JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, C03026, doi:10.1029/2008JC005109, 2009 Click Here for Full Article Seasonal upper circulation in the Sulu Sea from satellite altimetry data and a numerical model Shuqun Cai,1 Yinghui He,1,2 Shengan Wang,1 and Xiaomin Long1 Received 4 September 2008; revised 8 January 2009; accepted 27 January 2009; published 28 March 2009. [1] Eight years of Absolute Dynamic Topography from satellite altimetry data are used to study the seasonal variability of the circulation in the Sulu Sea (SS) through Empirical Orthogonal Function (EOF) analysis. It is revealed that first seasonal EOF mode shows a basin-scale anticyclonic/cyclonic circulation in summer/winter and second seasonal EOF mode shows a weak basin-scale anticyclonic/cyclonic meander flow from the Mindoro Strait to the Sibutu Passage, and the typical surface circulation structure in the SS is shown as a basin-scale anticyclonic/cyclonic circulation (or meander) centered at about 120.8°E, 8.6°N in August and December. According to the numerical experiments by a connected single-layer and two-layer model, it is shown that the upper circulation in the SS is closely related to the outflow via the Sibutu Passage and seasonal local wind stress. Either an outflow via the Sibutu Passage or the summer monsoon may cause an anticyclonic circulation in the SS, while the winter monsoon may cause a cyclonic circulation. Either an outflow via the Sibutu Passage or the winter monsoon would push the water out of the SS via the Balabac Strait but bring the water into the SS via the Mindoro Strait, while the summer monsoon would bring the water into the SS via the Balabac Strait but push the water out of the SS via the Mindoro Strait. Thus, in summer, the induced anticyclonic circulation with the negative relative vorticity is stronger in the SS but the water transports via the Mindoro and the Balabac straits are less, while in winter, the induced cyclonic circulation with the mainly positive relative vorticity is weaker but the water transports via the Mindoro and the Balabac straits are larger. The inflow via the Mindoro Strait is also significant since the outflow via the Sibutu Passage is mainly compensated by the inflow via the Mindoro Strait. The western strengthening near the Palawan Island and the asymmetry of the circulation in the SS is caused by the b effect. The transport via the Mindoro Strait is generally much larger than that via the Balabac Strait. An inflow into the SS via the Dipolog Strait contributes little to the circulation in the SS except for the current field near this strait. A stronger lower layer current than the upper layer one near the Mindoro Strait is also discussed. Citation: Cai, S., Y. He, S. Wang, and X. Long (2009), Seasonal upper circulation in the Sulu Sea from satellite altimetry data and a numerical model, J. Geophys. Res., 114, C03026, doi:10.1029/2008JC005109. 1. Introduction 30 m but the deepest depth about 960 m appears in the middle of the strait. The Dipolog Strait is very narrow, but its deepest [2] The Sulu Sea (SS) is almost a Mediterranean sea with depth is larger than 1000 m, it is connected with the Pacific via the greatest depth exceeding 5000 m in its southeast (Figure 1). the Surigao Strait. The Sibutu Passage is wide and deep with its It mainly connects with the South China Sea (SCS) through the deepest depth larger than 2000 m in the middle of the strait. Mindoro and the Balabac straits and with the Sulawesi Sea [3] Maybe the SS is too small to attract the oceanogra- through the Sibutu Passage. The Balabac Strait is narrow and phers’ attention; few studies on the circulation of the SS are shallow, across Section B (Figure 1a, 117°200E, 7° 8°200N), reported. According to the current charts by Wyrtki [1961], the sill depths are only about 10 m. The Mindoro Strait is also the surface circulation in the SS is mainly anticyclonic in narrow but much deeper, across Section A (120°50 121°50E, summer. Some regional model studies showed that the 12°150N),thesilldepthsatthewesternmostareshallowerthan annual mean upper circulation in the SS is cyclonic since the winter NE monsoon is dominant [Metzger and Hurlburt, 1LED, South China Sea Institute of Oceanology, Chinese Academy of 1996], and the southward transport flowing out of the SS via Sciences, Guangzhou, China. the Sibutu Passage into the Sulawesi Sea was more than 2 Graduate School of the Chinese Academy of Sciences, Beijing, China. 