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Richard B. Schahinger
and
John A. Church

Abstract

The Australian Coastal Experiment (ACE) in 1983–84 demonstrated the feasibility of using wind-forced coastal-trapped wave (CTW) theory to predict low-frequency alongshore currents and sea levels on the east Australian continental shelf. Moreover, it emphasized the importance of upstream boundary conditions for the CTW model to be of practical use. In ACE, the first three CTW modes at the upstream boundary of the model were obtained via an array of current meters across the continental shelf and slope. An alternative approach when such arrays are not available is to use the close relationship between the observed CTW modes in the southern ACE region and coastal sea level (or alongshore wind stress) in eastern Bass Strait to derive proxies for the modes from those readily obtainable variables. A CTW model that uses these revised modes as an upstream boundary condition is run for the ACE period, and predictions of alongshore current and sea level in the vicinity of Sydney compared with those from the standard model, as well as with the observed data. The respective hindcasts are qualitatively similar, though the revised model gives amplitudes that are about 10%–15% smaller than those from the standard model. An independent test of the scheme, made by running the revised model for the December 1984–April 1985 period, indicates predictions are in good agreement with the observed alongshore current record at a nearshore site some 400 km north of the model's upstream boundary.

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John A. Church
and
Howard J. Freeland

Abstract

The sea level on the southern Australian coast is examined for the source of the coastal-trapped wave energy observed during the Australian Coastal Experiment. Sea level, adjusted for atmospheric pressure, and atmospheric pressure are observed to propagate eastward at about 10 m s−1. At the lowest frequency examined (24-day period), some energy travels south along the west coast of Tasmania, but does not reach the east coast of mainland Australia, while some energy travels through Bass Strait to reach the east coast of mainland Australia. At the most energetic frequency (8-day period), adjusted sea levels are coherent over the 3700 km of coastline from southern Australia to the east coast, and much of the wind-forced coastal-trapped wave energy appears to travel through Bass Strait to the mainland east coast. We have not identified a mechanism for energy transfer through Bass Strait, and we do not know what fraction of the coastal-trapped wave energy incident on western Bass Strait actually reaches the east coast. It is suggested that at low frequencies the long wavelength waves are not affected by relatively small gaps in the coastline, but that at higher frequencies the wavelength is smaller and breaks in the coastline become more important. The first and second coastal-trapped wave modes observed at Cape Howe during the Australian Coastal Experiment are most coherent with the sea level at Lakes Entrance at the eastern edge of Bass Strait. It is suggested that these coastal-trapped wave modes are generated when the east-west flow through Bass Strait has to adjust to the narrow shelf of the east Australian coast and that the second mode is preferentially generated because its length scale (k−1) more closely approximates the north-south extent of this east–west flow.

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Trevor J. McDougall
and
John A. Church

Abstract

Current numerical models of ocean circulation parameterize diffusion using a diagonal diffusivity tensor in a horizontal/vertical coordinate system rather than in the isopycnal/diapycnal directions. It is pointed out that this procedure introduces a fictitious flux of density in the horizontal direction. A solution to this problem is available in the work of Redi. Also we show that care must be used in simple models of the deep ocean not to confuse the diapycnal velocity with the vertical velocity which simply occurs as a component of along isopycnal motion.

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Kewei Lyu
,
Xuebin Zhang
, and
John A. Church

Abstract

The ocean dynamic sea level (DSL) is an important component of regional sea level projections. In this study, we analyze mean states and future projections of the DSL from the global coupled climate models participating in phase 5 and phase 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively). Despite persistent biases relative to observations, both CMIP5 and CMIP6 simulate the mean sea level reasonably well. The equatorward bias of the Southern Hemisphere westerly wind stress is reduced from CMIP5 to CMIP6, which improves the simulated mean sea level in the Southern Ocean. The CMIP5 and CMIP6 DSL projections exhibit very similar features and intermodel uncertainties. With several models having a notably high climate sensitivity, CMIP6 projects larger DSL changes in the North Atlantic and Arctic associated with a larger weakening of the Atlantic meridional overturning circulation (AMOC). We further identify linkages between model mean states and future projections by looking for their intermodel relationships. The common cold-tongue bias leads to an underestimation of DSL rise in the western tropical Pacific. Models with their simulated midlatitude westerly winds located more equatorward tend to project larger DSL changes in the Southern Ocean and North Pacific. In contrast, a more equatorward location of the North Atlantic westerly winds or a weaker AMOC under current climatology is associated with a smaller weakening of the AMOC and weaker DSL changes in the North Atlantic and coastal Arctic. Our study provides useful emergent constraints for DSL projections and highlights the importance of reducing model mean-state biases for future projections.

