Gregory R. Foltz

Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes

AbstractMost studies of African dust and North Atlantic climate have been limited to the short time period since the satellite era (1980 onward), precluding the examination of their relationship on longer time scales. Here a new dust dataset with the record extending back to the 1950s is used to show a multidecadal covariability of North Atlantic SST and aerosol, Sahel rainfall, and Atlantic hurricanes. When the North Atlantic Ocean was cold from the late 1960s to the early 1990s, the Sahel received less rainfall and the tropical North Atlantic experienced a high concentration of dust. The opposite was true when the North Atlantic Ocean was warm before the late 1960s and after the early 1990s. This suggests a novel mechanism for North Atlantic SST variability—a positive feedback between North Atlantic SST, African dust, and Sahel rainfall on multidecadal time scales. That is, a warm (cold) North Atlantic Ocean produces a wet (dry) condition in the Sahel and thus leads to low (high) concentration of dust i...

Ocean- Atmosphere Interactions During Cyclone Nargis

Cyclone Nargis (Figure 1a) made landfall in Myanmar (formerly Burma) on 2 May 2008 with sustained winds of approximately 210 kilometers per hour, equivalent to a category 3–4 hurricane. In addition, Nargis brought approximately 600 millimeters of rain and a storm surge of 3–4 meters to the low-lying and densely populated Irrawaddy River delta. In its wake, the storm left an estimated 130,000 dead or missing and more than

Impact of Saharan Dust on Tropical North Atlantic SST

10 billion in economic losses. It was the worst natural disaster to strike the Indian Ocean region since the 26 December 2004 tsunami and the worst recorded natural disaster ever to affect Myanmar.

Impact of Barrier Layer Thickness on SST in the Central Tropical North Atlantic

Abstract A combination of satellite and in situ datasets is used to investigate the impact of interannual changes in atmospheric dust content on the sea surface temperature (SST) of the tropical North Atlantic Ocean. Throughout most of the region the authors find, in agreement with previous studies, that positive anomalies of dust are associated with a significant reduction in surface shortwave radiation (SWR), while negative anomalies of dust are associated with an enhancement of SWR. Statistical analysis for 1984–2000 suggests that changes in dustiness in the tropical North Atlantic (10°–25°N, 20°–60°W) explained approximately 35% of the observed interannual SST variability during boreal summer, when climatological dust concentrations are highest. Measurements from a long-term moored buoy in the central tropical North Atlantic are used to investigate the causes of anomalously cool SST that occurred in conjunction with a period of enhanced dustiness at the start of the unexpectedly quiet 2006 hurricane s...

Seasonal Mixed Layer Heat Balance of the Southwestern Tropical Indian Ocean

Measurements from three long-term moored buoys are used to investigate the impact of barrier layer thickness (BLT) on the seasonal cycle of sea surface temperature (SST) in the central tropical North Atlantic Ocean. It is found that seasonal variations of the BLT exert a considerable influence on SST through their modulation of the vertical heatflux at the base of the mixed layer, estimated as the residual in the mixed layer heat balance. Cooling associated with this term is strongest when the barrier layer is thin and the vertical temperature gradient at the base of the mixed layer is strong. Conversely, thick barrier layers are associated with a significant reduction in the vertical temperature gradient at the base of the mixed layer, which suppresses the upward transfer of cooler water into the mixed layer. Forced ocean and coupled ocean‐ atmosphere models that do not properly simulate the barrier layer may have difficulty reproducing the observed seasonal cycle of SST in the tropical North Atlantic.

The Role of Oceanic Heat Advection in the Evolution of Tropical North and South Atlantic SST Anomalies

Sea surface temperature (SST) in the southwestern tropical Indian Ocean exerts a significant influence on global climate through its influence on the Indian summer monsoon and Northern Hemisphere atmospheric circulation. In this study, measurements from a long-term moored buoy are used in conjunction with satellite, in situ, and atmospheric reanalysis datasets to analyze the seasonal mixed layer heat balance in the thermocline ridge region of the southwestern tropical Indian Ocean. This region is characterized by a shallow mean thermocline (90 m, as measured by the 208C isotherm) and pronounced seasonal cycles of Ekman pumping and SST (seasonal ranges of 20.1 to 0.6 m day 21 and 268–29.58C, respectively). It is found that surface heat fluxes and horizontal heat advection contribute significantly to the seasonal cycle of mixed layer heat storage. The net surface heat flux tends to warm the mixed layer throughout the year and is strongest during boreal fall and winter, when surface shortwave radiation is highest and latent heat loss is weakest. Horizontal heat advection provides warming during boreal summer and fall, when southwestward surface currents and horizontal SST gradients are strongest, and is close to zero during the remainder of the year. Vertical turbulent mixing, estimated as a residual in the heat balance, also undergoes a significant seasonal cycle. Cooling from this term is strongest in boreal summer, when surface wind and buoyancy forcing are strongest, the thermocline ridge is shallow (,90 m), and the mixed layer is deepening. These empirical results provide a framework for addressing intraseasonal and interannual climate variations, which are dynamically linked to the seasonal cycle, in the southwestern tropical Indian Ocean. They also provide a quantitative basis for assessing the accuracy of numerical ocean model simulations in the region.