2.5Sv(1Sv=1Â 106 m3 sÀ1), but Wajsowicz [1999] suggested that the transport via the Sibutu Passage was only Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JC005109$09.00 0.7 Sv. The global ocean circulation model study with 1/6° C03026 1of14 C03026 CAI ET AL.: CIRCULATION IN THE SULU SEA C03026 (a) (b) Figure 1. (a) Computational domain (including the SCS, SS, and part of the Pacific) surrounded by thick solid lines and coastal lines (double-dashed lines stand for open boundaries) and (b) topography of the Sulu Sea (unit is m). resolution by Fang et al. [2003] suggested that the annual the SS in depth. In the following, data and EOF analysis are mean transports from the SCS into the SS via the Mindoro given in section 2, the model description and choice of and the Balabac straits are 0.25 Sv and 1.35 Sv, respectively. parameters are given in section 3, in section 4, the numerical On the basis of the 900-year integration of a global ocean experimental results and discussions are presented, and circulation model with 1° coarse resolution driven by section 5 is the conclusion. ECMWF reanalysis wind data, Cai et al. [2005] studied the water exchange between the South China Sea and its 2. Data and Seasonal Variability of EOF Analysis adjacent oceans, and the simulated current charts suggested that the surface circulation of the SS is basically cyclonic in [5] The present study is based on the high-resolution winter. According to the preliminary numerical experiments (0.25°) Delayed Time Maps of ADT data with up to four by a connected single-layer and two-layer model with a satellites (Topex/Poseidon, Jason-1+ERS, Envisat and resolution of 100, Cai et al. [2008] found that because of the GFO) at a given time from the Web site http://www.aviso. local monsoon stress curl, the upper circulation in the SS is oceanobs.com. The data with a time step of 7 days from dominated by a weak anticyclonic eddy in summer and a June 2000 to May 2007 for the SS are extracted, then the strong cyclonic eddy in winter. However, no further inves- climatologic monthly means from January to December are tigation of the vorticity dynamics on the SS circulation has calculated for the seasonal variability of EOF analysis. The been carried out, e.g., what about the contributions of the data at the water depth shallower than 50 m and near the water exchange via the Sibutu Passage, Dipolog Strait and coast in the SS are excluded in the analysis. Finally, there Balabac Strait to the vorticities of the SS circulation? What are 344 effective grid points data used for the EOF analysis. about the contribution of the monsoon wind stress to the [6] Figure 2 shows the first two EOF modes for the seasonal upper circulation in the SS? Can we estimate the basic variability of the surface circulation in the SS. First EOF mode distributions of the transports via the Mindoro and the accounts for 71.46% of the total variance, it shows the basin- Balabac straits? It is necessary to employ a numerical model scale anticyclonic/cyclonic circulation centered at 120.6°E, to study these problems. Meanwhile, no satellite altimetry 8.4°N, and the time series has a minimum in August data are used to testify their numerical results. corresponding to the summer anticyclonic circulation and an [4] Satellite altimetry, which provides high time-space opposite maximum in December–January corresponding to the coverage unavailable from in situ data, has been widely winter cyclonic circulation in the basin. Second EOF mode employed to study the oceanic circulation variability. The accounts for 21.3% of the total variance, it shows a weak basin- major objective is to present the basic characteristics of the scale anticyclonic/cyclonic meander from the Mindoro Strait to seasonal mesoscale circulation in the SS by Empirical the Sibutu Passage, and the time series has a minimum in April Orthogonal Function (EOF) analysis using 8 years of corresponding to the anticyclonic meander flow from the Sibutu Absolute Dynamic Topography (ADT) from satellite altim- Passage to the Mindoro Strait and an opposite maximum in etry data, then, a connected single-layer and two-layer November corresponding to the cyclonic meander flow in an model used by Cai et al. [2008] is again employed to study opposite direction. To facilitate easy comparison with the the vorticity dynamics of the seasonal upper circulation in following numerical results, Figure 3 shows the combined first 2of14 C03026 CAI ET AL.: CIRCULATION IN THE SULU SEA C03026 Figure 2. Seasonal EOF modes of the Sulu Sea ADT variability: (a) spatial structure of first EOF, (b) time series of first EOF (unit is cm), (c) spatial structure of second EOF (unit is cm), and (d) time series of second EOF. EOF and second EOF modes in August and December. It is Figure 4 shows that the SS is controlled by the southwest shown that there is a basin-scale anticyclonic circulation cen- monsoon with the mainly negative wind stress curl in August tered at about 120.8°E, 8.6°N in August and a strong cyclonic while by the northeast monsoon with the mainly positive meander also centered at about the same site in December.