Tropical Pacific SSTs Research Articles

Holocene hydroclimatic variability recorded in sediments from Maddox Lake (northern California Coast Range)

AbstractPerspectives on past climate using lake sediments are critical for assessing modern and future climate change. These perspectives are especially important for water-stressed regions such as the western United States. One such region is northwestern California (CA), where Holocene-length hydroclimatic records are scarce. Here, we present a 9000-year, relative lake level record from Maddox Lake (CA) using a multi-indicator approach. The Early Holocene is characterized by variably low lake levels with a brief excursion to wetter climates/relative highstand ca. 8.4–8.06 cal ka BP, possibly related to the 8.2 ka cold event and changing Atlantic Meridional Overturning Circulation (AMOC). From 5.2–0.55 cal ka BP, Maddox Lake experienced a long-term regression, tracking changes in summer-winter insolation, tropical and northeast Pacific SSTs, and the southward migration of the ITCZ. This gradual regression culminated in a pronounced relative lowstand during the Medieval Climatic Anomaly (MCA). A marked relative highstand followed the MCA, correlative to the Little Ice Age. The latter reflects a far-field response to North Atlantic volcanism, solar variability, and possibly changes in AMOC and Arctic sea ice extent. Our results further confirm the hydroclimatic sensitivity of northwest California to various forcings including those emanating from the North Atlantic.

Atmospheric impacts of local sea surface temperatures versus remote drivers during strong South China Sea winter cold tongue events

AbstractSea surface temperatures (SSTs) in the western part of the South China Sea (SCS) are cooler than in the eastern part in boreal winter, owing to a winter climatological cold tongue (CT). In this study, using a regional atmospheric model configured for the Maritime Continent, we assess the atmospheric impacts of local (or SCS) SSTs versus those from remote drivers (e.g., western tropical Pacific SSTs) during strong CT events with anomalously cool SSTs. In the local run, more rainfall is observed over the eastern SCS, but no significant atmospheric impacts are found over the CT region when SSTs associated with strong CT events are imposed within the SCS while climatological conditions are imposed elsewhere. SCS SST anomalies during strong CT events do not significantly modify the regional wind circulation. The lack of atmospheric response to SSTs over the CT region may be explained by the wintertime mean SSTs (i.e., <27–28°C) over the CT region that are inadequate to trigger deep atmospheric convection, while eastern SCS SSTs are high enough. The increase of anomalous positive moist static energy (MSE) near the sea level over the eastern SCS indicates underlying warm eastern SCS SST anomalies could be influencing positive rainfall anomalies. In the remote run, imposing climatological SCS SSTs but remote SSTs and lateral boundary conditions linked to strong CT events results in cyclonic wind and positive rainfall anomalies over the eastern SCS and Philippines, which are a Matsuno–Gill response to the diabatic heating anomalies over the warm western tropical Pacific SST anomalies. Positive rainfall and cloud cover anomalies associated with the cyclonic wind anomalies are due to the anomalous positive MSE import into the eastern SCS by horizontal advection.

The role of interannual ENSO events in decadal timescale transitions of the Interdecadal Pacific Oscillation

The build-up of decadal timescale upper ocean heat content in the off-equatorial western tropical Pacific can provide necessary conditions for interannual El Niño/Southern Oscillation (ENSO) events to contribute to decadal timescale transitions of tropical Pacific SSTs to the opposite phase of the Interdecadal Pacific Oscillation (IPO). This can be viewed as a corollary to subseasonal westerly wind burst events contributing to El Niño interannual timescale transitions. A long pre-industrial control run with CESM1 is analyzed to show that there is a greater chance of ENSO activity to contribute to an IPO transition when off-equatorial western Pacific Ocean heat content reaches either a maximum (for El Niño to contribute to a transition to positive IPO) or minimum (for La Niña to contribute to a transition to negative IPO) as seen in observations. If above a necessary ocean heat content threshold, the convergence associated with westerly anomaly near-equatorial surface winds associated with El Niño activity can draw that heat content equatorward to sustain anomalously warm western and central Pacific SSTs. These are associated with positive precipitation and convective heating anomalies, a Gill-type response and wind stress curl anomalies that continue to feed warm water into the near-equatorial western Pacific. These conditions then sustain the decadal-timescale transition to positive IPO (with the opposite sign for transition to negative IPO). Associated central equatorial Pacific convective heating anomalies produce SLP and wind stress anomalies in the North and South Pacific that can excite westward-propagating off-equatorial oceanic Rossby waves to contribute to the western Pacific thermocline depth and consequent heat content anomalies.

