Abyssal Heat Budget in the Southwest Pacific Basin

AbstractThe abyssal Southwest Pacific Basin has warmed significantly between 1992-2017, consistent with warming along the bottom limb of the meridional overturning circulation seen throughout the global oceans. Here we present a framework for assessing the abyssal heat budget that includes the time-dependent unsteady effects of decadal warming and direct and indirect estimates of diapycnal mixing from microscale temperature measurements and finescale parameterizations. The unsteady terms estimated from the decadalwarming rate are shown to be within a factor of 3 of the steady state terms in the abyssal heat budget for the coldest portion of the water column and therefore, cannot be ignored. We show that a reduction in the lateral heat flux for the coldest temperature classes compensated by an increase in warmer waters advected into the basin has important implications for the heat balance and diffusive heat fluxes in the basin. Finally, vertical diffusive heat fluxes are estimated in different ways: using the newly available CTD-mounted microscale temperature measurements, a finescale strain parameterization, and a vertical kinetic energy parameterization from data along the P06 transect along 32.5°S. The unsteady-state abyssal heat budget for the basin shows closure within error estimates, demonstrating that (i) unsteady terms have become consequential for the heat balance in the isotherms closest to the ocean bottom and (ii) direct and indirect estimates from full depth GO-SHIP hydrographic transects averaged over similarly large spatial and temporal scales can capture the basin-averaged abyssal mixing needed to close the deep overturning circulation.

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Summer Heat and Ice Balances on Hodges Glacier, South Georgia, Falkland Islands Dependencies

Journal of Glaciology ◽

10.3189/s0022143000011606 ◽

1982 ◽

Vol 28 (99) ◽

pp. 221-238 ◽

Cited By ~ 4

Author(s):

I. G. G. Hogg ◽

J. G. Paren ◽

R. J. Timmis

Keyword(s):

Latent Heat ◽

Heat Balance ◽

Significant Contribution ◽

Heat Fluxes ◽

Falkland Islands ◽

Mountain Range ◽

Ablation Zone ◽

Net Radiation ◽

Summer Heat ◽

The Heat Balance

AbstractThe heat and ice balances of a temperate sub-Antarctic cirque glacier were measured through the 1973–74 melt season at an altitude midway between the climatic firn limit and the snout. The melt calculated from mean daily measurements at a single level of net radiation, wind-speed, temperature, and humidity agreed with that observed at nearby budget stakes. In the central ablation zone, radiation provided (54 ± 6)% and sensible fluxes (46 ± 6)% of the heat income through the summer, which was exceptionally warm and sunny. Latent-heat fluxes made no significant contribution to the heat balance. The calculation by Smith (1960) that the radiative, sensible, and latent heat fluxes contribute about equally to ablation in this zone has not been substantiated by measurement. The measured partition of the glacier’s heat balance suggested that maritime influences on its regime are mitigated by its position in the lee of a major mountain range.

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Towards measuring the meridional overturning circulation from space

Ocean Science ◽

10.5194/os-3-223-2007 ◽

2007 ◽

Vol 3 (2) ◽

pp. 223-228 ◽

Cited By ~ 3

Author(s):

D. Cromwell ◽

A. G. P. Shaw ◽

P. Challenor ◽

R. E. Houseago-Stokes ◽

R. Tokmakian

Keyword(s):

Climate Model ◽

Initial Step ◽

Meridional Overturning Circulation ◽

Ocean Bottom ◽

Bottom Pressure ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning ◽

Ocean Climate Model ◽

Future Work

Abstract. We present a step towards measuring the meridional overturning circulation (MOC), i.e. the full-depth water mass transport, in the North Atlantic using satellite data. Using the Parallel Ocean Climate Model, we simulate satellite observations of ocean bottom pressure and sea surface height (SSH) over the 20-year period from 1979–1998, and use a linear model to estimate the MOC. As much as 93.5% of the variability in the smoothed transport is thereby explained. This increases to 98% when SSH and bottom pressure are first smoothed. We present initial studies of predicting the time evolution of the MOC, with promising results. It should be stressed that this is an initial step only, and that to produce an actual working system for measuring the MOC from space would require considerable future work.

