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The Role of Oceanic Heat Advection in the Evolution of Tropical North and South Atlantic SST Anomalies*

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The Role of Oceanic Heat Advection in the Evolution of Tropical North and South Atlantic SST Anomalies* 6122 JOURNAL OF CLIMATE VOLUME 19 The Role of Oceanic Heat Advection in the Evolution of Tropical North and South Atlantic SST Anomalies* GREGORY R. FOLTZ AND MICHAEL J. MCPHADEN NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington (Manuscript received 21 November 2005, in final form 7 April 2006) ABSTRACT The role of horizontal oceanic heat advection in the generation of tropical North and South Atlantic sea surface temperature (SST) anomalies is investigated through an analysis of the oceanic mixed layer heat balance. It is found that SST anomalies poleward of 10° are driven primarily by a combination of wind- induced latent heat loss and shortwave radiation. Away from the eastern boundary, horizontal advection damps surface flux–forced SST anomalies due to a combination of mean meridional Ekman currents acting on anomalous meridional SST gradients, and anomalous meridional currents acting on the mean meridional SST gradient. Horizontal advection is likely to have the most significant effect on the interhemispheric SST gradient mode through its impact in the 10°–20° latitude bands of each hemisphere, where the variability in advection is strongest and its negative correlation with the surface heat flux is highest. In addition to the damping effect of horizontal advection in these latitude bands, evidence for coupled wind–SST feedbacks is found, with anomalous equatorward (poleward) SST gradients contributing to enhanced (reduced) west- ward surface winds and an equatorward propagation of SST anomalies. 1. Introduction ated with anomalous cross-equatorial surface winds and a meridional displacement of the ITCZ. An anomalous In contrast to the ENSO-dominated SST variability northward SST gradient is associated with anomalous in the tropical Pacific, SST variability in the tropical northward cross-equatorial winds, a northward dis- Atlantic on seasonal-to-decadal time scales involves placement of the ITCZ, reduced rainfall in Northeast two distinct modes. The first mode is similar to ENSO Brazil, and enhanced rainfall in sub-Saharan Africa and is associated with anomalous SST in the eastern (Lamb 1978; Hastenrath and Greischar 1993; Nobre equatorial Atlantic, a weakening of the trade winds in and Shukla 1996). the central basin, and a southward shift of convection in Previous studies have suggested that oscillations of the intertropical convergence zone (ITCZ; Carton and the gradient mode are caused by a combination of di- Huang 1994; Ruiz-Barradas et al. 2000). The second rect atmospheric forcing, coupled wind–evaporation– mode, which does not have a strong counterpart in the SST feedback, and horizontal oceanic heat advection tropical Pacific, is characterized by an anomalous me- (Carton et al. 1996; Chang et al. 2001; Czaja et al. 2002). ridional SST gradient centered near the equator, with The role of oceanic heat advection in the development anomalous SST signals extending into the subtropical of off-equatorial SST anomalies remains uncertain de- North and South Atlantic (Servain 1991; Nobre and spite having been the focus of several recent modeling Shukla 1996; Huang et al. 2004). This mode is associ- studies. Xie (1999) used a simple coupled model with a slab oceanic mixed layer to predict that meridional Ek- man advection will damp off-equatorial SST anomalies * Pacific Marine Environmental Laboratory Contribution in the tropical Atlantic. Chang et al. (2001) hypoth- Number 2914. esized that oscillations of the anomalous interhemi- spheric SST gradient in the tropical Atlantic are driven by an imbalance between positive ocean–atmosphere Corresponding author address: Gregory R. Foltz, NOAA/ Pacific Marine Environmental Laboratory, 7600 Sand Point Way feedback and negative oceanic heat advection feedback NE, Seattle, WA 98115. in the tropical North Atlantic, with surface fluxes lead- E-mail: [email protected] ing horizontal advection by ϳ2 yr. In contrast, Seager et © 2006 American Meteorological Society JCLI3961 1DECEMBER 2006 F O L T Z A N D MCPHADEN 6123 al. (2001) found no evidence of a significant phase lag olds et al. (2002) SST together with the monthly mean between surface fluxes and horizontal advection, sug- climatological mixed layer depths from de Boyer gesting that the role of oceanic heat advection is simply Montégut et al. (2004), available on a 2° ϫ 2° grid, and to damp the surface flux–forced SST anomalies. They from the World Ocean Database, available on a 1° ϫ 1° also showed that horizontal heat advection is strongest grid (Monterey and Levitus 1997). These SST and within 10° of the equator, where the mean Ekman cur- mixed layer depth datasets are also used in conjunction rents are the strongest. Both Chang et al. (2001) and with NCEP2 reanalysis wind velocity to estimate hori- Seager et al. (2001) found