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Matthew H. England

Abstract

A series of coarse-resolution models were integrated with a view to determining the most appropriate representation of the largest-scale water masses formed in the Southern Ocean. In particular, it was hoped that the models could realistically simulate Antarctic Bottom and Intermediate Water. The ocean model employed has a global domain with a realistic approximation of the continental outlines and bottom bathymetry. The subgrid-scale variation of bottom bathymetry is removed by spatial averaging over each grid box. The annual mean forcing at the sea surface is derived from climatological fields of temperature, salinity, and wind stress. It is found that the salinity of shelf water in the Weddell and Ross seas is critical if the model is to appropriately simulate the world's intermediate and bottom water masses. If the surface layer is too fresh in the Weddell and Ross seas, any bottom water formed adjacent to Antarctica is significantly less dense than in the real ocean. Furthermore, surface water at about 60°S (normally the region of intermediate water formation) strongly contributes to the model ocean's bottom water. This leaves the simulated bottom water too fresh and warm. On the other hand, with sufficiently salty bottom-water formed in the extreme Southern Ocean, a low-salinity tongue of intermediate water develops at 60°S. It is suggested that the sea-ice component of climate models is critical if the simulation is to capture the high-salinity shelf water and bottom-water formation adjacent to Antarctica and, in turn, allow for a realistic tongue of low-salinity Antarctic Intermediate Water (AAIW). The bathymetry of the Drake Passage is shown to determine the shape and strength of an intense meridional overturning cell in the Southern Ocean. By properly representing the northward extent of the Drake Passage, the formation and equatorward spreading of AAIW is simulated realistically. The scheme of AAIW formation obtained is quite different from the classical notion of circumpolar subduction of surface water at the polar front.

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Matthew H. England

Abstract

A hierarchy of coarse-resolution World Ocean experiments were integrated with a view to determining the most appropriate representation of the global-scale water masses in ocean general circulation models. The largest-scale response of the simulated ocean to the prescribed forcing in each model run is described. The World Ocean model eventually has a realistic approximation of continental outlines and bottom bathymetry. The model forcing at the sea surface is derived from climatological fields of temperature, salinity, and wind stress. The first experiment begins with a quite unrealistic and idealized World Ocean. Subsequent experiments then employ more realistic surface boundary conditions, model geometry, and internal physical processes. In all, 16 changes to the model configuration are investigated.

A fundamental dynamical constraint in the Drake Passage gap appears to limit the outflow rate of bottom water in the Antarctic region. This constraint acts to decouple the extreme Antarctic waters from the rest of the World Ocean. In a similar manner, including a surface wind stress acts to decouple the two hemispheres by limiting near-surface meridional flows across the equator. In the Atlantic basin, this decoupling becomes negligible when North Atlantic Deep Water (NADW) production is simulated. It is found that the representation of low salinity Antarctic Intermediate Water (AAIW) is sensitive to the level of horizontal diffusion employed by the model, as well as the chosen geometry of the Drake Passage gap and the amount of buoyancy provided by the model's deep water. For example, provided that lateral diffusion rates are not too excessive, a fresh tongue of AAIW is simulated if either sufficiently dense bottom water is formed off Antarctica, or if enough NADW outflows into the Southern Ocean. The inclusion of an isopycnal mixing scheme is shown to improve the representation of AAIW in coarse-resolution models.

The rate of horizontal diffusion and the relative location of the Drake Passage gap to the polar westerlies determines the shape and strength of an intense meridional overturning cell in the Southern Ocean. The inclusion of an isopycnal mixing scheme does not affect this circulation pattern significantly. On the other hand, the intensification of NADW production can substantially weaken the downwelling component of this cell by drawing more water of Southern Ocean origin northward. Accurately simulating NADW production and outflow requires a complete seasonal cycle in thermohaline forcing in the North Atlantic. The return path of NADW is primarily via a “cold water route” (i.e., the Drake Passage), although sufficiently strong NADW formation sees some return flow via the Agulhas leakage (i.e., the “warm water route”). By the last experiment of the present study, the model reproduces the subtle vertical layering of deep and intermediate water masses quite accurately. This represents a major success for the coarse-resolution multilevel ocean model.

