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R. T. Sutton, P-P. Mathieu, M. Collins, and B. Dong
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R. T. Sutton, S. P. Jewson, and D. P. Rowell

Abstract

The tropical Atlantic region, unlike the tropical Pacific, is not dominated by any single mode of climate variability such as the El Niño–Southern Oscillation (ENSO). Rather, this region is subject to multiple competing influences of comparable importance. The nature and potential predictability of these various influences has been investigated by analysis of an ensemble of atmospheric GCM integrations forced with observed SST for the period December 1948–November 1993.

The dominant modes of internal atmospheric and SST-forced variability are determined. Internal variability in the tropical Atlantic region is dominated by the equatorward extension of extratropical patterns, especially the North Atlantic oscillation. Three different SST-forced signals are identified. These are (a) a remote response to ENSO, (b) a response to the so-called Atlantic Dipole SST pattern, and (c) a response to equatorial Atlantic SST anomalies. The spatial structure and seasonality of these different elements of climate variability are diagnosed and feedbacks onto the ocean are assessed. The evidence presented supports the possibility of ENSO-like variability in the equatorial Atlantic, but does not support the suggestion that the Atlantic Dipole is a coupled ocean–atmosphere mode of variability.

An important feature of this study is that the results include quantitative estimates of the comparative importance, in different regions and different seasons, of the various influences on tropical Atlantic climate variability. These estimates are used to assess the potential predictability of key climatic indices.

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P-P. Mathieu, R. T. Sutton, B. Dong, and M. Collins

Abstract

The predictability of winter climate over the North Atlantic–European (NAE) region during ENSO events is investigated. Rather than employing traditional composite analyses, the authors focus on the impacts of six individual events: three El Niño events and three La Niña events. The investigation is based on the analysis of ensemble simulations with an atmospheric GCM forced with prescribed sea surface temperatures (SST) for the period December 1985–May 2001, and on observations. Model experiments are used to separate the respective roles of SST anomalies in the Indo-Pacific basin and in the Atlantic basin.

A significant (potentially predictable) climate signal is found in the NAE region for all six ENSO events. However, there are notable differences in the impacts of individual El Niño and La Niña events. These differences arise not simply from atmospheric internal variability but also because the atmosphere is sensitive to specific features of the SST anomaly fields that characterize the individual events. The different impacts arise partly from differences in Indo-Pacific SST and partly from differences in Atlantic SST. SST anomalies in both ocean basins can influence tropical convection and excite a Rossby wave response over the North Atlantic. The evidence presented here for the importance of Atlantic Ocean conditions argues that, in the development of systems for seasonal forecasting, attention should not be focused too narrowly on the tropical Pacific Ocean.

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Buwen Dong, Rowan T. Sutton, Len Shaffrey, and Nicholas P. Klingaman

Abstract

There is still no consensus about the best methodology for attributing observed changes in climate or climate events. One widely used approach relies on experiments in which the time periods of interest are simulated using an atmospheric general circulation model (AGCM) forced by prescribed sea surface temperatures (SSTs), with and without estimated anthropogenic influences. A potential limitation of such experiments is the lack of explicit atmosphere–ocean coupling; therefore a key question is whether the attribution statements derived from such studies are in fact robust. In this research the authors have carried out climate model experiments to test attribution conclusions in a situation where the answer is known—a so-called perfect model approach. The study involves comparing attribution conclusions for decadal changes derived from experiments with a coupled climate model (specifically an AGCM coupled to an ocean mixed-layer model) with conclusions derived from parallel experiments with the same AGCM forced by SSTs derived from the coupled model simulations. Results indicate that attribution conclusions for surface air temperature changes derived from AGCM experiments are generally robust and not sensitive to air–sea coupling. However, changes in seasonal mean and extreme precipitations, and circulation in some regions, show large sensitivity to air–sea coupling, notably in the summer monsoons over East Asia and Australia. Comparison with observed changes indicates that the coupled simulations generally agree better with observations. These results demonstrate that the AGCM-based attribution method has limitations and may lead to erroneous attribution conclusions in some regions for local circulation and mean and extreme precipitation. The coupled mixed-layer model used in this study offers an alternative and, in some respects, superior tool for attribution studies.

