Effect of Drake and Panamanian gateways on the circulation of an ocean model

PALEOCEANOGRAPHY, VOL. 8, NO. 4, PAGES 409-426, AUGUST 1993

EFFECT OF DRAKE AND PANAMANIAN GATEWAYS ON THE CIRCULATION OF AN OC•AN MODEL

Uwe Mikolajewicz and Ernst Maier-Reimer

Max-Planck-Institut f'tir Meteorologie Hamburg, Federal Republic of Germany

Thomas J. Crowley and Kwang-Yul Kim

Applied Research Corporation College Station, Texas

Abstract. Geologic studies indicate that prior to --40 Ma the Drake Passage was closed and the Central American Isthmus was open. The effect of these changes has been examined in an ocean general circulation model. Several sensitivity experiments were conducted, all with atmospheric forcing and other boundary conditions from the present climate, but with different combinations of closed and open gateways. In the first experiment, the only change involved closure of the Drake Passage. In agreement with earlier studies the barrier modified the

geostrophic balance that now maintains the circumpolar flow in the southern ocean, with the net effect being decreased transport of the Antarctic Current and an approximate fourfold increase in outflow of Antarctic deep-bottom waters. The very large increase in Antarctic outflow suppresses North Atlantic Deep Water (NADW) formation. In addition to corroboration of results from earlier studies, our simulations provide several new insights into the role of a closed Drake Passage. A more geologically realistic closed Drake/open central American isthmus experiment produces essentially the same pattern of deepwater circulation from the first experiment, except that Antarctic outflow is about 20% less than the first experiment. The resultant unipolar deepwater circulation pattern for the second experiment is consistent with paleoceanographic observations from the early Cenozoic. A third

Copyright 1993 by the American Geophysical Union.

Paper number 93PA00893. 0883- 8305/93/93PA-00893510.00

experiment involved an open Drake and open central American isthmus. In this experiment, Antarctic outflow is diminished to slightly above present levels but NADW production is still low due to free exchange of low-salinity surface water between the North Pacific and North Atlantic. The low level of thermohaline overturn should have reduced oceanic

productivity in the Oligocene (.--30 Ma), a result in agreement with geologic observations. Finally, simulations with an energy balance model demonstrate that the changes in surface heat flux south of 60øS due to breaching of the Drake barrier do not result in temperature changes large enough to have triggered Antarctic glaciation. This last result suggests that some other factor (CO27) may be required for Antarctic ice sheet expansion in the Oligocene (-•30-34 Ma). Our results lend further support to the concept that even in the absence of changing boundary conditions due to ice sheet growth, variations in the geometry of the ocean basins can significantly influence ocean circulation patterns and the sediment record. The results also suggest that the primary polarities of the Cenozoic deepwater circulation may have been controlled by opening and closing of these two gateways.

INTRODUCTION

The climate effect of ocean gateways has long been of interest to geologists [e.g., Berggren and Hollister, 1974; Haq, 1981]. The first modeling study to examine this effect was that of Gill and Bryan [1971], who found that in an idealized ocean model a closed Drake Passage should have increased outflow of Antarctic deep-bottom waters to the world ocean. A similar response with the National Oceanic

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410 Mikolajewicz et al.: Ocean Gateway Experiments

and Atmospheric Administration Geophysical Fluid Dynamics Laboratory (NOAA/GFDL) ocean general circulation model (GCM) was demonstrated by Cox [ 1989] and England [ 1992]. Sensitivity experiments with an open central American isthmus also caused large changes in the thermohaline circulation of an ocean model [Maier-Reimer et al., 1990].

In this paper we revisit the Drake Passage problem in order to reexamine its effect on the ocean

circulation. In addition to conducting an experiment similar to Cox [1989] with a different ocean GCM, we conduct geologically more realistic simulations with various combinations of a closed/open Drake and a closed/open central American isthmus and compare the results with the geologic record. We also use an energy balance model to test Kennett's [ 1977] idea that changes in surface circulation in the southern ocean may have been large enough to trigger Antarctic glaciation. Although Kennett's work addressed the opening between Australia and Antarctica (at least 50 Ma), our results are relevant to the role of barriers in the evolution of southern

hemisphere climate. Geologic studies place some constraints on timing

of Drake Passage changes. Presence of South American land mammals (marsupials) on the Antarctic Peninsula somewhere between 40 and 55

