SIO 210: Introduction to Physical Oceanography

Pacific equatorial circulation and ENSO (interannual Variability)

Lynne Talley, Fall, 2013


Reading:
  • DPO sections S7.9 (7.9), 10.7, 10.8 (see lecture for which sections to emphasize)

  • Additional:
  • Stewart text, chapter 14
  • Tomczak and Godfrey, ch. 19
  • Wallace, J. M. and S. Vogel, 1994. Reports to the Nation on Our Changing Planet: El Nino and climate prediction. UCAR Office for Interdisciplinary Earth Studies, Boulder, CO. (Look in the rightmost sidebar for "Reports to the Nation")

    Links to powerpoints

    Watch powerpoint (Pacific equatorial circulation and ENSO)
    Download powerpoint (Pacific equatorial circulation and ENSO)
    Download pdf (Pacific equatorial circulation and ENSO)

    NOAA National Weather Service Climate Prediction Center is an excellent resource, with information on each of the types of variability, types of data used to monitor them, and current conditions.
    Climate Prediction Center's Forecast Forum
    TOGA TAO website for tropical Pacific data display and delivery (ENSO utility)

    1. Equatorial circulation, with emphasis on the Pacific Ocean

    The trade winds are relatively steady easterlies. They are driven by warm waters in the western region and cooler waters in the east, which creates rising air in the west and sinking air in the east, and a thermally direct flow from east to west to feed this (Walker cell). (The tropical region in the atmosphere is roughly from 20S to 20N.)

    In the ocean the true equatorial region is much narrower - about 2 degrees wide. Easterly trade winds at the equator drive (1) poleward Ekman transport and (2) westward surface flow, as follows:

    The easterly trade winds cause northward Ekman transport just to the north of the equator and southward Ekman transport just to the south of the equator. This causes upwelling at the equator. As a result, the pycnocline shoals towards the equator. This drives a westward geostrophic flow at the sea surface, much like the eastern boundary current flow.

    Directly on the equator, the effect of rotation on the circulation vanishes, and so the concepts of geostrophic and Ekman flow do not apply. At the equator, the easterly trade winds push the surface water directly (frictionally) from east to west. This water piles up gently in the western Pacific (0.5 meters higher there than in the eastern Pacific).

    Figure. El Nino sea surface height images from satellite altimetry (NASA/JPL). Look for one that is labelled either as "normal" or "La Nina" to get a sense of the mean sea surface height distribution at the equator. Look at images from 1997 forespecially good views of the El Nino state.

    The pycnocline is deeper in the west also as a result, and much warmer water is found there ("warm pool").

    Upwelling in the east draws cool water to the surface because of the shallow pycnocline there, but intense eastward-flowing upwelling in the west cannot create cold water at the surface there because of the thickness of the warm pool.

    Because the sea surface is higher in the west than in the east, there is a pressure difference that causes the flow just beneath the surface layer to be eastward. This strong eastward flow is the Equatorial Undercurrent. It is centered at about 150 to 200 meters depth. EUC speeds are in excess of 100 cm/sec. The current is exceptionally thin vertically (about 150 meters thick).

    The Equatorial Undercurrent shoals towards the east, as does the pycnocline. The shoaling is associated with upwelling of cool water in the central/eastern Pacific, giving rise to the "cold tongue" in non-El Nino years.

    Below the Equatorial Undercurrent, the equatorial currents are complex. The narrow equatorial region is a waveguide for waves with a lot of vertical structure. These waves decay away quickly away from the equator. The quasi-permanent current structure reflects this complexity. A series of "stacked jets" is found on the equator down to about 1000 m.

    Steady trade winds, which cause equatorial upwelling, are more prevalent in the east than in the west. There is seasonality in the winds, and equatorial upwelling is weaker in the northern winter and spring, giving rise to mini-El Nino conditions (topic 8) each year in the eastern equatorial Pacific.

    When the trade winds weaken or even reverse, the flow of water westward at the equator weakens or reverses and upwelling weakens or stops. Surface waters in the eastern Pacific warm significantly since upwelling is no longer bringing the cool waters to the surface. The deep warm pool in the western Pacific thins as its water sloshes eastward along the equator in the absence of the trade winds which maintain it.

    Off the equator, flow is geostrophic. Just north and south of the equator are found the North and South Subsurface Countercurrents ("Tsuchiya jets"), which flow eastward and sometimes appear to be slightly deeper poleward extensions of the EUC.

    North of the equator (5N to 10N) in the Pacific and Atlantic is found the intense North Equatorial Countercurrent. This is driven by cyclonic wind stress curl associated with the Intertropical Convergence Zone. This current probably reaches very deep into the ocean. It is the southern side of a very long and narrow cyclonic circulation.

    The north side of the circulation is part of the "North Equatorial Current", which is also the westward flow of the subtropical gyre. The North Equatorial Current in the Pacific reaches the western boundary and splits into the Kuroshio (northward flow for the subtropical gyre) and into the Mindanao Current (southward flow for the tropical cyclonic gyre).

