David Stevens

University of East Anglia, School of Mathematics, Faculty Member

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Unimpressed with the Academia.edu. Research Interests "none" indeed! Furthermore the system won't let me change it.

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Daniel D. Hutto

University of Wollongong

Michael Spivey

University of California, Merced

Andrej Dujella

University of Zagreb

Don Ross

University College Cork

Louis de Saussure

University of Neuchâtel

Armando Marques-Guedes

UNL - New University of Lisbon

John Sutton

Macquarie University

Eros Carvalho

Universidade Federal do Rio Grande do Sul

Fabio Cuzzolin

Oxford Brookes University

Rory Smead

Northeastern University

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Papers by David Stevens

Simulations of two last glacial maximum ocean states

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A numerical ocean circulation model of the Norwegian and Greenland Seas

The dynamics and thermodynamics of the Norwegian and Greenland Seas are investigated using a thre... more The dynamics and thermodynamics of the Norwegian and Greenland Seas are investigated using a three-dimensional primitive equation ocean circulation model The horizontal resolution of the model is 1° in the zonal direction and 0 .5° in the meridional direction . The vertical structure is described by 15 levels . The model is driven by both annual mean and seasonally varying wind and thermohalme forcing . The connections of the Norwegian and Greenland Seas with the North Atlantic and Arctic Ocean are modelled with an open boundary condition . The simulated currents are in reasonable agreement with the observed circulation.

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An additional deep water mass in Drake Passage as revealed by 3He data

We present 3 He data from a repeat section across Drake Passage, from three sections off the Sout... more We present 3 He data from a repeat section across Drake Passage, from three sections off the South American continent in the Pacific, at 28 S, 35 S, and 43 S, and from three sections in the Atlantic, eastward of the Malvinas, close to 35 W, and near the Greenwich Meridian. In Drake Passage, a distinct high-3 He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3 He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88 W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high-3 He feature can be ruled out. We explain the feature by addition of high-3 He waters present at the 43 S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep-Sea Research I 49 (2002) 681), that is strongly related to the high-3 He feature. Employing multiparameter water mass analysis (including 3 He as a parameter), we find that deep waters as met at the 43 S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35 W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.071.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3 He signature in Drake Passage is essentially confined north of the Polar Front. Further downstream the signature crosses this front, to the extent that at 35 W the contributions south and north of it are of similar magnitude. At the same time, the 3 He levels north of the front are reduced due to a substantial admixture of low-3 He North Atlantic Deep Water, such that 3 He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow from the Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass. r

$Fig. 2. 5°He isolines (%) for sections 1-8 of Fig. 1, data points are indicated by dots and station numbers are given along the top of each section. Only the depth ranges of interest are shown. 6*He color scales are given in the margins (note different color scales), numerical values are given aligned with the 6°He isolines. a isolines are shown as white lines with values (kgm °) aligned. Numbers close to data points in (f-h) are the deduced SPDSW fractions, see Section 4. Approximate front positions according to the definitions of Orsi et al. (1995) are indicated (SAF—Subantactic Front, PF—Polar Front, SACCF—Southern ACC Front). (a) Section 1 (METEOR M11/5, 1990; WOCE section A21) across Drake Passage; north is to the right. For secondary PF (in parentheses) see text. (b) Section 2 (JAMES CLARK ROSS 40, 1999) across Drake Passage. (c) Section 3 (SONNE 102, 1995), southeast Pacific 28°S off the South American continent. Distance and depth scales are identical for (c-e). (d) Section 4, same as (c), but 35°S. (e) Section 5, same as (c), but 43°S. (f) Section 6 (JAMES CLARK ROSS 40, 1999), Falkland Plateau-Georgia Basin. Station numbers up to 140 are in the Georgia Basin, and the break between essentially zonal and meridional direction (Fig. 1) occurs at Sta. 142; the horizontal scale is distance preserving. West is to the left, so that the SAF appears to the left of the PF, in contrast to (g) and (h). The numbers next to the 5°He data points represent fractional SPDSW contributions (%; see Section 4), to be distinguished from the 6*He values shown at the 5°He isolines. Distance and depth scales are identical for (f-h). (g) Section 7 (JAMES CLARK ROSS 10, 1995; WOCE section A23), Scotia Sea—Argentine Basin meridional section, ca. 61—-38°S, 32-37° W; fractional SPDSW contributions as indicated in (f). North is to the right. (h) Section 8 (METEOR M11/5, 1990; WOCE section A12), Greenwich Meridian—Cape Basin meridional section, about 58.5-36°S; fractional SPDSW contributions as indicated in (f).$

