What causes trade winds to change?

Hi Jeff,

As with other wind systems, the key to the strength of the Pacific Trade Winds is a pressure gradient. The Trades are a characteristic of the Equatorial Intertropical Convergence Zone (ICT), a region of very stable air with a pressure of 1010hPa give or take just two hPa either way.

The systems that impart energy and momentum into the Trade Winds are the Subtropical Ridges, areas of high pressure situated at about 30° N and S of the Equator. These high pressure systems migrate poleward during the summer month and then equatorward during winter.

The higher the pressure in the subtropical ridges and the closer they move to the Equator the more pronounced the pressure gradient becomes and the faster the Trade Winds move.

The location and intensity of the subtropical ridges is influences by both the Southern Oscillation and the Pacific Decadal Oscillation.

During cooling ENSO episodes the ridge axes are skewed giving a northward tilt, whilst the positive ENSO phases favour a southerly tilt. This angle influences the transport of heat within and around Tropical regions, both in the atmosphere and the oceans.

The trend in recent decades has been a shift toward the positive El Nino episodes and a southerly bias in the ridges. This induces warming that imparts extra energy into the subtropical highs leading to greater lofting of the air-masses and a heightened pressure gradient, which in turn strengthens the Trades (it also has all the other effects associated with El Nino episodes).

As the same time that the Trades have intensified, the PDO has been in a negative trend and this is causing enhanced circulation of cool water from deeper in the oceans, this cool water is brought to the surface creating a greater temperature difference between the ocean and the atmosphere. This accelerates the flow of heat from the atmosphere into the ocean whilst at the same time it enhances the pressure and temperature gradient so intensifying the Trades.

Another likely factor is the Atlantic Multidecadal Oscillation (AMO), although in a different ocean to the Pacific Trade Winds, it does have the effect of amplifying the strength of the subtropical ridges.

The AMO switched to positive in 1995 and it’s likely this contributed to the strengthening Trade Winds. When the PDO switched to negative in 1998 this would have further strengthened the winds and all the while the Oceanic Nino Index has shown a positive trend, having peaked in approx 1994.

A note of caution, ocean dynamics are complex and there is a great deal still to be learned. The above answer is based on what we know at present, there may well be other factors.

What Causes The Trade Winds

I would suspect that temperature causes the trade winds to change. High temperatures cause strong trade winds, initiating a la Nina. Low temperatures slow down the trade winds, initiating an el Nino. If I am correct, I have described a negative feedback system. This does not mean that ENSO will stop global warming. La Nina conditions speed up the heat transfer between the oceans and the air. As the oceans heat up, la Ninas will no longer be associated with historic low temperatures. In fact, the most recent la Nina years, 2008, 2011 and 2012 were warmer than any el Nino years prior to 1998.

Realclimate.org http://www.realclimate.org/index.php/archives/2013... cites this paper:

M.A. Balmaseda, K. Mogensen, and A.T. Weaver, "Evaluation of the ECMWF ocean reanalysis system ORAS4", Quarterly Journal of the Royal Meteorological Society, vol. 139, pp. 1132-1161, 2013. http://dx.doi.org/10.1002/qj.2063 to reach the conclusion that "An intensification of the trades has affected surface ocean currents called the subtropical gyres, and these changes have resulted in a predominance of the La Nina state. The La Nina phase is associated with a lower global mean temperature than usual." It is NOT made clear what has caused the "intensification of the trades" in the first place.

I'm really curious as too which trade wind. If the link you provided is accentuating all trades winds (sources of origin) then its dead as. wrong.

ed: I'm not in disagreement about a super strong La Nina event. I just haven't read anything that postulates a definitive/quantifiable , verifiable correlation. It's all been a really good speculation at the best. Got anything else, with real foundation?

"There was a recent question posed by Ottawa Mike regarding changes in trade winds in which he claimed that it was just another reason to explain the 'pause' or 'hiatus' in surface warming."

