Mechanism of Water Temperature and

Circulation Variability in the Yellow Sea

Hao Wei, Chengyi Yuan, Xiaofan Luo Tianjin University of Science and Technology

Zhihua Zhang

State Oceanic Administration, China

Youyu Lu Fisheries and Oceans Canada

1

Physical Conditions in the Yellow Sea

• Water temperature & salinity variability

-- Seasonal cycle well described from observations (e.g. Atlas)

-- Long-term (> inter-annual) variations derived from scarce observations

• Circulation -- Basic circulation patterns &

seasonal variations described, e.g., wind-driven coastal currents,

influence of river runoff, Yellow Sea Warm Current (YSWC)

2

Su et al. 2005 3

Circulation Patterns in (a) Winter (b) Summer -- mostly inferred from hydrographic obs

Outstanding questions for forcing mechanism: -- Local wind & heat linked to large-scale atmosphere forcing (e.g.

East Asian Monsoon)? -- Lateral forcing by large-scale ocean processes (Kuroshio &

branches)? -- Role of YSWC in hydrographic variability? Approach: -- Large-scale model used for regional study -- Validation with observations -- Heat budget analysis -- Model sensitivity study

4

This study: focus on Yellow Sea

Global model: horizontal resolution 1° lat/lon ; 46 vertical levels; simulates large-scale, low-frequency variation; does not resolve meso-scale eddies, fronts, shelf processes

Northwest Pacific model: horizontal resolution 1/4° lat/lon; resolves large-scale shelf processes & only permits eddies

A 2-Way Nested Model Based on NEMO -- to study large-scale ocean influence on regional seas;

hindcast 1948-2007 with CORE forcing

5

Seasonal Cycle of Surface Temperature

6

Seasonal Cycle of Bottom Temperature

7

Winter & Summer Temperature

Bottom & surface T: winter consistent; summer inconsistent 8

1985 1990 1995 2000 20053.54.55.56.57.58.5

Year

SST

in F

eb(u

nit:

o C)

(a)

1985 1990 1995 2000 200523.024.025.026.027.028.0

Year

SST

in A

ug(u

nit:

o C)

(b)ModelAVHRR

1980 1985 1990 19956.08.0

10.012.014.0

Year

SST

in F

eb(u

nit:

o C)

(c)

1980 1985 1990 199515.0

20.0

25.0

30.0

YearSS

T in

Aug

(uni

t: o C

)

(d)Modelin-situ

1980 1985 1990 19954.06.08.0

10.012.0

Year

Bot

tom

Tem

pera

ture

in F

eb(u

nit:

o C)

(e)

1980 1985 1990 19957.08.09.0

10.011.0

Year

Bot

tom

Tem

pera

ture

in A

ug(u

nit:

o C)

(f)

Entire YS

Central YS

Topic #1: Forcing of Temperature Seasonal Cycle

Total budget: Surface heat flux 8-13 times larger than lateral flux 9

But: There is spatial non-homogeneity, especially in cooling season

In central YS, heat imported by YSWC still significant to compensate surface heat loss. But, is YSWC important for inter-annual T variation? 10

Small heat content changes

Large surface heat loss

Topic #2: Forcing of Winter Temperature Variations

Total budget: Variations of Winter (February) SST; shifting from cool to warm phases at late 1980s

During cooling season (September to March), lateral heat flux << surface heat flux; also surface latent & sensible heat fluxes dominate 11

Link to large-scale atmospheric forcing

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

Arc

tic O

scill

atio

n In

dex

Year

P = 0.10L = 20.00H = 2.00

1989RSI = 0.29

Average : -0.69 Average : 0.27

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 0.1

0.2

0.3

0.4

Reg

ime

Shift

Inde

x

Cooling season (September to March): temperature changes have negative correlation with East Asian Winter Monsoon index & positive correlation with winter Arctic Oscillation index

12

Again: spatial non-homogeneity in cooling season

In central YS, low correlation between variations in heat content and surface heat flux Suggesting localized importance of heat transport by YSWC Further analyses show VT variation is caused mainly by T, not V

13

Small heat content changes

Topic #3: Forcing of Yellow Sea Warm Current

• YSWC variation is not important for Yellow Sea temperature, but may be essential for salinity & nutrients

• Forcing mechanisms of YSWC

-- branching of Kuroshio (Uda 1934; Guan 1963, 1985)

-- compensation of wind-driven coastal currents (Hsueh 1988; Yuan & Hsueh 2010)

-- surface cooling & mixing over variable topography (Xie et al 2002; Huang et al 2005)

-- modified “Island Rule” (Lin & Yang 2010)

• Approach: -- Model sensitivity study, HEAT run forced by varying surface heat flux but climatological wind stress 14

REF HEAT

Bottom Temperature Anomaly: January 1979-1987

15 T variability mainly caused by surface heat flux

HEAT REF

Northward Flow Anomaly @ 35°N: January 1979-1987

19 YSWC variability mainly caused by wind

YSWC KCC

TWC

•Wind ： –τy*sin60° + τx*cos60°（35°N,123°E） •YSWC compensates coastal currents, both wind driven •YSWC influenced by 33°N West •YSWC less influenced by TWC •TWC is possibly large-scale forced

Wind Stress & Volume Transport across sections @ 33, 35°N (January)

20

Summary

• Two-way nested global-NWP model enables simulation of influences of local & remote forcing on Yellow Sea

• Total heat budget analysis -- Seasonal T, and variation of winter T, are primary caused

by surface heat flux -- Winter T Turbulent Heat Fluxes Wind EAWM AO • Cooling season in central YS, heat transport by YSWC

important -- for balancing surface heat loss -- for inter-annual variation of heat content -- but, variation of lateral heat transport is mainly caused by

T, not YSWC 21

Summary (Cont’d)

• YSWC variability: mainly caused by wind, compensates to coastal current, confirmed by model tests

• Ongoing work: -- Summer temperature variability (link to EASM?) -- Salinity budget; -- East China Sea; Kuroshio variability -- Improve model by including tides • Implication: -- Regional models may be sufficient for central YS -- Predictability is associated with large-scale atmospheric forcing -- Some analogy with Gulf of St. Lawrence & east Scotian Shelf? 22