$The data-providing cruises tions for each of the two oceans are based. For the other sections no intercalibration is available. The JCR 40 nutrient and oxygen d ata were adjusted to the METEOR data, and the SONNE data to those of P19 (deep data only; the 28°S and 35°S SONNE sections meet P19 at 88°W). data for the 43°S SONNE sec the 1967 Scorpio section at (Warren, 1973), which we simi We augmented the ion using data from the same latitude arly adjusted to P19 using stations close to the crossing of the two sections. The nutrient data partly do not meet WOCE precision standards (WCRP, 1988). As a consequence, the phosphate data of JCR 40 and the nitrate data of JCR 10 nor (PF) are omitted. h of the Polar Front the data for the METEOR Drake Passage (WOCE section A21) and JCR 10 (WOCE section A23) sections have been reported previously (Riith, 1998). We additionally use data from the Pacific WOCE section P19 (88°W) (Lupton, 1998). Helium isotope sections are presented in the common delta notation, 6°He, which denotes the fractional difference of a measured *He/*He ratio from that of atmospheric helium (7He/*He)atm = 1.384 x 107°; Clarke et al., 1976). The 1-Sigma precision of the 5°He data is generally +0.2% (6°He is a robust parameter such that offsets between different data sets can be ignored in the present context). The ocean—atmo- sphere solubility equilibrium which as mentioned governs the 5°*He values in the mixed layer, corresponds to approximately 5*He=—1.7% (Benson and Krause, 1980).$

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Antarctic Circumpolar Current Response to Zonally-Averaged Winds

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Can limited ocean mixing buffer rapid climate change

A B S T R A C T It has been argued that diapycnal mixing has a strongly stabilizing role in the g... more A B S T R A C T It has been argued that diapycnal mixing has a strongly stabilizing role in the global thermohaline circulation (THC). Negative feedback between THC transport and low-latitude buoyancy distribution is present in theory based on thermocline scaling, but is absent from Stommel's classical model. Here, it is demonstrated that these two models can be viewed as opposite limits of a single theory. Stommel's model represents unlimited diapycnal mixing, whereas the thermocline scaling represents weak mixing. The latter limit is more applicable to the modern ocean, and previous studies suggest that it is associated with a more stable THC. A new box model, which can operate near either limit, is developed to enable explicit analysis of the transient behaviour. The model is perturbed from equilibrium with an increase in surface freshwater forcing, and initially behaves as if the only feedbacks are those present in Stommel's model. The response is buffered by any upper ocean horizontal mixing, then by propagation of salinity anomalies, each of which are stabilizing mechanisms. However, negative feedback associated with limited diapycnal mixing only prevents thermohaline catastrophe in a modest parameter domain. This is because the time-scale associated with vertical advective-diffusive balance is much longer than the time required for the THC to change mode. The model is then tuned to allow equilibrium THC transport to be independent of the rate of mixing. The equilibrium surface salinity difference controls the classical THC-transport/salinity positive feedback, whereas the equilibrium interior density difference controls the mean-flow negative feedback. When mixing is strong, unrealistic vertical homogenization occurs, causing a convergence in surface and interior meridional gradients. This reduces positive feedback, and increases stability, in the tuned model. Therefore, Stommel's model appears to overestimate, rather than underestimate, THC stability to high-frequency changes in forcing.

