Hawaii FEP - Western Pacific Fishery Council

Near the equator, intense solar heating causes air to rise and water to evaporate, thus resulting inareas of low pressure. Air flowing from higher trade wind pressure areas move to the lowpressure areas such as the Intertropical Convergence Zone (ITCZ) and the South PacificConvergence Zone (SPCZ), which are located around 5° N and 30° S, respectively. Convergingtrade winds in these areas do not produce high winds, but instead often form areas that lacksignificant wind speeds. These areas of low winds are known as the “doldrums.” Theconvergence zones are associated near ridges of high sea–surface temperatures, withtemperatures of 28° C and above, and are areas of cloud accumulation and high rainfall amounts.The high rainfall amounts reduce ocean water salinity levels in these areas (Sturman andMcGowan 2003).The air that has risen in equatorial region fans out into the higher troposphere layer of theatmosphere and settles back toward Earth at middle latitudes. As air settles toward Earth, itcreates areas of high pressure known as subtropical high-pressure belts. One of these highpressureareas in the Pacific is called the “Hawaiian High Pressure Belt,” which is responsiblefor the prevailing trade wind pattern observed in the Hawaiian Islands (Sturman and McGowan2003).The Aleutian Low Pressure System is another prominent weather feature in the Pacific Oceanand is caused by dense polar air converging with air from the subtropical high-pressure belt. Asthese air masses converge around 60° N, air is uplifted, creating an area of low pressure. Whenthe relatively warm surface currents (Figure 8) meet the colder air temperatures of subpolarregions, latent heat is released, which causes precipitation. The Aleutian Low is an area wherelarge storms with high winds are produced. Such large storms and wind speeds have the abilityto affect the amount of mixing and upwelling between ocean layers (e.g., mixed layer andthermocline, Polovina et al. 1994).The Hawaii Archipelago is subject to high wave energy produced from weather systemsgenerated off the Aluetian Islands and other areas of the North Pacific. Such waves can havemajor effects on the nearshore environment. For example, high wave energies can break offpieces of coral, move underwater boulders, shift large volumes of sand, and erode islands (Grigg2002).The dynamics of the air–sea interface do not produce steady states of atmospheric pressuregradients and ocean circulation. As discussed in the previous sections, there are consistentweather patterns (e.g., ITCZ) and surface currents (e.g., north equatorial current); however,variability within the ocean–atmosphere system results in changes in winds, rainfall, currents,water column mixing, and sea-level heights, which can have profound effects on regionalclimates as well as on the abundance and distribution of marine organisms.One example of a shift in ocean–atmospheric conditions in the Pacific Ocean is El Niño–Southern Oscillation (ENSO). ENSO is linked to climatic changes in normal prominent weatherfeatures of the Pacific and Indian Oceans, such as the location of the ITCZ. ENSO, which canoccur every 2–10 years, results in the reduction of normal trade winds, which reduces theintensity of the westward flowing equatorial surface current (Sturman and McGowan 2003). Inturn, the eastward flowing countercurrent tends to dominate circulation, bringing warm, low-44