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L. J. Zedel
and
John A. Church

Abstract

Computationally simple criteria for the identification of spurious Doppler profiler velocity estimates are described and evaluated. Their effectiveness is determined by changes in the confidence intervals and mean values of Doppler shear profiles. Our tests show that indicators based on measures of acoustic signal quality (signal-to-noise ratio, and spectral width) do not provide a mean reduction in confidence intervals. We show that a combination of velocity quality screening criteria can reduce the confidence intervals of velocity estimates made with a 150 kHz profiler by as much as 25 percent. After screening, both components of horizontal currents are about as accurate, almost independent of depth, to at least 240 m. More importantly, data screening also removes a depth-dependent bias (as large as 0.05 m s−1) from velocity profiles. For the indicators tested, we recommend threshold values for our system configuration; these values can serve as guidelines to suitable settings in other applications. To obtain maximum accuracy of shear profiles (and absolute profiles when accurate navigation data are available) some form of data screening is essential.

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Quran Wu
,
Xuebin Zhang
,
John A. Church
, and
Jianyu Hu

Abstract

The modulation of the full-depth global integrated ocean heat content (GOHC) by El Niño–Southern Oscillation (ENSO) has been estimated in various studies. However, the quantitative results and the mechanisms at work remain uncertain. Here, a dynamically consistent ocean state estimate is utilized to study the large-scale integrated heat content variations during ENSO events for the global ocean. The full-depth GOHC exhibits a cooling tendency during the peak and decaying phases of El Niño, which is a result of the negative surface heat flux (SHF) anomaly in the tropics (30°S–30°N), partially offset by the positive SHF anomaly at higher latitudes. The tropical SHF anomaly acts as a lagged response to damp the convergence of oceanic heat transport, which redistributes heat from the extratropics and the subsurface layers (100–440 m) into the upper tropical oceans (0–100 m) during the onset and peak of El Niño. These results highlight the global nature of the oceanic heat redistribution during ENSO events, as well as how the redistribution process affects the full-depth GOHC. The meridional heat exchange across 30°S and 30°N is driven by ocean current anomalies, while multiple processes contribute to the vertical heat exchange across 100 m simultaneously. Heat advection due to unbalanced mass transport is distinguished from the mass balanced one, with significant contributions from the meridional and zonal overturning cells being identified for the latter in the vertical direction. Results presented here have implications for monitoring the planetary energy budget and evaluating ENSO’s global imprints on ocean heat content in different estimates.

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John A. Church
,
T. M. Joyce
, and
James F. Price

Abstract

CTD and acoustic Doppler current profiler data are analyzed for the response of the upper ocean to rapidly moving Hurricane Gay. Currents were observed within about two days of the hurricane passage and were dominated by a blue-shifted inertial oscillation confined largely to the mixed layer. The amplitude of the current was strongly asymmetric about the track, with the largest amplitude, 1 m s−1, about 80 km to the right of the track. There was also an asymmetry of the mixed layer depth and temperature with deeper and cooler values to the right of the track. The gradient Richardson number was found to have low values, in some places less than 1/4, in a layer from 10 to 30 m thick below the base of the surface mixed layer. This suggests that shear flow instability played an important role in the vertical mixing that occurred within the mixed layer and upper thermocline.