A Strong Atlantic Meridional Mode Event in 2009: The Role of Mixed Layer Dynamics*

Abstract The role of horizontal oceanic heat advection in the generation of tropical North and South Atlantic sea surface temperature (SST) anomalies is investigated through an analysis of the oceanic mixed layer heat balance. It is found that SST anomalies poleward of 10° are driven primarily by a combination of wind-induced latent heat loss and shortwave radiation. Away from the eastern boundary, horizontal advection damps surface flux–forced SST anomalies due to a combination of mean meridional Ekman currents acting on anomalous meridional SST gradients, and anomalous meridional currents acting on the mean meridional SST gradient. Horizontal advection is likely to have the most significant effect on the interhemispheric SST gradient mode through its impact in the 10°–20° latitude bands of each hemisphere, where the variability in advection is strongest and its negative correlation with the surface heat flux is highest. In addition to the damping effect of horizontal advection in these latitude bands, e...

The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing

In the first half of 2009, anomalous cooling of sea surface temperatures (SSTs) in the equatorial North Atlantic (ENA; 28‐128N) triggered a strong Atlantic meridional mode event. During its peak in April‐May, SSTs in the ENA were 18C colder than normal and SSTs in the equatorial South Atlantic (58S‐08) were 0.58C warmer than normal. Associated with the SST gradient were anomalous northerly winds, an anomalous southwardshiftoftheintertropicalconvergencezone,andseverefloodingin NortheastBrazil.Thisstudyuses in situ and satellite observations to examine the mechanisms responsible for the anomalous cooling in the ENAduringborealwinterandspringof2009.Itisfoundthatthecoolingwasinitiatedbystrongerthannormal trade winds during January and February 2009 associated with an anomalous strengthening of the subtropical North Atlantic high pressure system. Between 68 and 128N, unusually strong trade winds cooled the ocean through wind-induced evaporation and deepened the mixed layer anomalously by 5‐20 m. Closer to the equator, surface equatorial winds responded to the anomalous interhemispheric SST gradient, becoming northwesterly between the equator and 68N. The anomalous winds drove upwelling of 0.5‐1 m day 21 during March‐April, a period when there is normally weak downwelling. The associated vertical turbulent heat flux at the base of the mixed layer led to unusually cool SSTs in the central basin, further strengthening the anomalous interhemispheric SST gradient. These results emphasize the importance of mixed layer dynamics in the evolution of the meridional mode event of 2009 and the potential for positive coupled feedbacks between wind-induced upwelling and SST in the ENA.

Tropical instability vortices in the Atlantic Ocean

AbstractTwo decades of drifter and satellite data allow the authors to describe the seasonal evolution of the surface circulation of the Arabian Sea, which reverses annually with the Indian monsoon winds. This study finds several features that advance current understanding. Most significantly, northward flow appears along the length of the western boundary, together with a weak anticyclone at 6°N (a precursor to the Great Whirl) as early as March or April, one or two months before the southwest monsoon winds. This circulation is driven by planetary waves, which are initiated by wind curl forcing during the previous southwest monsoon, leading the authors to speculate that there is an oceanic mechanism through which one monsoon may precondition the next. Second, the authors find that the eastward South Equatorial Counter Current (SECC) is present year-round, fed by the northward East African Coastal Current (EACC). During the southwest monsoon the EACC overshoots the equator and splits, feeding both northwa...

Mixed Layer Heat Balance on Intraseasonal Time Scales in the Northwestern Tropical Atlantic Ocean

[1] This paper examines the physical characteristics and dynamics of Atlantic tropical instability vortices based on observational data from 1997 to 2000 and a high-resolution numerical model simulation. Most prominent during boreal summer, the vortices are characterized by 400 km spatial scales, westward translation at 0–40 cm s � 1 , anticyclonic circumferential velocities of 30–110 cm s � 1 , and SSTanomalies that decrease from 2� Ci n midbasin to 0.5� C or less near the western boundary. In contrast, the sea level anomalies grow as the vortices progress westward, reaching maximum amplitudes of 14 cm near 40� W. The large circumferential velocities, leading to relative vorticities of � 4 � 10 � 6 s � 1 and Rossby numbers of 0.5, indicate that centrifugal effects may play an important role in the vortices’ vorticity balance. We address the vortices’ vertical structure and vorticity dynamics by examination of a high-resolution numerical model. There is a reasonably good agreement between the model and observations. Simulated vortices are confined mostly to the mixed layer, which increases westward in depth from 30 to 100 m near the western boundary. In the eastern basin, potential vorticity within the vortices is not conserved but decreases because of wind stress forcing. In contrast, in the western basin, decreases in potential vorticity are due mostly to the vortices’ acquisition of Southern Hemisphere water. We estimate an annualized cross-equatorial transport of up to 1.2 Sv associated with the vortices, which is eventually contributed to the North Brazil Current system. INDEX TERMS: 4231 Oceanography: General: Equatorial oceanography; 4520 Oceanography: Physical: Eddies and mesoscale processes; 4528 Oceanography: Physical: Fronts and jets; KEYWORDS: tropics, instability, Atlantic