Observational Evidence for Trends and Variability in the Hadley Cell Based on 15 Years of Scatterometer Ocean Surface Wind Estimates

Recent evidence suggests that the tropics have expanded over the last few decades by a very rough 1⁰per decade. This is considered to be an atmospheric response to the observed tropical ocean warming trend. If continued, the expansion of the tropics (the Hadley cell) could have a substantial impact on water resources and the ecology of the sub-tropics. Until now, understanding of the mechanisms governing the changing width of the tropics has been confined to models and proxies because of the unavailability of systematic observations of the large-scale circulation.

Ocean surface vector winds, derived from satellite scatterometer observations, provide for the first time an accurate depiction of the large-scale circulation and allow the study of the Hadley cell evolution through analysis of its surface branch.  The launch of NASA’s QuikSCAT in 1999 marked the beginning of routine global observations of the surface vector winds and provided a consistent 10-year record.  In this study we determine the extent of the Hadley cell as defined by the subtropical zero-crossing of the zonally-averaged zonal wind component. We analyze the intensity of the Hadley cell as defined by the magnitude of the tropical convergence and we study the variations in the mean location of the peak surface convergence, indicative of the variations in the ITCZ mean location.  Our analysis revealed a couple of interesting results: i) The first half of the 10-year record shows two distinct cycles in the width of the Hadley cell while the later part of the record shows a steady increase in that width, as has been shown by others (~1 deg/decade, both south and north, for a total of about 2 deg/decade); ii) The two cycles in the 1999-2004 time period are likely a reflection of the modulation of the Hadley cell by the La Nina (1999) /El Nino (2002) events that dominated this period; iii) Analyzing the time series of 3-month running averages reveals the seasonal variation of the Hadley cell extent and width.

The launches of the ASCAT instruments on the METOP series, beginning in 2006 will assure the continuation of the climate data record of near-surface winds over the oceans. We performed similar analysis of the Hadley cell using the wind estimates from ASCAT and we found similar variability.  However, analyzing the trend in the joint QuikSCAT / ASCAT record appears more challenging.  Indeed, we found an apparent discontinuity in the signal when the data source changes from one observing system to another! 

This brings an important point regarding identifying trends in a multi-sensor data record.  Cross-platform calibration has been widely acknowledged in the past as a possible source for discontinuity in the data records which, if unaccounted for, could introduce spurious trends.  However, another source, namely the diurnal variability of the geophysical variables, has not been given a similar attention so far.  Yet, the diurnal signal is intertwined in the seasonal, annual and inter-annual signals as the different satellite-based instruments observe the geophysical variables at different local time-of-day.

To investigate the diurnal variability in the ocean surface winds, prior the launch of RapidScat, we analyzed records from the several tandem missions (QuikSCAT and SeaWinds; QuikSCAT and ASCAT; ASCAT and ISRO’s OSCAT), during times of overlap.  Our analyses support the notion about the significance of the diurnal signal.

Fortunately, the RapidScat mission makes it possible to resolve, for the first time, the details of the diurnal signal in the large-scale circulation. NASA’s ISS-RapidScat was launched on September 21^st2014 and began providing high-quality data just couple of weeks later.  The ISS (International Space Station) orbit provides unique opportunity to help understand and untangle the diurnal signal.  Our analyses of the RapidScat observations show the presence of a clear semidiurnal signal in the width of the Hadley cell.  This helps explain previously found discrepancies and will help in establishing the trend in the Hadley cell width and intensity.  More importantly, this points to a clear need to understand and resolve the diurnal signal before merging wind observations from different missions to form a consistent climate record that will allow us to study trends in the Earth’s system.

The research described in this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA).