Sunday, May 26, 2013

Manus is for [Cloud] Lovers

by Jess Conroy

We’re back from our latest trek to the ends of the earth. In pursuit of the rain once again, we headed out to the far western Pacific a couple weeks ago, to the remote island of Manus, off the coast of Papua New Guinea.  This time, we also brought our Picarro instrument along for the ride!
Manus Island, the perfect place to study tropical rainfall!
Why here, of all places?  To complement our rain and seawater collections on Christmas Island and in the Galápagos, we wanted to collect waters from a site bathed in the tepid waters of the West Pacific Warm Pool. I can testify that it was far more delightful to dip my toes in Manus seawater, compared to the chilly waters off the Galápagos!
Grabbing a water sample from a coastal lagoon.

Additionally, to understand the convective clouds that so often drape across the Manus sky, the US Department of Energy and the Australian Bureau of Meteorology have parked loads of fancy instruments here. So, coupled with water vapor isotope measurements from our Picarro, we will be able to place our precipitation isotope measurements in a very rich atmospheric context.
Team Manus: David Noone, me, and Jessica Moerman
standing proudly at the site with our automated rain collector!

What exactly are we looking for? Once again, we are trying to dissect the relationship between stable isotopes in water molecules and climate. In warm, wet, tropical places like Manus, there are usually fewer heavy oxygen and hydrogen isotopes in the rain. And, with more rain, there are fewer and fewer of the heavy isotopes. This important relationship, dubbed the ‘amount effect,’ is used by many paleoclimatologists, along with measurements of fossil water archived in things like speleothems and sediments, to estimate how wet or dry it was many decades, centuries, and even millennia before we started measuring precipitation rates.
Saying goodbye to the Manus weather observers, who will
continue to take our water samples for the next few years.

But, the question we are asking now is, why does this relationship exist? Interestingly, the mechanism for this relationship that I learned in my graduate classes back in the day is only really part of the story (we think). For example, it was once thought that since tropical rain is so intense, all the heavy isotopes rain out initially, making the subsequent precipitation lighter and lighter isotopically. But in the last few years, many new insights about the physics of the ‘amount effect’ suggest other causes of this relationship, like the evaporation of rain beneath the clouds, and the recycling of vapor with very low isotope values from the evaporated rain back into the rain clouds. But these insights come from models, not observations. So, our hope and goal is that, equipped with mountains of atmospheric data from Manus--like cloud base height, 3 different kinds of radar, temperature and humidity profiles through the atmosphere, and rain drop size distributions, for starters, we can start to understand this relationship between precipitation isotopes and rain amount, ultimately improving our interpretation of all those records of past rainfall from paleoclimate records.