2.1 Mass coupling

The ice entrainment from the ridge into the stream would couple the mass balances of the two, which are discussed successively below. For simplicity, we assume the surface displacement to be sufficiently slow that steady-state mass balance holds. Over the ridge, the longitudinal driving stress is

(1)$$\tau _0 = \rho _{\rm i}gh_0^2 /l, \;$$

and, applying a linear flow law (see Part 1: Ou, Reference Ou2021), the vertical-averaged longitudinal creep is (van der Veen, Reference van der Veen2013, Eqn (5.30))

(2)$$u_0 = h_0\tau _0/( {3\nu } ) , \;$$

where ν is the ice viscosity (Doake and Wolff, Reference Doake and Wolff1985). The lowering of the stream surface would induce an inward stress τ _i of

(3)$$\tau _{\rm i} = \rho _{\rm i}gh_0^2 \eta /w_0, \;$$

where w ₀ is the catchment width over the ridge and

(4)$$\eta \equiv ( h_0-h_1) /h_0\ll 1$$

is the fractional surface depression of the stream, assumed small at the outset for simplicity. Since this depression is explicitly calculated in the model, this approximation will be checked a posteriori for consistency. The inward stress would drive a vertical-averaged inward flow:

(5)$$v_{\rm i} = h_0\tau _{\rm i}/( {3\nu } ) , \;$$

accompanied by a mass flux

(6)$$q_{\rm_ i} = h_0v_{\rm i}.$$

Because of the assumed steady-state mass balance, this influx is supplied by the accumulation over the catchment width of the ridge minus the longitudinal flux divergence, or

(7)$$q_{\rm _i} = w_0( {\dot{a}-h_0u_0/l} ) \sim w_0\dot{a}.$$

We have neglected the longitudinal flux divergence as it is more than an order smaller than the accumulation for the standard case (Appendix). Combining Eqns (3)–(7), we link the two internal variables η and w ₀:

(8)$$\eta = \displaystyle{{\dot{a}} \over {u_0}}\displaystyle{{w_0^2 } \over {h_0l}}.$$

Physically, a greater depression implies a greater inflow, which needs to be supplied by the accumulation over a wider catchment zone. Based on the functional form of this equation, we define the width scale

(9)$$[ w ] \equiv ( {h_0l} ) ^{1/2}, \;$$

so this equation becomes

(10)$$\eta = \omega w_0 ^{ { \prime ^{\vskip 2pt 2}}} , \;$$

where primed variables are non-dimensionalized and

(11)$$\omega = \dot{a}/u_0$$

is a dimensionless parameter measuring the strength of the accumulation hence referred to as the accumulation parameter.

We next consider the mass balance of the stream, which states

(12)$$\dot{a}w_1 + \dot{a}w_0 = w_1q_1/l, \;$$

or the accumulation and the influx from the ridge (the second term, see Eqn (7)) are balanced by the longitudinal divergence of the stream flux. This equation can also be interpreted as the mass balance of the combined stream/ridge pair: that is, owing to the negligible longitudinal flux of the ridge, the total accumulation over the pair is discharged through the fast stream. As discussed in Part 1 (Ou, Reference Ou2021), applying the global momentum balance and the linear flow law to the lateral strain, the stream velocity, averaged across the stream, is (Raymond, Reference Raymond1996)

(13)$$u_1 = \displaystyle{{\tau _0-\tau _{\rm b}} \over {3\nu h_0}}w_1^2 , \;$$

where τ _b is the (unknown) mean basal stress. Note that because of the small surface depression (4), the same driving stress τ ₀ applies over the stream/ridge pair. Defining additional scales

(14)$$[ \tau ] = \tau _0, \;$$

(15)$$[ u ] = u_0\displaystyle{{{[ w ] }^2} \over {h_0^2 }}, \;$$

and

(16)$$[ q ] = h_0[ u ] , \;$$

the velocity and stream flux are non-dimensionalized to

(17)$$u_1 ^ {\hskip1pt{\prime}} = q_1^ {\hskip1pt{\prime}} = ( {1-\tau_{\rm b} ^{\hskip2pt {\prime}} } ) w_1 ^{\hskip2pt {\prime} ^{\hskip1pt2} } , \;$$

and Eqn (12) becomes

(18)$$w_0^ {\hskip1pt{\prime}} = w_1^ {\hskip1pt{\prime}} ( {q_1^ {\hskip1pt{\prime}} -\omega } ) /\omega , \;$$

which links the catchment width to the stream width.

