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Alberto Troccoli and Keith Haines

Abstract

A data analysis using conductivity–temperature–depth (CTD) measurements in the western tropical Pacific is carried out to get an estimate of the timescale over which temperature–salinity (T–S) relationships are preserved. Results show that the T–S preservation holds for periods on the order of a few weeks.

A new method for assimilating upper-ocean temperature profiles with salinity adjustments into numerical ocean models is then proposed. The approach would use a T–S relation that is more local in space and time than is the climatological T–S relation used in previous studies. The assimilation method avoids convective instability as the temperature data are introduced.

The CTD data and instantaneous fields from an ocean model are used to test the assimilation method by combining one profile with another. These tests recover the salinity profiles and the 0–500-m dynamic height very well (differences are smaller than 1 dyn cm). By contrast, analyses that used a climatological T–S relation did not provide a good salinity profile or dynamic height (errors were greater than 3 dyn cm).

If used for data assimilation, the method would allow the recovery of a good salinity and density field when only temperature data were available, at intervals of, say, two to four weeks. There is evidence that the same conclusions could be drawn for many other oceanic areas.

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Chris Old and Keith Haines

Abstract

A study of the formation and propagation of volume anomalies in North Atlantic Mode Waters is presented, based on 100 yr of monthly mean fields taken from the control run of the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3). Analysis of the temporal and spatial variability in the thickness between pairs of isothermal surfaces bounding the central temperature of the three main North Atlantic subtropical mode waters shows that large-scale variability in formation occurs over time scales ranging from 5 to 20 yr. The largest formation anomalies are associated with a southward shift in the mixed layer isothermal distribution, possibly due to changes in the gyre dynamics and/or changes in the overlying wind field and air–sea heat fluxes. The persistence of these anomalies is shown to result from their subduction beneath the winter mixed layer base where they recirculate around the subtropical gyre in the background geostrophic flow. Anomalies in the warmest mode (18°C) formed on the western side of the basin persist for up to 5 yr. They are removed by mixing transformation to warmer classes and are returned to the seasonal mixed layer near the Gulf Stream where the stored heat may be released to the atmosphere. Anomalies in the cooler modes (16° and 14°C) formed on the eastern side of the basin persist for up to 10 yr. There is no clear evidence of significant transformation of these cooler mode anomalies to adjacent classes. It has been proposed that the eastern anomalies are removed through a tropical–subtropical water mass exchange mechanism beneath the trade wind belt (south of 20°N). The analysis shows that anomalous mode water formation plays a key role in the long-term storage of heat in the model, and that the release of heat associated with these anomalies suggests a predictable climate feedback mechanism.

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Keith Haines and Chris Old

Abstract

A study of thermally driven water mass transformations over 100 yr in the ocean component of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) is presented. The processes of surface-forced transformations, subduction and mixing, both above and below the winter mixed layer base, are quantified. Subtropical Mode Waters are formed by surface heat fluxes and subducted at more or less the same rate. However, Labrador Seawater and Nordic Seawater classes (the other main subduction classes) are primarily formed by mixing within the mixed layer with very little formation directly from surface heat fluxes. The Subpolar Mode Water classes are dominated by net obduction of water back into the mixed layer from below.

Subtropical Mode Water (18°C) variability shows a cycle of formation by surface fluxes, subduction ∼2 yr later, followed by mixing with warmer waters below the winter mixed layer base during the next 3 yr, and finally obduction back into the mixed layer at 21°C, ∼5 yr after the original formation. Surface transformation of Subpolar Mode Waters, ∼12°C, are led by surface transformations of warmer waters by up to 5 yr as water is transferred from the subtropical gyre. They are also led by obduction variability from below the mixed layer, by ∼2 yr. The variability of obduction in Subpolar Mode Waters also appears to be preceded, by 3–5 yr, by variability in subduction of Labrador Sea Waters at ∼6°C. This supports a mechanism in which southward-propagating Labrador seawater anomalies below the subpolar gyre can influence the upper water circulation and obduction into the mixed layer.

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Alan D. Fox and Keith Haines

Abstract

This paper presents results from a global ocean model with ¼° resolution and 36 vertical levels, forced with European Centre for Medium-Range Weather Forecasts (ECMWF) winds and with applied altimetric sea level anomalies and temperature profile assimilation over the period 1992–96. Comparison with World Ocean Circulation Experiment data indicates the important role of temperature profile assimilation in maintaining the sharp thermocline gradients. Diagnostics of Walin-type water mass transformations over the North Atlantic are shown, which are implied by the procedure of assimilation. It is seen that the altimeter assimilation contributes very little to water transformation but the temperature profile assimilation effectively prevents all drift in water volumes for potential temperatures θ 0 > 7°C. Furthermore, the temperature profile assimilation is effective at producing subtropical mode waters at a rate of 16 Sv, which the poor representation of surface fluxes in this model run is unable to do. The possibility for interpreting the assimilation transformation fluxes in terms of deficiencies in physical processes such as air–sea fluxes and internal mixing is then discussed. The paper represents a new use of data assimilation methodology in order to quantify the physical biases in the fundamental processes of surface forcing and mixing in a way that is independent of explicit model parameterizations.