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Circulation and water mass modification in the Brazil-Malvinas Confluence

The confluence between the Brazil Current and the Malvinas Current [the Brazil-Malvinas Confluenc... more The confluence between the Brazil Current and the Malvinas Current [the Brazil-Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.48 (0.08), 0.38 (0.02), and 0.68C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.48 (0.07) and 0.78C (0.11), respectively.

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Comparison of two time-variant forced eddy-permitting global ocean models with hydrography of the Scotia Sea

A comparison between hydrographic observations and output from two realistically forced z-level g... more A comparison between hydrographic observations and output from two realistically forced z-level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12-0.40) and too warm in POCM_4C (by >2°C) suggesting problems with the airsea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C Ocean Modelling 9 (2005) 105-132 www.elsevier.com/locate/ocemod although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution.

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A Decomposition of the Atlantic Meridional Overturning

A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shear... more A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure-dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean-Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25°S and 50°N. A stronger dependency on the external mode and Ekman components between 8°and 35°N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.

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Coupled ocean-atmosphere interactions between the Madden-Julian Oscillation and synoptic scale variability over the warm pool

A principal component analysis of the combined fields of sea surface temperature (SST) and surfac... more A principal component analysis of the combined fields of sea surface temperature (SST) and surface zonal and meridional wind reveals that the dominant mode of intraseasonal (30 to 70 day) covariability during northern winter in the tropical Eastern Hemisphere is that of the Madden-Julian oscillation (MJO). Regression calculations show that the submonthly (30-day high-pass filtered) surface wind variability is significantly modulated during the MJO. Regions of increased (decreased) submonthly surface wind variability propagate eastward, approximately in phase with the intraseasonal surface westerly (easterly) anomalies of the MJO. Because of the dependence of the surface latent heat flux on the magnitude of the total wind speed, this systematic modulation of the submonthly surface wind variability produces a significant component in the intraseasonal latent heat flux anomalies, which partially cancels the latent heat flux anomalies due to the slowly varying intraseasonal wind anomalies, particularly south of 10°S.

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Deep and Bottom Waters in the Eastern Scotia Sea Rapid Changes in Properties and Circulation

Two meridional hydrographic transects (in 1995 and 1999) across the eastern Scotia Sea are used t... more Two meridional hydrographic transects (in 1995 and 1999) across the eastern Scotia Sea are used to investigate variability in the deep and bottom waters between the South Scotia Ridge and South Georgia. There is a significant warming of the warm deep water (WDW) south of the southern boundary of the Antarctic Circumpolar Current (ACC); waters are approximately 0.1Њ-0.2ЊC warmer in 1999 than 1995. This is due mainly to raised WDW potential temperatures in the Weddell Sea being fed through to the Scotia Sea as the WDW flows northeastward in the Weddell Gyre. There is a warming of the Weddell Sea Deep Water (WSDW) of approximately 0.05ЊC across the whole extent of the section, and an accompanying change in salinity that maintains the potential temperature-salinity relationship. This is caused by variability in the properties of the water overflowing the South Scotia Ridge, rather than enhanced outflow of the bottom layer of the Scotia Sea or movements of the ACC fronts, and may be related to changes in the intensity of the Weddell Gyre circulation. Consideration of other works suggests that the colder WSDW of 1995 is likely to be the anomalous case, rather than the warmer WSDW of 1999. The 1999 section reveals an inflow of Lower WSDW from east of the South Sandwich Arc via the Georgia Passage; this is constrained to the south of the southern boundary, and is not apparent in the 1995 measurements. Meanders in the southern boundary at Georgia Passage are likely to play a role in controlling the inflow of Lower WSDW, although changes in the peak density of the WSDW flowing across the South Scotia Ridge may be important also, with a denser inflow from the south acting to preclude an inflow of similar density from the northeast.