Ma [Woodburne and Zinsmeister, 1982] requires a connection up to that time. The best estimate for the opening of the Drake Passage is approximately 30 Ma [Barker and Burrell, 1977]. These changes occur around the same time as significant cooling on Antarctica at 34-35 Ma [Shackleton and Kennett, 1975; Zachos et al., 1992] and a major sea level fall about 30-32 Ma [Vail et al., 1977; Miller et al., 1991 ] that correlates with evidence for grounded ice in the Ross Sea [Barrett et al., 1987]. These near- coincidences support Kennett's basic idea if it is translated from Australia-Antarctica to the Drake

Passage.

MODEL

The model, known as the Hamburg large-scale geostrophic model, is based on the standard set of equations used in numerical models. These are the conservation laws for salt, heat, and momentum, the latter in a linearized form. The equation for the vertical component of momentum is replaced by the hydrostatic approximation. The model was proposed by Hasselmann [1982], is described more fully by Maier-Reimer et al. [1993], and has been used in a number of climate and paleoclimate applications [Maier-Reimer and Mikolajewicz, 1989; Maier- Reimer et al., 1990; Mikolajewicz and Maier-Reimer, 1990; Mikolajewicz et al., 1990]. Briefly, the circulation is divided into a barotropic component, consisting of the vertically averaged current and surface displacement fields, and a baroclinic component, consisting of the residual current after

subtraction of the barotropic component. The formulation of the model is fully implicit, thus allowing a time step of one month. Equatorial Kelvin waves are formally included but because of the coarse grid and the relatively long time step in integration, they are strongly damped. Outside the equatorial regions, the motion is essentially geostrophic, with frictional effects acting primarily at the boundaries and the equator. On the time scales of interest sea ice and the salinity and the temperature fields are the only prognostic variables, to which the velocities and the surface elevation equilibrate almost immediately.

For the present application the model was run on a 3.5 ø X 3.5 ø grid, with 11 vertical levels, realistic bottom topography, and a full seasonal cycle. The circulation was driven by the observed wind stress field from Hellermann and Rosenstein [ 1983] and air temperatures from the Comprehensive Ocean- Atmosphere Data Set [Slutz et al., 1985; Woodruff et al., 1987]. The model was first spun up with present geography and a homogeneous ocean of 2.5øC and 34.5 %0 for a period of 10,000 years, with a full seasonal cycle. Including the seasonal cycle is important because it strongly influences the regions and frequency of overturning in high latitudes. This first experiment (termed the standard run) utilized prescribed temperatures and freshwater fluxes, with damping constraining ocean surface temperature and salinity variations to observations (the latter from Levitus [1982]). At the end of this experiment, we analyzed the fresh water fluxes needed to match the observed surface salinity field. In the second step of our experiment (termed the control run) the damping term was replaced by prescription of the previously analyzed fresh water fluxes.

The above type of "mixed boundary conditions" (fixed surface temperatures but potential for variable salinities) are known to favor the existence of multiple steady states, as shown analytically for simple box models by Stommel [1961], by Bryan [1986] for an oceanic GCM, and by Manabe and Stouffer [1989] for a coupled ocean-atmosphere GCM. With modern forcing and topography, our model is capable of exhibiting at least three distinct equilibria. The region with the most pronounced differences between the modes is the Atlantic. Figure 1 displays the zonally averaged mass transport stream function of the Atlantic for the three modes. The first mode corresponds to the present circulation and is our control run. The second mode with strongly reduced formation of North Atlantic Deep Water (NADW) has many characteristics of the circulation for the last glacial maximum as shown by Heinze et al. [1991] with an oceanic carbon cycle model that was compared with •513C data from sediment cores [Duplessy et al., 1988]. The third mode is characterized by a complete absence of NADW formation and is very similar to the modern

Mikolajewicz et al.' Ocean Gateway Experiments 411

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Fig. 1. The three modes of circulation in the Hamburg model for present levels of atmospheric forcing (a) "NADW ON", meaning that NADW production is comparable to observations; (b) "NADW REDUCED", meaning that production is about one-half of observations; and (c) "NO NADW", in which NADW production is essentially shut down. Negative sign indicates counterclockwise meridional overturning as viewed from the reader's perspective.