Eliminating the catchment width $w_0^ {\hskip1pt{\prime}}$ from Eqns (10) and (18), we arrive at

(19)$$\eta = w_1^ {\hskip1pt{\prime}\hskip1pt ^2}[ {( {1-\tau_{\rm b}^ {\hskip1pt{\prime}} } ) w{_1 ^ {\hskip1pt{\prime}} }^2-\omega } ] ^2/\omega , \;$$

which contains only stream variables. As alluded to earlier, the additional degrees of freedom introduced by the entrainment have been closed by the viscous flow law and the mass balance over the ridge.

2.2. Meltwater balance

For a temperate bed, the basal heat balance is the same as the meltwater balance, and its difference from Part 1 is that the lateral dispersal of the meltwater, no longer curbed by the topographic trough, must be included. Since the stream is defined by an effective pressure lower than half the driving stress (Tulaczyk and others, Reference Tulaczyk, Kamb and Engelhardt2000, see Fig. 1), one expects the efficiency of the meltwater dispersal to play a key role in constraining the stream width. To maintain the high water-pressure necessary for the sliding motion, the drainage system must be of the distributed type, which may take the form of linked cavities, a thin water film or their hybrid (Fountain and Walder, Reference Fountain and Walder1998), but for our purpose, the distinction is immaterial since it can be absorbed into the effective hydraulic conductivity. For simplicity and indeed with stronger observational evidence (Engelhardt and Kamb, Reference Engelhardt and Kamb1997), we take the water film of uniform depth d as the proxy medium for the water transport.

Since the hydrological timescale is short compared with the timescale of the ice variability (Fricker and others, Reference Fricker, Scambos, Bindschadler and Padman2007; see Section 2.3), we assume a steady-state heat balance of the form (Kyrke-Smith and others, Reference Kyrke-Smith, Katz and Fowler2014)

(20)$$u_1\tau _{\rm b} = \dot{g}\left({\displaystyle{{h_0} \over {h_1}}-1} \right) + \displaystyle{{\rho _{\rm i}Ld^3} \over {12\mu }}\displaystyle{{2( {\tau_0-\tau_{\rm b}} ) } \over {w_1^2 }}, \;$$

which states that the frictional heating (the first term) is balanced by the conductive cooling in excess of the geothermal flux (the second term) and the drainage cooling associated with the outward meltwater flux (the last term). For the conductive cooling, we have assumed an englacial conductive equilibrium (Part 1) and noted that the marginal warmth only slightly perturbs the stream-wide conductive flux because of its narrowness (Harrison and others, Reference Harrison, Echelmeyer and Larsen1998) hence neglected. For the meltwater drainage, we have applied a scaled Darcy's law neglecting the surface slope because the stream resembles a plug flow (Joughin and others, Reference Joughin, Tulaczyk, Bindschadler and Price2002, Fig. 6) and noted that the effective pressure has end values as indicated in Figure 1. It is noted that the outward meltwater flux is wholly determined by the stream properties, and it can be accommodated by the ridge because of the slight set-back of the frozen bed (Fig. 1). Since both freezing and melting are allowed in the hydrological balance, there is no need to distinguish the expanding versus the shrinking stream (Haseloff and others (Reference Haseloff, Schoof and Gagliardini2018).

When non-dimensionalized, the heat balance (20) becomes

(21)$$0 = \underbrace{{\alpha u_1^ {\hskip1pt{\prime}} \tau _{\rm b}^ {\hskip1pt{\prime}} }}_{{\dot{\,f}}}-\underbrace{\eta }_{{\dot{c}}}-\underbrace{{\beta ( {1-\tau_{\rm b}^ {\hskip1pt{\prime}} } ) /w{_1^ {\hskip1pt{\prime}} }^2}}_{{\dot{d}}}, \;$$

where

(22)$$\alpha \equiv [ u ] [ \tau ] /\dot{g}$$

is the ‘heating’ parameter measuring the strength of the frictional heating and

(23)$$\beta = \displaystyle{{\rho _{\rm i}Ld^3} \over {12\dot{g}\mu }}\displaystyle{{2\;[ \tau ] } \over {{[ w ] }^2}}$$

is the ‘drainage’ parameter measuring the efficiency of the meltwater dispersal. Since this drainage parameter differs from the hydraulic conductivity only by a numerical factor, they are referred to interchangeably. We have used the symbols $\dot{f}$, $\dot{c}$ and $\dot{d}$ for the ‘frictional’ heating, ‘conductive’ cooling and ‘drainage’ cooling, respectively. Substituting from Eqns (17) and (19), we combine the mass and heat balances into a single equation governing the two prognostic variables: the half-width $w_1^ {\hskip1pt{\prime}}$ and the basal stress $\tau _{\rm b}^ {\hskip1pt{\prime}}$ of the stream:

