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Decadal Spinup of the South Pacific Subtropical Gyre

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Decadal Spinup of the South Pacific Subtropical Gyre 162 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37 Decadal Spinup of the South Pacific Subtropical Gyre D. ROEMMICH,J.GILSON, AND R. DAVIS Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California P. SUTTON National Institute of Water and Atmospheric Research, Wellington, New Zealand S. WIJFFELS CSIRO Marine Research, Hobart, Tasmania, Australia S. RISER School of Oceanography, University of Washington, Seattle, Washington (Manuscript received 17 May 2005, in final form 15 November 2005) ABSTRACT An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment (WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170°W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40°S, 170°W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and Argo float trajectories at 1000 m confirms the gyre spinup during the 1990s. The signals in sea surface height, dynamic height, and velocity all peaked around 2003 and subsequently began to decline. The 1990s increase in wind-driven circulation resulted from decadal intensification of wind stress curl east of New Zealand—variability associated with an increase in the atmosphere’s Southern Hemisphere annular mode. It is suggested (based on altimetric height) that mid- latitude gyres in all of the oceans have been affected by variability in the atmospheric annular modes on decadal time scales. 1. Introduction large-scale ocean circulation could be viewed only as a static or time-invariant entity because of limited data. Prior to the World Ocean Circulation Experiment This has now changed with the WOCE survey, followed (WOCE) experiment of the 1990s, there was no sys- by repeat hydrographic transects of the Climate Vari- tematic collection of global subsurface ocean data. The ability and Predictability (CLIVAR) program, sus- long-running expendable bathythermograph networks tained satellite altimetric height measurements, and the have a strong Northern Hemisphere bias and only fol- global Argo array of profiling floats. The ocean’s role in low shipping routes. Except for a few locations where climate variability can now be observed. The present repeated surveys were carried out, the pre-WOCE work was stimulated by a study of global ocean heat storage from 1993 to 2003 (Willis et al. 2004). An in- triguing finding was that the maximum in zonally inte- Corresponding author address: Dean Roemmich, Scripps Insti- tution of Oceanography, 9500 Gilman Dr., La Jolla, CA 92093- grated warming over the past decade occurred at 40°S, Ϫ2 0230. where the4Wm of warming in the upper 750 m was E-mail: [email protected] more than 4 times the global average. In the present DOI: 10.1175/JPO3004.1 © 2007 American Meteorological Society JPO3004 FEBRUARY 2007 ROEMMICH ET AL. 163 work, this spatial inhomogeneity in ocean warming is Pacific between 1991 and 1996, plus middepth velocity explained by the deepening of isopycnal surfaces that measurements from autonomous floats. A key WOCE signal the spinup of deep ocean gyres. transect along 170°W in 1996 repeated earlier occupa- The most energetic patterns of extratropical variabil- tions of that line in 1968 and 1990, and was repeated ity in the lower atmosphere are the annular modes—the again in 2001. Last, the Argo profiling float project de- Northern Hemisphere annular mode (NAM), or Arctic ployed floats over most of the South Pacific in 2004. As Oscillation, and its counterpart, the Southern Hemi- Argo matures and provides denser coverage, it be- sphere annular mode (SAM), or Antarctic Oscillation. comes a subsurface counterpart to SSH, measuring Positive values of these indices correspond to strength- time variability in dynamic height on large spatial ened subpolar westerly winds and weakened subtropi- scales. The floats also provide direct measurements of cal westerlies and, therefore, to increased wind stress velocity at 1000-m depth, continuing the WOCE time curl over the midlatitude ocean gyres. Positive trends in series. It is the combination of all of these datasets that the amplitude of both annular modes over two or more makes the present study possible. In the following we decades have been described, beginning about 1970 in will first describe the 12-yr variability in SSH and then the case of the NAM (Thompson et al. 2000) and 1979 its relation to subsurface changes in dynamic height and or earlier in the case of the SAM (Thompson et al. velocity observed in the decade between WOCE