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Monday, February 9, 2009

Recharging The Pacific Warm Pool Part 2

I’ve moved to WordPress.  This post can now be found at Recharging The Pacific Warm Pool Part 2
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Significant Decreases in Cloud Amount Over The Pacific Warm Pool During El Nino Events

INTRODUCTION

In Recharging The Pacific Warm Pool (Part 1), I illustrated a process by which the warm water that is transferred during an El Nino from the Pacific Warm Pool to the Eastern Equatorial Pacific is then returned to the Western Equatorial Pacific (and the Pacific Warm Pool) during the subsequent La Nina. Much of that warm water, though, on its return to the West Pacific (and East Indian Oceans) appears to remain on the surface, raising the SST anomalies of the East Pacific and West Pacific Oceans in a positive step change. Refer to:
Can El Nino Events Explain All of the Global Warming Since 1976? – Part 1
Can El Nino Events Explain All of the Global Warming Since 1976? – Part 2
Supplement To “Can El Nino Events Explain All Of The Warming Since 1976?”
Supplement 2 To “Can El Nino Events Explain All Of The Warming Since 1976?”

If the warm water that was below the surface of the PWP was then spread across the surface of the East Indian and West Pacific Oceans, how then does the warm water in the PWP recharge so quickly? This quick replenishment time (~ 2 years) is visible in the West Equatorial Pacific Warm Water Volume in the following graph, Figure 2, after the 97/98 El Nino. Not only did it recharge, the Warm Water Volume increased in the West Equatorial Pacific. After the 1986/87/88 El Nino, it appears it only took ~1.5 years for the Equatorial Pacific Warm Water Volume to recharge.
http://i34.tinypic.com/xfyro1.jpg
Figure 1

THERE ARE SUBSTANTIAL DECREASES IN CLOUD COVER OVER THE PWP DURING EL NINO EVENTS

And these substantial decreases in cloud cover cause significant increases in Downward Shortwave Radiation (DSR) in the Pacific Warm Pool.

The Pavlakis et al (2008) paper “ENSO Surface Shortwave Radiation Forcing over the Tropical Pacific” identifies the variations in surface downward shortwave radiation over portions of the Pacific Oceans caused by El Nino-produced cloud cover changes.
http://www.atmos-chem-phys-discuss.net/8/6697/2008/acpd-8-6697-2008-print.pdf

Keep in mind when reading the following that shortwave radiation is visible light, with a small portion of ultraviolet, and that longwave radiation is infrared, with a small portion of microwave. Also keep in mind that visible light (shortwave) warms the top hundred or so meters of the sea. Infrared (longwave) warms the top few centimeters only and through mixing from waves (advection) warms the mixing layer of the sea. By doing so infrared is said to lock in more ocean heat.

The following are quotes from the Pavlakis et al paper that are intended to provide background information.

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.”

Discussing the topic of this post, the Pacific Warm Pool, Pavlakis et al also state, “A high correlation was also found over the western Pacific (10S–5N, 120–140E), where the mean DSR anomaly values range from +20Wm−2 to −20Wm−2 during El Nino and La Nina episodes, respectively.”

To put that into perspective, the variation in TSI over the last three solar cycles was approximately 1 watt/meter^2.

Figure 2 Cell a is a comparative graph of NINO3.4 SST anomalies and Downward Shortwave Radiation anomaly (DSR-A) at the surface for the Western Pacific (10S–5N, 120–140E) (the West Pacific Warm Pool) from the K. G. Pavlakis et al (2008) paper, their Figure 8. The correlation is quite good.
http://i41.tinypic.com/2435kbb.jpg
Figure 2

Recall that Pavlakis et al stated, “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.” If the variability of DSR reflects mostly the fluctuations in cloud cover, let’s examine a time series graph of cloud cover over the Pacific Warm Pool. Refer to Figure 3, which illustrates Cloud Amount Anomaly from July 1983 to June 2006. There are significant decreases in Indo-Pacific Warm Pool Cloud Amount during El Nino events.
http://i44.tinypic.com/e6q146.jpg
Figure 3

The Cloud Amount dataset is from the International Satellite Cloud Climatology Program (ISCCP), available through the KNMI Climate Explorer website on a user-selected coordinate basis:
http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere
The ISCCP webpage is here:
http://isccp.giss.nasa.gov/

Decreases in cloud amount cause significant increases Downward Shortwave Radiation (visible light) at the surface. So let’s invert the Cloud Amount data and compare it to NINO3.4 SST anomaly data, in an effort to replicate the Pavlakis comparative chart. Refer to Figure 4. Again, the correlation is quite good.
http://i43.tinypic.com/nq73sz.jpg
Figure 4

It appears the drop in cloud amount during El Nino events would be a significant factor in recharging the Pacific Warm Pool.

1 comment:

Erl said...

Great post Bob. Its good to have the numbers on the cloud variation. I think you will find that cloud loss is linked to 200hPa temperature which has a dual maxima in the tropics each year the greater achieved in the run from January through to April. Should be a correlation with the intensity and number of stratospheric warming events in the Arctic.

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