An asymmetric upwind flow, Yellow Sea Warm Current: 2. Arrested topographic waves in response to the northwesterly wind

Abstract

[1] A warm and salty water mass exists along the Yellow Sea Trough (YST) in winter. This oceanic water mass is distinct from the ambient shelf water and is distributed on the western side of the YST. It has long been reasoned that a Yellow Sea Warm Current (YSWC) must exist. A recent observational study indeed supports the existence of the YSWC and shows that its position moved progressively westward as the warm water intrudes further shoreward toward the northwest. In this paper, we explain mechanisms for sustaining the YSWC and for its westward displacement. The northwesterly monsoonal wind prevails in the winter and is directed against the YSWC. The crosstrough scale is small compared with the spatial scale of monsoonal variation, so one can assume, to the first order, that the wind stress is uniform across the trough. The curl of depth-averaged wind stress has opposite signs on the two sides of the trough. Consequently, two oppositely rotating gyres develop initially and they converge along the trough giving rise to a barotropic upwind flow. But this upwind flow lasts only for a few days as the two gyres evolve and propagate as topographic waves. For a northerly wind, both gyres move westward since the positive (negative) potential vorticity flux on the western (eastern) side of the trough pushes the water toward shore (trough). If the bottom friction is negligible, the steady response becomes a large anticyclonic gyre over the trough and the upwind current is squeezed toward the shore line. In this case, no YSWC is sustained along or near the trough. This runaway warm current can be arrested by a moderate bottom friction. We therefore propose that the YSWC is actually arrested topographic waves in response to local wind stress forcing.Citation: Lin, X., and J. Yang (2011), An asymmetric upwind flow, Yellow Sea Warm Current: 2. Arrested topographic waves in response to the northwesterly wind,

Evolution of wind‐driven flows in the Yellow Sea during winter

“…A deep trough with a maximum depth of more than 80 m is located in the central area of the YS. In winter, the warm and salty Yellow Sea Warm Current (YSWC) flows into the YS along the west flank of the trough [ Lin et al ., ; Lin and Yang , ; Lü et al ., ]. It is widely recognized that the YSWC is an upwind flow driven by the northwesterly wind in winter [ Park , ; Hsueh , ; Lin and Yang , ].…”

“…The downwind flow at the surface and the upwind flow at the bottom driven by northwesterly wind in the YS have been widely studied [ Park , ; Hsueh and Pang , ; Hsueh and Yuan , ; Isobe , ; Lin and Yang , ]. During winter, the northwesterly wind drags water from north to south in the upper part of the YS so that downwind flows occur along the Korean and Chinese coasts.…”

“…During winter, the northwesterly wind drags water from north to south in the upper part of the YS so that downwind flows occur along the Korean and Chinese coasts. These flows, known as the Korean Coastal Current (KCC) and Chinese Coastal Current (CCC), respectively (Figure a), cause a pressure gradient to the north that generates an upwind flow in the central deep region [ Park , ; Lin and Yang , ].…”

“…Although some observations in the YS show significant upwind flows with 1 or 2 day lags to the northerly wind [ Hsueh , ; Teague and Jacobs , ; Lin et al ., ], it is not easy to quantify the relationship between the northwesterly wind and the responding flows owing to limited observations in time and space. Numerical experiments could be used to determine the connection between the wind and the flows [ Takahashi et al ., ; Lin and Yang , ].…”

Evidences of Intermittent Wind-Induced Flow in the Yellow Sea obtained from AVHRR SST Data

“…If the net total transport along the basin vanishes, the downwind flow should occur in shallow regions and the upwind flow, in deep regions (Csanady 1982;Park 1986). It should be mentioned, however, that the Cheju Warm Current water can also move westward across the trough, on which some mechanisms are proposed (Huang et al 2005;Lie et al 2009;Lin and Yang 2011). The observational evidence of the intermittent upwind or downwind flow provided by Hsueh (1988) is only for a particular winter.…”