and 2000; Marshall 2003; Renwick 2004). The decadal Argo. Last, the decadal change in wind forcing is de- trends, plus large interannual variability represented in scribed. the northern and southern annular modes, have pro- vided a substantial fraction of the low-frequency wind 2. The South Pacific decadal signal in sea surface forcing for subtropical and subpolar ocean circulation height variability. In the North Atlantic, Curry and McCartney (2001) The maximum in zonally integrated ocean warming described an increase of about 30% in the Gulf Stream at 40°S was observed by Willis et al. (2004) in both and North Atlantic Current transport between 1970 subsurface temperature, 0–750 m, and in satellite SSH, and 1995, forced by increases in wind stress and buoy- which is dominated by thermosteric effects on interan- ancy fluxes. They showed a similarity between this nual time scales. There are now 12 complete years of transport variability and the North Atlantic Oscillation, high-precision altimetric height data from 1993 to 2004. which is closely related to the NAM. The North Atlan- In Fig. 1 the time mean altimetric height for each 2-yr tic has had relatively intensive hydrographic and me- interval is plotted as the difference from the initial 2-yr teorological sampling, and sustained time series ocean mean, 1993–94. These figures use the altimetric height records exist at a few locations. Similar studies of long- product by the Archiving, Validation and Interpreta- term ocean variability have not been possible else- tion of Satellite Oceanographic Data Project (AVISO; where. Ducet et al. 2000), which merges Ocean Topography Results from a coupled model were used by Hall and Experiment (TOPEX)/Poseidon and European Re- Visbeck (2002) to describe the impacts of changes in mote Sensing Satellite (ERS) series altimeter data for the SAM on the circulation of the Southern Ocean. improved spatial coverage. Their focus was on high latitudes, where strengthened Over the complete 12-yr span of the SSH dataset westerlies resulted in increased northward Ekman (Fig. 1), there is both a spatial mean increase in SSH transport, divergence of the surface transport near of a few centimeters, corresponding to the global rise Antarctica, and acceleration of the Antarctic Circum- over this period, and a much greater increase in the polar Current. At middle latitudes around 40°S they western South Pacific at 40°S. East of New Zealand noted increases in Ekman convergence, SST, and west- the increase in SSH is more than 12 cm at 170°W, di- ward flow as effects of an increased SAM. Our descrip- minishing eastward to about 5 cm at 120°W. Most of tion will correspond to this midlatitude part of their the total SSH rise in this feature occurs in the early domain, and more specifically, in the South Pacific sec- years of the record between 1993–94 and 1999–2000 tor. (Figs. 1a–c). The present study utilizes a combination of datasets. While the increase in SSH east of New Zealand is the High-precision altimetric sea surface height (SSH) data most prominent signal in the 12-yr record, there are have been collected continuously since late 1992. The other substantial interannual anomalies in the South SSH dataset provides essential space–time context for Pacific as well. A high in SSH appears at the northern interpreting sparser in situ datasets. The WOCE hydro- edge of the domain in Fig. 1 in the early years and then graphic survey included many transects in the South coalesces with the high east of New Zealand in 2001–02. 164 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37 FIG. 1. The 2-yr time means of satellite altimetric height, subtracting the initial 2-yr mean (1993–94) from each: e.g., (a) 1995–96 minus 1993–94; (e) 2003–04 minus 1993–94. Another high at about 45°S, 110°W increases in mag- 3. Spinup of the deep subtropical gyre nitude through most of the record until 2001–02 and then subsides in 2003–04. a. Argo-minus-WOCE dynamic height change In the following section it is seen that the pattern of At the ocean surface, the South Pacific subtropical SSH increase east of New Zealand is aligned more gyre circulation is centered near 17°S (Reid 1986, Fig. closely with the deep circulation of the subtropical gyre 5). The change in sea surface height from the eastern in the South Pacific than with the surface circulation. boundary to the gyre center along 17°S is about 70 cm. The decadal SSH signal is indicative of changes extend- As is characteristic of subtropical gyres, the gyre center ing from the sea surface to depths greater than 1800 m.
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