(24)$$ 0 = \underbrace{{\alpha w{_1 {^{\hskip-1pt\prime}} }^2( {1-\tau_{\rm b}{^{\hskip-1pt\prime}} } ) \tau _{\rm b}{^{\hskip-1pt\prime}} }}_{{\dot{\,f}}}-\underbrace{{w{_1 {^{\hskip-1pt\prime}} }^2{[ {( {1-\tau_{\rm b} {^{\hskip-1pt\prime}} } ) w{_1 {^{\hskip-1pt\prime}} }^2 - \omega } ] }^2/\omega }}_{{\dot{c}}} - \underbrace{{\beta ( {1-\tau_{\rm b} {^{\hskip-1pt\prime}} } ) /w{_1 {^{\hskip-1pt\prime}} }^2}}_{{\dot{d}}}, \;$$

which contains three dimensionless parameters: ω, α and β.

To tidy up Eqn (24), we use the shorthands

(25)$$\tau \equiv \tau _{\rm b}^ {\hskip1pt{\prime}} , \;$$

(26)$$\zeta \equiv w_1 ^ {\hskip1pt{\prime} \hskip1pt^2} , \;$$

to yield

(27)$$0 = \underbrace{{\alpha \zeta ( {1-\tau } ) \tau }}_{{\dot{\,f}}}-\underbrace{{\zeta {[ {( {1-\tau } ) \zeta -\omega } ] }^2/\omega }}_{{\dot{c}}}-\underbrace{{\beta ( {1-\tau } ) /\zeta }}_{{\dot{d}}}.$$

Since this equation is quadratic in τ, it can be easily solved for τ(ζ), which is the solid curve drawn in Figure 2 for the standard case of (ω, α, β = 0.21,  2.3,  0.24). This curve bisects the net cooling and heating regions, as indicated by the encircled signs, and it has a minimum at some intermediate width marked by box C. The dashed line corresponds to zero surface depression η, which intersects the solid curve at box B. Since the surface depression (10) cannot be negative, it is set to zero to the left of the dashed line wherein the heat balance (27) with $\dot{c} = 0$ would extend the solid curve from boxes B to A. As we shall see later, box A marks both the onset and termination of the stream, whose half-width is then given by

(28)$$w_2 ^ {\hskip1pt{\prime}} = ( {\beta /\alpha } ) ^{1/4}.$$

Fig. 2. Steady-state heat balance (solid curve) on (ζ,τ) phase space for the standard case of (ω, α, β = 0.21, 2.3, 0.24). For a wide stream (of greater ζ), the balance is between the frictional heating $( \dot{f})$ and conductive cooling $( \dot{c})$ whereas for a narrow stream between the frictional heating $( \dot{f})$ and the drainage cooling $( \dot{d})$. To the left of the dashed line, the surface depression (η) is set to zero. Solid arrow indicates the stream evolution from its onset at box A to box B when the conductive cooling commences and to box C when the basal stress is at its minimum and the stream attains stationarity. The dash-dotted line is the entropy production (σ) whose maximum roughly aligns with the minimum basal stress.

It is observed that the solid curve representing Eqn (27) is U-shaped with a minimum in the basal stress, which can be understood by noting that: (a) because the three terms have different power dependence on the stream width, the drainage cooling would dominate at small width, the conductive cooling at large width, and the frictional heating possibly at intermediate width; and (b) because the frictional heating contains an extra appearance of the basal stress, it is overtaken by cooling when the bed is weak. The minimum basal stress thus occurs at some intermediate width when the drainage cooling and conductive cooling are of comparable strength. We shall argue next that following its onset, the stream would evolve toward this minimum when it attains stationarity.

