Equatorial Currents Before, During, and After The 1997/98 El Nino

I’ve moved to WordPress.  This post can now be found at Equatorial Currents Before, During, and After The 1997/98 El Nino

####################
INTRODUCTION

Most discussions of El Nino events center primarily on Sea Surface Temperature anomalies. Sometimes they include descriptions of surface (trade) winds. Rarely do they include illustrations of surface currents, but the changes in surface currents are a significant part of ENSO because they help show the transport of warm water from the West Pacific Warm Pool to the East Equatorial Pacific. This post illustrates the surface currents before, during, and after the 1997/98 El Nino event. The changes are significant and obvious.

SOURCE OF SURFACE CURRENT ILLUSTRATIONS

I recently found a NASA educational website called “Ocean Motion and surface currents”.
http://oceanmotion.org/index.htm
Lesson 4 for students is called “Global Ocean Warming”.
http://oceanmotion.org/guides/ow_4/ow_student_4.htm
I then discovered what they called their “Ocean Surface Current Data Visualizer”.
http://oceanmotion.org/html/resources/oscar.htm
By selecting the Regions (W Eq Pacific, Cen Eq Pacific, E Eq Pacific), the Parameter (Direction) and the Months and Years, I prepared the maps used in the post and video.

REGIONS ILLUSTRATED

Figure 1 shows the locations of the three Equatorial Pacific Surface Current Maps (Western, Central, and Eastern) provided by the “Ocean Surface Current Data Visualizer”. There is a significant overlap between the Central and Eastern regions.

http://s5.tinypic.com/of0d1t.jpg
Figure 1

OCTOBER 1996 – BEFORE THE 1997/98 EL NINO

Figures 2, 3, and 4 illustrate the color-coded surface currents for the Western, Central, and Eastern Equatorial Pacific on October 1, 1996. Eastward currents are shown in blue, while westward currents are shown in a color that I’ll call khaki, for lack of a better word. The eastward (blue) current is the Equatorial Pacific Counter Current. North and South of it are the westward (khaki) North and South Pacific Equatorial Currents. In these illustrations, the South Pacific Equatorial Current appears to be the largest of the three. The NINO3.4 SST anomaly for October 1996 was neutral (-0.299 deg C, based on ERSST.v2 data).

West Equatorial Pacific – October 1996
http://s5.tinypic.com/a4n86c.jpg
Figure 2
############

Central Equatorial Pacific – October 1996

http://s5.tinypic.com/5a2b8k.jpg
Figure 3
############

East Equatorial Pacific – October 1996

http://s5.tinypic.com/2ronuww.jpg
Figure 4

JULY 1997 – DURING THE 1997/98 EL NINO

Between October 1996 and July 1997, the trade winds had relaxed, two Kelvin waves had traveled east across the Equatorial Pacific, and warm water that was once “contained” by the West Pacific Warm Pool has been “sloshing” east. By July 1997, the NINO3.4 SST anomalies had risen to 1.84 deg C. And as illustrated in Figures 5, 6, and 7, the Pacific Equatorial Counter Current had expanded significantly, an indication that eastward flow from the West Pacific Warm Pool to the Eastern Equatorial Pacific had, in fact, increased.

West Equatorial Pacific – July 1997

http://s5.tinypic.com/xofcj5.jpg
Figure 5
############

Central Equatorial Pacific – July 1997
http://s5.tinypic.com/2mws6dv.jpg
Figure 6
############

East Equatorial Pacific – July 1997
http://s5.tinypic.com/2v8mxs4.jpg
Figure 7

NOVEMBER 1997 – THE PEAK OF THE 1997/98 EL NINO

The NINO3.4 SST anomaly had risen to 2.79 deg C. The flow in the Pacific Equatorial Counter Current remained at high levels. Refer to Figures 8, 9, and 10.

West Equatorial Pacific – November 1997

http://s5.tinypic.com/2wdymog.jpg
Figure 8
############

Central Equatorial Pacific – November 1997

http://s5.tinypic.com/2mnf5p2.jpg
Figure 9
############

East Equatorial Pacific – November 1997

http://s5.tinypic.com/2vjapvn.jpg
Figure 10

FEBRUARY 1998 – A FEW MONTHS AFTER THE PEAK OF THE 1997/98 EL NINO

By February 1998, NINO3.4 SST anomalies had dropped by more than 0.6 deg C to 2.16 deg C. The high Pacific Equatorial Counter Current flow rates had subsided as illustrated in Figures 11, 12, and 13.

West Equatorial Pacific – February 1998

http://s5.tinypic.com/2w4cx9l.jpg
Figure 11
############

Central Equatorial Pacific – February 1998
http://s5.tinypic.com/2ak0hv6.jpg
Figure 12
############

East Equatorial Pacific – February 1998

http://s5.tinypic.com/65y0dk.jpg
Figure 13

JULY 1998 – THE SECOND MONTH OF LA NINA SST ANOMALIES

In July 1998, NINO3.4 SST anomalies had reached -1.14 deg C. The Pacific Equatorial Counter Current is still at extremely low levels. Refer to Figures 14, 15, and 16.

West Equatorial Pacific – July 1998

http://s5.tinypic.com/zvqwcy.jpg
Figure 14
############

Central Equatorial Pacific – July 1998

http://s5.tinypic.com/f9p6v4.jpg
Figure 15
############

East Equatorial Pacific – July 1998

http://s5.tinypic.com/mw7ic5.jpg
Figure 16

A FILM ILLUSTRATING THE CHANGES IN FLOW

The following film illustrates the abrupt changes in the size of the Pacific Equatorial Counter Current at the beginning and end of the 1997/98 El Nino. Note that the two surges in eastward flow that are visible in the West Equatorial Pacific maps appear to coincide with the two Kelvin waves that preceded the 1997/98 El Nino. I’ll address this in another post.