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Monday, August 3, 2009

La Nina Events Are Not The Opposite Of El Nino Events

I’ve moved to WordPress.  This post can now be found at La Nina Events Are Not The Opposite Of El Nino Events
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.

Figure 1

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.
Figure 2

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.
Figure 3

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.
Figure 4

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.
Figure 5

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:


magellan said...

I always enjoy your reporting and analysis of ocean conditions.

At what point during this 'El Nino' year do you think it will manifest itself as being large, small or even meeting the ONI 5 month (IIRC) standard classification?

I've read some weather bloggers opinions that the current spike in June (for surface) and July (for satellite) is not related to El Nino, but is merely a release of heat similar to 2007, will descend downward as the year progresses placing El Nino as insignificant.

What's your take?

Bob Tisdale said...

d: Sorry. I don't make ENSO predictions. It's too volatile. But I will show why I believe it's unlikely to be a significant El Nino like the 1997/98 El Nino. I posted the following links at Lucia's TheBlackboard yesterday.

Here’s the equatorial subsurface temperature anomalies for June 1997 from ECMWF:

And then there’s June 2009, comparatively wimpy:

There aren’t any 6.5+ deg C subsurface anomalies this year (thankfully).

You wrote, "...the current spike in June (for surface) and July (for satellite) is not related to El Nino, but is merely a release of heat similar to 2007..."

Care to expand on that? Or some links? The first question that comes to mind is a release of heat from where? Do the the weather bloggers who make these claims provide comparative graphs of NINO3.4 SST anomalies and global surface temperature or TLT? Because if they had they'd note that most of the recent rises in surface and TLT anomalies are ENSO related.

John said...

What my mind is trying to wrap around is how the step el nino's you have referenced work.

In prior posts of yours, you've noted that ocean heat content has risen (though hopefully flattened lately).

Could these El Nino's be a response to the rising OHC? Thus, may we not have more of these steps in the future, leading to future warming? I guess the way my mind is trying to see it is rising temperatures via AGW lead to increased OHC, which lead to more frequent El Ninos, creating more step-ups, leading to future warming.

Am I missing something in my logic? I am hoping I am wrong, but for some reason this sticks in my head.

Bob Tisdale said...

John: You asked, "Could these El Nino's be a response to the rising OHC?"

Why don't we turn that around? Why can't El Nino events cause the rise in OHC? It's an idea I've been pondering lately. There are dips in OHC during minor El Nino events as heat is exchanged between the tropical Pacific and the atmosphere. The rise in TLT warms the oceans away from the equatorial Pacific. This can be seen in the SST anomaly animations I've posted recently. If La Nina events were of equal and opposite magnitude, the La Nina event would remove some of that heat from the atmosphere, returning part of it to the tropical Pacific. The remainder of the heat is radiated to space.

El Nino events cause shifts in cloud amount over the tropical Pacific that help the PWP "recharge," which is an increase in OHC there.

The magnitude and frequency of El Nino events have been higher than La Nina events since the mid-1970s.

Significant El Nino events that are not suppressed by volcanic aerosols, the 1986/87/88 and 1997/98 El Nino events, cause upward step changess in SST anomalies for the East Indian and West Pacific Oceans.

I think I'll elaborate on this and post it to see the responses I get.


John said...

Many thanks for the response.

Do you have any theories on why there has been the divergence between El Nino and La Nina frequency?

If La Nina is a response to El Nino, wouldn't we expect to see a more reasonable correlation in number between the two?

I saw the recent paper from Australia/New Zealand (I forget the authors) showing that La Ninas were more frequent in the 70s and El Ninos more frequent now.

It seems odd that if La Nina is a natural response shifting the waters back west to the PWP that the correlation wouldn't be closer to 1:1.

Thanks in advance.

Bob Tisdale said...

John: You wrote, "If La Nina is a response to El Nino, wouldn't we expect to see a more reasonable correlation in number between the two?"

You're assuming that a traditional El Nino event is occuring every time NINO3.4 SST anomalies rise above the threshold of 0.5 deg C. El Nino events are not the same.