The Impact of SST Biases in the Tropical East Pacific and Agulhas Current Region on Atmospheric Stationary Waves in the Southern Hemisphere

AbstractClimate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) vary significantly in their ability to simulate the phase and amplitude of atmospheric stationary waves in the midlatitude Southern Hemisphere. These models also suffer from a double intertropical convergence zone (ITCZ), with excessive precipitation in the tropical eastern South Pacific, and many also suffer from a biased simulation of the dynamics of the Agulhas Current around the tip of South Africa. The intermodel spread in the strength and phasing of SH midlatitude stationary waves in the CMIP archive is shown to be significantly correlated with the double-ITCZ bias and biases in the Agulhas Return Current. An idealized general circulation model (GCM) is used to demonstrate the causality of these links by prescribing an oceanic heat flux out of the tropical east Pacific and near the Agulhas Current. A warm bias in tropical east Pacific SSTs associated with an erroneous double ITCZ leads to a biased representation of midlatitude stationary waves in the austral hemisphere, capturing the response evident in CMIP models. Similarly, an overly diffuse sea surface temperature gradient associated with a weak Agulhas Return Current leads to an equatorward shift of the Southern Hemisphere jet by more than 3° and weak stationary wave activity in the austral hemisphere. Hence, rectification of the double-ITCZ bias and a better representation of the Agulhas Current should be expected to lead to an improved model representation of the austral hemisphere.

A Control of ENSO Transition Complexity by Tropical Pacific Mean SSTs Through Tropical‐Subtropical Interaction

AbstractEl Niño–Southern Oscillation (ENSO) transitions from one event to another in complex ways. Using observational analyses and forced atmospheric model experiments, we show that a preceding ENSO event can activate a subtropical Pacific forcing mechanism to trigger another ENSO event during the following year. These tropical‐subtropical Pacific interactions result in a cyclic ENSO transition if the two ENSO events are of opposite signs or a multiyear ENSO transition if they are of the same sign. The preceding ENSO event should excite deep convections in the tropical Pacific in order to activate the subtropical Pacific mechanism. This requirement enables mean temperatures in the cold tongue and warm pool to respectively control how easily the cyclic and multiyear transitions can occur. A future warmer tropical Pacific is projected to decrease the frequency of occurrence of multiyear ENSO transitions but increase the occurrence of cyclic ENSO transitions.

한반도 홀로세 후기 기후와 적도 태평양 해수면 온도 간의 연관성

한반도 홀로세 후기 기후와 적도 태평양 해수면 온도 간의 연관성

The Link between ENSO-like Forcing and Hydroclimate Variability of Coastal East Asia during the Last Millennium

Inconsistent reconstructions of East Asian hydroclimate for the last millennium significantly limit our understanding of the mechanisms behind climate variability during the medieval climate anomaly (MCA) and little ice age (LIA) in the region. In this study, we present new high-resolution multiproxy records (diatom, δ13C, C/N, TS) from the Mulyoungari swamp, Jeju Island, South Korea. Our results indicate that El Niño southern oscillation-like variations caused the dry MCA/wet LIA pattern in the study area. Recent paleo-ENSO studies generally support the hypothesis that the MCA was characterized by more persistent El Niño-like conditions. During El Niño events, the genesis of typhoons affecting coastal East Asia tends to diminish because of warm anomalies of eastern tropical Pacific (ETP) SSTs and downward motions over the western tropical Pacific. Therefore, coastal East Asia likely experienced a decline in typhoon-related precipitation during the MCA, in contrast to monsoon-dominated northern China. Our results additionally imply that SST anomalies in the ETP need to be carefully checked to better understand current hydroclimate variability in coastal East Asia, one of the most populated areas on earth.

A meridional dipole in premonsoon Bay of Bengal tropical cyclone activity induced by ENSO

AbstractAnalysis of Bay of Bengal tropical cyclone (TC) track data for the months of May–June during 1979–2014 reveals a meridional dipole in TC intensification: TC intensification rates increased significantly in the northern region and decreased in the southern region. The dipole is consistent with changes in the large‐scale TC environment estimated using the Genesis Potential Index (GPI) for the same period. While an increase in lower troposphere cyclonic vorticity and midtroposphere humidity in the northern Bay of Bengal made the environment more favorable for TC intensification, enhanced vertical wind shear in the southern Bay of Bengal tended to reduce TC development. These environmental changes were associated with a strengthening of the monsoon circulation for the months of May–June, driven by a La Niña‐like shift in tropical Pacific SSTs and associated tropical wave dynamics. Finally, analysis of a suite of climate models from the Coupled Model Intercomparison Project Phase 5 archive shows that most models correctly reproduce the link between ENSO and premonsoon Bay of Bengal TC activity at interannual timescales, demonstrating the robustness of our main conclusions.

