Knowledge of the processes giving rise to present-day climate variability and past climate change is required in order to anticipate the influence of human activity on climate. My research efforts acknowledge that to fully understand, model and predict changes in climate characteristics that have a large impact on society (especially temperature and precipitation patterns), a fully coupled ocean-atmosphere perspective is needed - one that accounts for changes in important variables such as the thermal structure of the slowly adjusting ocean. Complimenting observations with theory, I endeavor to accompany complex simulations of climate phenomena, with simple models capturing the essential dynamics required to explain unanswered questions within climate science.

What controls the mean east-west sea surface temperature gradient in the equatorial Pacific: the role of cloud albedo

In the context of approximately steady-state past and future climates, this research focused primarily on what controls the mean east-west SST gradient in the equatorial Pacific. This gradient is a key feature of tropical climate - tightly coupled to the atmospheric Walker circulation and the oceanic east-west thermocline tilt, it effectively defines tropical climate conditions. In the Pacific, its presence permits the El Niño-Southern Oscillation. What determines this temperature gradient within the fully coupled ocean-atmosphere system is therefore a pertinent question in climate dynamics, critical for understanding past and future climates.

Figure 1. The east-west upper-ocean (0-50m) temperature gradient (ΔTuo) as a function of the Pacific meridional albedo gradient (Δα) for the pre-industrial CMIP5 runs (black dots), several CESM sensitivity experiments (red crosses), and the observed value (blue dot). The regions used to evaluate ΔTuo and Δα are shown as boxes superimposed on the present-day observed SST field. The observed values of Δα and ΔTuo are based on the CERES-EBAF and WOA2009 datasets, respectively. Adapted from Burls and Fedorov (2014a).

Using a comprehensive coupled model - the Community Earth System Model (CESM) from the National Center for Atmospheric Research - we demonstrate how the gradient in cloud albedo between the tropics and mid-latitudes (Δα) sets the mean east-west SST gradient in the equatorial Pacific (Burls and Fedorov, 2014a). To change Δα in our numerical experiments, we change the optical properties of clouds by modifying the atmospheric water path, but only in the shortwave radiation scheme of the model. When we vary Δα from approximately -0.15 to 0.1, the east-west SST contrast in the equatorial Pacific reduces from 7.5°C to below 1°C and the Walker circulation nearly collapses. These experiments reveal a near linear dependence between Δα and the zonal temperature gradient, which generally agrees with results from the CMIP5 pre-industrial control simulations (Figure 1). We explain the close relation between the two variables using an energy balance model incorporating the essential dynamics of the warm pool, cold tongue, and Walker circulation complex.

Changes in the gradient in cloud albedo between the tropics and mid-latitudes therefore present a mechanism for long-term changes in the east-west tropical Pacific SST gradient and associated Walker circulation that is directly relevant to studies of both future and past climates. Our experiments suggest that the Walker circulation will decrease in response to a reduction in extra-tropical albedo, as warmer water is upwelled in the east and convection shifts eastward, occurring more uniformly across the tropical Pacific basin. At the same time the strength of Hadley circulation also reduces in response to the associated weakening of the meridional SST gradient. These results suggest that correctly capturing the cloud albedo response to anthropogenic forcing is an important factor in determining the response of large-scale SST gradients, Walker and Hadley circulation, and the hydrological cycle.

Simulating Warm Pliocene Conditions

Sensitivity studies and paleoclimate simulations provide valuable tests for current theories, and the state-of-the-art models used to simulate the response of Earth's climate to elevated CO2 levels. Uncertainties in the forecasts provided by climate models are largely associated with the parameterization of sub-grid scale phenomena such as atmospheric convection in the tropics or processes in the deep ocean. One way to constrain these uncertainties is to reproduce and study climatic conditions of the past as suggested by paleodata.

With respect to past climates, the results shown in Figure 1 provide an indication of the spatial extent and magnitude of the cloud albedo changes and radiative fluxes required to shift between present-day and warm worlds such as the early Pliocene - a period 4-5 million years ago when the equatorial Pacific east-west SST gradient, and meridional SST gradients were much weaker than today. The climate of the early Pliocene epoch is a particularly interesting period in Earth's history. With a continental configuration similar to present-day, and atmospheric CO2 concentrations similar to the anthropogenically forced CO2 levels seen today, it arguably provides one of the closest analogues to modern greenhouse gas conditions. The current anthropogenically forced rise in CO2 has the potential to trigger a number of feedbacks which could potentially push us back into the warm world of the early Pliocene, such as melting of glaciers, altering the hydrological cycle, and increasing the depth of the oceanic thermocline separating warm surface waters from colder deeper water. Could this rise in CO2 restore the warm conditions of the early Pliocene? To answer this question a better understanding of the processes responsible for the warm conditions in the early Pliocene is needed.

