Quantifying the Role of Mixed Layer Entrainment for Water Mass Transformation in the North Atlantic

AbstractIn this study, the salinity budget of the North Atlantic subtropical salinity maximum region for control volumes bounded by isohaline surfaces is analyzed. The authors provide closed budgets based on output from a high-resolution numerical simulation and partial budgets based on analyses of observational climatologies of hydrography and surface fluxes. With this choice of control volume, advection is eliminated from the instantaneous volume-integrated salt budget, and time-mean advection is eliminated from the budget evaluated from time-averaged data. In this way, the role of irreversible mixing processes in the maintenance and variability of the salinity maximum are more readily revealed. By carrying out the analysis with both near-instantaneous and time-averaged model output, the role of mesoscale eddies in stirring and mixing for this water mass is determined. This study finds that the small-scale mixing acting on enhanced gradients generated by the mesoscale eddies is approximately equal to that acting on the large-scale gradients estimated from climatological-mean conditions. The isohaline salinity budget can be related to water mass transformation rates associated with surface forcing and mixing processes in a straightforward manner. The authors find that the surface net evaporation in the North Atlantic salinity maximum region accounts for a transformation of 7 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of water across the 37-psu isohaline outcrop into the salinity maximum in the simulation, whereas the estimate based on climatological observations is 9 to 10 Sv.

Download Full-text

Water mass transformation and the North Atlantic Current in three multicentury climate model simulations

Journal of Geophysical Research Atmospheres ◽

10.1029/2012jc008021 ◽

2012 ◽

Vol 117 (C11) ◽

pp. n/a-n/a ◽

Cited By ~ 16

Author(s):

H. R. Langehaug ◽

P. B. Rhines ◽

T. Eldevik ◽

J. Mignot ◽

K. Lohmann

Keyword(s):

North Atlantic ◽

Climate Model ◽

Water Mass ◽

North Atlantic Current ◽

Model Simulations ◽

The North ◽

The North Atlantic ◽

Climate Model Simulations ◽

Water Mass Transformation ◽

Atlantic Current

Download Full-text

The assessment of salinity impact on surface water mass transformation in the North Atlantic

IOP Conference Series Earth and Environmental Science ◽

10.1088/1755-1315/606/1/012026 ◽

2020 ◽

Vol 606 ◽

pp. 012026

Author(s):

V M Kukushkin ◽

M Yu Markina

Keyword(s):

Surface Water ◽

North Atlantic ◽

Water Mass ◽

The North ◽

The North Atlantic ◽

Water Mass Transformation

Download Full-text

Response of the North Atlantic Thermohaline Circulation and Ventilation to Increasing Carbon Dioxide in CCSM3

Journal of Climate ◽

10.1175/jcli3757.1 ◽

2006 ◽

Vol 19 (11) ◽

pp. 2382-2397 ◽

Cited By ~ 79

Author(s):

Frank O. Bryan ◽

Gokhan Danabasoglu ◽

Norikazu Nakashiki ◽

Yoshikatsu Yoshida ◽

Dong-Hoon Kim ◽

...

Keyword(s):

North Atlantic ◽

Control Experiment ◽

Thermohaline Circulation ◽

Water Mass ◽

Overturning Circulation ◽

The North ◽

Freshwater Fluxes ◽

The North Atlantic ◽

Atlantic Thermohaline Circulation ◽

Water Mass Transformation

Abstract The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1% per year compound increase in CO2 is examined in three configurations of the Community Climate System Model version 3 that differ in their component model resolutions. The strength of the Atlantic overturning circulation declines at a rate of 22%–26% of the corresponding control experiment maximum overturning per century in response to the increase in CO2. The mean meridional overturning and its variability on decadal time scales in the control experiments, the rate of decrease in the transient forcing experiments, and the rate of recovery in periods of CO2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO2 in the framework of the water-mass transformation function, we show that the decline in the overturning is driven by decreasing density of the subpolar North Atlantic due to increasing surface heat fluxes. While there is an intensification of the hydrologic cycle in response to increasing CO2, the net effect of changes in surface freshwater fluxes on those density classes that are involved in deep-water formation is to increase their density; that is, changes in surface freshwater fluxes act to maintain a stronger overturning circulation. The differences in the control experiment overturning strength and the response to increasing CO2 are well predicted by the corresponding differences in the water-mass transformation rate. Reduction of meridional heat transport and enhancement of meridional salt transport from mid- to high latitudes with increasing CO2 also act to strengthen the overturning circulation. Analysis of the trends in an ideal age tracer provides a direct measure of changes in ocean ventilation time scale in response to increasing CO2. In the subpolar North Atlantic south of the Greenland–Scotland ridge system, there is a significant increase in subsurface ages as open-ocean deep convection is diminished and ventilation switches to a predominance of overflow waters. In middle and low latitudes there is a decrease in age within and just below the thermocline in response to a decrease in the upwelling of old deep waters. However, when considering ventilation within isopycnal layers, age increases for layers in and below the thermocline due to the deepening of isopycnals in response to global warming.