Refer to my posts on El Nino Modoki:


And refer to my post showing the relationship between the significant El Nino events and the subordinate aftereffect El Ninos:


Anonymous said...

Hi Bob, thanks for the article. I got into a rather lengthy discussion the other day about ocean heat dissipation- premise being that energy travels from hot to cold, surface energy should diffuse both up and down. We decided to accept that a majority would dissipate downwards since the ocean was colder than the atmosphere above. Do the trade winds pull more heat than not? And do the currents move more heat up or down? Thanks, par5

par5 said...

Hi Bob. I got into a lengthy discussion a couple of days ago about heat dissipation in the oceans. We decided to accept that heat travels both ways, but mostly down because the ocean is colder than the atmosphere above. We also agreed that the trade winds would pull more heat off of the surface. Is this a trade off in La Nina conditions?

Bob Tisdale said...

Par5:. Regarding tropical Pacific trade winds during La Nina events, the trade winds are higher than normal during ENSO-neutral conditions. They pick up more heat and moisture as they travel west, which would cool the ocean surface more than normal. But the increase in trade winds also decrease cloud amounts in the eastern tropical Pacific, don’t they? This would increase downwelling shortwave radiation and increase the OHC. Pavlakis et al found that DSR was the greatest contributor to net heat flux into tropical Pacific Ocean and that total cloud amount over the tropical Pacific dictated DSR.

The following is from my post “Recharging the Pacific Warm Pool Part 2”

Pavlakis et al state, “The net heat flux into the ocean is a small residual of four terms; the net shortwave radiation at the surface (NSR), the latent heat loss, the sensible heat transfer and the net downwelling longwave radiation at the Earth’s surface (NSL). The NSL is the difference between the downward longwave radiation (DLR) at the Earth’s surface and the Earth’s surface thermal emission.”

They continue, “The NSR is the difference between the downwelling shortwave radiation (DSR) and the reflected radiation from the ocean surface. However, the reflected term is more than one order of magnitude smaller than the DSR, since the ocean albedo is less than 0.07. Thus, DSR dominates the net shortwave flux budget. The variability of DSR, the component of the net heat into the ocean with the largest magnitude, reflects mostly fluctuations in cloud cover caused by variations in atmospheric circulation and thus, it is very important in order to describe and study the intensity or duration of ENSO events.”

Regarding your question about currents, I haven’t studied all of the subsurface currents (and there are many) in tropical Pacific to answer this. But if they’re causing warmer- or cooler-than-normal water to be subducted in one location, water is rising in another.

par5 said...

Thanks Bob, sorry about the dupes, my comp keeps freezing. This may be the paper I was looking for. Enjoy your day, James

Kipp Alpert Photography said...

Hi Bob,beautiful work here.I will take the time soon and study your graphs, along with your comments.I would like to learn this kind of Science.A fresh look, not a contentious debate. I have posted two comments on my own blog, that you may find interesting.If you want to talk about anything I will comment back. You should ask Brett to post your graphs and comments. We have a couple of bona fide sceptics along with the warmers right now. Soon;kippalpert1.wordpress.com

Kipp Alpert said...

Bob: Did you get my comments.I posted two topics on comments, if you want to talk about a topic leave your post.I will read your work within the week.Kippalpert1.wordpress.com

Bob Tisdale said...

Kipp: To build a base of visitors, it takes a long time and a lot of commenting at other blogs (WITH LINKS TO THE POSTS AT YOUR OWN BLOG ON SIMILAR SUBJECTS). You can't expect readers to visit from other photography blogs if you've only got one post at your blog.

The blogger initiates interest with a post of a new topic on a new thread, but, again, it takes a while of doing that with no visitors to get people interested in what you're writing and showing.

I'm lucky in one respect. I get very few comments for the number of visitors...I don't have to reply to a lot or moderate them. Most people comment when Anthony Watts cross-posts my work at WUWT.

My average daily traffic is about 20 times more than it was a year ago, and I've got over 200 posts and probably 2,000+ graphs to reference.



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