NOTES AND CORRESPONDENCE. Influence of Global Warming on Baroclinic Rossby Radius in the Ocean: A Model Intercomparison

Transcription

1 1354 J O U R N A L O F C L I M A T E VOLUME 19 NOTES AND CORRESPONDENCE Influence of Global Warming on Baroclinic Rossby Radius in the Ocean: A Model Intercomparison OLEG A. SAENKO Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, Victoria, British Columbia, Canada (Manuscript received 4 May 2005, in final form 22 July 2005) ABSTRACT Results from eight ocean atmosphere general circulation models are used to evaluate the influence of the projected changes in the oceanic stratification on the first baroclinic Rossby radius of deformation in the ocean, associated with atmospheric CO 2 increase. For each of the models, an oceanic state corresponding to the A1B stabilization experiment (with atmospheric CO 2 concentration of 720 ppm) is compared to a state corresponding to the preindustrial control experiment (with atmospheric CO 2 concentration of 280 ppm). In all of the models, the first baroclinic Rossby radius increases with increasing oceanic stratification in the warmer climate. There is, however, a considerable range among the models in the magnitude of the increase. At the latitudes of intense eddy activity associated with instability of western boundary currents (around ), the increase reaches 4 km on average, or about 15% of the local baroclinic Rossby radius. Some of the models predict an increase of the baroclinic Rossby radius by more than 20% at these latitudes under the applied forcing. It is therefore suggested that in a plausible future warmer climate, the characteristic length scale of mesoscale eddies, as well as boundary currents and fronts, may increase. In addition, since the speed of long baroclinic Rossby waves is proportional to the squared baroclinic Rossby radius of deformation, the results suggest that the time scale for large-scale dynamical oceanic adjustment may decrease in the warmer climate, thereby increasing the frequency of long-term climate variability where the oceanic Rossby wave dynamics set the dominant period. Finally, the speed of equatorial Kelvin waves and Rossby waves, carrying signals along the equator, including those related to ENSO, is projected to increase. 1. Introduction The first baroclinic Rossby radius of deformation plays a fundamental role in ocean dynamics. It is a natural scale that often is associated with boundary phenomena, such as boundary currents and fronts, and with eddies (Gill 1982). In addition, the speed of long (nondispersive) baroclinic Rossby waves, which are one of the key players in the large-scale oceanic adjustment to perturbations, is proportional to the squared baroclinic Rossby radius of deformation. Outside of nearequatorial latitudes, the first baroclinic Rossby radius can be formally defined as the distance, 1, that first baroclinic gravity waves of speed c 1 propagate over time f 1 (Gill 1982): Corresponding author address: Dr. Oleg A. Saenko, Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, Victoria, BC V8W 2Y2, Canada. oleg.saenko@ec.gc.ca 1 c 1 f, 1 where f is the planetary vorticity. Values of the first baroclinic Rossby radius in the ocean vary with latitude, ranging from a few kilometers at high latitudes to greater than 100 km near the equator. The speed of baroclinic gravity waves, and hence the baroclinic Rossby radius, is proportional to stratification (Gill 1982; see also next section). Under unperturbed climate conditions, the large-scale structure of the oceanic stratification does not change much, if averaging over a long enough period of time is applied. However, if the climate system is forced by, for example, a positive radiative forcing, the ocean, particularly the upper ocean, warms up and hence its stratification may change. It therefore seems reasonable to expect that the baroclinic Rossby radius may change with changing climate conditions. The question then is 2006 American Meteorological Society

