Xianghui,

These papers could be useful for you:

Article Processes Controlling the Mean Tropical Pacific Precipitatio...

Article Processes Controlling the Mean Tropical Pacific Precipitatio...

Article Precipitation and sea-surface salinity in the tropical Pacific Ocean

Thank Araghi for your recommendated articles about the mean precipitation distributions over the tropical Pacific.

What really confuses me is the variation of the precipitation distributions, e.g., on the interannual timescale. Like many articles mentioned (e.g., Zheng_etal_GRL_2014, Xie_Yang_GRL_2014, Xie_etal_JClim_2018), the precipitation anomalies over the eastern Pacific during the strong El Nino events (always being eastern Pacific type) are much near the equator (see the figure attached); however, they are almost located on the north of the equator during the weak El Nino events (always being central Pacific type). But there is no complete physical explanation on this.

From the distributions of the underlying sea surface temperature anomalies (SSTA), we can not get the collocated patterns between the precipitation anomalies and SSTA. So why does there exist precipitation anomalies over the eastern Pacific when no strong SSTA exist in the region during the weak El Nino? and why does they locate on the north of the equator?

One of the important factors affecting precipitation in the tropical Pacific is the quasi-biennial stratospheric oscillation (QBO), which has a strong impact on the tropical troposphere and convective cloud systems. The major challenge is to separate the QBO signal from other forcings such as ENSO. The general methodology is discussed by Liess and Geller in the paper: “On the relationship between QBO and distribution of tropical deep convection”. The text is available at:

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JD016317

If we assume (following the Gaia hypothesis of Lovelock) that humans are part of nature, all our actions are "natural". They only add a few oscillations to the rich symphony of sounds, it is not easy to de-convolute what we hear. Precipitation is one of the best examples when the inherently chaotic system is forced by the QBO and other factors mentioned in the responses of this thread.

Over the Pacific ocean at this point of time and Season, there would be an active ITCZ (Inter Tropical Convergence Zone). Most of the weather activity happening over this region could be attributed to transient wave perturbations moving along this low pressure zone. Whenever the convectively active phase of Madden Julian Oscillation (MJO) enters this region, ie, Pacific, the ITCZ over the region might get activated hosting a series of Tropical Cyclones. Meso-scale convective Complexes are also another causative factor for precipitation over the region, under warm Sea Surface Temperatures coinciding with conducive convective environment.

Xianghui,

I attended a talk where a professor from Cambridge University said that he had found that convection was most often the result of chemistry not physics. In the case of tropical convection this implies that it is caused by the increase in (light) water vapour in the air and not by the change in temperature. In other words it is not hot air rising, it is less dense moist air. Of course the moisture content of air depends on the SST, as does the air temperature, so it is difficult to separate the two effects.

The other point is that air cannot rise unless there is denser air to replace it. Dry air could be supplied from the continents but in summer it will be warmer and less dense than oceanic air.

I imagine tropical rainfall is mainly the result of convection, so if you were not aware of those two principles, then perhaps they can help you to solve your problem.

Regards, Alastair.

Xianghui,

The moisture budget and MSE budget may be helpful to understand the physical processes influencing the precipitation, e.g., the gross moisture stability, horizontal advection of moist enthalpy, heat flux, etc. For the attached figure, I guess the meridional advection (modulated by anomalous meridional wind and climatological specific humidity) may play a dominant role in the different precipitation distributions.

Janusz Pudykiewicz, Kenneth M Towe

Muhammad Mubashar Dogar

Thanks very much for your responses, and I have learned many from them. But there still are some questions confusing me.

On the interannual time scale, dose the meridional advection cause the anomalous precipitation pattern? or on the other way? If the meridional advection is the cause, then what causes the changes of it or the meridional wind?

If it is the precipitation causes the meridional wind, then what causes the changes of the precipitation? the underlying sea surface temperature only? since the precipitation pattern is always corresponding to the ENSO phase, e.g., less precipitation in La Nina and more in El Nino.

Xianghui,

As I understand it, the trade winds blow warm surface water from the tropical Pacific into the Warm Pool. Although some of that water can escape to the Indian Ocean through the Malacca Strait etc, sea level in the warm pool does rise until the trade winds weaken.

When that happens the warm water, which has been heate further in the warm pool flows back into the western tropicla Pacific. This further weakens the trade winds, and the eastern tropical Pacific sea surface temperature then rises.