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Annie P. S. Wong
,
Nathaniel L. Bindoff
, and
John A. Church

Abstract

Comparisons of hydrographic conditions in the North and South Pacific Oceans in the 1960s and 1985–94 have been made along five World Ocean Circulation Experiment sections. Below the seasonal mixed layer, statistically significant temporal differences in salinity and temperature have been detected in the water masses that occur in the upper 2000 dbar of the water column. These water mass property differences have been used to estimate the freshwater and heat storage trends in the Pacific over the study period. Along 24°N, 10°N, and 17°S, where either North Pacific Intermediate Water or Antarctic Intermediate Water is present, the upper waters have increased in salinity, while the intermediate and deep waters have decreased in salinity. Although the depth-integrated salinity changes observed along these sections are small, the regional redistribution of freshwater associated with the water mass changes is significant and implies significant redistribution of surface freshwater fluxes over the Pacific. Heat loss has occurred along 47°N and 17°S, but significant warming has occurred along 24° and 10°N, giving the Pacific a net heat gain of 1.79 × 108 J m−2. The resulting steric sea level change for the area in the Pacific between 60°N and 31.5°S over the roughly 20-yr study period is estimated to be a rise of 0.85 mm yr−1, consistent with those in existing literature, but larger than that estimated from numerical models reported in the Intergovernmental Panel on Climate Change Second Assessment Report.

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Kewei Lyu
,
Xuebin Zhang
,
John A. Church
, and
Quran Wu

Abstract

The Southern Hemisphere oceans absorb most of the excess heat stored in the climate system due to anthropogenic warming. By analyzing future climate projections from a large ensemble of the CMIP5 models under a high emission scenario (RCP8.5), we investigate how the atmospheric forcing and ocean circulation determine heat uptake and redistribution in the Southern Hemisphere oceans. About two-thirds of the net surface heat gain in the high-latitude Southern Ocean is redistributed northward, leading to enhanced and deep-reaching warming at middle latitudes near the boundary between the subtropical gyres and the Antarctic Circumpolar Current. The projected magnitudes of the ocean warming are closely related to the magnitudes of the wind and gyre boundary poleward shifts across the models. For those models with the simulated gyre boundary biased equatorward, the latitude where the projected ocean warming peaks is also located farther equatorward and a larger poleward shift of the gyre boundary is projected. In a theoretical framework, the subsurface ocean changes are explored using three distinctive processes on the temperature–salinity diagram: pure heave, pure warming, and pure freshening. The enhanced middle-latitude warming and the deepening of isopycnals are attributed to the pure heave and pure warming processes, likely related to the wind-driven heat convergence and the accumulation of extra surface heat uptake by the background ocean circulation, respectively. The equatorward and downward subductions of the surface heat and freshwater input at high latitudes (i.e., pure warming and pure freshening processes) result in cooling and freshening spiciness changes on density surfaces within the Subantarctic Mode Water and Antarctic Intermediate Water.

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Yi Jin
,
Xuebin Zhang
,
John A. Church
, and
Xianwen Bao

Abstract

Projections of future sea level changes are usually based on global climate models (GCMs). However, the changes in shallow coastal regions, like the marginal seas near China, cannot be fully resolved in GCMs. To improve regional sea level simulations, a high-resolution (~8 km) regional ocean model is set up for the marginal seas near China for both the historical (1994–2015) and future (2079–2100) periods under representative concentration pathways (RCPs) 4.5 and 8.5. The historical ocean simulations are evaluated at different spatiotemporal scales, and the model is then integrated for the future period, driven by projected monthly climatological climate change signals from eight GCMs individually via both surface and open boundary conditions. The downscaled ocean changes derived by comparing historical and future experiments reveal greater spatial details than those from GCMs, such as a low dynamic sea level (DSL) center of −0.15 m in the middle of the South China Sea (SCS). As a novel test, the downscaled results driven by the ensemble mean forcings are almost identical with the ensemble average results from individually downscaled cases. Forcing of the DSL change and increased cyclonic circulation in the SCS are dominated by the climate change signals from the Pacific, while the DSL change in the East China marginal seas is caused by both local atmosphere forcing and signals from the Pacific. The method of downscaling developed in this study is a useful modeling protocol for adaptation and mitigation planning for future oceanic climate changes.

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