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Matthew H. England

Abstract

The age of water in the World Ocean is studied using a passive age tracer introduced into a global ocean model. Additional information is derived from a transient “dye” tracer that tracks the time-dependent spreading of surface waters into the model ocean interior. Of particular interest is the nature of ocean ventilation over the 10–100-yr timescale, as well as the simulated age of deep and bottom water masses. In the upper model levels young water is found to correspond with regions of convergence (and downwelling) in the surface Ekman layer. Upwelling and convection are both shown to age the upper ocean by entraining older waters into the surface mixed layer. In the deep model levels, water age varies greatly between oceans, with young water found in convectively active regions (in the North Atlantic and in the Ross and Weddell Seas), and old water found in the deep North Pacific. The oldest water mass mixture (located at 2228-m depth in the western Pacific Ocean) is dated at 1494 years, made up of a combination of sources of water whose age varies between 500 and 5000 years. In the bottom layers of the model, Antarctic Bottom Water ventilates the extreme Southern Ocean over a 50–100-yr timescale, whereas the age approaches 1000 years in the northern limit of the Pacific basin. An analysis of age on the σ t = 27.4 kg m−3 isopycnal surface shows North Atlantic Deep Water (NADW) leaving the Atlantic Ocean with an average age of 300 years, although part of this water mass mixture is as young as 60 years. The young signal of NADW penetrates the Indian Ocean and the South Pacific via the circumpolar current over a timescale as short as 15 years, although water penetrating the far deep North Pacific is not detected in significant quantities (using a 10% concentration criterion) until about 500 years after the NADW formation time. A volumetric census of age in the World Ocean model shows relative maxima at 2°–3°C, 1200 years (corresponding to water in the deep North Pacific), and at 3°C, 300–500 years (corresponding to water in the deep Atlantic Ocean). The parameterization of mixing in the ocean model partly determines age, with an isopycnal mixing scheme reducing the deep and bottom water ventilation timescale by about 30%. By monitoring the gradual penetration of surface dye into the most remote ocean grid boxes, the time taken to ventilate the entire World Ocean model can be estimated to be around 5000 years.

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Peter R. Oke
and
Matthew H. England

Abstract

The oceanic response to imposed changes in the latitude of the subpolar westerly winds (SWWs) over the Southern Ocean is assessed in a global ocean model. The latitude changes are achieved by applying a zonally uniform zonal wind stress anomaly that is quasi-sinusoidal in latitude, with a positive (negative) band to the south (north) of about 50°S. This form of anomaly is chosen because it projects onto the Antarctic Oscillation, also known as the Southern Hemisphere annular mode, that is known to have a long-term trend. The response to both long-term trend and quasi-decadal periodic changes is examined in the latitude of the SWWs. In the long-term trend case, a 5.4° poleward shift of the SWWs over a 100-yr simulation is found to cause the poleward heat transport to increase by an average of 25% between 50°S and the equator. This change is primarily due to greater northward Ekman transport of cold water and its associated cooling of Subantarctic Mode Water (SAMW) by up to 0.5°C in the central-south Pacific. The authors also find that the rate of formation of Antarctic Intermediate Water increases as the SWWs shift poleward, resulting in cooling and freshening at intermediate depths. In the periodic experiment, where the SWWs axis has a range of 5.4° latitude, the poleward heat transport, North Atlantic Deep Water outflow and the overturning of Antarctic Bottom Water are all modulated by 20%–30%. Significant cooling is found at intermediate and upper-level water depths in the trend experiment and temperature fluctuations with a range of up to 0.4°C in the periodic experiment. These changes are of the same magnitude and form as that recently observed at intermediate depths in the Southern Ocean. The authors conclude that latitudinal shifts of the SWWs may play a significant role in generating observed temperature fluctuations at intermediate water depths.