A Multimodel Ensemble Pattern Regression Method to Correct the Tropical Pacific SST Change Patterns under Global Warming

Abstract This study develops a new observational constraint method, called multimodel ensemble pattern regression (EPR), to correct the projections of regional climate change by the conventional unweighted multimodel mean (MMM). The EPR method first extracts leading modes of historical bias using intermodel EOF analysis, then builds up the linear correlated modes between historical bias and change bias using multivariant linear regression, and finally estimates the common change bias induced by common historical bias. Along with correcting common change bias, the EPR method implicitly removes the intermodel uncertainty in the change projection deriving from the intermodel diversity in background simulation. The EPR method is applied to correct the patterns of tropical Pacific SST changes using the historical and representative concentration pathway 8.5 (RCP8.5) runs in 30 models from phase 5 of CMIP (CMIP5) and observed SSTs. The common bias patterns of the tropical Pacific SSTs in historical runs, including the excessive cold tongue, the southeastern warm bias, and the narrower warm pool, are estimated to induce La Niña–like change biases. After the estimated common change biases are removed, the corrected SST changes display a pronounced El Niño–like pattern and have much greater zonal gradients. The bias correction decreases by around half of the intermodel uncertainties in the MMM SST projections. The patterns of corrected tropical precipitation and circulation change are dominated by the enhanced SST change patterns, displaying a pronounced warmer-get-wetter pattern and a decreased Walker circulation with decreased uncertainties.

Atmosphere and Ocean Origins of North American Droughts*

Abstract The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability. Up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988. During the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States. Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts.

The Amundsen Sea low

ABSTRACTWe develop a climatology of the Amundsen Sea low (ASL) covering the period 1979–2008 using ECMWF operational and reanalysis fields. The depth of the ASL is strongly influenced by the phase of the Southern annular mode (SAM) with positive (negative) mean sea level pressure anomalies when the SAM is negative (positive). The zonal location of the ASL is linked to the phase of the mid‐tropospheric planetary waves and the low moves west from close to 110°W in January to near 150°W in June as planetary waves 1 to 3 amplify and their phases shift westwards. The ASL is deeper by a small, but significant amount, during the La Niña phase of El Niño‐Southern Oscillation (ENSO) compared to El Niño. The difference in depth of the low between the two states of ENSO is greatest in winter. There is no statistically significant difference in the zonal location of the ASL between the different phases of ENSO. Over 1979–2008 the low has deepened in January by 1.7 hPa dec−1 as the SAM has become more positive. It has also deepened in spring and autumn as the semi‐annual oscillation has increase in amplitude over the last 30 years. An increase in central pressure and eastward shift in March has occurred as a result of a cooling of tropical Pacific SSTs that altered the strength of the polar front jet. Copyright © 2012 Royal Meteorological Society

Interdecadal modulation of the influence of La Niña events on mei-yu rainfall over the Yangtze River valley

The aim of this study was to investigate changes in the relationship between mei-yu rainfall over East China and La Nia events in the late 1970s, a period concurrent with the Pacific climate shift, using meiyu rainfall data and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. This relationship was modulated by the climate shift: Before the 1977/1978 climate shift and after the 1992/1993 climate shift, mei-yu rainfall levels were above normal in most La Nia years, whereas during the period 1979-1991, mei-yu rainfall was usually below normal levels in La Nia years. Both composite analyses and results from an atmospheric general circulation model show remarkable detail in terms of La Nia's impacts on mei-yu rainfall in the late 1970s due to the change in the mean climatic state over the tropical Pacific. After the late 1970s, the tropical Pacific SSTs were warmer, and the mean state of low-level anticyclone circulation over the western North Pacific (WNP) weakened. Superimposed on La Nia-related cyclonic anomaly over the WNP, anticyclonic circulation weakened. Prior to the late 1970s, the mean state of low-level anticyclone circulation over the WNP was stronger and was less affected by La Nia-related anomalous cyclones. Anticyclone circulation may have brought moisture to the Yangtze River valley, leading to above-normal rainfall.

