Chapter IV: Abrupt transitions and hysteresis in glacial ocean circulation
4.3 Results
to layer 2 versus layer 3. A φvalue of 1 means that all ofTN ADW goes into layer 3 and aφvalue of 0 means that all ofTN ADW goes into layer 2.
At the surface in the Southern Ocean, we calculate the isopycnal outcrop position by solving the full residual-mean equation. At steady-state, transport in the Southern Ocean, ψr es, is the balance between mean wind-driven flow, ¯ψ, and eddy-driven flow,ψ∗(J. Marshall and Radko, 2003). If ¯ψ andψ∗are exactly equal, then there is no overturning transport. However, if the system is not in steady-state, this need not hold. Therefore, all three components of the residual circulation ( ¯ψ, ψ∗, andψr es) can contribute to the isopycnal outcropping position (althoughψr es is the only one of the three that modifies water masses). Outcropping positions evolve according to the equation:
dy dt = 1
hm
ψ¯ +ψ∗−ψr es = 1 hm
− τ
ρ0f +K s− Tr es
LA,P
, (4.11)
where hm is the depth of the mixed layer, τ is the wind stress (held at a constant value ofτ0for the experiments presented here),ρ0is the reference density, f is the coriolis parameter,K is the along-isopycnal diffusivity,sis the isopycnal slope, and Tr es is as defined in Equation 4.9.
The evolution equations for y and z are solved using a fourth-order Runge-Kutta method. We initialize the model for 10 years using a time step of 0.1 days, then run with a time step of 0.5 days for the rest of the simulation (5000 years). In order to speed up the run time of our model, for the full simulation we only save output every 50 model years. Our relatively simple model allows us to explore parameter space and run simulations for durations that would be out of reach for more complex GCMs. As a point of comparison, we can run our model for 100,000 years at 0.5-day resolution in≈6.5 hours on a single processor. Compare this to the CCSM3 model configuration used to run the TraCE simulation, which requires 62 CPU hours to simulate a single year (Z Liu et al., 2009; Collins et al., 2006; Yeager et al., 2006).
97 the strength of North Atlantic Deep water formation,TN ADW, the density of North Atlantic Deep Water, φ, and the position of the sea ice edge ysi. We also looked at the effect of having variable vertical diffusivity in the basin. Specifically, we wanted to assess which conditions led to abrupt changes in basin stratification and which conditions were associated with a “figure-eight” and “two-cell” circulation configuration.
Transient interhemispheric adjustment
In our first set of experiments, we examine the delay time between the Northern Hemisphere forcing and the Southern Hemisphere response at DO events (which are characterized by abrupt changes in NADW flux). We find an adjustment timescale that is associated with volume convergence into a density class due to an abrupt change in NADW formation flux. This timescale goes as the volume of the density class into which the flux perturbation occurs over the magnitude of the perturbation:
τtr ans ∼V/FV. For our generalized Atlantic basin geometry and typical NADW flux values, this timescale is≈200 years. Mechanistically, this adjustment arises because flux convergence advects isopycnals in the basin, rapidly altering basin stratification.
If, for example, NADW rapidly decreases, mass converges in the upper ocean and diverges at mid-depth. This causes an increase in mid-depth stratification and a decrease in upper ocean stratification. In the Southern Ocean, these changes in basin stratification are mapped to the surface by sloping isopycnals. Isopycnal slopes are generally constant in time due to eddy saturation (Munday, Johnson, and D. P. Marshall, 2013; Thompson, Hines, and Adkins,submitted).
These changes in isopycnal outcrop position could lead to changes in Southern Hemisphere temperature, consistent with the bipolar seesaw. When NADW is strong (in a Northern Hemisphere warm phase), isopycnals tend to outcrop further to the north exposing more of the Southern Ocean surface to colder, denser layers (Figure 4.3). This should lead to cooling in the Southern Hemisphere. When NADW is weak (in a Northern Hemisphere cold phase), isopycnals tend to outcrop closer to the Antarctic continent, exposing more of the Southern Ocean surface area to warmer, lighter layers. This should lead to warming the Southern Hemisphere (Thompson, Hines, and Adkins,submitted).
There is an interesting feedback between changes in basin stratification and changes in residence time of different density classes. For a Northern Hemisphere cooling event, where NADW flux abruptly decreases, mass converges in the uppermost layer,
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0
5 10 15 20
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0
-1 -0.8 -0.6 -0.4 -0.2 0
-4000 -3000 -2000 -1000 0
0 -0.5 -1 Fb
0
12 3
Atl. IndoPac.
Time (yr)
NADW (Sv)Outcrop positionInterface depth (m)
Figure 4.3: Example of model response in transient experiment. Top panel shows NADW forcing, middle panel shows outcrop position (as fraction of total ACC width), and bottom panel shows interface depth in the basin. Legend shows colors for the three interfaces and shading in middle panel indicates regions of positive and negative buoyancy forcing at the surface of the Southern Ocean,Fbplotted to right.