Pacific. All modes were stable for more than 4000 model years. We note that the stability of the modes is very different. Apparently the third mode is the most stable one and is most likely to occur from an arbitrarily chosen initial field.

The first mode of the model does a reasonably good job of simulating the present surface and deep ocean circulation patterns [Maier-Reimer et al., 1993]. It has a realistic "conveyer belt" [Broecker et al., 1988] structure, with a thermohaline cell in the North Atlantic, cross-equatorial heat transport in the Atlantic [cf. Hastenrath, 1982], deep-bottom water outflow from the Southern Ocean, and •4C patterns approximately in line with GEOSECS measure- ments.

Three main scenarios were examined: (1) closed Drake Passage; (2) closed Drake Passage and open central American isthmus; and (3) an open Drake Passage and open central American isthmus. In the second case we removed all land on North America between 30øN and the equator. The final scenario re- examined the results of Maier-Reimer et al. [1990], which had a narrower opening across Central America. The differing width of this opening is not considered to greatly affect the results.

Several different sill depths were used in the simulations. Our baseline cases specified sills at 2700 m for both the Drake and central American (Panama) gateways. We also conducted two additional sensitivity experiments, one with a shallow Drake opening (700 m) and a 2700 m Panamanian sill, the other with a 2700 m Drake opening and a very deep (4100 m) Panamanian sill. For the perturbation runs we started with the steady state of the control run and then modified the barriers. The total length of the perturbation run was 4500 years, with the average of the last 500 years being used in the analysis.

RESULTS

General pattern of deepwater changes. The effects of the Drake Passage on the global meridional circulation is illustrated in Figure 2. Relative to the control, the closed Drake Passage (Figure 2b) results in a stronger meridional overturning cell in the southern ocean (Figure 2a). Although this result is similar to that of Cox [ 1989] with the NOAA/GFDL model, forcing in the two model runs was different. The GFDL run used prescribed temperatures and salinities; the latter feature permit the freshwater flux to vary with time. The MPI run used prescribed temperatures but does not allow the freshwater flux to vary with time (see above discussion).

In general, the trend is similar for both the closed Drake/closed isthmus (Figure 2b) and closed Drake/open isthmus runs (Figure 2c), except that Antarctic outflow in the latter run is reduced

approximately 20%. Opening of the Drake Passage (Figure 2d) resulted in a significant reduction in Antarctic outflow (Figure 2d), with levels now only slightly (-15%) above that of the control run.

A result of particular interest involves the reduced level of NADW production in the sensitivity experiments. This latter result is manifested in terms of absence of a meridional overturning cell in the northern hemisphere (cf. Figures 2a and 2b). This response reflects the enhanced upwelling of Antarctic water in the North Atlantic, which reduces NADW outflow to the south. This reaction reduces

advection of salty water to the north, increasing the residence time of surface water in the subpolar Atlantic, further reducing salinity and convection. NADW production remains suppressed even after the Drake Passage opens (Figure 2d) because of exchange of low salinity water between the North

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Fig. 2. Global-averaged mcridional circulation (Sv) for (a) control; (b) closed Drake Passage; (c) closed Drake Passage and open central American isthmus; and (d) open Drake and open central American isthmus. Negative sign indicates counterclockwise mcridional overturning as viewed from the readcr's perspective.

Mikolajewicz et al.: Ocean Gateway Experiments 413

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Pacific and North Atlantic [Maier-Reimer et al., 1990].

We did several experiments to identify different stable modes for the closed-Drake configuration. Similar to the experiments described by Maier- Reimer et al. [1990] we were not successful. This result cannot be taken as a strict proof of the nonexistence of such stable modes. A strong increase of the net evaporation in the North Atlantic could lead to the existence of multiple steady states. For example, to discriminate between the purely topographical effect of the closure of the Drake Passage and a "halocline catastrophe" response, we performed an experiment with a closed Drake passage and prescribed modern surface salinity. In this run the formation of NADW was reduced to

about half the value of the reference run (results not shown). The evaporation in the North Atlantic had to be increased by 0.1 Sv in order to maintain modern surface salinities. This alteration must be

compared to a total net evaporation of the Atlantic (north of 30øS) to slightly more than 0.2 Sv in the reference run. Thus alternate modes of circulation

for the closed Drake case are possible, but freshwater flux changes would have to be quite large. Such changes may however explain episodic turn-on of NADW in the earliest Oligocene (34 Ma [Miller, 1992]).