Atlantic and Pacific sea surface temperatures and corn yields in the southeastern USA: lagged relationships and forecast model development

AbstractSea surface temperatures (SSTs) can provide valuable predictive information for regional climate and agricultural yields in many parts of the world. This study was conducted to identify relationships between Atlantic and Pacific SSTs and corn yields in Alabama, Florida, and Georgia and to use these relationships to develop forecasts that can be used at lead times prior to spring planting. The relationships between seasonal SSTs and detrended county corn yields were analysed using singular value decomposition (SVD) analysis and confirmed using principal component analysis (PCA). Using a Monte Carlo approach, field‐significant results were found with SSTs in the July–September (JAS−1) and October–December (OND−1) seasons in the previous year and with the January–March (JFM) season of the current. A significant portion of the variability of SSTs with corn yield residuals was connected with anomalous heating and cooling associated with the El Niño Southern Oscillation (ENSO). Based on the results found by SVD analysis and confirmed by PCA, indices of spatially averaged SSTs in regions of the North Pacific and Atlantic Oceans were derived. Using these indices along with the Niño3.4 index to represent tropical Pacific SSTs, cross‐validated multiple linear regression models were developed to predict corn yield residuals using index values in the JAS−1 and OND−1 seasons. Using the cross‐validated models 91.5 and 98.4% of forecasted county corn yield residuals showed predictive skill (based on tercile hit scores) with seasonal index values in the JAS−1 and OND−1 seasons, respectively. The results of the models indicate that the indices of SSTs show significant predictability with corn yield residuals at lead times up to 4–7 months prior to spring planting and are a significant improvement over the use of an index of ENSO alone. Copyright © 2010 Royal Meteorological Society

Tropical Oceanic Causes of Interannual to Multidecadal Precipitation Variability in Southeast South America over the Past Century*

Abstract Observations, atmosphere models forced by historical SSTs, and idealized simulations are used to determine the causes and mechanisms of interannual to multidecadal precipitation anomalies over southeast South America (SESA) since 1901. About 40% of SESA precipitation variability over this period can be accounted for by global SST forcing. Both the tropical Pacific and Atlantic Oceans share the driving of SESA precipitation, with the latter contributing the most on multidecadal time scales and explaining a wetting trend from the early midcentury until the end of the last century. Cold tropical Atlantic SST anomalies are shown to drive wet conditions in SESA. The dynamics that link SESA precipitation to tropical Atlantic SST anomalies are explored. Cold tropical Atlantic SST anomalies force equatorward-flowing upper-tropospheric flow to the southeast of the tropical heating anomaly, and the vorticity advection by this flow is balanced by vortex stretching and ascent, which drives the increased precipitation. The 1930s Pampas Dust Bowl drought occurred, via this mechanism, in response to warm tropical Atlantic SST anomalies. The atmospheric response to cold tropical Pacific SSTs also contributed. The tropical Atlantic SST anomalies linked to SESA precipitation are the tropical components of the Atlantic multidecadal oscillation. There is little evidence that the large trends over past decades are related to anthropogenic radiative forcing, although models project that this will cause a modest wetting of the climate of SESA. As such, and if the Atlantic multidecadal oscillation has shifted toward a warm phase, it should not be assumed that the long-term wetting trend in SESA will continue. Any reversal to a drier climate more typical of earlier decades would have clear consequences for regional agriculture and water resources.

Great Plains Drought in Simulations of the Twentieth Century

Abstract Coupled global circulation models (CGCMs) have been widely used to explore potential future climate change. Before these climate projections can be trusted, the ability of the models to simulate present-day climate must be assessed. This study evaluates the ability of three CGCMs that participated in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change to simulate long-term drought over the Great Plains region with the same frequency and intensity as was observed during the twentieth century. The three models evaluated in this study are the Geophysical Fluid Dynamics Laboratory Coupled Model, version 2.0 (CM2.0); the National Centers for Atmospheric Research Community Climate System Model, version 3 (CCSM3); and third climate configuration of the Met Office Unified Model (HadCM3). The models are shown to capture the broad features of the climatology of the Great Plains, with maximum precipitation occurring in early summer, as observed. However, all of the models overestimate annual precipitation rates. Also, in CCSM3, precipitation and evapotranspiration experience unrealistic decreases between the months of June and August. Long-term droughts are found in each simulation of the twentieth century that are comparable in duration, severity, and spatial extent as has been observed. However, the processes found to be associated with simulated long-term droughts vary among the models. In both CM2.0 and HadCM3, low-frequency variations in Great Plains precipitation are found to correspond with low-frequency variations in tropical Pacific SSTs. In CCSM3, on the other hand, there appears to be no significant correlation between tropical Pacific SST variability and Great Plains precipitation. Strong land–atmosphere coupling in CCSM3 may explain the persistence of long-term droughts in this model.

