It is often said that "climate is what you expect, weather is what you get". But while this may be true in a general sense (summers are warmer than winters) the degree to which it is true can vary significantly from year to year, decade to decade, century to century and over longer time periods. Some of this variability is internally generated by the climate system. Some is the result of human activity. I am involved in a number of projects to examine how this variability works.
The periodic variability of the temperatures and winds known as El Nino has impacts across the world. With graduate student Alexi Russell, I've been working to look at the role of east-west gradients in tropical temperature. In Russell and Gnanadesikan (2014), we show that such gradients in the Pacific are not only the result of random changes in the amplitude of El Nino but actually change the strength with which the ocean and atmosphere couple to each other. In Gnanadesikan et al. (subm. int. rev.) we show that the amplitude of El Nino in climate models is affected by lateral mixing, with higher mixing states producing a more unstable climate and more active ENSO. Alexi is also looking at the source of changes in the freezing level height over the Andes. Associated with an anomalous warming of this region, these changes are likely responsible for the rapid retreat of Andean glaciers.
Because ozone heats the stratosphere, one impact of ozone depletion has been a cooling of this region in the Southern Hemisphere spring. It now appears that a number of other changes have occcured over at the same time as ozone hole developed from the 1960s to the 1990s 1. the Antarctic jet stream i shifted southward 2. warmer waters moved southward, undermining ice shelves along the Antarctic Peninsula 3. sea ice did not retreat, but actually expanded in the winter. A joint project with MIT, Columbia University and NCAR explores whether these phenonmena are connected, or whether other explanations (natural variability, global warming) could be responsible. Student Jordan Thomas recently published a paper showing that models predict a relatively large variability in surface pressures and jet latitudes, so that these processes may well have contributed to the observed changes in the historical period. Other work is going on to understand the impact of increasing carbon dioxide. A particular puzzle that has emerged is why carbon uptake depends on lateral mixing, but heat uptake does not.
The retreat of Arctic sea ice has been in the news lately as ships begin to transit the Northwest Passage and Russia moves towards claiming rights to exploit mineral resources in the region. But it is far from clear why Arctic Sea is retreating so quickly. A recently funded NOAA proposal examines the role of three processes, ocean color , freshwater buildup, and the turbulent transfer of heat between ice and ocean. A key question is whether changes in all of these processes act to store heat more efficiently in the summer, and then return it to the ice more efficiently during the winter. Graduate students Grace Kim (ocean color) and Eshwan Ramudu (ocean-ice heat transfer) are exploring different parts of this problem.
Aerosols play an important role in absorbing and re-rediating heat in the atmosphere. But the roles of aerosols in fully coupled models have not been fully explored. Graduate student Anna Scott, who is working on the impacts of cities on climate, has found that the net impact of black carbon produced by Asian megacities is to cool the surface of the North Pacific. Graduate student Asha Jordan has found a moistening impact of Saharan dust.
Russell, A.. and A. Gnanadesikan, Understanding long-period variability in ENSO amplitude, J. Climate, 27, 4037-4051, 2014.
Dezfuli, A., B. Zaitchik and A. Gnanadesikan, Zonally asymmetric rainfall variability in South Equatorial Africa, J. Climate, 28, 809-818, 2015.
Thomas, J., D.W. Waugh and A. Gnanadesikan, Southern Hemisphere extratropical variability: Recent trends and natural variability, Geophys. Res. Lett., doi:10.1002/2015GL064521, 2015.
Scott, A., A. Gnanadesikan, M.A. Pradal and D.W. Waugh, North Pacific atmospheric cooling from an abrupt increase in black carbon in a coupled climate model, in rev. Geophys. Res. Lett.
Gnanadesikan,A., A.M. Russell, M.A. Pradal and R. Abernathey, Impact of lateral mixing on El Nino in a coupled climate model, subm. int. rev. for Journal of Advances in Modeling the Earth System.