In addition to our "main sequence" of simulations we also examined the sensitivity of the model to stipulation of changes in sill depth. Two additional experiments were conducted, one with a shallow (700 m) sill in the Drake Passage, the other with a deep (4100 m)opening between the Atlantic and Pacific across central America. The effect of a

shallow sill in the Drake Passage is intermediate between a deep sill and full closure (Figure 3a). However, a deep opening across central America (Figure 3b) enhances Antarctic outflow levels over a shallower sill. The center of the overturning cell also increases in depth approximately 700 m in the southern ocean (cf. Figures 2b and 3b).

Enhanced Antarctic outflow with a deeper central American isthmus is related to the horizontal pressure gradients across the deep sill. The coldest Antarctic water is in the Atlantic sector. With a deep sill across central America, this water can now flow into the Pacific, thus preventing "backup" of sinking waters at the point of origin. The dramatic nature of this response can be seen in the 3000-m velocity fields (Plates lb and lc), which illustrate almost a reversal in flow patterns over much of the Pacific basin. Particularly prominent is a change from a northward flowing western boundary current in the South Pacific with a 2700 m sill across the isthmus (Plate lb) to a southward flowing western boundary current with the 4100 m sill depth (Plate 1 c). Note that a comparison of the 2700 m isthmus sill depth run (Plate lb) with the control (Plate la) indicates that the most significant area of the southern ocean

circumpolar flow affected by breaching of the Drake Passage is in the South Pacific. Unfortunately, we are unable to determine from deep-sea hiatus records [Keller et al., 1989] whether there is any support for this model prediction in the geologic record.

Another manifestation of the deep ocean response is obtained by examining the "Quick •4C" fields (Figure 4). This is an artificial tracer in the model, with concentration defined as 1 in the atmosphere, a piston velocity of 6 m/yr, and a decay of 1/8237 yr-1. In the Atlantic, the Quick •4C fields reflect changing level of Antarctic and NADW production. At the equator, the deepwater age is approximately 10% less in the Drake Passage experiment, but it is 10% more at 30øN -- a result reflecting diminished production of young NADW.

Thermocline response. Examination of the global meridional circulation for the different scenarios

(Figures 2a-2d) indicates that changes vs. the present vary from roughly a factor of two increase to a reduction by onehalf (the factor of 2 increase reflects a fourfold increase of AABW but an elimination of

NADW). These changing levels of overturn may have affected the oceanic thermocline. For example, simple upwelling-diffusion models of the thermocline [Munk, 1966] suggest that the general structure of the thermocline is controlled by a balance between upwelling in the oceanic interior and downward diffusion of heat. The rate of upwelling is directly proportional to the rate of deepwater overturn. Although this model is an oversimplifica- tion of the real-world picture [e.g., Jenkins, 1988] it is nevertheless useful as a starting point of inquiry into the relationship between past changes in overturn, vertical structure, and the potential effects on ocean productivity.

To test the Munk model we compared (Figure 5) the mean annual temperature profiles in the model at three different locations for a closed Drake/open isthmus (strong circulation) and open Drake/open isthmus (weak circulation). For the two Pacific sites (30øS and the equator), we found only modest changes in the vertical temperature profiles. However, in the South Atlantic (30øS), the region most immediately influenced by Drake Passage changes, the changes were larger. The two most significant changes involved a cooling of the deep waters (due to absence of NADW) and a weakening of the thermocline for the open Drake/open isthmus configuration.

Although the ocean GCM yields some changes in the thermocline, they are not as large as those produced by a simple Munk-type upwelling- diffusion model. A direct comparison (Figure 6) of the Hamburg model results with those produced by a type of upwelling-diffusion model commonly used in transient CO2 climate studies [Kim et al., 1992] indicates that varying the upwelling rate in the simple model results in greater changes in the thermocline than occurs in the ocean GCM. These results imply

Mikolajewicz et al ß Ocean Gateway Experiments 415

a limit to some applications of the upwelling- diffusion model. However, because net thermohaline changes for transient CO2 runs are relatively small (some reduction in NADW but increase in Antarctic outflow [cf. Cubasch et al., 1992]), the upwelling-diffusion approach may still be useful for those investigations.