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J. R. Elliott, S. P. Jewson, and R. T. Sutton

Abstract

The El Niño–Southern Oscillation (ENSO) has far-reaching impacts on global climate via “teleconnections” associated with wavelike or other disturbances that are excited in the tropical Pacific. These teleconnections may influence the air–sea heat fluxes, either by altering the latent and sensible heat fluxes through a change in low-level wind speed or direction or by altering the degree of cloud cover and thus the radiation budget. The anomalous fluxes can generate sea surface temperature (SST) anomalies that can in turn feed back on the atmospheric circulation. These effects are explored for the 1997/98 ENSO event using a novel and powerful modeling technique in which a coupled ocean–atmosphere model (the U.K. Hadley Centre HadCM3 model) is forced to follow observed tropical Pacific SSTs using a strong thermal relaxation, while elsewhere the model is allowed to vary freely. This is an extension of previous studies in which the impact of ENSO was investigated using an atmospheric model coupled to an ocean mixed layer model. The authors focus on the impact of ENSO on the Atlantic Ocean. Model results are compared both with historical records of the Atlantic response to El Niño and with SST observations during the 1997/98 event. The model simulates well the warming of the tropical North Atlantic that is typical of El Niño events. In addition, it identifies a significant positive anomaly in the South Atlantic in the autumn of 1997/98 that was also observed and appears to be a feature of the Atlantic response to El Niño that has not previously been noted. The results suggest that many other large SST anomalies observed in the Atlantic during 1997/98 were not part of the response to El Niño.

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Nicholas T. Luchetti, Jessica R. P. Sutton, Ethan E. Wright, Michael C. Kruk, and John J. Marra

Abstract

There are more than 2,000 islands across Hawaii and the U.S.-Affiliated Pacific Islands (USAPI), where freshwater resources are heavily dependent upon rainfall. Many of the islands experience dramatic variations in precipitation during the different phases of the El Niño–Southern Oscillation (ENSO). Traditionally, forecasters in the region relied on ENSO climatologies based on spatially limited in situ data to inform their seasonal precipitation outlooks. To address this gap, a unique NOAA/NASA collaborative project updated the ENSO-based rainfall climatology for the Exclusive Economic Zones (EEZs) encompassing Hawaii and the USAPI using NOAA’s PERSIANN Climate Data Record (CDR). The PERSIANN-CDR provides a 30-yr record of global daily precipitation at 0.25° resolution (∼750 km2 near the equator). This project took place over a 10- week NASA DEVELOP National Program term and resulted in a 478-page climatic reference atlas. This atlas is based on a 30-yr period from 1 January 1985 through 31 December 2014 and complements station data by offering an enhanced spatial representation of rainfall averages.

Regional and EEZ-specific maps throughout the atlas illustrate the percent departure from average for each season based on the Oceanic Niño Index (ONI) for different ENSO phases. To facilitate intercomparisons across locations, this percentage-based climatology was provided to regional climatologists, forecasters, and outreach experts within the region. Anomalous wet and dry maps for each ENSO phase are used by the regional constituents to better understand precipitation patterns across their regions and to produce more accurate forecasts to inform adaptation, conservation, and mitigation options for drought and f looding events.

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D. Roemmich, J. Gilson, R. Davis, P. Sutton, S. Wijffels, and S. Riser

Abstract

An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment (WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170°W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40°S, 170°W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and Argo float trajectories at 1000 m confirms the gyre spinup during the 1990s. The signals in sea surface height, dynamic height, and velocity all peaked around 2003 and subsequently began to decline. The 1990s increase in wind-driven circulation resulted from decadal intensification of wind stress curl east of New Zealand—variability associated with an increase in the atmosphere’s Southern Hemisphere annular mode. It is suggested (based on altimetric height) that midlatitude gyres in all of the oceans have been affected by variability in the atmospheric annular modes on decadal time scales.

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S. Venzke, M. R. Allen, R. T. Sutton, and D. P. Rowell

Abstract

Decadal fluctuations in the climate of the North Atlantic–European region may be influenced by interactions between the atmosphere and the Atlantic Ocean, possibly as part of a coupled ocean–atmosphere mode of variability. For such a mode to exist, a consistent atmospheric response to fluctuations in North Atlantic sea surface temperatures (SST) is required. Furthermore, this response must provide feedbacks to the ocean. Whether a consistent response exists, and whether it yields the required feedbacks, are issues that remain controversial. Here, these issues are addressed using a novel approach to analyze an ensemble of six integrations of the Hadley Centre atmospheric general circulation model HadAM1, all forced with observed global SSTs and sea-ice extents for the period 1949–93.

