Brief Overview

The Atlantic Ocean is the second largest ocean, covering nearly one-fifth of the Earth’s surface. It is bounded by Africa and Europe in the east, and North and South America to the west. Sea Surface Temperature (SST) in the Atlantic Ocean plays a key role in modulating and interacting with weather and climate systems. Notable features of the Atlantic include the Azores and St. Helena high-pressure systems in the subtropics, the Intertropical Convergence Zone (ITCZ), which is located north of the equator, typically between 5-7oN. Additionally, the Atlantic is impacted by African easterly waves/disturbances that originate in Africa and move westward across the Atlantic Ocean. These disturbances often are the genesis of Atlantic hurricanes.

Seasonal Cycle
Tropical Atlantic variability is dominated by seasonal cycle . SSTs are warmest during boreal spring when the sun is closest to the equator, providing maximum solar radiation. During this period, the trade winds over the equator are weakest. As the sun moves northward, the trade winds along the equator intensify, causing the uplifting of the thermocline (upwelling) and leading to a cooling of SSTs in the eastern equatorial Atlantic. A distinct cold tongue forms during summer, with the coldest SST centered slightly south of the equator and extending along the eastern coast of Africa. The rapid SST cooling during summer season enhances ocean-continent thermal contrast, and hence impacts the development of west African monsoon

Atlantic Meridional Mode
Tropical Atlantic variability is often linked to two dominant patterns of coupled atmosphere-ocean climate modes. The most dominant mode is the cross-equatorial meridional gradient of SST anomalies (SSTA) in the tropical Atlantic. Such north-south contrast is often referred to as meridional mode. The meridional mode is often measured using the meridional gradient index (MGI). MGI is calculated by the area-averaged SST difference between the northern tropical Atlantic (60°W-30°W,5°N-20°N) and the southern tropical Atlantic (30°W-10°E, 20°S-0°N). A strong positive MGI is associated with warmer SSTs in the northern tropical Atlantic and cooler SSTs in the southern tropical Atlantic. This pattern is accompanied by anomalous near-surface winds that blow toward the warmer hemisphere. Additionally, a strong positive MGI is linked to wetter conditions in the Sahel, and drier conditions in Northeast Brazil and in regions surrounding the Gulf of Guinea. Conversely, a strong negative MGI index is linked to opposite precipitation patterns.

Atlantic Niño
The second dominant mode is the so-called A Atlantic Niño (or zonal mode). It is characterized by warmer than average SSTs in the eastern equatorial Atlantic and weaker than average trade winds over the east-central equatorial Atlantic. Atlantic Niño is sometimes called “ENSO’s little brother”. The Atlantic 3 index (ATL3), defined as area-averaged SST in the cold tongue region (20°W-0, 3°S-3°N), is often used to monitor the Atlantic Niño. The Atlantic zonal mode has three phases: positive, negative, and neutral. The positive phase (Atlantic Niño) is associated with above average SSTs in the eastern equatorial Atlantic, weaker trade winds over the central-eastern equatorial Atlantic, reduced rainfall in the Sahel, and an increased frequency of flooding in northeastern South America and the Gulf of Guinea. Conversely, the negative phase (Atlantic Niña) is associated with cooler than average SSTs in the eastern equatorial Atlantic, stronger than normal trade winds over the equatorial Atlantic, and dryer conditions in the Gulf. Neutral conditions occur when SSTs are near normal in the eastern equatorial Atlantic Ocean. Due to recent strong warming trend, a relative ATL3 has been proposed, which is defined as the difference between the ATL3 index and global mean SSTA. In addition to the MGI and Atlantic Niño, tropical Atlantic SST variability is also influenced by ENSO and the North Atlantic Oscillation (NAO). The relative importance of these factors varies from year to year.

Atlantic Multidecadal Oscillation
Beyond interannual time scale, SSTs in the North Atlantic (0-60oN) exhibit multidecadal variation with a dominant time scale of 60-80 years. This variability is named as the Atlantic Multidecadal Oscillation (AMO), defined as the SSTA averaged in 0-60°N, 80°W-0) or as the Atlantic Multidecadal Variability (AMV) to reflect its broad frequency range of fluctuations on the multidecadal time scale. Spatially, the AMO/AMV is characterized by SST anomalies (SSTAs) of the same sign across the North Atlantic, with larger anomalies extending from the Newfoundland to the eastern Atlantic. Meanwhile, to suppress the long-term trend, a relative AMO index was defined as the difference between the AMO index and global mean SSTA. As a key driver of multidecadal climate variations, the AMO affects European and North American climate over this time scale. AMO also plays an important role in modulating Atlantic hurricane activity. The warm phase of AMO is associated with an active era of Atlantic hurricane and vice versa. In addition to AMO, SST anomalies in the hurricane main development region (MDR, 90°W-18°W, 9°N-21°N) are another important factor affecting hurricane development. Strong SST warming in the MDR favors hurricane formation and intensification.