that the dominant compo- zontal mixed layer Ekman heat advection. nent of the heat advection term is the mean meridional To estimate geostrophic currents, we have obtained currents acting on anomalous SST gradients. Joyce et ERS-1/2/TOPEX/Poseidon/Jason altimeter sea level al. (2004) showed that anomalous wind stress curl as- anomaly data for the time period 1992–2002 from the sociated with the gradient mode forces anomalous Collect Localisation Satellites (CLS) Space Oceanog- cross-equatorial oceanic heat transport with a lag of a raphy Division. The anomalies are referenced to the few months, damping the existing anomalous SST gra- 1993–99 mean and are mapped according to the meth- dient. Their results suggest that the action of anoma- odology of Ducet et al. (2000). The data are available lous wind-forced meridional currents on the mean me- every 7 days on a 1⁄3° ϫ 1⁄3° grid. We have added the ridional SST gradient may play an important role in the 1993–99 mean dynamic topography (Rio and Hernan- evolution of the gradient mode. dez 2004) to the sea level anomaly data in order to In this study we use a combination of satellite and estimate the total geostrophic surface currents. We atmospheric reanalysis data to examine the role of hori- have converted each of the aforementioned datasets zontal oceanic heat advection in the evolution of SST from its original resolution to a 2° ϫ 2° ϫ monthly grid anomalies in the tropical North and South Atlantic. We for consistent analysis across all fields. focus on the region poleward of 5° of latitude, where currents can be estimated diagnostically from available 3. Methodology surface wind and sea level datasets. To examine the role of horizontal advection in the 2. Data evolution of tropical Atlantic SST anomalies, we con- Surface latent and sensible heat fluxes are obtained sider the mixed layer heat balance, which can be written from a combination of satellite and reanalysis products ѨT ⌬ Ϫ ١ ona1° ϫ 1° grid for the time period 1981–2002 (Yu et ϭϪ h Ѩ hv · T H Twe al. 2004). The reanalysis products consist of the Na- t tional Centers for Environmental Prediction–Depart- 0 q Ϫ q ͒ ͑ ϩ 0 Ϫh ˆ ١ Ϫ ment of Energy (NCEP–DOE) reanalysis 2 (hereafter · ͵ vˆT dz ␳ , 1 Ϫ cp NCEP2 reanalysis; Kanamitsu et al. 2002) and the Eu- h ropean Centre for Medium-Range Weather Forecasts following Moisan and Niiler (1998). The terms repre- (ECMWF) 40-yr reanalysis (Simmons and Gibson sent, from left to right, local storage, horizontal advec- 2000). Surface turbulent fluxes were computed from tion, entrainment, vertical temperature/velocity covari- these products using daily mean wind speed, SST, and ance, net surface heat flux adjusted for the penetration humidity in the bulk flux algorithm developed from the of light below the mixed layer (q0), and vertical turbu- Coupled Ocean–Atmosphere Response Experiment lent diffusion at the base of the mixed layer (qϪh). Here (COARE; Fairall et al. 2003) and are an improvement h is the depth of the mixed layer; T and ␷ are tempera- over the turbulent fluxes from atmospheric reanalyses ture and velocity, respectively, vertically averaged from (Yu et al. 2004). Surface shortwave radiation is ob- the surface to a depth of Ϫh; Tˆ and vˆ are deviations tained from the satellite-based dataset of Zhang et al. from the vertical average; H is the Heavyside unit func- ϫ ⌬ ϭ Ϫ (2004), which is available during 1983–2004 on a 2.5° tion, T T TϪh; and we is entrainment velocity. We 2.5° ϫ monthly grid. Surface longwave radiation is ob- neglect entrainment, the vertical temperature differ- ϫ ϫ tained from the NCEP2 reanalysis on a 2° 2° daily ence/velocity shear covariance term, and qϪh since we grid for the time period 1979–2004. We also use the Yu cannot reliably estimate them. The results of Carton et et al. (2004) wind speed and specific humidity, along al. (1996) and Foltz et al. (2003) indicate that these with weekly Reynolds et al. (2002) SST, to estimate terms are likely of minor importance in comparison to separately the wind speed and humidity contributions surface fluxes in the tropical North and South Atlantic. to the latent heat flux. For all terms in (1) we form anomalies by removing To estimate mixed layer heat storage, we use Reyn- the monthly mean seasonal cycle and then smooth with 6124 JOURNAL OF CLIMATE VOLUME 19 a 5-month running mean to emphasize variability on they act to heat the mixed layer. LHF and SHF are interannual and longer time scales. obtained from the Yu et al. (2004) dataset. SWR is The mixed layer depth, h, which contributes to the obtained from the Zhang et al. (2004) dataset, and storage and advection terms in (1), is affected by the LWR is obtained from the NCEP2 reanalysis. Follow- vertical distributions of both temperature and salinity. ing Wang and McPhaden (1999), we model the amount We therefore use the density-based climatological of shortwave radiation penetrating the mixed layer as ϭ Ϫ0.04h mixed layer depth estimates of de Boyer Montégut et Qpen 0.47Qsfc e , where Qsfc is the surface short- al.
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