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UC Irvine UC Irvine Previously Published Works UC Irvine UC Irvine Previously Published Works Title Coupling between Wind-Driven Currents and Midlatitude Storm Tracks Permalink https://escholarship.org/uc/item/644887v9 Journal Journal of Climate, 14(6) Authors Primeau, F. Cessi, P. Publication Date 2001-03-01 DOI 10.1175/1520-0442(2001)014<1243:CBWDCA>2.0.CO;2 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California 15 MARCH 2001 PRIMEAU AND CESSI 1243 Coupling between Wind-Driven Currents and Midlatitude Storm Tracks FRANCËOIS PRIMEAU AND PAOLA CESSI Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California (Manuscript received 15 December 1999, in ®nal form 30 May 2000) ABSTRACT A model for the interaction between the midlatitude ocean gyres and the wind stress is formulated for a shallow-water, spherical hemisphere with ®nite thermocline displacement and the latitudinal dependence of the long Rossby wave speed. The oceanic currents create a temperature front at the midlatitude intergyre boundary that is strongest near the western part of the basin. The intergyre temperature front affects the atmospheric temperature gradient in the storm track region, increasing the eddy transport of heat and the surface westerlies. The delayed adjustment of the gyres to the wind stress causes the westerly maximum to migrate periodically in time with a decadal period. The behavior of the model in a spherical geometry is qualitatively similar to that in a quasigeostrophic setting except that here the coupled oscillation involves oceanic temperature anomalies that circulate around the subpolar gyre, whereas the quasigeostrophic calculations favor the subtropical gyre. Another difference is that here there is a linear relationship between the period of the coupled oscillation and the delay time for the adjustment of ocean gyres to changes in the wind stress. This result departs from the quasigeostrophic result, in which the advection timescale also in¯uences the period of the decadal oscillation. 1. Introduction vertical shear. Depending on the particular con®guration The possibility of midlatitude ocean±atmosphere in- of the climatological storm track with respect to the teractions is exciting, because it brings into play the SST-induced heating anomaly, the anomalous eddy ¯ux- ocean in a role beyond that of a passive integrator of es can reinforce or reduce the perturbation in the mean noisy atmospheric forcing. As such, it allows for the ¯ow. Because the eddy ¯uxes are mostly due to transient possibility of enhanced predictability in midlatitude eddies, this scenario emphasizes the necessity of ap- weather patterns on decadal timescales. propriately resolving or parameterizing the baroclinic Although there is some observational evidence of a processes that maintain the storm track in midlatitudes. correlation between decadal ¯uctuations in atmospheric In a previous study, Cessi (2000) has formulated a sea level pressure (SLP) and in sea surface temperatures model that captures a feedback loop between storm (SST) anomalies (e.g., Nakamura et al. 1997; Trenberth tracks and the oceanic currents in which the SST, and Hurrell 1994), the dominant mechanism for the cou- through their coupling to the atmospheric heat budget, pling has not been determined. One of the obstacles in in¯uences the baroclinic eddy activity in the atmosphere analyzing this process, besides the obvious inadequacy and consequently the surface wind stress that drives the of the observational database on decadal timescales, is wind-driven ¯ows advecting the SSTs. The model cou- that different atmospheric general circulation models ples two simple modules for the ocean and atmosphere; (AGCM) respond very differently to similarly pre- namely, Stommel's model for the ocean gyres and scribed SST anomalies [cf. Peng and Whitaker (1999) Green's (1970) parameterization for baroclinic eddies and the references therein]. The discrepancies in the for the midlatitude transport of heat and momentum in response are largely due to differences in the models' the atmosphere. The only prescribed forcing in the mod- climatologies and eddy statistics. This is a crucial prob- el is the net absorbed shortwave heat ¯ux at the top of lem if, as suggested by Peng and Whitaker (1999), the the atmosphere, and although the basic strati®cations in main effect of anomalous SST is to alter the eddy ¯uxes the atmosphere and ocean are prescribed, the momentum of heat and vorticity via small changes in the mean and heat budgets, based on conservation laws, produce a remarkably realistic climatologyÐsurface westerlies at midlatitudes, with a well-de®ned storm track forced by surface heat ¯uxes on the ¯anks of the intergyre Corresponding author address: Francois Primeau, Canadian Centre thermal front. Furthermore, the delayed adjustment of for Climate Modelling and Analysis, Meteorological