Geophysical Research Letters

North Pacific halocline and cold climate induced by Panamanian Gateway closure in a coupled ocean-atmosphere GCM

Authors


Abstract

[1] The influence of the closure of the Panamanian Gateway during the late Cenozoic on climate in and around the North Pacific is investigated by using a coupled ocean-atmosphere general circulation model with an open and closed gateway. In the case of an open (closed) gateway, deep convection is present (absent) in the North Pacific. The deep convection is associated with surface saline water transported from the subtropical Atlantic through the open gateway to the North Pacific. On the other hand, with the closed gateway, the lack of saline water transport from the Atlantic induces halocline formation over the subarctic Pacific with cold climate. The deep convection in the North Pacific leads to a vigorous thermohaline circulation with larger meridional heat transport, and causes warmer climate in and around the North Pacific. These results are generally consistent with paleoceanographic and paleoclimatic estimates related to the closure of the Panamanian Gateway.

1. Introduction

[2] The motion of plates caused a number of tectonic changes involving the closure and opening of oceanic gateways, which may have induced reorganization of ocean general circulation and climate changes. A prime example is the Panamanian gateway which was open during most of the Cenozoic. Gradual shoaling of the gateway started 16 million years ago (Ma) in the Miocene and led to final closure at about 3 Ma in the middle Pliocene [Droxler et al., 1998]. Paleoceanographic proxies suggest that the closure of the gateway caused a series of dramatic changes in the Atlantic and Pacific Oceans, including the intensification of the thermohaline circulation (THC) in the North Atlantic [Haug and Tiedemann, 1998], the establishment of Atlantic-Pacific surface-water salinity contrast [Haug et al., 2001], the collapse of North Pacific deepwater formation [Blanc and Duplessy, 1982], and the onset of stratification in the subarctic North Pacific [Haug et al., 1999; Sigman et al., 2004].

[3] Several numerical models investigated the effects of the Panamanian gateway, especially that on the Atlantic Ocean by using an ocean GCM [Maier-Reimer et al., 1990; Mikolajewicz et al., 1993; Nisancioglu et al., 2003] or a coupled ocean-atmosphere model [Murdock et al., 1997; Mikolajewicz and Crowley, 1997; Prange and Schulz, 2004; von der Heydt and Dijkstra, 2005; Klocker et al., 2005]. Although paleoceanographic studies showed that an open gateway changed the conditions in the Pacific Ocean drastically, as mentioned above, previous modeling studies did not address the changes in the Pacific Ocean. In the present paper, therefore, we aim to identify the climate changes in and around the Pacific Ocean by using a coupled ocean-atmosphere general circulation model (GCM) with topography that includes either an open or a closed Panamanian Gateway. An advantage in this study is that the use of a coupled GCM allows us to examine changes in air temperature and precipitation.

2. Model Experiments

[4] The model used in the present study is a GFDL coupled ocean-atmosphere GCM with flux adjustment [Manabe et al., 1991]. The resolution of the atmospheric part is R15 with 9 levels, while that of the ocean part is 3.75°(lon.) × 4.4°(lat.) with 12 levels. The model represents positive and negative feedbacks of salt and heat transports on the THC reasonably well [Manabe and Stouffer, 2000; Stouffer and Manabe, 2003].

[5] Two experiments, named Close Experiment (CE) and Open Experiment (OE), are carried out. In OE, four ocean grid boxes are removed from the surface to the 9th level (2559 m) in order to represent the Panamanian Gateway. Equilibrium solutions are obtained after 5000-year time integrations in both experiments, and the final 100 years are used for analyses.

3. Oceanic Changes

[6] Figure 1 shows the zonally averaged streamfunction in the Pacific basin for both CE and OE. Almost no THC is found in CE (Figure 1a), consistent with present-day observations. In OE, however, deep convection occurs in the North Pacific, and a vigorous THC reaches depths of 2000 m with a maximum intensity of 12Sv (Figure 1b). In the subarctic North Atlantic, deep convection accompanied by the THC with an overturning strength of 16 Sv found in CE is absent in OE (not shown) as is the case for most ocean GCM studies [e.g., Maier-Reimer et al., 1990; Mikolajewicz et al., 1993; Murdock et al., 1997].

Figure 1.

Streamfunction of the meridional circulation in the North Pacific, averaged over the 100-yr period for equilibrium solutions with (a) closed and (b) open gateway. Units are in Sverdrups (106m3s−1). Contour interval is 2 Sv.

[7] The presence and absence of deep convection in the North Pacific is closely associated with sea surface salinity distribution. Sea surface salinities are generally higher in the subarctic North Atlantic than in the subarctic North Pacific in CE, while the opposite is true for OE (Figure 2). Since water density in high-latitudes is more dependent on salinity, surface water in the subarctic North Pacific (subarctic North Atlantic) is relatively heavier than the water in the other basin with the open (closed) gateway.

Figure 2.

Geographical distribution of sea surface salinity (psu), averaged over the 100-yr period for equilibrium solutions with (a) closed and (b) open gateway. Contour interval is 0.5 psu.

