3704 JOURNAL OF CLIMATE VOLUME 14 Coupled Ocean±Atmosphere Dynamics in a Simple Midlatitude Climate Model DAVID FERREIRA AND CLAUDE FRANKIGNOUL Laboratoire d'OceÂanographie Dynamique et de Climatologie, Unite mixte de recherche CNRS-ORSTOM-UPMC, Universite Pierre et Marie Curie, Paris, France JOHN MARSHALL Program in Atmospheres, Oceans, and Climate, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts (Manuscript received 22 February 2000, in ®nal form 16 January 2001) ABSTRACT Midlatitude air±sea interactions are investigated by coupling a stochastically forced two-layer quasigeostrophic channel atmosphere to a simple ocean model. The stochastic forcing has a large-scale standing pattern to simulate the main modes of low-frequency atmospheric variability. When the atmosphere interacts with an oceanic mixed layer via surface heat exchanges, the white noise forcing generates an approximately red noise sea surface temperature (SST) response. As the SST adjusts to the air temperature changes at low frequency, thus decreasing the heat ¯ux damping, the atmospheric spectra are slightly reddened, the power enhancement increasing with the zonal scale because of atmospheric dynamics. Decadal variability is enhanced by considering a ®rst baroclinic oceanic mode that is forced by Ekman pumping and modulates the SST by entrainment and horizontal advection. The ocean interior is bounded at its eastern edge, and a radiation condition is used in the west. Primarily in wintertime conditions, a positive feedback takes place between the atmosphere and the ocean when the atmo- spheric response to the SST is equivalent barotropic. Then, the ocean interior modulates the SST in a way that leads to a reinforcement of its forcing by the wind stress, although the heat ¯ux feedback is negative. The coupled mode propagates slowly westward with exponentially increasing amplitude, and it is fetch limited. The atmospheric and SST spectral power increase at all periods longer than 10 yr when the coupling with the ocean interior occurs by entrainment. When it occurs by advection, the power increase is primarily found at near- decadal periods, resulting in a slightly oscillatory behavior of the coupled system. Ocean dynamics thus leads to a small, but signi®cant, long-term climate predictability, up to about 6 yr in advance in the entrainment case. 1. Introduction and Deser 1995). As discussed by Deser and Blackmon There has been growing interest in understanding the (1993), the decadal SST ¯uctuations in the North At- nature of the air±sea interactions controlling extratrop- lantic are related to the wintertime atmospheric anom- ical sea surface temperature (SST) variability on cli- alies in broadly the same way as on shorter timescales, matic timescales. As reviewed by Frankignoul (1985), with stronger winds overlying cooler SSTs, while dif- the interannual SST anomalies mainly re¯ect the re- ferent relations are seen on interdecadal and longer time- sponse of the oceanic surface mixed layer to the day- scales. Halliwell and Mayer (1996) showed that in the to-day changes in the local air±sea ¯uxes, which act as westerlies local anomalous turbulent heat ¯ux is effec- a stochastic forcing. Away from frontal zones, the main tive in forcing the wintertime SST anomalies down to SST anomaly forcing is by surface heat exchanges and the decadal period, although in the western North At- vertical entrainment, and turbulent heat ¯uxes also con- lantic, the geostrophic variability strongly modulates the tribute to SST anomaly damping (Frankignoul et al. SST ¯uctuations (Halliwell 1998). The decadal signal 1998). Although the decay time of the SST anomalies may have an enhanced variance around periods of about is typically 3 months, that of the heat content in the 12±14 yr (Deser and Blackmon 1993; Sutton and Allen mixed layer is longer and wintertime SST anomalies 1997), but a dominant SST timescale does not require generally recur during the following autumn (Alexander an active ocean±atmospheric coupling; it could arise through the interplay between stochastic atmospheric forcing with a ®xed spatial pattern and oceanic advec- Corresponding author address: Claude Frankignoul, Laboratoire tion (Saravanan and McWilliams 1998), or re¯ect SST d'OceÂanographie Dynamique et de Climatologie, Unite mixte de re- cherche CNRS-ORSTOM-UPMC, Case 100, Universite Pierre et Ma- modulation by stochastically forced geostrophic ¯uc- rie Curie, 4 place Jussieu, 75252 Paris, France. tuations (Frankignoul et al. 1997; Jin 1997). E-mail: [email protected] General circulation model (GCM) experiments with q 2001 American Meteorological Society 1SEPTEMBER 2001 FERREIRA ET AL. 3705 Decadal oscillations sustained by a positive midlati- tude ocean±atmosphere feedback were invoked by Latif and Barnett (1994) to explain decadal variability in the North Paci®c in the ECHO coupled general circulation model. In their scenario, in an anomalously strong sub- tropical gyre, the