observed in the 2013/14 drought of CA. EOF2 is tightly and significantly correlated with CA winter precipitation. EOF2 is associated with warm western‒cool eastern tropical Pacific, resembling a mirror image of canonical El Niño events. In particular, it is found that, since the mid-1990s, sea surface temperatures (SSTs) in the western tropical Pacific have been more tightly correlated with EOF2 and with the variability of CA precipitation. A diagnostic regression model based on the west‒east SST difference in the tropical Pacific developed for two recent decades (1994/95–2014/15) has been found to capture the slow-moving interannual variability of the CA winter precipitation (about 50%). The regression model performs well, especially for the central and northern CA precipitation, where the impacts of El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on precipitation are indecisive. Our results emphasize the significant role of the western tropical Pacific SST forcing in the recent past, and in turn on CA droughts and potentially other precipitation extremes.

The Role of Extratropical Ocean Warming in the Coupled Climate Response to Arctic Sea Ice Loss

AbstractThe role of extratropical ocean warming in the coupled climate response to Arctic sea ice loss is investigated using coupled atmosphere–ocean general circulation model (AOGCM) and uncoupled atmospheric-only (AGCM) experiments. Coupled AOGCM experiments driven by sea ice albedo reduction and greenhouse gas–dominated radiative forcing are used to diagnose the extratropical sea surface temperature (SST) response to sea ice loss. Sea ice loss is then imposed in AGCM experiments both with and without these extratropical SST changes, which are found to extend beyond the regions where sea ice is lost. Sea ice loss in isolation drives warming that is confined to the Arctic lower troposphere and only a weak atmospheric circulation response. When the extratropical SST response caused by sea ice loss is also included in the forcing, the warming extends into the Arctic midtroposphere during winter. This coincides with a stronger atmospheric circulation response, including an equatorward shift in the eddy-driven jet, a deepening of the Aleutian low, and an expansion of the Siberian high. Similar results are found whether the extratropical SST forcing is taken directly from the AOGCM driven by sea ice loss, or whether they are diagnosed using a two-parameter pattern scaling technique where tropical adjustment to sea ice loss is removed. These results suggest that AGCM experiments that are driven by sea ice loss and only local SST increases will underestimate the Arctic midtroposphere warming and atmospheric circulation response to sea ice loss, compared to AOGCM simulations and the real world.

Roles of SST versus Internal Atmospheric Variability in Winter Extreme Precipitation Variability along the U.S. West Coast

The U.S. West Coast exhibits large variability of extreme precipitation during the boreal winter season (December–February). Understanding the large-scale forcing of such variability is important for improving prediction. This motivates analyses of the roles of sea surface temperature (SST) forcing and internal atmospheric variability on extreme precipitation on the U.S. West Coast. Observations, reanalysis products, and an ensemble of Atmospheric Model Intercomparison Project (AMIP) experiments from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are analyzed. It is found that SST forcing only accounts for about 20% of the variance of both extreme and nonextreme precipitation in winter. Under SST forcing, extreme precipitation is associated with the Pacific–North American teleconnection, while nonextreme precipitation is associated with the North Pacific Oscillation. The remaining 80% of extreme precipitation variations can be explained by internal atmospheric dynamics featuring a circumglobal wave train with a cyclonic circulation located over the U.S. West Coast. The circumglobal teleconnection manifests from the mid- to high-latitude intrinsic variability, but it can also emanate from anomalous convection over the tropical western Pacific, with stronger tropical convection over the Maritime Continent setting the stage for more extreme precipitation in winter. Whether forced by SST or internal atmospheric dynamics, atmospheric rivers are a common and indispensable feature of the large-scale environment that produces concomitant extreme precipitation along the U.S. West Coast.

Pacific Decadal Oscillation: Tropical Pacific Forcing versus Internal Variability

The Pacific decadal oscillation (PDO) is the leading mode of sea surface temperature (SST) variability over the North Pacific (north of 20°N). Its South Pacific counterpart (south of 20°S) is the South Pacific decadal oscillation (SPDO). The effects of tropical eastern Pacific (TEP) SST forcing and internal atmospheric variability are investigated for both the PDO and SPDO using a 10-member ensemble tropical Pacific pacemaker experiment. Each member is forced by the historical radiative forcing and observed SST anomalies in the TEP region. Outside the TEP region, the ocean and atmosphere are fully coupled and freely evolve. The TEP-forced PDO (54% variance) and SPDO (46% variance) are correlated in time and exhibit a symmetric structure about the equator, driven by the Pacific–North American (PNA) and Pacific–South American teleconnections, respectively. The internal PDO resembles the TEP-forced component but is related to internal Aleutian low (AL) variability associated with the Northern Hemisphere annular mode and PNA pattern. The internal variability is locally enhanced by barotropic energy conversion in the westerly jet exit region around the Aleutians. By contrast, barotropic energy conversion is weak associated with the internal SPDO, resulting in weak geographical preference of sea level pressure variability. Therefore, the internal SPDO differs from the TEP-forced component, featuring SST anomalies along ~60°S in association with the Southern Hemisphere annular mode. The limitations on isolating the internal component from observations are discussed. Specifically, internal PDO variability appears to contribute significantly to the North Pacific regime shift in the 1940s.