[8] The salinity distributions also exhibit substantial differences in subsurface layers. Figure 3 shows that a prominent halocline, north of 50°N in CE, develops in the North Pacific as observed today, while in OE the salinity is almost vertically uniform with values of about 34.4 in practical salinity units (psu). This indicates that stratification is weak (strong) in OE (CE), consistent with the presence (absence) of deep convection. Another interesting feature of the salinity distribution is that the low-salinity tongue corresponding to North Pacific Intermediate Water south of 45°N is found only in CE. Consequently, open and closed Panamanian gateways induce significantly different THCs not only in the North Atlantic, but also in the North Pacific, due to differences in the salinity distribution.

Figure 3.

The latitude-depth distribution of zonal-mean salinity (psu) in the North Pacific ocean, averaged over the 100-yr period for equilibrium solutions with (a) closed and (b) open gateway. Contour interval is 0.2 psu.

[9] The differences between the salinity distributions of OE and CE are mainly due to the differences in current distributions (Figure 4). When the Panamanian Gateway is open, saline water flows out from the subtropical Atlantic to the North Pacific in the surface layer. The saline water from the gateway is then carried to the subarctic North Pacific by a clockwise mean circulation, which consists of the North Equatorial Current around 15°N, the Kuroshio, and its extension. In OE, enhanced Kuroshio and Kuroshio extension and the overall northward components of interior currents in the shallow layer between the mid- and high-latitudes (i.e., 45°N) transport saline water to the subarctic North Pacific efficiently. In the North Atlantic, the Gulf Stream and its extension are substantially weakened, when the gateway is open, consistent with the lack of a strong THC.

Figure 4.

Geographical distribution of current vectors at depths of 170 m, averaged over the 100-yr period for equilibrium solutions with (a) closed and (b) open gateway. Arrow scaling is in units of cm s−1.

4. Impact on Surface Climate

[10] Differences in the THC in the North Pacific result in different meridional oceanic heat transports. Figure 5 shows that the THC in OE is stronger than that in CE, and transports about 80% more heat across 30°N. The heat carried by the ocean is released to the atmosphere between 30°N and 50°N. Thus, the mid-latitude atmosphere in OE receives more heat from the North Pacific than in CE.

Figure 5.

Meridional heat transport (PW = 1015 W) in the North Pacific, averaged over the 100-yr period for equilibrium solutions with closed (dashed) and open (solid) gateway. Positive (negative) values indicate northward (southward) oceanic heat transport. In the open experiment, only transport north of the gateway is plotted.

[11] The differences in the heating of the atmosphere cause lower (higher) air temperatures over the North Pacific and North America in CE (OE) (Figure 6a). Consistently, snow depth is greater over Alaska and western Canada with isolated anomalies in eastern Canada larger in CE than in OE (Figure 6b). The snow depth difference is not due to differences in amounts of precipitation, but due to the fact that lower temperatures in CE increase the ratio of snow to rain which results in larger snow depths.

Figure 6.

Geographical distribution of the difference (closed minus open) in (a) surface air temperature (°C) and (b) snow depth in water equivalent units (cm), averaged over the 100-yr period for equilibrium solutions.

5. Discussion and Conclusions

[12] The present study compared the differences resulting from an open and closed Panamanian Gateway by using a coupled GCM. We focused our attention on the influences in the Pacific Ocean and adjacent regions, which were not addressed by previous numerical studies focusing on the Atlantic Ocean. The present results suggest that before the closure of the Panamanian Gateway, a stronger THC existed in the North Pacific associated with more saline water in the subarctic North Pacific and a vertically uniform salinity distribution as in OE. On the other hand, the closure of the gateway may have ceased the THC in the North Pacific, and resulted in the formation of a strong halocline. The presence or absence of the strong THC further influences the atmosphere through heat flux from the ocean; surface air temperatures are lower and snow depth over land around the North Pacific are greater when the gateway is closed. Haug et al. [2005] investigated the influences of freshwater input into the subarctic Pacific on stratification and sea surface temperature there and on ice sheet growth by using an Earth system model of intermediate complexity (CLIMBER-2). Their results are highly consistent with our coupled GCM results.

[13] The results of the present model may be validated by comparing them with paleoceanographic and paleoclimate studies. In association with the closure of the Panamanian Gateway, a series of drastic changes were reported, for example, halocline formation in the North Pacific [Haug et al., 1999; Sigman et al., 2004], a weakened Kuroshio [Blanc and Duplessy, 1982], larger sea-ice cover in the Bering and Okhotsk Seas [Blanc and Duplessy, 1982], and a wider area covered by glaciers in northeastern Asia [Maslin et al., 1996]. These results are consistent with our findings. Therefore, the closure of the Panamanian Gateway is likely to have influenced the Pacific Ocean as well as the previously studied Atlantic Ocean.

Acknowledgments

[14] We thank R. Stouffer for his help in using the GFDL model and S. Manabe for his fruitful advice and suggestions. A. Abe-Ouchi and H. Yih are acknowledged for their valuable discussions. We are also grateful to the two reviewers whose comments improved the manuscript of this paper. This study was supported by grant-in-aid for scientific research kaken-hi #15540417 (SM) and by the 21st Century Center of Excellence Program on “Neo-Science of Natural History” headed by H. Okada (SM and HS); both funds were provided by the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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