    In the southern hemisphere there isn't usually a strong counterpart to the North Equatorial Countercurrent. Most of the time the westward flow of the northern part of the subtropical gyre appears to merge smoothly with the westward surface in the tropics and at the equator. This is called the "South Equatorial Current".

    In the Indian Ocean, the winds have very strong seasonality (monsoon). The equatorial current system is sensitive to the seasonality and complete reversals of currents occur. (See topic 8.)


    2. Introduction to climate variability

    Large-scale circulation, water properties, winds, heating/cooling, precipitation all have variability as well as the mean conditions considered thus far. The ocean is also an important part of some climate phenomena; it is "coupled" to the atmosphere for some types of dynamics, meaning that the atmosphere acts on the ocean to cause it to change which in turn act on the atmosphere to cause it to change.

    Feedbacks can be either positive or negative. Positive feedback mean that the coupling between the systems acts to increase the initially small change. Negative feedback means that the coupling acts to decrease the initial change.

    Variability in the large spatial scales is considered here at several time scales. We focus in this lecture on ENSO as a major example of interannual variation in which the ocean is major player.

    Time scales of variation:
    Intraseasonal variability: at time scales of 30-60 days, usually discussed for the atmosphere, but also involving the ocean in the tropics.
    Seasonal or annual variability: time scales of 1 year, obviously forced by the solar heating cycle. A prominent example of coupled seasonal variability is the monsoon.
    Interannual variability: time scales of 2 to about 7 years (poorly defined upper limit). Prominent example with ocean coupling is ENSO.
    Decadal variability: time scales of 1 to 3 decades (poorly defined again). Examples involving or affecting the ocean are the North Atlantic Oscillation, the Pacific Decadal Oscillation, the northern and southern annular modes.
    Centennial variability: time scales of centuries
    Glacial-interglacial variability: times scales of thousands of years, forced by orbital changes.

    The climate time scales associated with the ocean include seasonal variations (changes in surface temperature, associated with heating/cooling, indirectly associated with mixed layer depth and mechanisms), interannual variations (time scale for passage of large-scale waves such as Kelvin and Rossby waves, associated with earth's rotation, from one side of the ocean to another), decadal to centennial variations (time scale for gyre circulation), millenial variations (time scale for ocean overturn).

    Notes on presentation of climate data.
    Most climate data for interannual and decadal variations are presented in terms of anomalies (deviations) from the climatology (mean). The "mean" climatology is defined in many different ways. It can be the total mean of all data over the observing record - for instance 20 years of a particular data product. It can be a monthly climatology, which would be the monthly mean over the years in the observing record. For the anomalies to be relevant, the mean must be representative of the average of the normal extremes, and hence the record must be long enough to define a mean.


    3. El Nino Southern Oscillation (ENSO)

    There are many excellent websites concerning El Nino. A good entry point produced by NOAA is:
    http://www.pmel.noaa.gov/tao/index.shtml
    Another excellent entry point, based on satellite altimetry data, and produced by NASA is:
    http://topex-www.jpl.nasa.gov/science/el-nino.html

    The tropical Pacific is the seat of the global climate cycle variously known as "El Nino/La Nino" or the El Nino Southern Oscillation, which occurs every two to seven years. The name El Nino originated long ago from the recognition of increases in rainfall and decreases in fisheries along Peru around Christmas time. It has since been learned that these changes are part of a much larger climate pattern. The atmosphere and ocean are closely coupled in the equatorial region. Normal conditions include trade winds blowing from east to west; they result from air rising over a pool of warm water in the west ("warm pool") and air sinking over a tongue of cold water ("cold tongue") along the equator in the east (Walker cell). Clouds form and precipitate where air rises, and so normally there is rainfall in the western equatorial Pacific and dryness in the eastern Pacific. Associated with the trade winds, the atmospheric pressure is high in the east and low in the west.

    The trade winds force two things in the equatorial ocean: a westward surface current which carries warm water westward right along the equator (South Equatorial Current), and secondly upwelling along the equator due to poleward flow of the upper 30 to 50 meters of water just off the equator (Ekman transport). The poleward flow results from the earth's rotation which causes flow to be to the right of a force like the wind in the northern hemisphere and to the left in the southern hemisphere. Because the surface water carried to the west creates a thick layer of warm water there, the upwelled water is colder in the east than in the west. (Also, sea level is higher in the west due to the pileup of water there.) Thus the winds maintain the ocean temperature difference which then drives the winds.

    Under normal conditions, winds blowing equatorward along the west coast of the Americas cause the surface waters to move offshore (Ekman layer). This results in upwelling along these coasts, which draws up water from about 100 meters depth which is rich in nutrients (Peru and California Currents). This sustains large fisheries in these coastal regions.