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Matthew D. Palmer and Keith Haines

Abstract

This paper presents a new analysis of ocean heat content changes over the last 50 yr using isotherms by calculating the mean temperature above the 14°C isotherm and the depth of the 14°C isotherm as separate variables. A new quantity called the “relative heat content” (“RHC”) is introduced, which represents the minimum local heat content change over time, relative to a fixed isotherm. It is shown how mean temperature and isotherm depth changes make separable and additive contributions to changes in RHC.

Maps of RHC change between 1970 and 2000 show similar spatial patterns to a traditional fixed-depth ocean heat content change to 220 m. However, the separate contributions to RHC suggest a more spatially uniform contribution from warming above the isotherm, while isotherm depth changes show wind-driven signals, of which some are identifiable as being related to the North Atlantic Oscillation. The time series show that the warming contribution to RHC dominates the global trend, while the depth contribution only dominates on the basin scale in the North Atlantic. The RHC shows minima associated with the major volcanic eruptions (particularly in the Indian Ocean), and these are entirely contributed by mean temperature changes rather than isotherm depth changes. The depth change contributions to RHC are strongly affected by the recently reported XBT fall-rate bias, whereas the mean temperature contributions are not. Therefore, only the isotherm depth change contributions to RHC will need to be reassessed as fall-rate-corrected data become available.

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Paul G. Myers and Keith Haines

Abstract

A 100-yr integration of a Mediterranean Sea model is performed with surface restoring to monthly varying T, S, followed by 100 years of integration with surface fluxes of heat and freshwater alone. The fluxes are diagnosed from the restoring boundary run and are shown to agree favorably with the observations. The model properties remain stable under flux forcing, reproducing all major water masses and a realistic thermohaline circulation, although with more variability than the restoring run. Annual average production of Levantine Intermediate Water (LIW) is 1.3 ± 0.3 Sv on a core density of σ θ = 29.05. A major component of variation is on a timescale of 2–3 yr and is correlated with changes in surface salinity in the Levantine by 0.1 psu, which are in turn related to a variable path for the modified Atlantic water in the eastern basin. Smaller variations in production of deep waters in the Adriatic and Western Mediterranean are also found to be significantly correlated with the LIW production at lags of 2 and 6 yr respectively.

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Peili Wu, Keith Haines, and Nadia Pinardi

Abstract

This paper presents a scenario for understanding the unexpectedly large changes of deep waters and thermohaline circulation in the Eastern Mediterranean during the past decade. It is demonstrated as a possible deep-water renewal mechanism in the Eastern Mediterranean in a numerical simulation with a high-resolution model, which has successfully reproduced the observed 1987 and 1995 regimes as shown by the two Meteor cruises. A budget study of the model simulation has shown that more than 75% of the salt added to the deep layers of the Eastern Mediterranean could have come from the top 1000 m by a salinity redistribution process triggered by intensive cooling over the Aegean Sea. A water transformation process analysis is carried out in the model simulation to reveal how colder and fresher dense deep water formed in the Aegean is turned into the Eastern Mediterranean Deep Water (EMDW) as observed. Surface heat and freshwater fluxes are diagnosed to show the roles of each component during the transition.

About a 10% to 15% (11% or 6.6 cm yr−1 in this experiment) increase of freshwater loss over the Eastern Mediterranean plus some cold winters, which are able to decrease the sea surface temperature in the Aegean by an additional 1°–2°C, would be capable of switching the major EMDW formation site from the Adriatic to the Aegean Sea and altering the EMDW structure from the pre-1987 state to the 1995 regime as revealed by the two Meteor cruises, M5 and M31. It does not necessarily require a large increase of EP in the Aegean itself to produce the salty bottom water observed in the Cretan Sea (the south Aegean). An increased transport of Levantine Intermediate Water (LIW) into the Aegean would increase the salinity of the Aegean bottom water, but the major salinity increase for the new EMDW occurs in the Cretan Sea where the colder but fresher Aegean bottom water meets the LIW and convects to the bottom. Such internal convection causes the temperature to drop but the salinity to rise in the deep layers of the Cretan Sea. When the Cretan deep water flows through Kasos Strait outside of the Cretan Arc, it falls again to the bottom of the Levantine Basin and spreads into the Ionian Sea. During this second course of internal convection the water is diluted further before it sinks to the bottom due to mixing with old EMDW.