Download Full-text

Water mass transformation in the North Atlantic over 1985-2002 simulated in an eddy-permitting model

Ocean Science ◽

10.5194/os-1-127-2005 ◽

2005 ◽

Vol 1 (2) ◽

pp. 127-144 ◽

Cited By ~ 28

Author(s):

R. Marsh ◽

S. A. Josey ◽

A. J. G. de Nurser ◽

B. A. Cuevas ◽

A. C. Coward

Keyword(s):

North Atlantic ◽

General Circulation ◽

Water Mass ◽

Circulation Model ◽

Labrador Sea ◽

Mode Water ◽

The North ◽

Transformation Rates ◽

The North Atlantic ◽

Water Mass Transformation

Abstract. Water mass transformation in the North Atlantic is examined in an eddy-permitting simulation with the OCCAM ocean general circulation model, forced by realistic surface fluxes over the period 1985-2002. Three Atlantic regions are considered - the subtropics, mid-latitudes, the northeast Atlantic - along with the Labrador Sea. The oceanic boundaries of each region coincide with hydrographic sections occupied in recent years. These regions broadly represent the formation sites of Eighteen Degree Water (EDW), Subtropical Mode Water (STMW), Subpolar Mode Water (SPMW) and Labrador Sea Water (LSW). Water mass budgets are obtained for each region and year. Terms in the budget comprise surface-forced transformation rates, boundary exchanges and unsteadiness. Transformation rates due to "total mixing" are obtained as the difference between net and surface transformation rates. Transports at the boundaries are evaluated alongside recent hydrographic section datasets, while surface-driven and mixing-driven transformation rates are compared with estimates based on air-sea flux datasets and inverse analysis of hydrographic data. In general OCCAM compares well with the observations, although two particular discrepancies are identified: deep overflows at high latitudes too light by around 0.2 kg m-3 and spurious heat gain of up to 100 Wm-2 east of the Grand Banks. Over 1985-2002, there is considerable variability on a range of timescales, in the annual surface-driven and mixing-driven formation rates of all four water masses. In the case of EDW and STMW, surface-driven and mixing-driven formation rates largely cancel. This is not so for SPMW and LSW, leading to regional net formation rates of up to 17 Sv and 15 Sv, respectively. In particular, OCCAM successfully simulates the strong LSW formation event of 1989-1994.

Download Full-text

Water Mass Transformation in the North Atlantic and Its Impact on the Meridional Circulation: Insights from an Ocean Model Forced by NCEP–NCAR Reanalysis Surface Fluxes

Journal of Climate ◽

10.1175/1520-0442(2003)016<3085:wmtitn>2.0.co;2 ◽

2003 ◽

Vol 16 (19) ◽

pp. 3085-3110 ◽

Cited By ~ 41

Author(s):

Sergey K. Gulev ◽

Bernard Barnier ◽

Herve Knochel ◽

Jean-Marc Molines ◽

Melanie Cottet

Keyword(s):

North Atlantic ◽

Water Mass ◽

Meridional Circulation ◽

Ocean Model ◽

Surface Fluxes ◽

The North ◽

The North Atlantic ◽

Water Mass Transformation

Download Full-text

Water mass transformation in the North Atlantic over 1985–2002 simulated in an eddy-permitting model

Ocean Science Discussions ◽

10.5194/osd-2-63-2005 ◽

2005 ◽

Vol 2 (2) ◽

pp. 63-104 ◽

Cited By ~ 1

Author(s):

R. Marsh ◽

S. A. Josey ◽

A. J. G. Nurser ◽

B. A. de Cuevas ◽

A. C. Coward

Keyword(s):

North Atlantic ◽

Water Mass ◽

Surface Fluxes ◽

Mode Water ◽

The North ◽

Transformation Rates ◽

Consumption Rates ◽

Surface Transformation ◽

The North Atlantic ◽

Water Mass Transformation

Abstract. Water mass transformation in the North Atlantic is examined in an eddy-permitting simulation with the OCCAM ocean general circulation model, forced by realistic surface fluxes over the period 1985–2002. Three regions are considered: the Subtropics, the Mid-latitudes and the Northeast Atlantic. The oceanic boundaries of each region coincide with hydrographic sections occupied in recent years. These regions broadly represent the formation sites of Subtropical Mode Water (STMW) and Subpolar Mode Water (SPMW). A water mass budget is obtained for each region and year. Terms in the budget comprise surface-forced transformation rates, boundary exchanges and unsteadiness. Unsteadiness is relatively small, so that regional net water mass transformation is largely balanced by net boundary exchanges. Transformation rates due to mixing are then obtained as the difference between net and surface transformation rates. Transports at the boundaries are compared with recent observations, and reasonable agreement is obtained. For the period 1985-1993, model surface transformation rates are broadly in agreement with equivalent rates computed using the globally-balanced SOC fluxes of heat and freshwater, derived from ship observations. In each Atlantic region, surface transformation rates reach 10-15 Sv, based on both model and SOC fluxes. Higher spatial and temporal resolution in the OCCAM surface fluxes may improve the realism of surface transformation rates in some regions, notably the Labrador Sea. However, the unrealistic location of the North Atlantic Current too far south leads to spurious surface heating east of the Grand Banks in OCCAM. Period-mean transformation rates due to mixing reveal the formation of intermediate waters in each region due to the "consumption" of lighter and denser waters formed by surface fluxes, and are comparable with recent inverse estimates. There is, however, strong interannual-to-decadal variability in the consumption rates. In the subtropics, STMW consumption rates co-vary with STMW formation rates. In the subpolar gyre, anomalies in SPMW consumption rate appear to lag anomalies in the surface formation rate by up to 4 years.

Download Full-text