I didn't claim anything, I just reported it. Here is the first paragraph of the article I referenced:

"Stronger than normal trade winds in the central Pacific are the main cause of a 13-year halt in global surface temperatures increases, an Australian study reveals." http://www.abc.net.au/science/articles/2014/02/10/...

There's the claim. I didn't make that statement. Further, my question had nothing to do with the excuse or the details of the "new" excuse. It was about other statements from the scientist. I guessed you missed all that since your answer didn't address what I was asking in the least.

Now having said all that, my answer thus far has done the same as what I just accused you of so now that I've got the details out of the way I'll go ahead and answer your question directly and on topic.

<I don't know>

If you look at ENSO forecasts it's pretty clear that we don't really know much more than a few months in advance what ENSO is going to do. I'd say that's pretty good evidence we also don't know much about the more complex interactions of the other factors you mention. I'm going to go out on a limb and also suggest including cloud processes as a major player.

However, what I do know is that there have been many papers suggesting that the Pacific trade winds are going to weaken in a man made global warming world. England's paper claims recent increased trade winds so something's gotta give. Again, the dominant cause?

</I don't know>

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Edit: I actually just finished reading your reference. The final section is titled Uncertain Outcome and it follows with many statements along that theme:

"... it is not clear at this stage which way ENSO variability will tip. As far as we know, it could intensify, weaken, or even undergo little change depending on the balance of changes in the underlying processes."

That's about as uncertain as it gets. There's no "likely" and not even a "possible" or "may". It's full on "we don't know".

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Edit2: Here is how questions like this spur me to more research and and that has led me to this statement regarding the pause in the last AR5 WG1 Chapter 10:

" The observed recent warming hiatus, defined as the reduction in GMST trend during 1998–2012 as compared to the trend during 1951–2012, is attributable in roughly equal measure to a cooling contribution from internal variability and a reduced trend in external forcing (expert judgement, medium confidence).The forcing trend reduction is primarily due to a negative forcing trend from both volcanic eruptions and the downward phase of the solar cycle. However, there is low confidence in quantifying the role of forcing trend in causing the hiatus because of uncertainty in the magnitude of the volcanic forcing trends and low confidence in the aerosol forcing trend. Many factors, in addition to GHGs, including changes in tropospheric and stratospheric aerosols, stratospheric water vapour, and solar output, as well as internal modes of variability, contribute to the year-to-year and decade- to-decade variability of GMST.

The Atlantic Multi-decadal Oscillation (AMO) could be a confounding influence but studies that find a significant role for the AMO show that this does not project strongly onto 1951–2010 temperature trends." http://www.climatechange2013.org/images/report/WG1...

If you do a search on that document, there is one mention of the PDO in the the text not related to the hiatus which states:

"For other climate phenomena, such as ENSO, Indian Ocean Dipole (IOD), Pacific Decadal Oscillation (PDO), and monsoons, there are large observational and modelling uncertainties (see Section 9.5 and Chapter 14), and there is low confidence that changes in these phenomena, if observed, can be attributed to human-induced influence."

Low confidence, lots of uncertainty. And that's fresh off the press. Now, this guy England just tosses the hiatus explanation of the IPCC in trash. The excuse turnstile is spinning at a very rapid rate, perhaps unprecedented.

Oh, forgot to mention that in that chapter of the AR5 WG1 report, Pacific trade winds was mentioned once, in the title of one reference which was used in a section about sea levels.

It is a good question, in theory a warming world would produce less temperature difference between tropical and polar latitudes, therefore reducing atmospheric flow of water and wind. and perhaps the the trade winds.

However I not sure it would be that simple weather is chaotic and we don't really know how it would respond.

If ENSO reduced or disappeared it just means that warm/cold wet/dry periods would be less pronounced, average surface temperatures should stay the same, just less fluctuations.

But this is speculation how precipitation, snowfall, albedo would be affected is in unknown territory.