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The Observed North Atlantic Meridional Overturning Circulation: Its Meridional Coherence and Ocean Bottom Pressure

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-026.1 ◽

2014 ◽

Vol 44 (2) ◽

pp. 517-537 ◽

Cited By ~ 19

Author(s):

Shane Elipot ◽

Eleanor Frajka-Williams ◽

Chris W. Hughes ◽

Josh K. Willis

Keyword(s):

Time Series ◽

North Atlantic ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Ocean Bottom ◽

Bottom Pressure ◽

Overturning Circulation ◽

Ocean Bottom Pressure ◽

Meridional Overturning

Abstract Analyses of meridional transport time series from the Rapid Climate Change–Meridional Overturning Circulation (RAPID MOC) array at 26°N and from Argo float and altimetry data at 41°N reveal that, at semiannual and longer time scales, the contribution from the western boundary dominates the variability of the North Atlantic meridional overturning circulation (MOC), defined as the transport in the upper 1000 m of the ocean. Because the variability of the western boundary contribution is associated with a geostrophic overturning, it is reflected in independent estimates of transports from gradient of ocean bottom pressure (OBP) relative to and below 1000 m on the continental slope of the western boundary at three nominal latitudes (26°, 39°, and 42.5°N). Time series of western meridional transports relative to and below 1000 m derived from the OBP gradient, or equivalently derived from the transport shear profile, exhibit approximately the same phase relationship between 26° and 39°–42.5°N as the western contribution to the geostrophic MOC time series do: the western geostrophic MOC at 41°N precedes the MOC at 26°N by approximately a quarter of an annual cycle, resulting in a zero correlation at this time scale. This study therefore demonstrates how OBP gradients on basin boundaries can be used to monitor the MOC and its meridional coherence.

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An abrupt slowdown of Atlantic Meridional Overturning Circulation during 1915–1935 induced by solar forcing in a coupled GCM

Climate of the Past Discussions ◽

10.5194/cpd-10-2519-2014 ◽

2014 ◽

Vol 10 (3) ◽

pp. 2519-2546 ◽

Cited By ~ 1

Author(s):

P. Lin ◽

Y. Song ◽

Y. Yu ◽

H. Liu

Keyword(s):

Abrupt Change ◽

Polar Vortex ◽

Atlantic Meridional Overturning Circulation ◽

Total Solar Irradiance ◽

Heat Fluxes ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

High Latitudes ◽

Overturning Circulation ◽

Meridional Overturning

Abstract. In this study, we explore an abrupt change of Atlantic Meridional Overturning Circulation (AMOC) apparent in the historical run simulated by the second version of the Flexible Global Ocean–Atmosphere–Land System model – Spectral Version 2 (FGOALS-s2). The abrupt change is noted during the period from 1915 to 1935, in which the maximal AMOC value is weakened beyond 6 Sv (1 Sv = 106 m3 s−1). The abrupt signal first occurs at high latitudes (north of 46° N), then shifts gradually to middle latitudes (∼35° N) three to seven years later. The weakened AMOC can be explained in the following. The weak total solar irradiance (TIS) during early twentieth century decreases pole-to-equator temperature gradient in the upper stratosphere. The North polar vortex is weakened, which forces a negative North Atlantic Oscillation (NAO) phase during 1905–1914. The negative phase of NAO induces anomalous easterly winds in 50–70° N belts, which decrease the release of heat fluxes from ocean to atmosphere and induce surface warming over these regions. Through the surface ice–albedo feedback, the warming may lead to continuously melting sea ice in Baffin Bay and Davis Strait, which results in freshwater accumulation. This can lead to salinity and density reductions and then an abrupt slowdown of AMOC. Moreover, due to increased TIS after 1914, the enhanced Atlantic northward ocean heat transport from low to high latitudes induces an abrupt warming of sea surface temperature or upper ocean temperature in mid–high latitudes, which can also weaken the AMOC. The abrupt change of AMOC also appears in the PiControl run, which is associated with the lasting negative NAO phases due to natural variability.