2 1 APRIL 2006 N O T E S A N D C O R R E S P O N D E N C E 1355 how significant could such a change be in, for example, a climate with higher CO 2 level? We address this question using results from several climate models. Since the magnitude of the change is likely to depend on the magnitude of the forcing, one would like to consider a realistic case. The purpose of this note therefore is, using a simple procedure (see next section), to evaluate whether a plausible global warming scenario could lead to noticeable changes in the first baroclinic Rossby radius of deformation in the future ocean. Here we consider a hypothetical future climate, where concentrations of greenhouse gases in the atmosphere have stabilized at the level implied by the 720-ppm stabilization experiment [Special Report on Emissions Scenarios (SRES) A1B] adopted by the Intergovernmental Panel on Climate Change (IPCC). According to this scenario, the level of CO 2 in the atmosphere reaches 720 ppm by the end of the twentyfirst century, which is roughly in between the other two widely discussed scenarios, B1 and A2. 2. The procedure and the data A standard procedure for evaluating the baroclinic Rossby radii of deformation is to use a linearized quasigeostrophic potential vorticity equation. Under certain assumptions (Gill 1982), the vertical dependence can be separated from the horizontal and time dependence, leading to an eigenvalue problem of Sturm Liouville form. For many purposes, a sufficiently accurate solution of this eigenvalue problem can be obtained by employing the so-called Wentzel Kramers Brillouin (WKB) method (Gill 1982; Chelton et al. 1998). Using this method, the phase speed of the first baroclinic gravity wave is 0 c 1 1 H N z dz, 2 where N is a buoyancy frequency, computed following the procedure outlined in section d of appendix B in Chelton et al. (1998); H(x, y) is the depth of the ocean. Using this procedure and data from different climate models, we first evaluate the warming-induced changes in the speed of the first baroclinic gravity waves. Then, these changes are converted into the changes in the first baroclinic Rossby radius of deformation using Eq. (1) outside of 10 from the equator. The use of the WKB method seems to give particularly accurate results for evaluating km (see Fig. 1 in Chelton et al. 1998), as is typically found farther than about 10 from the equator. The data we use are from the coupled models submitted for inclusion in the IPCC s Fourth Assessment Report, archived and provided by the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Two sets of data are used here. One of them, representing the hypothetical future climate, corresponds to the 720-ppm stabilization experiment (SRES A1B). The other, against which the changes in the baroclinic Rossby radius are compared, corresponds to the PCMDI preindustrial control experiment, where atmospheric CO 2 was held fixed at the 280-ppm level. All quantities discussed below were computed using 3D monthly data of ocean potential temperature and salinity and then averaged over 10 yr to produce corresponding climatological distributions. Since vertical integration in Eq. (2) is taken over the whole depth of the ocean, the term stabilization used by the PCMDI warrants discussion. It takes a long time for the deep ocean to stabilize. For example, the time scale for vertical mixing in the ocean is H 2 /k, where k is the vertical diffusivity. Reasonable values for k give thousands of years for, and a comparable time scale can be obtained by considering either the rate of isopycnal mixing or the rate of deep water production. In the 720-ppm stabilization experiment, the participating models were run for 200 yr with fixed concentration of greenhouse gases, which is not enough for the deep ocean to stabilize. Some of the models were not run for the stabilization at all, whereas some others were run for only 100 yr. Therefore, to keep the number of the models participating in this calculation relatively large, we use the data from the last decade of the first 100 yr of the SRES A1B 720-ppm stabilization experiment (corresponding to a nominal period of time between 2191 and 2200; using the data between 2181 and 2200 does not change the results). From the above discussion, however, it seems likely that the models with significantly different mixing parameters could be at different stages of stabilization after 100 yr of integration. This, as well as many other factors, may have contributed to the differences between the model results. A comprehensive discussion of all potential sources of the differences is beyond the scope here. The models are listed in Table Results The speed of the first baroclinic gravity waves corresponding to the preindustrial control experiment is shown in Fig. 1 (top). In general agreement with observational estimates (Chelton et al. 1998), the models simulate values of c 1 in the range of 2 3 ms 1 in low latitudes, decreasing toward higher latitudes. The re-