    During an El Nino, the normal easterly trade winds weaken. In the western equatorial region they actually shift to being westerlies. The difference in atmospheric pressure between the central and western Pacific thus also decreases. (The pressure difference between Tahiti and Darwin, Australia, called the Southern Oscillation Index, is often taken as a measure of El Nino, hence the commonly-used name El Nino/Southern Oscillation.) The westward flow of ocean water at the equator then slows, which leads to a draining of the western warm pool towards the east. Equatorial ocean upwelling is reduced, which results in warmer sea surface temperatures in the eastern Pacific. As the western warm pool cools slightly and the central and eastern equatorial Pacific warm, this further decreases the strength of the tradewinds. This is therefore a positive feedback and the El Nino keeps growing.

    During an El Nino, the large atmospheric convection cell over Indonesia moves eastward. This results in drought in the western Pacific, including over Indonesia and Australia, and increased rainfall in the central and eastern Pacific, for instance at Christmas Island, the Galapagos and Ecuador.

    During an El Nino, the warm water in the eastern Pacific spreads to the west coast of the Americas and splits to flow north and south there. The normal upwelling off northern Peru weakens and also draws up only warm, nutrient-poor equatorial water. The result is a decline in production in this important fisheries area. If the El Nino is particularly strong, its effect in the ocean can reach as far north as the California coast.

    The opposite phase of the El Nino is called La Nina. (The name was coined to mean the opposite of El Nino.) La Nina is particularly strong occurrence of "normal" conditions. La Nina and El Nino are the two opposite phases of the same cycle. During La Nina, the tradewinds are especially strong, and the warm pool in the western Pacific and cold water at the equator in the central and eastern Pacific are particularly well-developed. Sea level is especially high in the western Pacific. Rainfall is strong in the western Pacific, with little rainfall in the eastern Pacific.

    El Nino/La Nina affects from the tropical Pacific can reach far to the northeast and southeast through the atmosphere. The cycle thus affects South and Central America and when especially strong, can impact the United States. During an especially strong El Nino, such as occurred in 1997, record rainfall is found in California, tornados and storms in the southeast U.S.

    El Nino occurs irregularly, but generally every three to seven years, which is the timescale for east-west propagation of large, slow ocean waves across the equatorial region. Major progress has been made in predicting an El Nino about one year in advance because the sequence of events in an El Nino is often the same. Thus detection of early signs of El Nino, such as the appearance of warm water in the eastern tropical Pacific or a change in the strength of the trade winds, often allows prediction of changes in rainfall and air temperature later in the year throughout the Pacific region. An ocean-atmosphere observing network and computer models now assist in observing and forecasting El Nino occurrences.

    The strength of El Nino varies greatly over an even more irregular time scale of about ten to thirty years. For instance El Nino's in the 1940's were strong, followed by several decades of weak events, and then followed by very strong El Nino's again in the 1980's and 1990's. Long records of El Nino's have been extracted from atmospheric pressure observations at Tahiti and Darwin, and from growth and properties of the annual accretion in coral heads in the tropical Pacific. This so-called decadal modulation of El Nino is much less well-understood at this time than El Nino itself.

    Involvement of ITCZ not detailed here. Kelvin wave mechanism for sloshing of warm pool eastward during El Nino.

    Southern Oscillation Index time series to 2003, from the NCEP Climate Diagnostics Bulletin

    Equatorial SST anomalies time series to 2003.

    NOAA/NESDIS global SST maps


    Study questions:

    Equatorial circulation

    1. Why is there such strong upwelling at the equator?
    2. What are the principle zonal currents in the equatorial Pacific?
    3. What is thought to drive the Equatorial Undercurrent?
    4. Where does the water in the Equatorial Undercurrent come from?
    5. What are the similarities between currents in the eastern boundary current regions and the equatorial region?

    El Nino

    6. About how often do El Ninos occur and what sets this timescale?
    7. What happens to the trades along the equator at the beginning of an El Nino? What is this usually in response to?
    8. What happens to precipitation patterns during an El Nino?
    9. What is the Southern Oscillation Index? At which phase of the El Nino/La Nina is the pressure at Tahiti high?
    10. Why is biological productivity along Peru decreased during an El Nino?

    Study calculations
    1. A steady westward wind blows across an ocean basin which is 5000 km wide. The surface wind stress measured at 5o north and 5o south is 0.05 N/m2. Calculate the net Ekman mass transport at these two latitudes. What is the average rate of upwelling between 5o north and 5o south.

    2. Suppose the water upwelling between 5o north and 5o south has a temperature of 24o and the mean ocean heating over this region is 100 W/m2. What is the mean temperature of the water flowing across 5o latitude in the Ekman layer?


    Further reading and sources for viewgraphs

    Rasmussen, E.M. and T.H. Carpenter, 1982. Variations in tropical sea surface temperature and surface wind fields associated with the Southern Oscillation/El Nino. Monthly Weather Rev., 110, 354-384.

    Philander, S.G., 1990. El Nino, La Nina and the Southern Oscillation. Academic Press. Look at the Introduction and Chapter 1.1, 1.2, 1.4, 1.5, and 1.7

    Intergovernmental Oceanographic Commission technical series, 40: Oceanic interdecadal climate variability (strong emphasis on Pacific)


    SIO 210 HOME November 24, 2013