2.3 Stationary state

A stream is initiated from a creep state when the bed first becomes temperate and the meltwater production has weakened the bed to below the driving stress, that is, along the top boundary in Figure 2 with a small width. A narrow stream is subjected to effective drainage of the meltwater, which would widen the stream toward box A when it attains a steady-state meltwater balance. The process is rapid since it is governed by the short hydrological timescale. For this reason, we shall identify box A as the onset of the stream, as alluded to earlier. Beyond box A, we discern possible positive feedback between the stream widening and the bed softening: a slight widening hence faster sliding motion would increase the frictional heating to soften the bed, which would augment the ice flux hence the entrainment to further widen the stream. This positive feedback would propel the stream toward box C when the bed is at its softest to thwart the positive feedback and the stream attains stationarity. Physically, this is when the increasing conductive cooling eventually overcomes the frictional heating to stabilize the bed strength.

It should be noted that although the solid curve represents a steady-state balance, it is the small deviation from this balance that propels the stream evolution. As this evolution involves the slower mass adjustment than the hydrological processes, its timescale should be that of the former, which stems ultimately from the sliding-induced thinning. It is thus of the form

(29)$$[ t ] = \displaystyle{{\eta h_0} \over {[ q ] /l}}$$

(30)$$ = \eta l/[ u ] , \;$$

or it is simply the longitudinal transit time multiplied by the fractional surface depression. For the standard case (Appendix), this transit time is ~2 ka and with η ≈ 0.1 based on the later solution, we have an evolution timescale that is of order 200 years hence centennial, which is consistent with that characterizes the ice stream variation over the Siple coast (Catania and others, Reference Catania, Hulbe, Conway, Scambos and Raymond2012). Having identified this timescale, we can infer the temporal behavior of the stream/ridge pair along the solution continuum without explicit time integration. This is the same approach as that taken in Part 1 (Ou, Reference Ou2021) wherein its utility in depicting the surge cycle has been demonstrated.

In addition to the positive feedback discerned above, we offer next a complementary rationale based on the non-equilibrium thermodynamics (NT). We note that because of the high Reynolds number, the substream water film is likely turbulent (Kamb, Reference Kamb1987), so the effective hydraulic conductivity would be subjected to microscopic fluctuations, whose fractional amplitude can be taken to be the ratio of the turbulent to the mean water velocity. Although Ou (Reference Ou2018) shows that such fluctuations would propel the system toward maximized entropy production (MEP) in a different context, the MEP has been applied over a wide range of phenomena as a veritable generalization of the second law to the NT system (Kleidon and Lorenz, Reference Kleidon and Lorenz2005). In our application, the entropy production σ is the product of the thermodynamic flux $( \dot{d})$ and the thermodynamic force (1 − τ) that drives this flux, or

(31)$$\sigma = \dot{d}( {1-\tau } ) $$

(32)$$ = \beta ( {1-\tau } ) ^2/\zeta .$$

We plot the entropy production along the solid curve in Figure 2 (the dash-dotted line) from which we observe that its maximum roughly aligns with the minimum basal stress. Because of the appearance of the stream width in Eqn (32), there is a slight misalignment of the two, which is of little practical consequence for our purpose. The application of the NT renders a different expression of the evolution timescale: for a fractional fluctuation amplitude ɛ, this timescale should be ɛ ⁻² of the hydrological timescale, based on Ou (Reference Ou2018). As a plausible example, we take the turbulent velocity of the meltwater to be an order smaller than its mean velocity, and a hydrological timescale of order 1 year (Fricker and others, Reference Fricker, Scambos, Bindschadler and Padman2007), then the evolution timescale could be centennial, similar to that governing the ice mass balance.

Based on above reasonings, we postulate that the stream would evolve on centennial timescale toward the minimum basal stress when it attains stationarity. We shall next examine this stationary state and compare it with observations.

2.4 Application

With the stationary state being of minimum basal stress, we differentiate Eqn (27) against ζ and set dτ/dζ to zero to derive