3 1356 J O U R N A L O F C L I M A T E VOLUME 19 TABLE 1. The climate models results that are used in this study. N Originating group(s) IPCC I.D. 1 Canadian Centre for Climate Modelling and Analysis CGCM3.1(T47) 2 Météo-France/Center National de Recherches Météorologiques (CNRM) CNRM CM3 3 U.S. Department of Commerce/National Oceanic and Atmospheric Administration/ GFDL CM2.0 Geophysical Fluid Dynamics Laboratory (GFDL) 4 LASG/Institute of Atmospheric Physics, China FGOALS-g1.0 5 Institut Pierre Simon Laplace (IPSL), France IPSL CM4 6 Center for Climate System Research (University of Tokyo), National Institute for Environmental Studies, and Frontier Research Center for Global Change Model for Interdisciplinary Research on Climate (MIROC3.2) (medres) (JAMSTEC), Japan 7 Meteorological Research Institute, Japan MRI CGCM Hadley Centre for Climate Prediction and Research, Met Office, United Kingdom (UKMO) UKMO Third Hadley Centre Coupled Ocean Atmosphere GCM (HadCM3) duction of c 1 from low to high latitudes reflects a tendency of the oceanic stratification to decrease toward higher latitudes. An exception is the increase of c 1 north of about 65 N, largely reflecting the increase of oceanic stratification poleward of 65 N associated with a strong halocline in the Arctic Ocean. The considerable differences in c 1 between the models may arise from a number of factors, ranging from differences in representation of physical processes in these coupled models to adopted resolution. For example, there is a significant difference among the models in the number of vertical levels used to resolve the vertical structure of the ocean. In some of the models, up to 50% of the total number of vertical levels is used to resolve the uppermost 300 m, which is less than 10% of the mean depth of the ocean. However, since Eq. (2) assumes both a computation of vertical derivative of density and an integration of buoyancy frequency from the surface to the ocean bottom, a high vertical resolution of the whole water column is desirable. There is general agreement between the models in that in the warmer climate, the zonally averaged speed of baroclinic gravity waves increases essentially at all latitudes (Fig. 1, bottom). However, the magnitude of the increase differs significantly between the models. On average, the largest increase of c 1 is simulated in the Arctic Ocean, although the spread between the model results is also largest there. One of the models [the Meteorological Research Institute Coupled GCM (MRI CGCM2.3.2)] shows very weak response of c 1 in the northern polar latitudes. (It should be noted that Arctic Ocean is typically very poorly resolved by the current generation of global ocean atmosphere models.) At southern high latitudes, the disagreement between the models is also large, with some of the models showing little change in c 1 south of 60 S, whereas some others show a relatively large response. Between 40 S and 40 N, agreement is somewhat better in that all of the models at least show an increase of c 1. The increase, however, varies significantly, from 0.2 to 0.5 m s 1, being about 0.3 m s 1 on average. According to Eq. (1), an increase of c 1 must translate into an increase of 1. At the equator, however, the increase of c 1 is by itself of particular interest. This is because equatorial Kelvin waves propagate eastward with the speed of baroclinic gravity waves, whereas the equatorial Rossby waves propagate westward with the speed that is also linearly related to the speed of baroclinic gravity waves. These waves play a fundamental role in equatorial dynamics and climate, including ENSO. Our calculations indicate that, averaged between the models the increase of c 1 at the equator is 0.36 m s 1, which is 13.7% relative to the preindustrial model mean of 2.62 m s 1. This suggests that the warming of the ocean in response to the plausible increase of atmospheric CO 2 may result in a nonnegligible increase in the propagation of signals along the equator, including those related to ENSO. Since these propagation speeds influence the period of ENSO, their increase may contribute to a tendency for a decreased ENSO period under increased atmospheric CO 2 in these same models, described by Merryfield (2006). The zonally averaged first baroclinic Rossby radii of deformation, corresponding to the preindustrial experiment, are shown in Fig. 2 (top). The inverse dependence of 1 on planetary vorticity away from the equator, implied by Eq. (1), clearly dominates its latitudinal structure. Near the equator 1 (0.5 c 1 / ) 1/2, where df/dy (Gill 1982), which gives values km (not shown). In agreement with observational estimates (Chelton et al. 1998), the simulated Rossby radii decrease from more than 150 km in the Tropics to just a few kilometers in polar regions. All models show a local maximum of 1 in the Arctic Ocean, associated with local maximum in c 1. The changes in the baroclinic Rossby radius are