The restriction of the large thermocline changes in the GCM to the South Atlantic is of interest because

early Deep-Sea Drilling Project results from the South Atlantic and Indian Ocean indicated chalk

layers of the coccolith Braarudosphaera [e.g., Wise and Hsii, 1971]. Berger [1981] conjectured that these layers represented monospecific blooms resulting from periodic overturning of the water column. Our results support Berger's hypothesis, for we simulate the weakest thermocline in precisely

the region where the Braarudosphaera chalks are found.

Surface changes. The modified deepwater circulation also affects the surface circulation and

resultant fields such as poleward ocean heat transport. As the most convenient way to examine the circulation pattern is to plot the differences from the present, we illustrate as an example the difference in circulation between the closed Drake/open isthmus and the open Drake/open isthmus (Plate 2c). The difference map is plotted to show the type of changes that might occur in a geologic column if an investigator were to examine changes between the Eocene (40 Ma) and Oligocene (30 Ma). The figure illustrates the general strengthening of the Antarctic Circumpolar Current (ACC) that results from breaching of the Drake barrier. This result is

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consistent with the calculations of Hidaka and

Tsuchiya [1953] emphasizing the importance of frictional drag for Antarctic circumpolar flow. As will be demonstrated in the discussion section, the surface circulation response immediately follows from the changes in the thermohaline circulation.

Of perhaps equal interest to the Antarctic changes are the far-field variations in the surface circulation

response to the opening of the Drake Passage (Plate 2c). There is a reduction in the strength of the Brazil Current, a strengthening of North Equatorial Current flow along the north coast of South America, reduced equatorward transport in the eastern South Pacific, and reduced transport of the Pacific western boundary current. This latter response is quite intriguing, for it suggests that if an investigator were to detect an Oligocene diminution in Kuroshio Current fauna in a Japanese section, the reduction may reflect the oceanic adjustments to an event occurring halfway around the word.

Poleward heat transport in the southern hemisphere increases for the altered barriers cases (Figure 7). These changes reflect in part the fact that, when NADW production is reduced, there is less near- surface transfer of heat from the South Atlantic to the

North Atlantic [cf. Crowley, 1992]; heat storage in the South Atlantic therefore increases. Enhanced

Antarctic outflow also increases poleward heat transport in the southern hemisphere.

Because opening of the Drake Passage results in significant changes in the thermohaline circulation and Antarctic surface circulation, we decided to test Kennett's [1977] hypothesis that changes in the Southern Ocean circulation should have significantly affected poleward ocean heat transport and possibly triggered Antarctic glaciation. To test this idea, we had to take an indirect route, because the model's atmosphere is fixed at present forcing for all experiments. We therefore examined the field for

changes in ocean heat flux before and after opening of the Drake Passage (Figure 8). These changes are not zonally symmetric south of the Antarctic convergence; there are regions of net increase and decrease that exceed 50 W/m 2.

In order to examine the potential surface temperature response to the ocean heat flux changes, we inserted the altered heat flux field from Figure 8 as a bottom boundary condition into a version of a two-dimensional (latitude-longitude) energy balance model originally developed by North et al. [1983]. The significant modification we make is that the lower boundary condition in the ocean is now specified in terms of ocean heat flux. In order to calculate the temperature change due to changes in ocean heat flux, we used the following energy balance model:

3T(7, t) C(7) at + A + BT- V- [D(x)VT] = Q(r-', t), where x is latitude, C (•')is the position-dependent heat capacity, B is the radiative damping term, and D(x) is diffusion. The forcing, O(•', t), represents the vertical heat flux as a function of position and time, and A is the global mean value of 0(3', t). The purpose of A is to subtract the added heat flux from the ocean to maintain the overall energy balance. The model and parameters are described in detail by North et al. [1983]. This model and its descendants have been repeatedly compared with atmospheric GCMs and observations and have shown a sensitivity comparable to both [Crowley et al., 1991; Kim and North, 1991 ]. The version we employ has previously been used in a paleoceanographic application to examine the effect of changes in the strength of the North Atlantic thermohaline circulation on northern hemisphere temperatures [Crowley and Kim, 1992].