Characterizing the forced atmospheric response is complicated by the presence of internal variability. A generalization of principal component analysis is used to estimate the common forced response given the knowledge of internal variability provided by the ensemble. In the North Atlantic region a remote atmospheric response to El Niño–Southern Oscillation and a further response related to a tripole pattern in North Atlantic SST are identified. The latter, which is most consistent in spring, involves atmospheric circulation changes over the entire region, including a dipole pattern in sea level pressure often associated with the North Atlantic oscillation. Only over the tropical/subtropical Atlantic, however, does it account for a substantial fraction of the total variance. How the atmospheric response could feed back to affect the ocean, and in particular the SST tripole, is investigated. Several potential feedbacks are identified but it has to be concluded that, because of their marginal consistency between ensemble members, a coupled mode that relied on these feedbacks would be susceptible to disruption by internal atmospheric variability. Notwithstanding this conclusion, the authors’ results suggest that predictions of SST evolution could be exploited to predict some aspects of atmospheric variability over the North Atlantic, including fluctuations in spring of the subtropical trade winds and the higher latitude westerlies.

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W. M. L. Morawitz, P. J. Sutton, P. F. Worcester, B. D. Cornuelle, J. F. Lynch, and R. Pawlowicz

Abstract

All available temperature data, including moored thermistor, hydrographic, and tomographic measurements, have been combined using least-squares inverse methods to study the evolution of the three-dimensional temperature field in the Greenland Sea during winter 1988/89. The data are adequate to resolve features with spatial scales of about 40 km and larger. A chimney structure reaching depths in excess of 1000 m is observed to the southwest of the gyre center during March 1989. The chimney has a spatial scale of about 50 km, near the limit of the spatial resolution of the data, and a timescale of about 10 days. The chimney structure breaks up and disappears in only 3–6 days. A one-dimensional vertical heat balance adequately describes changes in total heat content in the chimney region from autumn 1988 until the time of chimney breakup, when horizontal advection becomes important. A simple, one-dimensional mixed layer model is surprisingly successful in reproducing autumn to winter bulk temperature and salinity changes, as well as the observed evolution of the mixed layer to depths in excess of 1000 m. Uncertainties in surface freshwater fluxes make it difficult to determine, whether net evaporation minus precipitation, or ice advection, is responsible for the observed depth-averaged salinity increase front autumn to winter in the chimney region. Rough estimates of the potential energy balance in the mixed layer suggest that potential energy changes are reasonably consistent with turbulent kinetic energy (TKE) production terms. Initially the TKE term parameterizing wind forcing and sheer production is important, but as the mixed layer deepens the surface buoyancy production term dominates. The estimated average annual deep-water production rate in the Greenland Sea for 1988/89 is about 0.1 Sverdrups, comparable to production rates during the 1980s and early 1990s derived from tracer measurements. The location of the deep convection observed appears to be sensitively linked to the amount of Arctic Intermediate Water (AIW) present from autumn through spring. Although ATW is an important source of salt for the surface waters, too much AIW overstratifies the water column, preventing deep convection from occurring.

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A. Timmermann, Y. Okumura, S.-I. An, A. Clement, B. Dong, E. Guilyardi, A. Hu, J. H. Jungclaus, M. Renold, T. F. Stocker, R. J. Stouffer, R. Sutton, S.-P. Xie, and J. Yin

Abstract

The influences of a substantial weakening of the Atlantic meridional overturning circulation (AMOC) on the tropical Pacific climate mean state, the annual cycle, and ENSO variability are studied using five different coupled general circulation models (CGCMs). In the CGCMs, a substantial weakening of the AMOC is induced by adding freshwater flux forcing in the northern North Atlantic. In response, the well-known surface temperature dipole in the low-latitude Atlantic is established, which reorganizes the large-scale tropical atmospheric circulation by increasing the northeasterly trade winds. This leads to a southward shift of the intertropical convergence zone (ITCZ) in the tropical Atlantic and also the eastern tropical Pacific. Because of evaporative fluxes, mixing, and changes in Ekman divergence, a meridional temperature anomaly is generated in the northeastern tropical Pacific, which leads to the development of a meridionally symmetric thermal background state. In four out of five CGCMs this leads to a substantial weakening of the annual cycle in the eastern equatorial Pacific and a subsequent intensification of ENSO variability due to nonlinear interactions. In one of the CGCM simulations, an ENSO intensification occurs as a result of a zonal mean thermocline shoaling.

Analysis suggests that the atmospheric circulation changes forced by tropical Atlantic SSTs can easily influence the large-scale atmospheric circulation and hence tropical eastern Pacific climate. Furthermore, it is concluded that the existence of the present-day tropical Pacific cold tongue complex and the annual cycle in the eastern equatorial Pacific are partly controlled by the strength of the AMOC. The results may have important implications for the interpretation of global multidecadal variability and paleo-proxy data.

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