This experiment was run at φ= 1, meaning all of the NADW flux went from layer 1 into layer 3.
reducing stratification in the upper ocean and increasing stratification in the deep ocean (see Figure 4.3). As a consequence, diffusive upwelling for the deepest layers increases, since it is inversely dependent on the layer thickness (Thompson, Hines, and Adkins,submitted). This increase in deep circulation as a result of a deepening of basin stratification is another mechanism to explain the bipolar seesaw, and it is perhaps more physical than the “density vacuum” mechanism (Broecker, 1998), which relies on heat from the thermocline mixing down to the deep ocean.
We tested the idea that this same mechanism could be responsible for radiocarbon observations from the deep and abyssal South Atlantic during the late part of the last glacial period by adding a simple radiocarbon tracer to the model. Unlike a simple residence time calculation, radiocarbon tracer has a strong “memory” of past
99 changes in circulation, since it decays with a 5730-year half-life. We simulated a 25 kyr period from 35 ka to 10 ka, encompassing Heinrich Stadial 2 (an abrupt shut- down in NADW formation preceding the LGM), the LGM, and Heinrich Stadial 1 (a second abrupt shut-down in NADW formation immediately following the LGM).
Our simulation was able to qualitatively match deep (layer 3) and abyssal (layer 4) radiocarbon records from the South Atlantic (L. C. Skinner et al., 2010; Barker et al., 2010). These records show similar radiocarbon values preceding the LGM, separation in radiocarbon during the LGM, with oldest values in the deep (layer 3), and convergence in radiocarbon following the LGM (Thompson, Hines, and Adkins, submitted).
Abrupt transitions in steady-state experiments
As can be seen in Figure 4.3, interface 2 is more sensitive to changes in NADW flux than any of the other interfaces. In an attempt to more precisely determine the response of interface 2 to changes in NADW flux, we conducted a series of exper- iments where we ran the time-dependent model out to steady-state. In particular, we were interested in the model response to different NADW fluxes and different NADW densities (φ). We variedφ between 0 (all NADW goes into layer 2) and 1 (all NADW goes into layer 3), in increments of 0.1. For each value of φwe ran a
‘loop’ of NADW values—starting at a NADW flux of 4 Sv, we ran the model for 5000 years, we saved the final values for each interface depth and outcrop position, and then we used them as initial conditions for the next iteration, with a NADW flux of 5 Sv. To start, we ran each loop from a NADW flux of 4 Sv to 20 Sv and back to 4 Sv, in increments of 1 Sv. In some loops, there were abrupt transitions, particularly in the outcrop position of interface 2. For these loops, we re-ran them with increased NADW resolution (down to 0.25 Sv) in the vicinity of the transition (Figure 4.4).
For values of φbetween 0.4 and 1, the NADW loop is characterized by an abrupt transition in the outcrop position of interface 2 in the Atlantic (yA
2) from a region of negative surface buoyancy forcing to a region of positive surface buoyancy forcing.
For these loops with abrupt transitions in outcrop position, there is also hysteresis in outcrop position as a function of NADW flux—the transition from outcropping in a negative buoyancy flux region to a positive buoyancy flux region occurs at a higher NADW flux when NADW is increasing than when NADW is decreasing (Figure 4.4). The width of the hysteresis loop increases as φ decreases from 1 to 0.4, but once φ is below 0.4, the hysteresis (and the abrupt transition in outcrop position)
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 NADW flux (Sv)
-0.9 -0.85 -0.8 -0.75 -0.7 -0.65 -0.6
y2A outcrop position
0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9
-10 0 8-1
Fb (x10-8 m2 s-3) Φ = 1
Φ = 0
Figure 4.4: Outcrop position of interface 2 in the Atlantic (yA
2) for all values ofφ. φ varies from 0 to 1 in increments of 0.1. Each loop is a different single color; dashed lines mark the up-going limb of the loop (i.e. increasing from 4 Sv to 20 Sv) and solid lines mark the down-going limb of the loop. Circles mark each NADW flux value that was run. Shading in the background indicates regions of positive and negative surface buoyancy flux, Fb(plotted in full to right). Horizontal line marks theFb=0 contour.
disappears.
In the vicinity of the hysteresis loop, the amount of time it takes for the model to reach steady-state also increases (Figure 4.5). Forφvalues of 0–0.3, where there is no abrupt transition inyA
2 or hysteresis, equilibration times are short (50–100 years).
The equilibration time was calculated by finding the maximum in the first derivative of the timeseries foryA
2.
To test the Ferrari et al. (2014) mechanism, we ran a series of experiments where we changed the extent of sea ice in the Southern Ocean (ysi, see Eq. 4.7). Increasing this parameter extends sea ice further across the Southern Ocean and decreasing this parameter (to more negative values) contracts sea ice closer to the southern boundary of the domain. As ysi increases, the magnitude of the jump in yA
2 also increases;
as ysi decreases, the magnitude of the jump in yA
2 decreases. The hysteresis in yA
2
101
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
10 12 14 16 18 20
NADW flux (Sv)
4 6 8
500 1000 1500 2000 2500 3000 3500 4000 4500 Equilibration
time (yr)
Figure 4.5: Equilibration time for interface yA
2 for all values ofφ as a function of NADW flux. Filled circles are for the up-going limb of the loop and unfilled circles are for the down-going limb. Color and size of circle corresponds to equilibration time.
outcrop position still remains, however (Figure 4.6).