(33)$$0 = \alpha \zeta ( {1-\tau } ) \tau -\zeta [ {( {1-\tau } ) \zeta -\omega } ] [ {3( {1-\tau } ) \zeta -\omega } ] /\omega + \beta ( {1-\tau } ) /\zeta $$

which can be combined with Eqn (27) to yield

(34)$$\tau = 1-\omega ( {1 + \beta /\zeta^3} ) /\zeta .$$

The equation, being non-transcendental, allows the following procedure for calculating the stationary state. For a guessed ζ, we calculate τ from this equation, both are then substituted into Eqn (27) to check the equality, and the initial ζ is then adjusted until this equality is satisfied. For the standard case shown in Figure 2, the calculated variables have the values of $w_1^ {\hskip1pt{\prime}} = 0.80, \;\,\,\tau _{\rm b} ^ {\hskip1pt{\prime}} = 0.37$ and hence $\eta = 0.11, \;\,\,w_0 ^ {\hskip1pt{\prime}} = 0.73$ and $u_1^ {\hskip1pt{\prime}} = 0.40$. Since the surface depression is much smaller than unity, it supports the approximation (4), as alluded to earlier. Although, as we shall see later, all these values undergo cyclic changes when streams interact with one another, to the degree that the stationary state represents an extremum of the cycle, it has the observational counterpart hence can be compared. Dimensionally, for the standard case, the driving stress thus is 0.23 b (shorthand for bar), the basal stress is 0.09 b, the stream width is 32 km, the catchment width is 15 km, the stream velocity is 77 m a⁻¹ and the stream surface is 110 m lower than the ridge, all seems quite reasonable except for the stream velocity seems low (Joughin and others, Reference Joughin, Tulaczyk, Bindschadler and Price2002).

We next examine the dependence of the stationary state on the dimensionless parameters, as shown in Figure 3 on the (ω, β) space. The axes are in the binary logarithm spanning half to double the standard values (solid squares); the solid lines are for the standard α and the dashed lines for the doubled α. To understand the dependence, we show in Figure 4 how the stationary state defined by the minimum basal stress (box C) would be displaced (solid squares) when the dimensionless parameters are doubled from their standard values (the altered basal heat balance in the dashed lines). A greater ω, say, due to greater accumulation, would raise the stream surface to reduce the conductive cooling, which, as can be inferred from the double-headed arrow on the right, would lower box C and push it to the right. The stationary state, thus, is characterized by a smaller depression, wider stream and weaker bed whereas at the same time Eqn (10) implies a narrower catchment; the wider stream and weaker bed reinforce each other to augment the stream velocity (17); all are as observed in Figure 3.

Fig. 3. State variables plotted against the accumulation (ω) and drainage (β) parameters: (a) stream half-width; (b) basal stress; (c) surface depression; (d) catchment width and (e) stream velocity. Dashed contours are for the doubled heating parameter (α).

Fig. 4. As shown in Figure 2 but showing the modification of the heat-balance curve (solid to dashed lines) and displacement of the stationary state (box C to solid squares) by the doubling of the dimensionless parameters (single-headed arrows).

A greater β implies stronger drainage cooling, which, as can be inferred from the double-headed arrows on the left, would raise the left limb of the solid curve to push box C upward and to the right. The stationary state, thus, is characterized by deeper depression, wider stream and stronger bed, and from Eqn (10) a wider catchment; the stronger bed and wider stream however counter each other to render little net effect on the stream velocity (17); all are as observed in Figure 3.

A greater α implies stronger frictional heating, which would lower box C to weaken the bed, but with little effect on the stream width (only slightly narrower). The weakening of the bed, however, is observed from Eqn (19) to increase the depression and with that the catchment width (10); the weakening bed also speeds up the stream (17); all are again as observed in Figure 3.

Among the external parameters, the most uncertain are perhaps the ice viscosity ν and the water-film depth d. From scale definitions, the dimensionless parameters have the following power dependence on these two parameters: ω ~ ν, α ~ ν ⁻¹, β ~ d ³, so the ice viscosity has no effect on the drainage parameter whereas the water-film depth only affects the drainage parameter. Quantitatively, the doubling and halving of the ice viscosity falls within the parameter domain of Figure 3, which, however, can only accommodate 30% change in the water-film depth. Although one may appraise the sensitivity of the glacial properties from Figure 3, one should be mindful of the limit of such inferences given the crudeness of the model. Perhaps more significantly, Figure 3 suggests relative robustness of the standard solution over considerable ranges of the external conditions; the stream properties, thus, can be summarized crudely as follows: its width is about twice the geometric mean of its height and length (9), its basal stress is less than half of the driving stress, its surface depression is an order smaller than its height and it moves about two orders faster than the creep; all these are consistent with the observed Ross ice streams (Shabtaie and others, Reference Shabtaie, Bentley, Bindschadler and MacAyeal1988; Joughin and others, Reference Joughin, Tulaczyk, Bindschadler and Price2002).