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Dynamical ocean forcing of the Madden-Julian Oscillation at lead times of up to five months

The authors show that a simple three-dimensional ocean model linearized about a resting basic sta... more The authors show that a simple three-dimensional ocean model linearized about a resting basic state can accurately simulate the dynamical ocean response to wind forcing by the Madden-Julian oscillation (MJO). This includes the propagation of equatorial waves in the Indian Ocean, from the generation of oceanic equatorial Kelvin waves to the arrival of downwelling oceanic equatorial Rossby waves in the western Indian Ocean, where they have been shown to trigger MJO convective activity. Simulations with idealized wind forcing suggest that the latitudinal width of this forcing plays a crucial role in determining the potential for such feedbacks. Forcing the model with composite MJO winds accurately captures the global ocean response, demonstrating that the observed ocean dynamical response to the MJO can be interpreted as a linear response to surface wind forcing. The model is then applied to study ''primary'' Madden-Julian events, which are not immediately preceded by any MJO activity or by any apparent atmospheric triggers, but have been shown to coincide with the arrival of downwelling oceanic equatorial Rossby waves. Case study simulations show how this oceanic equatorial Rossby wave activity is partly forced by reflection of an oceanic equatorial Kelvin wave triggered by a westerly wind burst 140 days previously, and partly directly forced by easterly wind stress anomalies around 40 days prior to the event. This suggests predictability for primary Madden-Julian events on times scales of up to five months, following the reemergence of oceanic anomalies forced by winds almost half a year earlier.

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Eddy formation behind the tropical island of Aldabra

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Eddy heat fluxes from direct current measurements of the Antarctic Polar Front in Shag Rocks Passage

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Frontal Structure of the Antarctic Circumpolar Current in the South Indian Ocean

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Glacial thermohaline circulation states of the northern Atlantic: the compatibility of modelling and observations

Observational evidence from deep-sea cores suggests that the ocean circulation during the last gl... more Observational evidence from deep-sea cores suggests that the ocean circulation during the last glacial cycle was highly variable, at times occupying states very different from those found today. Modelling can be used to dynamically constrain the possible circulation states compatible with observations, thus guiding both understanding of past climate but also the geographical and scientific thrust of future palaeoceanographic research. The Last Glacial Maximum has been extensively studied and here, using carbon isotopes, among other variables, the most likely thermohaline state consistent with palaeoclimatic data is constructed. Past and new modelling efforts for the Last Glacial Maximum are then examined, to contrast questions resolvable by the modelling/data comparison with those that remain unanswered. This shows modelling evidence to confirm the prevailing view of intermediate-depth North Atlantic Deep Water being produced at the Last Glacial Maximum, in combination with deep water production around Antarctica. The potential sites for this deep and intermediate water production are defined by the basic state of the thermohaline circulation. However, their relative importance is a function of small perturbations in the surface temperature and salinity fields brought about by active coupling between the ocean and atmosphere. Regions where these water masses may have been produced at the Last Glacial Maximum are suggested.

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High mixing rates in the abyssal Southern Ocean

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Interannual Variability of the tropical Atlantic independent of and associated with ENSO Part I The north tropical Atlantic

The interannual variability of the tropical Atlantic ocean-atmosphere system is examined using 50... more The interannual variability of the tropical Atlantic ocean-atmosphere system is examined using 50 years of sea-surface temperature (SST) and re-analysis data, and satellite data when available. A singular value decomposition analysis of 12-to 72-month bandpass filtered SST and zonal wind stress reveals two dominant modes of interannual variability. The SST anomalies are confined to the North Tropical Atlantic (NTA) in the first mode and extend over the equatorial and South Tropical Atlantic in the second mode. No evidence is found for an Atlantic SST dipole. The structure of the first (NTA) mode is examined in detail here, while the second mode has been described in a companion paper. In particular, the relationship of the NTA mode with El Niño-Southern Oscillation (ENSO) is investigated. There are 12 NTA events (seven warm and five cold) that are associated with ENSO, and 18 NTA events (seven warm and 11 cold) that are independent of ENSO.