4 1 APRIL 2006 N O T E S A N D C O R R E S P O N D E N C E 1357 FIG. 1. (top) Zonally averaged phase speed of first baroclinic gravity waves corresponding to the preindustrial experiment and (bottom) its change in the 720-ppm A1B stabilization experiment. FIG. 2. (top) Zonally averaged first baroclinic Rossby radius of deformation corresponding to the preindustrial experiment and (bottom) its change in the 720-ppm A1B stabilization experiment. shown in Fig. 2 (bottom). As implied by the changes in c 1, the baroclinic Rossby radius increases, essentially at all latitudes and in all models. The most notable absolute increase of 1 is projected at low latitudes where values of 1 are large so that their relative change is small. Toward the midlatitudes, however, the relative change of 1 increases (Fig. 4, top), reaching 10% 20% at of latitude in most of the models, while some of the models show a much larger increase. In the real ocean, these are the latitudes of intense generation of mesoscale eddies due to instability of western boundary currents. These eddies play an important role in the exchange of heat and other properties between the subtropical and subpolar regions. Since a characteristic horizontal scale of the eddies appears to be linked to the baroclinic Rossby radius of deformation, the climate model results suggest that the scale of these eddies, as well as the scale of boundary currents and fronts, may increase in the warmer climate. In addition, most models show a significant increase of 1 in the Arctic Ocean (Fig. 2, bottom). This result, however, should be considered with caution, given that the Arctic Ocean is poorly resolved in these global models, as we have already noted. If this prediction were to be realized, the scale of the Arctic Ocean eddies and the width of the currents would increase accordingly. Standard linear theory (Gill 1982) predicts that the speed of long extratropical baroclinic Rossby waves is proportional to 1 2 [more recent developments in the theory, which bring the speed of the oceanic Rossby waves in closer agreement with satellite altimetric observations, are given in Killworth et al. (1997)]. The

5 1358 J O U R N A L O F C L I M A T E VOLUME 19 quantity 1 2 and its projected change according to the models are shown in Fig. 3. The results indicate that, unless fully compensated by the changes in the background oceanic circulation, the propagation speed of baroclinic Rossby waves is likely to increase at each latitude in a warmer climate. The relative changes projected by the models in 1 2 are shown in Fig. 4 (bottom). Although the spread between the models is large, most of the models predict an increase of 1 2 from about 20% in the subtropics to 30% 40% at the midlatitudes, although again some of the models show a much larger increase. Thus, according to these climate models, the time FIG. 4. (top) Relative changes in zonally averaged first baroclinic Rossby radius of deformation and (bottom) its squared values. FIG. 3. (top) Zonally averaged 1 2 corresponding to the preindustrial experiment and (bottom) its change in the 720-ppm A1B stabilization experiment. Notice that some of the models show absolute changes in 1 2 that differ from what might be expected based on the corresponding changes in 1, since , where the overbar denotes averaging. scale for large-scale dynamical oceanic adjustment is likely to decrease in the warmer climate. Furthermore, the frequency of decadal climate variability, where the oceanic Rossby wave dynamics play an important role (e.g., Latif and Barnett 1996) may increase. For example, using a simple coupled model of decadal oscillations, such as the one proposed in Münnich et al. (1998), it is straightforward to demonstrate that the 0.3 ms 1 increase of the baroclinic gravity wave speed results in the increase of the dominant decadal frequency by a factor of , depending on other parameters in that model. Finally, to give an idea about the geographical differences and the evolution in time, Fig. 5 shows the changes in the gravity wave speed in the Pacific Ocean and in the Atlantic Ocean by the end of the twenty-first century and twenty-second century. First, it is clear that