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b TEMPERATURE

Fig. 6. Comparison of thermocline changes in the Hamburg model with those produced in an upwelling-diffusion model [Kim et al., 1992]. (a) Hamburg model results replotted from the South Atlantic transect (cf. Figure 5); (b) upwelling- diffusion model results with base level upwelling rates of 4 rn/yr and modified rates of 8 and 2 rn/yr. The simulated changes in the upwelling-diffusion model reflect those simulated for cases with a strong thermohaline circulation in the South Atlantic

(D00P27, cf. Figure 5) and a weak thermohaline circulation (D27P27, cf. Figure 5).

The EBM response to imposed surface heat flux changes is large (5ø-6øC) in some regions south of the Antarctic convergence (Figure 9). However, the responses are not of uniform sign. To examine the integrated response, we examined the zonal average (Figure 10). Here the changes are much smaller (IøC) and still not of uniform sign. The globally averaged difference in surface temperature response between the runs is only 0.12øC. For comparison 15180 studies from the Southern Ocean indicate that

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the largest cooling in the Early Oligocene (34 Ma) was 4ø-5øC [Shackleton and Kennett, 1975].

At first glance the small average temperature response for the altered circulation may seem surprising, for the total circulation changes are large. However, it must be recalled that circulation only redistributes heat on the planet. The only reason the globally averaged computed temperature difference for the two runs is not 0.0øC is because of the slight errors introduced by the smoothing and truncation at spherical harmonic degree 11. Even south of the polar front the circulation changes are not uniform, as they are of opposite signs in different sectors of the southern ocean.

DISCUSSION

Interpretation of deepwater changes. A qualitative argument for the strong increase of the southern meridional overturning cell may be derived from the linearized equations of motion for the density driven part:

ut- fv + Px = 0

vt+fu+py=0

wt + go + Pz = 0

where O is the buoyancy and p the pressure, normalized by a reference density O0. The zonal component of the vorticity (o) is

• = Wy - Vz

From this the time derivative follows from vt - wt:

ot = + fuz- gOy We note that in the zonal average we have Vz < 0, W y>0, p <0, and Ivzl >> Iw I, which implies y . . a----Vz. •or the modern Antarcnc C•rcumpolar Current (ACC) we have Uz > 0 almost everywhere and f < 0. Thus we see that a stronger ACC reduces the forcing in the meridional circulation. If we plausibly assume (supported by our model) that the zonal average of Uz was reduced in the closed barrier case, the strengthening of the meridional turnover is an immediate consequence. The enhanced meridional velocity is then in geostrophic balance with an east-west pressure gradient, which at least in a zonal average can develop only on a closed barrier.

Model uncertainties. Although the results of our calculations extend our understanding beyond that which can be obtained by conjecture alone, there are of course limits to their applicability to understanding Cenozoic climate change. The most obvious limitation is use of present geography and boundary conditions. Because ice volume and (probably) carbon dioxide changed over the Cenozoic, these variations should also have affected ocean circulation

patterns. Another important limitation involves the fact that our model is not eddy-resolving, and such features are important for understanding the dynamics of the ACC [e.g., McWilliams et al., 1978; Wolff et al., 1991]. Furthermore, different ocean models might produce different results. For example, the ACC overturning cell in the GFDL model [Toggweiler et al., 1989] has a different strength than the one illustrated in Figure 2a. Sensitivity experiments at Hamburg indicate that this difference is a consequence of choice of appropriate formulation of boundary conditions rather than

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90øN , , , , , |

90ø8

Fig. 8. Differences in surhce heat flux (W/m 2) between closed Drake/open isthmus and open Dr•e/open isthmus experiments (D27P27 - D0027). The differences •e plotted to illustram the changes that occur after the Drake Passage opens. The sign of the response indicates heat flux to (positive) or from (negative) the atmosphere. •ese values •e the input m •e EBM c•culafion (Fig•e 9). Nora that the sign of the anom•ies is opposite to that calculated by the ocean GCM (which calculates heat flux changes into the ocean) and has been changed in order to obtain the co•ect sign of response in the EBM. Values have been smoothed by expan•ng original 5 ø X 5 ø input in spherical h•onics and truncating at de•ee 11.

inherently different model physics. Nevertheless, the differences indicate that some model results may vary somewhat depending on choice of parameters. Another potentially important source of errors is the strongly simplified description of the atmospheric behavior by the mixed boundary conditions and the forcing with modem climatology.