Because of the mass and thermal coupling, a stream/ridge pair must be considered as a single physical entity. Although the pair could attain stationarity when isolated from other pairs, we shall next see that such isolation is untenable and they invariably interact (Catania and others, Reference Catania, Hulbe, Conway, Scambos and Raymond2012).

3.1 Centerline separation

Let us imagine the initiation of the streams when the bed first becomes temperate as the ice sheet thickens. Without the topographic constriction, the streams would be randomly distributed and coalesce to form bigger ones as they grow (Kyrke-Smith and others, Reference Kyrke-Smith, Katz and Fowler2014). Through entrainment of the adjacent ice, they would self-organize into stream/ridge pair differentiated by wet/frozen beds, and the pair would grow to the stationary state discussed above if it is isolated from the neighboring pair. The situation can be visualized via a transect of the surface height across neighboring pairs as shown in Figure 5a. The left and right pairs are distinguished by solid and dashed lines, respectively, with the flat surfaces representing the streams and the curved surfaces, their catchment zones; the shaded strip is the ridge outside the catchment zones that enables the isolation of the two pairs. Such a configuration, however, is untenable for the following reason: the shaded strip, being outside the catchment zone, is not subjected to cold advection hence remains warm-based; it is thus ripe for initiating an embryonic stream to interact with the existing ones, contradicting their isolation.

Fig. 5. Transverse sections of the surface height of the interacting stream/ridge pairs. The left and right pairs are drawn in solid and dashed lines, with the flat and curve surfaces indicating the stream and the ridge, respectively. The centerline distance w _c (outermost vertical lines) decreases from (a) to (c). (a) The pairs are isolated from each other by an ice strip (shaded). (b) The streams vary between boxes D (slow) and C (fast) of Figure 6. (c) The streams vary between boxes A (stagnant) and C (fast) of Figure 6.

Fig. 6. Interaction between an active growing stream (solid arrow) and a passive narrowing one (dashed arrow), resulting in a cycle of: (a) smaller amplitude between fast/slow streams and (b) maximum amplitude between fast/stagnant streams.

If we denote the ‘centerline’ separation by $w_{\rm c} ^ {\hskip1pt{\prime}}$, it then has an upper bound of

(35)$$w_{\rm c} ^ {\hskip1pt{\prime}} = 2( w_1 ^ {\hskip1pt{\prime}} + w_0^ {\hskip1pt{\prime}} ) , \;$$

the right-hand side pertaining to the stationary stream/ridge pair derived in the previous section. Because of this upper bound, there is inherent interaction of the pairs with the widening one squeezing the other one. Because of this compensating behavior, the two stream beds obviously may not be weakening at the same time, so how does this reconcile with the minimization of the basal stress? For this, we posit the following hierarchy: the minimizing tendency applies only for the self-propelled stream growth; once it attains the maximum width, the tendency ceases and the stream can shrink passively if forced by the neighboring stream.

We visualize the interaction as shown in Figure 5b, which depicts the two extrema of the cycle within the stationary centerlines. As the left pair widens (the solid arrow), propelled by the weakening stream bed, it would narrow the right pair (the dashed arrow); when the left stream attains its maximum width, the right stream would be at its narrowest, which then begins to grow, reversing its prior passive role. One may also visualize the interaction on the phase space (Fig. 6a) whereby boxes D and C represent the two extrema of the cycle, and the solid and dashed arrows correspond to that shown in Figure 5b, namely, the active growth of the left stream from boxes D to C causes the passive shrinkage of the right stream from boxes C to D.

For a further decrease of the centerline separation, the narrowing stream may reach box A (Fig. 6b) of zero flow, which is characterized by half-width $w_2^ {\hskip1pt{\prime}}$ (28) and zero depression hence catchment width, as depicted in Figure 5c. The centerline separation thus is

(36)$$w_{\rm c}^ {\hskip1pt{\prime}} = w_1 ^ {\hskip1pt{\prime}} + w_0^ {\hskip1pt{\prime}} + w_2^ {\hskip1pt{\prime}} .$$

We shall next argue that this centerline separation, in fact, constitutes its lower bound. If this was not the case, the narrower stream would have stagnated before the wider stream reaches its maximum width, that is, the wider stream is not yet passive hence must be continually growing; and since the stagnation of the narrower stream would initiate an embryonic stream, both streams would be growing at the same time to push their centerlines apart, which cannot be stationary.