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Christopher M. Thomas, Bo Dong, and Keith Haines

Abstract

The NASA Energy and Water Cycle Study (NEWS) climatology is a self-consistent coupled annual and seasonal cycle solution for radiative, turbulent, and water fluxes over Earth’s surface using Earth observation data covering 2000–09. Here we seek to improve the NEWS solution, particularly over the ocean basins, by considering spatial covariances in the observation errors (some evidence for which is found by comparing five turbulent flux products over the oceans) and by introducing additional horizontal transports from ocean reanalyses as weak constraints. By explicitly representing large error covariances between surface heat flux components over the major ocean basins we retain the flux contrasts present in the original data and infer additional heat losses over the North Atlantic Ocean, more consistent with a strong Atlantic overturning. This change does not alter the global flux balance but if only the errors in evaporation and precipitation are correlated then those fluxes experience larger adjustments (e.g., the surface latent heat flux increases to 85 ± 2 W m−2). Replacing SeaFlux v1 with J-OFURO v3 (Japanese Ocean Flux Data Sets with Use of Remote Sensing Observations) ocean fluxes also leads to a considerable increase in the global latent heat loss as well as a larger North Atlantic heat loss. Furthermore, including a weak constraint on the horizontal transports of heat and freshwater from high-resolution ocean reanalyses improves the net fluxes over the North Atlantic, Caribbean Sea, and Arctic Ocean, without any impact on the global flux balances. These results suggest that better characterized flux uncertainties can greatly improve the quality of the optimized flux solution.

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Stephen M. Grey, Keith Haines, and Alberto Troccoli

Abstract

In this paper, decadal evolution of warm and cold anomalies in the subtropical and subpolar gyres of the North Atlantic in the 300–500-m and 100–250-m depth ranges is described. A series of pentadally averaged objective maps of upper-ocean thermal anomalies, from bathythermograph data, are presented. Warm and cold anomalies in the western subtropical gyre are succeeded by similar anomalies in the subpolar gyre and the east Atlantic and subtropical return flow. Major warm and cold anomalies in the 1950s and 1970s, respectively, are similar to those described previously in SSTs, although there is more temporal continuity in the subsurface anomalies.

Two very strong events in the subtropical gyre are identified, a cold anomaly in 1966–72 and an intense warm anomaly in 1988–94, that show the greatest temperature anomalies in the North Atlantic during the period of the study. Interisotherm thickness anomalies are shown for the subtropical gyre during these periods. In the warm period, mode waters are both warmer (18°–19°C) and of greater volume than on average, and lie in a narrow band south of the Gulf Stream above a depressed thermocline with warm temperature anomalies to at least 800-m depth. In the cold period, the predominant mode water temperature is closer to 17°C, but there is reduced water formation overall with a raised thermocline and cold temperature anomalies down to 600-m depth. The bowl of the gyre is flat during the cold period, and the implied recirculation may be weaker and extend farther to the south. The changes appear to be consistent with the intensification of the subtropical gyre in the warm period and a spindown in the cold period, although the relative roles of wind stress and air–sea heat fluxes in these changes need to be determined.

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Debbie Clifford, Robert Gurney, and Keith Haines

Abstract

Understanding links between the El Niño–Southern Oscillation (ENSO) and snow would be useful for seasonal forecasting, as well as for understanding natural variability and interpreting climate change predictions. Here, a 545-yr run of the third climate configuration of the Met Office Unified Model (HadCM3), with prescribed external forcings and fixed greenhouse gas concentrations, is used to explore the impact of ENSO on snow water equivalent (SWE) anomalies. In North America, positive ENSO events reduce the mean SWE and skew the distribution toward lower values, and vice versa during negative ENSO events. This is associated with a dipole SWE anomaly structure, with anomalies of opposite sign centered in western Canada and the central United States. In Eurasia, warm episodes lead to a more positively skewed distribution and the mean SWE is raised. Again, the opposite effect is seen during cold episodes. In Eurasia the largest anomalies are concentrated in the Himalayas. These correlations with February SWE distribution are seen to exist from the previous June–July–August (JJA) ENSO index onward, and are weakly detected in 50-yr subsections of the control run, but only a shifted North American response can be detected in the analysis of the 40-yr ECMWF Re-Analysis (ERA-40). The ENSO signal in SWE from the long run could still contribute to regional predictions, although it would only be a weak indicator.

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