Service of Can- ada, University of Victoria, P.O. Box 1700, Victoria, BC V8W 2Y2, the gyres by slowly propagating baroclinic Rossby Canada. waves produced a self-sustained oscillation of the at- E-mail: [email protected] mospheric storm track. q 2001 American Meteorological Society 1244 JOURNAL OF CLIMATE VOLUME 14 Recently, Miller et al. (1998) have shown that the In the atmosphere, we consider the zonally averaged, oceanic component of the feedback loop is in place. An vertically integrated heat and momentum balances. Thus increase in the intensity of the westerly winds across the redistribution of heat and momentum by baroclinic the midlatitudes in the late 1970s to early 1980s pro- eddies must be parameterized in terms of the zonally vided the opportunity for a case study of the oceanic averaged quantities. We adopt the parameterizations of response to a persistent change in the wind stress. Spe- Green (1970) and Stone (1972) as detailed in the fol- ci®cally, there is evidence of a spinup of the gyres with lowing. a western-intensi®ed thermocline response, together with an SST signal in the Kuroshio±Oyashio extension. a. Vertically integrated zonally averaged heat budget Encouraged by the recent observations, we revisit Cessi's model and inquire into the effects of the ide- The atmosphere is assumed to adjust instantaneously alized model con®guration. One unsatisfactory aspect to the ocean, so that the zonally and vertically integrated that can be abandoned without sacri®cing the simplicity heat budget is given by of the model, is the idealized b-plane geometry. The planetary scale of the wind-driven ocean circulation ` 1 ](cosf^yu&) C r dz suggests that a more realistic spherical geometry be pa E a cosf ]f 0 [] used. On a sphere, the westward phase speed of Rossby waves becomes a function of latitude, an effect that 5^Qio&2^Q &2s^Fao&, (1) might be important given the quasigeostrophic result where the angle brackets indicate a zonal average, and that the timescale for Rossby waves to cross the basin f is the latitude. Here, Q is the net absorbed short- sets the period of the oscillation in conjunction with the i wave heat ¯ux at the top of the atmosphere. The Q gyre advection time. In the present study, we have re- o is the outgoing longwave radiation that is parameter- formulated Cessi's (2000) model by recasting the ocean ized by linearizing the ``gray Stefan±Boltzmann law,'' module in terms of the planetary geostrophic equations. Q 5 G(u)su 4 , about a mean value, Q (in kelvins), The planetary geostrophic equations, unlike the quasi- o so that geostrophic equations can retain the full variation of the Coriolis parameter and are not restricted to small hor- Qo 5 A 1 Bus. (2) izontal variations in the thickness of the wind-driven layer. The constants A and B are prescribed, and if the at- The plan for the paper is as follows. In section 2, we mosphere were in radiative equilibrium they would de- describe the atmospheric module, comprising vertically termine the mean surface temperature, as well as the integrated, zonally averaged heat (section 2a) and mo- difference in temperature between the pole and the equa- mentum (section 2b) budgets. In section 3, we present tor. The term F is the ¯ux of heat from the atmosphere the oceanic module based on the planetary geostrophic ao equations for the momentum budget (sections 3a, 3b) into the ocean. The zonally averaged air±sea heat ¯ux and the thermodynamic budget, based on an advection± must be weighed by the fraction of a latitude circle, s, diffusion equation for the SST (section 3c). In section occupied by the ocean. Following Haney (1971), the 4, we present the climatology of the model, and in sec- ¯ux of heat through the ocean's surface is given by the tion 5, we describe the variability produced by the cou- approximation pled model that we contrast with the results obtained Fao 5 l(us 2 Ts), (3) from a quasigeostrophic b-plane formulation by Cessi (2000). In section 6, a box model is analyzed to clarify where Ts is the SST, and l is the bulk heat transfer the dependence of the oscillation's period on some of coef®cient. The Cpa is the speci®c heat of the atmosphere the parameters. Finally in section 7, we present a dis- at constant pressure, and r is the density of the atmo- cussion and summarize the results. sphere, assumed to be a function of height only. The zonally averaged heat ¯ux ^uy&, is parameterized to be down the mean gradient, that is, 2. The model atmosphere k ]^u& The strategy of the model is similar to that used in ^uy&52 . (4) a ]f Cessi (2000) except that spherical polar coordinates are used. We make the simplifying assumption that on the The form (4) has been adopted by Green (1970) as a timescales of interest, that is, much longer than a month, plausible representation of the heat ¯ux by midlatitude the atmosphere is in equilibrium with the ocean.
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