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Interannual Variability of the tropical Atlantic independent of and associated with ENSO Part II The south tropical Atlantic

Two dominant ocean-atmosphere modes of variability on interannual timescales were defined in part... more Two dominant ocean-atmosphere modes of variability on interannual timescales were defined in part I of this work, namely, the North Tropical Atlantic (NTA) and South Tropical Atlantic (STA) modes. In this paper we focus on the STA mode that covers the equatorial and sub-tropical South Atlantic. We show that STA events occurring in conjunction with ENSO have a preference for the southern summer season and seem to be forced by an atmospheric wave train emanating from the central tropical Pacific and travelling via South America, in addition to the more direct ENSO-induced change in the Walker Circulation. They are lagged by one season from the peak of ENSO. These events show little evidence for other-than-localised coupled ocean-atmosphere interaction.

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Impact of relative atmosphere-ocean resolution on coupled climate models

The joint DEFRA/NERC-funded UK-Japan Climate Collaboration (UJCC) project, together with the NCAS... more The joint DEFRA/NERC-funded UK-Japan Climate Collaboration (UJCC) project, together with the NCAS-Climate UK-HiGEM project , have been developing coupled models based on the Met Office Hadley Centre's HadGEM1 model (Johns et al., 2006, a configuration of the Met Office Unified ModelTM(UK)), but with enhanced resolution. Using atmospheric models with 150km, 90km and 60km mesh sizes at mid-latitudes, and ocean models at 1 degree (with enhancement to 1/3 degree meridionally at the equator) and 1/3 degree, we have formed a matrix of models in which we can attempt to understand the relative importance of model resolution in a coupled framework (in a similar way to the Japanese CCSR/NIES/FRCGC groups with their MIROC3.2 coupled model (Hasumi and Emori, 2004) using T42, T106 and T213 atmosphere models and 1.4 degree and 0.25 degree ocean models). Integrating such models for 50-100 years has been made possible through use of the Japanese Earth Simulator Impact of relative atmosphere-ocean resolution on coupled climate models super computer. Here we describe results from versions of the coupled HadGEM model (150km atmosphere, 1-1/3 degree ocean) and HiGEM model (90km atmosphere, 1/3 degree ocean) as well as the intermediate models.

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Importance of oceanic resolution and mean state on the extra-tropical response to El Nino in a matrix of coupled models

The extra-tropical response to El Niño in configurations of a coupled model with increased horizo... more The extra-tropical response to El Niño in configurations of a coupled model with increased horizontal resolution in the oceanic component is shown to be more realistic than in configurations with a low resolution oceanic component. This general conclusion is independent of the atmospheric resolution. Resolving small-scale processes in the ocean produces a more realistic oceanic mean state, with a reduced cold tongue bias, which in turn allows the atmospheric model component to be forced more realistically. A realistic atmospheric basic state is critical in order to represent Rossby wave propagation in response to El Niño, and hence the extra-tropical response to El Niño. Through the use of high and low resolution configurations of the forced atmospheric-only model component we show that, in isolation, atmospheric resolution does not significantly affect the simulation of the extra-tropical response to El Niño. It is demonstrated, through perturbations to the SST forcing of the atmospheric model component, that biases in the climatological SST field typical of coupled model configurations with low oceanic resolution can account for the erroneous atmospheric basic state seen in these coupled model configurations. These results highlight the importance of resolving small-scale oceanic processes in producing a realistic large-scale mean climate in coupled models, and suggest that it might may be possible to ''squeeze out'' valuable extra performance from coupled models through increases to oceanic resolution alone.

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Simulations of two last glacial maximum ocean states

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A numerical ocean circulation model of the Norwegian and Greenland Seas

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An additional deep water mass in Drake Passage as revealed by 3He data