Kuroshio transports more warm water northward and increases the SST in the western and central Paci®c. The atmospheric response to the SST anomaly is such that it sustains the latter via a positive heat ¯ux feedback while decreasing the wind stress curl, which after some delay spins down the subtropical gyre and eventually yields a SST anomaly of the reversed sign. GroÈtzner et al. (1998) have suggested that a similar coupling is at play in the North Atlantic. However, the midlatitude ocean is less active in ECHAM1/LSG and the heat ¯ux feedback negative (Zorita and Frankignoul 1997; Frankignoul et al. 2000), and the ocean is pri- marily passive in the Geophysical Fluid Dynamics Lab- oratory model (Delworth 1996). Since cause and effect are dif®cult to distinguish in coupled simulations, more studies are needed to understand the nature of the air± sea coupling and establish statistical signatures that could be tested with observed or coupled model data. Early basic studies of the midlatitude coupling have been reviewed by Frankignoul (1985) and Barsugli and FIG. 1. (top) NAO pattern obtained as the leading mode (47% of Battisti (1998). The latter coupled a stochastically the variance) of wintertime sea level pressure over the North Atlantic forced slab (or energy balance) atmosphere to a slab from an EOF analysis. Monthly anomalies were computed from the ocean via surface heat exchanges. The coupling allowed NCEP reanalysis for the period 1948±99 and then averaged to form for an adjustment between air temperature and SST that wintertime (Dec±Mar) anomalies. (bottom) The power spectrum (ar- bitrary units) of the ®rst principal component was computed using reduced the heat ¯ux feedback at low frequencies, en- the multitaper method. The power law in dashed line was obtained hancing the oceanic and atmospheric variability (see by a least squares ®t for periods between 2 and 20 yr. also Saravanan and McWilliams 1998). Longer time- scales are introduced in the coupled system by the ocean dynamics. Frankignoul et al. (1997, hereafter FMZ) prescribed SST anomalies generally suggest that there showed that the baroclinic response of the ocean to sto- is some atmospheric response in fall and winter, but it chastic wind stress forcing was red with a decadal dom- is model dependent. For example, in Kushnir and Held inant timescale, and Jin (1997) that the geostrophic cur- (1996), the response to a North Atlantic SST anomaly rents distorted the climatological SST gradient, leading is very weak and baroclinic, while in Palmer and Sun (for realistic SST damping time) to a very weak decadal (1985), Ferranti et al. (1994), and Peng et al. (1995), it peak under a spatially organized wind stress forcing, is stronger and equivalent barotropic, the SST anomaly and a more pronounced one with active atmospheric primarily displacing the storm track and altering the coupling. However, the coupling was based on ad hoc upper-tropospheric eddy vorticity ¯ux. Rodwell et al. assumptions for the zonally averaged quantities. A bal- (1999) showed that much of the observed low-frequency ance between meridional wind anomalies and low-level variability of the North Atlantic oscillation (NAO) can convergence, plus the assumption that the latter is pro- be reproduced by an atmospheric GCM forced by the portional to the SST perturbation, was used instead by observed changes in SST and sea ice. In addition, Czaja White et al. (1998) to study the wind stress feedback and Frankignoul (1999) showed that North Atlantic SST upon free oceanic Rossby waves. Empirical coupling anomalies have an impact on the observed atmospheric approaches have also been used, as in Weng and Neelin variability in late spring and early winter. These studies (1998) and Neelin and Weng (1999), who used the re- suggest that the ocean could lead to a small predict- sponse of atmospheric GCMs to prescribed SST to spec- ability of the wintertime NAO, consistent with the slight ify the wind stress and heat ¯ux feedback acting on SST redness of its spectrum discussed in Hurrel and van anomaly, or in Xu et al. (1998), who coupled an oceanic Loon (1997) and Wunsch (1999). The NAO redness is GCM to a statistical atmosphere derived from a coupled illustrated in Fig. 1, where the spectral slope is slightly GCM, although this procedure biases air±sea ¯uxes to- steeper than in Wunsch (1999) because the NAO is de- ward positive feedback (Frankignoul 1999). A dynam- ®ned by a large-scale pattern rather than by the tradi- ically more consistent approach, albeit simple enough tional NAO index. to be tractable analytically, was used by Goodman and 3706 JOURNAL OF CLIMATE VOLUME 14 TABLE 1. Standard value of the parameters. b 1.8 3 10211 m21 s21 f 8 3 1025 s21 21 co 22cms La 700 km 23 ra 1.3 kg m 3 23 ro 10 kg m 26 23 21 Cpo 4 3 10 kg m K Ha 10 km Ho 4500 m 23 CD 1.5 3 10 26 21 gr 1.5 3 10 s 26 21 ga 4 3 10 s 27 21 gs 1 3 10 s 28 21 ge 3 3 10 s 21 U1 18ms 21 U2 7ms k 4 3 1027 m21 FIG.