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Caroline C. Ummenhofer
and
Matthew H. England

Abstract

Interannual extremes in New Zealand rainfall and their modulation by modes of Southern Hemisphere climate variability are examined in observations and a coupled climate model. North Island extreme dry (wet) years are characterized by locally increased (reduced) sea level pressure (SLP), cold (warm) sea surface temperature (SST) anomalies in the southern Tasman Sea and to the north of the island, and coinciding reduced (enhanced) evaporation upstream of the mean southwesterly airflow. During extreme dry (wet) years in South Island precipitation, an enhanced (reduced) meridional SLP gradient occurs, with circumpolar strengthened (weakened) subpolar westerlies and an easterly (westerly) anomaly in zonal wind in the subtropics. As a result, via Ekman transport, anomalously cold (warm) SST appears under the subpolar westerlies, while anomalies of the opposite sign occur farther north. The phase and magnitude of the resulting SST and evaporation anomalies cannot account for the rainfall extremes over the South Island, suggesting a purely atmospheric mode of variability as the driving factor, in this case the Southern Annular Mode (SAM). New Zealand rainfall variability is predominantly modulated by two Southern Hemisphere climate modes, namely, the El Niño–Southern Oscillation (ENSO) and the SAM, with a latitudinal gradation in influence of the respective phenomena, and a notable interaction with orographic features. While this heterogeneity is apparent both latitudinally and as a result of orographic effects, climate modes can force local rainfall anomalies with considerable variations across both islands. North Island precipitation is for the most part regulated by both local air–sea heat fluxes and circulation changes associated with the tropical ENSO mode. In contrast, for the South Island the influence of the large-scale general atmospheric circulation dominates, especially via the strength and position of the subpolar westerlies, which are modulated by the extratropical SAM.

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Matthew H. England
and
Fei Huang

Abstract

The Indonesian Throughflow (ITF) variability is assessed using a retrospective analysis of the global ocean based on the Simple Ocean Data Assimilation (SODA) experiment spanning the period 1950–99. A comparison between the 1983–95 observed ITF, and the simulated ITF suggests a reasonably accurate reconstruction of ocean circulation in the vicinity of the ITF during the available measurement record. A wavelet analysis shows that once the seasonal cycle is removed, the dominant variation of the ITF anomaly is an interannual oscillation with a period of about 4–7 yr. This interannual variability is significantly correlated with the El Niño–Southern Oscillation (ENSO) pattern, with the ITF lagging the ENSO cycle by 8–9 months. This suggests that large-scale tropical ocean–atmosphere interaction plays an important role in the interannual variability of the ITF. Regional upper-ocean heat content variability might also play a role in controlling interannual fluctuations of the ITF transport via geostrophic flows, though it could equally be ITF variations that establish heat content anomalies downstream of the Indonesian archipelago. The model heat transport associated with the ITF is in good agreement with the limited observational record available. Resultant variability in annual mean ITF heat transport is in the range 0.4–1.2 PW, which is significantly correlated with ITF and ENSO indices.

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Willem P. Sijp
and
Matthew H. England

Abstract

This study shows that a reduction in vertical mixing applied inside the Atlantic basin can drastically increase North Atlantic Deep Water (NADW) stability with respect to freshwater perturbations applied to the North Atlantic. This is contrary to the notion that the stability of the ocean’s thermohaline circulation simply scales with vertical mixing rates. An Antarctic Intermediate Water (AAIW) reverse cell, reliant upon upwelling of cold AAIW into the Atlantic thermocline, is found to be associated with stable states where NADW is collapsed. Transitions between NADW “on” and “off” states are characterized by interhemispheric competition between this AAIW cell and the NADW cell. In contrast to the AAIW reverse cell, NADW eventually upwells outside the Atlantic basin and is thus not as sensitive to changes in vertical mixing within the Atlantic. A reduction in vertical mixing in the Atlantic weakens the AAIW reverse cell, resulting in an enhanced stability of NADW formation. The results also suggest that the AAIW reverse cell is responsible for the stability of NADW collapsed states, and thereby plays a key role in maintaining multiple equilibria in the climate system. A global increase of vertical mixing in the model results in significantly enhanced NADW stability, as found in previous studies. However, an enhancement of vertical mixing applied only inside the Atlantic Ocean results in a reduction of NADW stability. It is concluded that the stability of NADW formation to freshwater perturbations depends critically on the basin-scale distribution of vertical mixing in the world’s oceans.