For the above reasons, some caution is required before emphasizing details of the model results. However, because our principal result depends on the geostrophic relationship, and this result seems robust, we believe that our main conclusions about effects of a barrier are not a consequence of any model idiosyncrasies. Nevertheless, we recognize that future experiments with more realistic boundary conditions may well alter the magnitude of the thermohaline response in the Southern Ocean.

It is also important to emphasize that our results represent a sensitivity experiment rather than a simulation. Before a completely satisfactory understanding of early Cenozoic climates can be obtained, it is necessary to alter geography and atmospheric forcing. Barron and Peterson [1991] have conducted one such set of experiments, utilizing early versions of the National Center for Atmospheric Research atmospheric and ocean models. They report no such results as ours. However, their model utilized only four levels in the vertical and mean-annual forcing, so it may not resolve some of the deepwater changes we examine and which exert such a strong influence on surface flow. It is likely that a combination of continued sensitivity experiments and simulations will

eventually lead us to a better understanding of early Cenozoic ocean circulation. For example, Barron and Peterson [1991] demonstrated that with an Eocene (40 Ma) geography and higher CO2 levels, their ocean model produces warm, saline bottom water in the subtropics. This result is in agreement with geologic conjectures for that time interval [Brass et al., 1982] and some observations [Kennett and Stott, 1991 ].

Implications for Cenozoic paleoceanography. It is of interest to compare the model results with the general patterns of deepwater circulation inferred from paleoceanographic records. Most studies indicate that, although NADW formation may have been sporadically active during the Oligocene and early-middle Miocene (15-30 Ma), a more consistently active phase was not initiated until after 10 Ma [Miller and Fairbanks, 1985; Woodruff and Savin, 1989; Delaney, 1990; Wright et al., 1991; Miller, 1992]. Prior to that time, there appears to have been one primary source of deepwater formation, and that was in the high latitudes of the southern hemisphere [Katz and Miller, 1991].

Overall our results agree with the paleoceanographic observations except for the fact that the more active late Miocene phase of NADW formation was initiated prior to final closure of the central American isthmus (-3 Ma). In our previous study [Maier-Reimer et al., 1990], we conjectured that shallowing of the isthmus may have significantly restricted circulation prior to final closure. Geologic observations provide some support for this conjecture. Benthic foraminifera indicate that a sill


AT (øC) DUE TO HEAT FLUX ANOMALY

t80øW 120øW 60øW GM. 60 ø g 120 ø Ig 180 ø g

Fig. 9. Regional surface temperature response to heat flux perturbation stipulated in Figure 8. additional comments in Figure 8 caption.

See

appears to have been present at-1000 m by -12 Ma [Keller and Barron, 1983; Duque-Caro, 1990]. Although Atlantic-Pacific exchange continued into the Pliocene, various biotic indices suggest increased divergence of Atlantic-Pacific traits, especially after 6 Ma [Keller et al., 1987; Duque-Caro, 1990]. Indeed, benthic foraminifera indicate some marginal basins appear to have filled to within 200 m depth by this time, and some terrestrial vertebrates (raccoons, ground sloths) had already crossed the barrier by 9.3-8.0 Ma [Marshall, 1985]. These results all support our earlier conjecture as to reasons for early switch-on of NADW.

Simulated changes in the thermohaline circulation may also have significantly affected cycling of nutrients and carbonate preservation during the Cenozoic. For example, a twofold increase in thermohaline overturn should result in generally higher levels of oceanic productivity. Reduced levels of overturn should correspondingly decrease productivity. Berger [ 1981] has noted the overall levels of low accumulation rates in the Oligocene (30 Ma) that may reflect the processes we examine in this study.