Thus the flow of water causes the trade winds to weaken, and the weakened trade winds allow the flow of water. In other words it is a feedback process, not a simple cause and effect.

So it maybe that the advection causes the SST to rise which causes more rainfall and convection which causes more advection.

Ken,

El Nino is natural. You seem to be obsessed with denying the dangers of global warming. That is not what this question is about.

On the other hand, there is a paper here which might be related to Xianghui's question:

Broecker, W. S., and A. E. Putnam. “Hydrologic Impacts of Past Shifts of Earth’s Thermal Equator Offer Insight into Those to Be Produced by Fossil Fuel CO2.” Proceedings of the National Academy of Sciences 110, no. 42 (October 15, 2013): 16710–15.

https://doi.org/10.1073/pnas.1301855110.

Alastair Bain McDonald,

Thanks for your answers.

As you mentioned, the classic ENSO theories focus mostly on the zonal branch of the tropical Pacific, i.e., the zonal wind, zonal current, Walker circulation, Warm Pool, Cold Tongue, precipitation on the western and central Pacific, …

However, there seems rare studies interested in the meridional branch, especially over the eastern Pacific region, e.g., the precipitation anomalies over the eastern Pacific are always located on the north of the equator, corresponding to which the surface wind anomalies over the Cold Tongue region always have the meridional component.

Prof. Kenneth M Towe

Thanks for your answer. I want to know what controls the pattern and intensity of the precipitation anomalies over the tropical eastern Pacific region. They are always located on the north of the equator, with positive values during El Nino and negative values during La Nina.

Easterly Jet is one of the major factors controlling the fluctuations of rainfall intensity in the tropic.

Xianghui Fang

As you may have guessed, I am not an expert on El Nino so probably should not be posting but I thought I could give you a few clues from a system engineering perspective, to help you with your project.

My second post was not about the mechanism of El Nino but rather I was trying to make the point that it illustrates the mechanism of a feedback loop where there is no ultimate cause.

However, applying my first principle, that dense air can only rise if there is a source of denser air to replace it, does suggest an answer to your problem. Could it be that cold air from south west North America is the driver of convection in the south east North Pacific? In the autumn and winter the temperature of the air there, e.g. California, will cool, and when its density falls below that of the maritime air it could cause the stratus cloud to rise and produce precipitation.

Just a thought,

Cheers, Alastair.

Hi,

Year to year variations in tropical rainfall in the Pacific sector are governed mainly by the El Nino Southern Oscillation (ENSO) which varies on timescales of 2-5 years. The positive (El Nino) phase of ENSO causes increased rainfall in the central and eastern equatorial Pacific and decreased rainfall in the tropical west pacific. It does this through changes in the atmospheric circulation (including divergence and convergence rates), surface ocean temperatures and associated rainfall which break down the Pacific easterly trade winds in El Nino and strengthen them during La Nina. There are also remote effects on rainfall in other regions. You can read more about this here: Article Tropical Rainfall, Rossby Waves and Regional Winter Climate ...

On longer timescales, systematic changes in tropical rainfall are likely from systematic changes in tropical sea surface temperatures. These longer term changes can be read about here:Article Surface Warming Patterns Drive Tropical Rainfall Pattern Res...

Best

Adam

Adam A. Scaife

Thanks very much for your help. These papers are very helpful to understand the interannual variations of the rainfall over the tropical Pacific, especially the connections between which and the ENSO.

By the way, I want to recommend two recent papers, which investigated the year to year variations of the eastern Pacific ITCZ during the boreal spring, to whom interested in this topic.

Article Eastern pacific ITCZ dipole and ENSO diversity

Article Explaining the Year-to-Year Variability of the Eastern Pacif...

Best

Xianghui

@ Xianghui Fang

As mentioned severally here, ENSO is the major driver of interannual rainfall variability in the pacific. However, what hasn't been mentioned is the fact that Pacific Decadal Oscillation (PDO) is a major driver of decadal changes in rainfall anomalies. I am currently working on a study which shows that these two oscillatory modes of ocean-climate variability influence atmospheric pressure and wind patterns (including stress, vorticity, speed) and drive changes in sea surface temperature, sea surface evaporation and atmospheric humidity. This is a coupled ocean-atmosphere mechanism that forces anomalous weather and climate systems.

Cheers