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Andréa S. Taschetto
and
Matthew H. England

Abstract

This study investigates interseasonal and interevent variations in the impact of El Niño on Australian rainfall using available observations from the postsatellite era. Of particular interest is the difference in impact between classical El Niño events wherein peak sea surface temperature (SST) anomalies appear in the eastern Pacific and the recently termed El Niño “Modoki” events that are characterized by distinct warm SST anomalies in the central Pacific and weaker cold anomalies in the west and east of the basin. A clear interseasonal and interevent difference is apparent, with the maximum rainfall response for Modoki events occurring in austral autumn compared to austral spring for classical El Niños. Most interestingly, the Modoki and non-Modoki El Niño events exhibit a marked difference in rainfall impact over Australia: while classical El Niños are associated with a significant reduction in rainfall over northeastern and southeastern Australia, Modoki events appear to drive a large-scale decrease in rainfall over northwestern and northern Australia. In addition, rainfall variations during March–April–May are more sensitive to the Modoki SST anomaly pattern than the conventional El Niño anomalies to the east.

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Willem P. Sijp
and
Matthew H. England

Abstract

The effect of the position of the Southern Hemisphere subpolar westerly winds (SWWs) on the thermohaline circulation (THC) of the World Ocean is examined. The latitudes of zero wind stress curl position exert a strong control on the distribution of overturning between basins in the Northern Hemisphere. A southward wind shift results in a stronger Atlantic THC and enhanced stratification in the North Pacific, whereas a northward wind shift leads to a significantly reduced Atlantic THC and the development of vigorous sinking (up to 1500-m depth) in the North Pacific. In other words, the Atlantic dominance of the meridional overturning circulation depends on the position of the zero wind stress curl over the Southern Ocean in the experiments. This position has a direct influence on the surface salinity contrast between the Pacific and the Atlantic, which is then further amplified by changes in the distribution of Northern Hemisphere sinking between these basins. The results show that the northward location of the SWW stress maximum inferred for the last glacial period may have contributed to significantly reduced North Atlantic Deep Water formation during this period, and perhaps an enhanced and deeper North Pacific THC. Also, a more poleward location of the SWW stress maximum in the current warming climate may entail stronger salinity stratification of the North Pacific.

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Willem P. Sijp
and
Matthew H. England

Abstract

The absence of the Drake Passage (DP) gateway in coupled models generally leads to vigorous Antarctic bottom water (AABW) formation, Antarctic warming, and the absence of North Atlantic deep-water (NADW) formation. Here the authors show that this result depends critically on atmospheric moisture transport by midlatitude storms. The authors use coupled model simulations employing geometries different only at the location of DP to show that oceanic circulation similar to that of the present day is possible when DP is closed and atmospheric moisture transport values enhanced by Southern Ocean storm activity are used. In this case, no Antarctic warming occurs in conjunction with DP closure. The authors also find that the changes in poleward heat transport in response to the establishment of the Antarctic Circumpolar Current (ACC) are small. This result arises from enhanced atmospheric moisture transport at the midlatitudes of the Southern Hemisphere (SH), although the values used remain within a range appropriate to the present day. In contrast, homogeneous or (near) symmetric moisture diffusivity leads to strong SH sinking and the absence of a stable Northern Hemisphere (NH) overturning state, a feature familiar from previous studies. The authors’ results show that the formation of NADW, or its precursor, may have been possible before the opening of the DP at the Eocene/Oligocene boundary, and that its presence depends on an interplay between the existence of the DP gap and the hydrological cycle across the midlatitude storm tracks.

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