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Paul J. Durack and Susan E. Wijffels

Abstract

Using over 1.6 million profiles of salinity, potential temperature, and neutral density from historical archives and the international Argo Program, this study develops the three-dimensional field of multidecadal linear change for ocean-state properties. The period of analysis extends from 1950 to 2008, taking care to minimize the aliasing associated with the seasonal and major global El Niño–Southern Oscillation modes. Large, robust, and spatially coherent multidecadal linear trends in salinity to 2000-dbar depth are found. Salinity increases at the sea surface are found in evaporation-dominated regions and freshening in precipitation-dominated regions, with the spatial pattern of change strongly resembling that of the mean salinity field, consistent with an amplification of the global hydrological cycle. Subsurface salinity changes on pressure surfaces are attributable to both isopycnal heave and real water-mass modification of the temperature–salinity relationship. Subduction and circulation by the ocean’s mean flow of surface salinity and temperature anomalies appear to account for most regional subsurface salinity changes on isopycnals. Broad-scale surface warming and the associated poleward migration of isopycnal outcrops drive a clear and repeating pattern of subsurface isopycnal salinity change in each independent ocean basin. Qualitatively, the observed global multidecadal salinity changes are thus consonant with both broad-scale surface warming and the amplification of the global hydrological cycle.

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Gregory C. Johnson, Sabine Mecking, Bernadette M. Sloyan, and Susan E. Wijffels

Abstract

Decadal changes of abyssal temperature in the Pacific Ocean are analyzed using high-quality, full-depth hydrographic sections, each occupied at least twice between 1984 and 2006. The deep warming found over this time period agrees with previous analyses. The analysis presented here suggests it may have occurred after 1991, at least in the North Pacific. Mean temperature changes for the three zonal and three meridional hydrographic sections analyzed here exhibit abyssal warming often significantly different from zero at 95% confidence limits for this time period. Warming rates are generally larger to the south, and smaller to the north. This pattern is consistent with changes being attenuated with distance from the source of bottom water for the Pacific Ocean, which enters the main deep basins of this ocean southeast of New Zealand. Rough estimates of the change in ocean heat content suggest that the abyssal warming may amount to a significant fraction of upper World Ocean heat gain over the past few decades.

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J. Mauro Vargas-Hernández, Susan E. Wijffels, Gary Meyers, André Belo do Couto, and Neil J. Holbrook

Abstract

Studies of decadal-to-multidecadal ocean subsurface temperature variability are fundamental to improving the understanding of low-frequency climate signals. The present study uses the Simple Ocean Data Assimilation (SODA) version 2.2.4 product for the period 1950–2007 to identify decadal modes of variability that characterize the upper Indo-Pacific Ocean temperature structure (5–466-m depth). An empirical orthogonal function (EOF) analysis of the 10-yr low-pass filtered temperature field applied across four depths shows that the dominant mode is characterized by a long-term temperature trend, with warming at the surface and cooling at the thermocline depth connecting the tropical western Pacific with the southern Indian Ocean via the Indonesian Seas. EOF analysis of the detrended 10-yr filtered temperature data and correlation analyses of the EOF time series with established large-scale climate indices identified the interdecadal Pacific oscillation as EOF1, the North Pacific Gyre Oscillation as EOF2, and the decadal component of El Niño Modoki as EOF3 (respectively, modes 2, 3, and 4 of the nondetrended data). EOF2 identifies the Atlantic multidecadal oscillation when the analysis is applied to sea surface temperature anomalies only, suggesting that the surface is forced dominantly by fluxes associated with global-scale weather patterns, while the subsurface is dominantly forced by internal dynamics of the Pacific Ocean. This paper demonstrates that the decadal-to-interdecadal temperature variability in SODA has a pronounced vertical extension through the upper ocean. The upper thermocline accounts for most of the variance in the analysis. These results reinforce the importance of examining the subsurface ocean in climate dynamics studies that seek to understand the ocean’s role.

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Susan E. Wijffels, Josh Willis, Catia M. Domingues, Paul Barker, Neil J. White, Ann Gronell, Ken Ridgway, and John A. Church

Abstract

A time-varying warm bias in the global XBT data archive is demonstrated to be largely due to changes in the fall rate of XBT probes likely associated with small manufacturing changes at the factory. Deep-reaching XBTs have a different fall rate history than shallow XBTs. Fall rates were fastest in the early 1970s, reached a minimum between 1975 and 1985, reached another maximum in the late 1980s and early 1990s, and have been declining since. Field XBT/CTD intercomparisons and a pseudoprofile technique based on satellite altimetry largely confirm this time history. A global correction is presented and applied to estimates of the thermosteric component of sea level rise. The XBT fall rate minimum from 1975 to 1985 appears as a 10-yr “warm period” in the global ocean in thermosteric sea level and heat content estimates using uncorrected data. Upon correction, the thermosteric sea level curve has reduced decadal variability and a larger, steadier long-term trend.

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Véronique Lago, Susan E. Wijffels, Paul J. Durack, John A. Church, Nathaniel L. Bindoff, and Simon J. Marsland

Abstract

The ocean’s surface salinity field has changed over the observed record, driven by an intensification of the water cycle in response to global warming. However, the origin and causes of the coincident subsurface salinity changes are not fully understood. The relationship between imposed surface salinity and temperature changes and their corresponding subsurface changes is investigated using idealized ocean model experiments. The ocean’s surface has warmed by about 0.5°C (50 yr)−1 while the surface salinity pattern has amplified by about 8% per 50 years. The idealized experiments are constructed for a 50-yr period, allowing a qualitative comparison to the observed salinity and temperature changes previously reported. The comparison suggests that changes in both modeled surface salinity and temperature are required to replicate the three-dimensional pattern of observed salinity change. The results also show that the effects of surface changes in temperature and salinity act linearly on the changes in subsurface salinity. Surface salinity pattern amplification appears to be the leading driver of subsurface salinity change on depth surfaces; however, surface warming is also required to replicate the observed patterns of change on density surfaces. This is the result of isopycnal migration modified by the ocean surface warming, which produces significant salinity changes on density surfaces.

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Alexander Sen Gupta, Les C. Muir, Jaclyn N. Brown, Steven J. Phipps, Paul J. Durack, Didier Monselesan, and Susan E. Wijffels

Abstract

Even in the absence of external forcing, climate models often exhibit long-term trends that cannot be attributed to natural variability. This so-called climate drift arises for various reasons including the following: perturbations to the climate system on coupling component models together and deficiencies in model physics and numerics. When examining trends in historical or future climate simulations, it is important to know the error introduced by drift so that action can be taken where necessary. This study assesses the importance of drift for a number of climate properties at global and local scales. To illustrate this, the present paper focuses on simulated trends over the second half of the twentieth century. While drift in globally averaged surface properties is generally considerably smaller than observed and simulated twentieth-century trends, it can still introduce nontrivial errors in some models. Furthermore, errors become increasingly important at smaller scales. The direction of drift is not systematic across different models or variables, as such drift is considerably reduced in the multimodel mean. Despite drift being primarily associated with ocean adjustment, it is also apparent in atmospheric variables. For example, most models have local drift magnitudes in surface air and ocean temperatures that are typically between 15% and 35% of the twentieth-century simulation trend magnitudes for 1950–2000. Below depths of 1000–2000 m, drift dominates over any forced trend in most regions. As such steric sea level is strongly affected and for some models and regions the sea level trend direction is reversed. Thus depending on the application, drift may be negligible or may make up an important part of the simulated trend.

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