With the above, we have deduced upper and lower bounds of a stationary centerline separation given by Eqns (35) and (36), respectively; but for an initial value problem of the stream generation, once the lower bound is reached, the centerline separation has attained stationarity hence there is no tendency for its further expansion, or the system has reached an equilibrium.

3.2 Equilibrium state

With the above rationale, we thus define the equilibrium state of the interacting streams as defined by the stationary centerlines of minimum separation. Within the centerlines, the neighboring stream/ridge pairs exhibit compensating cycles of maximum amplitude, varying between the fastest (and widest) and stagnated (and narrowest) streams. To aid the visualization, we draw the time progression of the interacting streams in Figure 7, which will be compared later with that of the Ross ice streams B (left, also named after Whillans) and C (right).

Fig. 7. Time progression of the interacting streams, with their surfaces marked by stripes and maximum velocity by arrows. Following its reactivation, the left stream widens, deepens and speeds up. Squeezed by the left stream/ridge pair, the right stream narrows, shoals and slows to stagnation. Dashed line tracks the migration of the ridge, which is of constant height. The figure can be representative of the Ross ice streams B (left) and C (right) from 450 to 150 years ago.

The reactivation of the left stream causes it to widen and weaken its bed (the solid arrow from boxes A to C in Fig. 6b), both acting to speed up the flow (17); the increasing ice flux lowers its surface to entrain greater inward flux from the ridge (19) accompanied by growing catchment zone (10) until the stream bed reaches the minimum strength. The widening of the left stream/ridge pair would narrow the right stream and strengthen its bed (the dashed arrow from boxes C to A in Fig. 6b), being constricted to its right by the stationary centerline. This narrowing slows the stream and reduces the catchment flux until its surface rises to the ridge (box B). With the surface pinned to the ridge height, the narrowing continues to strengthen the bed until it reaches the driving stress and the flow comes to a halt (box A). Since the surface is already flush with the ridge and the bed remains temperate, it is preconditioned for reactivation to repeat the cycle.

To contrast the time progression of the ice streams with the surge cycles of the topography-confined glaciers, we juxtapose them in Figure 8. The widths (striped upper surfaces), the basal stress (striped lower surfaces), the surface height (solid lines) and the ice velocity (dashed lines) are drawn; the shaded columns signify the abrupt activation and termination of the surge.

Fig. 8. Time progression of the ice stream (left) and cyclic surge of the topographic glacier (right). Surface height in solid lines, velocity in dashed lines, striped top and bottom are width and basal stress, respectively. Shaded columns represent onset and termination of the surge characterized by short hydrological timescale. The figure shows the sharp contrast between the two cyclic variations.

At the surge onset, without the meltwater drainage, the basal stress quickly decreases to zero, causing the surge. The ensuing thinning augments the conductive cooling, which strengthens the bed and slows the flow. When the basal stress reaches about half the driving stress, the bed freezes over rapidly to enter the creep phase. There are, thus, three timescales characterizing the surge cycles: the onset and termination of the surge governed by the short hydrological timescale, the surge and creep durations characterized by the surface displacement divided by thinning rate and accumulation, respectively, with the former typically much shorter. For the stream, on the contrary, the meltwater drainage somewhat smooths the basal stress, so there is only one centennial timescale governing its time variation.

The differing causality of the surge from that of the stream manifests in sharply different relation among its state variables. During the surge, the causality goes from thinning to stronger bed to slower flow, resulting in a positive correlation between the surface height and the ice velocity. For the stream, on the contrary, the causality goes from widening to weaker bed to faster flow to lower surface, which results in anticorrelation between the surface height and the ice velocity. Because of these opposite correlations, they should be discernible from observations, as discussed next.

3.3 Observation

Although the Northeast Greenland Ice Stream has the moniker of an ice stream, it is steered by the bed trough (Joughin and others, Reference Joughin, Fahnestock, MacAyeal, Bamber and Gogineni2001) to fall in the purview of Part 1 (Ou, Reference Ou2021) wherein its deduced properties are discussed. Although the Siple coast does not have a flat bed, the relatively low relief does not strongly constrain the Ross ice streams, which represent unique examples of the ‘pure’ ice streams (Bentley, Reference Bentley1987; Bennett, Reference Bennett2003) for which our model may be applicable. Since our model is predicated on coupled stream/ridge pairs, its applications are further limited to ISB and ISC for the reason that ISA is bounded on one side by the Transantarctic Mountains, and that ISD and ISE have poorly developed interstream ridge before they merge into one stream. In addition, only ISB and ISC have their time evolution over the last millennium sufficiently documented (Hulbe and Fahnestock, Reference Hulbe and Fahnestock2007; Catania and others, Reference Catania, Hulbe, Conway, Scambos and Raymond2012) to test our model.