6 1 APRIL 2006 N O T E S A N D C O R R E S P O N D E N C E 1359 FIG. 5. Zonally averaged changes in phase speed of first baroclinic gravity waves (top) in the Pacific Ocean and (bottom) in the Atlantic Ocean. The changes correspond to two time intervals, and , relative to the preindustrial control experiment. The computations are done using the results from the GFDL CM2.0 model, where the vertical structure in the ocean is resolved by the largest number of levels (50) and where the changes in the wave speed are representative of the model-mean response (see Fig. 1, bottom). the changes may notably differ between the two oceans. Second, the oceanic stratification, as represented here by the speed of baroclinic gravity waves, and hence the baroclinic Rossby radius of deformation, will continue to increase significantly after the concentration of CO 2 in the atmosphere is held fixed, which is also expected. 4. Discussion and conclusions We have used results from eight ocean atmosphere general circulation models to evaluate the influence of the projected changes in the oceanic stratification on the first baroclinic Rossby radius of deformation in the ocean, associated with atmospheric CO 2 increase. For each of the models, an oceanic state corresponding to the A1B stabilization experiment (with final specified atmospheric CO 2 concentration of 720 ppm) is compared to a state corresponding to the preindustrial control experiment (with atmospheric CO 2 concentration of 280 ppm). In all the models, the first baroclinic Rossby radius increases with increasing oceanic stratification in the warmer climate. There is, however, a considerable range among the models in the magnitude of the increase. At the latitudes of intense eddy activity associated with instability of western boundary currents (around latitude), the increase reaches 4 km on average, or about 15% of the local baroclinic Rossby radius. Some of the models predict an increase of the baroclinic Rossby radius by more than 20% at these latitudes under the applied forcing. It is therefore suggested that in a plausible future warmer climate, the characteristic scale of mesoscale eddies, as well as boundary currents and fronts, may increase accordingly. In addition, since the speed of long baroclinic Rossby waves is proportional to the squared baroclinic Rossby radius of deformation, the results suggest that the time scale for large-scale dynamical oceanic adjustment may decrease in the warmer climate. Relative to the preindustrial climate, the squared baroclinic Rossby radius increases from about 20% in the subtropics to 30% 40% at the midlatitudes in most of the models. This, in addition to a number of oceanic implications, has a potential to affect the time scale of atmospheric extratropical variability and, in particular, the decadal climate variability where propagation of long Rossby waves from east to west across the ocean sets the dominant frequency. Finally, the speed of equatorial Kelvin waves and Rossby waves, carrying signals along the equator, including those related to ENSO, is projected to increase. Acknowledgments. I acknowledge the international modeling groups for providing their data for analysis, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for collecting and archiving the model data, the JSC/CLIVAR Working Group on Coupled Modelling (WGCM) and their Coupled Model Intercomparison Project (CMIP) and Climate Simulation Panel for organizing the model data analysis activity, and the IPCC WG1 TSU for technical support. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by the Office of Science, U.S. Department of Energy. I am grateful to Bill Merryfield, George Boer, and Ken Denman for useful discussion and comments.

7 1360 J O U R N A L O F C L I M A T E VOLUME 19 REFERENCES Chelton, D. B., R. A. deszoeke, M. G. Schlax, K. E. I. Naggar, and N. Siwertz, 1998: Geographical variability of the firstbaroclinic Rossby radius of deformation. J. Phys. Oceanogr., 28, Gill, A. E., 1982: Atmosphere-Ocean Dynamics. Academic Press, 662 pp. Killworth, P. D., D. B. Chelton, and R. A. De Szoeke, 1997: The speed of observed and theoretical long extratropical planetary waves. J. Phys. Oceanogr., 27, Latif, M., and T. P. Barnett, 1996: Decadal climate variability over the North Pacific and North America: Dynamics and predictability. J. Climate, 9, Merryfield, W. J., 2006: Changes to ENSO under CO 2 doubling in a multimodel ensemble. J. Climate, in press. Münnich, M., M. Latif, S. Venzke, and E. Maier-Reimer, 1998: Decadal oscillations in a simple coupled model. J. Climate, 11,