Qinyu Liu

A gap in the Indo-Pacific warm pool over the South China Sea in boreal winter: Seasonal development and interannual variability

[1] The Indo-Pacific warm water pool in boreal winter shows a conspicuous gap over the South China Sea (SCS) where sea surface temperature (SST) is considerably lower than over the oceans both to the west and east. The formation mechanisms for the climatology and interannual variability of SCS SST in boreal winter are investigated using a suite of new satellite measurements. The winter SCS is divided into two parts by the axis of the maximum northeasterly monsoonal winds. The positive wind curl in the southeastern half of the ocean drives a cyclonic gyre circulation in the deep basin. As its western boundary current, an intense southward flow is found south of Vietnam on the continental slope separating the Sunda Shelf to the west and the deep SCS basin to the east. This slope current exceeds 0.5 m s(-1) in speed and advects cold water from the north. This cold advection results in a distinct cold tongue in the winter SST climatology. Both the slope current and the cold tongue are strongest in November to February. This winter cold tongue displays considerable interannual variability that is highly correlated with eastern equatorial Pacific SST. In an El Nino the winter monsoon weakens, causing the SCS ocean circulation to spin down. The reduced western boundary current and its thermal advection result in a warming in the SCS winter cold tongue. Both SST variance and its correlation with the El Nino-Southern Oscillation peak along the climatological cold tongue indicate that ocean dynamics are an important player in SCS climate variability.

Linking Observations of the Asian Monsoon to the Indian Ocean SST: Possible Roles of Indian Ocean Basin Mode and Dipole Mode

Abstract The authors investigate the relationship between sea surface temperature (SST) in the tropical Indian Ocean (TIO) and the seasonal atmosphere circulation in the Asian monsoon region (AMR) using the maximum covariance analyses (MCAs). The results show that the Asian monsoon circulation is significantly correlated with two dominant SST anomaly (SSTA) modes: the Indian Ocean Basin mode (IOB) and the Indian Ocean dipole mode (IOD). The peak SSTA of the IOB appears in spring and has a much stronger relationship with the Asian summer monsoon than the peak of the IOD does, whereas the peak SSTA for the IOD appears in fall and shows a stronger link to the Asian winter monsoon than to the Asian summer monsoon. In addition, the IOB in spring has a relatively stronger link with the atmospheric circulation in summer than in other seasons. The large-scale atmospheric circulation and SSTA patterns of the covariability of the first two dominant MCA modes are described. For the first MCA mode, a warm IOB, persis...

Impact of Heating Anomalies Associated with Rainfall Variations over the Indo-Western Pacific on Asian Atmospheric Circulation in Winter

Observational data show that the dominant mode of the boreal winter rainfall anomalies in the tropical Indo-Western Pacific (IWP) is a west-east dipolar pattern, which is called the Indo-Western Pacific Dipole (IWPD) mode and is related to El Niño-Southern Oscillation. It is found that corresponded to the IWPD mode is a new atmospheric teleconnection pattern—a wave train pattern emitted from the IWP toward Asia and the northwest Pacific in winter. During the positive (negative) phase of the IWPD, the teleconnection pattern features the negative (positive) anomalies of 200-hPa geopotential height (H200) centered at 30°N, 110°E and the positive (negative) anomalies of H200 centered at 45°N, 140°E. The teleconnection pattern represents the dominant mode of the boreal winter H200 anomaly over Asia. A series of simple atmospheric model experiments are performed to confirm that this winter teleconnection pattern is induced by the heating anomalies associated with the IWPD, and the heating anomalies over the equatorial central Pacific are not important to this teleconnection pattern from the IWP toward Asia and the northeast Pacific. The IWPD is strengthened after the climate regime shift of the 1970s, which leads to a stronger teleconnection pattern.

Evidence of a Barrier Layer in the Sulu and Celebes Seas

Variability in the surface isothermal and mixed layers of the Sulu and Celebes Seas is examined using the conductivity‐temperature‐depth data from the Navy’s Master Oceanographic Observational Data Set (MOODS). Vertical gradient is calculated to determine isothermal layer depth with a criterion of 0.05 8 Cm 21 for temperature profiles and mixed layer depth with a criterion of 0.015 kg m24 for density profiles. When the isothermal layer depth is larger than the mixed layer depth, the barrier layer occurs. This study shows that the barrier layer occurs often in the Sulu and Celebes Seas. In the Sulu Sea, the barrier layer has seasonal variability with a minimum occurrence (38%) and a minimum thickness (3 m) in May and a maximum occurrence (94%) and a maximum thickness (36.5 m) in September. In the Celebes Sea, the barrier layer thickness changes from a maximum (49.7‐ 62.0 m) in March‐April to a minimum (9.6 m) in June. Possible mechanisms responsible for the barrier layer formation are discussed. In the Sulu Sea, the barrier layer may be formed by both rainfall and stratification; in the Celebes Sea, a rain-formed mechanism seems a major factor.