The inferred changing levels of oceanic productivity due to decreased thermohaline overturn may complicate previous interpretations of Cenozoic trends in variations of the calcium carbonate

compensation depth (CCD). For example, van Andel et al. [1975] note a 1-km depression in the CCD in the Oligocene. Berger [1981] attributed this depression to mass transfer of carbonate from the continental shelves to the deep ocean as a result of general reduction in sea level at this time. However, Broecker [ 1971 ] has pointed out that reduced rates of thermohaline overturn may also result in better preservation of carbonate. At present, approximately 80% of the carbonate formed in surface waters must

be dissolved at depths in order to maintain a steady supply of carbonate to surface waters. Broecker pointed out that if overturn decreased, then preservation would increase since not as much carbonate would need to be dissolved in order to

maintain a steady state. These arguments suggest that there is a need for a more quantitative examination of the relative importance of shelf/basin fractionation and thermohaline overturn for the

depression of the CCD in the mid-Cenozoic. Applications to Antarctic glaciation. Our results

provide only partial support for Kennett's [1977] hypothesis about the effect of meridional barriers on

AT (øC) DUE TO HEAT FLUX ANOMALY

i i i i ! i

90 70 50 30 10 -10 -30 -50 -70 -90

LATITUDE

Fig. 10. •onal-avcrag½ of computed rcmp½•arur½ response of Figure 9.


Antarctic circumpolar flow. They indicate that changes in Southern Ocean barriers significantly affect Antarctic outflow. However, the changes do not seem to be sufficient to explain estimated temperature patterns in the southern hemisphere in the early Cenozoic. Our results furthermore suggest that breaching of the final barrier to ACC flow (the Drake Passage) may be more important than breaching of the first barrier (Australia-Antarctica).

Even if other model simulations were to produce higher sea surface temperatures in the Southern Ocean in the early Cenozoic, it is not clear they would be sufficient to explain early Cenozoic waxmth on Antarctica. Sensitivity experiments with an atmospheric model [Oglesby, 1989] suggest that, even if sea surface temperatures were higher around Antarctica in the early Cenozoic, these changes alone may not have been sufficient to maintain an ice-free interior. An earlier study [Crowley et al., 1986] suggesting that deglaciation could have been due to enhanced seasonal warming on the combined Australian-Antarctic landmass has also been

overturned by subsequent calculations [Hyde et al., 1990; Oglesby, 1991]. These results suggest that some other factor (lower CO27) may be required for Antarctic ice sheet expansion in the Oligocene (•-30- 34 Ma). However, because of the fairly close timing between postulated opening/deepening of the Drake Passage and Antarctic glaciation (see introduction), it is possible that the large ocean circulation changes we simulate may have affected the CO2 levels of the atmosphere. This result may eventually be testable with coupled ocean-carbon models.

SUMMARY AND CONCLUSIONS

1. An ocean general circulation model has been used to test the effect of a closed Drake Passage on the circulation. Additional experiments examined the combined effects of a closed Drake Passage and open central American isthmus. All experiments were conducted with present atmospheric forcing and present geography.

2. Results indicate that a meridional barrier in the

Southern Ocean modifies the geostrophic balance, resulting in greater outflow of Antarctic deep-bottom water to the world ocean and weaker transport in the Antarctic Circumpolar Current.

3. Enhanced outflow of Antarctic deep-bottom waters suppresses deepwater production in the North Atlantic.

4. Energy balance simulations demonstrate that the changes in surface heat flux south of 60øS due to breaching of the Drake barrier do not result in large- scale temperature changes sufficient to have triggered Antarctic glaciation. This last result suggests that some other factor (CO27) may be required for Antarctic ice sheet expansion in the Oligocene (•-30- 34 Ma). Because of the fairly close timing between postulated opening/deepening of the Drake Passage and Antarctic glaciation, changes in the thermohaline

circulation may have affected the CO2 levels of the atmosphere, thus indirectly influencing Antarctic climates.

5. Changing levels of the thermohaline circulation should have affected oceanic productivity. In particular our results agree with observations of low productivity in the Oligocene and inferences of a weak thermocline in the South Atlantic at that time.

6. The results provide further support to the importance of geometry on the evolution of Cenozoic ocean circulation patterns. The results also suggest that the primary polarities of the Cenozoic deepwater circulation can be explained by opening and closing of these two gateways.

Acknowledgments. This work was supported in part by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 318 (U.M.) and NSF grant OCE-9115729 (TJC). We thank S. Baum, K. Bryan, K. Miller, A. Weaver, J.-O. Wolff, and an anonymous reviewer for comments, and H. H6ck and N. Noreiks for assistance with the

figures.

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Effect of Drake and Panamanian gateways on the circulation of an ocean model

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