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Estimating Air–Sea Heat Fluxes in Semienclosed Basins: The Case of the Gulf of Elat (Aqaba)

Journal of Physical Oceanography ◽

10.1175/2008jpo3858.1 ◽

2009 ◽

Vol 39 (1) ◽

pp. 185-202 ◽

Cited By ~ 20

Author(s):

Moshe Ben-Sasson ◽

Steve Brenner ◽

Nathan Paldor

Keyword(s):

Heat Flux ◽

Surface Temperature ◽

Sea Surface Temperature ◽

Air Temperature ◽

Heat Balance ◽

Evaporation Rate ◽

Heat Fluxes ◽

Sea Surface ◽

Turbulent Heat ◽

The Heat Balance

Abstract Meteorological and oceanographic data collected at the head of the Gulf of Elat were used to compute the air–sea heat flux components and the heat storage in the water column, which are in turn used to estimate the heat balance of this semienclosed basin. The solar radiation was measured directly, whereas the longwave (LW) cooling and the turbulent heat fluxes (latent, LH; sensible, SH) were computed from commonly used bulk formulas. Nine formulas for LW and four formulas for LH + SH were tested for a total of 36 possible combinations. Independent estimates for the bounds on the advective heat flux through the straits and results from a one-dimensional mixed layer model provided criteria to help identify the best choice of bulk formulas for the gulf. It was concluded that the LW formula of Bignami together with the turbulent flux formulas of Kondo provide the best estimate of the heat balance of the gulf. Based on this, the annual mean evaporation is 1.6–1.8 m yr−1, with a minimum of 1 m yr−1 in (the long) summer and a maximum of 3–4 m yr−1 in (the short) winter. The increase in evaporation rate during the winter results from the instability of the atmosphere at that time when the sea surface temperature exceeds the air temperature; in the summer, when the air temperature is much higher than the sea surface temperature, evaporation nearly stops due to the atmospheric stability. This estimated evaporation rate for the gulf, which is similar for all four of the LH formulas considered, is significantly smaller than values commonly quoted in the literature. Finally, in contrast to previous studies, it is found that the advective heat flux from the Straits of Tiran is large and significant in spring, reaching an estimated value of over 125 W m−2, but its annually averaged value is only about 35–40 W m−2.

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Turning Ocean Mixing Upside Down

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0244.1 ◽

2016 ◽

Vol 46 (7) ◽

pp. 2239-2261 ◽

Cited By ~ 70

Author(s):

Raffaele Ferrari ◽

Ali Mashayek ◽

Trevor J. McDougall ◽

Maxim Nikurashin ◽

Jean-Michael Campin

Keyword(s):

Boundary Layers ◽

Numerical Models ◽

Meridional Overturning Circulation ◽

Small Scale ◽

Ocean Bottom ◽

Overturning Circulation ◽

Bottom Boundary ◽

Bottom Boundary Layers ◽

Meridional Overturning ◽

Stratified Ocean

AbstractIt is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters that sink to the ocean bottom at high latitudes. However, the observational evidence that the strong turbulent fluxes generated by small-scale mixing in the stratified ocean interior are more vigorous close to the ocean bottom boundary than above implies that small-scale mixing converts light waters into denser ones, thus driving a net sinking of abyssal waters. Using a combination of theoretical ideas and numerical models, it is argued that abyssal waters upwell along weakly stratified boundary layers, where small-scale mixing of density decreases to zero to satisfy the no density flux condition at the ocean bottom. The abyssal ocean meridional overturning circulation is the small residual of a large net sinking of waters, driven by small-scale mixing in the stratified interior above the bottom boundary layers, and a slightly larger net upwelling, driven by the decay of small-scale mixing in the boundary layers. The crucial importance of upwelling along boundary layers in closing the abyssal overturning circulation is the main finding of this work.

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Monitoring Atlantic overturning circulation and transport variability with GRACE-type ocean bottom pressure observations – a sensitivity study

Ocean Science ◽

10.5194/os-11-953-2015 ◽

2015 ◽

Vol 11 (6) ◽

pp. 953-963 ◽

Cited By ~ 1

Author(s):

K. Bentel ◽

F. W. Landerer ◽

C. Boening

Keyword(s):