Figure 2. Pliocene warming relative to preindustrial conditions as simulated by a fully coupled climate model with modified cloud forcing. Superimposed on these SST anomalies are the observed differences between modern and early Pliocene SST estimates at available sites. Taken from Burls and Fedorov (2014b).

By imposing certain changes to cloud albedo within a fully coupled climate simulation (CESM) we can reproduce the structurally different SST patterns of the early Pliocene (Burls and Fedorov, 2014b). Most notably the reduced zonal and meridional SST gradients, weaker Hadley and Walker circulations, and stable warm pool temperature relative to modern day climate. Subsequent questions lie in exploring the feasibility and possible cause of the required albedo change, such as changes in atmospheric aerosols.

The role of ocean dynamics within tropical Atlantic climate variability: an ocean energetics perspective

During my time in the Department of Oceanography at the University of Cape Town, I investigated the role that oceanic processes play in the generation and evolution of coupled ocean-atmosphere variability in the tropical Atlantic Ocean - a region of great importance for African and South American climate. I focused on variability at the seasonal and inter-annual scales, as these are the time scales that have the largest impact on flood and drought events that often devastate large parts of Africa. Viewing upper-ocean variability within the tropical Atlantic from an energetics perspective (Figure 3), I assessed the role of ocean dynamics, in particular the role of ocean memory.

Figure 3. A schematic of the key energetic processes associated with upper ocean variability along the equator. Wind stress acting on surface currents generates wind power, feeding into the kinetic energy evolution equation, a portion of this wind power is converted to buoyancy power an exchange term with the available potential energy equation. Positive buoyancy power increases available potential energy, vertically displacing isopycnals. When integrated over a tropical ocean basin available potential energy succinctly quantifies the east-west slope of the thermocline and is strongly correlated with the east-west SST gradient.

Based on ocean reanalysis data, and a simulation of the tropical Atlantic region that I configured and integrated using the Regional Ocean Modeling System (ROMS-TAtl), I investigated the feedbacks between the equatorial ocean thermocline, the work done by tropical wind stress and surface heat fluxes. The results that I obtained highlighted the impact that small basin size has on the behavior and predictability of inter-annual, tropical Atlantic climate variability. Unlike in the Pacific where seasonal and inter-annual variability involve distinctly different physical processes, my results showed that the latter is a modulation of the former in the Atlantic, whose seasonal cycle has similarities with El Niño and La Niña in the Pacific (Burls et al., 2011). The ocean memory mechanism associated with inter-annual fluctuations in equatorial Atlantic Sea Surface Temperature (SST) appears to operate on much shorter time scales than that associated with the ENSO. Ocean memory in the Atlantic is largely associated with inter-annual modulations of a seasonally active delayed negative feedback response (Figure 4, Burls et al., 2012). Observed differences between the ENSO in the Pacific and inter-annual variability in the Atlantic (referred to in the literature as the Atlantic zonal mode) can then be accounted for in terms of these distinctions. We showed anomalous wind power over the tropical Atlantic to be a potential predictor for anomalous events within the equatorial Atlantic. However, because these events are due to a modulation of seasonally active coupled processes, and not independent processes operating on inter-annual time scales as seen in the Pacific, the lead time of this potential predictability is limited.

Figure 4. Tropical Atlantic available potential energy (APE) vs wind power phase diagrams for the 1988 eastern equatorial Atlantic warm event and 1992 cold event. The respective years are shown in black while the climatological phase diagram is shown in gray. Focusing on the climatology, between April and September a circular relationship between APE and wind power suggests that a seasonally excited thermocline mode of coupled variability plays an active role in the tropical Atlantic seasonal cycle (Burls et al., 2011). This figure illustrates that inter-annual variability is largely associated with a modification of the dynamic ocean processes that regulate eastern-central basin SST seasonally (Burls et al., 2012).