Based on Figure 5c, we have drawn in Figure 7 the projected time evolution of ISB (left) and ISC (right), and the time proceeds from ~450 years to 150 years ago. It is observed that, following the reactivation of ISB, it would widen and thin while ISC would narrow, thicken and eventually stagnate, a prognosis that is indeed consistent with the observed scenario (Catania and others, Reference Catania, Hulbe, Conway, Scambos and Raymond2012).

There is extensive discussion of the ISC stagnation, which is mostly viewed as a happenstance with myriad precedents (Anandakrishnan and others, Reference Anandakrishnan, Alley, Jacobel and Conway2001; Bennett, Reference Bennett2003). Among them, the ice and water piracies by ISB are inferred from the current surface relief (Alley and others, Reference Alley, Anandakrishnan, Bentley and Lord1994), which, however, is likely a result rather than the cause of the ISC stagnation (Price and others, Reference Price, Bindschadler, Hulbe and Joughin2001). Additionally, we note that the meltwater cannot be transported across the frozen ridge between streams, and that the upper-glacier supply of the meltwater is not required for maintaining the stream, which after all is initiated by the local production of the meltwater; then the ice piracy is at odds with the observed thickening of ISC before its stagnation. As regards the drainage switch to the channelized system (Retzlaff and Bentley, Reference Retzlaff and Bentley1993), there is no observational evidence of conduits under ISC and in fact the bed conditions of ISB and ISC are quite similar (Kamb, Reference Kamb, Alley and Bindschadler2001). The loss of the meltwater would expose the sticky spots, as inferred from seismic data (Anandakrishnan and others, Reference Anandakrishnan, Alley, Jacobel and Conway2001), which however is a mediating agent rather than a cause of the stagnation. Although the grounding line has retreated in the last hundreds of years, it is likely a response to the stagnation (Hulbe and Fahnestock, Reference Hulbe and Fahnestock2007) since ice flow is relatively unaffected by the back-stress from the ice shelf (Anandakrishnan and Alley, Reference Anandakrishnan and Alley1997). However, others have likened the ISC stagnation to the surge termination by thinning-induced freeze-on (Hulbe and Fahnestock, Reference Hulbe and Fahnestock2004; Bougamont and others, Reference Bougamont, Price, Christoffersen and Payne2011), which is at odds with the observation that the ISC bed remains wet (Bentley and others, Reference Bentley, Lord and Liu1998) and its stagnation is preceded by thickening – not thinning (Catania and others, Reference Catania, Hulbe, Conway, Scambos and Raymond2012).

Instead, based on positive comparison of Figure 7 with observations, we offer the following explanation of the ISC stagnation. First, it is not a happenstance but a recurring event of the natural cycles. Second, the cycles are the outcome of invariable interaction among streams with the neighboring ones exhibiting compensating cycles. As such, the widening of ISB following its reactivation ~450 years ago would narrow ISC to augment the meltwater drainage, which then strengthens the bed to cause the stagnation. Although the loss of lubricant has been mentioned in the literature (Catania and others, Reference Catania, Hulbe, Conway, Scambos and Raymond2012), it is largely overlooked, as attested by the above-referenced reviews, and our model provides a concrete affirmation of the mechanism.

There are additional points worth noting about ISC. Although its stagnation can be caused by narrowing-induced loss of the meltwater, as indicated by the dashed arrow in Figure 6b, the bed strengthening might not fully catch up with the narrowing to maintain the heat balance, so the stream might veer slightly into the cooling zone to effectuate partial freezing. This may possibly explain the accretional ice layers recently discovered under ISC (Vogel and others, Reference Vogel2005). To the degree that the meltwater drainage is operative when ISC stagnates, the bed must remain temperate and wet. This differs fundamentally from the surge termination, which, once commences, the positive feedback between the strengthening bed and reduced heating lead unceasingly to a thorough freeze-on (see Part 1); there is no allowance for a temperate and wet bed, contrary to the observed ISC.