ENSEMBLES: A new multi‐model ensemble for seasonal‐to‐annual predictions—Skill and progress beyond DEMETER in forecasting tropical Pacific SSTs

A new 46‐year hindcast dataset for seasonal‐to‐annual ensemble predictions has been created using a multi‐model ensemble of 5 state‐of‐the‐art coupled atmosphere‐ocean circulation models. The multi‐model outperforms any of the single‐models in forecasting tropical Pacific SSTs because of reduced RMS errors and enhanced ensemble dispersion at all lead‐times. Systematic errors are considerably reduced over the previous generation (DEMETER). Probabilistic skill scores show higher skill for the new multi‐model ensemble than for DEMETER in the 4–6 month forecast range. However, substantially improved models would be required to achieve strongly statistical significant skill increases. The combination of ENSEMBLES and DEMETER into a grand multi‐model ensemble does not improve the forecast skill further. Annual‐range hindcasts show anomaly correlation skill of ∼0.5 up to 14 months ahead. A wide range of output from the multi‐model simulations is becoming publicly available and the international community is invited to explore the full scientific potential of these data.

Drought reconstruction in the Qilian Mountains over the last two centuries and its implications for large-scale moisture patterns

We present a composite tree-ring chronology from two sites of Qilian Juniper (Sabina przewalskii) in the northwestern Qilian Mountains (QM), Northwestern China. Precipitation in June was found to be the main limiting factor for tree-growth. The tree rings are also significantly and positively correlated with June precipitation over large areas of the northern Tibetan Plateau (TP). The authors thus consider that the tree-ring based drought reconstruction from 1803–2006 is representative of a large area drought history. During the reconstruction period, persistent and severe dry epochs occurred in the 1820s–1830s, 1870s–1880s, 1920s, and 1950s–1960s, and persistent wet periods were found from 1803–1810s, 1890s–1920s, and 1970s–1980s. The severe dry and wet periods are similar to those found over the northeastern TP, indicating the potential linkages of the drought regimes between them. Comparison with global SST indicates that the drought variability is closely related to the tropical Pacific and Arctic Ocean SSTs, suggesting the connection of regional moisture variations to the Asian monsoon and westerly belt circulations, respectively.

Fluctuations of glaciers in the tropical Andes over the last millennium and palaeoclimatic implications: A review

The aim of this paper is to document the evolution of glaciers in the tropical Andes (between 10°N and 16°S) for the last millennium, based on moraines dated by lichenometry and radiocarbon, lake sediment records and historical documents. Viewed collectively, several glacial advances occurred synchronously. The first advance is dated around AD 1200–1350. The maximum glacial extent (MGE), defined as the furthest down-valley extent recorded synchronously by the majority of glaciers, occurred around 1630–1680 in Bolivia and Peru (the outer tropics) and around AD 1730 in Ecuador, Colombia and Venezuela (the inner tropics). Subsequently, during the 18th and 19th centuries, glaciers retreated continuously with only minor synchronous readvances. In the outer tropics, minor glacial advances occurred around 1730, 1760, 1800, 1850, and 1870. In the inner tropics, synchronous minor advances occurred around 1760, 1820 and 1880. Between the MGE and the early 20th century, glaciers lost about 30% of their total length. The retreat was slow between the 17th and 18th centuries but then became more marked. Use of the accumulation area ratio (AAR) method or historical observations in the different cordilleras revealed an increase in the equilibrium line altitude (ELA) of about 300 m from the MGE onward. Palaeoclimatic hypotheses, based on glaciological models run in different countries, suggest a cool and humid period in the 16–18th centuries followed by a colder and drier period in the 19th century. The reduction of glaciers observed from the middle of the 19th century is due to increasingly warmer conditions than before. Here, quantitative estimates are proposed to explain the evolution of the glaciers. In Venezuela, results indicate for the period 1250–1820 that mean air temperatures were 3.2 ± 1.4 °C cooler and precipitation was about 22% higher than at present. In Ecuador, temperatures of between 0.8° and 1.1 °C lower than today, and between 25% and 35% higher accumulation than today, appear to have occurred in the 18th century, followed by a short drier but colder period at the beginning of the 19th century. In Bolivia, the MGE could be a consequence of a decrease in temperature of 1.1 to 1.2 °C, and a 20 to 30% increase in accumulation or an increase in cloudiness of about 1–2/10. We discuss not only external climatic forcing but also the coincidence between glacier expansion and a decrease in solar irradiance. Dating uncertainties, however, have made the role of volcanism in glacier fluctuations impossible to determine. Finally, we discuss the relationships between ENSO and glacier fluctuations in recent centuries, which do not match directly with current knowledge of modern teleconnections between tropical Pacific SSTs and variations in glacier mass balance.

