>In my post Regression Analyses Do Not Capture The Multiyear Aftereffects Of Significant El Nino Events, I provided a detailed explanation of significant El Nino events. Click on the link above and scroll down to “EL NINO OVERVIEW”. What I did not explain in that post is that La Nina events are not the opposite of El Nino events. The following discussion assumes that readers have read the “EL NINO OVERVIEW” and the linked posts.
The North and South Equatorial Currents and the Equatorial Counter Current of the tropical Pacific are shown in Figure 1. The map indicates the existence of these currents, but does not show which currents dominate and which of those currents change significantly during El Nino and La Nina events.
The relative sizes of the North and South Equatorial Currents and the Equatorial Counter Current for an ENSO-neutral month (October 1996) are shown in Figure 2. 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.
Note: Figures 2, 3 and 4 are from my post Equatorial Currents Before, During, and After The 1997/98 El Nino.
During a significant El Nino, tropical Pacific trade winds relax and warm waters from the Western Equatorial Pacific and from below the surface of the Pacific Warm Pool slosh to the east. The current that carries the water east during the El Nino is the Pacific Equatorial Counter Current. The Equatorial Counter Current reaches the size shown in Figure 3 during El Nino events. In fact, this illustration is from the 1997/98 El Nino.
At the end of the El Nino, the trade winds resume, and the Pacific Equatorial Counter Current relaxes. Refer to Figure 4. The North and South Equatorial Currents become dominant. This change in direction of equatorial flow takes what remains of warm water that had traveled east during the El Nino and returns it to the west and the Pacific Warm Pool.
During the La Nina, convection, total cloud amount, and precipitation follow the warmer water west and the trade winds increase as the warmer water and convection travel farther west. This raises the thermocline in the eastern Pacific, raising lower temperature waters to the surface, increasing the temperature gradient from west to east, which increases the strength of the trade winds. The North and South Equatorial Currents remain dominant. Figure 5 illustrates the western equatorial currents in May 2000, which is still part of the 1998/99/00 La Nina.
The following s the video from the post Equatorial Currents Before, During, and After The 1997/98 El Nino.
Video: Equatorial Pacific Current Changes During The 1997/98 El Nino
The La Nina occurs when and because the coupled atmosphere-ocean processes overreact during their attempt to return to a “normal” state.
In effect, the La Nina is only an exaggeration of the “normal” ENSO-neutral state. On the other hand, an El Nino involves the reversal of “normal” current flow for a large portion of the equatorial Pacific.
The surface current map (Figure 1) was cropped from:
The detailed surface current maps (Figures 2 through 5) are from: