Decadal-scale Variability in the Closure of the Wind-driven Gyres in the Western Equatorial Pacific and Implications for Heat and Salt Transport Into the Indian Ocean
Roxana C. Wajsowicz (Dept of Meteorology/JCESS, University of Maryland, 3433 Computer and Space Science Building, College Park, MD 20742, U.S.A.)
e-mail: roxana@atmos.umd.edu
ABSTRACT
There is significant interannual and decadal-scale variability in the wind-stress curl over the North Pacific Ocean. In the context of a Sverdrup model, variability in the latitude of the streamline bounding the southern extent of the northern tropical gyre may produce variations in the relative fractions of the Mindanao Current and South Equatorial Current (SEC) which make up the throughflow into the Indian Ocean. FSU wind stress data give that this latitude has become more northerly over the last 30 years, which when substituted into a simple composition model gives a greater contribution from the warmer, saltier SEC. Historical salinity proflles for the region show that over the depths of the SEC salinity core, the equatorial Pacific has become fresher indicating that the northern extent of the SEC retroflection has become more southerly, which once again when substituted into the simple composition model gives a greater contribution from the SEC to the throughflow. In agreement with these observations and simple model, the profiles show that parts of the Indonesian archipelago and eastern Indian Ocean have indeed become saltier. In terms of the general circulation, these results suggest that the northward cross-equatorial transport of mass, heat and salt has decreased over the last 30 years in the Pacific Ocean. These quantities have instead passed into the southern Indian Ocean, where they have likely influenced the strength of the Indian monsoon.
INTRODUCTION
The total amount of rainfall over Indian during the southwest monsoon has considerable interannual and decadal-scale variability, see e.g. Kripalani & Kulkarni (1997). Although meteorologists have been. studying the phenomenon for more than twenty years, the causes and links with ENSO and observed Pacific decadal-scale variability remain a mystery. It is assumed that coupled modes of the ocean-atmosphere and land-atmosphere systems are involved, but their nature has not been identified. From an oceanographic perspective, a likely senario is that in strong monsoon years, warm SST anomalies in the Arabian Sea occur, which promote the development of convective rolls in the atmosphere, so yielding more rainfall. Air-sea interaction displacing the limb of the Walker circulation over the Bay of Bengal is another possibility. An interesting alternative was proposed by Wajsowicz (1999b), who noted that COADS data showed anomalously large evaporation rates over the southern Indian Ocean during the months of a strong southwest monsoon, which resulted in a positive anomaly in cross-equatorial moisture flux.
