Equatorial Annual Cycle ShangPing Xie IPRCMet University of

Equatorial Annual Cycle Shang-Ping Xie IPRC/Met, University of Hawaii Power. Point file available at http: //iprc. soest. hawaii. edu/~xie/ppt/annual. ppt References Mitchell, T. P. and J. M. Wallace, 1992: The annual cycle in equatorial convection and sea surface temperature. J. Climate, 5, 1140 -1156. Mitchum, G. T. , and Lukas, R. 1990: Westward propagation of annual sea level and wind signals in the western Pacific Ocean. J. Climate, 3, 1102 -1110. Xie, S. -P. , 1994: On the genesis of the equatorial annual cycle. J. Climate, 7, 2008 -2013. Xie, S. -P. , 2003: The shape of continents, air-sea interaction, and the rising branch of the Hadley circulation. In The Hadley Circulation: Past, Present and Future, H. F. Diaz and R. S. Bradley (eds. ), Cambridge University Press. Available in pdf at http: //iprc. soest. hawaii. edu/~xie

90 W, Eq SST 165 W, 20 N

SST Wind Cloud SST: Mean & Annual Harmonic Xie (2004, in Hadley Circulation …)

Equatorial Annual Cycle T u v Ø Why annual? Ø Why Strong in the east? Ø Why propagate westward?

t = A(x) e i[ wt - f(x) ] ty ty tx A(x) Lukas and Firing (1985) tx f(x)

SST, Precipitation and Surface Winds Mar-Apr Aug-Sept

August-May Difference OLR SST Sea surface height (cm) Mitchell and Wallace (1992)

Buoy Measurements at 110 W, Eq. Xie (1994)

Simple Theory of Equatorial Annual Cycle 1 D Ocean Linearization (coupling)

Ø Northward displaced ITCZ ( >0) Annual frequency (V’); Ø Tilt of thermocline H(x) Stronger annual cycle in the east; Ø Prevailing easterlies ( <0) Westward phase propagation. (Xie 1994, J. Climate, p. 2008) Evaporation: E= Upwelling: -1< Xie 1998, J. Climate, Eq. (2. 5), p. 191. <0

Temperature along equator SST’ & u’ at Eq Veq +

Model Results Xie 1994, J. Climate

Response to cross-equatorial winds

Dynamic Adjustment Mitchum and Lukas (1990)

SSH Annual Harmonic (Upper) Amplitude of the annual harmonic of sea level height (SSH; shade > 5 cm) measured by T/P satellite. (Lower-right) Longitude-time section along 5 N and (lower-left) latitude-time section at 165 W of SSH.

Equatorial Annual Cycle in the Atlantic Depth (m) Ocean dynamics play a more important role Houghton (1983, JPO, p. 2070)

Annual cycle in the equatorial oceans I year Mitchell and Wallace (1992) Role of Air-sea interaction

Monsoon Cold tongue Surface wind (m/s) and precipitation (mm/day) Monsoon effect June-April diff in APR run with cold tongue removed Cold tongue effect CTL-APR anomalies in June Okumura and Xie (2004, J. Climate)

Seasonal cycle of equatorial zonal wind: (1) Local air-sea interaction CTL run June April Longitude Ueq (m/s) CTL-APR run

Summary Ø Northward displaced ITCZ Annual frequency (V’) Ø Tilt of thermocline Stronger annual cycle in the east Ø Prevailing easterlies Westward phase propagation Ø While secondary in the eastern Pacific, ocean dynamics are important for equatorial annual cycle in the Atlantic. Ø Atlantic equatorial cycle is strongly influenced by continents and African monsoon in particular.

Atlantic equatorial cold tongue African monsoon + --

Wind Stirring Ocean Storms Experiments (47. 5 N, 139. 25 W) tx ty Sept. 1, 1987 Large and Crawford (1995, JPO, p. 2831)

80 60 40 20 Kessler