Time Series ◽

Large Scale ◽

Sensitivity Study ◽

Ocean Basin ◽

Meridional Overturning Circulation ◽

Western Slope ◽

Ocean Bottom ◽

Bottom Pressure ◽

Overturning Circulation ◽

Ocean Bottom Pressure

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism for large-scale northward heat transport and thus plays an important role for global climate. Relatively warm water is transported northward in the upper layers of the North Atlantic Ocean and, after cooling at subpolar latitudes, sinks down and is transported back south in the deeper limb of the AMOC. The utility of in situ ocean bottom pressure (OBP) observations to infer AMOC changes at single latitudes has been characterized in the recent literature using output from ocean models. We extend the analysis and examine the utility of space-based observations of time-variable gravity and the inversion for ocean bottom pressure to monitor AMOC changes and variability between 20 and 60° N. Consistent with previous results, we find a strong correlation between the AMOC signal and OBP variations, mainly along the western slope of the Atlantic Basin. We then use synthetic OBP data – smoothed and filtered to resemble the resolution of the GRACE (Gravity Recovery and Climate Experiment) gravity mission, but without errors – and reconstruct geostrophic AMOC transport. Due to the coarse resolution of GRACE-like OBP fields, we find that leakage of signal across the step slopes of the ocean basin is a significant challenge at certain latitudes. Transport signal rms is of a similar order of magnitude as error rms for the reconstructed time series. However, the interannual AMOC anomaly time series can be recovered from 20 years of monthly GRACE-like OBP fields with errors less than 1 sverdrup in many locations.

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Early-onset of Atlantic Meridional Overturning Circulation weakening in response to atmospheric CO2 concentration

npj Climate and Atmospheric Science ◽

10.1038/s41612-021-00182-x ◽

2021 ◽

Vol 4 (1) ◽

Author(s):

Mihai Dima ◽

Denis R. Nichita ◽

Gerrit Lohmann ◽

Monica Ionita ◽

Mirela Voiculescu

Keyword(s):

Numerical Simulations ◽

North Atlantic ◽

Co2 Concentration ◽

Atlantic Meridional Overturning Circulation ◽

Heat Fluxes ◽

Meridional Overturning Circulation ◽

Sea Surface ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning

AbstractThe Atlantic Meridional Overturning Circulation (AMOC), a tipping component of the climate system, is projected to slowdown during the 21st century in response to increased atmospheric CO2 concentration. The rate and start of the weakening are associated with relatively large uncertainties. Observed sea surface temperature-based reconstructions indicate that AMOC has been weakening since the mid-20th century, but its forcing factors are not fully understood. Here we provide dynamical observational evidence that the increasing atmospheric CO2 concentration affects the North Atlantic heat fluxes and precipitation rate, and weakens AMOC, consistent with numerical simulations. The inferred weakening, starting in the late 19th century, earlier than previously suggested, is estimated at 3.7 ± 1.0 Sv over the 1854–2016 period, which is larger than it is shown in numerical simulations (1.4 ± 1.4 Sv).

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Monitoring Atlantic overturning circulation variability with GRACE-type ocean bottom pressure observations – a sensitivity study

Ocean Science Discussions ◽

10.5194/osd-12-1765-2015 ◽

2015 ◽

Vol 12 (4) ◽

pp. 1765-1791 ◽

Cited By ~ 1

Author(s):

K. Bentel ◽

F. W. Landerer ◽

C. Boening

Keyword(s):

Large Scale ◽

Global Climate ◽

Sensitivity Study ◽

Ocean Basin ◽

Meridional Overturning Circulation ◽

Western Slope ◽

Ocean Bottom ◽

Bottom Pressure ◽

Overturning Circulation ◽

Ocean Bottom Pressure

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism for large-scale northward heat transport and thus plays an important role for global climate. Relatively warm water is transported northward in the upper layers of the North Atlantic Ocean, and after cooling at subpolar latitudes, sinks down and is transported back south in the deeper limb of the AMOC. The utility of in-situ ocean bottom pressure (OBP) observations to infer AMOC changes at single latitudes has been characterized in recent literature using output from ocean models. We extend the analysis and examine the utility of space-based observations of time-variable gravity and the inversion for ocean bottom pressure to monitor AMOC changes and variability between 20 and 60° N. Consistent with previous results, we find a strong correlation between the AMOC signal and OBP variations, mainly along the western slope of the Atlantic basin. We then use synthetic OBP data – smoothed and filtered to resemble the resolution of the GRACE gravity mission – and reconstruct geostrophic AMOC transport. Due to the coarse resolution of GRACE-like OBP fields, we find that leakage of signal across the step slopes of the ocean basin is a significant challenge at certain latitudes. However, overall, the inter-annual AMOC anomaly time series can be recovered from 20 years of monthly GRACE-like OBP fields with errors less than 1 Sverdrup.

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