Rick Lumpkin NOAA/AOML/PhOD Miami, Florida USA Surface currents of the Indian Ocean Data Buoy Cooperation Panel (DBCP) 20 th Session – October 2004 Chennai, India

SVP surface drifting buoys (drifters) Spherical surface float –SST, submergence –Satellite transmitter Drogue: –holey sock design –center depth: 15 m

Data density Drifter days per square degree before after 2001 Oct.1985-Apr drifter-years 701 drifters

Observations by year Blue: drogued Red: undrogued

Pseudo-Eulerian averages 2° x 1° bins, Principal Component orientation. Blue: >20 drifter days. Yellow: 5-20 dd Red: <5 dd (every other bin shown)

MEAN ANNUAL SEMIANNUAL EDDY SST, U, V: X= + X a,1 cos(2  t)+ X a,2 sin(2  t) + X sa,1 cos(4  t) + X sa,2 sin(4  t) + X eddy

Mean currents Shaded: mean speed (cm/s)

Mean zonal currents Solid: total currents Dashed: Ekman- removed currents SEC ECC

Seasonal variability NE Monsoon SW Monsoon Shading: SST anomaly Arrows: surface currents

North Equatorial Current/ Southwest Monsoon Current Total currents Ekman- removed

Monsoon-driven changes

Amplitude of annual variations A= 1/2  U ann 2 + V ann 2 Equator: Ekman dynamics “break down”

semiannual variations

Where is “annual mean” meaningful?

Quantifying modes of seasonal variation NE/SW Monsoon Equatorial convergence / divergence; East Indian Current Semiannual response on equator, reflected in Arabian/Bengal sub-basins “EOFs”: Empirical Orthogonal Functions Ranked by variance explained. Currents: calculated for principal component.

Conclusions Data density: most observations in ; good historical coverage for most of basin; poor coverage in western equatorial zone Mean currents: SW (summer) monsoon pattern dominates Seasonal changes: Resolution of seasonal amplitudes and phases throughout most of basin and in sub-basins. Primary mode: monsoon; higher modes associated with equatorial semiannual fluctuations and divergence/convergence, reflected in northern sub- basins.

NOAA/AOML/PhOD Miami, Florida USA Rick Lumpkin