Tropical Pacific Forcing of North American Medieval Megadroughts: Testing the Concept with an Atmosphere Model Forced by Coral-Reconstructed SSTs*

Abstract The possible role that tropical Pacific SSTs played in driving the megadroughts over North America during the medieval period is addressed. Fossil coral records from the Palmyra Atoll are used to derive tropical Pacific SSTs for the period from a.d. 1320 to a.d. 1462 and show overall colder conditions as well as extended multidecadal La Niña–like states. The reconstructed SSTs are used to force a 16-member ensemble of atmosphere GCM simulations, each with different initial conditions, with the atmosphere coupled to a mixed layer ocean outside of the tropical Pacific. Model results are verified against North American tree ring reconstructions of the Palmer Drought Severity Index. A singular value decomposition analysis is performed using the soil moisture anomaly simulated by another 16-member ensemble of simulations forced by global observed SSTs for 1856–2004 and tree ring reconstructions of the Palmer Drought Severity Index for the same period. This relationship is used to transfer the modeled medieval soil moisture anomaly (relative to the modern simulation) into a model-estimated Palmer Drought Severity Index. The model-estimated Palmer Drought Severity Index reproduces many aspects of both the interannual and decadal variations of the tree ring reconstructions, in addition to an overall drier climate that is drier than the tree ring records suggest. The model-estimated Palmer Drought Severity Index simulates two previously identified “megadroughts,” a.d. 1360–1400 and a.d. 1430–60, with a realistic spatial pattern and amplitude. In contrast, the model fails to produce a period of more normal conditions in the early fifteenth century that separated these two megadroughts. The dynamical link between tropical SSTs and the North American megadroughts is akin to that operating in modern droughts. The model results are used to argue that the tropical Pacific played an active role in driving the megadroughts. However, the match between simulated and reconstructed hydroclimate is such that it is likely that both the coral-reconstructed SST anomalies contain significant errors and that SST anomalies in other basins also played a role in driving hydroclimate variations over North America during the late medieval period.

Pacific Sea Surface Temperatures in the Twentieth Century: An Evolution-Centric Analysis of Variability and Trend

Abstract A consistent analysis of natural variability and secular trend in Pacific SSTs in the twentieth century is presented. By focusing on spatial and temporal recurrence, but without imposition of periodicity constraints, this single analysis discriminates between biennial, ENSO, and decadal variabilities, leading to refined evolutionary descriptions, and between these natural variability modes and secular trend, all without advance filtering (and potential aliasing) of the SST record. SST anomalies of all four seasons are analyzed together using the extended-EOF technique. Canonical ENSO variability is encapsulated in two modes that depict the growth (east-to-west along the equator) and decay (near-simultaneous amplitude loss across the basin) phases. Another interannual mode, energetic in recent decades, is shown linked to the west-to-east SST development seen in post–climate shift ENSOs: the noncanonical ENSO mode. The mode is closely related to Chiang and Vimont’s meridional mode, and leads to some reduction in canonical ENSO’s oscillatory tendency. Pacific decadal variability is characterized by two modes: the Pan-Pacific mode has a horseshoe structure with the closed end skirting the North American coast, and a quiescent eastern equatorial Pacific. The mode exhibits surprising connections to the tropical/subtropical Atlantic, with correlations there resembling the Atlantic multidecadal oscillation. The second decadal mode—the North Pacific mode—captures the 1976/77 climate shift and is closer to Mantua’s Pacific decadal oscillation. This analysis shows, perhaps for the first time, the striking links of the North Pacific mode to the western tropical Pacific and Indian Ocean SSTs. The physicality of both modes is assessed from correlations with the Pacific biological time series. Finally, the secular trend is characterized: implicit accommodation of natural variability leads to a nonstationary SST trend, including midcentury cooling. The SST trend is remarkably similar to the global surface air temperature trend. Geographically, a sliver of cooling is found in the central equatorial Pacific in the midst of widespread but nonuniform warming in all basins. An extensive suite of sensitivity tests, including counts of the number of observational analogs of the modes in test analyses, supports the robustness of this analysis.