Sea surface height oscillation with quasi-four-month period along the continental slope in the northern South China Sea*

The sea surface height oscillation with a quasi-four-month period (SSHO4) along continental slope in the northern South China Sea (NSCS) is detected using satellite altimeter data and an ocean model simulation. The SSHO4 is at southwest of Dongsha Island, and is characterized by a wavelength of ∼600 km and a southwestward phase speed of ∼0.1 m/s. Crossing the climatological background SST front, geostrophic currents corresponding to the SSHO4 generally induce sea surface temperature (SST) “tongues” during January–March. The cold and warm SST tongues appear southwest of cyclonic and anticyclonic eddies, respectively. The distance between the warm and cold SST tongues is about half the wavelength of the SSHO4. The geostrophic currents play an important role in lateral mixing, as manifested by the SST tongue phenomena in the NSCS.

Decadal Variation of the Geostrophic Vorticity West of the Luzon Strait

Merged satellite altimeter data (from 1993 to 2008) are used to study the decadal variation of geostrophic vorticity west of the Luzon Strait. The decadal variation of the geostrophic vorticity west of the Luzon Strait indicates that the Kuroshio intrusion and separated anticyclonic eddies from the Kuroshio are stronger in 1995-2000 than that in 2001- 2004. Between 1995 and 2000, along with the intensification of the anticyclone separated from the Kuroshio intrusion west of the Luzon Strait, the cyclone south of the Kuroshio intrusion is strengthened. This result proved the close relationship between eddy shedding and the growth of the cyclone south of the Kuroshio front. In 1995-2000, when the anticyclonic vorticity west of the Luzon Strait was increased, at the same time, the decrease thermocline thickness and the Kuroshio transport east of Luzon Island are found by analyzing the Simple Ocean Data Assimilation datasets. This suggests that the decadal change in the Kuroshio intrusion and eddy shedding in the Luzon strait is influenced by the decadal change in thermocline thickness and Kuroshio transport east of the Luzon Island. The South China Sea (SCS) is a semi-enclosed, marginal basin of the western Pacific. The Luzon Strait, located from 18.5°N to 22°N with widths around 400km and depths over 2000 m, is the only deep channel connecting the SCS with the Pacific. The Kuroshio, as one of the western boundary currents of the North Pacific, deforms while crossing the Luzon Strait (1). Anticyclonic eddies are found to be separated intermittently from the Kuroshio deformation in the Luzon Strait (2-4). The eddy shedding phenomenon is reported as an event of non-deterministic nature and with large variety in frequency and behavior (4-7), which indicates variation of time scale longer than interannual may exist in these phenomena. Furthermore, these studies are mainly limited in the description of the Sea Surface Height (SSH) and the geostrophic velocity, in which the strength or the Geostrophic Vorticity (GV) of the anticyclonic eddies left unknown. The GV anomaly is the anomaly vorticity of the geostrophic circulation calculated from the SSH anomaly data. It is very important in the study of potential vorticity interaction between the SCS and the Pacific. Moreover, it is suggested that the separation of the anticyclonic eddy from the Kuroshio intrusion is related with the cyclone generated by frontal instability in the south the Kuroshio intrusion (8), while the existence of the cyclone has not been proved by observation. By studying the GV anomaly west of the Luzon Strait, the relationship between the anticyclone from the Kuroshio intrusion and the cyclone from the south of the Kuroshio front will be discussed.

Sea surface wind and cold tongue over the winter South China Sea

Taking advantage of new satellite observations, we have investigated the spatial distributions and interannual variations in sea surface wind (SSW) and sea surface temperature (SST) in boreal winter over the South China Sea (SCS) during 2000~2004. The northeast monsoon prevails in the winter SCS, because orographic forcing by mountains on Taiwan and Luzon, the Ekman pumping in the southeastern half of the basin and the two dipole of SSW curls form with relatively weak winds in southwest of Taiwan and Luzon Islands. This SSW spins up a cyclonic basin scale Gyre over the SCS and three or two eddies with sub-basin scale, with a southward western boundary current (WBC) on the coast of South Vietnam and the continental slope east of the Sunda Shelf. This southward WBC forms a distinct cold tongue along the Sunda Slope. When positive SSW curl is stronger, so is the cold tongue in 2000 and 2004. The opposite is true in 2001 and 2003.